1 .. SPDX-License-Identifier: GPL-2.0
7 /proc/sys/net/ipv4/* Variables
8 ==============================
11 - 0 - disabled (default)
14 Forward Packets between interfaces.
16 This variable is special, its change resets all configuration
17 parameters to their default state (RFC1122 for hosts, RFC1812
20 ip_default_ttl - INTEGER
21 Default value of TTL field (Time To Live) for outgoing (but not
22 forwarded) IP packets. Should be between 1 and 255 inclusive.
23 Default: 64 (as recommended by RFC1700)
25 ip_no_pmtu_disc - INTEGER
26 Disable Path MTU Discovery. If enabled in mode 1 and a
27 fragmentation-required ICMP is received, the PMTU to this
28 destination will be set to min_pmtu (see below). You will need
29 to raise min_pmtu to the smallest interface MTU on your system
30 manually if you want to avoid locally generated fragments.
32 In mode 2 incoming Path MTU Discovery messages will be
33 discarded. Outgoing frames are handled the same as in mode 1,
34 implicitly setting IP_PMTUDISC_DONT on every created socket.
36 Mode 3 is a hardened pmtu discover mode. The kernel will only
37 accept fragmentation-needed errors if the underlying protocol
38 can verify them besides a plain socket lookup. Current
39 protocols for which pmtu events will be honored are TCP, SCTP
40 and DCCP as they verify e.g. the sequence number or the
41 association. This mode should not be enabled globally but is
42 only intended to secure e.g. name servers in namespaces where
43 TCP path mtu must still work but path MTU information of other
44 protocols should be discarded. If enabled globally this mode
45 could break other protocols.
52 default 552 - minimum discovered Path MTU
54 ip_forward_use_pmtu - BOOLEAN
55 By default we don't trust protocol path MTUs while forwarding
56 because they could be easily forged and can lead to unwanted
57 fragmentation by the router.
58 You only need to enable this if you have user-space software
59 which tries to discover path mtus by itself and depends on the
60 kernel honoring this information. This is normally not the
70 fwmark_reflect - BOOLEAN
71 Controls the fwmark of kernel-generated IPv4 reply packets that are not
72 associated with a socket for example, TCP RSTs or ICMP echo replies).
73 If unset, these packets have a fwmark of zero. If set, they have the
74 fwmark of the packet they are replying to.
78 fib_multipath_use_neigh - BOOLEAN
79 Use status of existing neighbor entry when determining nexthop for
80 multipath routes. If disabled, neighbor information is not used and
81 packets could be directed to a failed nexthop. Only valid for kernels
82 built with CONFIG_IP_ROUTE_MULTIPATH enabled.
91 fib_multipath_hash_policy - INTEGER
92 Controls which hash policy to use for multipath routes. Only valid
93 for kernels built with CONFIG_IP_ROUTE_MULTIPATH enabled.
101 - 2 - Layer 3 or inner Layer 3 if present
103 fib_sync_mem - UNSIGNED INTEGER
104 Amount of dirty memory from fib entries that can be backlogged before
105 synchronize_rcu is forced.
107 Default: 512kB Minimum: 64kB Maximum: 64MB
109 ip_forward_update_priority - INTEGER
110 Whether to update SKB priority from "TOS" field in IPv4 header after it
111 is forwarded. The new SKB priority is mapped from TOS field value
112 according to an rt_tos2priority table (see e.g. man tc-prio).
114 Default: 1 (Update priority.)
118 - 0 - Do not update priority.
119 - 1 - Update priority.
121 route/max_size - INTEGER
122 Maximum number of routes allowed in the kernel. Increase
123 this when using large numbers of interfaces and/or routes.
125 From linux kernel 3.6 onwards, this is deprecated for ipv4
126 as route cache is no longer used.
128 neigh/default/gc_thresh1 - INTEGER
129 Minimum number of entries to keep. Garbage collector will not
130 purge entries if there are fewer than this number.
134 neigh/default/gc_thresh2 - INTEGER
135 Threshold when garbage collector becomes more aggressive about
136 purging entries. Entries older than 5 seconds will be cleared
137 when over this number.
141 neigh/default/gc_thresh3 - INTEGER
142 Maximum number of non-PERMANENT neighbor entries allowed. Increase
143 this when using large numbers of interfaces and when communicating
144 with large numbers of directly-connected peers.
148 neigh/default/unres_qlen_bytes - INTEGER
149 The maximum number of bytes which may be used by packets
150 queued for each unresolved address by other network layers.
153 Setting negative value is meaningless and will return error.
155 Default: SK_WMEM_MAX, (same as net.core.wmem_default).
157 Exact value depends on architecture and kernel options,
158 but should be enough to allow queuing 256 packets
161 neigh/default/unres_qlen - INTEGER
162 The maximum number of packets which may be queued for each
163 unresolved address by other network layers.
165 (deprecated in linux 3.3) : use unres_qlen_bytes instead.
167 Prior to linux 3.3, the default value is 3 which may cause
168 unexpected packet loss. The current default value is calculated
169 according to default value of unres_qlen_bytes and true size of
174 mtu_expires - INTEGER
175 Time, in seconds, that cached PMTU information is kept.
177 min_adv_mss - INTEGER
178 The advertised MSS depends on the first hop route MTU, but will
179 never be lower than this setting.
181 fib_notify_on_flag_change - INTEGER
182 Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/
183 RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed.
185 After installing a route to the kernel, user space receives an
186 acknowledgment, which means the route was installed in the kernel,
187 but not necessarily in hardware.
188 It is also possible for a route already installed in hardware to change
189 its action and therefore its flags. For example, a host route that is
190 trapping packets can be "promoted" to perform decapsulation following
191 the installation of an IPinIP/VXLAN tunnel.
192 The notifications will indicate to user-space the state of the route.
194 Default: 0 (Do not emit notifications.)
198 - 0 - Do not emit notifications.
199 - 1 - Emit notifications.
200 - 2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change.
204 ipfrag_high_thresh - LONG INTEGER
205 Maximum memory used to reassemble IP fragments.
207 ipfrag_low_thresh - LONG INTEGER
208 (Obsolete since linux-4.17)
209 Maximum memory used to reassemble IP fragments before the kernel
210 begins to remove incomplete fragment queues to free up resources.
211 The kernel still accepts new fragments for defragmentation.
213 ipfrag_time - INTEGER
214 Time in seconds to keep an IP fragment in memory.
216 ipfrag_max_dist - INTEGER
217 ipfrag_max_dist is a non-negative integer value which defines the
218 maximum "disorder" which is allowed among fragments which share a
219 common IP source address. Note that reordering of packets is
220 not unusual, but if a large number of fragments arrive from a source
221 IP address while a particular fragment queue remains incomplete, it
222 probably indicates that one or more fragments belonging to that queue
223 have been lost. When ipfrag_max_dist is positive, an additional check
224 is done on fragments before they are added to a reassembly queue - if
225 ipfrag_max_dist (or more) fragments have arrived from a particular IP
226 address between additions to any IP fragment queue using that source
227 address, it's presumed that one or more fragments in the queue are
228 lost. The existing fragment queue will be dropped, and a new one
229 started. An ipfrag_max_dist value of zero disables this check.
231 Using a very small value, e.g. 1 or 2, for ipfrag_max_dist can
232 result in unnecessarily dropping fragment queues when normal
233 reordering of packets occurs, which could lead to poor application
234 performance. Using a very large value, e.g. 50000, increases the
235 likelihood of incorrectly reassembling IP fragments that originate
236 from different IP datagrams, which could result in data corruption.
242 inet_peer_threshold - INTEGER
243 The approximate size of the storage. Starting from this threshold
244 entries will be thrown aggressively. This threshold also determines
245 entries' time-to-live and time intervals between garbage collection
246 passes. More entries, less time-to-live, less GC interval.
248 inet_peer_minttl - INTEGER
249 Minimum time-to-live of entries. Should be enough to cover fragment
250 time-to-live on the reassembling side. This minimum time-to-live is
251 guaranteed if the pool size is less than inet_peer_threshold.
254 inet_peer_maxttl - INTEGER
255 Maximum time-to-live of entries. Unused entries will expire after
256 this period of time if there is no memory pressure on the pool (i.e.
257 when the number of entries in the pool is very small).
264 Limit of socket listen() backlog, known in userspace as SOMAXCONN.
265 Defaults to 4096. (Was 128 before linux-5.4)
266 See also tcp_max_syn_backlog for additional tuning for TCP sockets.
268 tcp_abort_on_overflow - BOOLEAN
269 If listening service is too slow to accept new connections,
270 reset them. Default state is FALSE. It means that if overflow
271 occurred due to a burst, connection will recover. Enable this
272 option _only_ if you are really sure that listening daemon
273 cannot be tuned to accept connections faster. Enabling this
274 option can harm clients of your server.
276 tcp_adv_win_scale - INTEGER
277 Count buffering overhead as bytes/2^tcp_adv_win_scale
278 (if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
281 Possible values are [-31, 31], inclusive.
285 tcp_allowed_congestion_control - STRING
286 Show/set the congestion control choices available to non-privileged
287 processes. The list is a subset of those listed in
288 tcp_available_congestion_control.
290 Default is "reno" and the default setting (tcp_congestion_control).
292 tcp_app_win - INTEGER
293 Reserve max(window/2^tcp_app_win, mss) of window for application
294 buffer. Value 0 is special, it means that nothing is reserved.
298 tcp_autocorking - BOOLEAN
299 Enable TCP auto corking :
300 When applications do consecutive small write()/sendmsg() system calls,
301 we try to coalesce these small writes as much as possible, to lower
302 total amount of sent packets. This is done if at least one prior
303 packet for the flow is waiting in Qdisc queues or device transmit
304 queue. Applications can still use TCP_CORK for optimal behavior
305 when they know how/when to uncork their sockets.
309 tcp_available_congestion_control - STRING
310 Shows the available congestion control choices that are registered.
311 More congestion control algorithms may be available as modules,
314 tcp_base_mss - INTEGER
315 The initial value of search_low to be used by the packetization layer
316 Path MTU discovery (MTU probing). If MTU probing is enabled,
317 this is the initial MSS used by the connection.
319 tcp_mtu_probe_floor - INTEGER
320 If MTU probing is enabled this caps the minimum MSS used for search_low
325 tcp_min_snd_mss - INTEGER
326 TCP SYN and SYNACK messages usually advertise an ADVMSS option,
327 as described in RFC 1122 and RFC 6691.
329 If this ADVMSS option is smaller than tcp_min_snd_mss,
330 it is silently capped to tcp_min_snd_mss.
332 Default : 48 (at least 8 bytes of payload per segment)
334 tcp_congestion_control - STRING
335 Set the congestion control algorithm to be used for new
336 connections. The algorithm "reno" is always available, but
337 additional choices may be available based on kernel configuration.
338 Default is set as part of kernel configuration.
339 For passive connections, the listener congestion control choice
342 [see setsockopt(listenfd, SOL_TCP, TCP_CONGESTION, "name" ...) ]
345 Allows TCP to send "duplicate" SACKs.
347 tcp_early_retrans - INTEGER
348 Tail loss probe (TLP) converts RTOs occurring due to tail
349 losses into fast recovery (draft-ietf-tcpm-rack). Note that
350 TLP requires RACK to function properly (see tcp_recovery below)
360 Control use of Explicit Congestion Notification (ECN) by TCP.
361 ECN is used only when both ends of the TCP connection indicate
362 support for it. This feature is useful in avoiding losses due
363 to congestion by allowing supporting routers to signal
364 congestion before having to drop packets.
368 = =====================================================
369 0 Disable ECN. Neither initiate nor accept ECN.
370 1 Enable ECN when requested by incoming connections and
371 also request ECN on outgoing connection attempts.
372 2 Enable ECN when requested by incoming connections
373 but do not request ECN on outgoing connections.
374 = =====================================================
378 tcp_ecn_fallback - BOOLEAN
379 If the kernel detects that ECN connection misbehaves, enable fall
380 back to non-ECN. Currently, this knob implements the fallback
381 from RFC3168, section 6.1.1.1., but we reserve that in future,
382 additional detection mechanisms could be implemented under this
383 knob. The value is not used, if tcp_ecn or per route (or congestion
384 control) ECN settings are disabled.
386 Default: 1 (fallback enabled)
389 This is a legacy option, it has no effect anymore.
391 tcp_fin_timeout - INTEGER
392 The length of time an orphaned (no longer referenced by any
393 application) connection will remain in the FIN_WAIT_2 state
394 before it is aborted at the local end. While a perfectly
395 valid "receive only" state for an un-orphaned connection, an
396 orphaned connection in FIN_WAIT_2 state could otherwise wait
397 forever for the remote to close its end of the connection.
404 Enables Forward RTO-Recovery (F-RTO) defined in RFC5682.
405 F-RTO is an enhanced recovery algorithm for TCP retransmission
406 timeouts. It is particularly beneficial in networks where the
407 RTT fluctuates (e.g., wireless). F-RTO is sender-side only
408 modification. It does not require any support from the peer.
410 By default it's enabled with a non-zero value. 0 disables F-RTO.
412 tcp_fwmark_accept - BOOLEAN
413 If set, incoming connections to listening sockets that do not have a
414 socket mark will set the mark of the accepting socket to the fwmark of
415 the incoming SYN packet. This will cause all packets on that connection
416 (starting from the first SYNACK) to be sent with that fwmark. The
417 listening socket's mark is unchanged. Listening sockets that already
418 have a fwmark set via setsockopt(SOL_SOCKET, SO_MARK, ...) are
423 tcp_invalid_ratelimit - INTEGER
424 Limit the maximal rate for sending duplicate acknowledgments
425 in response to incoming TCP packets that are for an existing
426 connection but that are invalid due to any of these reasons:
428 (a) out-of-window sequence number,
429 (b) out-of-window acknowledgment number, or
430 (c) PAWS (Protection Against Wrapped Sequence numbers) check failure
432 This can help mitigate simple "ack loop" DoS attacks, wherein
433 a buggy or malicious middlebox or man-in-the-middle can
434 rewrite TCP header fields in manner that causes each endpoint
435 to think that the other is sending invalid TCP segments, thus
436 causing each side to send an unterminating stream of duplicate
437 acknowledgments for invalid segments.
439 Using 0 disables rate-limiting of dupacks in response to
440 invalid segments; otherwise this value specifies the minimal
441 space between sending such dupacks, in milliseconds.
443 Default: 500 (milliseconds).
445 tcp_keepalive_time - INTEGER
446 How often TCP sends out keepalive messages when keepalive is enabled.
449 tcp_keepalive_probes - INTEGER
450 How many keepalive probes TCP sends out, until it decides that the
451 connection is broken. Default value: 9.
453 tcp_keepalive_intvl - INTEGER
454 How frequently the probes are send out. Multiplied by
455 tcp_keepalive_probes it is time to kill not responding connection,
456 after probes started. Default value: 75sec i.e. connection
457 will be aborted after ~11 minutes of retries.
459 tcp_l3mdev_accept - BOOLEAN
460 Enables child sockets to inherit the L3 master device index.
461 Enabling this option allows a "global" listen socket to work
462 across L3 master domains (e.g., VRFs) with connected sockets
463 derived from the listen socket to be bound to the L3 domain in
464 which the packets originated. Only valid when the kernel was
465 compiled with CONFIG_NET_L3_MASTER_DEV.
467 Default: 0 (disabled)
469 tcp_low_latency - BOOLEAN
470 This is a legacy option, it has no effect anymore.
472 tcp_max_orphans - INTEGER
473 Maximal number of TCP sockets not attached to any user file handle,
474 held by system. If this number is exceeded orphaned connections are
475 reset immediately and warning is printed. This limit exists
476 only to prevent simple DoS attacks, you _must_ not rely on this
477 or lower the limit artificially, but rather increase it
478 (probably, after increasing installed memory),
479 if network conditions require more than default value,
480 and tune network services to linger and kill such states
481 more aggressively. Let me to remind again: each orphan eats
482 up to ~64K of unswappable memory.
484 tcp_max_syn_backlog - INTEGER
485 Maximal number of remembered connection requests (SYN_RECV),
486 which have not received an acknowledgment from connecting client.
488 This is a per-listener limit.
490 The minimal value is 128 for low memory machines, and it will
491 increase in proportion to the memory of machine.
493 If server suffers from overload, try increasing this number.
495 Remember to also check /proc/sys/net/core/somaxconn
496 A SYN_RECV request socket consumes about 304 bytes of memory.
498 tcp_max_tw_buckets - INTEGER
499 Maximal number of timewait sockets held by system simultaneously.
500 If this number is exceeded time-wait socket is immediately destroyed
501 and warning is printed. This limit exists only to prevent
502 simple DoS attacks, you _must_ not lower the limit artificially,
503 but rather increase it (probably, after increasing installed memory),
504 if network conditions require more than default value.
506 tcp_mem - vector of 3 INTEGERs: min, pressure, max
507 min: below this number of pages TCP is not bothered about its
510 pressure: when amount of memory allocated by TCP exceeds this number
511 of pages, TCP moderates its memory consumption and enters memory
512 pressure mode, which is exited when memory consumption falls
515 max: number of pages allowed for queueing by all TCP sockets.
517 Defaults are calculated at boot time from amount of available
520 tcp_min_rtt_wlen - INTEGER
521 The window length of the windowed min filter to track the minimum RTT.
522 A shorter window lets a flow more quickly pick up new (higher)
523 minimum RTT when it is moved to a longer path (e.g., due to traffic
524 engineering). A longer window makes the filter more resistant to RTT
525 inflations such as transient congestion. The unit is seconds.
527 Possible values: 0 - 86400 (1 day)
531 tcp_moderate_rcvbuf - BOOLEAN
532 If set, TCP performs receive buffer auto-tuning, attempting to
533 automatically size the buffer (no greater than tcp_rmem[2]) to
534 match the size required by the path for full throughput. Enabled by
537 tcp_mtu_probing - INTEGER
538 Controls TCP Packetization-Layer Path MTU Discovery. Takes three
542 - 1 - Disabled by default, enabled when an ICMP black hole detected
543 - 2 - Always enabled, use initial MSS of tcp_base_mss.
545 tcp_probe_interval - UNSIGNED INTEGER
546 Controls how often to start TCP Packetization-Layer Path MTU
547 Discovery reprobe. The default is reprobing every 10 minutes as
550 tcp_probe_threshold - INTEGER
551 Controls when TCP Packetization-Layer Path MTU Discovery probing
552 will stop in respect to the width of search range in bytes. Default
555 tcp_no_metrics_save - BOOLEAN
556 By default, TCP saves various connection metrics in the route cache
557 when the connection closes, so that connections established in the
558 near future can use these to set initial conditions. Usually, this
559 increases overall performance, but may sometimes cause performance
560 degradation. If set, TCP will not cache metrics on closing
563 tcp_no_ssthresh_metrics_save - BOOLEAN
564 Controls whether TCP saves ssthresh metrics in the route cache.
566 Default is 1, which disables ssthresh metrics.
568 tcp_orphan_retries - INTEGER
569 This value influences the timeout of a locally closed TCP connection,
570 when RTO retransmissions remain unacknowledged.
571 See tcp_retries2 for more details.
573 The default value is 8.
575 If your machine is a loaded WEB server,
576 you should think about lowering this value, such sockets
577 may consume significant resources. Cf. tcp_max_orphans.
579 tcp_recovery - INTEGER
580 This value is a bitmap to enable various experimental loss recovery
583 ========= =============================================================
584 RACK: 0x1 enables the RACK loss detection for fast detection of lost
585 retransmissions and tail drops. It also subsumes and disables
586 RFC6675 recovery for SACK connections.
588 RACK: 0x2 makes RACK's reordering window static (min_rtt/4).
590 RACK: 0x4 disables RACK's DUPACK threshold heuristic
591 ========= =============================================================
595 tcp_reordering - INTEGER
596 Initial reordering level of packets in a TCP stream.
597 TCP stack can then dynamically adjust flow reordering level
598 between this initial value and tcp_max_reordering
602 tcp_max_reordering - INTEGER
603 Maximal reordering level of packets in a TCP stream.
604 300 is a fairly conservative value, but you might increase it
605 if paths are using per packet load balancing (like bonding rr mode)
609 tcp_retrans_collapse - BOOLEAN
610 Bug-to-bug compatibility with some broken printers.
611 On retransmit try to send bigger packets to work around bugs in
614 tcp_retries1 - INTEGER
615 This value influences the time, after which TCP decides, that
616 something is wrong due to unacknowledged RTO retransmissions,
617 and reports this suspicion to the network layer.
618 See tcp_retries2 for more details.
620 RFC 1122 recommends at least 3 retransmissions, which is the
623 tcp_retries2 - INTEGER
624 This value influences the timeout of an alive TCP connection,
625 when RTO retransmissions remain unacknowledged.
626 Given a value of N, a hypothetical TCP connection following
627 exponential backoff with an initial RTO of TCP_RTO_MIN would
628 retransmit N times before killing the connection at the (N+1)th RTO.
630 The default value of 15 yields a hypothetical timeout of 924.6
631 seconds and is a lower bound for the effective timeout.
632 TCP will effectively time out at the first RTO which exceeds the
633 hypothetical timeout.
635 RFC 1122 recommends at least 100 seconds for the timeout,
636 which corresponds to a value of at least 8.
638 tcp_rfc1337 - BOOLEAN
639 If set, the TCP stack behaves conforming to RFC1337. If unset,
640 we are not conforming to RFC, but prevent TCP TIME_WAIT
645 tcp_rmem - vector of 3 INTEGERs: min, default, max
646 min: Minimal size of receive buffer used by TCP sockets.
647 It is guaranteed to each TCP socket, even under moderate memory
652 default: initial size of receive buffer used by TCP sockets.
653 This value overrides net.core.rmem_default used by other protocols.
654 Default: 131072 bytes.
655 This value results in initial window of 65535.
657 max: maximal size of receive buffer allowed for automatically
658 selected receiver buffers for TCP socket. This value does not override
659 net.core.rmem_max. Calling setsockopt() with SO_RCVBUF disables
660 automatic tuning of that socket's receive buffer size, in which
661 case this value is ignored.
662 Default: between 131072 and 6MB, depending on RAM size.
665 Enable select acknowledgments (SACKS).
667 tcp_comp_sack_delay_ns - LONG INTEGER
668 TCP tries to reduce number of SACK sent, using a timer
669 based on 5% of SRTT, capped by this sysctl, in nano seconds.
670 The default is 1ms, based on TSO autosizing period.
672 Default : 1,000,000 ns (1 ms)
674 tcp_comp_sack_slack_ns - LONG INTEGER
675 This sysctl control the slack used when arming the
676 timer used by SACK compression. This gives extra time
677 for small RTT flows, and reduces system overhead by allowing
678 opportunistic reduction of timer interrupts.
680 Default : 100,000 ns (100 us)
682 tcp_comp_sack_nr - INTEGER
683 Max number of SACK that can be compressed.
684 Using 0 disables SACK compression.
688 tcp_slow_start_after_idle - BOOLEAN
689 If set, provide RFC2861 behavior and time out the congestion
690 window after an idle period. An idle period is defined at
691 the current RTO. If unset, the congestion window will not
692 be timed out after an idle period.
697 Use the Host requirements interpretation of the TCP urgent pointer field.
698 Most hosts use the older BSD interpretation, so if you turn this on
699 Linux might not communicate correctly with them.
703 tcp_synack_retries - INTEGER
704 Number of times SYNACKs for a passive TCP connection attempt will
705 be retransmitted. Should not be higher than 255. Default value
706 is 5, which corresponds to 31seconds till the last retransmission
707 with the current initial RTO of 1second. With this the final timeout
708 for a passive TCP connection will happen after 63seconds.
710 tcp_syncookies - INTEGER
711 Only valid when the kernel was compiled with CONFIG_SYN_COOKIES
712 Send out syncookies when the syn backlog queue of a socket
713 overflows. This is to prevent against the common 'SYN flood attack'
716 Note, that syncookies is fallback facility.
717 It MUST NOT be used to help highly loaded servers to stand
718 against legal connection rate. If you see SYN flood warnings
719 in your logs, but investigation shows that they occur
720 because of overload with legal connections, you should tune
721 another parameters until this warning disappear.
722 See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.
724 syncookies seriously violate TCP protocol, do not allow
725 to use TCP extensions, can result in serious degradation
726 of some services (f.e. SMTP relaying), visible not by you,
727 but your clients and relays, contacting you. While you see
728 SYN flood warnings in logs not being really flooded, your server
729 is seriously misconfigured.
731 If you want to test which effects syncookies have to your
732 network connections you can set this knob to 2 to enable
733 unconditionally generation of syncookies.
735 tcp_fastopen - INTEGER
736 Enable TCP Fast Open (RFC7413) to send and accept data in the opening
739 The client support is enabled by flag 0x1 (on by default). The client
740 then must use sendmsg() or sendto() with the MSG_FASTOPEN flag,
741 rather than connect() to send data in SYN.
743 The server support is enabled by flag 0x2 (off by default). Then
744 either enable for all listeners with another flag (0x400) or
745 enable individual listeners via TCP_FASTOPEN socket option with
746 the option value being the length of the syn-data backlog.
748 The values (bitmap) are
750 ===== ======== ======================================================
751 0x1 (client) enables sending data in the opening SYN on the client.
752 0x2 (server) enables the server support, i.e., allowing data in
753 a SYN packet to be accepted and passed to the
754 application before 3-way handshake finishes.
755 0x4 (client) send data in the opening SYN regardless of cookie
756 availability and without a cookie option.
757 0x200 (server) accept data-in-SYN w/o any cookie option present.
758 0x400 (server) enable all listeners to support Fast Open by
759 default without explicit TCP_FASTOPEN socket option.
760 ===== ======== ======================================================
764 Note that additional client or server features are only
765 effective if the basic support (0x1 and 0x2) are enabled respectively.
767 tcp_fastopen_blackhole_timeout_sec - INTEGER
768 Initial time period in second to disable Fastopen on active TCP sockets
769 when a TFO firewall blackhole issue happens.
770 This time period will grow exponentially when more blackhole issues
771 get detected right after Fastopen is re-enabled and will reset to
772 initial value when the blackhole issue goes away.
773 0 to disable the blackhole detection.
775 By default, it is set to 1hr.
777 tcp_fastopen_key - list of comma separated 32-digit hexadecimal INTEGERs
778 The list consists of a primary key and an optional backup key. The
779 primary key is used for both creating and validating cookies, while the
780 optional backup key is only used for validating cookies. The purpose of
781 the backup key is to maximize TFO validation when keys are rotated.
783 A randomly chosen primary key may be configured by the kernel if
784 the tcp_fastopen sysctl is set to 0x400 (see above), or if the
785 TCP_FASTOPEN setsockopt() optname is set and a key has not been
786 previously configured via sysctl. If keys are configured via
787 setsockopt() by using the TCP_FASTOPEN_KEY optname, then those
788 per-socket keys will be used instead of any keys that are specified via
791 A key is specified as 4 8-digit hexadecimal integers which are separated
792 by a '-' as: xxxxxxxx-xxxxxxxx-xxxxxxxx-xxxxxxxx. Leading zeros may be
793 omitted. A primary and a backup key may be specified by separating them
794 by a comma. If only one key is specified, it becomes the primary key and
795 any previously configured backup keys are removed.
797 tcp_syn_retries - INTEGER
798 Number of times initial SYNs for an active TCP connection attempt
799 will be retransmitted. Should not be higher than 127. Default value
800 is 6, which corresponds to 63seconds till the last retransmission
801 with the current initial RTO of 1second. With this the final timeout
802 for an active TCP connection attempt will happen after 127seconds.
804 tcp_timestamps - INTEGER
805 Enable timestamps as defined in RFC1323.
808 - 1: Enable timestamps as defined in RFC1323 and use random offset for
809 each connection rather than only using the current time.
810 - 2: Like 1, but without random offsets.
814 tcp_min_tso_segs - INTEGER
815 Minimal number of segments per TSO frame.
817 Since linux-3.12, TCP does an automatic sizing of TSO frames,
818 depending on flow rate, instead of filling 64Kbytes packets.
819 For specific usages, it's possible to force TCP to build big
820 TSO frames. Note that TCP stack might split too big TSO packets
821 if available window is too small.
825 tcp_pacing_ss_ratio - INTEGER
826 sk->sk_pacing_rate is set by TCP stack using a ratio applied
827 to current rate. (current_rate = cwnd * mss / srtt)
828 If TCP is in slow start, tcp_pacing_ss_ratio is applied
829 to let TCP probe for bigger speeds, assuming cwnd can be
830 doubled every other RTT.
834 tcp_pacing_ca_ratio - INTEGER
835 sk->sk_pacing_rate is set by TCP stack using a ratio applied
836 to current rate. (current_rate = cwnd * mss / srtt)
837 If TCP is in congestion avoidance phase, tcp_pacing_ca_ratio
838 is applied to conservatively probe for bigger throughput.
842 tcp_tso_win_divisor - INTEGER
843 This allows control over what percentage of the congestion window
844 can be consumed by a single TSO frame.
845 The setting of this parameter is a choice between burstiness and
846 building larger TSO frames.
850 tcp_tw_reuse - INTEGER
851 Enable reuse of TIME-WAIT sockets for new connections when it is
852 safe from protocol viewpoint.
856 - 2 - enable for loopback traffic only
858 It should not be changed without advice/request of technical
863 tcp_window_scaling - BOOLEAN
864 Enable window scaling as defined in RFC1323.
866 tcp_wmem - vector of 3 INTEGERs: min, default, max
867 min: Amount of memory reserved for send buffers for TCP sockets.
868 Each TCP socket has rights to use it due to fact of its birth.
872 default: initial size of send buffer used by TCP sockets. This
873 value overrides net.core.wmem_default used by other protocols.
875 It is usually lower than net.core.wmem_default.
879 max: Maximal amount of memory allowed for automatically tuned
880 send buffers for TCP sockets. This value does not override
881 net.core.wmem_max. Calling setsockopt() with SO_SNDBUF disables
882 automatic tuning of that socket's send buffer size, in which case
883 this value is ignored.
885 Default: between 64K and 4MB, depending on RAM size.
887 tcp_notsent_lowat - UNSIGNED INTEGER
888 A TCP socket can control the amount of unsent bytes in its write queue,
889 thanks to TCP_NOTSENT_LOWAT socket option. poll()/select()/epoll()
890 reports POLLOUT events if the amount of unsent bytes is below a per
891 socket value, and if the write queue is not full. sendmsg() will
892 also not add new buffers if the limit is hit.
894 This global variable controls the amount of unsent data for
895 sockets not using TCP_NOTSENT_LOWAT. For these sockets, a change
896 to the global variable has immediate effect.
898 Default: UINT_MAX (0xFFFFFFFF)
900 tcp_workaround_signed_windows - BOOLEAN
901 If set, assume no receipt of a window scaling option means the
902 remote TCP is broken and treats the window as a signed quantity.
903 If unset, assume the remote TCP is not broken even if we do
904 not receive a window scaling option from them.
908 tcp_thin_linear_timeouts - BOOLEAN
909 Enable dynamic triggering of linear timeouts for thin streams.
910 If set, a check is performed upon retransmission by timeout to
911 determine if the stream is thin (less than 4 packets in flight).
912 As long as the stream is found to be thin, up to 6 linear
913 timeouts may be performed before exponential backoff mode is
914 initiated. This improves retransmission latency for
915 non-aggressive thin streams, often found to be time-dependent.
916 For more information on thin streams, see
917 Documentation/networking/tcp-thin.rst
921 tcp_limit_output_bytes - INTEGER
922 Controls TCP Small Queue limit per tcp socket.
923 TCP bulk sender tends to increase packets in flight until it
924 gets losses notifications. With SNDBUF autotuning, this can
925 result in a large amount of packets queued on the local machine
926 (e.g.: qdiscs, CPU backlog, or device) hurting latency of other
927 flows, for typical pfifo_fast qdiscs. tcp_limit_output_bytes
928 limits the number of bytes on qdisc or device to reduce artificial
929 RTT/cwnd and reduce bufferbloat.
931 Default: 1048576 (16 * 65536)
933 tcp_challenge_ack_limit - INTEGER
934 Limits number of Challenge ACK sent per second, as recommended
935 in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)
938 tcp_rx_skb_cache - BOOLEAN
939 Controls a per TCP socket cache of one skb, that might help
940 performance of some workloads. This might be dangerous
941 on systems with a lot of TCP sockets, since it increases
944 Default: 0 (disabled)
949 udp_l3mdev_accept - BOOLEAN
950 Enabling this option allows a "global" bound socket to work
951 across L3 master domains (e.g., VRFs) with packets capable of
952 being received regardless of the L3 domain in which they
953 originated. Only valid when the kernel was compiled with
954 CONFIG_NET_L3_MASTER_DEV.
956 Default: 0 (disabled)
958 udp_mem - vector of 3 INTEGERs: min, pressure, max
959 Number of pages allowed for queueing by all UDP sockets.
961 min: Below this number of pages UDP is not bothered about its
962 memory appetite. When amount of memory allocated by UDP exceeds
963 this number, UDP starts to moderate memory usage.
965 pressure: This value was introduced to follow format of tcp_mem.
967 max: Number of pages allowed for queueing by all UDP sockets.
969 Default is calculated at boot time from amount of available memory.
971 udp_rmem_min - INTEGER
972 Minimal size of receive buffer used by UDP sockets in moderation.
973 Each UDP socket is able to use the size for receiving data, even if
974 total pages of UDP sockets exceed udp_mem pressure. The unit is byte.
978 udp_wmem_min - INTEGER
979 Minimal size of send buffer used by UDP sockets in moderation.
980 Each UDP socket is able to use the size for sending data, even if
981 total pages of UDP sockets exceed udp_mem pressure. The unit is byte.
988 raw_l3mdev_accept - BOOLEAN
989 Enabling this option allows a "global" bound socket to work
990 across L3 master domains (e.g., VRFs) with packets capable of
991 being received regardless of the L3 domain in which they
992 originated. Only valid when the kernel was compiled with
993 CONFIG_NET_L3_MASTER_DEV.
1000 cipso_cache_enable - BOOLEAN
1001 If set, enable additions to and lookups from the CIPSO label mapping
1002 cache. If unset, additions are ignored and lookups always result in a
1003 miss. However, regardless of the setting the cache is still
1004 invalidated when required when means you can safely toggle this on and
1005 off and the cache will always be "safe".
1009 cipso_cache_bucket_size - INTEGER
1010 The CIPSO label cache consists of a fixed size hash table with each
1011 hash bucket containing a number of cache entries. This variable limits
1012 the number of entries in each hash bucket; the larger the value the
1013 more CIPSO label mappings that can be cached. When the number of
1014 entries in a given hash bucket reaches this limit adding new entries
1015 causes the oldest entry in the bucket to be removed to make room.
1019 cipso_rbm_optfmt - BOOLEAN
1020 Enable the "Optimized Tag 1 Format" as defined in section 3.4.2.6 of
1021 the CIPSO draft specification (see Documentation/netlabel for details).
1022 This means that when set the CIPSO tag will be padded with empty
1023 categories in order to make the packet data 32-bit aligned.
1027 cipso_rbm_structvalid - BOOLEAN
1028 If set, do a very strict check of the CIPSO option when
1029 ip_options_compile() is called. If unset, relax the checks done during
1030 ip_options_compile(). Either way is "safe" as errors are caught else
1031 where in the CIPSO processing code but setting this to 0 (False) should
1032 result in less work (i.e. it should be faster) but could cause problems
1033 with other implementations that require strict checking.
1040 ip_local_port_range - 2 INTEGERS
1041 Defines the local port range that is used by TCP and UDP to
1042 choose the local port. The first number is the first, the
1043 second the last local port number.
1044 If possible, it is better these numbers have different parity
1045 (one even and one odd value).
1046 Must be greater than or equal to ip_unprivileged_port_start.
1047 The default values are 32768 and 60999 respectively.
1049 ip_local_reserved_ports - list of comma separated ranges
1050 Specify the ports which are reserved for known third-party
1051 applications. These ports will not be used by automatic port
1052 assignments (e.g. when calling connect() or bind() with port
1053 number 0). Explicit port allocation behavior is unchanged.
1055 The format used for both input and output is a comma separated
1056 list of ranges (e.g. "1,2-4,10-10" for ports 1, 2, 3, 4 and
1057 10). Writing to the file will clear all previously reserved
1058 ports and update the current list with the one given in the
1061 Note that ip_local_port_range and ip_local_reserved_ports
1062 settings are independent and both are considered by the kernel
1063 when determining which ports are available for automatic port
1066 You can reserve ports which are not in the current
1067 ip_local_port_range, e.g.::
1069 $ cat /proc/sys/net/ipv4/ip_local_port_range
1071 $ cat /proc/sys/net/ipv4/ip_local_reserved_ports
1074 although this is redundant. However such a setting is useful
1075 if later the port range is changed to a value that will
1076 include the reserved ports.
1080 ip_unprivileged_port_start - INTEGER
1081 This is a per-namespace sysctl. It defines the first
1082 unprivileged port in the network namespace. Privileged ports
1083 require root or CAP_NET_BIND_SERVICE in order to bind to them.
1084 To disable all privileged ports, set this to 0. They must not
1085 overlap with the ip_local_port_range.
1089 ip_nonlocal_bind - BOOLEAN
1090 If set, allows processes to bind() to non-local IP addresses,
1091 which can be quite useful - but may break some applications.
1095 ip_autobind_reuse - BOOLEAN
1096 By default, bind() does not select the ports automatically even if
1097 the new socket and all sockets bound to the port have SO_REUSEADDR.
1098 ip_autobind_reuse allows bind() to reuse the port and this is useful
1099 when you use bind()+connect(), but may break some applications.
1100 The preferred solution is to use IP_BIND_ADDRESS_NO_PORT and this
1101 option should only be set by experts.
1104 ip_dynaddr - BOOLEAN
1105 If set non-zero, enables support for dynamic addresses.
1106 If set to a non-zero value larger than 1, a kernel log
1107 message will be printed when dynamic address rewriting
1112 ip_early_demux - BOOLEAN
1113 Optimize input packet processing down to one demux for
1114 certain kinds of local sockets. Currently we only do this
1115 for established TCP and connected UDP sockets.
1117 It may add an additional cost for pure routing workloads that
1118 reduces overall throughput, in such case you should disable it.
1122 ping_group_range - 2 INTEGERS
1123 Restrict ICMP_PROTO datagram sockets to users in the group range.
1124 The default is "1 0", meaning, that nobody (not even root) may
1125 create ping sockets. Setting it to "100 100" would grant permissions
1126 to the single group. "0 4294967295" would enable it for the world, "100
1127 4294967295" would enable it for the users, but not daemons.
1129 tcp_early_demux - BOOLEAN
1130 Enable early demux for established TCP sockets.
1134 udp_early_demux - BOOLEAN
1135 Enable early demux for connected UDP sockets. Disable this if
1136 your system could experience more unconnected load.
1140 icmp_echo_ignore_all - BOOLEAN
1141 If set non-zero, then the kernel will ignore all ICMP ECHO
1142 requests sent to it.
1146 icmp_echo_ignore_broadcasts - BOOLEAN
1147 If set non-zero, then the kernel will ignore all ICMP ECHO and
1148 TIMESTAMP requests sent to it via broadcast/multicast.
1152 icmp_ratelimit - INTEGER
1153 Limit the maximal rates for sending ICMP packets whose type matches
1154 icmp_ratemask (see below) to specific targets.
1155 0 to disable any limiting,
1156 otherwise the minimal space between responses in milliseconds.
1157 Note that another sysctl, icmp_msgs_per_sec limits the number
1158 of ICMP packets sent on all targets.
1162 icmp_msgs_per_sec - INTEGER
1163 Limit maximal number of ICMP packets sent per second from this host.
1164 Only messages whose type matches icmp_ratemask (see below) are
1165 controlled by this limit. For security reasons, the precise count
1166 of messages per second is randomized.
1170 icmp_msgs_burst - INTEGER
1171 icmp_msgs_per_sec controls number of ICMP packets sent per second,
1172 while icmp_msgs_burst controls the burst size of these packets.
1173 For security reasons, the precise burst size is randomized.
1177 icmp_ratemask - INTEGER
1178 Mask made of ICMP types for which rates are being limited.
1180 Significant bits: IHGFEDCBA9876543210
1182 Default mask: 0000001100000011000 (6168)
1184 Bit definitions (see include/linux/icmp.h):
1186 = =========================
1188 3 Destination Unreachable [1]_
1189 4 Source Quench [1]_
1192 B Time Exceeded [1]_
1193 C Parameter Problem [1]_
1198 H Address Mask Request
1199 I Address Mask Reply
1200 = =========================
1202 .. [1] These are rate limited by default (see default mask above)
1204 icmp_ignore_bogus_error_responses - BOOLEAN
1205 Some routers violate RFC1122 by sending bogus responses to broadcast
1206 frames. Such violations are normally logged via a kernel warning.
1207 If this is set to TRUE, the kernel will not give such warnings, which
1208 will avoid log file clutter.
1212 icmp_errors_use_inbound_ifaddr - BOOLEAN
1214 If zero, icmp error messages are sent with the primary address of
1215 the exiting interface.
1217 If non-zero, the message will be sent with the primary address of
1218 the interface that received the packet that caused the icmp error.
1219 This is the behaviour many network administrators will expect from
1220 a router. And it can make debugging complicated network layouts
1223 Note that if no primary address exists for the interface selected,
1224 then the primary address of the first non-loopback interface that
1225 has one will be used regardless of this setting.
1229 igmp_max_memberships - INTEGER
1230 Change the maximum number of multicast groups we can subscribe to.
1233 Theoretical maximum value is bounded by having to send a membership
1234 report in a single datagram (i.e. the report can't span multiple
1235 datagrams, or risk confusing the switch and leaving groups you don't
1238 The number of supported groups 'M' is bounded by the number of group
1239 report entries you can fit into a single datagram of 65535 bytes.
1241 M = 65536-sizeof (ip header)/(sizeof(Group record))
1243 Group records are variable length, with a minimum of 12 bytes.
1244 So net.ipv4.igmp_max_memberships should not be set higher than:
1246 (65536-24) / 12 = 5459
1248 The value 5459 assumes no IP header options, so in practice
1249 this number may be lower.
1251 igmp_max_msf - INTEGER
1252 Maximum number of addresses allowed in the source filter list for a
1258 Controls the IGMP query robustness variable (see RFC2236 8.1).
1260 Default: 2 (as specified by RFC2236 8.1)
1262 Minimum: 1 (as specified by RFC6636 4.5)
1264 force_igmp_version - INTEGER
1265 - 0 - (default) No enforcement of a IGMP version, IGMPv1/v2 fallback
1266 allowed. Will back to IGMPv3 mode again if all IGMPv1/v2 Querier
1267 Present timer expires.
1268 - 1 - Enforce to use IGMP version 1. Will also reply IGMPv1 report if
1269 receive IGMPv2/v3 query.
1270 - 2 - Enforce to use IGMP version 2. Will fallback to IGMPv1 if receive
1271 IGMPv1 query message. Will reply report if receive IGMPv3 query.
1272 - 3 - Enforce to use IGMP version 3. The same react with default 0.
1276 this is not the same with force_mld_version because IGMPv3 RFC3376
1277 Security Considerations does not have clear description that we could
1278 ignore other version messages completely as MLDv2 RFC3810. So make
1279 this value as default 0 is recommended.
1281 ``conf/interface/*``
1282 changes special settings per interface (where
1283 interface" is the name of your network interface)
1286 is special, changes the settings for all interfaces
1288 log_martians - BOOLEAN
1289 Log packets with impossible addresses to kernel log.
1290 log_martians for the interface will be enabled if at least one of
1291 conf/{all,interface}/log_martians is set to TRUE,
1292 it will be disabled otherwise
1294 accept_redirects - BOOLEAN
1295 Accept ICMP redirect messages.
1296 accept_redirects for the interface will be enabled if:
1298 - both conf/{all,interface}/accept_redirects are TRUE in the case
1299 forwarding for the interface is enabled
1303 - at least one of conf/{all,interface}/accept_redirects is TRUE in the
1304 case forwarding for the interface is disabled
1306 accept_redirects for the interface will be disabled otherwise
1313 forwarding - BOOLEAN
1314 Enable IP forwarding on this interface. This controls whether packets
1315 received _on_ this interface can be forwarded.
1317 mc_forwarding - BOOLEAN
1318 Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE
1319 and a multicast routing daemon is required.
1320 conf/all/mc_forwarding must also be set to TRUE to enable multicast
1321 routing for the interface
1324 Integer value used to differentiate the devices by the medium they
1325 are attached to. Two devices can have different id values when
1326 the broadcast packets are received only on one of them.
1327 The default value 0 means that the device is the only interface
1328 to its medium, value of -1 means that medium is not known.
1330 Currently, it is used to change the proxy_arp behavior:
1331 the proxy_arp feature is enabled for packets forwarded between
1332 two devices attached to different media.
1337 proxy_arp for the interface will be enabled if at least one of
1338 conf/{all,interface}/proxy_arp is set to TRUE,
1339 it will be disabled otherwise
1341 proxy_arp_pvlan - BOOLEAN
1342 Private VLAN proxy arp.
1344 Basically allow proxy arp replies back to the same interface
1345 (from which the ARP request/solicitation was received).
1347 This is done to support (ethernet) switch features, like RFC
1348 3069, where the individual ports are NOT allowed to
1349 communicate with each other, but they are allowed to talk to
1350 the upstream router. As described in RFC 3069, it is possible
1351 to allow these hosts to communicate through the upstream
1352 router by proxy_arp'ing. Don't need to be used together with
1355 This technology is known by different names:
1357 In RFC 3069 it is called VLAN Aggregation.
1358 Cisco and Allied Telesyn call it Private VLAN.
1359 Hewlett-Packard call it Source-Port filtering or port-isolation.
1360 Ericsson call it MAC-Forced Forwarding (RFC Draft).
1362 shared_media - BOOLEAN
1363 Send(router) or accept(host) RFC1620 shared media redirects.
1364 Overrides secure_redirects.
1366 shared_media for the interface will be enabled if at least one of
1367 conf/{all,interface}/shared_media is set to TRUE,
1368 it will be disabled otherwise
1372 secure_redirects - BOOLEAN
1373 Accept ICMP redirect messages only to gateways listed in the
1374 interface's current gateway list. Even if disabled, RFC1122 redirect
1377 Overridden by shared_media.
1379 secure_redirects for the interface will be enabled if at least one of
1380 conf/{all,interface}/secure_redirects is set to TRUE,
1381 it will be disabled otherwise
1385 send_redirects - BOOLEAN
1386 Send redirects, if router.
1388 send_redirects for the interface will be enabled if at least one of
1389 conf/{all,interface}/send_redirects is set to TRUE,
1390 it will be disabled otherwise
1394 bootp_relay - BOOLEAN
1395 Accept packets with source address 0.b.c.d destined
1396 not to this host as local ones. It is supposed, that
1397 BOOTP relay daemon will catch and forward such packets.
1398 conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay
1403 Not Implemented Yet.
1405 accept_source_route - BOOLEAN
1406 Accept packets with SRR option.
1407 conf/all/accept_source_route must also be set to TRUE to accept packets
1408 with SRR option on the interface
1415 accept_local - BOOLEAN
1416 Accept packets with local source addresses. In combination with
1417 suitable routing, this can be used to direct packets between two
1418 local interfaces over the wire and have them accepted properly.
1421 route_localnet - BOOLEAN
1422 Do not consider loopback addresses as martian source or destination
1423 while routing. This enables the use of 127/8 for local routing purposes.
1428 - 0 - No source validation.
1429 - 1 - Strict mode as defined in RFC3704 Strict Reverse Path
1430 Each incoming packet is tested against the FIB and if the interface
1431 is not the best reverse path the packet check will fail.
1432 By default failed packets are discarded.
1433 - 2 - Loose mode as defined in RFC3704 Loose Reverse Path
1434 Each incoming packet's source address is also tested against the FIB
1435 and if the source address is not reachable via any interface
1436 the packet check will fail.
1438 Current recommended practice in RFC3704 is to enable strict mode
1439 to prevent IP spoofing from DDos attacks. If using asymmetric routing
1440 or other complicated routing, then loose mode is recommended.
1442 The max value from conf/{all,interface}/rp_filter is used
1443 when doing source validation on the {interface}.
1445 Default value is 0. Note that some distributions enable it
1448 src_valid_mark - BOOLEAN
1449 - 0 - The fwmark of the packet is not included in reverse path
1450 route lookup. This allows for asymmetric routing configurations
1451 utilizing the fwmark in only one direction, e.g., transparent
1454 - 1 - The fwmark of the packet is included in reverse path route
1455 lookup. This permits rp_filter to function when the fwmark is
1456 used for routing traffic in both directions.
1458 This setting also affects the utilization of fmwark when
1459 performing source address selection for ICMP replies, or
1460 determining addresses stored for the IPOPT_TS_TSANDADDR and
1461 IPOPT_RR IP options.
1463 The max value from conf/{all,interface}/src_valid_mark is used.
1467 arp_filter - BOOLEAN
1468 - 1 - Allows you to have multiple network interfaces on the same
1469 subnet, and have the ARPs for each interface be answered
1470 based on whether or not the kernel would route a packet from
1471 the ARP'd IP out that interface (therefore you must use source
1472 based routing for this to work). In other words it allows control
1473 of which cards (usually 1) will respond to an arp request.
1475 - 0 - (default) The kernel can respond to arp requests with addresses
1476 from other interfaces. This may seem wrong but it usually makes
1477 sense, because it increases the chance of successful communication.
1478 IP addresses are owned by the complete host on Linux, not by
1479 particular interfaces. Only for more complex setups like load-
1480 balancing, does this behaviour cause problems.
1482 arp_filter for the interface will be enabled if at least one of
1483 conf/{all,interface}/arp_filter is set to TRUE,
1484 it will be disabled otherwise
1486 arp_announce - INTEGER
1487 Define different restriction levels for announcing the local
1488 source IP address from IP packets in ARP requests sent on
1491 - 0 - (default) Use any local address, configured on any interface
1492 - 1 - Try to avoid local addresses that are not in the target's
1493 subnet for this interface. This mode is useful when target
1494 hosts reachable via this interface require the source IP
1495 address in ARP requests to be part of their logical network
1496 configured on the receiving interface. When we generate the
1497 request we will check all our subnets that include the
1498 target IP and will preserve the source address if it is from
1499 such subnet. If there is no such subnet we select source
1500 address according to the rules for level 2.
1501 - 2 - Always use the best local address for this target.
1502 In this mode we ignore the source address in the IP packet
1503 and try to select local address that we prefer for talks with
1504 the target host. Such local address is selected by looking
1505 for primary IP addresses on all our subnets on the outgoing
1506 interface that include the target IP address. If no suitable
1507 local address is found we select the first local address
1508 we have on the outgoing interface or on all other interfaces,
1509 with the hope we will receive reply for our request and
1510 even sometimes no matter the source IP address we announce.
1512 The max value from conf/{all,interface}/arp_announce is used.
1514 Increasing the restriction level gives more chance for
1515 receiving answer from the resolved target while decreasing
1516 the level announces more valid sender's information.
1518 arp_ignore - INTEGER
1519 Define different modes for sending replies in response to
1520 received ARP requests that resolve local target IP addresses:
1522 - 0 - (default): reply for any local target IP address, configured
1524 - 1 - reply only if the target IP address is local address
1525 configured on the incoming interface
1526 - 2 - reply only if the target IP address is local address
1527 configured on the incoming interface and both with the
1528 sender's IP address are part from same subnet on this interface
1529 - 3 - do not reply for local addresses configured with scope host,
1530 only resolutions for global and link addresses are replied
1532 - 8 - do not reply for all local addresses
1534 The max value from conf/{all,interface}/arp_ignore is used
1535 when ARP request is received on the {interface}
1537 arp_notify - BOOLEAN
1538 Define mode for notification of address and device changes.
1540 == ==========================================================
1541 0 (default): do nothing
1542 1 Generate gratuitous arp requests when device is brought up
1543 or hardware address changes.
1544 == ==========================================================
1546 arp_accept - BOOLEAN
1547 Define behavior for gratuitous ARP frames who's IP is not
1548 already present in the ARP table:
1550 - 0 - don't create new entries in the ARP table
1551 - 1 - create new entries in the ARP table
1553 Both replies and requests type gratuitous arp will trigger the
1554 ARP table to be updated, if this setting is on.
1556 If the ARP table already contains the IP address of the
1557 gratuitous arp frame, the arp table will be updated regardless
1558 if this setting is on or off.
1560 mcast_solicit - INTEGER
1561 The maximum number of multicast probes in INCOMPLETE state,
1562 when the associated hardware address is unknown. Defaults
1565 ucast_solicit - INTEGER
1566 The maximum number of unicast probes in PROBE state, when
1567 the hardware address is being reconfirmed. Defaults to 3.
1569 app_solicit - INTEGER
1570 The maximum number of probes to send to the user space ARP daemon
1571 via netlink before dropping back to multicast probes (see
1572 mcast_resolicit). Defaults to 0.
1574 mcast_resolicit - INTEGER
1575 The maximum number of multicast probes after unicast and
1576 app probes in PROBE state. Defaults to 0.
1578 disable_policy - BOOLEAN
1579 Disable IPSEC policy (SPD) for this interface
1581 disable_xfrm - BOOLEAN
1582 Disable IPSEC encryption on this interface, whatever the policy
1584 igmpv2_unsolicited_report_interval - INTEGER
1585 The interval in milliseconds in which the next unsolicited
1586 IGMPv1 or IGMPv2 report retransmit will take place.
1588 Default: 10000 (10 seconds)
1590 igmpv3_unsolicited_report_interval - INTEGER
1591 The interval in milliseconds in which the next unsolicited
1592 IGMPv3 report retransmit will take place.
1594 Default: 1000 (1 seconds)
1596 ignore_routes_with_linkdown - BOOLEAN
1597 Ignore routes whose link is down when performing a FIB lookup.
1599 promote_secondaries - BOOLEAN
1600 When a primary IP address is removed from this interface
1601 promote a corresponding secondary IP address instead of
1602 removing all the corresponding secondary IP addresses.
1604 drop_unicast_in_l2_multicast - BOOLEAN
1605 Drop any unicast IP packets that are received in link-layer
1606 multicast (or broadcast) frames.
1608 This behavior (for multicast) is actually a SHOULD in RFC
1609 1122, but is disabled by default for compatibility reasons.
1613 drop_gratuitous_arp - BOOLEAN
1614 Drop all gratuitous ARP frames, for example if there's a known
1615 good ARP proxy on the network and such frames need not be used
1616 (or in the case of 802.11, must not be used to prevent attacks.)
1622 Allows you to write a number, which can be used as required.
1626 xfrm4_gc_thresh - INTEGER
1627 (Obsolete since linux-4.14)
1628 The threshold at which we will start garbage collecting for IPv4
1629 destination cache entries. At twice this value the system will
1630 refuse new allocations.
1632 igmp_link_local_mcast_reports - BOOLEAN
1633 Enable IGMP reports for link local multicast groups in the
1639 kuznet@ms2.inr.ac.ru
1646 delon.nicolas@wanadoo.fr
1651 /proc/sys/net/ipv6/* Variables
1652 ==============================
1654 IPv6 has no global variables such as tcp_*. tcp_* settings under ipv4/ also
1655 apply to IPv6 [XXX?].
1657 bindv6only - BOOLEAN
1658 Default value for IPV6_V6ONLY socket option,
1659 which restricts use of the IPv6 socket to IPv6 communication
1662 - TRUE: disable IPv4-mapped address feature
1663 - FALSE: enable IPv4-mapped address feature
1665 Default: FALSE (as specified in RFC3493)
1667 flowlabel_consistency - BOOLEAN
1668 Protect the consistency (and unicity) of flow label.
1669 You have to disable it to use IPV6_FL_F_REFLECT flag on the
1677 auto_flowlabels - INTEGER
1678 Automatically generate flow labels based on a flow hash of the
1679 packet. This allows intermediate devices, such as routers, to
1680 identify packet flows for mechanisms like Equal Cost Multipath
1681 Routing (see RFC 6438).
1683 = ===========================================================
1684 0 automatic flow labels are completely disabled
1685 1 automatic flow labels are enabled by default, they can be
1686 disabled on a per socket basis using the IPV6_AUTOFLOWLABEL
1688 2 automatic flow labels are allowed, they may be enabled on a
1689 per socket basis using the IPV6_AUTOFLOWLABEL socket option
1690 3 automatic flow labels are enabled and enforced, they cannot
1691 be disabled by the socket option
1692 = ===========================================================
1696 flowlabel_state_ranges - BOOLEAN
1697 Split the flow label number space into two ranges. 0-0x7FFFF is
1698 reserved for the IPv6 flow manager facility, 0x80000-0xFFFFF
1699 is reserved for stateless flow labels as described in RFC6437.
1706 flowlabel_reflect - INTEGER
1707 Control flow label reflection. Needed for Path MTU
1708 Discovery to work with Equal Cost Multipath Routing in anycast
1709 environments. See RFC 7690 and:
1710 https://tools.ietf.org/html/draft-wang-6man-flow-label-reflection-01
1714 - 1: enabled for established flows
1716 Note that this prevents automatic flowlabel changes, as done
1717 in "tcp: change IPv6 flow-label upon receiving spurious retransmission"
1718 and "tcp: Change txhash on every SYN and RTO retransmit"
1720 - 2: enabled for TCP RESET packets (no active listener)
1721 If set, a RST packet sent in response to a SYN packet on a closed
1722 port will reflect the incoming flow label.
1724 - 4: enabled for ICMPv6 echo reply messages.
1728 fib_multipath_hash_policy - INTEGER
1729 Controls which hash policy to use for multipath routes.
1731 Default: 0 (Layer 3)
1735 - 0 - Layer 3 (source and destination addresses plus flow label)
1736 - 1 - Layer 4 (standard 5-tuple)
1737 - 2 - Layer 3 or inner Layer 3 if present
1739 anycast_src_echo_reply - BOOLEAN
1740 Controls the use of anycast addresses as source addresses for ICMPv6
1748 idgen_delay - INTEGER
1749 Controls the delay in seconds after which time to retry
1750 privacy stable address generation if a DAD conflict is
1753 Default: 1 (as specified in RFC7217)
1755 idgen_retries - INTEGER
1756 Controls the number of retries to generate a stable privacy
1757 address if a DAD conflict is detected.
1759 Default: 3 (as specified in RFC7217)
1762 Controls the MLD query robustness variable (see RFC3810 9.1).
1764 Default: 2 (as specified by RFC3810 9.1)
1766 Minimum: 1 (as specified by RFC6636 4.5)
1768 max_dst_opts_number - INTEGER
1769 Maximum number of non-padding TLVs allowed in a Destination
1770 options extension header. If this value is less than zero
1771 then unknown options are disallowed and the number of known
1772 TLVs allowed is the absolute value of this number.
1776 max_hbh_opts_number - INTEGER
1777 Maximum number of non-padding TLVs allowed in a Hop-by-Hop
1778 options extension header. If this value is less than zero
1779 then unknown options are disallowed and the number of known
1780 TLVs allowed is the absolute value of this number.
1784 max_dst_opts_length - INTEGER
1785 Maximum length allowed for a Destination options extension
1788 Default: INT_MAX (unlimited)
1790 max_hbh_length - INTEGER
1791 Maximum length allowed for a Hop-by-Hop options extension
1794 Default: INT_MAX (unlimited)
1796 skip_notify_on_dev_down - BOOLEAN
1797 Controls whether an RTM_DELROUTE message is generated for routes
1798 removed when a device is taken down or deleted. IPv4 does not
1799 generate this message; IPv6 does by default. Setting this sysctl
1800 to true skips the message, making IPv4 and IPv6 on par in relying
1801 on userspace caches to track link events and evict routes.
1803 Default: false (generate message)
1805 nexthop_compat_mode - BOOLEAN
1806 New nexthop API provides a means for managing nexthops independent of
1807 prefixes. Backwards compatibilty with old route format is enabled by
1808 default which means route dumps and notifications contain the new
1809 nexthop attribute but also the full, expanded nexthop definition.
1810 Further, updates or deletes of a nexthop configuration generate route
1811 notifications for each fib entry using the nexthop. Once a system
1812 understands the new API, this sysctl can be disabled to achieve full
1813 performance benefits of the new API by disabling the nexthop expansion
1814 and extraneous notifications.
1815 Default: true (backward compat mode)
1817 fib_notify_on_flag_change - INTEGER
1818 Whether to emit RTM_NEWROUTE notifications whenever RTM_F_OFFLOAD/
1819 RTM_F_TRAP/RTM_F_OFFLOAD_FAILED flags are changed.
1821 After installing a route to the kernel, user space receives an
1822 acknowledgment, which means the route was installed in the kernel,
1823 but not necessarily in hardware.
1824 It is also possible for a route already installed in hardware to change
1825 its action and therefore its flags. For example, a host route that is
1826 trapping packets can be "promoted" to perform decapsulation following
1827 the installation of an IPinIP/VXLAN tunnel.
1828 The notifications will indicate to user-space the state of the route.
1830 Default: 0 (Do not emit notifications.)
1834 - 0 - Do not emit notifications.
1835 - 1 - Emit notifications.
1836 - 2 - Emit notifications only for RTM_F_OFFLOAD_FAILED flag change.
1840 ip6frag_high_thresh - INTEGER
1841 Maximum memory used to reassemble IPv6 fragments. When
1842 ip6frag_high_thresh bytes of memory is allocated for this purpose,
1843 the fragment handler will toss packets until ip6frag_low_thresh
1846 ip6frag_low_thresh - INTEGER
1847 See ip6frag_high_thresh
1849 ip6frag_time - INTEGER
1850 Time in seconds to keep an IPv6 fragment in memory.
1852 IPv6 Segment Routing:
1854 seg6_flowlabel - INTEGER
1855 Controls the behaviour of computing the flowlabel of outer
1856 IPv6 header in case of SR T.encaps
1858 == =======================================================
1859 -1 set flowlabel to zero.
1860 0 copy flowlabel from Inner packet in case of Inner IPv6
1861 (Set flowlabel to 0 in case IPv4/L2)
1862 1 Compute the flowlabel using seg6_make_flowlabel()
1863 == =======================================================
1868 Change the interface-specific default settings.
1870 These settings would be used during creating new interfaces.
1874 Change all the interface-specific settings.
1876 [XXX: Other special features than forwarding?]
1878 conf/all/disable_ipv6 - BOOLEAN
1879 Changing this value is same as changing ``conf/default/disable_ipv6``
1880 setting and also all per-interface ``disable_ipv6`` settings to the same
1883 Reading this value does not have any particular meaning. It does not say
1884 whether IPv6 support is enabled or disabled. Returned value can be 1
1885 also in the case when some interface has ``disable_ipv6`` set to 0 and
1886 has configured IPv6 addresses.
1888 conf/all/forwarding - BOOLEAN
1889 Enable global IPv6 forwarding between all interfaces.
1891 IPv4 and IPv6 work differently here; e.g. netfilter must be used
1892 to control which interfaces may forward packets and which not.
1894 This also sets all interfaces' Host/Router setting
1895 'forwarding' to the specified value. See below for details.
1897 This referred to as global forwarding.
1902 fwmark_reflect - BOOLEAN
1903 Controls the fwmark of kernel-generated IPv6 reply packets that are not
1904 associated with a socket for example, TCP RSTs or ICMPv6 echo replies).
1905 If unset, these packets have a fwmark of zero. If set, they have the
1906 fwmark of the packet they are replying to.
1910 ``conf/interface/*``:
1911 Change special settings per interface.
1913 The functional behaviour for certain settings is different
1914 depending on whether local forwarding is enabled or not.
1917 Accept Router Advertisements; autoconfigure using them.
1919 It also determines whether or not to transmit Router
1920 Solicitations. If and only if the functional setting is to
1921 accept Router Advertisements, Router Solicitations will be
1924 Possible values are:
1926 == ===========================================================
1927 0 Do not accept Router Advertisements.
1928 1 Accept Router Advertisements if forwarding is disabled.
1929 2 Overrule forwarding behaviour. Accept Router Advertisements
1930 even if forwarding is enabled.
1931 == ===========================================================
1935 - enabled if local forwarding is disabled.
1936 - disabled if local forwarding is enabled.
1938 accept_ra_defrtr - BOOLEAN
1939 Learn default router in Router Advertisement.
1943 - enabled if accept_ra is enabled.
1944 - disabled if accept_ra is disabled.
1946 ra_defrtr_metric - UNSIGNED INTEGER
1947 Route metric for default route learned in Router Advertisement. This value
1948 will be assigned as metric for the default route learned via IPv6 Router
1949 Advertisement. Takes affect only if accept_ra_defrtr is enabled.
1954 Default: IP6_RT_PRIO_USER i.e. 1024.
1956 accept_ra_from_local - BOOLEAN
1957 Accept RA with source-address that is found on local machine
1958 if the RA is otherwise proper and able to be accepted.
1960 Default is to NOT accept these as it may be an un-intended
1965 - enabled if accept_ra_from_local is enabled
1966 on a specific interface.
1967 - disabled if accept_ra_from_local is disabled
1968 on a specific interface.
1970 accept_ra_min_hop_limit - INTEGER
1971 Minimum hop limit Information in Router Advertisement.
1973 Hop limit Information in Router Advertisement less than this
1974 variable shall be ignored.
1978 accept_ra_pinfo - BOOLEAN
1979 Learn Prefix Information in Router Advertisement.
1983 - enabled if accept_ra is enabled.
1984 - disabled if accept_ra is disabled.
1986 accept_ra_rt_info_min_plen - INTEGER
1987 Minimum prefix length of Route Information in RA.
1989 Route Information w/ prefix smaller than this variable shall
1994 * 0 if accept_ra_rtr_pref is enabled.
1995 * -1 if accept_ra_rtr_pref is disabled.
1997 accept_ra_rt_info_max_plen - INTEGER
1998 Maximum prefix length of Route Information in RA.
2000 Route Information w/ prefix larger than this variable shall
2005 * 0 if accept_ra_rtr_pref is enabled.
2006 * -1 if accept_ra_rtr_pref is disabled.
2008 accept_ra_rtr_pref - BOOLEAN
2009 Accept Router Preference in RA.
2013 - enabled if accept_ra is enabled.
2014 - disabled if accept_ra is disabled.
2016 accept_ra_mtu - BOOLEAN
2017 Apply the MTU value specified in RA option 5 (RFC4861). If
2018 disabled, the MTU specified in the RA will be ignored.
2022 - enabled if accept_ra is enabled.
2023 - disabled if accept_ra is disabled.
2025 accept_redirects - BOOLEAN
2030 - enabled if local forwarding is disabled.
2031 - disabled if local forwarding is enabled.
2033 accept_source_route - INTEGER
2034 Accept source routing (routing extension header).
2036 - >= 0: Accept only routing header type 2.
2037 - < 0: Do not accept routing header.
2042 Autoconfigure addresses using Prefix Information in Router
2047 - enabled if accept_ra_pinfo is enabled.
2048 - disabled if accept_ra_pinfo is disabled.
2050 dad_transmits - INTEGER
2051 The amount of Duplicate Address Detection probes to send.
2055 forwarding - INTEGER
2056 Configure interface-specific Host/Router behaviour.
2060 It is recommended to have the same setting on all
2061 interfaces; mixed router/host scenarios are rather uncommon.
2063 Possible values are:
2065 - 0 Forwarding disabled
2066 - 1 Forwarding enabled
2070 By default, Host behaviour is assumed. This means:
2072 1. IsRouter flag is not set in Neighbour Advertisements.
2073 2. If accept_ra is TRUE (default), transmit Router
2075 3. If accept_ra is TRUE (default), accept Router
2076 Advertisements (and do autoconfiguration).
2077 4. If accept_redirects is TRUE (default), accept Redirects.
2081 If local forwarding is enabled, Router behaviour is assumed.
2082 This means exactly the reverse from the above:
2084 1. IsRouter flag is set in Neighbour Advertisements.
2085 2. Router Solicitations are not sent unless accept_ra is 2.
2086 3. Router Advertisements are ignored unless accept_ra is 2.
2087 4. Redirects are ignored.
2089 Default: 0 (disabled) if global forwarding is disabled (default),
2090 otherwise 1 (enabled).
2093 Default Hop Limit to set.
2098 Default Maximum Transfer Unit
2100 Default: 1280 (IPv6 required minimum)
2102 ip_nonlocal_bind - BOOLEAN
2103 If set, allows processes to bind() to non-local IPv6 addresses,
2104 which can be quite useful - but may break some applications.
2108 router_probe_interval - INTEGER
2109 Minimum interval (in seconds) between Router Probing described
2114 router_solicitation_delay - INTEGER
2115 Number of seconds to wait after interface is brought up
2116 before sending Router Solicitations.
2120 router_solicitation_interval - INTEGER
2121 Number of seconds to wait between Router Solicitations.
2125 router_solicitations - INTEGER
2126 Number of Router Solicitations to send until assuming no
2127 routers are present.
2131 use_oif_addrs_only - BOOLEAN
2132 When enabled, the candidate source addresses for destinations
2133 routed via this interface are restricted to the set of addresses
2134 configured on this interface (vis. RFC 6724, section 4).
2138 use_tempaddr - INTEGER
2139 Preference for Privacy Extensions (RFC3041).
2141 * <= 0 : disable Privacy Extensions
2142 * == 1 : enable Privacy Extensions, but prefer public
2143 addresses over temporary addresses.
2144 * > 1 : enable Privacy Extensions and prefer temporary
2145 addresses over public addresses.
2149 * 0 (for most devices)
2150 * -1 (for point-to-point devices and loopback devices)
2152 temp_valid_lft - INTEGER
2153 valid lifetime (in seconds) for temporary addresses.
2155 Default: 172800 (2 days)
2157 temp_prefered_lft - INTEGER
2158 Preferred lifetime (in seconds) for temporary addresses.
2160 Default: 86400 (1 day)
2162 keep_addr_on_down - INTEGER
2163 Keep all IPv6 addresses on an interface down event. If set static
2164 global addresses with no expiration time are not flushed.
2167 * 0 : system default
2170 Default: 0 (addresses are removed)
2172 max_desync_factor - INTEGER
2173 Maximum value for DESYNC_FACTOR, which is a random value
2174 that ensures that clients don't synchronize with each
2175 other and generate new addresses at exactly the same time.
2176 value is in seconds.
2180 regen_max_retry - INTEGER
2181 Number of attempts before give up attempting to generate
2182 valid temporary addresses.
2186 max_addresses - INTEGER
2187 Maximum number of autoconfigured addresses per interface. Setting
2188 to zero disables the limitation. It is not recommended to set this
2189 value too large (or to zero) because it would be an easy way to
2190 crash the kernel by allowing too many addresses to be created.
2194 disable_ipv6 - BOOLEAN
2195 Disable IPv6 operation. If accept_dad is set to 2, this value
2196 will be dynamically set to TRUE if DAD fails for the link-local
2199 Default: FALSE (enable IPv6 operation)
2201 When this value is changed from 1 to 0 (IPv6 is being enabled),
2202 it will dynamically create a link-local address on the given
2203 interface and start Duplicate Address Detection, if necessary.
2205 When this value is changed from 0 to 1 (IPv6 is being disabled),
2206 it will dynamically delete all addresses and routes on the given
2207 interface. From now on it will not possible to add addresses/routes
2208 to the selected interface.
2210 accept_dad - INTEGER
2211 Whether to accept DAD (Duplicate Address Detection).
2213 == ==============================================================
2215 1 Enable DAD (default)
2216 2 Enable DAD, and disable IPv6 operation if MAC-based duplicate
2217 link-local address has been found.
2218 == ==============================================================
2220 DAD operation and mode on a given interface will be selected according
2221 to the maximum value of conf/{all,interface}/accept_dad.
2223 force_tllao - BOOLEAN
2224 Enable sending the target link-layer address option even when
2225 responding to a unicast neighbor solicitation.
2229 Quoting from RFC 2461, section 4.4, Target link-layer address:
2231 "The option MUST be included for multicast solicitations in order to
2232 avoid infinite Neighbor Solicitation "recursion" when the peer node
2233 does not have a cache entry to return a Neighbor Advertisements
2234 message. When responding to unicast solicitations, the option can be
2235 omitted since the sender of the solicitation has the correct link-
2236 layer address; otherwise it would not have be able to send the unicast
2237 solicitation in the first place. However, including the link-layer
2238 address in this case adds little overhead and eliminates a potential
2239 race condition where the sender deletes the cached link-layer address
2240 prior to receiving a response to a previous solicitation."
2242 ndisc_notify - BOOLEAN
2243 Define mode for notification of address and device changes.
2245 * 0 - (default): do nothing
2246 * 1 - Generate unsolicited neighbour advertisements when device is brought
2247 up or hardware address changes.
2249 ndisc_tclass - INTEGER
2250 The IPv6 Traffic Class to use by default when sending IPv6 Neighbor
2251 Discovery (Router Solicitation, Router Advertisement, Neighbor
2252 Solicitation, Neighbor Advertisement, Redirect) messages.
2253 These 8 bits can be interpreted as 6 high order bits holding the DSCP
2254 value and 2 low order bits representing ECN (which you probably want
2259 mldv1_unsolicited_report_interval - INTEGER
2260 The interval in milliseconds in which the next unsolicited
2261 MLDv1 report retransmit will take place.
2263 Default: 10000 (10 seconds)
2265 mldv2_unsolicited_report_interval - INTEGER
2266 The interval in milliseconds in which the next unsolicited
2267 MLDv2 report retransmit will take place.
2269 Default: 1000 (1 second)
2271 force_mld_version - INTEGER
2272 * 0 - (default) No enforcement of a MLD version, MLDv1 fallback allowed
2273 * 1 - Enforce to use MLD version 1
2274 * 2 - Enforce to use MLD version 2
2276 suppress_frag_ndisc - INTEGER
2277 Control RFC 6980 (Security Implications of IPv6 Fragmentation
2278 with IPv6 Neighbor Discovery) behavior:
2280 * 1 - (default) discard fragmented neighbor discovery packets
2281 * 0 - allow fragmented neighbor discovery packets
2283 optimistic_dad - BOOLEAN
2284 Whether to perform Optimistic Duplicate Address Detection (RFC 4429).
2286 * 0: disabled (default)
2289 Optimistic Duplicate Address Detection for the interface will be enabled
2290 if at least one of conf/{all,interface}/optimistic_dad is set to 1,
2291 it will be disabled otherwise.
2293 use_optimistic - BOOLEAN
2294 If enabled, do not classify optimistic addresses as deprecated during
2295 source address selection. Preferred addresses will still be chosen
2296 before optimistic addresses, subject to other ranking in the source
2297 address selection algorithm.
2299 * 0: disabled (default)
2302 This will be enabled if at least one of
2303 conf/{all,interface}/use_optimistic is set to 1, disabled otherwise.
2305 stable_secret - IPv6 address
2306 This IPv6 address will be used as a secret to generate IPv6
2307 addresses for link-local addresses and autoconfigured
2308 ones. All addresses generated after setting this secret will
2309 be stable privacy ones by default. This can be changed via the
2310 addrgenmode ip-link. conf/default/stable_secret is used as the
2311 secret for the namespace, the interface specific ones can
2312 overwrite that. Writes to conf/all/stable_secret are refused.
2314 It is recommended to generate this secret during installation
2315 of a system and keep it stable after that.
2317 By default the stable secret is unset.
2319 addr_gen_mode - INTEGER
2320 Defines how link-local and autoconf addresses are generated.
2322 = =================================================================
2323 0 generate address based on EUI64 (default)
2324 1 do no generate a link-local address, use EUI64 for addresses
2325 generated from autoconf
2326 2 generate stable privacy addresses, using the secret from
2327 stable_secret (RFC7217)
2328 3 generate stable privacy addresses, using a random secret if unset
2329 = =================================================================
2331 drop_unicast_in_l2_multicast - BOOLEAN
2332 Drop any unicast IPv6 packets that are received in link-layer
2333 multicast (or broadcast) frames.
2335 By default this is turned off.
2337 drop_unsolicited_na - BOOLEAN
2338 Drop all unsolicited neighbor advertisements, for example if there's
2339 a known good NA proxy on the network and such frames need not be used
2340 (or in the case of 802.11, must not be used to prevent attacks.)
2342 By default this is turned off.
2344 enhanced_dad - BOOLEAN
2345 Include a nonce option in the IPv6 neighbor solicitation messages used for
2346 duplicate address detection per RFC7527. A received DAD NS will only signal
2347 a duplicate address if the nonce is different. This avoids any false
2348 detection of duplicates due to loopback of the NS messages that we send.
2349 The nonce option will be sent on an interface unless both of
2350 conf/{all,interface}/enhanced_dad are set to FALSE.
2358 Limit the maximal rates for sending ICMPv6 messages.
2360 0 to disable any limiting,
2361 otherwise the minimal space between responses in milliseconds.
2365 ratemask - list of comma separated ranges
2366 For ICMPv6 message types matching the ranges in the ratemask, limit
2367 the sending of the message according to ratelimit parameter.
2369 The format used for both input and output is a comma separated
2370 list of ranges (e.g. "0-127,129" for ICMPv6 message type 0 to 127 and
2371 129). Writing to the file will clear all previous ranges of ICMPv6
2372 message types and update the current list with the input.
2374 Refer to: https://www.iana.org/assignments/icmpv6-parameters/icmpv6-parameters.xhtml
2375 for numerical values of ICMPv6 message types, e.g. echo request is 128
2376 and echo reply is 129.
2378 Default: 0-1,3-127 (rate limit ICMPv6 errors except Packet Too Big)
2380 echo_ignore_all - BOOLEAN
2381 If set non-zero, then the kernel will ignore all ICMP ECHO
2382 requests sent to it over the IPv6 protocol.
2386 echo_ignore_multicast - BOOLEAN
2387 If set non-zero, then the kernel will ignore all ICMP ECHO
2388 requests sent to it over the IPv6 protocol via multicast.
2392 echo_ignore_anycast - BOOLEAN
2393 If set non-zero, then the kernel will ignore all ICMP ECHO
2394 requests sent to it over the IPv6 protocol destined to anycast address.
2398 xfrm6_gc_thresh - INTEGER
2399 (Obsolete since linux-4.14)
2400 The threshold at which we will start garbage collecting for IPv6
2401 destination cache entries. At twice this value the system will
2402 refuse new allocations.
2406 Pekka Savola <pekkas@netcore.fi>
2407 YOSHIFUJI Hideaki / USAGI Project <yoshfuji@linux-ipv6.org>
2410 /proc/sys/net/bridge/* Variables:
2411 =================================
2413 bridge-nf-call-arptables - BOOLEAN
2414 - 1 : pass bridged ARP traffic to arptables' FORWARD chain.
2419 bridge-nf-call-iptables - BOOLEAN
2420 - 1 : pass bridged IPv4 traffic to iptables' chains.
2425 bridge-nf-call-ip6tables - BOOLEAN
2426 - 1 : pass bridged IPv6 traffic to ip6tables' chains.
2431 bridge-nf-filter-vlan-tagged - BOOLEAN
2432 - 1 : pass bridged vlan-tagged ARP/IP/IPv6 traffic to {arp,ip,ip6}tables.
2437 bridge-nf-filter-pppoe-tagged - BOOLEAN
2438 - 1 : pass bridged pppoe-tagged IP/IPv6 traffic to {ip,ip6}tables.
2443 bridge-nf-pass-vlan-input-dev - BOOLEAN
2444 - 1: if bridge-nf-filter-vlan-tagged is enabled, try to find a vlan
2445 interface on the bridge and set the netfilter input device to the
2446 vlan. This allows use of e.g. "iptables -i br0.1" and makes the
2447 REDIRECT target work with vlan-on-top-of-bridge interfaces. When no
2448 matching vlan interface is found, or this switch is off, the input
2449 device is set to the bridge interface.
2451 - 0: disable bridge netfilter vlan interface lookup.
2455 ``proc/sys/net/sctp/*`` Variables:
2456 ==================================
2458 addip_enable - BOOLEAN
2459 Enable or disable extension of Dynamic Address Reconfiguration
2460 (ADD-IP) functionality specified in RFC5061. This extension provides
2461 the ability to dynamically add and remove new addresses for the SCTP
2464 1: Enable extension.
2466 0: Disable extension.
2471 Enable or disable pf (pf is short for potentially failed) state. A value
2472 of pf_retrans > path_max_retrans also disables pf state. That is, one of
2473 both pf_enable and pf_retrans > path_max_retrans can disable pf state.
2474 Since pf_retrans and path_max_retrans can be changed by userspace
2475 application, sometimes user expects to disable pf state by the value of
2476 pf_retrans > path_max_retrans, but occasionally the value of pf_retrans
2477 or path_max_retrans is changed by the user application, this pf state is
2478 enabled. As such, it is necessary to add this to dynamically enable
2479 and disable pf state. See:
2480 https://datatracker.ietf.org/doc/draft-ietf-tsvwg-sctp-failover for
2490 Unset or enable/disable pf (pf is short for potentially failed) state
2491 exposure. Applications can control the exposure of the PF path state
2492 in the SCTP_PEER_ADDR_CHANGE event and the SCTP_GET_PEER_ADDR_INFO
2493 sockopt. When it's unset, no SCTP_PEER_ADDR_CHANGE event with
2494 SCTP_ADDR_PF state will be sent and a SCTP_PF-state transport info
2495 can be got via SCTP_GET_PEER_ADDR_INFO sockopt; When it's enabled,
2496 a SCTP_PEER_ADDR_CHANGE event will be sent for a transport becoming
2497 SCTP_PF state and a SCTP_PF-state transport info can be got via
2498 SCTP_GET_PEER_ADDR_INFO sockopt; When it's diabled, no
2499 SCTP_PEER_ADDR_CHANGE event will be sent and it returns -EACCES when
2500 trying to get a SCTP_PF-state transport info via SCTP_GET_PEER_ADDR_INFO
2503 0: Unset pf state exposure, Compatible with old applications.
2505 1: Disable pf state exposure.
2507 2: Enable pf state exposure.
2511 addip_noauth_enable - BOOLEAN
2512 Dynamic Address Reconfiguration (ADD-IP) requires the use of
2513 authentication to protect the operations of adding or removing new
2514 addresses. This requirement is mandated so that unauthorized hosts
2515 would not be able to hijack associations. However, older
2516 implementations may not have implemented this requirement while
2517 allowing the ADD-IP extension. For reasons of interoperability,
2518 we provide this variable to control the enforcement of the
2519 authentication requirement.
2521 == ===============================================================
2522 1 Allow ADD-IP extension to be used without authentication. This
2523 should only be set in a closed environment for interoperability
2524 with older implementations.
2526 0 Enforce the authentication requirement
2527 == ===============================================================
2531 auth_enable - BOOLEAN
2532 Enable or disable Authenticated Chunks extension. This extension
2533 provides the ability to send and receive authenticated chunks and is
2534 required for secure operation of Dynamic Address Reconfiguration
2537 - 1: Enable this extension.
2538 - 0: Disable this extension.
2542 prsctp_enable - BOOLEAN
2543 Enable or disable the Partial Reliability extension (RFC3758) which
2544 is used to notify peers that a given DATA should no longer be expected.
2546 - 1: Enable extension
2552 The limit of the number of new packets that can be initially sent. It
2553 controls how bursty the generated traffic can be.
2557 association_max_retrans - INTEGER
2558 Set the maximum number for retransmissions that an association can
2559 attempt deciding that the remote end is unreachable. If this value
2560 is exceeded, the association is terminated.
2564 max_init_retransmits - INTEGER
2565 The maximum number of retransmissions of INIT and COOKIE-ECHO chunks
2566 that an association will attempt before declaring the destination
2567 unreachable and terminating.
2571 path_max_retrans - INTEGER
2572 The maximum number of retransmissions that will be attempted on a given
2573 path. Once this threshold is exceeded, the path is considered
2574 unreachable, and new traffic will use a different path when the
2575 association is multihomed.
2579 pf_retrans - INTEGER
2580 The number of retransmissions that will be attempted on a given path
2581 before traffic is redirected to an alternate transport (should one
2582 exist). Note this is distinct from path_max_retrans, as a path that
2583 passes the pf_retrans threshold can still be used. Its only
2584 deprioritized when a transmission path is selected by the stack. This
2585 setting is primarily used to enable fast failover mechanisms without
2586 having to reduce path_max_retrans to a very low value. See:
2587 http://www.ietf.org/id/draft-nishida-tsvwg-sctp-failover-05.txt
2588 for details. Note also that a value of pf_retrans > path_max_retrans
2589 disables this feature. Since both pf_retrans and path_max_retrans can
2590 be changed by userspace application, a variable pf_enable is used to
2595 ps_retrans - INTEGER
2596 Primary.Switchover.Max.Retrans (PSMR), it's a tunable parameter coming
2597 from section-5 "Primary Path Switchover" in rfc7829. The primary path
2598 will be changed to another active path when the path error counter on
2599 the old primary path exceeds PSMR, so that "the SCTP sender is allowed
2600 to continue data transmission on a new working path even when the old
2601 primary destination address becomes active again". Note this feature
2602 is disabled by initializing 'ps_retrans' per netns as 0xffff by default,
2603 and its value can't be less than 'pf_retrans' when changing by sysctl.
2607 rto_initial - INTEGER
2608 The initial round trip timeout value in milliseconds that will be used
2609 in calculating round trip times. This is the initial time interval
2610 for retransmissions.
2615 The maximum value (in milliseconds) of the round trip timeout. This
2616 is the largest time interval that can elapse between retransmissions.
2621 The minimum value (in milliseconds) of the round trip timeout. This
2622 is the smallest time interval the can elapse between retransmissions.
2626 hb_interval - INTEGER
2627 The interval (in milliseconds) between HEARTBEAT chunks. These chunks
2628 are sent at the specified interval on idle paths to probe the state of
2629 a given path between 2 associations.
2633 sack_timeout - INTEGER
2634 The amount of time (in milliseconds) that the implementation will wait
2639 valid_cookie_life - INTEGER
2640 The default lifetime of the SCTP cookie (in milliseconds). The cookie
2641 is used during association establishment.
2645 cookie_preserve_enable - BOOLEAN
2646 Enable or disable the ability to extend the lifetime of the SCTP cookie
2647 that is used during the establishment phase of SCTP association
2649 - 1: Enable cookie lifetime extension.
2654 cookie_hmac_alg - STRING
2655 Select the hmac algorithm used when generating the cookie value sent by
2656 a listening sctp socket to a connecting client in the INIT-ACK chunk.
2663 Ability to assign md5 or sha1 as the selected alg is predicated on the
2664 configuration of those algorithms at build time (CONFIG_CRYPTO_MD5 and
2665 CONFIG_CRYPTO_SHA1).
2667 Default: Dependent on configuration. MD5 if available, else SHA1 if
2668 available, else none.
2670 rcvbuf_policy - INTEGER
2671 Determines if the receive buffer is attributed to the socket or to
2672 association. SCTP supports the capability to create multiple
2673 associations on a single socket. When using this capability, it is
2674 possible that a single stalled association that's buffering a lot
2675 of data may block other associations from delivering their data by
2676 consuming all of the receive buffer space. To work around this,
2677 the rcvbuf_policy could be set to attribute the receiver buffer space
2678 to each association instead of the socket. This prevents the described
2681 - 1: rcvbuf space is per association
2682 - 0: rcvbuf space is per socket
2686 sndbuf_policy - INTEGER
2687 Similar to rcvbuf_policy above, this applies to send buffer space.
2689 - 1: Send buffer is tracked per association
2690 - 0: Send buffer is tracked per socket.
2694 sctp_mem - vector of 3 INTEGERs: min, pressure, max
2695 Number of pages allowed for queueing by all SCTP sockets.
2697 min: Below this number of pages SCTP is not bothered about its
2698 memory appetite. When amount of memory allocated by SCTP exceeds
2699 this number, SCTP starts to moderate memory usage.
2701 pressure: This value was introduced to follow format of tcp_mem.
2703 max: Number of pages allowed for queueing by all SCTP sockets.
2705 Default is calculated at boot time from amount of available memory.
2707 sctp_rmem - vector of 3 INTEGERs: min, default, max
2708 Only the first value ("min") is used, "default" and "max" are
2711 min: Minimal size of receive buffer used by SCTP socket.
2712 It is guaranteed to each SCTP socket (but not association) even
2713 under moderate memory pressure.
2717 sctp_wmem - vector of 3 INTEGERs: min, default, max
2718 Currently this tunable has no effect.
2720 addr_scope_policy - INTEGER
2721 Control IPv4 address scoping - draft-stewart-tsvwg-sctp-ipv4-00
2723 - 0 - Disable IPv4 address scoping
2724 - 1 - Enable IPv4 address scoping
2725 - 2 - Follow draft but allow IPv4 private addresses
2726 - 3 - Follow draft but allow IPv4 link local addresses
2731 The listening port for the local UDP tunneling sock. Normally it's
2732 using the IANA-assigned UDP port number 9899 (sctp-tunneling).
2734 This UDP sock is used for processing the incoming UDP-encapsulated
2735 SCTP packets (from RFC6951), and shared by all applications in the
2736 same net namespace. This UDP sock will be closed when the value is
2739 The value will also be used to set the src port of the UDP header
2740 for the outgoing UDP-encapsulated SCTP packets. For the dest port,
2741 please refer to 'encap_port' below.
2745 encap_port - INTEGER
2746 The default remote UDP encapsulation port.
2748 This value is used to set the dest port of the UDP header for the
2749 outgoing UDP-encapsulated SCTP packets by default. Users can also
2750 change the value for each sock/asoc/transport by using setsockopt.
2751 For further information, please refer to RFC6951.
2753 Note that when connecting to a remote server, the client should set
2754 this to the port that the UDP tunneling sock on the peer server is
2755 listening to and the local UDP tunneling sock on the client also
2756 must be started. On the server, it would get the encap_port from
2757 the incoming packet's source port.
2762 ``/proc/sys/net/core/*``
2763 ========================
2765 Please see: Documentation/admin-guide/sysctl/net.rst for descriptions of these entries.
2768 ``/proc/sys/net/unix/*``
2769 ========================
2771 max_dgram_qlen - INTEGER
2772 The maximum length of dgram socket receive queue