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Based on kernel version 3.19. Page generated on 2015-02-13 21:22 EST.

1	/proc/sys/net/ipv4/* Variables:
2	
3	ip_forward - BOOLEAN
4		0 - disabled (default)
5		not 0 - enabled
6	
7		Forward Packets between interfaces.
8	
9		This variable is special, its change resets all configuration
10		parameters to their default state (RFC1122 for hosts, RFC1812
11		for routers)
12	
13	ip_default_ttl - INTEGER
14		Default value of TTL field (Time To Live) for outgoing (but not
15		forwarded) IP packets. Should be between 1 and 255 inclusive.
16		Default: 64 (as recommended by RFC1700)
17	
18	ip_no_pmtu_disc - INTEGER
19		Disable Path MTU Discovery. If enabled in mode 1 and a
20		fragmentation-required ICMP is received, the PMTU to this
21		destination will be set to min_pmtu (see below). You will need
22		to raise min_pmtu to the smallest interface MTU on your system
23		manually if you want to avoid locally generated fragments.
24	
25		In mode 2 incoming Path MTU Discovery messages will be
26		discarded. Outgoing frames are handled the same as in mode 1,
27		implicitly setting IP_PMTUDISC_DONT on every created socket.
28	
29		Mode 3 is a hardend pmtu discover mode. The kernel will only
30		accept fragmentation-needed errors if the underlying protocol
31		can verify them besides a plain socket lookup. Current
32		protocols for which pmtu events will be honored are TCP, SCTP
33		and DCCP as they verify e.g. the sequence number or the
34		association. This mode should not be enabled globally but is
35		only intended to secure e.g. name servers in namespaces where
36		TCP path mtu must still work but path MTU information of other
37		protocols should be discarded. If enabled globally this mode
38		could break other protocols.
39	
40		Possible values: 0-3
41		Default: FALSE
42	
43	min_pmtu - INTEGER
44		default 552 - minimum discovered Path MTU
45	
46	ip_forward_use_pmtu - BOOLEAN
47		By default we don't trust protocol path MTUs while forwarding
48		because they could be easily forged and can lead to unwanted
49		fragmentation by the router.
50		You only need to enable this if you have user-space software
51		which tries to discover path mtus by itself and depends on the
52		kernel honoring this information. This is normally not the
53		case.
54		Default: 0 (disabled)
55		Possible values:
56		0 - disabled
57		1 - enabled
58	
59	fwmark_reflect - BOOLEAN
60		Controls the fwmark of kernel-generated IPv4 reply packets that are not
61		associated with a socket for example, TCP RSTs or ICMP echo replies).
62		If unset, these packets have a fwmark of zero. If set, they have the
63		fwmark of the packet they are replying to.
64		Default: 0
65	
66	route/max_size - INTEGER
67		Maximum number of routes allowed in the kernel.  Increase
68		this when using large numbers of interfaces and/or routes.
69		From linux kernel 3.6 onwards, this is deprecated for ipv4
70		as route cache is no longer used.
71	
72	neigh/default/gc_thresh1 - INTEGER
73		Minimum number of entries to keep.  Garbage collector will not
74		purge entries if there are fewer than this number.
75		Default: 128
76	
77	neigh/default/gc_thresh2 - INTEGER
78		Threshold when garbage collector becomes more aggressive about
79		purging entries. Entries older than 5 seconds will be cleared
80		when over this number.
81		Default: 512
82	
83	neigh/default/gc_thresh3 - INTEGER
84		Maximum number of neighbor entries allowed.  Increase this
85		when using large numbers of interfaces and when communicating
86		with large numbers of directly-connected peers.
87		Default: 1024
88	
89	neigh/default/unres_qlen_bytes - INTEGER
90		The maximum number of bytes which may be used by packets
91		queued for each	unresolved address by other network layers.
92		(added in linux 3.3)
93		Setting negative value is meaningless and will return error.
94		Default: 65536 Bytes(64KB)
95	
96	neigh/default/unres_qlen - INTEGER
97		The maximum number of packets which may be queued for each
98		unresolved address by other network layers.
99		(deprecated in linux 3.3) : use unres_qlen_bytes instead.
100		Prior to linux 3.3, the default value is 3 which may cause
101		unexpected packet loss. The current default value is calculated
102		according to default value of unres_qlen_bytes and true size of
103		packet.
104		Default: 31
105	
106	mtu_expires - INTEGER
107		Time, in seconds, that cached PMTU information is kept.
108	
109	min_adv_mss - INTEGER
110		The advertised MSS depends on the first hop route MTU, but will
111		never be lower than this setting.
112	
113	IP Fragmentation:
114	
115	ipfrag_high_thresh - INTEGER
116		Maximum memory used to reassemble IP fragments. When
117		ipfrag_high_thresh bytes of memory is allocated for this purpose,
118		the fragment handler will toss packets until ipfrag_low_thresh
119		is reached. This also serves as a maximum limit to namespaces
120		different from the initial one.
121	
122	ipfrag_low_thresh - INTEGER
123		Maximum memory used to reassemble IP fragments before the kernel
124		begins to remove incomplete fragment queues to free up resources.
125		The kernel still accepts new fragments for defragmentation.
126	
127	ipfrag_time - INTEGER
128		Time in seconds to keep an IP fragment in memory.
129	
130	ipfrag_max_dist - INTEGER
131		ipfrag_max_dist is a non-negative integer value which defines the
132		maximum "disorder" which is allowed among fragments which share a
133		common IP source address. Note that reordering of packets is
134		not unusual, but if a large number of fragments arrive from a source
135		IP address while a particular fragment queue remains incomplete, it
136		probably indicates that one or more fragments belonging to that queue
137		have been lost. When ipfrag_max_dist is positive, an additional check
138		is done on fragments before they are added to a reassembly queue - if
139		ipfrag_max_dist (or more) fragments have arrived from a particular IP
140		address between additions to any IP fragment queue using that source
141		address, it's presumed that one or more fragments in the queue are
142		lost. The existing fragment queue will be dropped, and a new one
143		started. An ipfrag_max_dist value of zero disables this check.
144	
145		Using a very small value, e.g. 1 or 2, for ipfrag_max_dist can
146		result in unnecessarily dropping fragment queues when normal
147		reordering of packets occurs, which could lead to poor application
148		performance. Using a very large value, e.g. 50000, increases the
149		likelihood of incorrectly reassembling IP fragments that originate
150		from different IP datagrams, which could result in data corruption.
151		Default: 64
152	
153	INET peer storage:
154	
155	inet_peer_threshold - INTEGER
156		The approximate size of the storage.  Starting from this threshold
157		entries will be thrown aggressively.  This threshold also determines
158		entries' time-to-live and time intervals between garbage collection
159		passes.  More entries, less time-to-live, less GC interval.
160	
161	inet_peer_minttl - INTEGER
162		Minimum time-to-live of entries.  Should be enough to cover fragment
163		time-to-live on the reassembling side.  This minimum time-to-live  is
164		guaranteed if the pool size is less than inet_peer_threshold.
165		Measured in seconds.
166	
167	inet_peer_maxttl - INTEGER
168		Maximum time-to-live of entries.  Unused entries will expire after
169		this period of time if there is no memory pressure on the pool (i.e.
170		when the number of entries in the pool is very small).
171		Measured in seconds.
172	
173	TCP variables:
174	
175	somaxconn - INTEGER
176		Limit of socket listen() backlog, known in userspace as SOMAXCONN.
177		Defaults to 128.  See also tcp_max_syn_backlog for additional tuning
178		for TCP sockets.
179	
180	tcp_abort_on_overflow - BOOLEAN
181		If listening service is too slow to accept new connections,
182		reset them. Default state is FALSE. It means that if overflow
183		occurred due to a burst, connection will recover. Enable this
184		option _only_ if you are really sure that listening daemon
185		cannot be tuned to accept connections faster. Enabling this
186		option can harm clients of your server.
187	
188	tcp_adv_win_scale - INTEGER
189		Count buffering overhead as bytes/2^tcp_adv_win_scale
190		(if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),
191		if it is <= 0.
192		Possible values are [-31, 31], inclusive.
193		Default: 1
194	
195	tcp_allowed_congestion_control - STRING
196		Show/set the congestion control choices available to non-privileged
197		processes. The list is a subset of those listed in
198		tcp_available_congestion_control.
199		Default is "reno" and the default setting (tcp_congestion_control).
200	
201	tcp_app_win - INTEGER
202		Reserve max(window/2^tcp_app_win, mss) of window for application
203		buffer. Value 0 is special, it means that nothing is reserved.
204		Default: 31
205	
206	tcp_autocorking - BOOLEAN
207		Enable TCP auto corking :
208		When applications do consecutive small write()/sendmsg() system calls,
209		we try to coalesce these small writes as much as possible, to lower
210		total amount of sent packets. This is done if at least one prior
211		packet for the flow is waiting in Qdisc queues or device transmit
212		queue. Applications can still use TCP_CORK for optimal behavior
213		when they know how/when to uncork their sockets.
214		Default : 1
215	
216	tcp_available_congestion_control - STRING
217		Shows the available congestion control choices that are registered.
218		More congestion control algorithms may be available as modules,
219		but not loaded.
220	
221	tcp_base_mss - INTEGER
222		The initial value of search_low to be used by the packetization layer
223		Path MTU discovery (MTU probing).  If MTU probing is enabled,
224		this is the initial MSS used by the connection.
225	
226	tcp_congestion_control - STRING
227		Set the congestion control algorithm to be used for new
228		connections. The algorithm "reno" is always available, but
229		additional choices may be available based on kernel configuration.
230		Default is set as part of kernel configuration.
231		For passive connections, the listener congestion control choice
232		is inherited.
233		[see setsockopt(listenfd, SOL_TCP, TCP_CONGESTION, "name" ...) ]
234	
235	tcp_dsack - BOOLEAN
236		Allows TCP to send "duplicate" SACKs.
237	
238	tcp_early_retrans - INTEGER
239		Enable Early Retransmit (ER), per RFC 5827. ER lowers the threshold
240		for triggering fast retransmit when the amount of outstanding data is
241		small and when no previously unsent data can be transmitted (such
242		that limited transmit could be used). Also controls the use of
243		Tail loss probe (TLP) that converts RTOs occurring due to tail
244		losses into fast recovery (draft-dukkipati-tcpm-tcp-loss-probe-01).
245		Possible values:
246			0 disables ER
247			1 enables ER
248			2 enables ER but delays fast recovery and fast retransmit
249			  by a fourth of RTT. This mitigates connection falsely
250			  recovers when network has a small degree of reordering
251			  (less than 3 packets).
252			3 enables delayed ER and TLP.
253			4 enables TLP only.
254		Default: 3
255	
256	tcp_ecn - INTEGER
257		Control use of Explicit Congestion Notification (ECN) by TCP.
258		ECN is used only when both ends of the TCP connection indicate
259		support for it.  This feature is useful in avoiding losses due
260		to congestion by allowing supporting routers to signal
261		congestion before having to drop packets.
262		Possible values are:
263			0 Disable ECN.  Neither initiate nor accept ECN.
264			1 Enable ECN when requested by incoming connections and
265			  also request ECN on outgoing connection attempts.
266			2 Enable ECN when requested by incoming connections
267			  but do not request ECN on outgoing connections.
268		Default: 2
269	
270	tcp_fack - BOOLEAN
271		Enable FACK congestion avoidance and fast retransmission.
272		The value is not used, if tcp_sack is not enabled.
273	
274	tcp_fin_timeout - INTEGER
275		The length of time an orphaned (no longer referenced by any
276		application) connection will remain in the FIN_WAIT_2 state
277		before it is aborted at the local end.  While a perfectly
278		valid "receive only" state for an un-orphaned connection, an
279		orphaned connection in FIN_WAIT_2 state could otherwise wait
280		forever for the remote to close its end of the connection.
281		Cf. tcp_max_orphans
282		Default: 60 seconds
283	
284	tcp_frto - INTEGER
285		Enables Forward RTO-Recovery (F-RTO) defined in RFC5682.
286		F-RTO is an enhanced recovery algorithm for TCP retransmission
287		timeouts.  It is particularly beneficial in networks where the
288		RTT fluctuates (e.g., wireless). F-RTO is sender-side only
289		modification. It does not require any support from the peer.
290	
291		By default it's enabled with a non-zero value. 0 disables F-RTO.
292	
293	tcp_keepalive_time - INTEGER
294		How often TCP sends out keepalive messages when keepalive is enabled.
295		Default: 2hours.
296	
297	tcp_keepalive_probes - INTEGER
298		How many keepalive probes TCP sends out, until it decides that the
299		connection is broken. Default value: 9.
300	
301	tcp_keepalive_intvl - INTEGER
302		How frequently the probes are send out. Multiplied by
303		tcp_keepalive_probes it is time to kill not responding connection,
304		after probes started. Default value: 75sec i.e. connection
305		will be aborted after ~11 minutes of retries.
306	
307	tcp_low_latency - BOOLEAN
308		If set, the TCP stack makes decisions that prefer lower
309		latency as opposed to higher throughput.  By default, this
310		option is not set meaning that higher throughput is preferred.
311		An example of an application where this default should be
312		changed would be a Beowulf compute cluster.
313		Default: 0
314	
315	tcp_max_orphans - INTEGER
316		Maximal number of TCP sockets not attached to any user file handle,
317		held by system.	If this number is exceeded orphaned connections are
318		reset immediately and warning is printed. This limit exists
319		only to prevent simple DoS attacks, you _must_ not rely on this
320		or lower the limit artificially, but rather increase it
321		(probably, after increasing installed memory),
322		if network conditions require more than default value,
323		and tune network services to linger and kill such states
324		more aggressively. Let me to remind again: each orphan eats
325		up to ~64K of unswappable memory.
326	
327	tcp_max_syn_backlog - INTEGER
328		Maximal number of remembered connection requests, which have not
329		received an acknowledgment from connecting client.
330		The minimal value is 128 for low memory machines, and it will
331		increase in proportion to the memory of machine.
332		If server suffers from overload, try increasing this number.
333	
334	tcp_max_tw_buckets - INTEGER
335		Maximal number of timewait sockets held by system simultaneously.
336		If this number is exceeded time-wait socket is immediately destroyed
337		and warning is printed. This limit exists only to prevent
338		simple DoS attacks, you _must_ not lower the limit artificially,
339		but rather increase it (probably, after increasing installed memory),
340		if network conditions require more than default value.
341	
342	tcp_mem - vector of 3 INTEGERs: min, pressure, max
343		min: below this number of pages TCP is not bothered about its
344		memory appetite.
345	
346		pressure: when amount of memory allocated by TCP exceeds this number
347		of pages, TCP moderates its memory consumption and enters memory
348		pressure mode, which is exited when memory consumption falls
349		under "min".
350	
351		max: number of pages allowed for queueing by all TCP sockets.
352	
353		Defaults are calculated at boot time from amount of available
354		memory.
355	
356	tcp_moderate_rcvbuf - BOOLEAN
357		If set, TCP performs receive buffer auto-tuning, attempting to
358		automatically size the buffer (no greater than tcp_rmem[2]) to
359		match the size required by the path for full throughput.  Enabled by
360		default.
361	
362	tcp_mtu_probing - INTEGER
363		Controls TCP Packetization-Layer Path MTU Discovery.  Takes three
364		values:
365		  0 - Disabled
366		  1 - Disabled by default, enabled when an ICMP black hole detected
367		  2 - Always enabled, use initial MSS of tcp_base_mss.
368	
369	tcp_no_metrics_save - BOOLEAN
370		By default, TCP saves various connection metrics in the route cache
371		when the connection closes, so that connections established in the
372		near future can use these to set initial conditions.  Usually, this
373		increases overall performance, but may sometimes cause performance
374		degradation.  If set, TCP will not cache metrics on closing
375		connections.
376	
377	tcp_orphan_retries - INTEGER
378		This value influences the timeout of a locally closed TCP connection,
379		when RTO retransmissions remain unacknowledged.
380		See tcp_retries2 for more details.
381	
382		The default value is 8.
383		If your machine is a loaded WEB server,
384		you should think about lowering this value, such sockets
385		may consume significant resources. Cf. tcp_max_orphans.
386	
387	tcp_reordering - INTEGER
388		Initial reordering level of packets in a TCP stream.
389		TCP stack can then dynamically adjust flow reordering level
390		between this initial value and tcp_max_reordering
391		Default: 3
392	
393	tcp_max_reordering - INTEGER
394		Maximal reordering level of packets in a TCP stream.
395		300 is a fairly conservative value, but you might increase it
396		if paths are using per packet load balancing (like bonding rr mode)
397		Default: 300
398	
399	tcp_retrans_collapse - BOOLEAN
400		Bug-to-bug compatibility with some broken printers.
401		On retransmit try to send bigger packets to work around bugs in
402		certain TCP stacks.
403	
404	tcp_retries1 - INTEGER
405		This value influences the time, after which TCP decides, that
406		something is wrong due to unacknowledged RTO retransmissions,
407		and reports this suspicion to the network layer.
408		See tcp_retries2 for more details.
409	
410		RFC 1122 recommends at least 3 retransmissions, which is the
411		default.
412	
413	tcp_retries2 - INTEGER
414		This value influences the timeout of an alive TCP connection,
415		when RTO retransmissions remain unacknowledged.
416		Given a value of N, a hypothetical TCP connection following
417		exponential backoff with an initial RTO of TCP_RTO_MIN would
418		retransmit N times before killing the connection at the (N+1)th RTO.
419	
420		The default value of 15 yields a hypothetical timeout of 924.6
421		seconds and is a lower bound for the effective timeout.
422		TCP will effectively time out at the first RTO which exceeds the
423		hypothetical timeout.
424	
425		RFC 1122 recommends at least 100 seconds for the timeout,
426		which corresponds to a value of at least 8.
427	
428	tcp_rfc1337 - BOOLEAN
429		If set, the TCP stack behaves conforming to RFC1337. If unset,
430		we are not conforming to RFC, but prevent TCP TIME_WAIT
431		assassination.
432		Default: 0
433	
434	tcp_rmem - vector of 3 INTEGERs: min, default, max
435		min: Minimal size of receive buffer used by TCP sockets.
436		It is guaranteed to each TCP socket, even under moderate memory
437		pressure.
438		Default: 1 page
439	
440		default: initial size of receive buffer used by TCP sockets.
441		This value overrides net.core.rmem_default used by other protocols.
442		Default: 87380 bytes. This value results in window of 65535 with
443		default setting of tcp_adv_win_scale and tcp_app_win:0 and a bit
444		less for default tcp_app_win. See below about these variables.
445	
446		max: maximal size of receive buffer allowed for automatically
447		selected receiver buffers for TCP socket. This value does not override
448		net.core.rmem_max.  Calling setsockopt() with SO_RCVBUF disables
449		automatic tuning of that socket's receive buffer size, in which
450		case this value is ignored.
451		Default: between 87380B and 6MB, depending on RAM size.
452	
453	tcp_sack - BOOLEAN
454		Enable select acknowledgments (SACKS).
455	
456	tcp_slow_start_after_idle - BOOLEAN
457		If set, provide RFC2861 behavior and time out the congestion
458		window after an idle period.  An idle period is defined at
459		the current RTO.  If unset, the congestion window will not
460		be timed out after an idle period.
461		Default: 1
462	
463	tcp_stdurg - BOOLEAN
464		Use the Host requirements interpretation of the TCP urgent pointer field.
465		Most hosts use the older BSD interpretation, so if you turn this on
466		Linux might not communicate correctly with them.
467		Default: FALSE
468	
469	tcp_synack_retries - INTEGER
470		Number of times SYNACKs for a passive TCP connection attempt will
471		be retransmitted. Should not be higher than 255. Default value
472		is 5, which corresponds to 31seconds till the last retransmission
473		with the current initial RTO of 1second. With this the final timeout
474		for a passive TCP connection will happen after 63seconds.
475	
476	tcp_syncookies - BOOLEAN
477		Only valid when the kernel was compiled with CONFIG_SYN_COOKIES
478		Send out syncookies when the syn backlog queue of a socket
479		overflows. This is to prevent against the common 'SYN flood attack'
480		Default: 1
481	
482		Note, that syncookies is fallback facility.
483		It MUST NOT be used to help highly loaded servers to stand
484		against legal connection rate. If you see SYN flood warnings
485		in your logs, but investigation	shows that they occur
486		because of overload with legal connections, you should tune
487		another parameters until this warning disappear.
488		See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.
489	
490		syncookies seriously violate TCP protocol, do not allow
491		to use TCP extensions, can result in serious degradation
492		of some services (f.e. SMTP relaying), visible not by you,
493		but your clients and relays, contacting you. While you see
494		SYN flood warnings in logs not being really flooded, your server
495		is seriously misconfigured.
496	
497		If you want to test which effects syncookies have to your
498		network connections you can set this knob to 2 to enable
499		unconditionally generation of syncookies.
500	
501	tcp_fastopen - INTEGER
502		Enable TCP Fast Open feature (draft-ietf-tcpm-fastopen) to send data
503		in the opening SYN packet. To use this feature, the client application
504		must use sendmsg() or sendto() with MSG_FASTOPEN flag rather than
505		connect() to perform a TCP handshake automatically.
506	
507		The values (bitmap) are
508		1: Enables sending data in the opening SYN on the client w/ MSG_FASTOPEN.
509		2: Enables TCP Fast Open on the server side, i.e., allowing data in
510		   a SYN packet to be accepted and passed to the application before
511		   3-way hand shake finishes.
512		4: Send data in the opening SYN regardless of cookie availability and
513		   without a cookie option.
514		0x100: Accept SYN data w/o validating the cookie.
515		0x200: Accept data-in-SYN w/o any cookie option present.
516		0x400/0x800: Enable Fast Open on all listeners regardless of the
517		   TCP_FASTOPEN socket option. The two different flags designate two
518		   different ways of setting max_qlen without the TCP_FASTOPEN socket
519		   option.
520	
521		Default: 1
522	
523		Note that the client & server side Fast Open flags (1 and 2
524		respectively) must be also enabled before the rest of flags can take
525		effect.
526	
527		See include/net/tcp.h and the code for more details.
528	
529	tcp_syn_retries - INTEGER
530		Number of times initial SYNs for an active TCP connection attempt
531		will be retransmitted. Should not be higher than 255. Default value
532		is 6, which corresponds to 63seconds till the last retransmission
533		with the current initial RTO of 1second. With this the final timeout
534		for an active TCP connection attempt will happen after 127seconds.
535	
536	tcp_timestamps - BOOLEAN
537		Enable timestamps as defined in RFC1323.
538	
539	tcp_min_tso_segs - INTEGER
540		Minimal number of segments per TSO frame.
541		Since linux-3.12, TCP does an automatic sizing of TSO frames,
542		depending on flow rate, instead of filling 64Kbytes packets.
543		For specific usages, it's possible to force TCP to build big
544		TSO frames. Note that TCP stack might split too big TSO packets
545		if available window is too small.
546		Default: 2
547	
548	tcp_tso_win_divisor - INTEGER
549		This allows control over what percentage of the congestion window
550		can be consumed by a single TSO frame.
551		The setting of this parameter is a choice between burstiness and
552		building larger TSO frames.
553		Default: 3
554	
555	tcp_tw_recycle - BOOLEAN
556		Enable fast recycling TIME-WAIT sockets. Default value is 0.
557		It should not be changed without advice/request of technical
558		experts.
559	
560	tcp_tw_reuse - BOOLEAN
561		Allow to reuse TIME-WAIT sockets for new connections when it is
562		safe from protocol viewpoint. Default value is 0.
563		It should not be changed without advice/request of technical
564		experts.
565	
566	tcp_window_scaling - BOOLEAN
567		Enable window scaling as defined in RFC1323.
568	
569	tcp_wmem - vector of 3 INTEGERs: min, default, max
570		min: Amount of memory reserved for send buffers for TCP sockets.
571		Each TCP socket has rights to use it due to fact of its birth.
572		Default: 1 page
573	
574		default: initial size of send buffer used by TCP sockets.  This
575		value overrides net.core.wmem_default used by other protocols.
576		It is usually lower than net.core.wmem_default.
577		Default: 16K
578	
579		max: Maximal amount of memory allowed for automatically tuned
580		send buffers for TCP sockets. This value does not override
581		net.core.wmem_max.  Calling setsockopt() with SO_SNDBUF disables
582		automatic tuning of that socket's send buffer size, in which case
583		this value is ignored.
584		Default: between 64K and 4MB, depending on RAM size.
585	
586	tcp_notsent_lowat - UNSIGNED INTEGER
587		A TCP socket can control the amount of unsent bytes in its write queue,
588		thanks to TCP_NOTSENT_LOWAT socket option. poll()/select()/epoll()
589		reports POLLOUT events if the amount of unsent bytes is below a per
590		socket value, and if the write queue is not full. sendmsg() will
591		also not add new buffers if the limit is hit.
592	
593		This global variable controls the amount of unsent data for
594		sockets not using TCP_NOTSENT_LOWAT. For these sockets, a change
595		to the global variable has immediate effect.
596	
597		Default: UINT_MAX (0xFFFFFFFF)
598	
599	tcp_workaround_signed_windows - BOOLEAN
600		If set, assume no receipt of a window scaling option means the
601		remote TCP is broken and treats the window as a signed quantity.
602		If unset, assume the remote TCP is not broken even if we do
603		not receive a window scaling option from them.
604		Default: 0
605	
606	tcp_thin_linear_timeouts - BOOLEAN
607		Enable dynamic triggering of linear timeouts for thin streams.
608		If set, a check is performed upon retransmission by timeout to
609		determine if the stream is thin (less than 4 packets in flight).
610		As long as the stream is found to be thin, up to 6 linear
611		timeouts may be performed before exponential backoff mode is
612		initiated. This improves retransmission latency for
613		non-aggressive thin streams, often found to be time-dependent.
614		For more information on thin streams, see
615		Documentation/networking/tcp-thin.txt
616		Default: 0
617	
618	tcp_thin_dupack - BOOLEAN
619		Enable dynamic triggering of retransmissions after one dupACK
620		for thin streams. If set, a check is performed upon reception
621		of a dupACK to determine if the stream is thin (less than 4
622		packets in flight). As long as the stream is found to be thin,
623		data is retransmitted on the first received dupACK. This
624		improves retransmission latency for non-aggressive thin
625		streams, often found to be time-dependent.
626		For more information on thin streams, see
627		Documentation/networking/tcp-thin.txt
628		Default: 0
629	
630	tcp_limit_output_bytes - INTEGER
631		Controls TCP Small Queue limit per tcp socket.
632		TCP bulk sender tends to increase packets in flight until it
633		gets losses notifications. With SNDBUF autotuning, this can
634		result in a large amount of packets queued in qdisc/device
635		on the local machine, hurting latency of other flows, for
636		typical pfifo_fast qdiscs.
637		tcp_limit_output_bytes limits the number of bytes on qdisc
638		or device to reduce artificial RTT/cwnd and reduce bufferbloat.
639		Default: 131072
640	
641	tcp_challenge_ack_limit - INTEGER
642		Limits number of Challenge ACK sent per second, as recommended
643		in RFC 5961 (Improving TCP's Robustness to Blind In-Window Attacks)
644		Default: 100
645	
646	UDP variables:
647	
648	udp_mem - vector of 3 INTEGERs: min, pressure, max
649		Number of pages allowed for queueing by all UDP sockets.
650	
651		min: Below this number of pages UDP is not bothered about its
652		memory appetite. When amount of memory allocated by UDP exceeds
653		this number, UDP starts to moderate memory usage.
654	
655		pressure: This value was introduced to follow format of tcp_mem.
656	
657		max: Number of pages allowed for queueing by all UDP sockets.
658	
659		Default is calculated at boot time from amount of available memory.
660	
661	udp_rmem_min - INTEGER
662		Minimal size of receive buffer used by UDP sockets in moderation.
663		Each UDP socket is able to use the size for receiving data, even if
664		total pages of UDP sockets exceed udp_mem pressure. The unit is byte.
665		Default: 1 page
666	
667	udp_wmem_min - INTEGER
668		Minimal size of send buffer used by UDP sockets in moderation.
669		Each UDP socket is able to use the size for sending data, even if
670		total pages of UDP sockets exceed udp_mem pressure. The unit is byte.
671		Default: 1 page
672	
673	CIPSOv4 Variables:
674	
675	cipso_cache_enable - BOOLEAN
676		If set, enable additions to and lookups from the CIPSO label mapping
677		cache.  If unset, additions are ignored and lookups always result in a
678		miss.  However, regardless of the setting the cache is still
679		invalidated when required when means you can safely toggle this on and
680		off and the cache will always be "safe".
681		Default: 1
682	
683	cipso_cache_bucket_size - INTEGER
684		The CIPSO label cache consists of a fixed size hash table with each
685		hash bucket containing a number of cache entries.  This variable limits
686		the number of entries in each hash bucket; the larger the value the
687		more CIPSO label mappings that can be cached.  When the number of
688		entries in a given hash bucket reaches this limit adding new entries
689		causes the oldest entry in the bucket to be removed to make room.
690		Default: 10
691	
692	cipso_rbm_optfmt - BOOLEAN
693		Enable the "Optimized Tag 1 Format" as defined in section 3.4.2.6 of
694		the CIPSO draft specification (see Documentation/netlabel for details).
695		This means that when set the CIPSO tag will be padded with empty
696		categories in order to make the packet data 32-bit aligned.
697		Default: 0
698	
699	cipso_rbm_structvalid - BOOLEAN
700		If set, do a very strict check of the CIPSO option when
701		ip_options_compile() is called.  If unset, relax the checks done during
702		ip_options_compile().  Either way is "safe" as errors are caught else
703		where in the CIPSO processing code but setting this to 0 (False) should
704		result in less work (i.e. it should be faster) but could cause problems
705		with other implementations that require strict checking.
706		Default: 0
707	
708	IP Variables:
709	
710	ip_local_port_range - 2 INTEGERS
711		Defines the local port range that is used by TCP and UDP to
712		choose the local port. The first number is the first, the
713		second the last local port number. The default values are
714		32768 and 61000 respectively.
715	
716	ip_local_reserved_ports - list of comma separated ranges
717		Specify the ports which are reserved for known third-party
718		applications. These ports will not be used by automatic port
719		assignments (e.g. when calling connect() or bind() with port
720		number 0). Explicit port allocation behavior is unchanged.
721	
722		The format used for both input and output is a comma separated
723		list of ranges (e.g. "1,2-4,10-10" for ports 1, 2, 3, 4 and
724		10). Writing to the file will clear all previously reserved
725		ports and update the current list with the one given in the
726		input.
727	
728		Note that ip_local_port_range and ip_local_reserved_ports
729		settings are independent and both are considered by the kernel
730		when determining which ports are available for automatic port
731		assignments.
732	
733		You can reserve ports which are not in the current
734		ip_local_port_range, e.g.:
735	
736		$ cat /proc/sys/net/ipv4/ip_local_port_range
737		32000	61000
738		$ cat /proc/sys/net/ipv4/ip_local_reserved_ports
739		8080,9148
740	
741		although this is redundant. However such a setting is useful
742		if later the port range is changed to a value that will
743		include the reserved ports.
744	
745		Default: Empty
746	
747	ip_nonlocal_bind - BOOLEAN
748		If set, allows processes to bind() to non-local IP addresses,
749		which can be quite useful - but may break some applications.
750		Default: 0
751	
752	ip_dynaddr - BOOLEAN
753		If set non-zero, enables support for dynamic addresses.
754		If set to a non-zero value larger than 1, a kernel log
755		message will be printed when dynamic address rewriting
756		occurs.
757		Default: 0
758	
759	ip_early_demux - BOOLEAN
760		Optimize input packet processing down to one demux for
761		certain kinds of local sockets.  Currently we only do this
762		for established TCP sockets.
763	
764		It may add an additional cost for pure routing workloads that
765		reduces overall throughput, in such case you should disable it.
766		Default: 1
767	
768	icmp_echo_ignore_all - BOOLEAN
769		If set non-zero, then the kernel will ignore all ICMP ECHO
770		requests sent to it.
771		Default: 0
772	
773	icmp_echo_ignore_broadcasts - BOOLEAN
774		If set non-zero, then the kernel will ignore all ICMP ECHO and
775		TIMESTAMP requests sent to it via broadcast/multicast.
776		Default: 1
777	
778	icmp_ratelimit - INTEGER
779		Limit the maximal rates for sending ICMP packets whose type matches
780		icmp_ratemask (see below) to specific targets.
781		0 to disable any limiting,
782		otherwise the minimal space between responses in milliseconds.
783		Note that another sysctl, icmp_msgs_per_sec limits the number
784		of ICMP packets	sent on all targets.
785		Default: 1000
786	
787	icmp_msgs_per_sec - INTEGER
788		Limit maximal number of ICMP packets sent per second from this host.
789		Only messages whose type matches icmp_ratemask (see below) are
790		controlled by this limit.
791		Default: 1000
792	
793	icmp_msgs_burst - INTEGER
794		icmp_msgs_per_sec controls number of ICMP packets sent per second,
795		while icmp_msgs_burst controls the burst size of these packets.
796		Default: 50
797	
798	icmp_ratemask - INTEGER
799		Mask made of ICMP types for which rates are being limited.
800		Significant bits: IHGFEDCBA9876543210
801		Default mask:     0000001100000011000 (6168)
802	
803		Bit definitions (see include/linux/icmp.h):
804			0 Echo Reply
805			3 Destination Unreachable *
806			4 Source Quench *
807			5 Redirect
808			8 Echo Request
809			B Time Exceeded *
810			C Parameter Problem *
811			D Timestamp Request
812			E Timestamp Reply
813			F Info Request
814			G Info Reply
815			H Address Mask Request
816			I Address Mask Reply
817	
818		* These are rate limited by default (see default mask above)
819	
820	icmp_ignore_bogus_error_responses - BOOLEAN
821		Some routers violate RFC1122 by sending bogus responses to broadcast
822		frames.  Such violations are normally logged via a kernel warning.
823		If this is set to TRUE, the kernel will not give such warnings, which
824		will avoid log file clutter.
825		Default: 1
826	
827	icmp_errors_use_inbound_ifaddr - BOOLEAN
828	
829		If zero, icmp error messages are sent with the primary address of
830		the exiting interface.
831	
832		If non-zero, the message will be sent with the primary address of
833		the interface that received the packet that caused the icmp error.
834		This is the behaviour network many administrators will expect from
835		a router. And it can make debugging complicated network layouts
836		much easier.
837	
838		Note that if no primary address exists for the interface selected,
839		then the primary address of the first non-loopback interface that
840		has one will be used regardless of this setting.
841	
842		Default: 0
843	
844	igmp_max_memberships - INTEGER
845		Change the maximum number of multicast groups we can subscribe to.
846		Default: 20
847	
848		Theoretical maximum value is bounded by having to send a membership
849		report in a single datagram (i.e. the report can't span multiple
850		datagrams, or risk confusing the switch and leaving groups you don't
851		intend to).
852	
853		The number of supported groups 'M' is bounded by the number of group
854		report entries you can fit into a single datagram of 65535 bytes.
855	
856		M = 65536-sizeof (ip header)/(sizeof(Group record))
857	
858		Group records are variable length, with a minimum of 12 bytes.
859		So net.ipv4.igmp_max_memberships should not be set higher than:
860	
861		(65536-24) / 12 = 5459
862	
863		The value 5459 assumes no IP header options, so in practice
864		this number may be lower.
865	
866		conf/interface/*  changes special settings per interface (where
867		"interface" is the name of your network interface)
868	
869		conf/all/*	  is special, changes the settings for all interfaces
870	
871	igmp_qrv - INTEGER
872		 Controls the IGMP query robustness variable (see RFC2236 8.1).
873		 Default: 2 (as specified by RFC2236 8.1)
874		 Minimum: 1 (as specified by RFC6636 4.5)
875	
876	log_martians - BOOLEAN
877		Log packets with impossible addresses to kernel log.
878		log_martians for the interface will be enabled if at least one of
879		conf/{all,interface}/log_martians is set to TRUE,
880		it will be disabled otherwise
881	
882	accept_redirects - BOOLEAN
883		Accept ICMP redirect messages.
884		accept_redirects for the interface will be enabled if:
885		- both conf/{all,interface}/accept_redirects are TRUE in the case
886		  forwarding for the interface is enabled
887		or
888		- at least one of conf/{all,interface}/accept_redirects is TRUE in the
889		  case forwarding for the interface is disabled
890		accept_redirects for the interface will be disabled otherwise
891		default TRUE (host)
892			FALSE (router)
893	
894	forwarding - BOOLEAN
895		Enable IP forwarding on this interface.
896	
897	mc_forwarding - BOOLEAN
898		Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE
899		and a multicast routing daemon is required.
900		conf/all/mc_forwarding must also be set to TRUE to enable multicast
901		routing	for the interface
902	
903	medium_id - INTEGER
904		Integer value used to differentiate the devices by the medium they
905		are attached to. Two devices can have different id values when
906		the broadcast packets are received only on one of them.
907		The default value 0 means that the device is the only interface
908		to its medium, value of -1 means that medium is not known.
909	
910		Currently, it is used to change the proxy_arp behavior:
911		the proxy_arp feature is enabled for packets forwarded between
912		two devices attached to different media.
913	
914	proxy_arp - BOOLEAN
915		Do proxy arp.
916		proxy_arp for the interface will be enabled if at least one of
917		conf/{all,interface}/proxy_arp is set to TRUE,
918		it will be disabled otherwise
919	
920	proxy_arp_pvlan - BOOLEAN
921		Private VLAN proxy arp.
922		Basically allow proxy arp replies back to the same interface
923		(from which the ARP request/solicitation was received).
924	
925		This is done to support (ethernet) switch features, like RFC
926		3069, where the individual ports are NOT allowed to
927		communicate with each other, but they are allowed to talk to
928		the upstream router.  As described in RFC 3069, it is possible
929		to allow these hosts to communicate through the upstream
930		router by proxy_arp'ing. Don't need to be used together with
931		proxy_arp.
932	
933		This technology is known by different names:
934		  In RFC 3069 it is called VLAN Aggregation.
935		  Cisco and Allied Telesyn call it Private VLAN.
936		  Hewlett-Packard call it Source-Port filtering or port-isolation.
937		  Ericsson call it MAC-Forced Forwarding (RFC Draft).
938	
939	shared_media - BOOLEAN
940		Send(router) or accept(host) RFC1620 shared media redirects.
941		Overrides ip_secure_redirects.
942		shared_media for the interface will be enabled if at least one of
943		conf/{all,interface}/shared_media is set to TRUE,
944		it will be disabled otherwise
945		default TRUE
946	
947	secure_redirects - BOOLEAN
948		Accept ICMP redirect messages only for gateways,
949		listed in default gateway list.
950		secure_redirects for the interface will be enabled if at least one of
951		conf/{all,interface}/secure_redirects is set to TRUE,
952		it will be disabled otherwise
953		default TRUE
954	
955	send_redirects - BOOLEAN
956		Send redirects, if router.
957		send_redirects for the interface will be enabled if at least one of
958		conf/{all,interface}/send_redirects is set to TRUE,
959		it will be disabled otherwise
960		Default: TRUE
961	
962	bootp_relay - BOOLEAN
963		Accept packets with source address 0.b.c.d destined
964		not to this host as local ones. It is supposed, that
965		BOOTP relay daemon will catch and forward such packets.
966		conf/all/bootp_relay must also be set to TRUE to enable BOOTP relay
967		for the interface
968		default FALSE
969		Not Implemented Yet.
970	
971	accept_source_route - BOOLEAN
972		Accept packets with SRR option.
973		conf/all/accept_source_route must also be set to TRUE to accept packets
974		with SRR option on the interface
975		default TRUE (router)
976			FALSE (host)
977	
978	accept_local - BOOLEAN
979		Accept packets with local source addresses. In combination with
980		suitable routing, this can be used to direct packets between two
981		local interfaces over the wire and have them accepted properly.
982		default FALSE
983	
984	route_localnet - BOOLEAN
985		Do not consider loopback addresses as martian source or destination
986		while routing. This enables the use of 127/8 for local routing purposes.
987		default FALSE
988	
989	rp_filter - INTEGER
990		0 - No source validation.
991		1 - Strict mode as defined in RFC3704 Strict Reverse Path
992		    Each incoming packet is tested against the FIB and if the interface
993		    is not the best reverse path the packet check will fail.
994		    By default failed packets are discarded.
995		2 - Loose mode as defined in RFC3704 Loose Reverse Path
996		    Each incoming packet's source address is also tested against the FIB
997		    and if the source address is not reachable via any interface
998		    the packet check will fail.
999	
1000		Current recommended practice in RFC3704 is to enable strict mode
1001		to prevent IP spoofing from DDos attacks. If using asymmetric routing
1002		or other complicated routing, then loose mode is recommended.
1003	
1004		The max value from conf/{all,interface}/rp_filter is used
1005		when doing source validation on the {interface}.
1006	
1007		Default value is 0. Note that some distributions enable it
1008		in startup scripts.
1009	
1010	arp_filter - BOOLEAN
1011		1 - Allows you to have multiple network interfaces on the same
1012		subnet, and have the ARPs for each interface be answered
1013		based on whether or not the kernel would route a packet from
1014		the ARP'd IP out that interface (therefore you must use source
1015		based routing for this to work). In other words it allows control
1016		of which cards (usually 1) will respond to an arp request.
1017	
1018		0 - (default) The kernel can respond to arp requests with addresses
1019		from other interfaces. This may seem wrong but it usually makes
1020		sense, because it increases the chance of successful communication.
1021		IP addresses are owned by the complete host on Linux, not by
1022		particular interfaces. Only for more complex setups like load-
1023		balancing, does this behaviour cause problems.
1024	
1025		arp_filter for the interface will be enabled if at least one of
1026		conf/{all,interface}/arp_filter is set to TRUE,
1027		it will be disabled otherwise
1028	
1029	arp_announce - INTEGER
1030		Define different restriction levels for announcing the local
1031		source IP address from IP packets in ARP requests sent on
1032		interface:
1033		0 - (default) Use any local address, configured on any interface
1034		1 - Try to avoid local addresses that are not in the target's
1035		subnet for this interface. This mode is useful when target
1036		hosts reachable via this interface require the source IP
1037		address in ARP requests to be part of their logical network
1038		configured on the receiving interface. When we generate the
1039		request we will check all our subnets that include the
1040		target IP and will preserve the source address if it is from
1041		such subnet. If there is no such subnet we select source
1042		address according to the rules for level 2.
1043		2 - Always use the best local address for this target.
1044		In this mode we ignore the source address in the IP packet
1045		and try to select local address that we prefer for talks with
1046		the target host. Such local address is selected by looking
1047		for primary IP addresses on all our subnets on the outgoing
1048		interface that include the target IP address. If no suitable
1049		local address is found we select the first local address
1050		we have on the outgoing interface or on all other interfaces,
1051		with the hope we will receive reply for our request and
1052		even sometimes no matter the source IP address we announce.
1053	
1054		The max value from conf/{all,interface}/arp_announce is used.
1055	
1056		Increasing the restriction level gives more chance for
1057		receiving answer from the resolved target while decreasing
1058		the level announces more valid sender's information.
1059	
1060	arp_ignore - INTEGER
1061		Define different modes for sending replies in response to
1062		received ARP requests that resolve local target IP addresses:
1063		0 - (default): reply for any local target IP address, configured
1064		on any interface
1065		1 - reply only if the target IP address is local address
1066		configured on the incoming interface
1067		2 - reply only if the target IP address is local address
1068		configured on the incoming interface and both with the
1069		sender's IP address are part from same subnet on this interface
1070		3 - do not reply for local addresses configured with scope host,
1071		only resolutions for global and link addresses are replied
1072		4-7 - reserved
1073		8 - do not reply for all local addresses
1074	
1075		The max value from conf/{all,interface}/arp_ignore is used
1076		when ARP request is received on the {interface}
1077	
1078	arp_notify - BOOLEAN
1079		Define mode for notification of address and device changes.
1080		0 - (default): do nothing
1081		1 - Generate gratuitous arp requests when device is brought up
1082		    or hardware address changes.
1083	
1084	arp_accept - BOOLEAN
1085		Define behavior for gratuitous ARP frames who's IP is not
1086		already present in the ARP table:
1087		0 - don't create new entries in the ARP table
1088		1 - create new entries in the ARP table
1089	
1090		Both replies and requests type gratuitous arp will trigger the
1091		ARP table to be updated, if this setting is on.
1092	
1093		If the ARP table already contains the IP address of the
1094		gratuitous arp frame, the arp table will be updated regardless
1095		if this setting is on or off.
1096	
1097	
1098	app_solicit - INTEGER
1099		The maximum number of probes to send to the user space ARP daemon
1100		via netlink before dropping back to multicast probes (see
1101		mcast_solicit).  Defaults to 0.
1102	
1103	disable_policy - BOOLEAN
1104		Disable IPSEC policy (SPD) for this interface
1105	
1106	disable_xfrm - BOOLEAN
1107		Disable IPSEC encryption on this interface, whatever the policy
1108	
1109	igmpv2_unsolicited_report_interval - INTEGER
1110		The interval in milliseconds in which the next unsolicited
1111		IGMPv1 or IGMPv2 report retransmit will take place.
1112		Default: 10000 (10 seconds)
1113	
1114	igmpv3_unsolicited_report_interval - INTEGER
1115		The interval in milliseconds in which the next unsolicited
1116		IGMPv3 report retransmit will take place.
1117		Default: 1000 (1 seconds)
1118	
1119	promote_secondaries - BOOLEAN
1120		When a primary IP address is removed from this interface
1121		promote a corresponding secondary IP address instead of
1122		removing all the corresponding secondary IP addresses.
1123	
1124	
1125	tag - INTEGER
1126		Allows you to write a number, which can be used as required.
1127		Default value is 0.
1128	
1129	Alexey Kuznetsov.
1130	kuznet@ms2.inr.ac.ru
1131	
1132	Updated by:
1133	Andi Kleen
1134	ak@muc.de
1135	Nicolas Delon
1136	delon.nicolas@wanadoo.fr
1137	
1138	
1139	
1140	
1141	/proc/sys/net/ipv6/* Variables:
1142	
1143	IPv6 has no global variables such as tcp_*.  tcp_* settings under ipv4/ also
1144	apply to IPv6 [XXX?].
1145	
1146	bindv6only - BOOLEAN
1147		Default value for IPV6_V6ONLY socket option,
1148		which restricts use of the IPv6 socket to IPv6 communication
1149		only.
1150			TRUE: disable IPv4-mapped address feature
1151			FALSE: enable IPv4-mapped address feature
1152	
1153		Default: FALSE (as specified in RFC3493)
1154	
1155	flowlabel_consistency - BOOLEAN
1156		Protect the consistency (and unicity) of flow label.
1157		You have to disable it to use IPV6_FL_F_REFLECT flag on the
1158		flow label manager.
1159		TRUE: enabled
1160		FALSE: disabled
1161		Default: TRUE
1162	
1163	auto_flowlabels - BOOLEAN
1164		Automatically generate flow labels based based on a flow hash
1165		of the packet. This allows intermediate devices, such as routers,
1166		to idenfify packet flows for mechanisms like Equal Cost Multipath
1167		Routing (see RFC 6438).
1168		TRUE: enabled
1169		FALSE: disabled
1170		Default: false
1171	
1172	anycast_src_echo_reply - BOOLEAN
1173		Controls the use of anycast addresses as source addresses for ICMPv6
1174		echo reply
1175		TRUE:  enabled
1176		FALSE: disabled
1177		Default: FALSE
1178	
1179	mld_qrv - INTEGER
1180		Controls the MLD query robustness variable (see RFC3810 9.1).
1181		Default: 2 (as specified by RFC3810 9.1)
1182		Minimum: 1 (as specified by RFC6636 4.5)
1183	
1184	IPv6 Fragmentation:
1185	
1186	ip6frag_high_thresh - INTEGER
1187		Maximum memory used to reassemble IPv6 fragments. When
1188		ip6frag_high_thresh bytes of memory is allocated for this purpose,
1189		the fragment handler will toss packets until ip6frag_low_thresh
1190		is reached.
1191	
1192	ip6frag_low_thresh - INTEGER
1193		See ip6frag_high_thresh
1194	
1195	ip6frag_time - INTEGER
1196		Time in seconds to keep an IPv6 fragment in memory.
1197	
1198	conf/default/*:
1199		Change the interface-specific default settings.
1200	
1201	
1202	conf/all/*:
1203		Change all the interface-specific settings.
1204	
1205		[XXX:  Other special features than forwarding?]
1206	
1207	conf/all/forwarding - BOOLEAN
1208		Enable global IPv6 forwarding between all interfaces.
1209	
1210		IPv4 and IPv6 work differently here; e.g. netfilter must be used
1211		to control which interfaces may forward packets and which not.
1212	
1213		This also sets all interfaces' Host/Router setting
1214		'forwarding' to the specified value.  See below for details.
1215	
1216		This referred to as global forwarding.
1217	
1218	proxy_ndp - BOOLEAN
1219		Do proxy ndp.
1220	
1221	fwmark_reflect - BOOLEAN
1222		Controls the fwmark of kernel-generated IPv6 reply packets that are not
1223		associated with a socket for example, TCP RSTs or ICMPv6 echo replies).
1224		If unset, these packets have a fwmark of zero. If set, they have the
1225		fwmark of the packet they are replying to.
1226		Default: 0
1227	
1228	conf/interface/*:
1229		Change special settings per interface.
1230	
1231		The functional behaviour for certain settings is different
1232		depending on whether local forwarding is enabled or not.
1233	
1234	accept_ra - INTEGER
1235		Accept Router Advertisements; autoconfigure using them.
1236	
1237		It also determines whether or not to transmit Router
1238		Solicitations. If and only if the functional setting is to
1239		accept Router Advertisements, Router Solicitations will be
1240		transmitted.
1241	
1242		Possible values are:
1243			0 Do not accept Router Advertisements.
1244			1 Accept Router Advertisements if forwarding is disabled.
1245			2 Overrule forwarding behaviour. Accept Router Advertisements
1246			  even if forwarding is enabled.
1247	
1248		Functional default: enabled if local forwarding is disabled.
1249				    disabled if local forwarding is enabled.
1250	
1251	accept_ra_defrtr - BOOLEAN
1252		Learn default router in Router Advertisement.
1253	
1254		Functional default: enabled if accept_ra is enabled.
1255				    disabled if accept_ra is disabled.
1256	
1257	accept_ra_from_local - BOOLEAN
1258		Accept RA with source-address that is found on local machine
1259	        if the RA is otherwise proper and able to be accepted.
1260	        Default is to NOT accept these as it may be an un-intended
1261	        network loop.
1262	
1263		Functional default:
1264	           enabled if accept_ra_from_local is enabled
1265	               on a specific interface.
1266		   disabled if accept_ra_from_local is disabled
1267	               on a specific interface.
1268	
1269	accept_ra_pinfo - BOOLEAN
1270		Learn Prefix Information in Router Advertisement.
1271	
1272		Functional default: enabled if accept_ra is enabled.
1273				    disabled if accept_ra is disabled.
1274	
1275	accept_ra_rt_info_max_plen - INTEGER
1276		Maximum prefix length of Route Information in RA.
1277	
1278		Route Information w/ prefix larger than or equal to this
1279		variable shall be ignored.
1280	
1281		Functional default: 0 if accept_ra_rtr_pref is enabled.
1282				    -1 if accept_ra_rtr_pref is disabled.
1283	
1284	accept_ra_rtr_pref - BOOLEAN
1285		Accept Router Preference in RA.
1286	
1287		Functional default: enabled if accept_ra is enabled.
1288				    disabled if accept_ra is disabled.
1289	
1290	accept_redirects - BOOLEAN
1291		Accept Redirects.
1292	
1293		Functional default: enabled if local forwarding is disabled.
1294				    disabled if local forwarding is enabled.
1295	
1296	accept_source_route - INTEGER
1297		Accept source routing (routing extension header).
1298	
1299		>= 0: Accept only routing header type 2.
1300		< 0: Do not accept routing header.
1301	
1302		Default: 0
1303	
1304	autoconf - BOOLEAN
1305		Autoconfigure addresses using Prefix Information in Router
1306		Advertisements.
1307	
1308		Functional default: enabled if accept_ra_pinfo is enabled.
1309				    disabled if accept_ra_pinfo is disabled.
1310	
1311	dad_transmits - INTEGER
1312		The amount of Duplicate Address Detection probes to send.
1313		Default: 1
1314	
1315	forwarding - INTEGER
1316		Configure interface-specific Host/Router behaviour.
1317	
1318		Note: It is recommended to have the same setting on all
1319		interfaces; mixed router/host scenarios are rather uncommon.
1320	
1321		Possible values are:
1322			0 Forwarding disabled
1323			1 Forwarding enabled
1324	
1325		FALSE (0):
1326	
1327		By default, Host behaviour is assumed.  This means:
1328	
1329		1. IsRouter flag is not set in Neighbour Advertisements.
1330		2. If accept_ra is TRUE (default), transmit Router
1331		   Solicitations.
1332		3. If accept_ra is TRUE (default), accept Router
1333		   Advertisements (and do autoconfiguration).
1334		4. If accept_redirects is TRUE (default), accept Redirects.
1335	
1336		TRUE (1):
1337	
1338		If local forwarding is enabled, Router behaviour is assumed.
1339		This means exactly the reverse from the above:
1340	
1341		1. IsRouter flag is set in Neighbour Advertisements.
1342		2. Router Solicitations are not sent unless accept_ra is 2.
1343		3. Router Advertisements are ignored unless accept_ra is 2.
1344		4. Redirects are ignored.
1345	
1346		Default: 0 (disabled) if global forwarding is disabled (default),
1347			 otherwise 1 (enabled).
1348	
1349	hop_limit - INTEGER
1350		Default Hop Limit to set.
1351		Default: 64
1352	
1353	mtu - INTEGER
1354		Default Maximum Transfer Unit
1355		Default: 1280 (IPv6 required minimum)
1356	
1357	router_probe_interval - INTEGER
1358		Minimum interval (in seconds) between Router Probing described
1359		in RFC4191.
1360	
1361		Default: 60
1362	
1363	router_solicitation_delay - INTEGER
1364		Number of seconds to wait after interface is brought up
1365		before sending Router Solicitations.
1366		Default: 1
1367	
1368	router_solicitation_interval - INTEGER
1369		Number of seconds to wait between Router Solicitations.
1370		Default: 4
1371	
1372	router_solicitations - INTEGER
1373		Number of Router Solicitations to send until assuming no
1374		routers are present.
1375		Default: 3
1376	
1377	use_tempaddr - INTEGER
1378		Preference for Privacy Extensions (RFC3041).
1379		  <= 0 : disable Privacy Extensions
1380		  == 1 : enable Privacy Extensions, but prefer public
1381		         addresses over temporary addresses.
1382		  >  1 : enable Privacy Extensions and prefer temporary
1383		         addresses over public addresses.
1384		Default:  0 (for most devices)
1385			 -1 (for point-to-point devices and loopback devices)
1386	
1387	temp_valid_lft - INTEGER
1388		valid lifetime (in seconds) for temporary addresses.
1389		Default: 604800 (7 days)
1390	
1391	temp_prefered_lft - INTEGER
1392		Preferred lifetime (in seconds) for temporary addresses.
1393		Default: 86400 (1 day)
1394	
1395	max_desync_factor - INTEGER
1396		Maximum value for DESYNC_FACTOR, which is a random value
1397		that ensures that clients don't synchronize with each
1398		other and generate new addresses at exactly the same time.
1399		value is in seconds.
1400		Default: 600
1401	
1402	regen_max_retry - INTEGER
1403		Number of attempts before give up attempting to generate
1404		valid temporary addresses.
1405		Default: 5
1406	
1407	max_addresses - INTEGER
1408		Maximum number of autoconfigured addresses per interface.  Setting
1409		to zero disables the limitation.  It is not recommended to set this
1410		value too large (or to zero) because it would be an easy way to
1411		crash the kernel by allowing too many addresses to be created.
1412		Default: 16
1413	
1414	disable_ipv6 - BOOLEAN
1415		Disable IPv6 operation.  If accept_dad is set to 2, this value
1416		will be dynamically set to TRUE if DAD fails for the link-local
1417		address.
1418		Default: FALSE (enable IPv6 operation)
1419	
1420		When this value is changed from 1 to 0 (IPv6 is being enabled),
1421		it will dynamically create a link-local address on the given
1422		interface and start Duplicate Address Detection, if necessary.
1423	
1424		When this value is changed from 0 to 1 (IPv6 is being disabled),
1425		it will dynamically delete all address on the given interface.
1426	
1427	accept_dad - INTEGER
1428		Whether to accept DAD (Duplicate Address Detection).
1429		0: Disable DAD
1430		1: Enable DAD (default)
1431		2: Enable DAD, and disable IPv6 operation if MAC-based duplicate
1432		   link-local address has been found.
1433	
1434	force_tllao - BOOLEAN
1435		Enable sending the target link-layer address option even when
1436		responding to a unicast neighbor solicitation.
1437		Default: FALSE
1438	
1439		Quoting from RFC 2461, section 4.4, Target link-layer address:
1440	
1441		"The option MUST be included for multicast solicitations in order to
1442		avoid infinite Neighbor Solicitation "recursion" when the peer node
1443		does not have a cache entry to return a Neighbor Advertisements
1444		message.  When responding to unicast solicitations, the option can be
1445		omitted since the sender of the solicitation has the correct link-
1446		layer address; otherwise it would not have be able to send the unicast
1447		solicitation in the first place. However, including the link-layer
1448		address in this case adds little overhead and eliminates a potential
1449		race condition where the sender deletes the cached link-layer address
1450		prior to receiving a response to a previous solicitation."
1451	
1452	ndisc_notify - BOOLEAN
1453		Define mode for notification of address and device changes.
1454		0 - (default): do nothing
1455		1 - Generate unsolicited neighbour advertisements when device is brought
1456		    up or hardware address changes.
1457	
1458	mldv1_unsolicited_report_interval - INTEGER
1459		The interval in milliseconds in which the next unsolicited
1460		MLDv1 report retransmit will take place.
1461		Default: 10000 (10 seconds)
1462	
1463	mldv2_unsolicited_report_interval - INTEGER
1464		The interval in milliseconds in which the next unsolicited
1465		MLDv2 report retransmit will take place.
1466		Default: 1000 (1 second)
1467	
1468	force_mld_version - INTEGER
1469		0 - (default) No enforcement of a MLD version, MLDv1 fallback allowed
1470		1 - Enforce to use MLD version 1
1471		2 - Enforce to use MLD version 2
1472	
1473	suppress_frag_ndisc - INTEGER
1474		Control RFC 6980 (Security Implications of IPv6 Fragmentation
1475		with IPv6 Neighbor Discovery) behavior:
1476		1 - (default) discard fragmented neighbor discovery packets
1477		0 - allow fragmented neighbor discovery packets
1478	
1479	optimistic_dad - BOOLEAN
1480		Whether to perform Optimistic Duplicate Address Detection (RFC 4429).
1481			0: disabled (default)
1482			1: enabled
1483	
1484	use_optimistic - BOOLEAN
1485		If enabled, do not classify optimistic addresses as deprecated during
1486		source address selection.  Preferred addresses will still be chosen
1487		before optimistic addresses, subject to other ranking in the source
1488		address selection algorithm.
1489			0: disabled (default)
1490			1: enabled
1491	
1492	icmp/*:
1493	ratelimit - INTEGER
1494		Limit the maximal rates for sending ICMPv6 packets.
1495		0 to disable any limiting,
1496		otherwise the minimal space between responses in milliseconds.
1497		Default: 1000
1498	
1499	
1500	IPv6 Update by:
1501	Pekka Savola <pekkas@netcore.fi>
1502	YOSHIFUJI Hideaki / USAGI Project <yoshfuji@linux-ipv6.org>
1503	
1504	
1505	/proc/sys/net/bridge/* Variables:
1506	
1507	bridge-nf-call-arptables - BOOLEAN
1508		1 : pass bridged ARP traffic to arptables' FORWARD chain.
1509		0 : disable this.
1510		Default: 1
1511	
1512	bridge-nf-call-iptables - BOOLEAN
1513		1 : pass bridged IPv4 traffic to iptables' chains.
1514		0 : disable this.
1515		Default: 1
1516	
1517	bridge-nf-call-ip6tables - BOOLEAN
1518		1 : pass bridged IPv6 traffic to ip6tables' chains.
1519		0 : disable this.
1520		Default: 1
1521	
1522	bridge-nf-filter-vlan-tagged - BOOLEAN
1523		1 : pass bridged vlan-tagged ARP/IP/IPv6 traffic to {arp,ip,ip6}tables.
1524		0 : disable this.
1525		Default: 0
1526	
1527	bridge-nf-filter-pppoe-tagged - BOOLEAN
1528		1 : pass bridged pppoe-tagged IP/IPv6 traffic to {ip,ip6}tables.
1529		0 : disable this.
1530		Default: 0
1531	
1532	bridge-nf-pass-vlan-input-dev - BOOLEAN
1533		1: if bridge-nf-filter-vlan-tagged is enabled, try to find a vlan
1534		interface on the bridge and set the netfilter input device to the vlan.
1535		This allows use of e.g. "iptables -i br0.1" and makes the REDIRECT
1536		target work with vlan-on-top-of-bridge interfaces.  When no matching
1537		vlan interface is found, or this switch is off, the input device is
1538		set to the bridge interface.
1539		0: disable bridge netfilter vlan interface lookup.
1540		Default: 0
1541	
1542	proc/sys/net/sctp/* Variables:
1543	
1544	addip_enable - BOOLEAN
1545		Enable or disable extension of  Dynamic Address Reconfiguration
1546		(ADD-IP) functionality specified in RFC5061.  This extension provides
1547		the ability to dynamically add and remove new addresses for the SCTP
1548		associations.
1549	
1550		1: Enable extension.
1551	
1552		0: Disable extension.
1553	
1554		Default: 0
1555	
1556	addip_noauth_enable - BOOLEAN
1557		Dynamic Address Reconfiguration (ADD-IP) requires the use of
1558		authentication to protect the operations of adding or removing new
1559		addresses.  This requirement is mandated so that unauthorized hosts
1560		would not be able to hijack associations.  However, older
1561		implementations may not have implemented this requirement while
1562		allowing the ADD-IP extension.  For reasons of interoperability,
1563		we provide this variable to control the enforcement of the
1564		authentication requirement.
1565	
1566		1: Allow ADD-IP extension to be used without authentication.  This
1567		   should only be set in a closed environment for interoperability
1568		   with older implementations.
1569	
1570		0: Enforce the authentication requirement
1571	
1572		Default: 0
1573	
1574	auth_enable - BOOLEAN
1575		Enable or disable Authenticated Chunks extension.  This extension
1576		provides the ability to send and receive authenticated chunks and is
1577		required for secure operation of Dynamic Address Reconfiguration
1578		(ADD-IP) extension.
1579	
1580		1: Enable this extension.
1581		0: Disable this extension.
1582	
1583		Default: 0
1584	
1585	prsctp_enable - BOOLEAN
1586		Enable or disable the Partial Reliability extension (RFC3758) which
1587		is used to notify peers that a given DATA should no longer be expected.
1588	
1589		1: Enable extension
1590		0: Disable
1591	
1592		Default: 1
1593	
1594	max_burst - INTEGER
1595		The limit of the number of new packets that can be initially sent.  It
1596		controls how bursty the generated traffic can be.
1597	
1598		Default: 4
1599	
1600	association_max_retrans - INTEGER
1601		Set the maximum number for retransmissions that an association can
1602		attempt deciding that the remote end is unreachable.  If this value
1603		is exceeded, the association is terminated.
1604	
1605		Default: 10
1606	
1607	max_init_retransmits - INTEGER
1608		The maximum number of retransmissions of INIT and COOKIE-ECHO chunks
1609		that an association will attempt before declaring the destination
1610		unreachable and terminating.
1611	
1612		Default: 8
1613	
1614	path_max_retrans - INTEGER
1615		The maximum number of retransmissions that will be attempted on a given
1616		path.  Once this threshold is exceeded, the path is considered
1617		unreachable, and new traffic will use a different path when the
1618		association is multihomed.
1619	
1620		Default: 5
1621	
1622	pf_retrans - INTEGER
1623		The number of retransmissions that will be attempted on a given path
1624		before traffic is redirected to an alternate transport (should one
1625		exist).  Note this is distinct from path_max_retrans, as a path that
1626		passes the pf_retrans threshold can still be used.  Its only
1627		deprioritized when a transmission path is selected by the stack.  This
1628		setting is primarily used to enable fast failover mechanisms without
1629		having to reduce path_max_retrans to a very low value.  See:
1630		http://www.ietf.org/id/draft-nishida-tsvwg-sctp-failover-05.txt
1631		for details.  Note also that a value of pf_retrans > path_max_retrans
1632		disables this feature
1633	
1634		Default: 0
1635	
1636	rto_initial - INTEGER
1637		The initial round trip timeout value in milliseconds that will be used
1638		in calculating round trip times.  This is the initial time interval
1639		for retransmissions.
1640	
1641		Default: 3000
1642	
1643	rto_max - INTEGER
1644		The maximum value (in milliseconds) of the round trip timeout.  This
1645		is the largest time interval that can elapse between retransmissions.
1646	
1647		Default: 60000
1648	
1649	rto_min - INTEGER
1650		The minimum value (in milliseconds) of the round trip timeout.  This
1651		is the smallest time interval the can elapse between retransmissions.
1652	
1653		Default: 1000
1654	
1655	hb_interval - INTEGER
1656		The interval (in milliseconds) between HEARTBEAT chunks.  These chunks
1657		are sent at the specified interval on idle paths to probe the state of
1658		a given path between 2 associations.
1659	
1660		Default: 30000
1661	
1662	sack_timeout - INTEGER
1663		The amount of time (in milliseconds) that the implementation will wait
1664		to send a SACK.
1665	
1666		Default: 200
1667	
1668	valid_cookie_life - INTEGER
1669		The default lifetime of the SCTP cookie (in milliseconds).  The cookie
1670		is used during association establishment.
1671	
1672		Default: 60000
1673	
1674	cookie_preserve_enable - BOOLEAN
1675		Enable or disable the ability to extend the lifetime of the SCTP cookie
1676		that is used during the establishment phase of SCTP association
1677	
1678		1: Enable cookie lifetime extension.
1679		0: Disable
1680	
1681		Default: 1
1682	
1683	cookie_hmac_alg - STRING
1684		Select the hmac algorithm used when generating the cookie value sent by
1685		a listening sctp socket to a connecting client in the INIT-ACK chunk.
1686		Valid values are:
1687		* md5
1688		* sha1
1689		* none
1690		Ability to assign md5 or sha1 as the selected alg is predicated on the
1691		configuration of those algorithms at build time (CONFIG_CRYPTO_MD5 and
1692		CONFIG_CRYPTO_SHA1).
1693	
1694		Default: Dependent on configuration.  MD5 if available, else SHA1 if
1695		available, else none.
1696	
1697	rcvbuf_policy - INTEGER
1698		Determines if the receive buffer is attributed to the socket or to
1699		association.   SCTP supports the capability to create multiple
1700		associations on a single socket.  When using this capability, it is
1701		possible that a single stalled association that's buffering a lot
1702		of data may block other associations from delivering their data by
1703		consuming all of the receive buffer space.  To work around this,
1704		the rcvbuf_policy could be set to attribute the receiver buffer space
1705		to each association instead of the socket.  This prevents the described
1706		blocking.
1707	
1708		1: rcvbuf space is per association
1709		0: rcvbuf space is per socket
1710	
1711		Default: 0
1712	
1713	sndbuf_policy - INTEGER
1714		Similar to rcvbuf_policy above, this applies to send buffer space.
1715	
1716		1: Send buffer is tracked per association
1717		0: Send buffer is tracked per socket.
1718	
1719		Default: 0
1720	
1721	sctp_mem - vector of 3 INTEGERs: min, pressure, max
1722		Number of pages allowed for queueing by all SCTP sockets.
1723	
1724		min: Below this number of pages SCTP is not bothered about its
1725		memory appetite. When amount of memory allocated by SCTP exceeds
1726		this number, SCTP starts to moderate memory usage.
1727	
1728		pressure: This value was introduced to follow format of tcp_mem.
1729	
1730		max: Number of pages allowed for queueing by all SCTP sockets.
1731	
1732		Default is calculated at boot time from amount of available memory.
1733	
1734	sctp_rmem - vector of 3 INTEGERs: min, default, max
1735		Only the first value ("min") is used, "default" and "max" are
1736		ignored.
1737	
1738		min: Minimal size of receive buffer used by SCTP socket.
1739		It is guaranteed to each SCTP socket (but not association) even
1740		under moderate memory pressure.
1741	
1742		Default: 1 page
1743	
1744	sctp_wmem  - vector of 3 INTEGERs: min, default, max
1745		Currently this tunable has no effect.
1746	
1747	addr_scope_policy - INTEGER
1748		Control IPv4 address scoping - draft-stewart-tsvwg-sctp-ipv4-00
1749	
1750		0   - Disable IPv4 address scoping
1751		1   - Enable IPv4 address scoping
1752		2   - Follow draft but allow IPv4 private addresses
1753		3   - Follow draft but allow IPv4 link local addresses
1754	
1755		Default: 1
1756	
1757	
1758	/proc/sys/net/core/*
1759		Please see: Documentation/sysctl/net.txt for descriptions of these entries.
1760	
1761	
1762	/proc/sys/net/unix/*
1763	max_dgram_qlen - INTEGER
1764		The maximum length of dgram socket receive queue
1765	
1766		Default: 10
1767	
1768	
1769	UNDOCUMENTED:
1770	
1771	/proc/sys/net/irda/*
1772		fast_poll_increase FIXME
1773		warn_noreply_time FIXME
1774		discovery_slots FIXME
1775		slot_timeout FIXME
1776		max_baud_rate FIXME
1777		discovery_timeout FIXME
1778		lap_keepalive_time FIXME
1779		max_noreply_time FIXME
1780		max_tx_data_size FIXME
1781		max_tx_window FIXME
1782		min_tx_turn_time FIXME
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