/proc/sys/net/ipv4/* Variables:

ip_forward - BOOLEAN

	0 - disabled (default)

	not 0 - enabled 

	Forward Packets between interfaces.

	This variable is special, its change resets all configuration

	parameters to their default state (RFC1122 for hosts, RFC1812

	for routers)

ip_default_ttl - INTEGER

	default 64

ip_no_pmtu_disc - BOOLEAN

	Disable Path MTU Discovery.

	default FALSE

IP Fragmentation:

ipfrag_high_thresh - INTEGER

	Maximum memory used to reassemble IP fragments. When 

	ipfrag_high_thresh bytes of memory is allocated for this purpose,

	the fragment handler will toss packets until ipfrag_low_thresh

	is reached.

ipfrag_low_thresh - INTEGER

	See ipfrag_high_thresh	

ipfrag_time - INTEGER

	Time in seconds to keep an IP fragment in memory.	

INET peer storage:

inet_peer_threshold - INTEGER

	The approximate size of the storage.  Starting from this threshold	

	entries will be thrown aggressively.  This threshold also determines

	entries' time-to-live and time intervals between garbage collection

	passes.  More entries, less time-to-live, less GC interval.

inet_peer_minttl - INTEGER

	Minimum time-to-live of entries.  Should be enough to cover fragment

	time-to-live on the reassembling side.  This minimum time-to-live  is

	guaranteed if the pool size is less than inet_peer_threshold.

	Measured in jiffies.

inet_peer_maxttl - INTEGER

	Maximum time-to-live of entries.  Unused entries will expire after

	this period of time if there is no memory pressure on the pool (i.e.

	when the number of entries in the pool is very small).

	Measured in jiffies.

inet_peer_gc_mintime - INTEGER

	Minimum interval between garbage collection passes.  This interval is

	in effect under high memory pressure on the pool.

	Measured in jiffies.

inet_peer_gc_maxtime - INTEGER

	Minimum interval between garbage collection passes.  This interval is

	in effect under low (or absent) memory pressure on the pool.

	Measured in jiffies.

TCP variables: 

tcp_syn_retries - INTEGER

	Number of times initial SYNs for an active TCP connection attempt

	will be retransmitted. Should not be higher than 255. Default value

	is 5, which corresponds to ~180seconds.

tcp_synack_retries - INTEGER

	Number of times SYNACKs for a passive TCP connection attempt will

	be retransmitted. Should not be higher than 255. Default value

	is 5, which corresponds to ~180seconds.

tcp_keepalive_time - INTEGER

	How often TCP sends out keepalive messages when keepalive is enabled.

	Default: 2hours.

tcp_keepalive_probes - INTEGER

	How many keepalive probes TCP sends out, until it decides that the

	connection is broken. Default value: 9.

tcp_keepalive_interval - INTEGER

	How frequently the probes are send out. Multiplied by

	tcp_keepalive_probes it is time to kill not responding connection,

	after probes started. Default value: 75sec i.e. connection

	will be aborted after ~11 minutes of retries.

tcp_retries1 - INTEGER

	How many times to retry before deciding that something is wrong

	and it is necessary to report this suspection to network layer.

	Minimal RFC value is 3, it is default, which corresponds

	to ~3sec-8min depending on RTO.

tcp_retries2 - INTEGER

	How may times to retry before killing alive TCP connection.

	RFC1122 says that the limit should be longer than 100 sec.

	It is too small number.	Default value 15 corresponds to ~13-30min

	depending on RTO.

tcp_orphan_retries - INTEGER

	How may times to retry before killing TCP connection, closed

	by our side. Default value 7 corresponds to ~50sec-16min

	depending on RTO. If you machine is loaded WEB server,

	you should think about lowering this value, such sockets

	may consume significant resources. Cf. tcp_max_orphans.

tcp_fin_timeout - INTEGER

	Time to hold socket in state FIN-WAIT-2, if it was closed

	by our side. Peer can be broken and never close its side,

	or even died unexpectedly. Default value is 60sec.

	Usual value used in 2.2 was 180 seconds, you may restore

	it, but remember that if your machine is even underloaded WEB server,

	you risk to overflow memory with kilotons of dead sockets,

	FIN-WAIT-2 sockets are less dangerous than FIN-WAIT-1,

	because they eat maximum 1.5K of memory, but they tend

	to live longer.	Cf. tcp_max_orphans.

tcp_max_tw_buckets - INTEGER

	Maximal number of timewait sockets held by system simultaneously.

	If this number is exceeded time-wait socket is immediately destroyed

	and warning is printed. This limit exists only to prevent

	simple DoS attacks, you _must_ not lower the limit artificially,

	but rather increase it (probably, after increasing installed memory),

	if network conditions require more than default value.

tcp_tw_recycle - BOOLEAN

	Enable fast recycling TIME-WAIT sockets. Default value is 1.

	It should not be changed without advice/request of technical

	experts.

tcp_max_orphans - INTEGER

	Maximal number of TCP sockets not attached to any user file handle,

	held by system.	If this number is exceeded orphaned connections are

	reset immediately and warning is printed. This limit exists

	only to prevent simple DoS attacks, you _must_ not rely on this

	or lower the limit artificially, but rather increase it

	(probably, after increasing installed memory),

	if network conditions require more than default value,

	and tune network services to linger and kill such states

	more aggressively. Let me to remind again: each orphan eats

	up to ~64K of unswappable memory.

tcp_abort_on_overflow - BOOLEAN

	If listening service is too slow to accept new connections,

	reset them. Default state is FALSE. It means that if overflow

	occurred due to a burst, connection will recover. Enable this

	option _only_ if you are really sure that listening daemon

	cannot be tuned to accept connections faster. Enabling this

	option can harm clients of your server.

tcp_syncookies - BOOLEAN

	Only valid when the kernel was compiled with CONFIG_SYNCOOKIES

	Send out syncookies when the syn backlog queue of a socket 

	overflows. This is to prevent against the common 'syn flood attack'

	Default: FALSE

	Note, that syncookies is fallback facility.

	It MUST NOT be used to help highly loaded servers to stand

	against legal connection rate. If you see synflood warnings

	in your logs, but investigation	shows that they occur

	because of overload with legal connections, you should tune

	another parameters until this warning disappear.

	See: tcp_max_syn_backlog, tcp_synack_retries, tcp_abort_on_overflow.

	syncookies seriously violate TCP protocol, do not allow

	to use TCP extensions, can result in serious degradation

	of some services (f.e. SMTP relaying), visible not by you,

	but your clients and relays, contacting you. While you see

	synflood warnings in logs not being really flooded, your server

	is seriously misconfigured.

tcp_stdurg - BOOLEAN

	Use the Host requirements interpretation of the TCP urg pointer field.

	Most hosts use the older BSD interpretation, so if you turn this on

	Linux might not communicate correctly with them.	

	Default: FALSE 

tcp_max_syn_backlog - INTEGER

	Maximal number of remembered connection requests, which are

	still did not receive an acknowledgement from connecting client.

	Default value is 1024 for systems with more than 128Mb of memory,

	and 128 for low memory machines. If server suffers of overload,

	try to increase this number. Warning! If you make it greater

	than 1024, it would be better to change TCP_SYNQ_HSIZE in

	include/net/tcp.h to keep TCP_SYNQ_HSIZE*16<=tcp_max_syn_backlog

	and to recompile kernel.

tcp_window_scaling - BOOLEAN

	Enable window scaling as defined in RFC1323.

tcp_timestamps - BOOLEAN

	Enable timestamps as defined in RFC1323.

tcp_sack - BOOLEAN

	Enable select acknowledgments (SACKS).

tcp_fack - BOOLEAN

	Enable FACK congestion avoidance and fast restransmission.

	The value is not used, if tcp_sack is not enabled.

tcp_dsack - BOOLEAN

	Allows TCP to send "duplicate" SACKs.

tcp_ecn - BOOLEAN

	Enable Explicit Congestion Notification in TCP.

tcp_reordering - INTEGER

	Maximal reordering of packets in a TCP stream.

	Default: 3	

tcp_retrans_collapse - BOOLEAN

	Bug-to-bug compatibility with some broken printers.

	On retransmit try to send bigger packets to work around bugs in

	certain TCP stacks.

tcp_wmem - vector of 3 INTEGERs: min, default, max

	min: Amount of memory reserved for send buffers for TCP socket.

	Each TCP socket has rights to use it due to fact of its birth.

	Default: 4K

	default: Amount of memory allowed for send buffers for TCP socket

	by default. This value overrides net.core.wmem_default used

	by other protocols, it is usually lower than net.core.wmem_default.

	Default: 16K

	max: Maximal amount of memory allowed for automatically selected

	send buffers for TCP socket. This value does not override

	net.core.wmem_max, "static" selection via SO_SNDBUF does not use this.

	Default: 128K

tcp_rmem - vector of 3 INTEGERs: min, default, max

	min: Minimal size of receive buffer used by TCP sockets.

	It is guaranteed to each TCP socket, even under moderate memory

	pressure.

	Default: 8K

	default: default size of receive buffer used by TCP sockets.

	This value overrides net.core.rmem_default used by other protocols.

	Default: 87380 bytes. This value results in window of 65535 with

	default setting of tcp_adv_win_scale and tcp_app_win:0 and a bit

	less for default tcp_app_win. See below about these variables.

	max: maximal size of receive buffer allowed for automatically

	selected receiver buffers for TCP socket. This value does not override

	net.core.rmem_max, "static" selection via SO_RCVBUF does not use this.

	Default: 87380*2 bytes.

tcp_mem - vector of 3 INTEGERs: min, pressure, max

	low: below this number of pages TCP is not bothered about its

	memory appetite.

	pressure: when amount of memory allocated by TCP exceeds this number

	of pages, TCP moderates its memory consumption and enters memory

	pressure mode, which is exited when memory consumtion falls

	under "low".

	high: number of pages allowed for queueing by all TCP sockets.

	Defaults are calculated at boot time from amount of available

	memory.

tcp_app_win - INTEGER

	Reserve max(window/2^tcp_app_win, mss) of window for application

	buffer. Value 0 is special, it means that nothing is reserved.

	Default: 31

tcp_adv_win_scale - INTEGER

	Count buffering overhead as bytes/2^tcp_adv_win_scale

	(if tcp_adv_win_scale > 0) or bytes-bytes/2^(-tcp_adv_win_scale),

	if it is <= 0.

	Default: 2

tcp_rfc1337 - BOOLEAN

	If set, the TCP stack behaves conforming to RFC1337. If unset,

	we are not conforming to RFC, but prevent TCP TIME_WAIT

	asassination.	

	Default: 0

ip_local_port_range - 2 INTEGERS

	Defines the local port range that is used by TCP and UDP to

	choose the local port. The first number is the first, the 

	second the last local port number. Default value depends on

	amount of memory available on the system:

	> 128Mb 32768-61000

	< 128Mb 1024-4999 or even less.

	This number defines number of active connections, which this

	system can issue simultaneously to systems not supporting

	TCP extensions (timestamps). With tcp_tw_recycle enabled

	(i.e. by default) range 1024-4999 is enough to issue up to

	2000 connections per second to systems supporting timestamps.

ip_nonlocal_bind - BOOLEAN

	If set, allows processes to bind() to non-local IP adresses,

	which can be quite useful - but may break some applications.

	Default: 0

ip_dynaddr - BOOLEAN

	If set non-zero, enables support for dynamic addresses.

	If set to a non-zero value larger than 1, a kernel log

	message will be printed when dynamic address rewriting

	occurs.

	Default: 0

icmp_echo_ignore_all - BOOLEAN

icmp_echo_ignore_broadcasts - BOOLEAN

	If either is set to true, then the kernel will ignore either all

	ICMP ECHO requests sent to it or just those to broadcast/multicast

	addresses, respectively.

icmp_destunreach_rate - INTEGER

icmp_paramprob_rate - INTEGER

icmp_timeexceed_rate - INTEGER

icmp_echoreply_rate - INTEGER (not enabled per default)

	Limit the maximal rates for sending ICMP packets to specific targets.

	0 to disable any limiting, otherwise the maximal rate in jiffies(1)

	See the source for more information.

icmp_ignore_bogus_error_responses - BOOLEAN

	Some routers violate RFC 1122 by sending bogus responses to broadcast

	frames.  Such violations are normally logged via a kernel warning.

	If this is set to TRUE, the kernel will not give such warnings, which

	will avoid log file clutter.

	Default: FALSE

(1) Jiffie: internal timeunit for the kernel. On the i386 1/100s, on the

Alpha 1/1024s. See the HZ define in /usr/include/asm/param.h for the exact

value on your system. 

igmp_max_memberships - INTEGER

	Change the maximum number of multicast groups we can subscribe to.

	Default: 20

conf/interface/*: 

conf/all/* is special and changes the settings for all interfaces.

	Change special settings per interface.

log_martians - BOOLEAN

	Log packets with impossible addresses to kernel log.

accept_redirects - BOOLEAN

	Accept ICMP redirect messages.

	default TRUE (host)

		FALSE (router)

forwarding - BOOLEAN

	Enable IP forwarding on this interface.

mc_forwarding - BOOLEAN

	Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE

	and a multicast routing daemon is required.

proxy_arp - BOOLEAN

	Do proxy arp.

shared_media - BOOLEAN

	Send(router) or accept(host) RFC1620 shared media redirects.

	Overrides ip_secure_redirects.

	default TRUE

secure_redirects - BOOLEAN

	Accept ICMP redirect messages only for gateways,

	listed in default gateway list.

	default TRUE

send_redirects - BOOLEAN

	Send redirects, if router. Default: TRUE

bootp_relay - BOOLEAN

	Accept packets with source address 0.b.c.d destined

	not to this host as local ones. It is supposed, that

	BOOTP relay daemon will catch and forward such packets.

	default FALSE

	Not Implemented Yet.

accept_source_route - BOOLEAN

	Accept packets with SRR option.

	default TRUE (router)

		FALSE (host)

rp_filter - BOOLEAN

	1 - do source validation by reversed path, as specified in RFC1812

	    Recommended option for single homed hosts and stub network

	    routers. Could cause troubles for complicated (not loop free)

	    networks running a slow unreliable protocol (sort of RIP),

	    or using static routes.

	0 - No source validation. 

	Default value is 0. Note that some distributions enable it

	in startip scripts.

Alexey Kuznetsov.

kuznet@ms2.inr.ac.ru

Updated by:

Andi Kleen

ak@muc.de

/proc/sys/net/ipv6/* Variables:

IPv6 has no global variables such as tcp_*.  tcp_* settings under ipv4/ also

apply to IPv6 [XXX?].

conf/default/*:

	Change the interface-specific default settings.

conf/all/*:

	Change all the interface-specific settings.  

	[XXX:  Other special features than forwarding?]

conf/all/forwarding - BOOLEAN

	Enable global IPv6 forwarding between all interfaces.  

	IPv4 and IPv6 work differently here; e.g. netfilter must be used 

	to control which interfaces may forward packets and which not.

	This also sets all interfaces' Host/Router setting 

	'forwarding' to the specified value.  See below for details.

	This referred to as global forwarding.

conf/interface/*:

	Change special settings per interface.

	The functional behaviour for certain settings is different 

	depending on whether local forwarding is enabled or not.

accept_ra - BOOLEAN

	Accept Router Advertisements; autoconfigure using them.

	Functional default: enabled if local forwarding is disabled.

			    disabled if local forwarding is enabled.

accept_redirects - BOOLEAN

	Accept Redirects.

	Functional default: enabled if local forwarding is disabled.

			    disabled if local forwarding is enabled.

autoconf - BOOLEAN

	Configure link-local addresses using L2 hardware addresses.

	Default: TRUE

dad_transmits - INTEGER

	The amount of Duplicate Address Detection probes to send.

	Default: 1

forwarding - BOOLEAN

	Configure interface-specific Host/Router behaviour.  

	Note: It is recommended to have the same setting on all 

	interfaces; mixed router/host scenarios are rather uncommon.

	FALSE:

	By default, Host behaviour is assumed.  This means:

	1. IsRouter flag is not set in Neighbour Advertisements.

	2. Router Solicitations are being sent when necessary.

	3. If accept_ra is TRUE (default), accept Router 

	   Advertisements (and do autoconfiguration).

	4. If accept_redirects is TRUE (default), accept Redirects.

	TRUE:

	If local forwarding is enabled, Router behaviour is assumed. 

	This means exactly the reverse from the above:

	1. IsRouter flag is set in Neighbour Advertisements.

	2. Router Solicitations are not sent.

	3. Router Advertisements are ignored.

	4. Redirects are ignored.

	Default: FALSE if global forwarding is disabled (default),

		 otherwise TRUE.

hop_limit - INTEGER

	Default Hop Limit to set.

	Default: 64

mtu - INTEGER

	Default Maximum Transfer Unit

	Default: 1280 (IPv6 required minimum)

router_solicitation_delay - INTEGER

	Number of seconds to wait after interface is brought up

	before sending Router Solicitations.

	Default: 1

router_solicitation_interval - INTEGER

	Number of seconds to wait between Router Solicitations.

	Default: 4

router_solicitations - INTEGER

	Number of Router Solicitations to send until assuming no 

	routers are present.

	Default: 3

IPv6 Update by:

Pekka Savola

pekkas@netcore.fi

$Id: ip-sysctl.txt,v 1.5 2002/08/19 13:34:26 blueflux Exp $