This is a reference of IP networking parameters that are configurable as described in our linux tweaking article -here-.

/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.
[email protected]

Updated by:
Andi Kleen
[email protected]

/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
[email protected]

$Id: ip-sysctl.txt,v 1.1.1.1 2002-08-19 13:34:26 blueflux Exp $