/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 somethig 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 aggressivley. 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
	occured 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 acknowldgement 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.

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.

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.

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. 

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
$Id: ip-sysctl.txt,v 1.13 2000/01/18 08:24:09 davem Exp $
