ip-sysctl.txt

ARM 嵌入式 系统 设计与实例开发 实验教材 二源码

/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