/proc/sys/net/ipv4/icmp_destunreach_rate
If the kernel decides that it can’t deliver a packet, it will drop it, and send the source of the packet an ICMP notice to this effect.
/proc/sys/net/ipv4/icmp_echo_ignore_all
Don’t act on echo packets at all. Please don’t set this by default, but if you are used as a relay in a DoS attack, it may be useful.
/proc/sys/net/ipv4/icmp_echo_ignore_broadcasts [Useful]
If you ping the broadcast address of a network, all hosts are supposed to respond. This makes for a dandy denial-of-service tool. Set this to 1 to ignore these broadcast messages.
/proc/sys/net/ipv4/icmp_echoreply_rate
The rate at which echo replies are sent to any one destination.
/proc/sys/net/ipv4/icmp_ignore_bogus_error_responses
Set this to ignore ICMP errors caused by hosts in the network reacting badly to frames sent to what they perceive to be the broadcast address.
/proc/sys/net/ipv4/icmp_paramprob_rate
A relatively unknown ICMP message, which is sent in response to incorrect packets with broken IP or TCP headers. With this file you can control the rate at which it is sent.
/proc/sys/net/ipv4/icmp_timeexceed_rate
This the famous cause of the ’Solaris middle star’ in traceroutes. Limits number of ICMP Time Exceeded messages sent.
/proc/sys/net/ipv4/igmp_max_memberships
Maximum number of listening igmp (multicast) sockets on the host. FIXME: Is this true?
/proc/sys/net/ipv4/inet_peer_gc_maxtime
FIXME: Add a little explanation about the inet peer storage?
Minimum interval between garbage collection passes. This interval is in effect under low (or absent) memory pressure on the pool. Measured in jiffies.
/proc/sys/net/ipv4/inet_peer_gc_mintime
Minimum interval between garbage collection passes. This interval is in effect under high memory pressure on the pool. Measured in jiffies.
/proc/sys/net/ipv4/inet_peer_maxttl
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.
/proc/sys/net/ipv4/inet_peer_minttl
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.
/proc/sys/net/ipv4/inet_peer_threshold
The approximate size of the INET peer 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.
/proc/sys/net/ipv4/ip_autoconfig
This file contains the number one if the host received its IP configuration by RARP, BOOTP, DHCP or a similar mechanism. Otherwise it is zero.
/proc/sys/net/ipv4/ip_default_ttl
Time To Live of packets. Set to a safe 64. Raise it if you have a huge network. Don’t do so for fun - routing loops cause much more damage that way. You might even consider lowering it in some circumstances.
/proc/sys/net/ipv4/ip_dynaddr
You need to set this if you use dial-on-demand with a dynamic interface address. Once your demand interface comes up, any local TCP sockets which haven’t seen replies will be rebound to have the right address. This solves the problem that the connection that brings up your interface itself does not work, but the second try does.
/proc/sys/net/ipv4/ip_forward
If the kernel should attempt to forward packets. Off by default.
/proc/sys/net/ipv4/ip_local_port_range
Range of local ports for outgoing connections. Actually quite small by default, 1024 to 4999.
/proc/sys/net/ipv4/ip_no_pmtu_disc
Set this if you want to disable Path MTU discovery - a technique to determine the largest Maximum Transfer Unit possible on your path. See also the section on Path MTU discovery in the cookbook chapter.
/proc/sys/net/ipv4/ipfrag_high_thresh
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.
/proc/sys/net/ipv4/ip_nonlocal_bind
Set this if you want your applications to be able to bind to an address which doesn’t belong to a device on your system. This can be useful when your machine is on a non-permanent (or even dynamic) link, so your services are able to start up and bind to a specific address when your link is down.
/proc/sys/net/ipv4/ipfrag_low_thresh
Minimum memory used to reassemble IP fragments.
/proc/sys/net/ipv4/ipfrag_time
Time in seconds to keep an IP fragment in memory.
/proc/sys/net/ipv4/tcp_abort_on_overflow
A boolean flag controlling the behaviour under lots of incoming connections. When enabled, this causes the kernel to actively send RST packets when a service is overloaded.
/proc/sys/net/ipv4/tcp_fin_timeout
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.
/proc/sys/net/ipv4/tcp_keepalive_time
How often TCP sends out keepalive messages when keepalive is enabled.
Default: 2hours.
/proc/sys/net/ipv4/tcp_keepalive_intvl
How frequent probes are retransmitted, when a probe isn’t acknowledged.
Default: 75 seconds.
/proc/sys/net/ipv4/tcp_keepalive_probes
How many keepalive probes TCP will send, until it decides that the connection is broken.
Default value: 9.
Multiplied with tcp_keepalive_intvl, this gives the time a link can be nonresponsive after a keepalive has been sent.
/proc/sys/net/ipv4/tcp_max_orphans
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 remind you again: each orphan eats up to 64K of unswappable memory.
/proc/sys/net/ipv4/tcp_orphan_retries
How may times to retry before killing TCP connection, closed by our side. Default value 7 corresponds to 50sec-16min depending on RTO. If your machine is a loaded WEB server, you should think about lowering this value, such sockets may consume significant resources. Cf. tcp_max_orphans.
/proc/sys/net/ipv4/tcp_max_syn_backlog
Maximal number of remembered connection requests, which 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
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