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.
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.
The rate at which echo replies are sent to any one destination.
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.
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.
This is the famous cause of the 'Solaris middle star' in traceroutes. Limits
the rate of ICMP Time Exceeded messages sent.
Maximum number of listening igmp (multicast) sockets on the host.
FIXME: Is this true?
FIXME: Add a little explanation about the inet peer storage?
Miximum interval between garbage collection passes. This interval is in
effect under low (or absent) memory pressure on the pool. Measured in
Minimum interval between garbage collection passes. This interval is in
effect under high memory pressure on the pool. Measured in jiffies.
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.
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.
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.
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.
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.
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.
If the kernel should attempt to forward packets. Off by default.
Range of local ports for outgoing connections. Actually quite small by
default, 1024 to 4999.
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
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
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.
Minimum memory used to reassemble IP fragments.
Time in seconds to keep an IP fragment in memory.
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.
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.
How often TCP sends out keepalive messages when keepalive is enabled.
How frequent probes are retransmitted, when a probe isn't acknowledged.
Default: 75 seconds.
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
non-responsive after a keepalive has been sent.
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.
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.
Maximal number of remembered connection requests, which still did not
receive an acknowledgment 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
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
Bug-to-bug compatibility with some broken printers.
On retransmit try to send bigger packets to work around bugs in
certain TCP stacks.
How many times to retry before deciding that something is wrong
and it is necessary to report this suspicion to network layer.
Minimal RFC value is 3, it is default, which corresponds
to 3sec-8min depending on RTO.
How may times to retry before killing alive TCP connection.
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.
This boolean enables a fix for 'time-wait assassination hazards in tcp', described
in RFC 1337. If enabled, this causes the kernel to drop RST packets for
sockets in the time-wait state.
Use Selective ACK which can be used to signify that specific packets are
missing - therefore helping fast recovery.
Use the Host requirements interpretation of the TCP urg pointer
Most hosts use the older BSD interpretation, so if you turn this on
Linux might not communicate correctly with them.
Number of SYN packets the kernel will send before giving up on the new
To open the other side of the connection, the kernel sends a SYN with a
piggybacked ACK on it, to acknowledge the earlier received SYN. This is part
2 of the threeway handshake. This setting determines the number of SYN+ACK
packets sent before the kernel gives up on the connection.
Timestamps are used, amongst other things, to protect against wrapping
sequence numbers. A 1 gigabit link might conceivably re-encounter a previous
sequence number with an out-of-line value, because it was of a previous
generation. The timestamp will let it recognize this 'ancient packet'.
Enable fast recycling TIME-WAIT sockets. Default value is 1.
It should not be changed without advice/request of technical experts.
TCP/IP normally allows windows up to 65535 bytes big. For really fast
networks, this may not be enough. The window scaling options allows for
almost gigabyte windows, which is good for high bandwidth*delay products.