Linux Solutions

Welcome to the tutorial guide. The guide will provide a user with guidance and instructions on generic ipv4. Please note that the kernel has an internal clock which runs at HZ ticks per second. On Intel, HZ is mostly 100. So setting a *_rate file to, say 50, would allow for 2 packets per second.

/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 a user is used as a relay in a DoS attack, it may be useful.
/proc/sys/net/ipv4/icmp_echo_ignore_broadcasts [Useful]
If a user pings 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 a user can control the rate at which it is sent.
/proc/sys/net/ipv4/icmp_timeexceed_rate
This is the famous cause of the ‘Solaris middle star’ in traceroutes. Limits the rate 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? Miximum 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.
/proc/sys/net/ipv4/ip_dynaddr
A user can set this if he/she uses dial-on-demand with a dynamic interface address. If the 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 the interface 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
A user can set this if he/she wants to disable Path MTU discovery. This is a technique to determine the largest Maximum Transfer Unit possible on the path.
/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
A user can set this if he/she wants his/her applications to be able to bind to an address which doesn’t belong to a device on the system. This can be useful when a users’ machine is on a non-permanent (or even dynamic) link, so the services are able to start up and bind to a specific address when the 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 a user. 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, a user may restore it, but remember that if his/her machine is even underloaded WEB server, he/she will 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 non-responsive 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. A user should not rely on this or lower the limit artificially, but rather increase it, if network conditions require more than default value, and tune network services to linger and kill such states more aggressively.
/proc/sys/net/ipv4/tcp_orphan_retries
How may times to retry before killing TCP connection, closed by the user. Default value 7 corresponds to 50sec-16min depending on RTO. If a users machine is a loaded WEB server, a user 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 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. Please note that if P 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.
/proc/sys/net/ipv4/tcp_max_tw_buckets
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, a user_must_ not lower the limit artificially, but rather increase it (probably, after increasing installed memory), if network conditions require more than default value.
/proc/sys/net/ipv4/tcp_retrans_collapse
Bug-to-bug compatibility with some broken printers. On retransmit try to send bigger packets to work around bugs in certain TCP stacks.
/proc/sys/net/ipv4/tcp_retries1
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.
/proc/sys/net/ipv4/tcp_retries2
How may times to retry before killing alive TCP connection. RFC 1122 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.
/proc/sys/net/ipv4/tcp_rfc1337
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. Default: 0
/proc/sys/net/ipv4/tcp_sack
Use Selective ACK which can be used to signify that specific packets are missing - therefore helping fast recovery.
/proc/sys/net/ipv4/tcp_stdurg
Use the Host requirements interpretation of the TCP urg pointer field. Most hosts use the older BSD interpretation, so if a user turns this on Linux might not communicate correctly with them. Default: FALSE
/proc/sys/net/ipv4/tcp_syn_retries
Number of SYN packets the kernel will send before giving up on the new connection.
/proc/sys/net/ipv4/tcp_synack_retries
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.
/proc/sys/net/ipv4/tcp_timestamps
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’.
/proc/sys/net/ipv4/tcp_tw_recycle
Enable fast recycling TIME-WAIT sockets. Default value is 1. It should not be changed without advice/request of technical experts.
/proc/sys/net/ipv4/tcp_window_scaling
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.

If you followed this tutorial guide then you would have learnt about generic ipv4