arp — Linux ARP kernel module.
This kernel protocol module implements the Address Resolution Protocol defined in RFC 826. It is used to convert between Layer2 hardware addresses and IPv4 protocol addresses on directly connected networks. The user normally doesn't interact directly with this module except to configure it; instead it provides a service for other protocols in the kernel.
A user process can receive ARP packets by using packet(7) sockets. There is
also a mechanism for managing the ARP cache in user-space by
using netlink(7) sockets. The ARP
table can also be controlled via ioctl(2) on any
PF_INET socket.
The ARP module maintains a cache of mappings between hardware addresses and protocol addresses. The cache has a limited size so old and less frequently used entries are garbage-collected. Entries which are marked as permanent are never deleted by the garbage-collector. The cache can be directly manipulated by the use of ioctls and its behavior can be tuned by the sysctls defined below.
When there is no positive feedback for an existing mapping
after some time (see the sysctls below) a neighbor cache
entry is considered stale. Positive feedback can be gotten
from a higher layer; for example from a successful TCP ACK.
Other protocols can signal forward progress using the
MSG_CONFIRM flag to sendmsg(2). When there is
no forward progress ARP tries to reprobe. It first tries to
ask a local arp daemon app_solicit times for an
updated MAC address. If that fails and an old MAC address is
known an unicast probe is send ucast_solicit times. If that
fails too it will broadcast a new ARP request to the network.
Requests are only send when there is data queued for
sending.
Linux will automatically add a non-permanent proxy arp entry when it receives a request for an address it forwards to and proxy arp is enabled on the receiving interface. When there is a reject route for the target no proxy arp entry is added.
Three ioctls are available on all PF_INET sockets. They take a pointer to a
struct arpreq as
their parameter.
struct arpreq { struct sockaddr arp_pa;struct sockaddr arp_ha;int arp_flags;struct sockaddr arp_netmask;char arp_dev[16];};
SIOCSARP, SIOCDARP and SIOCGARP respectively set, delete and get
an ARP mapping. Setting & deleting ARP maps are
privileged operations and may only be performed by a
process with the CAP_NET_ADMIN capability or an effective
UID of 0.
arp_pa must be
an AF_INET socket and
arp_ha must have
the same type as the device which is specified in
arp_dev. arp_dev is a zero-terminated
string which names a device.
arp_flagsflag meaning ATF_COM Lookup complete ATF_PERM Permanent entry ATF_PUBL Publish entry ATF_USETRAILERS Trailers requested ATF_NETMASK Use a netmask ATF_DONTPUB Don't answer
If the ATF_NETMASK flag is
set, then arp_netmask should be valid.
Linux 2.2 does not support proxy network ARP entries, so
this should be set to 0xffffffff, or 0 to remove an
existing proxy arp entry. ATF_USETRAILERS is obsolete and should
not be used.
ARP supports a sysctl interface to configure parameters
on a global or per-interface basis. The sysctls can be
accessed by reading or writing the /proc/sys/net/ipv4/neigh/*/* files or
with the sysctl(2) interface. Each
interface in the system has its own directory in
/proc/sys/net/ipv4/neigh/. The setting in the `default'
directory is used for all newly created devices. Unless
otherwise specified time-related sysctls are specified in
seconds.
The maximum number of jiffies to delay before replying to a IPv6 neighbor solicitation message. Anycast support is not yet implemented. Defaults to 1 second.
The maximum number of probes to send to the user
space ARP daemon via netlink before dropping back to
multicast probes (see mcast_solicit).
Defaults to 0.
Once a neighbor has been found, the entry is
considered to be valid for at least a random value
between base_reachable_time/2
and 3*base_reachable_time/2.
An entry's validity will be extended if it receives
positive feedback from higher level protocols.
Defaults to 30 seconds.
Delay before first probe after it has been decided that a neighbor is stale. Defaults to 5 seconds.
How frequently the garbage collector for neighbor entries should attempt to run. Defaults to 30 seconds.
Determines how often to check for stale neighbor entries. When a neighbor entry is considered stale it is resolved again before sending data to it. Defaults to 60 seconds.
The minimum number of entries to keep in the ARP cache. The garbage collector will not run if there are fewer than this number of entries in the cache. Defaults to 128.
The soft maximum number of entries to keep in the ARP cache. The garbage collector will allow the number of entries to exceed this for 5 seconds before collection will be performed. Defaults to 512.
The hard maximum number of entries to keep in the ARP cache. The garbage collector will always run if there are more than this number of entries in the cache. Defaults to 1024.
The minimum number of jiffies to keep an ARP entry in the cache. This prevents ARP cache thrashing if there is more than one potential mapping (generally due to network misconfiguration). Defaults to 1 second.
The maximum number of attempts to resolve an address by multicast/broadcast before marking the entry as unreachable. Defaults to 3.
When an ARP request for a known proxy-ARP address
is received, delay up to proxy_delay jiffies
before replying. This is used to prevent network
flooding in some cases. Defaults to 0.8 seconds.
The maximum number of packets which may be queued to proxy-ARP addresses. Defaults to 64.
The number of jiffies to delay before retransmitting a request. Defaults to 1 second.
The maximum number of attempts to send unicast
probes before asking the ARP daemon (see app_solicit).
Defaults to 3.
The maximum number of packets which may be queued for each unresolved address by other network layers. Defaults to 3.
The struct arpreq
changed in Linux 2.0 to include the arp_dev member and the ioctl
numbers changed at the same time. Support for the old ioctls
was dropped in Linux 2.2.
Support for proxy arp entries for networks (netmask not equal 0xffffffff) was dropped in Linux 2.2. It is replaced by automatic proxy arp setup by the kernel for all reachable hosts on other interfaces (when forwarding and proxy arp is enabled for the interface).
The neigh/*
sysctls did not exist before Linux 2.2.
Some timer settings are specified in jiffies, which is architecture- and kernel version-dependent; see time(7).
There is no way to signal positive feedback from user space. This means connection oriented protocols implemented in user space will generate excessive ARP traffic, because ndisc will regularly reprobe the MAC address. The same problem applies for some kernel protocols (e.g., NFS over UDP).
This man page mashes IPv4 specific and shared between IPv4 and IPv6 functionality together.
RFC 826 for a description of ARP.
RFC 2461 for a description of IPv6 neighbor discovery and the base algorithms used.
Linux 2.2+ IPv4 ARP uses the IPv6 algorithms when applicable.
This page is part of release 2.79 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
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t This man page is Copyright (C) 1999 Matthew Wilcox <willybofh.ai>. Permission is granted to distribute possibly modified copies of this page provided the header is included verbatim, and in case of nontrivial modification author and date of the modification is added to the header. Modified June 1999 Andi Kleen $Id: arp.7,v 1.10 2000/04/27 19:31:38 ak Exp $ |