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arp(7) - phpMan

ARP(7) Linux Programmer's Manual ARP(7)

NAME
arp - Linux ARP kernel module.

DESCRIPTION
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 mecha‐
nism 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 AF_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 /proc interfaces described below.

When there is no positive feedback for an existing mapping after some time (see the /proc
interfaces 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 for‐
ward 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, a uni‐
cast probe is sent ucast_solicit times. If that fails too, it will broadcast a new ARP
request to the network. Requests are sent only when there is data queued for sending.

Linux will automatically add a nonpermanent 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.

Ioctls
Three ioctls are available on all AF_INET sockets. They take a pointer to a struct arpreq
as their argument.

struct arpreq {
struct sockaddr arp_pa; /* protocol address */
struct sockaddr arp_ha; /* hardware address */
int arp_flags; /* flags */
struct sockaddr arp_netmask; /* netmask of protocol address */
char arp_dev[16];
};

SIOCSARP, SIOCDARP and SIOCGARP respectively set, delete and get an ARP mapping. Setting
and deleting ARP maps are privileged operations and may be performed only by a process
with the CAP_NET_ADMIN capability or an effective UID of 0.

arp_pa must be an AF_INET address 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_flags │
├────────────────┬────────────────────┤
│flag │ 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 sup‐
port 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.

/proc interfaces
ARP supports a range of /proc interfaces to configure parameters on a global or per-inter‐
face basis. The interfaces can be accessed by reading or writing the
/proc/sys/net/ipv4/neigh/*/* files. 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 interfaces are specified in
seconds.

anycast_delay (since Linux 2.2)
The maximum number of jiffies to delay before replying to a IPv6 neighbor solicita‐
tion message. Anycast support is not yet implemented. Defaults to 1 second.

app_solicit (since Linux 2.2)
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.

base_reachable_time (since Linux 2.2)
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 proto‐
cols. Defaults to 30 seconds. This file is now obsolete in favor of base_reach‐
able_time_ms.

base_reachable_time_ms (since Linux 2.6.12)
As for base_reachable_time, but measures time in milliseconds. Defaults to 30000
milliseconds.

delay_first_probe_time (since Linux 2.2)
Delay before first probe after it has been decided that a neighbor is stale.
Defaults to 5 seconds.

gc_interval (since Linux 2.2)
How frequently the garbage collector for neighbor entries should attempt to run.
Defaults to 30 seconds.

gc_stale_time (since Linux 2.2)
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.

gc_thresh1 (since Linux 2.2)
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.

gc_thresh2 (since Linux 2.2)
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.

gc_thresh3 (since Linux 2.2)
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.

locktime (since Linux 2.2)
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 net‐
work misconfiguration). Defaults to 1 second.

mcast_solicit (since Linux 2.2)
The maximum number of attempts to resolve an address by multicast/broadcast before
marking the entry as unreachable. Defaults to 3.

proxy_delay (since Linux 2.2)
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.

proxy_qlen (since Linux 2.2)
The maximum number of packets which may be queued to proxy-ARP addresses. Defaults
to 64.

retrans_time (since Linux 2.2)
The number of jiffies to delay before retransmitting a request. Defaults to 1 sec‐
ond. This file is now obsolete in favor of retrans_time_ms.

retrans_time_ms (since Linux 2.6.12)
The number of milliseconds to delay before retransmitting a request. Defaults to
1000 milliseconds.

ucast_solicit (since Linux 2.2)
The maximum number of attempts to send unicast probes before asking the ARP daemon
(see app_solicit). Defaults to 3.

unres_qlen (since Linux 2.2)
The maximum number of packets which may be queued for each unresolved address by
other network layers. Defaults to 3.

VERSIONS
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/* interfaces did not exist before Linux 2.2.

BUGS
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-ori‐
ented 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 together functionality that is IPv4-specific with functionality that
is shared between IPv4 and IPv6.