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
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
behaviour 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 neighbour cache entry is considered
stale. Positive feedback can be gotten from a higher layer; for exam-
ple 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 sockaddr arp_pa; /* protocol address */
struct sockaddr arp_ha; /* hardware address */
int arp_flags; /* flags */
struct sockaddr arp_netmask; /* netmask of protocol address */
SIOCSARP, SIOCDARP and SIOCGARP respectively set, delete and get an ARP
mapping. Setting & deleting ARP maps are privileged operations and may
| 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 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
neighbour solicitation message. Anycast support is not yet
implemented. Defaults to 1 second.
The maximum number of probes to send to the user space ARP dae-
mon via netlink before dropping back to multicast probes (see
mcast_solicit). Defaults to 0.
Once a neighbour 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 pro-
tocols. Defaults to 30 seconds.
Delay before first probe after it has been decided that a neigh-
bour is stale. Defaults to 5 seconds.
How frequently the garbage collector for neighbour entries
should attempt to run. Defaults to 30 seconds.
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 multi-
cast/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 sec-
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 unre-
solved address by other network layers. Defaults to 3.
Some timer settings are specified in jiffies, which is architecture
related. On the Alpha a jiffy is 1/1024 of a second, on most other
architectures it is 1/100s.
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
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.
RFC826 for a description of ARP.
RFC2461 for a description of IPv6 neighbour discovery and the base
Linux 2.2+ IPv4 ARP uses the IPv6 algorithms when applicable.
Linux Man Page 1999-06-03 arp(7)