ARP(7) Linux Programmers 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 doesnt 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 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.
Ioctls
Three ioctls are available on all PF_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 & 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_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 | Dont 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.
Sysctls
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.
anycast_delay
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.
app_solicit
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.
base_reachable_time
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 entrys validity will be
extended if it receives positive feedback from higher level pro
tocols. Defaults to 30 seconds.
delay_first_probe_time
Delay before first probe after it has been decided that a neigh
bor is stale. Defaults to 5 seconds.
gc_interval
How frequently the garbage collector for neighbor entries should
attempt to run. Defaults to 30 seconds.
gc_stale_time
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
The minimum number of entries to keep in the ARP cache. The
garbage collector will not run if there are fewer than this num
ber of entries in the cache. Defaults to 128.
gc_thresh2
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
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
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.
mcast_solicit
The maximum number of attempts to resolve an address by multi
cast/broadcast before marking the entry as unreachable.
Defaults to 3.
proxy_delay
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
onds.
proxy_qlen
The maximum number of packets which may be queued to proxy-ARP
addresses. Defaults to 64.
retrans_time
The number of jiffies to delay before retransmitting a request.
Defaults to 1 second.
ucast_solicit
The maximum number of attempts to send unicast probes before
asking the ARP daemon (see app_solicit). Defaults to 3.
unres_qlen
The maximum number of packets which may be queued for each unre
solved 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 inter
faces (when forwarding and proxy arp is enabled for the interface).
The neigh/* sysctls 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 oriented protocols implemented in user space will gen
erate 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.
SEE ALSO
capabilities(7), ip(7)
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.
COLOPHON
This page is part of release 3.05 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/.
Linux 2007-07-27 ARP(7)
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