arp(7) Miscellaneous Information 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 ad-
dresses 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 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 con-
sidered stale. Positive feedback can be gotten from a higher layer;
for example from a successful TCP ACK. Other protocols can signal for-
ward 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 lo-
cal arp daemon app_solicit times for an updated MAC address. If that
fails and an old MAC address is known, a unicast 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 re-
ceives 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 tar-
get, 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 capabil-
ity 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 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.
/proc interfaces
ARP supports a range of /proc interfaces to configure parameters on a
global or per-interface basis. The interfaces can be accessed by read-
ing 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 de-
vices. Unless otherwise specified, time-related interfaces are speci-
fied in seconds.
anycast_delay (since Linux 2.2)
The maximum number of jiffies to delay before replying to a IPv6
neighbor solicitation message. Anycast support is not yet im-
plemented. Defaults to 1 second.
app_solicit (since Linux 2.2)
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 (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 ex-
tended if it receives positive feedback from higher level proto-
cols. Defaults to 30 seconds. This file is now obsolete in fa-
vor of base_reachable_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 neigh-
bor 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 be-
fore 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 num-
ber 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. De-
faults 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 po-
tential mapping (generally due to network misconfiguration).
Defaults to 1 second.
mcast_solicit (since Linux 2.2)
The maximum number of attempts to resolve an address by multi-
cast/broadcast before marking the entry as unreachable. De-
faults 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 sec-
onds.
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 second. This file is now obsolete in favor of re-
trans_time_ms.
retrans_time_ms (since Linux 2.6.12)
The number of milliseconds to delay before retransmitting a re-
quest. 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 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/* 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-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 together functionality that is IPv4-specific with
functionality that is shared between IPv4 and IPv6.
SEE ALSO
capabilities(7), ip(7), arpd(8)
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.
Linux man-pages 6.7 2023-10-31 arp(7)
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