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 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 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, 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 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 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 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 solicitation 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 protocols. Defaults to 30
seconds. This file is now obsolete in favor 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
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 network
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 second. 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.
