NAME | SYNOPSIS | DESCRIPTION | DIAGNOSTICS | ATTRIBUTES | SEE ALSO
pseudo-device ether
The Address Resolution Protocol (ARP) is a protocol used to map dynamically between Internet host addresses and Ethernet addresses. It is used by all the 10Mb/s Ethernet interface drivers. It is not specific to Internet protocols or to 10Mb/s Ethernet, but the current implementation only supports that combination.
ARP caches Internet-Ethernet address mappings. When an interface requests a mapping for an address not in the cache, ARP queues the message which requires the mapping and broadcasts a message on the associated network requesting the address mapping. If a response is provided, the new mapping is cached and any pending message is transmitted. ARP will queue at most one packet while waiting for a response to a mapping request; only the most recently "transmitted" packet is kept. If the target host does not respond after several requests, the host is considered to be down for a short period (normally 20 seconds), allowing an error to be returned to transmission attempts during this interval. The error is EHOSTDOWN for a non-responding destination host, and EHOSTUNREACH for a non-responding router.
The ARP cache is stored in the system routing table
as dynamically-created host routes. The route to a directly-attached Ethernet
network is installed as a cloning route (one with
the RTF_CLONING
flag set). This causes routes to
individual hosts on that network to be created on demand. These routes time
out periodically (normally 20 minutes after validation; entries are not validated
when not in use). An entry for a host which is not responding is a reject route (one with the RTF_REJECT
flag set).
ARP entries may be added, deleted or changed using the arp(1M) utility. Entries added manually may be temporary or permanent, and may be published. Entries which are published cause the system to respond to ARP requests for that host as if it were the target of the request.
In the past, ARP was used to negotiate the use of a trailer encapsulation. This is no longer supported.
ARP watches passively for hosts impersonating the local host (in other words, a host which responds to an ARP mapping request for the local host's address).
The error message: "duplicate IP address %x!! sent from ethernet address: %x:%x:%x:%x:%x:%x." indicates that ARP has discovered another host on the local network which responds to mapping requests for its own Internet address with a different Ethernet address, generally indicating that two hosts are attempting to use the same Internet address.
See attributes(5) for descriptions of the following attributes:
ATTRIBUTE TYPE | ATTRIBUTE VALUE |
---|---|
Interface Stability | Evolving |
arp(1M), ifconfig(1M), route(1M), inet(7P), route(7P)
An Ethernet Address Resolution Protocol, Plummer D., RFC826
Trailer Encapsulations, Leffler S.J., Karels M.J., RFC893
NAME | SYNOPSIS | DESCRIPTION | DIAGNOSTICS | ATTRIBUTES | SEE ALSO