ARP

The Address Resolution Protocol (ARP) provides IPv4 hosts within a broadcast domain with a means to discover what (if any) other host owns a network address. It is replaced by the Neighbor Discovery Protocol in IPv6. The ARP cache has a fixed size, and older entries are garbage-collected when necessary (unless marked "permanent"; see below).

ARP packet types

This nomenclature comes from RFC 3927, which specifies all-zero destination hardware addresses for broadcasting. All-ones is more common.

• ARP Announcement -- Source and destination network address are equal. Destination hardware address is all zeros. Source hardware address is the sending station's MAC.
  • Used to claim an address in LLMNR for zero-configuration networking
• ARP Probe -- Source network address is all zeroes, as is destination hardware address. Source hardware address is the sending station's MAC. Destination network address is the address being probed.
  • Used to check address suitability for LLMNR in zero-configuration networking
• Gratuitous ARP -- ARP for which there was no request
• Inverse ARP -- Opcodes 8 and 9 allow for Inverse ARP: "I know this hardware address; what's the physical address?"
• Reverse ARP -- Opcodes 3 and 4 allow for Reverse ARP: "I know my hardware address; what's my physical address?" Replaced by DHCP/BOOTP.

Linux ARP states

These are encoded in the ndm_state bitmask of a RTM_NEWNEIGH/RTM_GETNEIGH netlink message (valid ndm_flags values include NTF_PROXY for proxy ARP entries and NTF_ROUTER for IPv6 routers).

state	meaning	transitions
permanent	never undergoes verification or expiry, and is not subject to LRU	none (manually configured)
noarp	normal expiration, never verified "A device with no destination cache"	use resets use counter
reachable	normal expiration	use resets timer
stale	still usable, needs verification	use resets timer and returns to reachable timer changes state to delay
delay	schedule ARP request, needs verification	use resets timer and returns to reachable timer changes state to probe, issuing ARP request
probe	need ARP reply for verification (reprobing)	use resets timer and returns to reachable timer reissues ARP requests or moves to failed
incomplete (new entry)	need ARP reply for first use	use resets timer and returns to reachable timer reissues ARP requests or moves to failed
failed	no response received / invalid cache entry	timer governs reissuing of ARP requests use resets timer and moves to reachable

See Also

• Linux's rtnetlink(7) man page
• "The ARP State Machine" James Westall's CpSc 853 "Protocol Implementation" class notes
• "2.1 Address Resolution Protocol (ARP)" Martin Brown's "Guide to IP Layer Network Administration with Linux"
• "Gratuitous ARP in DHCP vs IPv4 ACD Draft" 2004-04-02 (IETF DHC working group)