arp(7)

✨ NAME ✨

arp — the Linux ARP kernel module, bestie 💖 (yes this is section 7 of the manual pages, we're in the ~miscellaneous~ zone, iykyk)

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🦋 DESCRIPTION 🦋

Okay so. This kernel protocol module implements the Address Resolution Protocol defined in RFC 826. What does it do? It converts between Layer2 hardware addresses and IPv4 protocol addresses on directly connected networks. That's it, that's the gig 💅

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. It's giving backstage crew energy, not main character energy, and honestly? Iconic 🫶

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. So many doors into the same room, slay 🚪✨

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. Bring your tuning fork 🔥

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. Not me knocking on doors nobody has mail for 💀

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. Periodt.

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🌸 Ioctls 🌸

Three ioctls are available on all AF_INET sockets. They take a pointer to a struct arpreq as their argument. Behold, the sacred text 📜:

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. Bestie do NOT try this from your regular user account, it's not going to go how you want 😭

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.

Now the arp_flags moment, besties 👑:

• 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. Sorry to that flag, she's retired 🎀

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💜 /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. Nagging detail but it matters, fr fr ⏰

• 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.

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⭐ 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. Rip to them 🪦

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 interfaces (when forwarding and proxy arp is enabled for the interface). Glow-up, honestly ✨

The neigh/* interfaces did not exist before Linux 2.2.

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💀 BUGS 💀

Some timer settings are specified in jiffies, which is architecture- and kernel version-dependent; see time(7). So like, your "1 second" might be vibing differently depending on your machine. Not great, not terrible, just the lore 🙈

There is no way to signal positive feedback from user space. This means connection-oriented protocols implemented in user space will generate excessive ARP traffic, because ndisc will regularly reprobe the MAC address. The same problem applies for some kernel protocols (e.g., NFS over UDP). We are all suffering together 💅

This man page mashes together functionality that is IPv4-specific with functionality that is shared between IPv4 and IPv6. Yeah. She's a mashup. We don't talk about it 😭

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🔥 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. The crossover episode nobody asked for but honestly it ate 👑

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okay bestie that's the whole page, you're officially ARP-literate, go configure something responsibly 🫶

this lil rewrite was serving Linux man-pages 6.20, dated 2026-02-08, and it was a whole vibe 🌸✨💖