Ioctls

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.

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 ATF_USETRAILERS is obsolete and should not be used.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

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.