ARP(9) | Kernel Developer's Manual | ARP(9)

# NAME

`arp`, `arp_ifinit`, `arpresolve`, `arpintr` —
externally visible ARP functions

# SYNOPSIS

`#include <netinet/if_inarp.h>`

void
`arp_ifinit`(struct ifnet *ifp, struct ifaddr *ifa);

int
`arpresolve`(struct ifnet *ifp, const struct rtentry *rt, struct mbuf *m, const struct sockaddr *dst, void *desten, size_t destlen);

void
`arpintr`();

# DESCRIPTION

The `arp` functions provide the interface between the `arp` module and the network drivers which need `arp` functionality. Such drivers must request the arp attribute in their "files" declaration.

`arp_ifinit`()
  Sets up the `arp` specific fields in ifa. Additionally, it sends out a gratuitous `arp` request on ifp, so that other machines are warned that we have a (new) address and duplicate addresses can be detected.

  You must call this in your drivers' ioctl function when you get a SIOCSIFADDR request with an AF_INET address family.

`arpresolve`()
  is called by network output functions to resolve an IPv4 address. If no rt is given, a new one is looked up or created. If the passed or found rt does not contain a valid gateway link level address, a pointer to the packet in m is stored in the route entry, possibly replacing older stored packets, and an `arp` request is sent instead. When an `arp` reply is received, the last held packet is send. Otherwise, the looked up address is returned and written into the storage desten points to. `arpresolve`() returns 1, if a valid address was stored to desten, and the packet can be sent immediately. Else a 0 is returned.
`arpintr`()
  When an `arp` packet is received, the network driver (class) input interrupt handler queues the packet on the arpintrq queue, and requests an `arpintr`() soft interrupt callback. `arpintr`() dequeues the packets, performs sanity checks and calls (for IPv4 `arp` packets, which are the only ones supported currently) the `in_arpinput`() function. `in_arpinput`() either generates a reply to request packets, and adds the sender address translation to the routing table, if a matching route entry is found. If the route entry contained a pointer to a held packet, that packet is sent.

# CODE REFERENCES

The ARP code is implemented in sys/net/if_arp.h, sys/netinet/if_inarp.h and sys/netinet/if_arp.c.

# SEE ALSO

ether_ifattach(9)

Plummer, D., "RFC826", An Ethernet Address Resolution Protocol.

# STANDARDS

RFC 826

# HISTORY

Rewritten to support other than Ethernet link level addresses in NetBSD 1.3.

# AUTHORS

UCB CSRG (original implementation)
Ignatios Souvatzis (support for non-Ethernet)

September 1, 2019 | NetBSD 9.0