arp.4p

<B>Digital的Unix操作系统VAX 4.2源码</B>

.\" SCCSID: @(#)arp.4p	2.1	3/10/87
.\" SCCSID: @(#)arp.4p	2.1	3/10/87
.TH arp 4p
.SH Name
arp \- Address Resolution Protocol
.SH Syntax  
.B "pseudo-device ether"
.SH Description
.NXR "Address Resolution Protocol"
.NXR "Address Resolution Protocol" "communicating with non-ARP system"
.NXR "Ethernet address" "mapping to Internet address"
.NXR "Internet address" "mapping to Ethernet address"
The
ARP protocol is used to map dynamically between DARPA Internet
and 10Mb/s Ethernet addresses.  It is
used by all the 10Mb/s Ethernet interface drivers.
.PP
The ARP protocol 
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
messages are transmitted.  The ARP protocol 
queues only the most recently ``transmitted''  
packet while waiting for a mapping request to be responded to.
.PP
To enable communications with systems which do not use ARP, ioctls
are provided to enter and delete
entries in the Internet-to-Ethernet tables.
The usage is:
.EX 0
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <net/if.h>
struct arpreq arpreq;

ioctl(s, SIOCSARP, (caddr_t)&arpreq);
ioctl(s, SIOCGARP, (caddr_t)&arpreq);
ioctl(s, SIOCDARP, (caddr_t)&arpreq);
.EE
.PP
Each ioctl takes the same structure as an argument.
SIOCSARP sets an ARP entry, SIOCGARP gets an ARP entry, and SIOCDARP
deletes an ARP entry.
These ioctls may be applied to any socket descriptor
.I s,
but only by the superuser.
The
.I arpreq
structure contains:
.EX 0
/*
 * ARP ioctl request
 */
struct arpreq {
    struct sockaddr   arp_pa;     /* protocol address */
    struct sockaddr   arp_ha;     /* hardware address */
    int               arp_flags;  /* flags */
};
/*  arp_flags field values */
#define ATF_COM  2   /* completed entry (arp_ha valid) */
#define	ATF_PERM 4   /* permanent entry */
#define	ATF_PUBL 8   /* publish (respond for other host) */
.EE
.PP
The address family for the
.I arp_pa
sockaddr must be AF_INET; for the 
.I arp_ha
sockaddr, it must be AF_UNSPEC.
The only flag bits that can be written are ATF_PERM and ATF_PUBL.
ATF_PERM causes the entry to be permanent if the ioctl call succeeds.
The ioctl may fail if more
than four permanent Internet host addresses hash to the same slot.
ATF_PUBL specifies that the ARP code
should respond to ARP requests for the
indicated host coming from other machines.  This lets a SUN act as an
ARP server, which can be used to make an ARP-only machine talk
to a non-ARP machine.
.PP
The ARP protocol watches passively for a host 
that responds to an ARP mapping request for the local host's address.
.SH Restrictions 
.NXR "Address Resolution Protocol" "restricted"
ARP packets on the Ethernet use only 42 bytes of data.  The smallest
legal Ethernet packet is 60 bytes, however, not including CRC.
Some systems may not enforce the minimum packet size.  
.SH Diagnostics
.NXR "Address Resolution Protocol" "diagnostics"
.B "duplicate IP address!! sent from Ethernet address: %x:%x:%x:%x:%x:%x"
.br
ARP has discovered another host on the local network that responds to
mapping requests for its own Internet address.
.SH See Also
inet(4f), arp(8c), ifconfig(8c)