Date: Tue, 22 Jan 2002 12:42:03 -0800 (PST) From: Kenneth Stailey <kstailey@yahoo.com> To: Kenneth Stailey <kstailey@yahoo.com>, freebsd-net@FreeBSD.ORG Subject: Re: ARP API documentation? Message-ID: <20020122204203.11832.qmail@web10001.mail.yahoo.com> In-Reply-To: <20020122202554.77694.qmail@web10005.mail.yahoo.com>
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--- Kenneth Stailey <kstailey@yahoo.com> wrote:
>
> --- Ruslan Ermilov <ru@FreeBSD.ORG> wrote:
> > On Tue, Jan 22, 2002 at 09:22:12AM -0800, Kenneth Stailey wrote:
> > > Hi,
> > >
> > > I search and search but cannot find any documentation on the user-space
> > level
> > > API for ARP. I know of only two code examples in the tree
> > (usr.sbin/arp/arp.c
> > > and usr.sbin/ppp/arp.c) and I do not wish to play guessing games about
> the
> > > protocol.
> > >
> > > It's clear that you open a PF_ROUTE raw socket and write a message to it
> to
> > > manipulate the ARP tables. Is the format of the message documented
> > anywhere?
> > >
> > route(4).
>
> How is ARP documented without using the word "ARP"?
>
> $ man 4 route | grep -i arp | wc -l
> 0
>
> Seems the old ioctl(2) interface to ARP had better documentation than that.
Where's stuff like this, only for new API?
% man 4 arp
ARP(4P) UNIX Programmer's Manual ARP(4P)
NAME
arp - Address Resolution Protocol
SYNOPSIS
/sys/conf/SYSTEM:
NETHER 1 # ether pseudo-device
DESCRIPTION
ARP is a protocol used to dynamically map between DARPA
Internet and 10Mb/s Ethernet addresses. It is used by all
the 10Mb/s Ethernet interface drivers. It is not specific
to Internet protocols or to 10Mb/s Ethernet, but this imple-
mentation currently supports only that combination.
ARP 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 message is transmitted.
ARP will queue at most one packet while waiting for a map-
ping request to be responded to; only the most recently
``transmitted'' packet is kept.
To facilitate communications with systems which do not use
ARP, ioctls are provided to enter and delete entries in the
Internet-to-Ethernet tables. Usage:
#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);
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 s, but only by the super-user. The
arpreq structure contains:
/*
* 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 0x02/* completed entry (arp_ha valid) */
Printed 1/8/98 August 1, 1987 1
ARP(4P) UNIX Programmer's Manual ARP(4P)
#define ATF_PERM 0x04 /* permanent entry */
#define ATF_PUBL 0x08 /* publish (respond for other host)
*/
#define ATF_USETRAILERS 0x10 /* send trailer packets to host */
The address family for the arp_pa sockaddr must be AF_INET;
for the arp_ha sockaddr it must be AF_UNSPEC. The only flag
bits which may be written are ATF_PERM, ATF_PUBL and
ATF_USETRAILERS. ATF_PERM causes the entry to be permanent
if the ioctl call succeeds. The peculiar nature of the ARP
tables may cause the ioctl to fail if more than 8 (per-
manent) 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 allows a host to act as an ``ARP server,'' which may be
useful in convincing an ARP-only machine to talk to a non-
ARP machine.
ARP is also used to negotiate the use of trailer IP encapsu-
lations; trailers are an alternate encapsulation used to
allow efficient packet alignment for large packets despite
variable-sized headers. Hosts which wish to receive trailer
encapsulations so indicate by sending gratuitous ARP trans-
lation replies along with replies to IP requests; they are
also sent in reply to IP translation replies. The negotia-
tion is thus fully symmetrical, in that either or both hosts
may request trailers. The ATF_USETRAILERS flag is used to
record the receipt of such a reply, and enables the
transmission of trailer packets to that host.
ARP watches passively for hosts impersonating the local host
(i.e. a host which responds to an ARP mapping request for
the local host's address).
DIAGNOSTICS
duplicate IP address!! sent from ethernet address:
%x:%x:%x:%x:%x:%x. ARP has discovered another host on the
local network which responds to mapping requests for its own
Internet address.
SEE ALSO
ec(4), de(4), il(4), inet(4F), arp(8C), ifconfig(8C)
``An Ethernet Address Resolution Protocol,'' RFC826, Dave
Plummer, Network Information Center, SRI.
``Trailer Encapsulations,'' RFC893, S.J. Leffler and M.J.
Karels, Network Information Center, SRI.
BUGS
ARP packets on the Ethernet use only 42 bytes of data; how-
ever, the smallest legal Ethernet packet is 60 bytes (not
including CRC). Some systems may not enforce the minimum
packet size, others will.
Printed 1/8/98 August 1, 1987 2
% uname -a
BSD pdp11.xxxx.com 2.11 2.11 BSD UNIX #11: Fri Jan 4 14:59:26 EST 2002
kstailey@pdp11.xxxx.com:/usr/src/sys/HIPPON pdp11
> > Cheers,
> > --
> > Ruslan Ermilov Oracle Developer/DBA,
> > ru@sunbay.com Sunbay Software AG,
> > ru@FreeBSD.org FreeBSD committer,
> > +380.652.512.251 Simferopol, Ukraine
> >
> > http://www.FreeBSD.org The Power To Serve
> > http://www.oracle.com Enabling The Information Age
>
>
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