Date: Fri, 11 Aug 1995 14:09:26 -0400 From: Garrett Wollman <wollman@halloran-eldar.lcs.mit.edu> To: current@freebsd.org Subject: Another way of finding the destination of a received UDP packet Message-ID: <9508111809.AA07865@halloran-eldar.lcs.mit.edu>
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Earlier we had a discussion on the lengths that various programs
needed to go to in order to figure out the destination address to
which a packet which was just received had been sent. I was leafing
through someone else's copy of Stevens volume 1, and noticed a
description of the behavior documented here (taken from ip(4)):
If the IP_RECVDSTADDR option is enabled on a SOCK_DGRAM socket, the
recvmsg call will return the destination IP address for a UDP datagram.
The msg_control field in the msghdr structure points to a buffer that
contains a cmsghdr structure followed by the IP address. The cmsghdr
fields have the following values:
cmsg_len = sizeof(struct in_addr)
cmsg_level = IPPROTO_IP
cmsg_type = IP_RECVDSTADDR
So it is possible to avoid binding a socket for every interface.
Instead, a program can create a single socket for listening, and then
create and destroy one for each message sent in reply to a previous
query.
I don't propose to actually implement this in anything myself... It
might be useful or necessary to also provide an IP_RECVIF or some such
for certain applications which care about interfaces.
-GAWollman
--
Garrett A. Wollman | Shashish is simple, it's discreet, it's brief. ...
wollman@lcs.mit.edu | Shashish is the bonding of hearts in spite of distance.
Opinions not those of| It is a bond more powerful than absence. We like people
MIT, LCS, ANA, or NSA| who like Shashish. - Claude McKenzie + Florent Vollant
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