Date: Tue, 25 Jun 1996 16:17:15 +0100 (BST) From: Jonathan Laventhol <jcl@pavilion.co.uk> To: hackers@freebsd.org Subject: Multiple delivery of UDP and IP aliases Message-ID: <199606251517.QAA13345@deputy.pavilion.co.uk>
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Dear BSD Clever People --
(Please respond directly: I'm not on the mailing lists. Thanks.)
Anybody understand the following:
I'm getting multiple delivery of UDP packets under particular
circumstances to do with SO_REUSEADDR and aliases on interfaces.
Briefly, a packet sent to an alias is getting delivered twice,
to different sockets, whereas the packet to the main address
only gets delivered once. This is a real problem to do with
named serving on alias ip addresses, but I've got test code if
anyone wants to play with it.
Get an interface with two alias addresses:
ifconfig de0 inet 192.9.200.1 netmask 255.255.255.0
ifconfig de0 inet alias 192.9.200.2 netmask 255.255.255.255
ifconfig de0 inet alias 192.9.200.3 netmask 255.255.255.255
Get two DGRAM sockets, both marked SO_REUSEADDR:
bind socket1 to 192.9.200.2 port 2000
bind socket0 to INADDR_ANY port 2000
Get another host, 192.9.200.9.
Transmit a UDP packet
from to arrives
192.9.200.9 192.9.100.1 socket0
192.9.200.9 192.9.200.2 *** socket1 and socket0 ***
192.9.200.9 192.9.200.3 socket0
But ... get two DGRAM sockets, both marked SO_REUSEADDR:
bind socket1 to 192.9.200.1 port 2000 (the 'real' address)
bind socket0 to INADDR_ANY port 2000
Transmit a UDP packet
from to arrives
192.9.200.9 192.9.100.1 socket1
192.9.200.9 192.9.200.2 socket0
192.9.200.9 192.9.200.3 socket0
The duplication looks suspect to me. I (briefly) tried to find it
in the kernel but didn't see it. I did, however, find this
intriguing comment in udp_usrreq.c
* udp_usrreq.c,v 1.12 1995/05/30 08:10:02 rgrimes Exp
/*
* Deliver a multicast or broadcast datagram to *all* sockets
* for which the local and remote addresses and ports match
* those of the incoming datagram. This allows more than
* one process to receive multi/broadcasts on the same port.
* (This really ought to be done for unicast datagrams as
* well, but that would cause problems with existing
* applications that open both address-specific sockets and
* a wildcard socket listening to the same port -- they would
* end up receiving duplicates of every unicast datagram.
* Those applications open the multiple sockets to overcome an
* inadequacy of the UDP socket interface, but for backwards
* compatibility we avoid the problem here rather than
* fixing the interface. Maybe 4.5BSD will remedy this?)
*/
Anybody know a) where the duplication I'm reporting happens, and
b) what the 'inadequacy of the UDP socket interface' is?
All help gratefully received.
Jonathan.
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J o n a t h a n L a v e n t h o l
T e c h n i c a l D i r e c t o r
P a v i l i o n I n t e r n e t p l c
aqua house 24 old steine brighton bn1 1el
01273 607072 phone . 01273 607073 fax
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