Re: UDP sockets bound to ANY send answers with wrong src ip address

On Wed, Jun 09, 2004 at 02:25:39PM +0300, Denis Vlasenko wrote:

I observe that UDP sockets listening on ANY
send response packets with ip addr derived from
ip address of interface which is used to send 'em
instead of using dst ip address of client's packet.

I was bitten by this with DNS and NTP.

This is the responsibility of the program. Unless the UDP packet is
bound to a particular address, or the program specifies an IP, the
kernel will pick one. For this reason both the BIND and NTP daemons
open a socket for each interface so they can control this. netstat on
my machine shows:

udp        0      0 192.168.1.225:123       0.0.0.0:*                           
udp        0      0 127.0.0.1:123           0.0.0.0:*                           
udp        0      0 0.0.0.0:123             0.0.0.0:*                        

for the NTP server. The DNS has similar machanism. Remember, UDP
doesn't involve connections, so there is no concept of "replying" to a
packet, the program has to manage that itself.

In your example, if you tell netcat which address to bind to, it will
work.

Hope this helps,
-- 
Martijn van Oosterhout   [off-list ref]   http://svana.org/kleptog/

Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a
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else to do the other 95% so you can sue them.