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 <kleptog@svana.org> http://svana.org/kleptog/ > Patent. n. Genius is 5% inspiration and 95% perspiration. A patent is a > tool for doing 5% of the work and then sitting around waiting for someone > else to do the other 95% so you can sue them.
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