Fw: Dcc servers not answering

Vernon Schryver vjs@calcite.rhyolite.com
Mon Jul 15 14:44:43 UTC 2002


> From: "Ray Gardener" <R.A.Gardener@shu.ac.uk>

> ...
> > My guess about the problem is that you have a nearby firewall or other
> > mechanism that filters all UDP packets for port 6277.
>
> I queried this our filewall guys and they reckoned that there are no blocks
> on the network to stop dcc working. (I have also checked the local firewall
> on my Redhat box BTW)
>
> I can connect to each of the dcc servers above  using
>
> telnet <hostname> 6277

`telnet <hostname> 6277` is a good way to look for a firewall, except
that it looks for TCP instead of UDP filtering.  The DCC inter-server
"flooding" uses TCP, but the client-server interactions use UDP.

> and when I bring up my server on localhost I can see that machine using
> spamassassin (although cdcc still tells me that it is not responding)

What kind of server to you run on localhost? dccd, sendmail, or something
else?  What machine does SpamAssassin see, one of the public DCC servers?
If so, how does it see it?  As far as I know, SpamAssassin does not
include any DCC code, and so cannot talk to DCC servers directly.


Given the many clients of the public DCC servers and that all of the
servers all fail the same for you, the problem must be fairly close
to your system.  Either the UDP packets from your system are not
getting out or the UDP responses are not getting back.  It could
be a bug in the DCC code or your system, but since RedHat is fairly
popular among DCC users, that sounds less likely than a firewall.

Do you have a version of `ping` that can use UDP instead of ICMP?

What happens when you try `traceroute -p 6277 -M 10 z.dcc-servers.net`
and smaller values of the -M initial TTL value?  If you have a version
of `traceroute` that uses UDP, that understands that has -M vary the
TTL, then that would send UDP packets to port 6277 and check for a
filter on out-going UDP packets to port 6277.  I can't think of a way
to check for incoming UDP packets from port 6277.

It might be effective to use `tcpdump` or another packet snooping
tool to look for UDP packets to and from port 6277.  If my guess is
right, tcpdump on your machine or a nearby system will see the 
DCC/UDP/IP packets to port 6277 leaving your system but no responses.


Vernon Schryver    vjs@rhyolite.com



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