Posted by Gordon Burditt on 05/15/06 07:41

>I wrote a PHP script that needs to connect to port 53 on UDP and on
>some (shared) servers it doesn't seem to be working. This makes me

Port 53 on the DESTINATION system, port 53 on the LOCAL system, or
both? Are you sure the destination system is running a server on
port 53?

If the local system is running its own DNS server (may be a good
idea for a hosting web server) on port 53, you can't bind to port
53 on the local system because the nameserver is already using it.
Also, binding to low-numbered ports ( < 1024 ) may require root
privilege. But to talk to port 53 you don't need port 53 on your
end.

>currious - is it possible that these servers connections on specific
>ports has been disallowed? If so, how would one go about doing this?

Firewalls, either on the host itself (Linux and FreeBSD have things
like ipf and ipfw in the kernel if it's built that way), or it could
be a nearby Cisco router). Port 53 isn't something normally blocked,
though. It's sometimes the *ONLY* UDP let through.

>Also, how might I go about detecting it?

I think you just did. Try to send a packet and see what comes back
(ICMP port unreachable, ICMP host unreachable, ICMP prohibited, nothing,
or a response) with something like tcpdump.

Gordon L. Burditt

[Back to original message]