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Old 12-08-2003, 10:34 PM   #1
Registered: Oct 2002
Distribution: RedHat 9
Posts: 43

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firewall prevents dns access

Hi Everyone,

I upgraded to RH9 a few months back and have had a lot of trouble getting my dialup connection up and running again, I've managed to solve all of the problems except one which is related to my firewall. I can't access/ping any websites by name, only by numerical address, which sounds like a DNS problem. If I either turn off my firewall completely, or issue an "ipchains --flush" as root, then everything works fine.

disabling my firewall to get on the net sounds like asking for trouble to me. There must be a way to allow DNS packets through but keep the rest of the stuff out. I've tried bringing the firewall security level down to medium and allowing incoming DHCP, but that didn't help. Does anyone know how I can fix this?

I would greatly appreciate any suggestions you might have. Many thanks in advance.
Old 12-08-2003, 10:51 PM   #2
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Registered: Oct 2003
Location: Zurich, Switzerland
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Is your /etc/resolv.conf looking ok? Is it ok to recieve dns queries (domain/53)?
Old 12-08-2003, 11:02 PM   #3
Registered: Oct 2002
Distribution: RedHat 9
Posts: 43

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Hi ToniT,

thanks for your reply. I'm still a bit of a newbie so I'm not sure how things should look. This is what my /etc/resolv.conf looks like:

nameserver #kppp temp entry
nameserver #kppp temp entry

and these nameservers work fine as long as my firewall is down. Am I missing something in this file?

Thanks for your help.
Old 12-09-2003, 12:46 AM   #4
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Registered: Jul 2003
Location: Silicon Valley, USA
Distribution: OpenBSD 4.6, OS X 10.6.2, CentOS 4 & 5
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ipchains is deprecated. If you're using a 2.4.x kernel you want to use iptables. iptables should be doing stateful inspection so you shouldn't have to modify the firewall at all. In fact, your problem is pretty bizarre because if you can get all traffic except DNS, there shouldn't be any reason why DNS specifically is blocked.

In any case, you need to allow outbound connections to 53/TCP & UDP. If your firewall is not behaving statefully (really weird) then you need to allow return responses from 53/TCP & UDP to ports (TCP and UDP) over 1023. Again, if you have a stateful firewall, you won't need to do any of this.


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