RE: [squid-users] DNS lookup failure when transparent proxy

>>> On 5/13/05, Steven Morris <steven.morris@tripleplay-services.com> wrote:
>>> Hi,
>>>
>>> I've installed squid version 2.5 release 9 on redhat linux fedora core 3

>>> and
>>> setup a proxy server (with 2 ethernet ports) between my LAN and the
>>> internet.
>>> I've successfully configured Squid so the proxy server runs
transparently
>>> and intercepts all http requests from clients on the LAN.
>>> When I enter IP address's (including the IP address for google) in the
>>> client's web browser URL, the pages are served fine, but when I enter a
>>> domain name in the URL,
>>> the browser returns the 'Page Cannot be displayed message'.
>>>
>>> The client machine and proxy (in /etc/resolv.conf) both know our ISP
>>> nameservers IP address and I've configured the proxy server firewall to
>>> allow DNS lookups via UDP on port 53.
>>>
>>> If the client browser is configured to use the proxy server (rather than
>>> have it run transparently), DNS lookups work fine and domain name URL
>>> pages
>>> are displayed.
>>> I would however, really like to get DNS lookups working with a
transparent
>>> proxy setup.
>>>
>>> I can't see it myself but would this involve changing settings in
>>> squid.conf?.. or perhaps some form of iptables forwarding command for
DNS
>>> responses on UDP port 53? (ie to forward incoming
>>> responses from the nameserver to the proxy back to the client machines
on
>>> the LAN).
>>>
>>> Any possible solutions would be greatly appreciated.
>>>
>>> Regards,
>>>
>>> Steve
>>>
>>>
>> ----- Original Message -----
>> From: "dev singh" <dev.pratap@gmail.com>
>> To: "Steven Morris" <steven.morris@tripleplay-services.com>
>> Cc: <squid-users@squid-cache.org>
>> Sent: Friday, May 13, 2005 1:22 PM
>> Subject: Re: [squid-users] DNS lookup failure when transparent proxy
>>
>>
>> Hi steve,
>>
>> Start caching DNS on proxy server and put ur server's private ip as
>> the primary DNS on client and the things will go.
>>
>> I think the problem with ur existing configuration is that the private
>> ip which u r using on ur clients , they are not known by ur DNS
>> server.
>>
>> r u avle to ping ur dns server from ur clinet . if u r able to do that
>> than ur configuration will work otherwise it won't.
>>
>> For more detail reason kindly give a rough sketch of ur setup
>> Regards
>> dev
>>
> -----Original Message-----
> From: Steven Morris [mailto:steven.morris@tripleplay-services.com]
> Sent: Friday, May 13, 2005 6:40 AM
> To: dev singh
> Cc: squid-users@squid-cache.org
> Subject: Re: [squid-users] DNS lookup failure when transparent proxy
>
>
> Hi Dev,
>
> Thanks very much for your response.
>
> Am I correct in thinking that I should set my LAN client's primary DNS
> server address to the IP address of the proxy server?
> You mentioned I should start caching DNS on the proxy server.. how would I

> go about this? Do I need to configure something on
> the proxy server / squid.conf to enable this?
>
> I'm not currently able to ping our ISP dns server from the client. I can
> however (as expected) ping the dns server from the proxy server.
>
> My setup involves the following:
> Proxy server with two ethernet ports.
> eth0: A LAN with the client machines attached.. these are the clients
whose
> http requests are being served transparently by the proxy server.
> eth1: Another LAN containing a wireless router with connection to the
> internet.
>
> In squid.conf, I've setup the configuration for a transparent proxy:
> httpd_accel_host virtual
> httpd_accel_port 80
> httpd_accel_with_proxy on
> httpd_accel_uses_host_header on
>
> I've set the port used by clients to acces squid (http_port 3128) and ran
> the iptables command that redirects incoming tcp packets on port 80 to
port
> 3128.
> Apart from this (and some configuration to ACL's in squid.conf), I've not
> changed anything from the default installation of squid.
>
> Regards
> Steve
>
>

Basically what's happening is your clients have no DNS server to query (they
can't ping the ISP DNS server, so likely can't query it), so don't know how
to translate domain names into IP addresses. When the clients are set up to
explicitly use the Proxy, they just send the request to it and let it work
out all the details (including DNS lookups).

Running a caching DNS server is not too difficult (especially if you have
managed DNS servers to query), and you can find instructions scattered
around the net. FC3 comes with BIND 9 (use YUM to install it if it's not on
the system), and if you edit the /etc/named.conf file to add your ISP DNS
servers as forwarders, you should be pretty set. "man named.conf" will give
you a good idea of how to add forwarders. If it's not enough, I'd be happy
to help off list.

Your other option (assuming that you have NAT working) is to allow the
clients behind the Squid server to use the ISP nameservers directly.

Chris
Received on Fri May 13 2005 - 12:16:23 MDT