[j-nsp] controlling the source IP for the Dns Proxy feature

[Ben Dale]

Hi Andy,

I have come across this exact issue using the feature.

There was a good reason why we didn't use default address selection that escapes me just now, but I had a slight advantage in that I was using route-based VPNs, so I was able to number the st0 interface with a /32 from the corporate range and source my queries from there.

Unfortunately policy-based VPNs are extremely limiting when it comes to things like this.  I can't think of too many scenarios where I'd even use them any more.  If you don't have too many sites, I'd seriously consider migrating them across.
 
Cheers,

Ben

On 16 Oct 2014, at 1:28 am, Andy Litzinger <andy.litzinger.lists at gmail.com> wrote:

> Hello,
> is anyone out there using the dns-proxy feature for the branch SRX?  Are
> there any clever tricks for specifying the source address the SRX uses to
> query name servers?  It does not appear to be a config option.
> 
> with the default config it appears to use the IP of the outbound
> interface.  If I add the config statement 'set system default address
> selection' i can influence it to use the lo0.0 address, which can solve my
> issue, but not in a way i prefer.
> 
> my exact problem is the following:
> I have an SRX 220H in a remote office. It has an trust and untrust zone.
> users sit on the trust zone and receive dhcp from the SRX and use the SRX
> as their default gateway and dns server.  There is a policy based vpn that
> connects from the untrust zone to our corp HQ.  I have the dns-proxy config
> set up so that if a dns request is going to an intranet zone, e.g.
> corp.example.com, then it should use DNS servers that live across the
> tunnel in our corp HQ.  If they are looking up anything else, they use
> google dns servers.
> 
> here's the relevant config:
> dns-proxy {
>    interface {
>        <interface facing users>;
>    }
>    default-domain * {
>        forwarders {
>            8.8.8.8;
>            8.8.4.4;
>        }
>    }
>    default-domain corp.example.com {
>        forwarders {
>            <corp hq name server1>;
>            <corp hq name server 2>;
>        }
>    }
> }
> 
> the problem is without the 'default address selection' set the SRX tries to
> use the untrust interface IP as the source IP to query our corp HQ name
> servers, but the traffic doesn't enter the tunnel because it doesn't match
> the vpn policy.  And I can't change the policy to allow it because the
> untrust interface IP is the endpoint of the tunnel.  It looks like the
> source zone of the dns query is junos-host- is it possible maybe to set up
> a junos-host zone to untrust zone NAT when going to corp-hq IP space?
> 
> or is there another clever solution?
> 
> thanks!
> -andy
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