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

Andy Litzinger Andy.Litzinger at theplatform.com
Wed Oct 15 18:35:49 EDT 2014

I'd happily use route-based vpns if they are supported in my use case.
Based on Kbs and internet lore, it seemed policy based was the best bet
for stability.

My two tunnel endpoint devices are the SRX and a Cisco ASA.
On the SRX side I've got a single subnet but on the ASA side I've got two
subnets.  I would happily use a simple policy on the ASA side like 'permit
ip any <SRX side IP subnet> <SRX side mask>' if i was confident I wasn't
going to have squirrely issues with connectivity.

What do you think?


On 10/15/14 3:22 PM, "Ben Dale" <bdale at comlinx.com.au> wrote:

>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.
>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
>> 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
>> 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
>> as their default gateway and dns server.  There is a policy based vpn
>> connects from the untrust zone to our corp HQ.  I have the dns-proxy
>> 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 {
>>  ;
>>  ;
>>        }
>>    }
>>    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
>> 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
>> 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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