The Sophos XG135 is not returning any DNS name resolution if we don’t “reconnect” the RJ45 cable.

Question

Hello ! Can someone support me in a &ldquo;weird&rdquo; issue please ? 
 
 Problem : The Sophos XG135 is not returning any DNS name resolution if we don&rsquo;t &ldquo;reconnect&rdquo; the RJ45 cable. 
 When any PC on Windows 10 boots, they cannot access the DNS server which is our Sophos, but they can ping IP any ip address. 
 However, when we disconnect/reconnect the RJ45 cable, they can ping the server by the DNS name. It means that Sophos &ldquo;wakes-up&rdquo; and provide the DNS answer so the computer can ping the requested server. 
 We did an ipconfig/all before and after the reconnect and there are no difference ! 
 
 Regarding our setup, we are in a workgroup without a full domain setup. We added a &ldquo;suffix&rdquo; to each computer so it directly add the domain name which it does a DNS request. 
 Any idea where the issue could be coming from ? 
 
 Thanks in advance ! 
 Sam 
 
 *************************************************************************************** 
 
 Configuration : 
 
 Sophos XG135 buy on end of 2020. 
 Firmware version : SFOS 18.0.5 MR-5-Build586 
 Internal subnetwork : 192.168.1.0/24 
 Remote subnetwork : 10.0.81.0/24

DNS configuration : 
 
 Firewall rules :

NAT rules :

Manipulation : 
 
 After disconnect and re-connect RJ45 cable

*************************************************************************************** 
 
 It very weird. It only works when we disconnect and re-connect the RJ45 cable to connect on the DNS name but it works with the IP address. 
 
 It is the same problem for all computer of the company. 
 
 Is anybody have already encountered this error ? 
 
 Thanks in advance for your help. 
 
 Anthony

emmosophos · Accepted Answer

Hello Samuel, 
 Thank you for the logs and packet capture, so it seems your Windows computer prefers to use the Public DNS rather than the XG, when you disconnect the cable, your computer decides to start using the other DNS, and it is when it works, you could set the XG DNS as the primary manually in the computers, I take 192.168.1.1 is the XG and not another computer in your network doing the DNS. 
 Or you can simply remove the 8.8.8.8 from the XG, and let your computers use the XG for DNS resolution for local and external FQDN. 
 IF the XG isn&rsquo;t 192.168.1.1 and is another computer, as Scott suggested, remove this from the DNS of the XG, and add a DNS request route on the XG to point to 192.168.1.1 for your helvetik.local 
 Regards,

Scott_D_L · Answer

Hi Samuel, 
 What do you mean it didnt work when you tried to remove the DNS (192.168.1.1)? Are you saying it wouldn't let you remove it or you have the same issue? 
 I would make sure your 1st and 2nd DNS servers are external DNS servers. 
 Here's my current config. The third DNS server is an internal address in this case. The first two are external DNS servers(Google etc). Not pictured but below this is the DNS request route for my local domain DNS servers for internal resolution.

The Sophos XG135 is not returning any DNS name resolution if we don’t “reconnect” the RJ45 cable.

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