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DNS best practice

BAlfson

DNS Best Practice

You might have seen the model we use as I've described it in many places here:

  1. The 'Global' tab of 'Network Services >> DNS' lists "Internal (Network)" (also other internal networks, like "DMZ (Network)" and any "VPN Pool" if applicable) as 'Allowed networks'.
  2. On the 'Forwarders' tab, if you use or plan to use the SMTP Proxy, use an Availability Group containing the OpenDNS or Google (8.8.4.4 first, for speed) name servers in 'DNS Forwarders' (if using any spamhaus.org RBLs with the SMTP Proxy, don't use Google DNS). 'Use forwarders assigned by ISP' is not checked.*
    1. If the SMTP Proxy is not to be a part of your setup, don't add anything in 'DNS Forwarders' and do select 'Use forwarders assigned by ISP'.  See the Change Log below concerning CDNs.
    2. Alternatively, if you're using a Microsoft CDN like Office365, do use the Availability Group approach above and add a Request Route for office365.com pointing at your ISP's name server.
  3. In 'Request Routing', the internal DNS is used for reverse DNS of internal IPs (for example if your internal subnet is 172.16.20.0/24, you would have "20.16.172.in-addr.arpa" in the 'Domain' field and your internal DNS server(s) in 'Target Servers'. With that, the UTM can list machine names instead of internal IP addresses in the reports.
  4. Also, in 'Request Routing', so the UTM can resolve internal FQDNs, add, for example 'yourdomain.loc -> {internal DNS server}'. Do the same for other domains for which you have Forward Lookup Zones in your internal DNS server.
  5. Configure Windows Server (or other) DHCP server for internal devices to point at your internal name server for DNS, then the UTM, then the OpenDNS or Google servers.
  6. The internal DNS server's first forwarder is to the UTM's DNS Proxy, then to the OpenDNS or Google servers.
  7. If you consistently have "connection to server timed out" issues and ECN is not selected ('Advanced' tab of 'QoS'), empty 'Allowed networks' in #1, configure the internal DNS server to bypass the UTM in #6. I suspect this is caused by a problem at the ISP.
  8. In Transparent mode Web Filtering, the client browser resolves FQDNs.  When Pharming Protection is enabled at the bottom of the 'Misc' tab, the Proxy will block a request with "Host Not Found" if it cannot resolve the FQDN to an IP.  If disabling Pharming Protection eliminates such blocks for you, then you have not followed #1 through #7.

We used to do it the other way, but comments by BarryG, BruceKConvergent and others convinced me to change our approach.

Cheers - Bob
* Caution: unchecking 'Use forwarders assigned by ISP' and failing to populate 'DNS Forwarders' will result in degraded performance as the ASG/UTM will fall back to the Root Name Servers.

Change Log: 2020-02-14 Based on a post by wolfman1, I added a warning in 2. about using Google if spamhaus.org is one of the RBLs used in the SMTP Proxy; 2017-11-13 Added 2.a and 2.b based on further info in Alex Busch's thread; 2017-11-12 Added the caveat to #2 about the SMTP Proxy because of Alex Busch's comments about Content Delivery Networks (CDNs); 2017-08-02 added #8 based on a comment by Sophos' Michael Dunn; 2017-06-09 added "VPN Pool" to #1; 2017-04-08 made #3 clearer based on a question by jlbrown also added "or Google" to #5 & #6; 2017-02-12 added 8.8.4.4 comment to #2 based on a comment here by rfcat_vk; 2017-01-14 added "in the 'Domain' field" in #3; 2015-09-25 In #7 corrected #5 to #6; 2015-09-24 changed Astaro to UTM and added #7 based on comments by vilic in DNS issue?; 2015-06-22 based on a thread by TCF, I improved the wording in #1, #2 & #4; 2015-06-20 changed from .local to .loc as reminded by bimmerdriver; 2015-03-20 Added title; 2014-10-04 DHCP and internal FQDNs; 2013-10-09 Added Availability Group idea from adrienjb in #2; 2013-02-04 reordered; 2012-08-20 Added "* Caution" note for #2 based on a suggestion by BarryG



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[edited by: FloSupport at 11:06 AM (GMT -7) on 18 Sep 2020]
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  • BAlfson said:

    DNS Best Practice

    You might have seen the model we use as I've described it in many places here:

    1. ...
    2. On the 'Forwarders' tab, use an Availability Group containing the OpenDNS or Google (8.8.4.4 first, for speed) name servers in 'DNS Forwarders'. 'Use forwarders assigned by ISP' is not checked.*
      ....

    As not everyone here is located in the US, if the UTM uses OpenDNS or GoogleDNS as forwarders, things like geolocation via DNS aren't working anymore. So a lot of CDN and Office365 may perform not in perfect condition.

    I think one should keep that in mind for best practices.

    Best

    Alex

    -

  • in reply to Alexander Busch

    Alexander Busch said:

     

     
    BAlfson

    DNS Best Practice

    You might have seen the model we use as I've described it in many places here:

    1. ...
    2. On the 'Forwarders' tab, use an Availability Group containing the OpenDNS or Google (8.8.4.4 first, for speed) name servers in 'DNS Forwarders'. 'Use forwarders assigned by ISP' is not checked.*
      ....

     

     

     

    As not everyone here is located in the US, if the UTM uses OpenDNS or GoogleDNS as forwarders, things like geolocation via DNS aren't working anymore. So a lot of CDN and Office365 may perform not in perfect condition.

    I think one should keep that in mind for best practices.

    Best

    Alex

     

     

    Not a problem; I live in Europe and have used both Google and OpenDNS as forwarders for a long time. I still get geolocation sites directed to European datacenters and also while using those I cannot watch the American Netflix series. Most geolocation works based on you own IP-address, so unless you start using VPN's or other masquerading technology to hide your public IP, its not a problem.

    Managing several Sophos firewalls both at work and at some home locations, dedicated to continuously improve IT-security and feeling well helping others with their IT-security challenges.

  • in reply to apijnappels

    That's what I thought, Arno, but my research confirms Alex' description.  I wonder if your experience is just a reflection of the improvement in the throughput over the backbone of the Internet or if some CDNs do redirects based on geoip as we assumed.

    Your experience with Netflix is also very interesting.  Could you expand on that a bit?  Maybe including samples from your Web Filtering log when using your ISP's DNS instead of OpenDNS/Google?

    Cheers - Bob

     
    Sophos UTM Community Moderator
    Sophos Certified Architect - UTM
    Sophos Certified Engineer - XG
    Gold Solution Partner since 2005
    MediaSoft, Inc. USA
  • in reply to BAlfson

    BAlfson said:

    That's what I thought, Arno, but my research confirms Alex' description.  I wonder if your experience is just a reflection of the improvement in the throughput over the backbone of the Internet or if some CDNs do redirects based on geoip as we assumed.

    Your experience with Netflix is also very interesting.  Could you expand on that a bit?  Maybe including samples from your Web Filtering log when using your ISP's DNS instead of OpenDNS/Google?

    Cheers - Bob

     

    On Netflix, there's no difference on using my providers' DNS address or the OpenDNS or Google's DNS settings. If i'm in the Netherlands, I get Dutch Netflix (and subtitles). Last week I was in France and got the French subtitles (still using OpenDNS). Unless I VPN to my own UTM, then I get the Dutch subtitles again.

    For Office365 our services are hosted in European datacentre (due to not affected by the Patriot act). So maybe that also influences where DNS requests go...

    If there's any other CDN that I could test latency to, I will happily make some tests using different DNS servers for lookup.

    Managing several Sophos firewalls both at work and at some home locations, dedicated to continuously improve IT-security and feeling well helping others with their IT-security challenges.

  • in reply to apijnappels

    apijnappels said:

     On Netflix, there's no difference on using my providers' DNS address or the OpenDNS or Google's DNS settings. If i'm in the Netherlands, I get Dutch Netflix (and subtitles). Last week I was in France and got the French subtitles (still using OpenDNS). Unless I VPN to my own UTM, then I get the Dutch subtitles again.

    For Office365 our services are hosted in European datacentre (due to not affected by the Patriot act). So maybe that also influences where DNS requests go...

    If there's any other CDN that I could test latency to, I will happily make some tests using different DNS servers for lookup.

     
    Hi Guys,
     
    just to add a bit of complexity to that. There is a thing called EDNS Client Subnet. See https://tools.ietf.org/html/rfc7871 . With that it would be possible for the cloud provider to get information of the IP or network originating the DNS query. So the geolocaiton problem should not affect user which are using public DNS Servers like Google.
    And after a little research, a lot of cloud providers and CDNs support EDNS - to specifically address this issue, see this. Except Microsoft! It is disappointing that Microsoft hasn't added support for EDNS in its geolocation mechanism used by Office 365.
    So at the moment I think the big problem is only with Office365.
     
    Best 
    Alex

    -

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