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Cisco 877W acting a a DNS server. Does it answer external DNS queries coming from the WAN

Unanswered Question
Oct 11th, 2014
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Hello,

I have a Cisco 877W running on my ADSL2+ service at home.

It is setup to act as a DNS server to answer DNS queries for my LAN and has the below commands as part of its configuration

 

ip dns server

!

ip dhcp pool LAN
   network 192.168.2.0 255.255.255.0
   default-router 192.168.2.254
   dns-server 8.8.8.8

 

 

My question is, when I scan my WAN IP for open ports, port 53 (DNS) is open. Does this mean my router will be acting as a DNS server for anyone on the internet who directs DNS queries to my WAN IP?

If so, am I able to turn off port 53 towards the Internet, or do I need to add an an access-list to only accept queries from my internal network.

Thanks for your feedback.

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Jody Lemoine Sat, 10/11/2014 - 05:58
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That's correct. The "ip dns server" command will answer queries on any interface.

Given that your DHCP server is telling your clients to use Google DNS and not your router, I would just turn the router's DNS server off with the "no ip dns server" command.

Setting up an ACL (and/or inspection or zone-based firewalling) on your Internet-facing interface is the best practice to protect your network in general, not just to prevent external DNS queries.

jared.j01 Sun, 10/12/2014 - 03:55
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Thanks very much for your reply.

I have disabled the router to be a DNS server and now the port is closed when I check using a website port scanner.

Will investigate ACL's/firewalls etc for general safety too.

Thanks again.

Jody Lemoine Sun, 10/12/2014 - 05:23
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I'm glad I could be of help.

If you found the information useful, I would appreciate it if you would mark it as correct and rate it accordingly.

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