DNS over TCP not udp

Question

A Network problem with ISP caused all UDP traffic to cease, but DNS still working though I think it is malfunctioning. How to prove for sure using software that my current DNS queries are resolved using TCP ?

You get a down vote for providing absolutely no details that would allow us to help you. That being said, fire up a network capture, filter the capture for DNS and look at the capture results. — joeqwerty, Sep 21 '13 at 16:58

score 0 · Answer 1 · answered Sep 21 '13 at 19:45

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Start up a packet capture using wireshark or tcpdump or whatever, logging packet contents. Then, do a DNS lookup and observe the TCP request and response.

answered Sep 21 '13 at 19:45

Falcon Momot

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score 0 · Accepted Answer · answered Sep 21 '13 at 19:46

I'm thinking it is pretty unlikely you're using tcp to resolve, but who knows. I've personally never had a resolver fallback to tcp.

The command for *nix-alikes to do as suggested by joequerty and Falcon Momot is

tcpdump -v 'port 53'

and look for the part of the line that says proto TCP or proto UDP.

In windows, install wireshark and look for port 53 traffic. It will tell you the protocol.

DNS over TCP not udp

2 Answers2