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DNS uses UDP packets normally. However when the response is larger than a certain size, it switches to TCP.

It would be very useful if we can specify TCP/UDP when running DNS query commands with DIG or NSLOOKUP for example.

Is there a way to do this with DIG?

madacoda
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2 Answers2

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With dig you can use +tcp.

man dig says:

+[no]tcp
Use [do not use] TCP when querying name servers. The default behavior is to use UDP unless a type any or ixfr=N query is requested, in which case the default is TCP. AXFR queries always use TCP.

+[no]vc
Use [do not use] TCP when querying name servers. This alternate syntax to +[no]tcp is provided for backwards compatibility. The "vc" stands for "virtual circuit".

So both +tcp and +vc does what you want, but +tcp is a bit more clear in what you're trying to do.

Using dig myself and tracing the traffic with Wireshark I see that dig google.com @8.8.8.8 uses UDP and dig google.com @8.8.8.8 +tcp uses TCP.

Bendilock
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The old tools call this "virtual circuit" (vc) mode:

dig google.com @8.8.8.8 +vc

nslookup -vc google.com 8.8.8.8

Windows nslookup (interactive mode):

nslookup> set vc
grawity
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