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© Rakhesh Sasidharan

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The dig command

Just a quick post on how to use the dig command. Had to troubleshooting some DNS issues today and nslookup just wasn’t cutting it so I downloaded BIND for Windows and ran dig from there.  (For anyone else that’s interested – if you download BIND (it’s a zip file), extract the contents, and copy dig.exe and all the .dll files elsewhere that’s all you need to run dig).

Here’s the simple way of running dig:

A bit confusing when you come from nslookup where you type the name server first and then the name and type. Here’s the nslookup syntax for reference:

Although all documentation shows the dig syntax as above I think the order can be changed too. Since the name server is identified by the @ sign you can specify it later on too:

I will use this syntax everywhere as it’s easier for me to remember.

You can complicate the simple syntax by adding options to dig. There’s two sort of options: query options and domain options (I made the names up). These are usually placed after the type but again the order doesn’t really matter:

  • Query options start with a - (dash) and look like the switches you have in most command line tools. These are useful to control the behavior of dig when making the query.
    • For instance: say I want to for dig to only use IPv4. I can add the option -4. Similarly for only IPv6 it is -6.
    • If I want to specify the port of the name server (i.e. apart from 53) the -p option is for that.
    • (Very useful) If I want to specify a different source port for the client (i.e. the port from which the outgoing request is made and to which the reply is sent, in case you want to eliminate any issues with that) the -b switch is for that. It takes as argument address#port, if you don’t care about the address leave it as 0.0.0.0#port.
    • Apart from specifying the name and type as above it is also possible to pass these as query options via -n and -t.
    • There are some more query options. Above are just the common ones I can remember.
  • Domain options start with a + (plus). These pertain to the name lookup you are doing.
    • For instance: typically DNS queries are performed using UDP. To use TCP instead do +tcp. Most of these options can be prefixed with a “no”, so if you explicitly want to not use TCP then you can do +notcp instead.
    • To specify the default domain name do +domain=xxx.
    • To turn off or on recursive mode do +norecurse or +recurse. (This is similar to the -recurse or -norecurse switch in nslookup).
      • Note: By default recursion is on.
      • A quick note on recursion: When recursion is on dig asks the name server you supply (or that used by the OS) for an answer and if that name server doesn’t have an answer the name server is expected ask other name servers and get back with an answer. Hence the term “recursive”. The name server will send a recursive query to the name server it knows of, who will send a recursive query to the name server it knows of, until an answer is got down the chain of delegation. 
      • A quick note on iterative (non-recursive queries): When recursion is off dig gets an answer from the first name server, then it queries the second name server, gets an answer and queries the third name server, and so on … until dig itself gets an answer from a server who knows.
    • The output from dig is usually verbose and shows all the sections. To keep it short use the +short option.
      • This doesn’t show the name of the server that dig got an answer from. To print this too use the +identify option.
    • To list all the authoritative name servers of a domain along with their SOA records use the +nssearch option. This disables recursion.
    • To list a trace of the delegation path use the +trace option. This too disables recursion. The delegation path is listed by dig contacting each name server down the chain iteratively.
    • Couple of output modifying options. Most of these require no changing:
      • By default the first line of the output shows the version of  dig, the query, and some extra bits of info. To hide that use +nocmd.
      • By default the answer section of replies is shown. To hide it do +noanswer.
      • By default the additional sections of a reply are shown. To hide it do +noadditional.
      • By default the authority section of a reply is shown. To hide it do +noauthority.
      • By default the question asked is shown as a comment (this is useful because you might type something like dig rakhesh.com which dig interprets as dig rakhesh.com A so this makes the question explicit). To hide this do +noquestion.
        • Note that this is not same as the query that is sent. The query is the question plus other flags like whether we want a recursive query etc. By default the query that is sent is not shown. To show that do +qr.
      • By default comments are printed. To hide it do +nocomments.
      • By default some stats are printed. To hide these do +nostats.
    • To change the timeout use +time=X. Default is 5 seconds. This is equivalent to nslookup -timeout=X.
    • To change the number of retries use +retry=X. Default is 3. This is equivalent to nslookup -retry=X.
    • There are some more domain options. Above are just the common ones I can remember.

An interesting thing about dig (since BIND 9) is that it can do multiple queries (the brackets below are not part of the command, I put them to show the multiple queries):

Each of these queries can take its own options but in addition to that you can also provide global options. So the full command when you have multiple queries and global options is as below (again, without the brackets):

To add to the fun you can even have one name server to be used for all queries and over-ride these via name servers for each query! Thus a fuller syntax is:

This is also why the options can be at the beginning of the query rather than at the end of the query. When you specify global options you can over-ride these per query (except the +[no]cmd option).

The dig command by rakhesh is licensed under a Creative Commons Attribution 4.0 International License.