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Julia Evans

How to use dig

• dns •

Hello! I talked to a couple of friends recently who mentioned they wished they knew how to use dig to make DNS queries, so here’s a quick blog post about it.

When I first started using dig I found it a bit intimidating – there are so many options! I’m going to leave out most of dig’s options in this post and just talk about the ones I actually use.

Also I learned recently that you can set up a .digrc configuration file to make its output easier to read and it makes it SO MUCH nicer to use.

I also drew a zine page about dig a few years ago, but I wanted to write this post to include a bit more information.

2 types of dig arguments: query and formatting

There are 2 main types of arguments you can pass to dig:

  1. arguments that tell dig what DNS query to make
  2. arguments that tell dig how to format the response

First, let’s go through the query options.

the main query options

The 3 things you usually want to control about a DNS query are:

  1. the name (like jvns.ca). The default is a query for the empty name (.).
  2. the DNS query type (like A or CNAME). The default is A.
  3. the server to send the query to (like 8.8.8.8). The default is what’s in /etc/resolv.conf.

The format for these is:

dig @server type name

Here are a couple of examples:

  • dig @8.8.8.8 jvns.ca queries Google’s public DNS server (8.8.8.8) for jvns.ca.
  • dig ns jvns.ca makes an query with type NS for jvns.ca

-x: make a reverse DNS query

One other query option I use occasionally is -x, to make a reverse DNS query. Here’s what the output looks like.

$ dig -x 172.217.13.174
174.13.217.172.in-addr.arpa. 72888 IN	PTR	yul03s04-in-f14.1e100.net.

-x isn’t magic – dig -x 172.217.13.174 just makes a PTR query for 174.13.217.172.in-addr.arpa.. Here’s how to make exact the same reverse DNS query without using -x.

$ dig ptr 174.13.217.172.in-addr.arpa.
174.13.217.172.in-addr.arpa. 72888 IN	PTR	yul03s04-in-f14.1e100.net.

I always use -x though because it’s less typing.

options for formatting the response

Now, let’s talk about arguments you can use to format the response.

I’ve found that the way dig formats DNS responses by default is pretty overwhelming to beginners. Here’s what the output looks like:

; <<>> DiG 9.16.20 <<>> -r jvns.ca
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 28629
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 4096
; COOKIE: d87fc3022c0604d60100000061ab74857110b908b274494d (good)
;; QUESTION SECTION:
;jvns.ca.			IN	A

;; ANSWER SECTION:
jvns.ca.		276	IN	A	172.64.80.1

;; Query time: 9 msec
;; SERVER: 192.168.1.1#53(192.168.1.1)
;; WHEN: Sat Dec 04 09:00:37 EST 2021
;; MSG SIZE  rcvd: 80

If you’re not used to reading this, it might take you a while to sift through it and find the IP address you’re looking for. And most of the time, you’re only interested in one line of this response (jvns.ca. 276 IN A 172.64.80.1).

Here are my 2 favourite ways to make dig’s output more manageable.

way 1: +noall +answer

This tells dig to just print what’s in the “Answer” section of the DNS response. Here’s an example of querying for the NS records for google.com.

$ dig +noall +answer ns google.com
google.com.		158564	IN	NS	ns4.google.com.
google.com.		158564	IN	NS	ns1.google.com.
google.com.		158564	IN	NS	ns2.google.com.
google.com.		158564	IN	NS	ns3.google.com.

The format here is:

NAME         TTL            TYPE   CONTENT
google.com   158564   IN    NS     ns3.google.com.

By the way: if you’ve ever wondered what IN means, it’s the “query class” and stands for “internet”. It’s basically just a relic from the 80s and 90s when there were other networks competing with the internet like “chaosnet”.

way 2: +short

This is like dig +noall +answer, but even shorter – it just shows the content of each record. For example:

$ dig +short ns google.com
ns2.google.com.
ns1.google.com.
ns4.google.com.
ns3.google.com.

you can put formatting options in digrc

If you don’t like dig’s default format (I don’t!), you can tell it to use a different format by default by creating a .digrc file in your home directory.

I really like the +noall +answer format, so I put +noall +answer in my ~/.digrc. Here’s what it looks like for me when I run dig jvns.ca using that configuration file.

$ dig jvns.ca
jvns.ca.		255	IN	A	172.64.80.1

So much easier to read!

And if I want to go back to the long format with all of the output (which I do sometimes, usually because I want to look at the records in the Authority section of the response), I can get a long answer again by running:

$ dig +all jvns.ca

dig +trace

The last dig option that I use is +trace. dig +trace mimics what a DNS resolver does when it looks up a domain – it starts at the root nameservers, and then queries the next level of nameservers (like .com), and so on until it reaches the authoritative nameserver for the domain. So it’ll make about 30 DNS queries. (I checked using tcpdump, it seems to make 2 queries to get A/AAAA records for each of the root nameservers so that’s already 26 queries. I’m not really sure why it does this because it should already have those IPs hardcoded, but it does.)

I find this mostly useful for understanding how DNS works though, I don’t think that I’ve used it to solve a problem.

why dig?

Even though there are simpler tools to make DNS queries (like dog and host), I find myself sticking with dig.

What I like about dig is actually the same thing I don’t like about dig – it shows a lot of detail!

I know that if I run dig +all, it’ll show me all of the sections of the DNS response. For example, let’s query one of the root nameservers for jvns.ca. The response has 3 sections I might care about – Answer, Authority, and Additional.

$ dig @h.root-servers.net. jvns.ca +all
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 18229
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 4, ADDITIONAL: 9
;; WARNING: recursion requested but not available

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;jvns.ca.			IN	A

;; AUTHORITY SECTION:
ca.			172800	IN	NS	c.ca-servers.ca.
ca.			172800	IN	NS	j.ca-servers.ca.
ca.			172800	IN	NS	x.ca-servers.ca.
ca.			172800	IN	NS	any.ca-servers.ca.

;; ADDITIONAL SECTION:
c.ca-servers.ca.	172800	IN	A	185.159.196.2
j.ca-servers.ca.	172800	IN	A	198.182.167.1
x.ca-servers.ca.	172800	IN	A	199.253.250.68
any.ca-servers.ca.	172800	IN	A	199.4.144.2
c.ca-servers.ca.	172800	IN	AAAA	2620:10a:8053::2
j.ca-servers.ca.	172800	IN	AAAA	2001:500:83::1
x.ca-servers.ca.	172800	IN	AAAA	2620:10a:80ba::68
any.ca-servers.ca.	172800	IN	AAAA	2001:500:a7::2

;; Query time: 103 msec
;; SERVER: 198.97.190.53#53(198.97.190.53)
;; WHEN: Sat Dec 04 11:23:32 EST 2021
;; MSG SIZE  rcvd: 289

dog also shows the records in the “additional” section , but it’s not super explicit about which is which (I guess the + means it’s in the additional section?). It doesn’t seem to show the records in the “Authority” section.

$ dog @h.root-servers.net. jvns.ca 
  NS ca.                2d0h00m00s A "c.ca-servers.ca."
  NS ca.                2d0h00m00s A "j.ca-servers.ca."
  NS ca.                2d0h00m00s A "x.ca-servers.ca."
  NS ca.                2d0h00m00s A "any.ca-servers.ca."
   A c.ca-servers.ca.   2d0h00m00s + 185.159.196.2
   A j.ca-servers.ca.   2d0h00m00s + 198.182.167.1
   A x.ca-servers.ca.   2d0h00m00s + 199.253.250.68
   A any.ca-servers.ca. 2d0h00m00s + 199.4.144.2
AAAA c.ca-servers.ca.   2d0h00m00s + 2620:10a:8053::2
AAAA j.ca-servers.ca.   2d0h00m00s + 2001:500:83::1
AAAA x.ca-servers.ca.   2d0h00m00s + 2620:10a:80ba::68
AAAA any.ca-servers.ca. 2d0h00m00s + 2001:500:a7::2

And host seems to only show the records in the “answer” section (in this case no records)

$ host jvns.ca h.root-servers.net
Using domain server:
Name: h.root-servers.net
Address: 198.97.190.53#53
Aliases:

Anyway, I think that these simpler DNS tools are great (I even made my own simple web DNS tool) and you should absolutely use them if you find them easier but that’s why I stick with dig. drill’s output format seems very similar to dig’s though, and maybe drill is better! I haven’t really tried it.

that’s all!

I only learned about .digrc recently and I love using it so much, so I hope it helps some of you spend less time sorting though dig output!

Someone on Twitter pointed out that it would be nice if there were a way to tell dig to show a short version of the response which also included the response’s status (like NOERROR, NXDOMAIN, SERVFAIL, etc), and I agree! I couldn’t find an option in the man page that does that though.

Debugging a weird 'file not found' error DNS "propagation" is actually caches expiring