Karan Sharma

Ramblings on tech, cycling and finance

11 Nov 2019

A quick primer on dig

Dig is a DNS lookup utility developed by BIND which helps a lot while troubleshooting DNS issues (which are more common than you probably think #hugops). I use dig fairly often and thought to write an introductory guide on how you can use dig with some practical examples that’ll help you dig through DNS issues faster (sorry for the lame pun, couldn’t resist.)

Basics

The most basic and common usage for dig is to query the authorative servers for a particular domain and retrieve the IP. If it’s an IPv4 then you should be looking at A record, while if it’s IPv6 then AAAA record is your friend. Let’s see the DNS records for the site you’re currently on:

➜  ~ dig mrkaran.dev

; <<>> DiG 9.10.6 <<>> mrkaran.dev
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 23292
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1220
;; QUESTION SECTION:
;mrkaran.dev.			IN	A

;; ANSWER SECTION:
mrkaran.dev.		60	IN	A	206.189.89.118

;; Query time: 6 msec
;; SERVER: 127.0.0.1#53(127.0.0.1)
;; WHEN: Tue Oct 29 23:13:31 IST 2019
;; MSG SIZE  rcvd: 67

This is the most basic example for dig. Let’s explore some of the additional options.

Keep it short

dig +short keeps the information to bare minimum and only displays the ANSWER.

dig +short mrkaran.dev
206.189.89.118

Nameserver details

If you want to find the Nameserver for your DNS records, you can use the query type ns.

$ dig mrkaran.dev ns +short
alec.ns.cloudflare.com.
cruz.ns.cloudflare.com.

ns is one of the many query types you can use to indicate which type of DNS record you want to fetch. Default is A record which returns the IPv4 address of the domain (unless it’s a root domain, in which case the default query type is NS). Some other examples of query types are mx, AAAA, TXT etc.

Fun Fact: ANY query type has become obsolete as per the new RFC8482 and DNS operators can choose to not respond to this query. The reason for this is that the payload response size for an ANY query is quite huge (since it has to return all type of DNS records) and this could affect the performance of authoritative servers in case of a DNS amplification attack.

Using different DNS server

Let’s say you want to switch to a different resolver, you can use @ followed by the address of your DNS server.

$ dig mrkaran.dev @9.9.9.9

Reverse DNS Lookup

This one’s actually pretty cool. dig -x lets you query the IP and retrieve the hostname details for that IP.

dig -x 206.189.89.118

Multiple queries

You can input a list of domain names and pass the file with the arg -f to dig.

$ cat digfile
mrkaran.dev
joinmastodon.org
zoho.com

To list down all MX records for the domains in a file, you can use something like:

$ dig -f digfile +noall mx +answer
mrkaran.dev.		242	IN	MX	10 mx.zoho.in.
mrkaran.dev.		242	IN	MX	20 mx2.zoho.in.
mrkaran.dev.		242	IN	MX	50 mx3.zoho.in.
joinmastodon.org.	21599	IN	MX	10 in1-smtp.messagingengine.com.
joinmastodon.org.	21599	IN	MX	20 in2-smtp.messagingengine.com.
zoho.com.		299	IN	MX	10 smtpin.zoho.com.
zoho.com.		299	IN	MX	20 smtpin2.zoho.com.
zoho.com.		299	IN	MX	50 smtpin3.zoho.com.

Search List

I learnt this recently while debugging a DNS issue in one of the Kubernetes pods. Dig doesn’t use search paths by default, so if you have a service say redis inside a namespace dig won’t fetch any result:

$ dig redis +short 
# empty output, indicates no record found

This is because a service name in Kubernetes is of the form service.namespace.svc.cluster.local. So, we should actually be querying for redis.myns.svc.cluster.local and we’ll get our result. But isn’t that too long and painful (sorry for the pun) to type?

So, there’s another option +search which can be used to find all domains matching the search path defined in /etc/resolv.conf namesever configurations.

$ cat /etc/resolv.conf
nameserver 10.100.0.10
search myns.svc.cluster.local svc.cluster.local cluster.local

We can now query for redis with this search list:

dig redis +search +short
10.100.32.73

DNSSec Validation

dig even lets you validate the DNS records you received using DNSSEC validation.

$ dig mrkaran.dev +dnssec
; <<>> DiG 9.10.6 <<>> mrkaran.dev +dnssec
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 36275
;; flags: qr rd ra ad; QUERY: 1, ANSWER: 2, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags: do; udp: 1452
;; QUESTION SECTION:
;mrkaran.dev.			IN	A

;; ANSWER SECTION:
mrkaran.dev.		20	IN	A	178.128.17.49
mrkaran.dev.		20	IN	RRSIG	A 13 2 20 20191112173050 20191110153050 34505 mrkaran.dev. Tl3zD6EqfVRvZi79ahePQcAXnbSUY9ZEYx/KwXnDUyonlrCKuBHzIYYC MJoVns410+sOwbIrcAdLgx+eiMYqRQ==

;; Query time: 65 msec
;; SERVER: 1.1.1.1#53(1.1.1.1)
;; WHEN: Mon Nov 11 22:01:01 IST 2019
;; MSG SIZE  rcvd: 163

The important bit to note here is the ad flag set which represents Authenticated Data. The records will only be returned if the validation succeeds (unless you also specify +cd which indicates Checking Disabled flag.)

On a server which doesn’t have DNSSEC enabled, you can see no records are returned with the +dnssec flag.

$ dig dnssec-failed.org +dnssec
; <<>> DiG 9.10.6 <<>> dnssec-failed.org +dnssec
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 19886
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;dnssec-failed.org.		IN	A

;; Query time: 335 msec
;; SERVER: 1.1.1.1#53(1.1.1.1)
;; WHEN: Mon Nov 11 22:03:50 IST 2019
;; MSG SIZE  rcvd: 35

That pretty much broadly covers some practical examples with dig. I will soon write a detailed post on how DNSSEC validation works and why it needs to be mainstream.

Fin!