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.)
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 18.104.22.168 ;; 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
dig +short mrkaran.dev 22.214.171.124
If you want to find the
Nameserver for your DNS records, you can use the query type
$ 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
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 @126.96.36.199
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 188.8.131.52
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.
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
dig even lets you validate the DNS records you received using
$ 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 184.108.40.206 mrkaran.dev. 20 IN RRSIG A 13 2 20 20191112173050 20191110153050 34505 mrkaran.dev. Tl3zD6EqfVRvZi79ahePQcAXnbSUY9ZEYx/KwXnDUyonlrCKuBHzIYYC MJoVns410+sOwbIrcAdLgx+eiMYqRQ== ;; Query time: 65 msec ;; SERVER: 220.127.116.11#53(18.104.22.168) ;; 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
$ 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: 22.214.171.124#53(126.96.36.199) ;; 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.