How to trace a DNS lookup

We want to find an IP address for ws-mt1.pusher.com. We could use dig to ask a well-known DNS server like 8.8.8.8 for an A record:

$ dig @8.8.8.8 ws-mt1.pusher.com A

; <<>> DiG 9.8.3-P1 <<>> @8.8.8.8 ws-mt1.pusher.com A
; (1 server found)
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 34532
;; flags: qr rd ra; QUERY: 1, ANSWER: 3, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;ws-mt1.pusher.com.		IN	A

;; ANSWER SECTION:
ws-mt1.pusher.com.	43	IN	CNAME	edge106-mt1.pusher.com.
edge106-mt1.pusher.com.	7183	IN	CNAME	ec2-54-197-34-162.compute-1.amazonaws.com.
ec2-54-197-34-162.compute-1.amazonaws.com. 85534 IN A 54.197.34.162

;; Query time: 13 msec
;; SERVER: 8.8.8.8#53(8.8.8.8)
;; WHEN: Sun Aug 27 10:28:51 2017
;; MSG SIZE  rcvd: 129

Here, dig found that ws-mt1.pusher.com. is at address 54.197.34.162. dig found this with just one network round trip, because 8.8.8.8 has cached all the answers. This obscures how 8.8.8.8 knows the answer to the question. Instead of directly asking 8.8.8.8, we can follow the authority all the way from the root name servers. The dig tool has an option to do this, +trace:

+[no]trace:

Toggle tracing of the delegation path from the root name servers for the name being looked up. Tracing is disabled by default. When tracing is enabled, dig makes iterative queries to resolve the name being looked up. It will follow referrals from the root servers, showing the answer from each server that was used to resolve the lookup.

Let’s trace the delegation path for ws-mt1.pusher.com!

$ dig @8.8.8.8 +trace +question ws-mt1.pusher.com A

; <<>> DiG 9.8.3-P1 <<>> @8.8.8.8 +trace +question ws-mt1.pusher.com A
; (1 server found)
;; global options: +cmd
;.				IN	NS
.			170574	IN	NS	a.root-servers.net.
...
.			170574	IN	NS	f.root-servers.net.
...
;; Received 228 bytes from 8.8.8.8#53(8.8.8.8) in 31 ms

;ws-mt1.pusher.com.		IN	A
com.			172800	IN	NS	a.gtld-servers.net.
...
com.			172800	IN	NS	i.gtld-servers.net.
...
;; Received 495 bytes from 198.97.190.53#53(198.97.190.53) in 107 ms

;ws-mt1.pusher.com.		IN	A
pusher.com.		172800	IN	NS	ns-74.awsdns-09.com.
pusher.com.		172800	IN	NS	ns-703.awsdns-23.net.
...
;; Received 203 bytes from 192.43.172.30#53(192.43.172.30) in 21 ms

;ws-mt1.pusher.com.		IN	A
ws-mt1.pusher.com.	60	IN	CNAME	edge121-mt1.pusher.com.
edge121-mt1.pusher.com.	7200	IN	CNAME	ec2-54-236-213-148.compute-1.amazonaws.com.
...
;; Received 250 bytes from 205.251.194.191#53(205.251.194.191) in 309 ms

dig asks 8.8.8.8, “What are the name servers for the root zone, .”? 8.8.8.8 responds: “The name servers for . are a.root-servers.net., ...” dig arbitrarily picks f.root-servers.net., and resolves it to 198.97.190.53.
dig asks 198.97.190.53, “what are the addresss for ws-mt1.pusher.com.?” 198.97.190.53 responds: “I don’t know. Ask one of the name servers for com., which are: a.gtld-servers.net., ...” dig arbitrarily picks i.gtld-servers.net., and resolves it to 192.43.172.30.
dig asks 192.43.172.30, “what are the addresses for ws-mt1.pusher.com.?” 192.43.172.30 responds: “I don’t know. Ask one of the name servers for pusher.com., which are: ns-74.awsdns-09.com., ...” dig arbitrarily picks ns-703.awsdns-23.net., and resolves it to 205.251.194.191.
dig asks 205.251.194.191, “what are the addresses for ws-mt1.pusher.com.?” 205.251.194.191 responds: “I don’t know, but it’s the same as ec2-54-236-213-148.compute-1.amazonaws.com. - look that up instead.”

Notice that, even here, this is not the whole process! When a name server responds saying “the name server is i.gtld-servers.net.”, you then need to perform a recursive DNS subquery to find out an address for i.gtld-servers.net.. dig omits this detail here.

Also, our search for an address did not complete! The search finished with a CNAME, meaning we have to start all over again looking for the new name ec2-54-236-213-148.compute-1.amazonaws.com.! Let’s do that:

$ dig @8.8.8.8 +trace +question ec2-54-236-213-148.compute-1.amazonaws.com. A

; <<>> DiG 9.8.3-P1 <<>> @8.8.8.8 +trace +question ec2-54-236-213-148.compute-1.amazonaws.com. A
; (1 server found)
;; global options: +cmd
;.				IN	NS
.			229778	IN	NS	d.root-servers.net.
...
;; Received 228 bytes from 8.8.8.8#53(8.8.8.8) in 24 ms

;ec2-54-236-213-148.compute-1.amazonaws.com. IN A
com.			172800	IN	NS	h.gtld-servers.net.
...
;; Received 492 bytes from 199.7.91.13#53(199.7.91.13) in 106 ms

;ec2-54-236-213-148.compute-1.amazonaws.com. IN A
amazonaws.com.		172800	IN	NS	u1.amazonaws.com.
...
;; Received 192 bytes from 192.55.83.30#53(192.55.83.30) in 65 ms

;ec2-54-236-213-148.compute-1.amazonaws.com. IN A
compute-1.amazonaws.com. 900	IN	NS	u1.amazonaws.com.
...
;; Received 438 bytes from 205.251.195.199#53(205.251.195.199) in 708 ms

;ec2-54-236-213-148.compute-1.amazonaws.com. IN A
ec2-54-236-213-148.compute-1.amazonaws.com. 604800 IN A	54.236.213.148
...
;; Received 358 bytes from 156.154.64.10#53(156.154.64.10) in 15 ms

Finally, we get an address 54.236.213.148! Naive DNS queries would take dozens of round trips, taking seconds to complete. This is why we have caching/TTLs and DNS recursion!

Tagged .

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How to trace a DNS lookup

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