Digging know how (howto)
Dig is one of the most important tools that every sysop uses in day-to-day work. It gives us the possibility to trace resolving path of domains, checking status of domain records or even getting whole definitions of records for a particular domain (under some circumstances…). We won’t write here an essay about DNS and it’s functionality – You san always read it here: http://en.wikipedia.org/wiki/Domain_Name_System or even better, here: https://webhostinggeeks.com/guides/dns/
Assuming, that You ave already dig installed (if not - try yum install bind-utils on CentOs or whatever else on different distros) We can start with explaining how to use properly dig.
1. Simple query
Let’s check Gamedesire’s domain www.gamedesire.com:
[docent@docent-desktop ganymede]$ dig sysop.gamedesire.com
; <<>> DiG 9.8.1-P1-RedHat-9.8.1-3.P1.fc15 <<>> sysop.gamedesire.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<
Above We can see a bunch of data.. “status: NOERROR” is answer status – NOERROR means, that query was resolved properly. ANSWER SECTION contains of information about resolved records (here We can see 2 A records for www.gamedesire.com: 174.123.95.100 and 174.123.95.101). Next We have AUTHORITY SECTION which gives us an answer to the question: “what are the authoritative DNS servers for this domain?”. We have also some statistic data (query time, ADDITIONAL SECTION with DNS servers info etc). And remember to check answer flags (in above example: qr rd ra). You can find explanation of those in RFC 1035 §4.1.1..
What happens when domain name is not resolved properly? Let’s try with non-existing sysop.gamedesire.com:
[docent@docent-desktop ganymede]$ dig sysop.gamedesire.com
; <<>> DiG 9.8.1-P1-RedHat-9.8.1-3.P1.fc15 <<>> sysop.gamedesire.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<
As we can see the answer status is NXDOMAIN here – it means NoneXistingDomain.
2. Querying specific records
There are plenty of DNS record types. Most commonly used are A, MX, TXT and CNAME. Here is the explanation and complete list of those types: http://en.wikipedia.org/wiki/List_of_DNS_record_types. So now – how can We query for a MX domain?
[docent@docent-desktop ganymede]$ dig mx google.com
; <<>> DiG 9.8.1-P1-RedHat-9.8.1-3.P1.fc15 <<>> mx google.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<
So We see above in ANSWER SECTION all found MX records. We can ask for any record type with this method.
3. Tracing DNS query
Every DNS query takes some hierarchical steps. Knowing those could be very helpful under some circumstances:
[docent@docent-desktop ganymede]$ dig +trace www.wikipedia.org
; <<>> DiG 9.8.1-P1-RedHat-9.8.1-3.P1.fc15 <<>> +trace www.wikipedia.org
;; global options: +cmd
. 209351 IN NS k.root-servers.net.
. 209351 IN NS m.root-servers.net.
. 209351 IN NS j.root-servers.net.
. 209351 IN NS l.root-servers.net.
. 209351 IN NS b.root-servers.net.
. 209351 IN NS f.root-servers.net.
. 209351 IN NS h.root-servers.net.
. 209351 IN NS g.root-servers.net.
. 209351 IN NS c.root-servers.net.
. 209351 IN NS e.root-servers.net.
. 209351 IN NS d.root-servers.net.
. 209351 IN NS i.root-servers.net.
. 209351 IN NS a.root-servers.net.
;; Received 512 bytes from 192.168.1.1#53(192.168.1.1) in 693 ms
org. 172800 IN NS b2.org.afilias-nst.org.
org. 172800 IN NS a0.org.afilias-nst.info.
org. 172800 IN NS a2.org.afilias-nst.info.
org. 172800 IN NS c0.org.afilias-nst.info.
org. 172800 IN NS b0.org.afilias-nst.org.
org. 172800 IN NS d0.org.afilias-nst.org.
;; Received 437 bytes from 128.8.10.90#53(128.8.10.90) in 379 ms
wikipedia.org. 86400 IN NS ns1.wikimedia.org.
wikipedia.org. 86400 IN NS ns0.wikimedia.org.
wikipedia.org. 86400 IN NS ns2.wikimedia.org.
;; Received 147 bytes from 199.19.54.1#53(199.19.54.1) in 332 ms
www.wikipedia.org. 3600 IN CNAME wikipedia-lb.wikimedia.org.
wikipedia-lb.wikimedia.org. 600 IN CNAME wikipedia-lb.esams.wikimedia.org.
wikipedia-lb.esams.wikimedia.org. 3600 IN A 91.198.174.225
;; Received 121 bytes from 208.80.152.142#53(208.80.152.142) in 157 ms
We see above all the DNS query steps – first question to the root servers for a proper TLD DNS server (org), next the question to the TLD org’s server for a proper wikipedia.org NS servers, and then the question to the wikipedia’s NS servers for a resolution to the name ‘wikipedia.org’ which appears to be a CNAMEs for something more…
4. Shortening the output
Digging is a quite verbose action – by design – much verbosity is good for debugging purposes. It’s good to know how can We reduce output of this command – for example when We would like to wrap dig command with some monitoring script. For this purpose I suggest to get known with:
+nostats
+nocmd
+noquestion
+short
Let’s try with the strongest one from above - +short will reduce all the dig’s “noise”:
[docent@docent-desktop ganymede]$ dig +short www.gamedesire.com
174.123.95.101
174.123.95.100
We can join +short with eg. “+trace”:
[docent@docent-desktop ganymede]$ dig +short +trace www.gamedesire.com
NS l.root-servers.net. from server 192.168.1.1 in 33 ms.
NS c.root-servers.net. from server 192.168.1.1 in 33 ms.
NS g.root-servers.net. from server 192.168.1.1 in 33 ms.
NS m.root-servers.net. from server 192.168.1.1 in 33 ms.
NS b.root-servers.net. from server 192.168.1.1 in 33 ms.
NS a.root-servers.net. from server 192.168.1.1 in 33 ms.
NS i.root-servers.net. from server 192.168.1.1 in 33 ms.
NS j.root-servers.net. from server 192.168.1.1 in 33 ms.
NS d.root-servers.net. from server 192.168.1.1 in 33 ms.
NS e.root-servers.net. from server 192.168.1.1 in 33 ms.
NS h.root-servers.net. from server 192.168.1.1 in 33 ms.
NS f.root-servers.net. from server 192.168.1.1 in 33 ms.
NS k.root-servers.net. from server 192.168.1.1 in 33 ms.
A 174.123.95.100 from server 207.218.247.135 in 173 ms.
A 174.123.95.101 from server 207.218.247.135 in 173 ms.
You can try yourself with other params.
5. Asking specific DNS server
It is very good practice during checking DNS resolutions (especially while transferring domains etc) to ask query to a couple of DNS servers. We can ask particular DNS server (but only when this server allows us to do so). Remember that by default dig uses DNSes listed in Your /etc/resolv.conf file. Let’s try to ask google’s DNSes first:
[docent@docent-desktop ganymede]$ dig +short @8.8.8.8 www.gamedesire.com
174.123.95.101
174.123.95.100
And now some ThePlanet's:
[docent@docent-desktop ganymede]$ dig +short @ns1.theplanet.com www.gamedesire.com
174.123.95.101
174.123.95.100
6. The authority
In this example:
[docent@docent-desktop ~]$ dig www.gamedesire.com @ns1.theplanet.com
; <<>> DiG 9.8.1-P1-RedHat-9.8.1-3.P1.fc15 <<>> www.gamedesire.com @ns1.theplanet.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<
We can see, that AA bit is set here (flags: qr aa rd). So ns1.theplanet.com is authoritative for gamedesire.com domain (as in AUTHORITY section). Now let’s try to dig it again using some other DNS server:
[docent@docent-desktop ~]$ dig www.gamedesire.com
; <<>> DiG 9.8.1-P1-RedHat-9.8.1-3.P1.fc15 <<>> www.gamedesire.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<
Here We can see, that there is no AA bit set, and also TTL values are lower than 86400 for www.gamedesire.com (We can see 19128 value). Why is that? Because this answer is cached somewhere in the middle and our current DNS server is not authorative for gamedesire.com . If We would repeat this query this TTL value would be dropping every each question. We can’t tell looking at above dig output where this query was cached – to know this We would have to repeat the query with recursion disabled and step manually through all the DNS tree (but in 9/10 cases It will be You local DNS cache… which is caching answers).
7. Tracing DIG execution
As I wrote above We can set +trace param using dig to trace resolving path in the DNS tree. But how can We trace what exact queries are sent and received? Surely with tcpdump:
[root@docent-desktop ~]# tcpdump -i p2p1 -s1024 udp port domain
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on p2p1, link-type EN10MB (Ethernet), capture size 512 bytes
10:54:17.047260 IP pma.local.40206 > 192.168.1.1.domain: 9836+ A? www.gamedesire.com. (36)
10:54:17.047721 IP pma.local.55428 > 192.168.1.1.domain: 59517+ PTR? 1.1.168.192.in-addr.arpa. (42)
10:54:17.079337 IP 192.168.1.1.domain > pma.local.55428: 59517 NXDomain* 0/1/0 (109)
10:54:17.080473 IP 192.168.1.1.domain > pma.local.40206: 9836 2/2/2 A 174.123.95.101, A 174.123.95.100 (146)
^C
4 packets captured
4 packets received by filter
0 packets dropped by kernel
From the tcpdump(8) manpage: Name server requests are formatted as:
src > dst: id op? flags qtype qclass name (len)
I rather suggest reading this manpages yourself – just look for “UDP Name Server Requests” section.
