How can I find the origins of conflicting DNS records?
12 Answers
You'll want the SOA (Start of Authority) record for a given domain name, and this is how you accomplish it using the universally available nslookup command line tool:
command line> nslookup
> set querytype=soa
> stackoverflow.com
Server: 217.30.180.230
Address: 217.30.180.230#53
Non-authoritative answer:
stackoverflow.com
origin = ns51.domaincontrol.com # ("primary name server" on Windows)
mail addr = dns.jomax.net # ("responsible mail addr" on Windows)
serial = 2008041300
refresh = 28800
retry = 7200
expire = 604800
minimum = 86400
Authoritative answers can be found from:
stackoverflow.com nameserver = ns52.domaincontrol.com.
stackoverflow.com nameserver = ns51.domaincontrol.com.
The origin (or primary name server on Windows) line tells you that ns51.domaincontrol is the main name server for stackoverflow.com.
At the end of output all authoritative servers, including backup servers for the given domain, are listed.
You used the singular in your question but there are typically several authoritative name servers, the RFC 1034 recommends at least two.
Unless you mean "primary name server" and not "authoritative name server". The secondary name servers are authoritative.
To find out the name servers of a domain on Unix:
% dig +short NS stackoverflow.com
ns52.domaincontrol.com.
ns51.domaincontrol.com.
To find out the server listed as primary (the notion of "primary" is quite fuzzy these days and typically has no good answer):
% dig +short SOA stackoverflow.com | cut -d' ' -f1
ns51.domaincontrol.com.
To check discrepencies between name servers, my preference goes to the old check_soa tool, described in Liu & Albitz "DNS & BIND" book (O'Reilly editor). The source code is available in http://examples.oreilly.com/dns5/
% check_soa stackoverflow.com
ns51.domaincontrol.com has serial number 2008041300
ns52.domaincontrol.com has serial number 2008041300
Here, the two authoritative name servers have the same serial number. Good.
You could find out the nameservers for a domain with the "host" command:
[davidp@supernova:~]$ host -t ns stackoverflow.com
stackoverflow.com name server ns51.domaincontrol.com.
stackoverflow.com name server ns52.domaincontrol.com.
I found that the best way it to add always the +trace option:
dig SOA +trace stackoverflow.com
It works also with recursive CNAME hosted in different provider. +trace trace imply +norecurse so the result is just for the domain you specify.
The term you should be googling is "authoritative," not "definitive".
On Linux or Mac you can use the commands whois, dig, host, nslookup or several others. nslookup might also work on Windows.
An example:
$ whois stackoverflow.com
[...]
Domain servers in listed order:
NS51.DOMAINCONTROL.COM
NS52.DOMAINCONTROL.COM
As for the extra credit: Yes, it is possible.
aryeh is definitely wrong, as his suggestion usually will only give you the IP address for the hostname. If you use dig, you have to look for NS records, like so:
dig ns stackoverflow.com
Keep in mind that this may ask your local DNS server and thus may give wrong or out-of-date answers that it has in its cache.
You can use the whois service. On a UNIX like operating system you would execute the following command. Alternatively you can do it on the web at http://www.internic.net/whois.html.
whois stackoverflow.com
You would get the following response.
...text removed here...
Domain servers in listed order: NS51.DOMAINCONTROL.COM NS52.DOMAINCONTROL.COM
You can use nslookup or dig to find out more information about records for a given domain. This might help you resolve the conflicts you have described.
I have found that for some domains, the above answers do not work. The quickest way I have found is to first check for an NS record. If that doesn't exist, check for an SOA record. If that doesn't exist, recursively resolve the name using dig and take the last NS record returned. An example that fits this is analyticsdcs.ccs.mcafee.com.
- Check for an NS record
host -t NS analyticsdcs.ccs.mcafee.com.
- If no NS found, check for an SOA record
host -t SOA analyticsdcs.ccs.mcafee.com.
- If neither NS or SOA, do full recursive and take the last NS returned
dig +trace analyticsdcs.ccs.mcafee.com. | grep -w 'IN[[:space:]]*NS' | tail -1
- Test that the name server returned works
host analyticsdcs.ccs.mcafee.com. gtm2.mcafee.com.
An easy way is to use an online domain tool. My favorite is Domain Tools (formerly whois.sc). I'm not sure if they can resolve conflicting DNS records though. As an example, the DNS servers for stackoverflow.com are
NS51.DOMAINCONTROL.COM
NS52.DOMAINCONTROL.COM
Unfortunately, most of these tools only return the NS record as provided by the actual name server itself. To be more accurate in determining which name servers are actually responsible for a domain, you'd have to either use "whois" and check the domains listed there OR use "dig [domain] NS @[root name server]" and run that recursively until you get the name server listings...
I wish there were a simple command line that you could run to get THAT result dependably and in a consistent format, not just the result that is given from the name server itself. The purpose of this for me is to be able to query about 330 domain names that I manage so I can determine exactly which name server each domain is pointing to (as per their registrar settings).
Anyone know of a command using "dig" or "host" or something else on *nix?
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