nslookup is a network administration tool for querying the Domain Name System (DNS) to obtain domain name or IP address mapping or any other specific DNS record.
It is also used to troubleshoot DNS related problems. This article provides few examples on using the nslookup command.
nslookup can operate on both “Interactive mode” and “Non-Interactive mode”. Interactive mode allows the user to query the DNS-Server about various host, and domains. Non-Interactive mode allows the user to query the information for a host or domain. In this article, all the commands explained are “Non-Interactive mode”.
1. nslookup – Simple Example
nslookup followed by the domain name will display the “A Record” ( IP Address ) of the domain.
$ nslookup redhat.com Server: 192.168.19.2 Address: 192.168.19.2#53 Non-authoritative answer: Name: redhat.com Address: 188.8.131.52
In the above output, server refers to the IP address of the DNS server. Then the below section provides the “A Record” ( IP Address ) of the domain “redhat.com”.
The default output of nslookup command is less cluttered than the default output of dig command. Some of you might be comfortable using dig command for DNS lookups.
2. Query the MX Record using -query=mx
MX ( Mail Exchange ) record maps a domain name to a list of mail exchange servers for that domain. The MX record tells that all the mails sent to “@redhat.com” should be routed to the Mail server in that domain.
$ nslookup -query=mx redhat.com Server: 192.168.19.2 Address: 192.168.19.2#53 Non-authoritative answer: redhat.com mail exchanger = 10 mx2.redhat.com. redhat.com mail exchanger = 5 mx1.redhat.com. Authoritative answers can be found from: mx2.redhat.com internet address = 184.108.40.206 mx1.redhat.com internet address = 220.127.116.11
In the above example, we have 2 MX records for the domain “redhat.com”. The number ( 5, 10 ), associated with the MX records tells the preference of mail server. Lower the number, higher the preference. So when a mail is sent to “@redhat.com”, first preference will be “mx1.redhat.com”, then “mx2.redhat.com”.
Authoritative Answer vs Non-Authoritative Answer
You may also noticed the keyword “Authoritative Answer” and “Non-Authoritative Answer” in the above output.
Any answer that originates from the DNS Server which has the complete zone file information available for the domain is said to be authoritative answer.
In many cases, DNS servers will not have the complete zone file information available for a given domain. Instead, it maintains a cache file which has the results of all queries performed in the past for which it has gotten authoritative response. When a DNS query is given, it searches the cache file, and return the information available as “Non-Authoritative Answer”.
3. Query the NS Record using -query=ns
NS ( Name Server ) record maps a domain name to a list of DNS servers authoritative for that domain. It will output the name serves which are associated with the given domain.
nslookup -type=ns redhat.com Server: 192.168.19.2 Address: 192.168.19.2#53 Non-authoritative answer: redhat.com nameserver = ns4.redhat.com. redhat.com nameserver = ns2.redhat.com. redhat.com nameserver = ns1.redhat.com. redhat.com nameserver = ns3.redhat.com. Authoritative answers can be found from: ns4.redhat.com internet address = 18.104.22.168 ns2.redhat.com internet address = 22.214.171.124 ns1.redhat.com internet address = 126.96.36.199 ns3.redhat.com internet address = 188.8.131.52
4. Query the SOA Record using -query=soa
SOA record ( start of authority ), provides the authoritative information about the domain, the e-mail address of the domain admin, the domain serial number, etc…
$ nslookup -type=soa redhat.com Server: 192.168.19.2 Address: 192.168.19.2#53 Non-authoritative answer: redhat.com origin = ns1.redhat.com mail addr = noc.redhat.com serial = 2012071601 refresh = 300 retry = 180 expire = 604800 minimum = 14400 Authoritative answers can be found from: ns1.redhat.com internet address = 184.108.40.206
- mail addr – specifies the mail address of the domain admin ( email@example.com )
- serial – sort of revision numbering system. The standard convention is to use “YYYYMMYYNN” format. ( 2012-07-16. 01 will be incremented, if more than one edit has taken place on a same day )
- refresh – specifies ( in seconds ), when the secondary DNS will poll the primary to see if the serial number has been increased. If increased, secondary will make a new request to copy the new zone file.
- retry – specifies the interval to re-connect with the Primary DNS
- expire – specifies the time that the secondary DNS will keep the cached zone file as valid
- minimum – specifies the time that the secondary DNS should cache the zone file
5. View available DNS records using -query=any
We can also view all the available DNS records using -query=any option.
$ nslookup -type=any google.com Server: 192.168.19.2 Address: 192.168.19.2#53 Non-authoritative answer: Name: google.com Address: 220.127.116.11 Name: google.com Address: 18.104.22.168 google.com nameserver = ns1.google.com. google.com nameserver = ns2.google.com. google.com origin = ns1.google.com mail addr = dns-admin.google.com serial = 2012071701 refresh = 7200 retry = 1800 expire = 1209600 minimum = 300 google.com mail exchanger = 20 alt1.aspmx.l.google.com. google.com mail exchanger = 30 alt2.aspmx.l.google.com. google.com mail exchanger = 40 alt3.aspmx.l.google.com. google.com mail exchanger = 50 alt4.aspmx.l.google.com. google.com mail exchanger = 10 aspmx-v4v6.l.google.com. google.com has AAAA address 2a00:1450:4002:801::1004 Authoritative answers can be found from: ns4.google.com internet address = 22.214.171.124 ns3.google.com internet address = 126.96.36.199
6. Reverse DNS lookup
You can also do the reverse DNS look-up by providing the IP Address as argument to nslookup.
$ nslookup 188.8.131.52 Server: 192.168.19.2 Address: 192.168.19.2#53 Non-authoritative answer: 184.108.40.206.in-addr.arpa name = origin-www2.redhat.com.
7. Using Specific DNS server
Instead of using default DNS server’s for querying, you can also specify a particular name server to resolve the domain name.
$ nslookup redhat.com ns1.redhat.com Server: 220.127.116.11 Address: 18.104.22.168#53 Name: redhat.com Address: 22.214.171.124
In the above command, we have used the ns1.redhat.com as the DNS server. Here you may notice that, we don’t get any “Non-authoritative answer:” header, since ns1.redhat.com has all the zone information of redhat.com
8. Change the port number to connect with
By default DNS servers uses the port number 53. If for any reasons, the port number got changed, then we can specify the port number using -port option
$ nslookup -port 56 redhat.com
9. Change timeout interval to wait for a reply
You can change the default timeout to wait for a reply using -timeout option.
$ nslookup -timeout=10 redhat.com
10. Enabling debug mode using -debug
You can turn on/off the debugging using -debug option in the command line
$ nslookup -debug redhat.com Server: 192.168.19.2 Address: 192.168.19.2#53 ------------ QUESTIONS: redhat.com, type = A, class = IN ANSWERS: -> redhat.com internet address = 126.96.36.199 ttl = 5 AUTHORITY RECORDS: ADDITIONAL RECORDS: ------------ Non-authoritative answer: Name: redhat.com Address: 188.8.131.52
The debug mode will display the packets information during searching.