Linux nslookup command

Updated: 09/22/2017 by Computer Hope

About nslookup

The nslookup command is used to query internet name servers interactively for information.

Overview

nslookup, which stands for "name server lookup", is a useful tool for finding out information about a named domain.

By default, nslookup will translate a domain name to an IP address (or vice versa). For instance, to find out what the IP address of microsoft.com is, you could run the command:

nslookup microsoft.com

...and you would receive a response like this:

Server:     8.8.8.8
Address:    8.8.8.8#53

Non-authoritative answer:
Name:    microsoft.com
Address: 134.170.185.46
Name:    microsoft.com
Address: 134.170.188.221

Here, 8.8.8.8 is the address of our system's Domain Name Server. This is the server our system is configured to use to translate domain names into IP addresses. "#53" indicates that we are communicating with it on port 53, which is the standard port number domain name servers use to accept queries.

Below this, we have our lookup information for microsoft.com. Our name server returned two entries, 134.170.185.46 and 134.170.188.221. This indicates that microsoft.com uses a round robin setup to distribute server load. When you access micrsoft.com, you may be directed to either of these servers and your packets will be routed to the correct destination.

You can see that we have received a "Non-authoritative answer" to our query. An answer is "authoritative" only if our DNS has the complete zone file information for the domain in question. More often, our DNS will have a cache of information representing the last authoritative answer it received when it made a similar query; this information is passed on to you, but the server qualifies it as "non-authoritative": the information was recently received from an authoritative source, but the DNS server is not itself that authority.

Reverse DNS Lookups

We can also perform the above operation in reverse by providing the IP address rather than the domain name. For instance, the command:

nslookup 134.170.185.46

...will return information resembling the following:

Server:     8.8.8.8
Address:    8.8.8.8#53

Non-authoritative answer:
46.185.170.134.in-addr.arpa    name = grv.microsoft.com.

Authoritative answers can be found from:

Querying The NS Record Of A Domain

The NS Record of a domain is a map of all name servers that are authoritative for that domain. You can query a domain's NS Record using the option -type=ns, like this:

nslookup -type=ns microsoft.com

...and the response will resemble the following:

Server:     8.8.8.8
Address:    8.8.8.8#53

Non-authoritative answer:
microsoft.com    nameserver = ns3.msft.net.
microsoft.com    nameserver = ns4.msft.net.
microsoft.com    nameserver = ns1.msft.net.
microsoft.com    nameserver = ns2.msft.net.

Authoritative answers can be found from:

This output gives us the names of the four microsoft.com name servers (which actually belong to the msft.net domain), according to our DNS's (non-authoritative) information. If there is an available source for authoritative answers, it is listed at the bottom of the output.

Querying The MX Record

The MX Record is a map of mail exchange servers for a domain. When you send email to a domain, for example "@microsoft.com", mail is routed to Microsoft's MX servers.

You can query a domain for its MX Record using the -type=mx option. For example:

nslookup -type=mx microsoft.com

...will respond with output resembling the following:

Server:     8.8.8.8
Address:    8.8.8.8#53

Non-authoritative answer:
microsoft.com    mail exchanger = 10 microsoft-com.mail.protection.outlook.com.

Authoritative answers can be found from:

Here, the mail exchanger address is prefixed with a number (10). If there were more than one mail exchanger, they would each have a different number, with the lower numbers representing a higher priority. So if there were another exchanger with the prefix 5, that server would take precedence over the server listed here.

Querying The SOA Record

The SOA ("Start Of Authority") Record for a domain provides technical information about the domain. It can be queried with the option -type=soa.

nslookup -type=soa microsoft.com
Server:     8.8.8.8
Address:    8.8.8.8#53

Non-authoritative answer:
microsoft.com
    origin = ns1.msft.net
    mail addr = msnhst.microsoft.com
    serial = 2014110802
    refresh = 7200
    retry = 600
    expire = 2419200
    minimum = 3600

Authoritative answers can be found from:

The information listed here is the cached version held by our domain name server. It includes:

  • origin: The authority from which the information originated.
  • mail addr: The email address of the domain administrator (The first dot would be an @ symbol in an email address, so here the email address is [email protected]).
  • serial: Revision data for this information, in the form YYYYMMDDNN. Here, the information is current as of August 11, 2014; 02 means it was the second revision made on that day.
  • refresh: A number representing the interval, in seconds, after which the secondary name server will check the primary name server for an updated revision of this information. This information tells us that the secondary Microsoft name server's information is never more than two hours (7200 seconds) out of date.
  • retry: The secondary nameserver will wait this many seconds before attempting to reconnect to the primary name server after a failed attempt.
  • expire: The secondary nameserver's cache of the primary nameserver's information will always be considered invalid after this many seconds.
  • minimum: The secondary nameserver's cache of the primary nameserver's information should not be refreshed if this amount of time has not elapsed since the last refresh.

Viewing All Available NS Records

We can also view all available NS records for a domain by using the option -type=any. For example:

nslookup -type=any microsoft.com
Server:     8.8.8.8
Address:    8.8.8.8#53

Non-authoritative answer:
Name:    microsoft.com
Address: 134.170.185.46
Name:    microsoft.com
Address: 134.170.188.221
microsoft.com    nameserver = ns3.msft.net.
microsoft.com    nameserver = ns4.msft.net.
microsoft.com    nameserver = ns1.msft.net.
microsoft.com    nameserver = ns2.msft.net.
microsoft.com
    origin = ns1.msft.net
    mail addr = msnhst.microsoft.com
    serial = 2014110802
    refresh = 7200
    retry = 600
    expire = 2419200
    minimum = 3600
microsoft.com    mail exchanger = 10 microsoft-com.mail.protection.outlook.com.
microsoft.com    text = "v=spf1 include:_spf-a.microsoft.com include:_spf-b.microsoft.com 
include:_spf-c.microsoft.com include:_spf-ssg-a.microsoft.com include:spf-a.hotmail.com 
ip4:147.243.128.24 ip4:147.243.128.26 ip4:147.243.128.25 ip4:147.243.1.47 ip4:147.243.1.48 
-all" microsoft.com    text = "FbUF6DbkE+Aw1/wi9xgDi8KVrIIZus5v8L6tbIQZkGrQ/rVQKJi8CjQbBtW
tE64ey4NJJwj5J65PIggVYNabdQ=="

Authoritative answers can be found from:

Querying Another DNS

By default, nslookup will query the same DNS the system is configured to use for all network operations. You can specify a custom DNS to query, however, by specifying it on the command line. For example:

nslookup microsoft.com ns1.msft.net

...will provide us with the authoritative answer to our previous query of microsoft.com:

Server:     ns1.msft.net
Address:    65.55.37.62#53

Name:    microsoft.com
Address: 134.170.185.46
Name:    microsoft.com
Address: 134.170.188.221

This is useful not only for obtaining authoritative information, but for finding out exactly what information a certain DNS currently has cached.

Querying On A Non-Standard Port

By default, domain name servers accept queries on port 53. If this is configured differently on the server you are trying to query, you can specify another port number using the -port= option:

nslookup -port=54 microsoft.com

Here, the query attempt failed because our DNS uses the standard port, number 53.

Debugging The Query Transaction

Advanced users may need to examine more closely the details of the query transaction. This can be achieved using the -debug option:

nslookup -debug microsoft.com
Server:     8.8.8.8
Address:    8.8.8.8#53

------------
    QUESTIONS:
    microsoft.com, type = A, class = IN
    ANSWERS:
    ->  microsoft.com
    internet address = 134.170.188.221
    ttl = 964
    ->  microsoft.com
    internet address = 134.170.185.46
    ttl = 964
    AUTHORITY RECORDS:
    ADDITIONAL RECORDS:
------------
Non-authoritative answer:
Name:    microsoft.com
Address: 134.170.188.221
Name:    microsoft.com
Address: 134.170.185.46

Interactive Mode

You can also run multiple commands interactively by running nslookup with no options:

nslookup

This will bring you to a special command prompt where you can use commands to perform any of the operations listed above. It will look like this:

>

At this prompt, you can simply type in a domain name or IP address to query your DNS for its basic information:

> microsoft.com
Server:     8.8.8.8
Address:    8.8.8.8#53

Non-authoritative answer:
Name:    microsoft.com
Address: 134.170.185.46
Name:    microsoft.com
Address: 134.170.188.221

...or, to submit a specific query type, you can set the type with set type=value, and then run the query. For instance, these two commands are the equivalent of running nslookup -type=any microsoft.com from the command line:

> set type=any
> microsoft.com
Server:     8.8.8.8
Address:    8.8.8.8#53

Non-authoritative answer:
Name:    microsoft.com
Address: 134.170.188.221
Name:    microsoft.com
Address: 134.170.185.46
microsoft.com    nameserver = ns2.msft.net.
microsoft.com    nameserver = ns3.msft.net.
microsoft.com    nameserver = ns4.msft.net.
microsoft.com    nameserver = ns1.msft.net.
microsoft.com
    origin = ns1.msft.net
    mail addr = msnhst.microsoft.com
    serial = 2014110802
    refresh = 7200
    retry = 600
    expire = 2419200
    minimum = 3600
microsoft.com    mail exchanger = 10 microsoft-com.mail.protection.outlook.com.
microsoft.com    text = "v=spf1 include:_spf-a.microsoft.com include:_spf-b.microsoft.com include:_spf-c.microsoft.com include:_spf-ssg-a.microsoft.com include:spf-a.hotmail.com ip4:147.243.128.24 ip4:147.243.128.26 ip4:147.243.128.25 ip4:147.243.1.47 ip4:147.243.1.48 -all"
microsoft.com    text = "FbUF6DbkE+Aw1/wi9xgDi8KVrIIZus5v8L6tbIQZkGrQ/rVQKJi8CjQbBtWtE64ey4NJJwj5J65PIggVYNabdQ=="

Authoritative answers can be found from:

When you are ready to exit nslookup's Interactive Mode, simply run the command exit:

> exit

...and you will be returned to the command line.

There are several other commands you can run from Interactive Mode; see Interactive Commands below for a list.

nslookup syntax

nslookup [-option] [name | -] [server]

Technical Description

nslookup is a program used to query Internet domain name servers for information.

nslookup has two modes: interactive and non-interactive. Interactive mode allows the user to query name servers for information about various hosts and domains or to print a list of hosts in a domain. Non-interactive mode is used to print just the name and requested information for a host or domain.

Arguments

Interactive mode is entered in the following cases:

  1. when no arguments are given (the default name server will be used)
  2. when the first argument is a hyphen ("-") and the second argument is the host name or Internet address of a name server.

Non-interactive mode is used when the name or Internet address of the host to be looked up is given as the first argument (which can be preceded by options). The optional second argument specifies the host name or address of a name server, to be used instead of the system's default DNS.

Options

Options can be specified on the command line if they precede the arguments and are prefixed with a hyphen. For example, to change the default query type to host information, and the initial timeout to 10 seconds, type:

nslookup -query=hinfo -timeout=10

The name of the option corresponds to the keyword of the set command. See below for a description of these keywords, and their available settings.

Interactive Commands

host [server] Look up information for host using the current default server, or server if specified. If host is an Internet address and the query type is A or PTR, the name of the host is returned. If host is a name and does not have a trailing period, the search list is used to qualify the name.

To look up a host not in the current domain, append a period to the name.
server domain, lserver domain Change the default server to domain; lserver uses the initial server to look up information about domain, while server uses the current default server. If an authoritative answer can't be found, the names of servers that might have the answer are returned.
exit Exits the program.
set keyword[=value] This command is used to change state information that affects the lookups. Valid keywords are:

all Prints the current values of the frequently used options to set. Information about the current default server and host is also printed.
class=value Change the query class to one of:

IN the Internet class
CH the Chaos class
HS the Hesiod class
ANY wildcard
The class specifies the protocol group of the information.

(Default = IN; abbreviation = cl)
[no]debug Turn on or off the display of the full response packet and any intermediate response packets when searching.

(Default = nodebug; abbreviation = [no]deb)
[no]d2 Turn debugging mode on or off. This displays more about what nslookup is doing.

(Default = nod2)
domain=name Sets the search list to name.
[no]search If the lookup request contains at least one period but doesn't end with a trailing period, append the domain names in the domain search list to the request until an answer is received.

(Default = search)
port=value Change the default TCP/UDP name server port to value.

(Default = 53; abbreviation = po)
querytype=value, type=value Specify the type of the information to be returned in response to your query. value may be one of the following, in upper or lower case:

a an IP address
any any and all available data
cname canonical name
gid a group identifier for a group name
hinfo CPU and type of operating system
mb mailbox domain name
mg mail group member
minfo mailbox or mail list information
mr mail rename domain name
mx mail exchanger
ns the name servers for the named zone
ptr if the query is an IP address, returns a server name; otherwise, a pointer to other information
soa the start of authority for the named zone
txt returns a textual string with relevant information
[no]recurse Tell the name server to query other servers if it does not have the information.

(Default = recurse; abbreviation = [no]rec)
retry=number Set the number of retries to number.
timeout=number Change the initial timeout interval for waiting for a reply to number seconds.
[no]vc Always use a virtual circuit when sending requests to the server.

(Default = novc)
[no]fail Try the next nameserver if a nameserver responds with SERVFAIL or a referral (nofail) or terminate query (fail) on such a response.

(Default = nofail)

Configuration Files

nslookup makes use of the configuration file /etc/nsresolv.conf, if it exists, for any special DNS configuration you may need. This file is managed using the command line program resolvconf.

Deprecation

nslookup was briefly classified as "deprecated" by the Internet Systems Consortium in favor of the tools host and dig. They later reversed this decision, but it's important to note that host and dig can provide you with the same functions as nslookup, at a somewhat higher level of industry acceptance.

nslookup examples

nslookup 204.228.150.3

Perform a reverse lookup of the IP address 204.228.150.3. Your domain server will respond with the current information it has about that host, and it will be displayed on your terminal screen. Output will resemble the following:

Server:     8.8.8.8
Address:    8.8.8.8#53

Non-authoritative answer:
3.150.228.204.in-addr.arpa    name = 204-228-150-3.xmission.com.

Authoritative answers can be found from:
nslookup -type=mx computerhope.com

Query your DNS for information about the mail exchange server(s) for the domain computerhope.com. Output will resemble the following:

Server:     8.8.8.8
Address:    8.8.8.8#53

Non-authoritative answer:
computerhope.com    mail exchanger = 10 mail.computerhope.com.

Authoritative answers can be found from:
nslookup -type=any google.com

Query your DNS for all available information related to the domain google.com. Output will resemble the following:

Server:     8.8.8.8
Address:    8.8.8.8#53

Non-authoritative answer:
Name:    google.com
Address: 74.125.226.167
Name:    google.com
Address: 74.125.226.165
Name:    google.com
Address: 74.125.226.169
Name:    google.com
Address: 74.125.226.163
Name:    google.com
Address: 74.125.226.168
Name:    google.com
Address: 74.125.226.174
Name:    google.com
Address: 74.125.226.162
Name:    google.com
Address: 74.125.226.160
Name:    google.com
Address: 74.125.226.161
Name:    google.com
Address: 74.125.226.166
Name:    google.com
Address: 74.125.226.164
google.com    has AAAA address 2607:f8b0:4006:809::1009
google.com    nameserver = ns3.google.com.
google.com    mail exchanger = 50 alt4.aspmx.l.google.com.
google.com    rdata_257 = \# 19 0005697373756573796D616E7465632E636F6D
google.com    nameserver = ns4.google.com.
google.com    text = "v=spf1 include:_spf.google.com ip4:216.73.93.70/31 ip4:216.73.93.72/31 ~all"
google.com    mail exchanger = 40 alt3.aspmx.l.google.com.
google.com    nameserver = ns1.google.com.
google.com    mail exchanger = 30 alt2.aspmx.l.google.com.
google.com    mail exchanger = 20 alt1.aspmx.l.google.com.
google.com    mail exchanger = 10 aspmx.l.google.com.
google.com
    origin = ns1.google.com
    mail addr = dns-admin.google.com
    serial = 2014110400
    refresh = 7200
    retry = 1800
    expire = 1209600
    minimum = 300
google.com    nameserver = ns2.google.com.

Authoritative answers can be found from:

dig — DNS lookup utility.
host — Convert a host name to an IP address and vice versa.
ping — Send ICMP ECHO_REQUEST packets to network hosts.