The Linux nslookup Command #

The Linux nslookup command is a powerful tool for querying DNS (Domain Name System) servers to obtain domain name or IP address mapping information. It helps troubleshoot DNS-related issues and verify configurations. The nslookup command is widely used for network diagnostics and debugging.

Key Usage and Syntax of the nslookup Command #

The basic syntax of the nslookup command is:

nslookup [OPTIONS] [DOMAIN|IP]

• OPTIONS: Additional flags to modify the command’s behavior, such as specifying a DNS server.
• DOMAIN: The domain name to resolve to an IP address.
• IP: The IP address to resolve to a domain name (reverse lookup).

nslookup Command Cheatsheet Table #

Command	Description
nslookup example.com	Resolve the domain name example.com to its IP address.
nslookup 8.8.8.8	Perform a reverse lookup on the IP address 8.8.8.8.
nslookup example.com 8.8.8.8	Query the DNS server 8.8.8.8 for the domain example.com.
nslookup -type=MX example.com	Retrieve the mail exchange (MX) records for example.com.
nslookup -type=NS example.com	Retrieve the name server (NS) records for example.com.
nslookup -type=TXT example.com	Retrieve the TXT records for example.com.

Interactive Mode #

The nslookup command supports an interactive mode where users can perform multiple queries in a session.

Start Interactive Mode #

nslookup

This command enters interactive mode, allowing you to type queries directly.

Example Commands in Interactive Mode #

• server 8.8.8.8: Switch the DNS server to 8.8.8.8.
• set type=MX: Set the query type to mail exchange (MX) records.
• example.com: Query the current DNS server for example.com.

Example Queries #

Resolving a Domain Name #

nslookup example.com

Output: #

Server:  8.8.8.8
Address: 8.8.8.8#53

Non-authoritative answer:
Name:    example.com
Address: 93.184.216.34

This output shows that the DNS server 8.8.8.8 resolved example.com to the IP address 93.184.216.34.

Reverse Lookup #

nslookup 8.8.8.8

Output: #

Server:  8.8.8.8
Address: 8.8.8.8#53

Non-authoritative answer:
8.8.8.8.in-addr.arpa   name = dns.google.

This output shows that 8.8.8.8 is resolved to dns.google.

Advanced Usage #

Querying Specific Record Types #

Use the -type option to query specific DNS records, such as MX, NS, or TXT:

nslookup -type=TXT example.com

Output: #

Server:  8.8.8.8
Address: 8.8.8.8#53

Non-authoritative answer:
example.com   text = "v=spf1 include:_spf.google.com ~all"

This output shows the SPF record for example.com.

Specifying a Custom DNS Server #

By default, nslookup uses the system’s configured DNS server. To use a different DNS server, specify it as a second argument:

nslookup example.com 1.1.1.1

This queries the 1.1.1.1 server for the domain example.com.

Using nslookup with RELIANOID Solutions #

In RELIANOID’s infrastructure, nslookup can be a valuable tool for debugging DNS configurations in load balancer and proxy environments. For example:

Verify Backend Server DNS Records #

nslookup backend1.realianoid.local

This ensures that backend servers are correctly resolved in the DNS system.

Test High Availability DNS Failover #

Use nslookup to test the resolution of failover IPs configured for RELIANOID’s load balancer. Ensure the DNS points to the active server:

nslookup vip1.realianoid.local

Monitor External DNS Providers #

For troubleshooting client access, test external DNS providers to verify their response times and records:

nslookup example.com 8.8.8.8

Summary #

The nslookup command is a versatile tool for network administrators, helping diagnose DNS issues and verify configurations. Whether resolving domain names, performing reverse lookups, or debugging advanced DNS setups, nslookup is indispensable for reliable network operations. With RELIANOID’s infrastructure, leveraging nslookup ensures seamless DNS functionality and enhances system resilience.