Example Uses of the Linux Host Command

The Linux host command finds the IP address for a domain and vice versa.

On its own, the host command returns a list of all the possible switches that can be used with it.

To get the list type the following into a terminal window:

host

The following results will be displayed:

• -a is equivalent to -v -t ANY.
• -c specifies query class for non-IN data.
• -C compares SOA records on authoritative nameservers.
• -d is equivalent to -v.
• -l lists all hosts in a domain, using AXFR.
• -i performs IP6.INT reverse lookup.
• -N changes the number of dots allowed before root lookup is done.
• -r disables recursive processing.
• -R specifies number of retries for UDP packets.
• -s stipulates that a SERVFAIL response should stop query.
• -t specifies the query type.
• -T enables TCP/IP mode.
• -v enables verbose output.
• -w specifies to wait forever for a reply.
• -W specifies how long to wait for a reply.
• -4 uses IPv4 query transport only.
• -6 uses IPv6 query transport only.
• -m sets the memory debugging flag.
• -V prints version number and exits.

To return the IP address for a domain name, simply type the following command:

host <domainname>

For example, to find the domain name for linux.lifewire.com, type the following command.

host linux.lifewire.com

The results from the host command will be as follows:

linux.lifewire.com is an alias for dynglbcs.lifewire.com.dynglbcs.lifewire.com has address 207.241.148.82

Of course, linux.lifewire.com is a subdomain for lifewire.com. Running the host command against the full lifewire.com domain name returns a different IP address.

lifewire.com has address 207.241.148.80

There is some further output from the host command against lifewire.com, as it displays how mail is handled.

For example:

lifewire.com mail is handled by 500 ALT4.ASPMX.L.Google.com

The opposite of returning the IP address from a domain name is returning the domain name from an IP address.

You can do this by typing the following into a terminal window:

host <IP Address>

For example, we know that 207.241.148.80 is the IP address for Lifewire.com. Type the following into a terminal window:

host 207.241.148.80

The results are as follows:

82.148.241.207.in-addr.arpa domain name pointer glbny.lifewire.com.

The host command by default returns just enough information but you can get a more detailed output by using either the -d or -v switch as follows:

host -d linux.lifewire.com

The results from the above command show the domain that was looked up along with any results. It also returns the SOA details for a domain.

SOA stands for Start of Authority. If you register a domain name and then host that domain with a web hosting company, the web hosting company must maintain an SOA for that domain. It provides a way of keeping track of domain names.

You can find the SOA details for a domain by typing the following command:

host -C <ipaddress>
host -C <domainname>

For example, type the following into a terminal window:

host -C lifewire.com

There are a number of results returned but they all contain the same fields which are as follows:

• The primary name server
• The responsible party for the domain
• A timestamp which changes when the domain is updated
• The number of seconds before the zone should be refreshed
• The number of seconds before a failed refresh should be retried
• The upper limit in seconds before a zone is no longer considered authoritative
• The negative result TTL

There are obviously many other switches, such as -l, which provides a listing, and -T, which searches using TCP/IP instead of UDP.

You will find that a lot of web servers will refuse these types of queries.

In general, you will probably only need to use the host command to return either the IP address for a domain name or the domain name for an IP address.