How can I resolve a hostname to an IP address in a Bash script?

Every system in a TCP/IP network is assigned a unique identifier known as IP address that helps to connect it with other system o the network and all over the internet. All the websites you access on the internet also have unique IP addresses. As it is difficult for everyone to remember the IP addresses of these websites, the DNS system comes which helps to translate these hard to remember IP addresses into human-readable names. With DNS, you no longer have to remember the IP addresses. Instead, you have to just have to remember the domain name and all done. Actually, on the backed, the DNS server takes the hostname and resolves it to an IP address which the browser or application then connects to.

In this article, we will explain how to resolve a hostname/domain name to an IPv4 and IPv6 address in a Bash script. However, before proceeding towards creating the script, let us review some of the commands that can be used to resolve the hostname/domain name to an IP address.

Ping

Ping is the most simple and built-in tool that is available on almost all operating systems. It is used to verify the reachability of a host in a network. However, we can also used it to find the IP address against any hostname/domain name. Use the following syntax to find the IP address of a targeted hostname/domain name:

$ ping target-host

Nslookup

Nslookup is widely used to resolve the hostname to an IP address. In order to use this command for an IP lookup, use the following syntax:

$ nslookup target-host

Host

Another command-line utility “host” can be used to find IP address against any hostname/domain name. In order to use this command, use the following syntax:

$ host target-host

Dig

Dig is another useful command line tool that is used to query various DNS related records. It can be used to find IP address against any hostname/domain name. Use Dig command in the following way to find an IP address against a specific hostname/domain name.

$ dig target-host +short

Bash script to resolve a hostname to an IP address

In order to use the bash script for an IP lookup, follow the below steps:

1. Create a bash file using any text editor. Here I will be using the Nano editor to create a script named “iplookup.sh”.
   $ sudo nano script.sh

2. Copy-paste the following lines in your script file. Note that, here in this script, I am specifying Google’s public DNS server for IP lookup. You can specify any other DNS server as per your environment.
   # Specify DNS server
   dnsserver="8.8.8.8"
   # function to get IP address
   function get_ipaddr {
     ip_address=""
       # A and AAA record for IPv4 and IPv6, respectively
       # $1 stands for first argument
     if [ -n "$1" ]; then
       hostname="${1}"
       if [ -z "query_type" ]; then
         query_type="A"
       fi
       # use host command for DNS lookup operations
       host -t ${query_type}  ${hostname} &>/dev/null ${dnsserver}
       if [ "$?" -eq "0" ]; then
         # get ip address
         ip_address="$(host -t ${query_type} ${hostname} ${dnsserver}| awk '/has.*address/{print $NF; exit}')"
       else
         exit 1
       fi
     else
       exit 2
     fi
   # display ip
    echo $ip_address
   }
   hostname="${1}"
   for query in "A-IPv4" "AAAA-IPv6"; do
     query_type="$(printf $query | cut -d- -f 1)"
     ipversion="$(printf $query | cut -d- -f 2)"
     address="$(get_ipaddr ${hostname})"
     if [ "$?" -eq "0" ]; then
       if [ -n "${address}" ]; then
       echo "The ${ipversion} adress of the Hostname ${hostname} is: $address"
       fi
     else
       echo "An error occurred"
     fi
   done
3. Once done, use Ctrl+O and Ctrl+X to save and exit the file respectively.
4. Now to find an IP address against a targeted hostname/domain name, run the script using the following syntax:
   $ ./script.sh target-host

   For instance, to resolve the IP address of “google.com”, the command would be:

   $ ./iplookup.sh google.com

   The output would be similar to this:

   Similarly, to resolve the IP address of “yahoo.com”, the command would be:

   $ ./iplookup.sh yahoo.com

   The output would be similar to this:

   That is all there is to it! In this article, we have learned to resolve the hostname to an IPv4 and IPv6 address using a bash script. We also learned some other command-line tools such as Ping, Nslookup, Host, and Dig that can be used to perform an IP lookup.

from Linux Hint https://ift.tt/3dIbHX6