Can resolve hostname linux

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:

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:

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:

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.

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:

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

# 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
#
ip_address = «»
# A and AAA record for IPv4 and IPv6, respectively
# $1 stands for first argument
if [ -n «$1» ] ; then
hostname = » $ »
if [ -z «query_type» ] ; then
query_type = «A»
fi
# use host command for DNS lookup operations
host -t $ $ &>/ dev / null $
if [ «$?» -eq «0» ] ; then
# get ip address
ip_address = » $(host -t $ $ $| awk ‘/has.*address/’) »
else
exit 1
fi
else
exit 2
fi
# display ip
echo $ip_address
stands for first argument
if [ -n »
ip_address = «»
# A and AAA record for IPv4 and IPv6, respectively
# $1 stands for first argument
if [ -n «$1» ] ; then
hostname = » $ »
if [ -z «query_type» ] ; then
query_type = «A»
fi
# use host command for DNS lookup operations
host -t $ $ &>/ dev / null $
if [ «$?» -eq «0» ] ; then
# get ip address
ip_address = » $(host -t $ $ $| awk ‘/has.*address/’) »
else
exit 1
fi
else
exit 2
fi
# display ip
echo $ip_address
» ] ; then
hostname = » $ »
if [ -z «query_type» ] ; then
query_type = «A»
fi
# use host command for DNS lookup operations
host -t $ $ &>/ dev / null $
if [ «$?» -eq «0» ] ; then
# get ip address
ip_address = » $(host -t $ $ $
hostname = » $ »
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 $) »
if [ «$?» -eq «0» ] ; then
if [ -n » $ » ] ; then
echo «The $ adress of the Hostname $ is: $address »
fi
else
echo «An error occurred»
fi
done

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

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.

About the author

Karim Buzdar

Karim Buzdar holds a degree in telecommunication engineering and holds several sysadmin certifications. As an IT engineer and technical author, he writes for various web sites. He blogs at LinuxWays.

Источник

Solved «ssh: Could not resolve hostname xxxx» error in Linux

In this article, we will see how to solve «ssh: Could not resolve hostname xxxx» error in Linux. Sometimes you might have observed that when you try to remotely connect a host using the hostname then you end up in having «ssh: Could not resolve hostname xxxx» error on the output. But if you try to connect the same host using its IP address instead of hostname then you are able to connect to that IP but not to the same host. These can happen due to multiple reasons.

You need to check the most suitable reason occurring in your case. Here we will go through all the possible reasons that can cause «ssh: Could not resolve hostname xxxx» error and the best steps that you can take to solve that error.

Solved «ssh: Could not resolve hostname xxxx» error in Linux

In my case, when I tried to connect the remote cyberhost using ssh root@cyberhost command then I noticed below ssh: Could not resolve hostname cyberhost: Temporary failure in name resolution error on the output as you can see below.

cyberithub@ubuntu:~$ ssh root@cyberhost ssh: Could not resolve hostname cyberhost: Temporary failure in name resolution

Reason #1: Problem with the Hostname Resolution

To fix the above error, I have created an entry in /etc/hosts file mapping the remote host name to the IP address. This will make ssh check the hosts file before checking with the DNS Server for hostname resolution.

cyberithub@ubuntu:~$ sudo nano /etc/hosts 127.0.0.1 localhost 192.168.0.102 cyberhost # The following lines are desirable for IPv6 capable hosts ::1 ip6-localhost ip6-loopback fe00::0 ip6-localnet ff00::0 ip6-mcastprefix ff02::1 ip6-allnodes ff02::2 ip6-allrouters

After putting the above entry, I again tried to connect cyberhost using ssh root@cyberhost command and this time I am able to connect successfully as you can see below.

cyberithub@ubuntu:~$ ssh root@cyberhost The authenticity of host 'cyberhost (192.168.0.102)' can't be established. ECDSA key fingerprint is SHA256:YUAj/JP3Uw5O8J4Z7iKqSEk5B6pgCOuNfcciXy51he4. Are you sure you want to continue connecting (yes/no/[fingerprint])? yes Warning: Permanently added 'cyberhost,192.168.0.102' (ECDSA) to the list of known hosts. root@cyberhost's password: Last login: Tue Feb 14 09:19:43 2023 O @ @ Issabel is a product meant to be configured through a web browser. @ @ O Any changes made from within the command line may corrupt the system @ O O configuration and produce unexpected behavior; in addition, changes O made to system files through here may be lost when doing an update. To access your Issabel System, using a separate workstation (PC/MAC/Linux) Open the Internet Browser using the following URL: Your opportunity to give back: http://www.patreon.com/issabel System load: 0.66 (1min) 1.00 (5min) 0.47 (15min) Uptime: 3 min Asterisk: OFFLINE Memory: [==========>--------------------------------------] 22% 401/1837M Usage on /: [===========>-------------------------------------] 24% 12/50G Swap usage: 0.0% SSH logins: 2 open sessions Processes: 195 total, 148 yours

Reason #2: Host Resolving to Invalid IP

If the hostname is already mapped to an IP Address in the Global DNS Server or to the local host file, then it might be possible that IP address of the host is now changed and the new assigned IP is currently not reachable may be due to lack of connectivity or the IP itself is not valid now. Regarding the older IP that you are able to connect could have assigned to some other host. You can run nslookup command to query the DNS Server and check the address.

cyberithub@ubuntu:~$ nslookup cyberhost Server: 192.168.0.146 Address: 192.168.0.146#53 Non-authoritative answer: Name: cyberhost Address: 192.168.0.106

Hope above given solution should be enough to solve your «ssh: Could not resolve hostname xxxx» error as well. Please let me know your feedback in the comment box !!

Источник

Ubuntu 12.04 — Can’t Resolve Hostname

I have a small dedi server on which I have installed Ubuntu 12.04. I access it via x2go since I have a desktop installed. All worked fine until 2 days ago when, after a reboot, I lost any Internet access and started getting «Can’t Resolve Hostname» errors. If I try to ping google I get:

ping: unknown host google.com 

To access some sites I added them to the hosts file and it works for most of them but not all. Of course this is just a temporary solution. If I look into «System Settings — Network», I get this: I am not very competent, so I don’t know what other info to post but please ask anything you wish me to find out. Thank you.

Please check the output of ps -ef | grep -i «network» — this is to check if the networking process is running at all. If its not running then do sudo service networking start and see what happens afterwards.

Hi heemayl thank you for your help, I executed both of your commands since I couldn’t interpret the results of the first one. This is the result: prntscr.com/5eoi78 Thank you

Did you change any network related settings recently? Give the output of ifconfig -a. Do you use static IP or DHCP?

Could you please post the output of nm-tool . Also, can you ping 8.8.8.8 (Google DNS, to make sure your internet connection is working at all). I suspect some faulty DNS config.

1 Answer 1

The problem is purely related to DNS. As there was no DNS nameserver entries in the /etc/resolv.conf file so the name resolution was failing while pinging by hostname to hosts outside your /etc/hosts entries.

In Ubuntu 12.04 the Network Manager package provides the network related functionality (rather than the old networking program), with the resolvconf (and dnsmasq to some extent) program providing the mass DNS functionality. But surprisingly in your case the resolvconf is not installed so we have to manually update the /etc/resolv.conf file.

So by running the command

echo «nameserver 8.8.8.8» | sudo tee /etc/resolv.conf

we are basically setting the Google’s free DNS server (8.8.8.8) as the nameserver.

This command will insert the text «nameserver 8.8.8.8» into the «/etc/resolv.conf» file and display the text on the screen too. In this way we have a working name resolver that will resolve the hostnames we give into IP addresses.

One very important thing to note here, we are using google’s DNS which is not ideal, you should use your ISP’s DNS here. Ask your ISP to give you their DNS address (can be multiple) and add the address as the nameserver. Although you can keep the Google’s DNS as the backup in case your ISP’s one fails for some reason. Let’s assume that your ISP’s DNS is vv.xx.yy.zz , so you need to run the following commands to make it as the primary DNS and keeping the Google’s DNS as a backup.

echo "nameserver vv.xx.yy.zz" | sudo tee /etc/resolv.conf && echo -e "nameserver 8.8.8.8" "\nnameserver 8.8.4.4" | sudo tee -a /etc/resolv.conf 

See the -a switch in tee command, that is used to append rather than overwrite. Here 8.8.4.4 is also Google’s DNS.

You can add as many nameservers as you want in /etc/resolv.conf but that would be overkill. Just keep it simple yet compact.

Источник