Linux RHCSA Part 27 - Networking - Network Monitoring and Analysis - DNS, DNS Client Troubleshooting.

When DNS problems occur, it is usually clear what we are dealing with because an appropriate error message informs us, e.g., "Can't be resolved" or "Unable to lookup". Although a given message does not always point to the solution, it is a good hint on where to start searching for the error and fixing it. 

# cat /etc/sysconfig/network

NETWORKING=yes

NETWORKING_IPV6=no

HOSTNAME=RHEL01

You can edit this file if you want to change the system's hostname. If you need to use a Fully Qualified Domain Name (FQDN), you do so in this file as well. You can use RHEL01.example.com as the full domain name for RHEL01. If you know the system's hostname, you should enforce the setting of where the server should send a query if it needs to know the name of another host. Two files contain information enabling DNS lookups: 

• /etc/hosts 

<IP> <fully qualified domain name> <alias> 

Let's see the current /etc/hosts file : 

We can see that at the moment, there is nothing in the file other than local entries. However, if a small environment (five or more machines) is running, using /etc/hosts will make more sense. You will need to add entries for each machine (remembering that this file is located locally on the machine). Here you can immediately see that such an approach can quickly get out of control, especially if you had to update the /etc/hosts file on many machines. Therefore, it becomes logical to use a DNS server to which queries will be directed instead of the local /etc/hosts file. The DNS server address is contained in the /etc/resolv.conf file. This solution allows querying the DNS server in the same way as with the local /etc/hosts file.

Let's see what the /etc/resolv.conf file currently looks like : 

The result of this command shows that the RHEL01 machine sends all DNS queries to the DNS server at address 172.168.1.1, i.e., to itself. Although we do not have a DNS server installed yet, this exercise will show how to configure the client correctly. The /etc/resolv.conf file can also look like this: 

The search option specifies the domain name that will be searched by default. If a client machine is trying to gain access and the hostname is not qualified as RHEL01.example.com, and the DNS server returns an error stating it could not find the host, let's try adding the domain option to the search. This is a typical configuration when client machines belong to a domain. Secondly, let's look at the list of servers available in the second DNS server if a given name is not available in the first one. 

Okay, we have the /etc/hosts and /etc/resolv.conf files configured. Now the question surely arises: where does the client query first? Naturally, there is a third file defining the search order in a single line. This file can be one of the last two, but it is usually the latter. 

Older file : 

Newer file : 

By default, the nsswitch.conf file is used, and for good reason. Sometimes in a specific environment, we want to look up a specific host and connect to it, and there is no need for all other hosts on the network to see it; for this purpose, we can easily use /etc/hosts. Once all files have been configured correctly, you can start testing whether names are being resolved correctly. The test can be performed in two ways - the first is using the "ping" command from one host to another, the next way is the "nslookup" command, querying for an IP address and receiving the associated hostname.

Using the ping command by hostname :

# ping RHEL02

Using the nslookup command :

# nslookup RHEL02

Network troubleshooting can be torture if you don't know what to look for and if you don't understand how networking works.