Using DNS Resolution

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The Universal Messaging server uses the domain name service (DNS) resolution to determine the fully qualified domain name (FQDN) of the host machine on which the Universal Messaging server is installed. At startup, the Universal Messaging server validates if the host on which the server runs is resolvable in the realm configuration.

In an inter-realm communication scenario, the Universal Messaging server performs a DNS resolution at startup to gather information about all realms in its namespace. When the Universal Messaging server updates for any changes in the namespace automatically, the server also performs a DNS resolution to verify the hostname. For example, the Universal Messaging server has two new realms in its namespace - UM1 and UM2. When starting up, the Universal Messaging server performs a DNS resolution on UM1 and UM2 and saves the resolved DNS information. When an auto-updater task is scheduled to check for any changes to UM1 and UM2, the server performs a DNS resolution again.

When a client creates a connection to the Universal Messaging server and establishes a session, the client also performs a reverse DNS resolution.

Note: Universal Messaging prioritizes the connection to non-loopback addresses over loopback addresses.

To verify that the DNS resolution on the Universal Messaging host machine can resolve the name of the host machine to the correct IP address and reverse the IP address of the host machine back to the correct hostname:

On Windows, open the Command Prompt terminal on the client machine or any machine that participates in the same Universal Messaging join, cluster, or zone and run the following commands:

To determine the FQDN, the IP address of the DNS, and the IP address of the host machine, enter the name of the Universal Messaging host machine, for example:
```
C:\>nslookup host_machine
Server: dns.localhost.com
Address: 10.60.24.25
Name: host_machine.localhost.com
Address: 10.60.25.85
```
To determine the FQDN, the IP address of the DNS, and the host machine, enter the IP address of the Universal Messaging host machine, for example:
```
C:\>nslookup 10.60.25.85
Server: dns.localhost.com
Address: 10.60.24.25
Name: host_machine.localhost.com
Address: 10.60.25.85
```
To verify the FQDN, the IP address of the DNS, and the name of the host machine, enter the reverse IP address of the Universal Messaging host machine and the FQDN of the DNS, for example:
```
C:\>nslookup -query=ptr 85.25.60.10.in-addr.arpa dns.localhost.com
Server: dns.localhost.com
Address: 10.60.24.2585.25.60.10.in-addr.arpa
name = host_machine.localhost.com
```

When a host machine obtains the IP address dynamically through the Dynamic Host Configuration Protocol (DHCP), many DNS servers do not return the correct name for a given IP address. Contact your IT department or network administrator to assign a static IP address to the host machine.