How to check DNS configuration

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Overview

Certain problems may occur when configuring new clients in SEP sesam if the DNS server is misconfigured or missing. SEP sesam needs a proper DNS to work and will not work with an IP address only. All DNS names must be correctly resolved (forward and reverse DNS lookup).

If the DNS server is missing, you will have to use the hosts file of the client and backup server to make systems available via a DNS name. The hosts file can be found in the following locations:

Linux
/etc/hosts
Windows
C:\Windows\system32\drivers\etc\hosts
Information sign.png Note
The hostname of the SEP sesam server may not include an underscore "_" sign. For hostname restrictions, see Restrictions on valid host names.

Tools to check DNS resolution

Several tools are available for checking the DNS resolution, however, SEP recommends that you use sm_setup check_resolution.

sm_setup check_resolution (recommended)

The SEP sesam sm_setup tool is part of the SEP sesam Client and Server installation and can be used from the command line to resolve DNS names. Before using this tool, you will have to set up a SEP sesam profile as described in the FAQ: What happens when I set a profile?

SEP recommends that you run this command on the backup server AND on the client with same arguments. It's important that the client and the backup server resolving themself correct.

Syntax
Client:~ # sm_setup check_resolution backupserver
Calling getaddrinfo with 'backupserver'

        Official name: backupserver.sep.de
        IPv4 Address #1: 172.16.1.146

Calling getnameinfo for IP Address #1 '172.16.1.146'

        Official name: backupserver.sep.de
        Alternate name: backupserver
Client:~ # sm_setup check_resolution client
Calling getaddrinfo with 'client'

        Official name: client.sep.de
        IPv4 Address #1: 172.16.1.145

Calling getnameinfo for IP Address #1 '172.16.1.145'

        Official name: client.sep.de
        Alternate name: client


Backupserver:~ # sm_setup check_resolution client
' Calling getaddrinfo with 'client'

        Official name: client.sep.de
        IPv4 Address #1: 172.16.1.145

Calling getnameinfo for IP Address #1 '172.16.1.145'

        Official name: client.sep.de
        Alternate name: client

Backupserver:~ # sm_setup check_resolution backupserver
Calling getaddrinfo with 'backupserver'

        Official name: backupserver.sep.de
        IPv4 Address #1: 172.16.1.146

Calling getnameinfo for IP Address #1 '172.16.1.146'

        Official name: backupserver.sep.de
        Alternate name: backupserver


The returned addresses and hostnames must match. If the reverse resolve returns an official name that is different from the name specified on the command line, there will be problems with backing up the client (see Common error messages).

nslookup (Windows and Linux)

The nslookup tool is a network administration command-line tool for querying the DNS to obtain a hostname or IP address.

It is useful for troubleshooting DNS issues but not for full hostname resolution as it ignores the hosts file. SEP sesam resolves its hostnames via "common library function" and will first use the hostname specified in the hosts file of the system. By default, nslookup will translate a domain name to an IP address (or vice versa).

Use the nslookup command to check if the name resolution forward with and without FQDN as well as reverse is correct. Check on the SEP sesam Server AND on the SEP sesam Client. If the DNS is not used and the verification is taking place over the etc/hosts file, use ping to verify individual clients.

Syntax
       nslookup {client}
       nslookup {IP-Address of client}                         # important reverse lookup
       nslookup {SEPsesam Server name}
       nslookup {IP-Address of SEPsesam Server}                # important reverse lookup

Example: check mysesam name resolution and reverse lookup:

     #>nslookup mysesam
     Server:   dns.domaine.de
     Address:  192.168.1.254
     Name:     mysesam.domaine.de
     Address:  192.168.1.1
     #>nslookup 192.168.1.1
     Server:   dns.domaine.de
     Address:  192.168.1.254
     Name:     mysesam.domaine.de
     Address:  192.168.1.1

host (Linux only)

The host command can also be used to resolve a hostname into an IP address and vice-versa. It defaults to the name server configured in /etc/resolv.conf but can also be used with a DNS server as an additional argument. It will query the DNS server of the system first.

Syntax
Client:~ # host backupserver
                     backupserver.sep.de has address 172.16.1.146

Client:~ # host 172.16.1.146
                     146.1.16.172.in-addr.arpa domain name pointer backupserver.sep.de

ping

ping is a network administration software utility used to test the reachability of a destination device on an IP network via ICMP echo request. It is not a proper tool for checking the DNS resolution and will not always be 100% correct. Although ping does resolve an IP address, it is not strictly a name server lookup tool and can return a potentially outdated cached result.

In addition, it is not possible to correctly reverse resolve the DNS names. For more details, see ping description on Wikipedia.

Common error messages

The following common error messages indicate that there is a problem with your name resolution:

CLIENT_HOSTNAME: Login to stpd from <CLIENT_HOSTNAME> to <SESAM_SERVER_HOSTNAME> incorrect.
Login incorrect. Client resolves his IP address [X.X.X.X] to [RANDOM_HOSTNAME], but server resolves it to [X.X.X.X]. Please adjust your name resolution.  (0)

In such case, check your name resolution (DNS or etc/hosts file). The SEP sesam Server and SEP sesam Client must be reachable with or without FQDN and should be able to resolve each other and also itself correctly, including the reverse lookup.

In case you have changed an entry in your DNS configuration, but Windows still reports a wrong hostname/IP, try to run ipconfig /flushdns as administrator.