sendmail, 4th Edition

Use a Different Name Server with dig(1)

Normally, dig(1) talks to the name server that is defined in your /etc/resolv.conf file. There will be times, however, when you will need to use a different name server. To illustrate, consider the need to move from one ISP to another. Let’s say your MX records are correct on the old ISP name servers, and you wish to make sure that they are correct on the new name servers before switching over to them. You could change your /etc/resolv.conf file to use the new name servers, but that isn’t advisable until you are certain the new name servers are working correctly. Instead, simply cause dig(1) itself to use the new name servers:

% dig @nameserver host

Here, the @ is immediately followed by the hostname or IP address of the name server to use instead of the default. The dig(1) program will perform its lookups directly using the name servers specified. Consider:

% dig +short mx your.domain
0 mail.your.domain
10 mail2.your.domain
% dig +short @123.45.67.89 your.domain
1 mailserver.new.isp
10 mail.your.domain

Here, we first look up the local domain using the current name servers (there is no @ argument) and find that the output from dig(1) is correct. We then look up the local domain at the new name server using its IP address (the @123.45.67.89) and discover that they are set up incorrectly. This discovery gives you time to fix your MX records on the new name servers before you actually switch services to them.