Wednesday, December 12, 2012

Sendmail: Bypass DNS and Forward Emails to Smart Host

Scenario: We need a dumb mail server that would forward all outgoing mails (originated in the server) to a relay host/smart host. We don't want our mail server to do any DNS queries (we leave the noble task for the smart host, after all, he's "smart").
Here's how it's done in sendmail-

  • Create a new file in the /etc/mail directory
vim  /etc/mail/service.switch

####### start of file #########

hosts files
aliases files

####### end of file ###########


  • We add the "relay host" IP to sendmail.mc
vim  /etc/mail/sendmail.mc

define(`SMART_HOST',`192.168.2.250')dnl 
### obviously, replace the relay host address based on your requirements
### end ###

m4 /etc/mail/senmdmail.mc > /etc/mail/sendmail.cf
service sendmail restart
NOTE: Make sure there is no dnl in the beginning of the line. The compiler will treat any starting with dnl as a comment.

And it's done. Now our mail server will not do any DNS queries and forward all outgoing mail to smart host located at 192.168.2.250.

Hope this helps :)


1 comment:

  1. Hi,

    This configuration helped me. Could you please give the configuration to bypass dns for aix. In aix i couldnt find sendmail.mc file

    ReplyDelete