SZÉPE Viktor writes:

Thank you for your support!

-nodnslookup is the solution for satellite type server which delivery
messages through one smarthost.

What should I do on normal (tcp/25,587,465 are open to the internet)
mail server where DNS lookup is necessary? Is there a way to exclude
localhost from DNS lookup on the initial (pre-EHLO) connection?

Looking at the code there does not appear to be a way to select -nodnslookup based on the connecting IP address.

There is a facility for selectively setting environment variables based on the connecting IP address, the smtpaccess list (see makesmttpaccess). But, currently nodnslookup just looks only at the parameter.

In tcpd.c, you can try changing

   if (nodnslookup)     return;

to something like

   if (nodnslookup || getenv("NODNSLOOKUP"))  return;

and then put<tab>allow,NODNSLOOKUP=1

into the smtpaccess file.

But why don't you just run bind locally, and have it handle DNS resolution for local zones. You can have it listen only on local IP addresses, and thusly inaccessible from the Internet, and then get some benefits of a local DNS lookup cache.

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