The Boston Diaries

The ongoing saga of a programmer who doesn't live in Boston, nor does he even like Boston, but yet named his weblog/journal “The Boston Diaries.”

Go figure.

Thursday, September 14, 2000

Expanding on EXPN

Once again Spring is having problems with mailing her journal entries, only this time it's with Yahoo.

What is it? EXPN is all the rage now? Must expand email addresses?


So I decide to fix it once and for all. I check the configuration file for Sendmail and don't see any obvious way to disable the EXPN command. Not wanting to hack the source code to remove the EXPN command I figure the next easiest way is to hack the actual binary and change any occurence of E-S-P-N-NULBYTE such that sendmail will no longer be able to actually respond to the EXPN command. I have some software I wrote years ago that makes this relatively easy to do.

So, I find the occurences of E-X-P-N-NULBYTE and make the changes.

No go. Sendmail still reponds to the EXPN command.

Okay, so next it's occurrences of e-x-p-n-NULLBYTE and that's when I find the curious string “noexpn” in the executable. Hmmmmmmmm … I think to myself. Might there actually be a way to disable the EXPN command?

So I search the site for noexpn and I find this:


Set the privacy options. “Privacy” is really a misnomer; many of these are just a way of insisting on stricter adherence to the SMTP protocol. The options can be selected from:

Allow open access
Insist on HELO or EHLO command before MAIL
Insist on HELO or EHLO command before EXPN
Disallow EXPN entirely
Insist on HELO or EHLO command before VRFY
Disallow VRFY entirely
Restrict mailq command
Restrict -q command line flag
Don't return success DSNs
Disallow essentially all SMTP status queries
Put X-Authentication-Warning: headers in messages

The goaway pseudo-flag sets all flags except restrictmailq and restrictqrun. If mailq is restricted, only people in the same group as the queue directory can print the queue. If queue runs are restricted, only root and the owner of the queue directory can run the queue. Authentication Warnings add warnings about various conditions that may indicate attempts to spoof the mail system, such as using an non-standard queue directory.

I don't know if it's A Good Thing or A Bad Thing that you can learn more about a program from scanning the executable than you could probably get reading the documentation.

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