Serendipity

What Is It ?

Qmail is a modern replacement for sendmail, written by Dan Bernstein, who also has a web page for qmail. qmail is a secure package. There was a $1,000 prize for anyone who can show otherwise, which went unclaimed. Qmail is being increasingly used by savvy and security-conscious admins on the net. You can read more on qmail, including many other useful patches which work with it, on the Qmail Users' web site.

I've created a simple patch to qmail, which greatly simplifies the use of it on dial-up hosts. Until now it was not quite ideal for dial-up use, as it assumes the remote delivery path is always available, which when using dial-on-demand meant by default it would dial up straight away to deliver. This required additional software such as the use of serialmail, or else stopping and restarting qmail upon connection to pick up new parameters.

With this patch, which modifies only qmail-send, it's possible to hold and release the remote delivery queue at will, using a new parameter "holdremote", which will be re-read when qmail is kicked with a SIGHUP. Thus outgoing mail can be queued as normal, but with the holdremote flag set it will not be delivered. When going online, arrange in your ip-up script for the parameter to be unset, tickle qmail-send with a SIGHUP, and Roberta is your mother's senior sibling - all queued remote mail will be immediately rescheduled. When you go offline, set the flag again and re-SIGHUP qmail-send: any futher mail will be queued for next time.

There is also a holdlocal option for symmetry and because it virtually fell out of the code, should anyone find a use for it.

Why use this patch ?

Advantages of my method over serialmail are:

• serialmail is additional software and configuration beyond the basic qmail package which a user may download first time; with my patch there is no additional software (except the patch, obviously) or configuration.
• serialmail requires a looping call should more mail be queued whilst online; this requires installation of still another DJB package (daemontools). With my patch, further mail is delivered instantly by qmail as designed, with no extra load or complexity.
• multiple serialmail instances can end up being run at the same time (for instance if using diald set up by a naive admin) unless, again, daemontools are used, thus resulting in duplicated mail; with my patch, this cannot happen as only a qmail parameter is changed.
• using serialmail to forward to a smarthost restricts the amount of mail which can be sent at once; by sending the mail directly it can potentially be sent faster, as the full bandwidth of your outgoing link can be used by dispatching several mails at the same time.
• Using a smarthost introduces an additional Single Point of Failure into your outgoing mail. Dispatching it directly to remote hosts using their MX records avoids this.

You do not need (though may still want) this patch if:

• you have a permanent network connection for outgoing remote mail.
• you have a dial-up link but not over IP.
• you send remote mail immediately it's queued via a dial-on-demand dial-up IP link and are happy to continue to do so (typically applies to people who don't pay their own phone charges :-)).
• you send remote mail to different destinations via two or more different links (e.g. internal-to-work and net-at-large). In this case you will require serialmail, which is an excellent package for this purpose.

Download this Patch

At the time of writing, qmail is at version 1.03. There are versions of this patch for use with both qmail 1.01 and qmail 1.03. You need the qmail sources, to which you add this patch using the following command:

zcat qmail-hold-<version>.pat.gz | patch -p0

Then compile and install - simple as winking !

Using the Patch

Here are some script excerpts which illustrate how I use it.

Firstly, an extract from the script which runs when my PPP link to my ISP comes up (in my case /etc/diald/ip-up):

{ rm -f /var/qmail/control/holdremote
/usr/bin/killall -HUP qmail-send
} &

The following should be added to the PPP down script, for me that is /etc/diald/ip-down:

{ echo 1 >/var/qmail/control/holdremote
chmod 644 /var/qmail/control/holdremote
/usr/bin/killall -HUP qmail-send
} &

Finally, in order to ensure that Qmail starts up in the held (offline) state, you need to add the following to the script which starts it (for me this is /etc/rc.d/init.d/sendmail.rc, in the "start" case):

echo 1 >/var/qmail/control/holdremote
chmod 644 /var/qmail/control/holdremote