Doorknob is meant to be used on machines where you want to forward all of the mail to one remote mail account. Example use cases are laptops where you want to forward all your mail to say gmail no matter who it is really for. Or routers. Or work machines with overly strict email policies :(
Doorknob has one simple config file and comes with a sample file that is fairly well documented. All mail goes to the One True Server (smtp-server) from the One True User (mail-from). Doorknob can even overwrite the From: in the header for those overly strict work policies.
If the recipient has an @ in the email (fred@gmail.com) then the address goes through unchanged. However if the recipient is local (root) then it is sent to the One True User. This means all your cron output gets sent to the right place.
Hint: It is recommended that HOSTNAME be set with the fully qualified host name.
Doorknob tries to be distro agnostic. This means you have to work a bit harder to install it.
Check the user settable portion at the top of the Makefile and make sure the settings are correct. The defaults should work. I recommend using BearSSL. See README.BearSSL.
make && sudo make install
A file called setup.sh should have been created. It should have all the defaults from the Makefile so that it is consistent with the executables. Take a good look at it since it must be run as root. When you are happy with it run:
sudo sh ./setup.sh
You now must edit /etc/doorknob.conf (or whatever you set CONFIGFILE to). This file will most likely contain your username and password, so keep the permissions tight.
You can test doorknob in the foreground using `sudo doorknob -d'. Send a couple of mails and make sure there are no errors.
You can now use your favorite init script or supervisor system to start doorknob. A sample rc.doorknob is included. The rc.doorknob was tested with Slackware and also with Ubuntu. For Ubuntu copy rc.doorknob to /etc/init.d/doorknob. This should work for RedHat/Centos too, but I haven't tested it.
Doorknob is not very bright. A basic assumption is that you are sending to a commercial grade mail server which will already be doing a lot of (too much?) checking and filtering of the email. Or you are on a local network where security is not really a consideration. So doorknob lets them do that job and just does as little as possible. Maybe doorknob isn't as dumb as he looks.
A note about SSL. Doorknob can be run with no certificates. This is easier, but we do not guarantee we are talking to the right server.
You can specify the cert files with 'cert' entries in the config file. You should have at least two files: one from the server and the trusted root.
Currently doorknob supports BearSSL. To verify the certs you can use the brssl program from BearSSL.
brssl verify -CA <cert-root-file> <server-cert-file>
You can safely skip this section. You only need this if you want to interface with doorknob directly.
I am going to assume the defaults here.
Doorknob watches the /var/spool/doorknob/queue directory for new files and then sends them. You should create the file somewhere else (/var/spool/doorknob/tmp is used by sendmail) and then move it into the queue directory.
The file name doesn't really matter as long as it isn't a hidden file. The recommended format should guarantee no collisions (except over NFS):
<seconds>.<microseconds>.<pid>
The format of the file is:
<raw to address ...>
<empty line>
<stuff>
A raw address is fred@gmail.com, not <fred@gmail.com> and certainly
not Fred Flintstone <fred@gmail.com>.
Seriously, unless rewriting the From: line, doorknob doesn't care what comes after the empty line.
A note about the raw to address. Doorknob assumes that the real SMTP server does not know who root, lisa, or fred are on your machine. To addresses that have an @ in them are sent through unmolested. But the first occurrence of an address without an @ is replaced with the mail-from address. All other occurrences are silently dropped. The To: field in the header should still contain the correct addresses.
The mail server I use now requires the From and the RCPT_TO match. The crontab daemon I use does not add the From. This created a problem and all my crontab email was rejected.
Since crontab is my main use case for doorknob, I had to fix it.
The doorknob sendmail will now add From: unknown if there is no From
and a Date if there is no date. Since I was already updating, might as
well add the date since I sort by date in my mail client.
I then enabled rewrite-from in my /etc/doorknob.conf. This takes the
unknown and corrects it. I am now getting crontab emails again.