Contents
Outgoing mail
Incoming mail
fetchmail
sendmail
procmail
IMAP server
Outgoing mail
Mail gets from the laptop (nettle/rocket) to the outside world through
the following sequence of steps:
- The email client (e.g. Thunderbird) on the laptop connects to a
"local SMTP server" (see below) and transfers the email to it.
- The local SMTP server does a DNS lookup of the domain part of the
email recipient to find the remote SMTP server for that domain.
For example, if the email is addressed to timufea@fastmail.co.uk, the
local SMTP server will do a DNS lookup of fastmail.co.uk to find which
SMTP server handles mail for that domain.
- The local SMTP server connects to the remote SMTP server and
transfers the email to it.
- The remote SMTP server may hand the email on to yet another SMTP
server, or it may hand it to a local delivery agent, which will put it
the recipient's mailbox.
Note that the local SMTP server is
acting both as a server (to Thunderbird) and as a client (to the remote
SMTP server). It's possible to have a whole chain of SMTP servers
relaying mail in this way before it reaches its final destination.
There are three main possibilities for what we may use as the "local
SMTP server":
- sendmail
running on the laptop
- sendmail running on sage
- an ISP's SMTP
server (e.g. mailhost.zen.co.uk
at Zen Internet)
Using sendmail on the
laptop is the simplest method, and the one that is most independent of
other machines.
The disadvantage of using sendmail
on the laptop is that outgoing mail (from any of our machines) appears
to come
from our fixed IP address (82.71.13.216). This IP address maps (under reverse DNS)
to sage.wingpath.co.uk.
But sendmail introduces
itself (using the SMTP
HELO command) to the
remote SMTP server as rocket.wingpath.private
(say). Some remote SMTP servers will not accept mail unless the
sender's HELO and
reverse-DNS hostnames match, and so will reject mail coming directly
from the laptops.
You can use either of the other alternatives to get around this
problem. Alternatively, you can upgrade to sendmail 8.14.0 or later and
configure the HELO name (see below for how to do this).
Incoming mail
Mail from the outside world ends up in IMAP mailboxes on sage through
the following sequence of steps:
- Someone
sends an email, which ends up in a mailbox at one the the sites where
we have email accounts (e.g. the wingpath.co.uk website,
fastmail.co.uk). These sites have POP3 or IMAP servers that allow us to
access the emails remotely.
- fetchmail polls external POP3
and IMAP mail servers, downloads any mail that it finds, and passes it
to sendmail.
- sendmail passes the mail to
procmail.
- procmail calls SpamAssassin to test whether the mail is spam,
and then it passes to the IMAP server.
All these processes (fetchmail,
sendmail, procmail, and the IMAP server)
run on sage.
The email client (Thunderbird/Mozilla) on the laptop connects to the
IMAP server on sage to fetch and display emails.
It is now also possible for an SMTP client to connect directly to
sendmail on sage, instead of going via the wingpath.co.uk website. Only
steps 3 and 4 occur when this is done. This works for email domains
(such as wingpath.co.uk) for which sage is the primary mail server.
Several configuration changes were needed to
enable this - see Allowing
direct mail to sage below.
fetchmail
fetchmail is provided in the
Slackware distribution.
fetchmail is configured using the file /etc/fetchmailrc.
This mainly lists the mail servers to be polled, the usernames &
passwords on each server, and the corresponding usernames on sage. See
the manual
page for (full!) details. Since the file contains passwords, it
should be readable only by root.
fetchmail is started from /etc/rc.d/rc.local.
sendmail
sendmail is an "SMTP server" -
essentially an email router, but with lots of bells and whistles added.
sendmail is provided in
the Slackware distribution, and is pre-configured to handle both
outgoing and incoming mail. Incoming mail is, by default, delivered to
local mailboxes (i.e. ordinary files in the local filing system).
Configuration of sendmail
is notoriously complicated - full details are on the sendmail website.
The following describes the little that I have done on nettle and sage.
The main sendmail
configuration file is /etc/mail/sendmail.cf,
but this shouldn't be edited directly (not that you would be tempted
to, since it is totally incomprehensible). You should instead edit the
file /usr/share/sendmail/cf/cf/sendmail.mc (which is only
slightly more comprehensible), and then generate /etc/mail/sendmail.cf
from it using the following procedure:
su -
cd /usr/share/sendmail/cf/cf
vim sendmail.mc
m4 sendmail.mc >sendmail.cf
cp sendmail.cf /etc/mail
/etc/rc.d/rc.sendmail restart
If sendmail.mc doesn't exist, make a copy of sendmail-slackware.mc
and use that.
I have made the following changes to the sendmail configuration on
nettle:
FEATURE(`local_procmail',`',`procmail
-t -Y -a $h -d $u')dnl
in sendmail.mc has been
changed to:
FEATURE(`local_procmail',`',`procmail
-t -Y -m USER=$u /etc/procmailrc')dnl
to stop
procmail
delivering to local mailboxes on nettle, and instead act simply as a
filter using rules from
/etc/procmailrc.
I did this so that I could re-direct mail arriving at nettle
to the
IMAP server on sage instead.
See
procmail below for how the re-direction
was configured.
define(`confHELO_NAME',
`sage.wingpath.co.uk')dnl
has been added immediately after the other
define lines. This tells
sendmail to identify itself to
the remote SMTP server (using the
HELO
command) as
sage.wingpath.co.uk
instead of
nettle.wingpath.private.
This is needed because outgoing mail appears to come
from our fixed IP address (82.71.13.216), which maps (under
reverse DNS)
to
sage.wingpath.co.uk.
Some SMTP servers will not accept mail unless the sender's
HELO
and reverse-DNS hostnames match.
Note that you need
sendmail
8.14.0 or later for this to work.
On sage, I have made the confHELO_NAME
change, and also the following:
FEATURE(`local_procmail',`',`procmail
-t -Y -a $h -d $u')dnl
has been changed to:
FEATURE(`local_procmail',`',`procmail
-t -Y -m USER=$u /etc/procmailrc')dnl
to stop
procmail
delivering to local mailboxes on sage, and instead act simply as a
filter using rules from
/etc/procmailrc.
This was done so that mail could be filtered using
SpamAssassin and then sent to the
IMAP server. See
procmail
below for how this was configured.
- The accept_unresolvable_domains,
nocanonify and accept_unqualified_senders
features have been enabled (uncommented). These were all found to be
needed to stop sendmail
rejecting mail, which fetchmail
then tried to deliver again, thus producing an infinite loop.
There are also several other sendmail
configuration files in /etc/mail.
If you edit any of these files, you have to run make in the /etc/mail directory afterwards.
procmail
IMAP server
Allowing direct mail to
sage
It is now also possible for an SMTP client to connect directly to
sendmail on sage, instead
of going via the wingpath.co.uk website. The DNS configuration (both on
vps.wingpath.co.uk, our primary DNS server, and on sage) has been
modified to make sage our primary mail host - this involved adding MX
entries.
The firewall setup on fennel had to be modified to forward SMTP
connection requests to sage. This was done by adding a rule in "Firewall Rules->Inbound services".
Several configuration changes were needed on sage:
- The PERMIT line
in the /etc/rc.d/rc.firewall
script was edited to include 25/tcp,
and then the rc.firewall
script was executed. This allows connections to port 25 (i.e. the SMTP
port) through sage's firewall.
- The following lines were added to /etc/mail/local-host-names:
sage.wingpath.co.uk
wingpath.co.uk
wingpath.com
This tells sendmail that
any mail arriving for these "host-names" should be delivered locally
(i.e. to sage).
- The following lines were added to /etc/mail/aliases:
frank: fog
sales: fog
support: fog
webadmin: fog
wingpath: fog
accounts: fog
f.ogorman: fog
fogorman: fog
frank.ogorman: fog
cromo: monica
m.croucher: monica
mcroucher: monica
monica.croucher:monica
monika: monica
and then make was
executed in the directory /etc/mail.
This tells sendmail
which mailbox to deliver mail to (e.g. mail for username frank@sage.wingpath.co.uk is
delivered to the fog
mailbox). Note that sendmail
rejects mail to unknown usernames.
- The line "sendmail: ALL"
was added to the file /etc/hosts.allow.
This file is one of the "TCP wrappers" configuration files used
primarily by inetd - but
it seems to be used by sendmail
too. The line was added to allow any host connect to sendmail.
- The command "/etc/rc.d/rc.sendmail
restart" was executed to get sendmail to re-read its
configuration files.