Courier worksheet

For scalability, we are going to arrange for exim to deliver all local mail in Maildir format. This creates a subdirectory called "Maildir" in the user's home directory, which in turn contains three subdirectories: new, cur and tmp. Messages are written into tmp, moved to new when delivery is complete, and moved to cur when read. Each message has a long unique filename based on the hostname and the time of day.

Because each message is stored in a separate file, it is much faster for the pop3 daemon to start up every time a user connects. It also allows for safe delivery onto a shared (NFS) disk backend.

Exim does not include any software for retrieving mail from a mailbox, so we need to install additional software. Courier is a mail system which includes a number of packages. In fact it has its own MTA, but we will ignore this (it is still under heavy development, and does not have the flexibility needed for an ISP environment). The components we are interested in are the IMAP/POP3 servers and 'sqwebmail', the webmail server.

You can get the entire courier system as one package (including the MTA), or just the components. We will get the authlib, pop3/imap and webmail components separately.

Remember: in the command examples given below, commands shown with the prompt "$" should be run as your normal non-root userid. Only those commands with prompt "#" need to be run as root.

0. Reconfigure exim for Maildir local delivery

Edit /usr/exim/configure, find the local_delivery transport and modify it as follows:

  driver = appendfile
  directory = $home/Maildir
# group = mail
# mode = 0660

Optionally you could add further parameters to this transport which let you impose quotas on your users, for example to limit all users to 10 megabytes of storage each:
  maildir_tag = ,S=$message_size
  quota_size_regex = ,S=(\d+)
  quota = 10M
  quota_warn_threshold = 90%
(Aside: this quota mechanism relies on users not meddling with the quota information which is stored within their maildir; in other words, users with shell access would be able to bypass their quota if they knew what they were doing)

Remember to HUP your exim daemon. Now test out your new configuration by delivering to some local account on your machine:

$ /usr/exim/bin/exim -bt localuser
  router = localuser, transport = local_delivery
$ /usr/exim/bin/exim localuser
Here is a test
$ cd /home/localuser/Maildir
$ ls
cur     new     tmp
$ ls new,S=426
$ cat new/*
Return-path: <>
Here is a test

Note: once you have changed to Maildir delivery, you will find that any local Unix MUA (which looks for new messages in /var/mail/username) will no longer see your incoming mail. How to fix this depends on which MUA you are using. Some examples:
Edit /usr/local/etc/Muttrc and put:
set spoolfile="~/Maildir/"

Not supported by default, patch available.

To be confirmed
Has direct access to /var/mail or Maildir directly; can also use POP3/IMAP to retrieve new mail.

You can get the entire courier system as one package (including the MTA), or just the components. We will get the authlib, pop3/imap and webmail components separately.

As with most software packages under FreeBSD, you have a choice of installing directly from source, or using the ports system. If you install from source you have the most control over which version is installed and which compilation options are used. However installing from packages is easier, gives you a record of which files where installed where, and installs the files in the "normal" places you'd expect for a FreeBSD system. In particular, the commands get installed in
/usr/local/bin and /usr/local/sbin, which is already in your $PATH. We will use the ports in this lab.

1. Install courier-authlib

The courier packages now share a single authentication library, courier-authlib. This package is responsible for looking up usernames and passwords - it can retrieve this information from various locations, including Unix system accounts (authpam), SQL databases (authmysql and authpgsql), LDAP databases (authldap), and local file databases (authuserdb). Having a separate package means that the same authentication configuration can now be shared by both POP3/IMAP and Webmail.

# cd /usr/ports/mail/courier-authlib-vchkpw

# make
# make install
# make clean         (optional step - deletes temporary files in 'work' subdir)

2. Configure and start courier-authlib

courier-authlib runs a pool of authentication daemons which perform the actual work; courier-imap and sqwebmail communicate with these daemons via a socket. So the next thing we need to do is to start the daemons. First you need to edit /etc/rc.conf:

# vi /etc/rc.conf
add the following line:

Courier-authlib itself has a single configuration file, /usr/local/etc/authlib/authdaemonrc. For the purposes of this exercise, we will turn on authentication debugging.

# cd /usr/local/etc/authlib
# vi authdaemonrc
change this line:

To save resources, you can also configure the authdaemond process not to try any authentication mechanisms which you know you don't need. For example, if all your authentication is only via PAM for Unix system passwords, then you can remove all the others:


Now we are ready to start the authentication daemons:

# /usr/local/etc/rc.d/ start
Starting courier_authdaemond.
# ps auxwww | grep authdaemond
root     36787  0.0  0.2  1220  720  p1  S    10:40AM   0:00.00 /usr/local/sbin/courierlogger
-pid=/usr/local/var/spool/authdaemon/pid -start /usr/local/libexec/courier-authlib/authdaemond
root     36788  0.0  0.2  1464  880  p1  S    10:40AM   0:00.00 /usr/local/libexec/courier-authlib/authdaemond
root     36789  0.0  0.2  1464  880  p1  S    10:40AM   0:00.00 /usr/local/libexec/courier-authlib/authdaemond
root     36790  0.0  0.2  1464  880  p1  S    10:40AM   0:00.00 /usr/local/libexec/courier-authlib/authdaemond
root     36791  0.0  0.2  1464  880  p1  S    10:40AM   0:00.00 /usr/local/libexec/courier-authlib/authdaemond
root     36792  0.0  0.2  1464  880  p1  S    10:40AM   0:00.00 /usr/local/libexec/courier-authlib/authdaemond
root     36793  0.0  0.2  1464  880  p1  S    10:40AM   0:00.00 /usr/local/libexec/courier-authlib/authdaemond

ps shows one courierlogger process, and six authdaemond processes (one master, five workers). If you didn't see "Starting courier_authdaemond" then you made a typing error.

3. Test courier-authlib

You can test the authentication system by itself; the "authtest" command sends requests down the authentication socket, and displays the responses which come back. Test using any Unix login account which already exists on your system.

# authtest brian             -- find an account called 'brian'
# authtest brian foo         -- check 'brian' has password 'foo'
# authenumerate              -- list all accounts

Try it also with a non-existent username, and with both the right password and a wrong password for an account, to confirm that passwords are being validated properly.

Because we enabled login debugging, you should find that each authentication request generates detailled information in /var/log/debug.log showing how the request is passed to each module in turn. Have a look in this file to confirm:

# less /var/log/debug.log

Further documentation for courier-authlib can be found on the web at, and is also installed in

4. Install courier-imap

Using ports, building courier-imap is straightforward:

# cd /usr/ports/mail/courier-imap
# make
# make install
# make clean         (optional step)

5. Configure and start courier-imap

You can choose to run POP3, IMAP, or both. There is a configuration file for each one:


The default configuration is acceptable in most cases. However for a large server you may wish to increase the maximum number of concurrent connections from the default of 40, if you have fairly powerful hardware:

# cd /usr/local/etc/courier-imap
# vi pop3d
# vi imapd

Then, you need to enable the daemon(s) which you wish to run in /etc/rc.conf

# vi /etc/rc.conf
add the following line(s):

And then run the startup script(s):

# /usr/local/etc/rc.d/ start
Starting courier_imap_pop3d.
# /usr/local/etc/rc.d/ start
Starting courier_imap_imapd.

6. Test POP3 and IMAP

Test using telnet: POP3 and IMAP are both text-based layer 7 protocols and you can drive them by hand.

# telnet localhost 110
Connected to
Escape character is '^]'.
+OK Hello there.
user username
+OK Password required.
pass password
+OK logged in.
+OK 26 49857
retr 1
+OK 1073 octets follow.
... message
+OK Bye-bye.
Connection closed by foreign host.

# telnet localhost 143
Connected to
Escape character is '^]'.
Copyright 1998-2005 Double Precision, Inc.  See COPYING for distribution information.
a login username password
a examine inbox
* FLAGS (\Answered \Flagged \Deleted \Seen \Recent)
* OK [PERMANENTFLAGS ()] No permanent flags permitted
* OK [UIDVALIDITY 989061119] Ok
a logout
* BYE Courier-IMAP server shutting down
a OK LOGOUT completed
Connection closed by foreign host.

NOTE: The daemons will fail to login if the mail directory does not exist, although current versions do now provide an error message. Hence you need to have delivered at least one message to the user, to create their mailbox, before they can login (or use the 'maildirmake' command to create it). Look for logging messages in
/var/log/maillog and /var/log/debug.log.

7. pop3 and imap over SSL

If you wish, you can choose to allow pop3 over SSL (port 995) and imap over SSL (port 993). The advantage is that, for clients which support it, the traffic is encrypted. The disadvantage is higher CPU load on your server for the encryption of data.

To run SSL you will need a certificate. For testing purposes you can use a 'self-signed' certificate. The pop3d.cnf and imapd.cnf files contain the
parameters for the Snakeoil certificate. You may edit this for you environment, but note that it is not proper certificate signed by a recognised CA.
Run the following scripts which will generate them for you:

# cd /usr/local/etc/courier-imap
# cp pop3d.cnf.dist pop3d.cnf
# cp imapd.cnf.dist imapd.cnf
# mkpop3dcert
# mkimapdcert

Next, enable the SSL daemons in /etc/rc.conf:

# vi /etc/rc.conf
courier_imap_pop3d_ssl_enable="YES"       # pop3 over ssl, port 995
courier_imap_imapd_ssl_enable="YES"       # imap over ssl, port 993

Then you start the servers:

# /usr/local/etc/rc.d/ start
Starting courier_imap_pop3d_ssl.
# /usr/local/etc/rc.d/ start
Starting courier_imap_imapd_ssl.

You can't use a regular telnet to test it, because all your communication needs to be encrypted, but openssl has an SSL client you can use to make an encrypted connection for testing:

# openssl s_client -connect localhost:995

If you were running the service commercially you would be better to get a proper certificate signed by a recognised CA, rather than using a self-signed certificate.

8. Install sqwebmail

Webmail is a very useful service to offer your clients - although you may need to be careful of the extra CPU load and bandwidth it might use.

Unlike many other webmail solutions, which use POP3 or IMAP to talk to the mail store, sqwebmail reads and writes Maildir directories directly. This makes it efficient in the case where POP/IMAP and webmail run on the same box, or where there is an NFS-shared mailstore.

sqwebmail is feature-rich, very customisable through HTML templates and stylesheets, supports multiple languages, and is simple to install (it runs as a single CGI). Note however that it is still under very active development and hence subject to change quite frequently.

If you don't have it, install and test Apache first:

# cd /usr/ports/www/apache13-modssl
# make all install clean
# vi /etc/rc.conf
# /usr/local/etc/rc.d/ start

Check your Apache install is working by pointing a web browser at http://localhost/

Now install sqwebmail:

# cd /usr/ports/mail/sqwebmail
# make WITH_CHARSET=all
# make install
# make clean       (optional step)

The option "WITH_CHARSET=all" allows sqwebmail to view messages in a wide range of character sets. This increases the size of the binary by about one megabyte with the extra translation tables which are included.

One other change is required: add the following line to /etc/crontab to periodically clean out old sessions:

0 * * * * bin /usr/local/share/sqwebmail/

Sqwebmail comes in two parts: a small CGI stub which sends HTTP requests down a socket; and a pool of daemons which perform the actual work. The CGI stub is installed in /usr/local/www/cgi-bin-dist by default, and there are some graphics installed in /usr/local/www/data-dist/sqwebmail/*. These locations will work for a default Apache install, but if you have changed the normal Apache configuration (e.g. DocumentRoot) then you may need to copy these somewhere else.

9. Configure and start sqwebmail

sqwebmail's main configuration file is /usr/local/etc/sqwebmail/sqwebmaild - however you almost certainly don't need to change it.

As usual, you will need to enable the sqwebmail daemon in /etc/rc.conf, and then call its startup script.

# vi /etc/rc.conf

# /usr/local/etc/rc.d/ start
Starting sqwebmaild.

10. Test sqwebmail

If everything is working correctly, you should be able to point a web browser at http://localhost/cgi-bin/sqwebmail/sqwebmail and be presented with a login screen, where you can enter a username and password and login.

If this does not work:

Further documentation for sqwebmail can be found at and installed in /usr/local/share/doc/sqwebmail/

11. Optional extra exercises

11.1 Give your neighbour a mail account on your system. Let them check that they can collect mail using POP3, IMAP and Webmail.

11.2 When users send mail via sqwebmail, we would like their IP address and login name to be recorded in one of the Received: headers to provide a security audit. This can be done by modifying the script '' which sqwebmail uses to send all outgoing mail.

# vi /usr/local/share/sqwebmail/
exec /usr/sbin/sendmail -oi -t $DSN -f "$1"
exec /usr/sbin/sendmail -oi -t -f "$1" -oMr "$SERVER_PROTOCOL" -oMa "$REMOTE_ADDR" -oMt "$2"
# /usr/local/etc/rc.d/ restart

After making this change, compose a mail via the sqwebmail interface. When it is delivered, check the full headers and look for the bottom Received: header which should record the source of the mail.

Received: from [] (
        by with HTTP/1.1 (Exim 4.43 (FreeBSD))
        id 1Cln2K-0000Pd-EP
        for; Tue, 04 Jan 2005 11:39:40 +0000

11.3 A number of behaviours of courier-imap and sqwebmail can be changed by means of "account options". These can be set globally, and overridden for individual accounts (although not for Unix system accounts). Try the following:

# vi /usr/local/etc/authlib/authdaemonrc
# /usr/local/etc/rc.d/ restart

You can see account options for an account using "authtest username", and list all accounts together with their options using "authenumerate -o"

The available options are:


Disable shared folder functionality (hides the 'key' icon in sqwebmail). Shared folders need additional setting up, and only work for systems with virtual accounts.


Disable these types of access from this account


Webmail: disable ability for users to set the From: header on outgoing mail


Webmail: disable ability for users to change their passwords


Webmail: add an X-Sender: header to outgoing mail


Webmail: use a text-only interface


Webmail: disable the "return receipt" functionality (Exim does not support this so we must disable it anyway)

12. Security and virtual accounts (advanced)

In the simple examples above, we have been using the system password file to authenticate users. When creating new "E-mail only" accounts on your system, you probably don't want your users to be able to login to Unix using ssh or telnet. To disable this, you can simply create their accounts with a nonexistent shell.

# pw useradd username -m -s /nonexistent

To improve security and scalability further, you may wish to keep all your mail accounts in a completely separate password file or database; these users won't be known to Unix at all. The mail directories and messages have to be owned by some Unix user, so we can choose to make them all owned by the 'mailnull' user.

You have many choices of authentication module: for example an LDAP database, a mysql or postgresql database, or a local dbm file (courier supports a format called 'userdb'). These databases will contain the mail login usernames and passwords, and the directories where the mail will be stored. You'll need to configure Courier to use this new database as a login source, and also configure Exim to read this source to determine whether a user exists and where to deliver mail to.

The following example shows how to do this with authuserdb, which is described in "man makeuserdb" and "man userdb". We'll create two users in the table, and two empty maildirs. We'll make them under directory /var/vmail01, which we'll assume is a fast SCSI hard drive. To support many domains we'll make the POP3 login be user@domain instead of just a username. We'll have a separate directory for each domain, and also use the first two characters of the username as subdirectories, so that if we have ten thousand users for one domain we don't end up with ten thousand accounts within the same directory.

First we make an empty userdb, and make sure it's not world readable

# touch /usr/local/etc/userdb
# chmod 600 /usr/local/etc/userdb

Next we create some accounts and empty maildirs:

# userdb set uid=26 gid=6 home=/var/vmail01/
# userdbpw -md5 | userdb set systempw
Password: wibble
Reenter password: wibble
# mkdir -p /var/vmail01/
# maildirmake /var/vmail01/
# chown -R mailnull:mail /var/vmail01/

# userdb set uid=26 gid=6 home=/var/vmail01/
# userdbpw -md5 | userdb set systempw
Password: boing
Reenter password: boing
# mkdir -p /var/vmail01/
# maildirmake /var/vmail01/
# chown -R mailnull:mail /var/vmail01/

(In real life you'd write a script to automate this process for creating new accounts). The compulsory fields we need to provide are "home" and numeric "uid" and "gid"; these are documented in "man makeuserdb".

Now we check the userdb contents - it's just a plain text file - then convert it into userdb.dat which is the fast indexed version that authuserdb reads. Note that authuserdb requires encrypted passwords.

# cat /usr/local/etc/userdb     home=/var/vmail01/f/|systempw=$1$96YgsKCe$0oey3dzw0mztdby6ICFxR0|gid=6|uid=26    home=/var/vmail01/f/|systempw=$1$nXNJyXcB$1mItZjaFmOV/3YHby8SGu0|gid=6|uid=26
# makeuserdb
# ls -l /usr/local/etc/userdb.dat
-rw-r--r--  1 root  wheel  65536 May 14 16:23 /usr/local/etc/userdb.dat

Now you can configure courier-imap and sqwebmail to login using these new accounts. If you leave authpam in the configuration then you can login with both the userdb accounts and the system accounts. If you removed authuserdb from the list of authentication modules earlier, then put it back now:

# vi /usr/local/etc/authlib/authdaemonrc
authmodulelist="authuserdb authpam"
# /usr/local/etc/rc.d/ stop
# /usr/local/etc/rc.d/ start

At this point you should be able to login using one of the new accounts, and see the (empty) mail directory.

# telnet localhost 110
Connected to
Escape character is '^]'.
+OK Hello there.
+OK Password required.
pass wibble
+OK logged in.
+OK 0 0
+OK Bye-bye.
Connection closed by foreign host.

If this doesn't work, follow the instructions for debugging authentication problems given earlier. Remember the authtest and authenumerate commands, and look in
/var/log/maillog and /var/log/debug.log

Now all that is necessary is for Exim to know how to deliver messages to these users. There are a couple of ways this can be done; Exim can be configured to read
/usr/local/etc/userdb.dat directly, or it can be configured to talk to courier's authdaemond process. Both can be set up using Exim's general-purpose configuration language.

It's a tribute to the flexibility of Exim that this can be done even though Exim does not have any specific features for using userdb or authdaemond, although it does mean that the configuration looks complicated at first glance. If you want to find out in detail how these configurations work, you will need to read the Exim documentation carefully.

Firstly, we need a separate list of which domains which need lookups in the userdb database; for simplicitly we will make this a plain text file which is searched linearly. If this gets large it can be converted into an indexed database.

# vi /usr/exim/vdomains

# vi /usr/local/etc/exim/configure
Change the local domains setting to say:
domainlist local_domains = @ : lsearch;/usr/exim/vdomains

To read /usr/local/etc/userdb.dat directly, we use the "dbmnz" lookup type. It's made a bit awkward because courier uses a vertical bar to separate fields, whereas Exim's "extract" operator expects fields separated by spaces, but we can use the regular expression substitution operator (sg) to convert this into the form we want.

[put this immediately after 'begin routers']
  driver = accept
  transport = local_delivery_userdb
  domains = lsearch;/usr/exim/vdomains
  address_data = ${lookup{$local_part@$domain}dbmnz{/usr/local/etc/userdb.dat}\
     {${sg{$value}{([^=]+)=([^|]+)\\|?}{\$1="\$2" }}}fail}
  # note the space between "\$2" and }}}

# If the address_data lookup succeeds, then we'll go to the transport.
# But if the address_data lookup fails, then we fall through to here; all
# remaining addresses in vdomains need to be bounced.

  driver = redirect
  domains = lsearch;/usr/exim/vdomains
  data = :fail:unknown user

[put this anywhere after 'begin transports']
  driver = appendfile
  directory = ${extract{home}{$address_data}}/${extract{mail}{$address_data}{$value}{Maildir}}/
  user = ${extract{uid}{$address_data}}
  group = ${extract{gid}{$address_data}}
  maildir_tag = ,S=$message_size
  quota_size_regex = ,S=(\d+)
  quota = ${if match{${extract{quota}{$address_data}}}{([0-9]+)S}{$1}{}}
  quota_filecount = ${if match{${extract{quota}{$address_data}}}{([0-9]+)C}{$1}{}}
  quota_warn_threshold = 90%

Once you've done this, test using

# /usr/exim/bin/exim -bt is undeliverable:
  unknown user
# /usr/exim/bin/exim -bt
  router = userdb, transport = local_delivery_userdb
# /usr/exim/bin/exim -v
Subject: test

  <= U=root P=local S=302
  => fred <> R=userdb T=local_delivery_userdb

It's even possible for Exim to send a request to courier's authdaemond process to perform the lookup, which has the advantage that it will work for any courier authentication module or combination of modules. However there are similar difficulties with parsing the response properly, and the debug output you get from courier is not as good as Exim produces.

[put this immediately after 'begin routers']

# We use manualroute with empty route_data as a dummy router, just to
# set address_data to the value read from the socket.

  driver = manualroute
  route_data =
  domains = lsearch;/usr/exim/vdomains
  address_data = ${sg{${readsocket{/usr/local/var/spool/authdaemon/socket}\
        {PRE . exim $local_part@$domain\n}}}{([^=]+)=([^\n]+)\n}{\$1="\$2" }}
  # note the space between "\$2" and }}

# Next, if the response contains HOME= then we know this address is valid
# and we can send it to the local delivery transport

  driver = accept
  transport = local_delivery_courier
  domains = lsearch;/usr/exim/vdomains
  condition = ${extract{HOME}{$address_data}{1}{0}}

# Otherwise, the address was bad. If it contains FAIL then it's a permanant
# failure, otherwise it's a temporary failure

  driver = redirect
  domains = lsearch;/usr/exim/vdomains
  data = ${if match{$address_data}{FAIL}{:fail:unknown user}fail}

  driver = appendfile
  directory = ${extract{HOME}{$address_data}}/${extract{MAILDIR}{$address_data}{$value}{Maildir}}/
  user = ${extract{UID}{$address_data}}
  group = ${extract{GID}{$address_data}}
  maildir_tag = ,S=$message_size
  quota_size_regex = ,S=(\d+)
  quota = ${if match{${extract{QUOTA}{$address_data}}}{([0-9]+)S}{$1}{}}
  quota_filecount = ${if match{${extract{QUOTA}{$address_data}}}{([0-9]+)C}{$1}{}}
  quota_warn_threshold = 90%

Finally, if you are building a box where you know that all the mail will be owned by the 'exim' or 'mailnull' user, then in fact the daemons no longer need to run with root privileges - they can run as the exim user only, as they never have to change to any other userid. This in theory should make your system more secure.

There are some steps you can take to do this for each daemon:

Set the 'deliver_drop_privilege' option in /usr/exim/configure. You'll probably also have to adjust permissions so that the exim user has access to the authdaemon socket:
# chown mailnull:mail /usr/local/var/spool/authdaemon
Edit /usr/local/etc/courier-imap/pop3d and imapd and add "-user=mailnull" to couriertcpd options, i.e.
TCPDOPTS="-nodnslookup -noidentlookup -user=mailnull"
In principle you can run the sqwebmaild startup script as user 'mailnull', using su:
# su mailnull -c "/usr/local/etc/rc.d/ start"
However, for this to work, you first need to sort out some permissions issues: you need to recompile and reinstall sqwebmail using
# make WITH_CACHEOWNER=mailnull
and change the entry in /etc/crontab to run as user 'mailnull' not user 'bin'. You also need to change ownerships of the configuration files and temporary directory:
# cd /usr/local/etc
# chown mailnull:mail sqwebmail/sqwebmaild authlib/authdaemonrc
# chown -R mailnull:mail /usr/local/var/sqwebmail