This is a quick’n’dirty tutorial on how to get e-mail working on a basic Linux (Redhat/Fedora/CentOS) installation like the one you may get if you deploy a node in AWS with a predefined AMI (Amazon Machine Image).
The Redhat linux distributions come by default with postfix as MTA (Mail Transport Agent) and Cyrus IMAP as the MDA (Mail Delivery Agent) so this small tutorial is focused on these. Having the packages installed we can go to the configuration files:
/etc/postfix/main.cf: the postfix file, where we configure which e-mails we accept and what to do with them.
/etc/imapd.conf: settings for Cyrus IMAP – where to look for the mailbox passwords, what authentication mechanisms should be supported for pop/imap.
/etc/sasl2/smtpd.conf: for smtp authentication, if an e-mail relay is required (NB: this file can also be located in /usr/lib/sasl2/smtpd.conf).
There is also another file, /etc/cyrus.conf that usually contains the proper defaults upon installation so there may be no need to look into that. It contains settings like the supported protocols (pop3/pop3s/imap/imaps) and the lmtp socket location (the interface between postfix and Cyrus IMAP).
In the postfix configuration file, the following settings are essential:
# cat /etc/postfix.main.cf ... mailbox_transport = lmtp:unix:/var/lib/imap/socket/lmtp virtual_transport = $mailbox_transport virtual_mailbox_domains = /etc/postfix/virtual_domains virtual_mailbox_maps = hash:/etc/postfix/virtual_maps ...
Explanation: the e-mails for the domains specified in virtual_mailbox_maps are to be, before anything else, accepted for further processing. The final mailbox must be determined by looking into virtual_mailbox_maps (if no such mailbox is determined, some error will be returned to the sender). The effective delivery should be done through the socket specified at mailbox_transport.
Some example file contents:
# cat /etc/postfix/virtual_domains brainware.ro # cat /etc/postfix/virtual_maps email@example.com brainware.ro/john.doe
The “virtual_maps” file is to be “hashed” with postmap (have a hashmap generated out of it):
# postmap hash:/etc/postfix/virtual_maps
On the Cyrus IMAP side, we must first check that the daemon listens on the proper lmtp socket (by default it should):
# cat /etc/cyrus.conf | grep lmtp lmtpunix cmd="lmtpd" listen="/var/lib/imap/socket/lmtp" prefork=1
NB: at this point one may want to disable SELinux in order to allow for the socket communication between postfix and Cyrus IMAP.
Now to the other options; assuming that we want corporate-style addresses (name.surname@domain), the mailbox namespace should be changed so that the separator becomes a different character than “.” (dot). This can be achieved easily:
# cat /etc/imapd.conf ... unixhierarchysep: 1 #this changes the separator to "/" ...
Other settings to look for in imapd.conf, apart from this one:
# cat /etc/imapd.conf ... admins: cyrus #to be able to run cyrus admin sasl_mech_list: plain login cram-md5 digest-md5 ntlm gssapi #full house sasl_pwcheck_method: auxprop #to look in /etc/sasldb2 for passwords virtdomains: userid #to determine the domain from the mailbox name (user@domain) allowanonymouslogin: no #security allowplaintext: yes #allow older login methods (e.g. testing with telnet) ...
The services should be restarted to activate the new configuration:
# service postfix restart Shutting down postfix: [ OK ] Starting postfix: [ OK ] # service cyrus-imapd restart Shutting down cyrus-imapd: [ OK ] Exporting cyrus-imapd databases: [ OK ] Importing cyrus-imapd databases: [ OK ] Starting cyrus-imapd: [ OK ]
We are now pretty close to wrapping things up. First let’s check /etc/sasldb2 for the proper access rights (NB: the passwords are not stored with any encryption in there!):
# ls -la /etc/sasldb2 -rw-r----- 1 cyrus mail 12288 Oct 15 12:34 sasldb2
Preparing for running cyradm (Cyrus IMAP admin) for the first time, we need to set the password for the user “cyrus” – the admin that we have specified in /etc/imapd.conf:
# saslpasswd2 -c "cyrus" Password: ...
Now we can run cyradm and define our first mailbox and set its password:
# cyradm -u cyrus localhost IMAP Password: localhost> cm firstname.lastname@example.org localhost> quit # saslpasswd2 -c "email@example.com" Password: ....
Let’s now test the pop3 protocol (plaintext) login with telnet:
# telnet localhost 110 Trying ::1... Connected to localhost. Escape character is '^]'. +OK ... user firstname.lastname@example.org +OK Name is a valid mailbox pass ... +OK Mailbox locked and ready list +OK scan listing follows ... quit +OK Connection closed by foreign host.
Are we good? Mostly yes. We may also want to implement e-mail relaying with smtp authentication. First we need to enable it in postfix:
# cat /etc/postfix/main.cf ... smtpd_sender_restrictions = permit_mynetworks, permit_sasl_authenticated smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination, permit smtpd_sasl_auth_enable = yes smtp_sasl_security_options = noanonymous ...
Then we need to configure the authentication mechanism in /etc/sasl2/smtpd.conf (please also note the alternate location of this file at the top of the document):
# cat /etc/sasl2/smtpd.conf pwcheck_method: auxprop #use /etc/sasldb2 auxprop_plugin: sasldb mech_list: plain login cram-md5 digest-md5 ntlm gssapi #full house
After another postfix restart it’s very likely that now we are good. We have a simple e-mail service running with postfix and Cyrus IMAP. Thank you for your read!