Differences between revisions 53 and 54
Revision 53 as of 2016-02-17 21:04:56
Size: 17188
Comment: change/add links with anchor
Revision 54 as of 2016-02-28 15:20:27
Size: 17199
Comment: minor formatting fixes
Deletions are marked like this. Additions are marked like this.
Line 275: Line 275:
Line 290: Line 289:
 1. do not use TLS/SSL at all (only unsecure connections are available)
 1. use TLS/SSL, if possible. Fall back to unsecure connections otherwise.
 1. only allow TLS/SSL (unsecure connections are not available)

The second option (called [[http://en.wikipedia.org/wiki/STARTTLS | STARTTLS ]]) is recommended for general purpose mail servers. It provides some sort of "compatibility mode". Secure data transfer is enabled but not enforced.
 1. Do not use TLS/SSL at all (only unsecure connections are available).
 1. Use TLS/SSL, if possible. Fall back to unsecure connections otherwise.
 1. Only allow TLS/SSL (unsecure connections are not available).

The second option (called [[WikiPedia:STARTTLS|STARTTLS]]) is recommended for general purpose mail servers. It provides some sort of "compatibility mode". Secure data transfer is enabled but not enforced.
Line 300: Line 299:
Recent postfix versions employ the parameter [[http://www.postfix.org/postconf.5.html#smtpd_tls_security_level | smtpd_tls_security_level]] to control TLS encryption (valid values are ''none'', ''may'' or ''encrypt'')

Previously two parameters ([[http://www.postfix.org/postconf.5.html#smtpd_use_tls | smtpd_use_tls]] and [[ http://www.postfix.org/postconf.5.html#smtpd_enforce_tls | smtpd_enforce_tls ]]) were used. They can be unset. See also Bugreport DebianBug:520936.

With following commands TLS is enforced (no STARTTLS) and the old configuration parameters are reset to default values: {{{
Recent Postfix versions employ the parameter [[http://www.postfix.org/postconf.5.html#smtpd_tls_security_level |smtpd_tls_security_level]] to control TLS encryption (valid values are ''none'', ''may'' or ''encrypt'').

Previously two parameters ([[http://www.postfix.org/postconf.5.html#smtpd_use_tls |smtpd_use_tls]] and [[ http://www.postfix.org/postconf.5.html#smtpd_enforce_tls |smtpd_enforce_tls ]]) were used. They can be unset. See also the Debian bug report DebianBug:520936.

With the following commands TLS is enforced (no STARTTLS) and the old configuration parameters are reset to default values: {{{
Line 348: Line 347:
It seems ''fetchmail'' is not able to setup a TLS connection to Postfix. (Not to be confused with ''fetchmail'' 's capabilities to ''fetch'' mails via TLS-connections.) It seems ''fetchmail'' is not able to setup a TLS connection to Postfix. (Not to be confused with ''fetchmail''{{{}}}'s capabilities to ''fetch'' mails via TLS connections.)
Line 377: Line 376:
Alternatively ''fetchmail'' can be instructed to use an external TLS-capable program*) to forward mails. This is not handled here. And if fetchmail and Postfix run on the same machine it does not make much sense anyway.

*) so called "lightwight" MTAs like ''msmtp'' or ''sSMTP''
----
Alternatively ''fetchmail'' can be instructed to use an external TLS-capable program<<FootNote(The so called "lightweight" MTAs like ''msmtp'' or ''sSMTP''.)>> to forward mails. This is not handled here. And if fetchmail and Postfix run on the same machine it does not make much sense anyway.
Line 441: Line 439:
----

Translation(s): English - Italiano


Postfix is a Secure Mail Transfer Agent.

Postfix

Installing and Configuring Postfix on Debian

  • Install postfix (this will remove exim since there can't be two mail systems). (If you have a website, choose "Internet Site" if the configuration prompts ask for it.):

apt-get install postfix
  • Check the log mail.log, mail.err, mail.info, mail.warn to see if postfix runs.

cat /var/log/mail.log
  • Add your domain to the config files, so others can't abuse your mailsystem:

postconf -e "myorigin = example.com"
  • Add your hostname (computer name). (Use command "hostname" at the command-line to display your hostname if not sure.)

postconf -e "myhostname=server1.example.com"
  • Now add the domain names that your system will handle.

postconf -e "relay_domains = example.com, example2.com, example3.com"
  • Reload Postfix Server:

postfix reload
  • Test the mailserver. Type

telnet localhost 25
  • You should see:

Trying 127.0.0.1...
Connected to localhost.localdomain.
Escape character is '^]'.
220 server1.example.com ESMTP Postfix (Debian/GNU)
  • Send an email to yourself:

mail from:<you@youremail.com>
rcpt to:<user@example.com>
data
To: user@example.com
From: you@youremail.com
Subject: Hey my first email
This is my first email on debian postfix after installing configuring it.
It was easy.
  • To end data hit enter, type in a dot, and hit enter again:

.
  • Then

quit
  • You're done. Type "mail" in the command-line terminal and see if you have some.
    • Now let's go to the next step:
    • If you have a router with firewall, enable port 25 and forward that port to your computer.
    • Enter your MX records in your domain provider. (e.g. godaddy.com or dnspark.com)
    • Check your mx records: go to http://www.iptools.biz/ locate "DNS lookup". From pulldown menu select "MX". Type in your domain name (ex. example.com). You should see some records there. If you don't see any MX records go back to previous step. You have to have MX record otherwise other computers won't be able to see you when sending emails.

    • Useful commands:

qshape
mailq
qshape deferred
postsuper
postsuper -r ALL   (requeue all emails)

anti-spam: smtp restrictions

  • The first fight starts at your server so this should be added to any email server that you setup. This makes sure that any computer that tries to send an email to you has a valid domain name. (spammers use ex. myhomepc as a domain name. This will stop them from spamming you.)
  • Insert this in your /etc/postfix/main.cf:

smtpd_recipient_restrictions = reject_invalid_hostname,
        reject_unknown_recipient_domain,
        reject_unauth_destination,
        reject_rbl_client sbl.spamhaus.org,
        permit

smtpd_helo_restrictions = reject_invalid_helo_hostname,
        reject_non_fqdn_helo_hostname,
        reject_unknown_helo_hostname

anti-spam: Using RBL Lists

  • RBL list is a list of domains which says whether they are spammers or not.

Insert this in your /etc/postfix/main.cf:

smtpd_client_restrictions = reject_rbl_client dnsbl.sorbs.net

See what rbl is about: http://www.us.sorbs.net/mailsystems/postfix.shtml

and avoid such blacklists

Debian Anti-Spam Anti-Virus Gateway Email Server

Forward Emails

  • Forwarding emails can be done via alias file located in /etc/aliases

  • Run this command to add alias maps:

postconf -e "alias_maps = hash:/etc/aliases"
  • You can now add your user to /etc/aliases like this:

root: lucas
  • You can forward your emails to a different email address

lucas: myemail@example.com
  • Or you could forward your email while still getting a copy in your local mailbox

lucas: lucas myemail@example.com
  • When done adding aliases run this command which will create a database like file.

newaliases
  • Reload postfix

/etc/init.d/postfix reload

Virtual Emails

  • If you want virtual emails such as abuse or postmaster you can do the following.

  • Run this command to add virtual alias maps:

postconf -e "virtual_alias_maps = hash:/etc/postfix/virtual"
  • Create a /etc/postfix/virtual file

vi /etc/postfix/virtual
  • And add your virtual emails

postmaster info@example.com
abuse info@example.com
someemail lucas
  • Create a database like file out of it

postmap /etc/postfix/virtual
  • Reload postfix

/etc/init.d/postfix reload

Maildir

  • To use maildir format in your mailbox which creates separate files for each email you can use the following commands:
  • Maildir has few advantages over mbox format. (It keeps emails in separate files, allows for multiple applications to read mail, etc.)
  • Issue these commands:

postconf -e "home_mailbox = Maildir/"
postconf -e "mailbox_command ="
  • You are done. Now your mail goes to Maildir format.

Mutt

  • If you want to read your new maildir format you have to tell mutt to use it as well. Edit this file:

vi /etc/Muttrc
  • Add these lines to the bottom of the file:

set folder="~/Maildir"
set mask="!^\\.[^.]"
set mbox="~/Maildir"
set record="+.Sent"
set postponed="+.Drafts"
set spoolfile="~/Maildir"
  • Now start mutt and send an email to yourself to see if it all works.

Postfix and mailing lists

Mailman with Postfix

(!) The instructions below are WRONG! You should not postfix-to-mailman.py and alias at the same time. Please read /etc/mailman/postfix-to-mailman.py instead.

  • Install mailman:

apt-get install mailman
  • When done type:

newlist mailman
  • Start mailman

/etc/init.d/mailman start

relay_domains = example.com, lists.example.com
  • In same file add ,hash:/var/lib/mailman/data/aliases after alias_maps

alias_maps = hash:/etc/aliases,hash:/var/lib/mailman/data/aliases
  • Now type:

postconf -e "transport_maps = hash:/etc/postfix/transport"
postconf -e "mailman_destination_recipient_limit = 1"
  • In /etc/postfix/master.cf add:

mailman unix  -       n       n       -       -       pipe
   flags=FR user=list
   argv=/var/lib/mailman/bin/postfix-to-mailman.py ${nexthop} ${user}
  • Edit or create /etc/postfix/transport. Add this line:

lists.example.com    mailman:
  • Then postmap it:

postmap /etc/postfix/transport
  • Now edit /etc/mailman/mm_cfg.py and add:

MTA = 'Postfix'
DEB_LISTMASTER = 'postmaster@example.com'
POSTFIX_STYLE_VIRTUAL_DOMAIN = ['lists.example.com']
  • Done. Now restart postfix, mailman

/etc/init.d/postfix reload
/etc/init.d/mailman restart
  • Create a mailing list:

newlist list_name
  • If you want archives add this to /etc/apache2/apache2.conf

Alias /pipermail/ /var/lib/mailman/archives/public/
Alias /images/mailman/ /usr/share/images/mailman/

Then you need to reload apache:

/etc/init.d/apache2 reload
  • Done. Go to

http://lists.yourwebsite.com/cgi-bin/mailman/listinfo/list_name/

Mailman Troubleshooting

Connection refused

Assuming your postfix is running and listening on localhost, another possible problem is that postfix is not configured to run in IPv6 mode, but your /etc/hosts file specifies ::1 as localhost. In that case mailman tries to send mails to ::1 which has no postfix listening, thus resulting in a (111, 'connection refused') error.

Postfix and TLS/SSL

Adding TLS/SSL

There are three options for transferring data to Postfix (smtpd):

  1. Do not use TLS/SSL at all (only unsecure connections are available).
  2. Use TLS/SSL, if possible. Fall back to unsecure connections otherwise.
  3. Only allow TLS/SSL (unsecure connections are not available).

The second option (called STARTTLS) is recommended for general purpose mail servers. It provides some sort of "compatibility mode". Secure data transfer is enabled but not enforced.

STARTTLS connections start unencrypted via the regular smtp port 25. If both sides agree the rest of the data transfer is encrypted, still using port 25.

Pure TLS/SSL uses it own port, usually smtps (465). See below.

Recent Postfix versions employ the parameter smtpd_tls_security_level to control TLS encryption (valid values are none, may or encrypt).

Previously two parameters (smtpd_use_tls and smtpd_enforce_tls) were used. They can be unset. See also the Debian bug report 520936.

With the following commands TLS is enforced (no STARTTLS) and the old configuration parameters are reset to default values:

~# postconf -e smtpd_tls_security_level=encrypt
~# postconf -# smtpd_use_tls
~# postconf -# smtpd_enforce_tls

Alternate TLS/SSL Ports

You may be interested in supporting the smtps and/or submission ports (see /etc/services) so that your mobile/remote users who may be on a system that blocks, filters or poorly proxies SMTP (port 25) traffic can still send mail through your server. Since these ports are not also used for MTA to MTA traffic, you can enforce extra restrictions such as requiring SSL/TLS.

We do this by modifying the file /etc/postfix/master.cf to run additional smtpd services with special parameters on dedicated ports.

Submission

The submission port (587), covered in RFC 2476, is reserved for mail user agents (MUA)/ mail submission agents (MSA) to send email to a mail transfer agent (MTA).

In order to enable an additional service edit the file /etc/postfix/master.cf.

In this example we disallow ETRN, require TLS and enable SASL Auth on the submission port.

submission inet n      -       -       -       -       smtpd
        -o smtpd_etrn_restrictions=reject
        -o smtpd_enforce_tls=yes 
        -o smtpd_sasl_auth_enable=yes

SMTPS

The smtps (or ssmtp) port (465) is the equivalent of https. The secure layer is expected from the get-go and not an optional negotiated parameter after connecting.

Whether the port is named smtps or ssmtp depends on the contents of your /etc/services file. On Debian both names seem to be defined. The output of netstat -tl shows ssmtp.

In order to enable an additional service edit the file /etc/postfix/master.cf.

On Debian there is already a prepared entry for smtps but commented out. Remove the "#" characters to enable it.

smtps     inet  n       -       -       -       -       smtpd
  -o syslog_name=postfix/smtps
  -o smtpd_tls_wrappermode=yes
  -o smtpd_sasl_auth_enable=yes
  -o smtpd_client_restrictions=permit_sasl_authenticated,reject
  -o milter_macro_daemon_name=ORIGINATING

Connections from Fetchmail to Postfix

It seems fetchmail is not able to setup a TLS connection to Postfix. (Not to be confused with fetchmail's capabilities to fetch mails via TLS connections.)

If Postfix is configured to only accept TLS connections (smtpd_tls_security_level=encrypt) fetchmail will fail with an error like "Must issue a STARTTLS command first".

One way to escape from this is to provide an unencrypted smtp service. Of course, this service should be available for a local fetchmail process only.

Edit /etc/postfix/master.cf and add

127.0.0.1:40025      inet  n       -       -       -       -       smtpd
    -o smtpd_tls_security_level=none

This will add an additional smtp service listening on port 40025 with TLS disabled but only accepting local connections.

Fetchmail has to be configured accordingly via the option smtphost.

Edit /etc/fetchmailrc

# Server options
poll ...

# User options
user a ...
        smtphost 127.0.0.1/40025
user b ...
        smtphost 127.0.0.1/40025

The smtphost option is a so called "user option". It must be added to every user section.

Alternatively fetchmail can be instructed to use an external TLS-capable program1 to forward mails. This is not handled here. And if fetchmail and Postfix run on the same machine it does not make much sense anyway.

Advanced options

SPF and multiple external ip addresses

Explanation

I have some systems that are networked on an internal private ip address subnet (192.168.0.0/16). For a few reasons I email reports and such to <user>@mail.internal where user is an address that is not valid for receiving mail via the external interfaces. These systems also share a public ip address subnet so they could email each other that way, but I'd prefer they didn't for local addresses. I have published SPF records for the public mail servers because all of our mail routes through those servers so if others care to check they can ignore email claiming to be from us but being delivered from other servers as per our SPF record.

Recently I have expanded the ip addresses these systems are using externally to support multiple instances of port-based services like https (adding :oddport doesn't impress the customers.) I could have expanded or added more liberal SPF record values, or added more forward and reverse DNS records but I wanted to stick with less ip addresses.

So to recap my system has:

  • eth1 <public ip with spf published>

  • eth1:1 <public ip for extra port-based services>

  • eth0 <private ip on>

By using the settings in /etc/postfix/master.cf, /etc/postfix/main.cf and /etc/postfix/transport as outlined above I was able to get my outgoing smtp traffic to use my SPF published ip address once again.

Make SPF and multiple external ip addresses

If you are trying to implement SPF records while binding to one external ip address and still working with dual-homed multiple ip aliased systems, or have any other reason to support multi-homed systems with multiple ip addresses but want to limit postfix to use only two of them try this.

  • /etc/postfix/master.cf

    • clone the smtp (not smtpd) service. Set the first one to use <spf published ip address> Rename the second to smtpinternal and use <internal ip address>

 smtp      unix  -       -       -       -       -       smtp
        -o smtp_bind_address=<spf published ip address>
 smtpinternal      unix  -       -       -       -       -       smtp
        -o smtp_bind_address=<internal ip address>
  • /etc/postfix/main.cf

    • Use transport_maps for routing

 transport_maps = hash:/etc/postfix/transport
  • /etc/postfix/transport

    • Map a transport for your internal domain.

 .internal smtpinternal:

Just postmap /etc/postfix/transport, invoke-rc.d postfix stop and invoke-rc.d postfix start and you should be in business. Email to <user>@<system>.internal will be delivered via the internal interface/ip address all other email will be delivered via default methods which means internet mail will go out the the spf published ip address.

Optional:

  • /etc/postfix/main.cf

    • Use the inet_interfaces setting to only listen on the ip addresses you want to.

       inet_interfaces = 127.0.0.1, <internal ip>, <spf published external ip>

Postfix and Sasl

External links

Please see Postfix/Tutorials



  1. The so called "lightweight" MTAs like msmtp or sSMTP. (1)