Postfix
Postfix is a mail transfer agent that according to its website:
- attempts to be fast, easy to administer, and secure, while at the same time being sendmail compatible enough to not upset existing users. Thus, the outside has a sendmail-ish flavor, but the inside is completely different.
This article builds upon Mail server. The goal of this article is to setup Postfix and explain what the basic configuration files do. There are instructions for setting up local system user-only delivery and a link to a guide for virtual user delivery.
Installation
Configuration
See Postfix Basic Configuration. Configuration files are in /etc/postfix by default. The two most important files are:
-
master.cf, defines what Postfix services are enabled and how clients connect to them, see master(5) -
main.cf, the main configuration file, see postconf(5)
Configuration changes need a postfix.service reload in order to take effect.
Aliases
See aliases(5).
You can specify aliases (also known as forwarders) in /etc/postfix/aliases.
You should map all mail addressed to root to another account since it is not a good idea to read mail as root.
Uncomment the following line, and change you to a real account.
root: you
Once you have finished editing /etc/postfix/aliases you must run the postalias command:
postalias /etc/postfix/aliases
For later changes you can use:
newaliases
~/.forward, e.g. /root/.forward for root. Specify the user to whom root mail should be forwarded, e.g. user@localhost.
/root/.forward
user@localhost
Local mail
To only deliver mail to local system users (that are in /etc/passwd) update /etc/postfix/main.cf to reflect the following configuration. Uncomment, change, or add the following lines:
myhostname = localhost mydomain = localdomain mydestination = $myhostname, localhost.$mydomain, localhost inet_interfaces = $myhostname, localhost mynetworks_style = host default_transport = error: outside mail is not deliverable
All other settings may remain unchanged. After setting up the above configuration file, you may wish to set up some #Aliases and then #Start Postfix.
Virtual mail
Virtual mail is mail that does not map to a user account (/etc/passwd).
See Virtual user mail system with Postfix, Dovecot and Roundcube for a comprehensive guide how to set it up.
Check configuration
Run the postfix check command. It should output anything that you might have done wrong in a config file.
To see all of your configs, type postconf. To see how you differ from the defaults, try postconf -n.
Start Postfix
newaliases at least once for Postfix to run, even if you did not set up any #Aliases.Start/enable the postfix.service.
TLS
For more information, see Postfix TLS Support.
Secure SMTP (sending)
By default, Postfix/sendmail will not send email encrypted to other SMTP servers. To use TLS when available, add the following line to main.cf:
/etc/postfix/main.cf
smtp_tls_security_level = may
To enforce TLS (and fail when the remote server does not support it), change may to encrypt. Note, however, that this violates RFC:2487 if the SMTP server is publicly referenced.
Secure SMTP (receiving)
By default, Postfix will not accept secure mail.
You need to obtain a certificate. Point Postfix to your TLS certificates by adding the following lines to main.cf:
/etc/postfix/main.cf
smtpd_tls_security_level = may smtpd_use_tls = yes smtpd_tls_cert_file = /path/to/cert.pem smtpd_tls_key_file = /path/to/key.pem
There are two ways to accept secure mail. STARTTLS over SMTP (port 587) and SMTPS (port 465). The latter was previously deprecated but was reinstated by RFC:8314.
To enable STARTTLS over SMTP (port 587), uncomment the following lines in master.cf:
/etc/postfix/master.cf
submission inet n - n - - smtpd -o syslog_name=postfix/submission -o smtpd_tls_security_level=encrypt -o smtpd_sasl_auth_enable=yes -o smtpd_tls_auth_only=yes -o smtpd_reject_unlisted_recipient=no # -o smtpd_client_restrictions=$mua_client_restrictions # -o smtpd_helo_restrictions=$mua_helo_restrictions # -o smtpd_sender_restrictions=$mua_sender_restrictions -o smtpd_recipient_restrictions= -o smtpd_relay_restrictions=permit_sasl_authenticated,reject -o milter_macro_daemon_name=ORIGINATING
The smtpd_*_restrictions options remain commented because $mua_*_restrictions are not defined in main.cf by default. If you do decide to set any of $mua_*_restrictions, uncomment those lines too.
To enable SMTPS (port 465), uncomment the following lines in master.cf:
/etc/postfix/master.cf
smtps inet n - n - - smtpd -o syslog_name=postfix/smtps -o smtpd_tls_wrappermode=yes -o smtpd_sasl_auth_enable=yes -o smtpd_reject_unlisted_recipient=no # -o smtpd_client_restrictions=$mua_client_restrictions # -o smtpd_helo_restrictions=$mua_helo_restrictions # -o smtpd_sender_restrictions=$mua_sender_restrictions -o smtpd_recipient_restrictions= -o smtpd_relay_restrictions=permit_sasl_authenticated,reject -o milter_macro_daemon_name=ORIGINATING
The rationale surrounding the $smtpd_*_restrictions lines is the same as above.
Tips and tricks
Blacklist incoming emails
Manually blacklisting incoming emails by sender address can easily be done with Postfix.
Create and open /etc/postfix/blacklist_incoming file and append sender email address:
user@example.com REJECT
Then use the postmap command to create a database:
# postmap hash:blacklist_incoming
Add the following code before the first permit rule in main.cf:
smtpd_recipient_restrictions = check_sender_access hash:/etc/postfix/blacklist_incoming
Finally restart postfix.service.
Hide the sender's IP and user agent in the Received header
This is a privacy concern mostly, if you use Thunderbird and send an email. The received header will contain your LAN and WAN IP and info about the email client you used. (Original source: AskUbuntu) What we want to do is remove the Received header from outgoing emails. This can be done by the following steps:
Add the following line to main.cf:
smtp_header_checks = regexp:/etc/postfix/smtp_header_checks
Create /etc/postfix/smtp_header_checks with this content:
/^Received: .*/ IGNORE /^User-Agent: .*/ IGNORE
Finally, restart postfix.service.
Postfix in a chroot jail
Postfix is not put in a chroot jail by default. The Postfix documentation [1] provides details about how to accomplish such a jail. The steps are outlined below and are based on the chroot-setup script provided in the Postfix source code.
First, go into the master.cf file in the directory /etc/postfix and change all the chroot entries to 'yes' (y) except for the services qmgr, proxymap, proxywrite, local, and virtual
Second, create two functions that will help us later with copying files over into the chroot jail (see last step)
CP="cp -p"
cond_copy() {
# find files as per pattern in $1
# if any, copy to directory $2
dir=`dirname "$1"`
pat=`basename "$1"`
lr=`find "$dir" -maxdepth 1 -name "$pat"`
if test ! -d "$2" ; then exit 1 ; fi
if test "x$lr" != "x" ; then $CP $1 "$2" ; fi
}
Next, make the new directories for the jail:
set -e umask 022
POSTFIX_DIR=${POSTFIX_DIR-/var/spool/postfix}
cd ${POSTFIX_DIR}
mkdir -p etc lib usr/lib/zoneinfo test -d /lib64 && mkdir -p lib64
Find the localtime file
lt=/etc/localtime if test ! -f $lt ; then lt=/usr/lib/zoneinfo/localtime ; fi if test ! -f $lt ; then lt=/usr/share/zoneinfo/localtime ; fi if test ! -f $lt ; then echo "cannot find localtime" ; exit 1 ; fi rm -f etc/localtime
Copy localtime and some other system files into the chroot's etc
$CP -f $lt /etc/services /etc/resolv.conf /etc/nsswitch.conf etc $CP -f /etc/host.conf /etc/hosts /etc/passwd etc ln -s -f /etc/localtime usr/lib/zoneinfo
Make sure resolv.conf is owned by root:
chown root /var/spool/postfix/etc/resolv.conf
Copy required libraries into the chroot using the previously created function cond_copy
cond_copy '/usr/lib/libnss_*.so*' lib cond_copy '/usr/lib/libresolv.so*' lib cond_copy '/usr/lib/libdb.so*' lib
And do not forget to reload Postfix.
DANE (DNSSEC)
Resource Record
DANE supports several types of records, however not all of them are suitable in Postfix.
Certificate usage 0 is unsupported, 1 is mapped to 3 and 2 is optional, thus it is recommendet to publish a "3" record. More on Resource Records.
Configuration
Opportunistic DANE is configured this way:
/etc/postfix/main.cf
smtpd_use_tls = yes smtp_dns_support_level = dnssec smtp_tls_security_level = dane
/etc/postfix/master.cf
dane unix - - n - - smtp -o smtp_dns_support_level=dnssec -o smtp_tls_security_level=dane
To use per-domain policies, e.g. opportunistic DANE for example.org and mandatory DANE for example.com, use something like this:
/etc/postfix/main.cf
indexed = ${default_database_type}:${config_directory}/
# Per-destination TLS policy
#
smtp_tls_policy_maps = ${indexed}tls_policy
# default_transport = smtp, but some destinations are special:
#
transport_maps = ${indexed}transport
transport
example.com dane example.org dane
tls_policy
example.com dane-only
smtp_tls_security_level to dane-only. Be aware that this makes Postfix tempfail (respond with a 4.X.X error code) on all deliveries that do not use DANE at all!Full documentation is found here.
Extras
- PostfixAdmin — A web-based administrative interface for Postfix.
Postgrey
Postgrey can be used to enable greylisting for a Postfix mail server.
Installation
Install the postgrey package. To get it running quickly edit the Postfix configuration file and add these lines:
/etc/postfix/main.cf
smtpd_recipient_restrictions = check_policy_service inet:127.0.0.1:10030
Then start/enable the postgrey service. Afterwards, reload the postfix service. Now greylisting should be enabled.
Configuration
Configuration is done via editing the postgrey.service file. First copy it over to edit it.
# cp /usr/lib/systemd/system/postgrey.service /etc/systemd/system/
Whitelisting
To add automatic whitelisting (successful deliveries are whitelisted and do not have to wait any more), you could add the --auto-whitelist-clients=N option and replace N by a suitably small number (or leave it at its default of 5).
...actually, the preferred method should be the override:
cat /etc/systemd/system/postgrey.service.d/override.conf
[Service]
ExecStart=
ExecStart=/usr/bin/postgrey --inet=127.0.0.1:10030 \
--pidfile=/run/postgrey/postgrey.pid \
--group=postgrey --user=postgrey \
--daemonize \
--greylist-text="Greylisted for %%s seconds" \
--auto-whitelist-clients
To add your own list of whitelisted clients in addition to the default ones, create the file /etc/postfix/whitelist_clients.local and enter one host or domain per line, then restart postgrey.service so the changes take effect.
Troubleshooting
If you specify --unix=/path/to/socket and the socket file is not created ensure you have removed the default --inet=127.0.0.1:10030 from the service file.
For a full documentation of possible options see perldoc postgrey.
SpamAssassin
This section describes how to integrate SpamAssassin.
SpamAssassin stand-alone generic setup
Edit /etc/postfix/master.cf and add the content filter under smtp.
smtp inet n - n - - smtpd -o content_filter=spamassassin
Also add the following service entry for SpamAssassin
spamassassin unix - n n - - pipe
flags=R user=spamd argv=/usr/bin/vendor_perl/spamc -e /usr/bin/sendmail -oi -f ${sender} ${recipient}
Now you can start and enable spamassassin.service.
SpamAssassin combined with Dovecot LDA / Sieve (Mailfiltering)
Set up LDA and the Sieve-Plugin as described in Dovecot#Sieve. But ignore the last line mailbox_command... .
Instead add a pipe in /etc/postfix/master.cf:
dovecot unix - n n - - pipe
flags=DRhu user=vmail:vmail argv=/usr/bin/vendor_perl/spamc -u spamd -e /usr/lib/dovecot/dovecot-lda -f ${sender} -d ${recipient}
And activate it in /etc/postfix/main.cf:
virtual_transport = dovecot
Alternately, if you do not want to use virtual transports you can use the
mailbox_command. This runs
with the local user and group, whereas the pipe runs with with the specified user using the user setting.
mailbox_command = /usr/bin/vendor_perl/spamc -u spamd -e /usr/lib/dovecot/dovecot-lda -f "$SENDER" -a "$RECIPIENT"
SpamAssassin combined with Dovecot LMTP / Sieve
Set up the LMTP and Sieve as described in Dovecot#Sieve.
Edit /etc/dovecot/conf.d/90-plugin.conf and add:
sieve_before = /etc/dovecot/sieve.before.d/ sieve_extensions = +vnd.dovecot.filter sieve_plugins = sieve_extprograms sieve_filter_bin_dir = /etc/dovecot/sieve-filter sieve_filter_exec_timeout = 120s #this is often needed for the long running spamassassin scans, default is otherwise 10s
Create the directory and put spamassassin in as a binary that can be ran by dovecot:
# mkdir /etc/dovecot/sieve-filter # ln -s /usr/bin/vendor_perl/spamc /etc/dovecot/sieve-filter/spamc
Create a new file, /etc/dovecot/sieve.before.d/spamassassin.sieve which contains:
require [ "vnd.dovecot.filter" ]; filter "spamc" [ "-d", "127.0.0.1", "--no-safe-fallback" ];
Compile the sieve rules spamassassin.svbin:
# cd /etc/dovecot/sieve.before.d # sievec spamassassin.sieve
Finally, restart dovecot.service.
Rule-based mail processing
With policy services one can easily finetune Postfix' behaviour of mail delivery. postfwd and policyd (policyd-mysqlAUR, policyd-pgsqlAUR or policyd-sqliteAUR) provide services to do so. This allows you to e.g. implement time-aware grey- and blacklisting of senders and receivers as well as SPF policy checking.
Policy services are standalone services and connected to Postfix like this:
/etc/postfix/main.cf
smtpd_recipient_restrictions = ... check_policy_service unix:/run/policyd.sock check_policy_service inet:127.0.0.1:10040
Placing policy services at the end of the queue reduces load, as only legitimate mails are processed. Be sure to place it before the first permit statement to catch all incoming messages.
Sender Policy Framework
To use the Sender Policy Framework with Postfix, you can install python-spf-engineAUR, python-postfix-policyd-spfAUR or postfix-policyd-spf-perlAUR.
With spf-engine or python-postfix-policyd-spf
Edit /etc/python-policyd-spf/policyd-spf.conf to your needs. An extensively commented version can be found at /etc/python-policyd-spf/policyd-spf.conf.commented.
Pay some extra attention to the HELO check policy, as standard settings strictly reject HELO failures.
In main.cf file, add a timeout for the policyd:
/etc/postfix/main.cf
policy-spf_time_limit = 3600s
Then add a transport
/etc/postfix/master.cf
policy-spf unix - n n - 0 spawn
user=nobody argv=/usr/bin/policyd-spf
Lastly you need to add the policyd to the smtpd_recipient_restrictions. To minimize load put it to the end of the restrictions but above any reject_rbl_client DNSBL line:
/etc/postfix/main.cf
smtpd_recipient_restrictions=
...
permit_sasl_authenticated
permit_mynetworks
reject_unauth_destination
check_policy_service unix:private/policy-spf
Now reload the postfix service.
You can test your setup with the following:
/etc/python-policyd-spf/policyd-spf.conf
defaultSeedOnly = 0
With postfix-policyd-spf-perl
Do the same process with postfix as with python-postfix-policyd-spf, but with the following differences:
Timeout for the policyd in main.cf file:
/etc/postfix/main.cf
policy_time_limit = 3600
Transport:
/etc/postfix/master.cf
policy unix - n n - 0 spawn
user=nobody argv=/usr/lib/postfix/postfix-policyd-spf-perl
Add the policyd to the smtpd_recipient_restrictions:
check_policy_service after reject_unauth_destination or else your system can become an open relay./etc/postfix/main.cf
smtpd_recipient_restrictions=
...
reject_unauth_destination
check_policy_service unix:private/policy
...
Sender Rewriting Scheme
To use the Sender Rewriting Scheme with Postfix, install postsrsdAUR and adjust the settings:
/etc/postsrsd/postsrsd
SRS_DOMAIN=yourdomain.tld SRS_EXCLUDE_DOMAINS=yourotherdomain.tld,yet.anotherdomain.tld SRS_SEPARATOR== SRS_SECRET=/etc/postsrsd/postsrsd.secret SRS_FORWARD_PORT=10001 SRS_REVERSE_PORT=10002 RUN_AS=postsrsd CHROOT=/usr/lib/postsrsd
Enable and start the daemon, making sure it runs after reboot as well. Then configure Postfix accordingly by tweaking the following lines:
/etc/postfix/main.cf
sender_canonical_maps = tcp:localhost:10001 sender_canonical_classes = envelope_sender recipient_canonical_maps = tcp:localhost:10002 recipient_canonical_classes= envelope_recipient,header_recipient
Restart Postfix and start forwarding mail.
Troubleshooting
Warning: "database /etc/postfix/*.db is older than source file .."
If you get one or both warnings with journalctl:
warning: database /etc/postfix/virtual.db is older than source file /etc/postfix/virtual warning: database /etc/postfix/transport.db is older than source file /etc/postfix/transport
Then you can fix it by using these commands, depending on the messages you get:
postmap /etc/postfix/transport postmap /etc/postfix/virtual
And restart postfix.service.
Host or domain name not found. Name service error for name=...
If you get the following warning with journalctl:
Host or domain name not found. Name service error for name=...
It could be that you are running Postfix in a chroot and /etc/resolv.conf is missing. If so, you can fix this by:
mkdir -p /var/spool/postfix/etc cp /etc/resolv.conf /var/spool/postfix/etc/resolv.conf
And restart postfix.service.