#!/bin/bash # # Postfix (SMTP) # -------------- # # Postfix handles the transmission of email between servers # using the SMTP protocol. It is a Mail Transfer Agent (MTA). # # Postfix listens on port 25 (SMTP) for incoming mail from # other servers on the Internet. It is responsible for very # basic email filtering such as by IP address and greylisting, # it checks that the destination address is valid, rewrites # destinations according to aliases, and passses email on to # another service for local mail delivery. # # The first hop in local mail delivery is to Spamassassin via # LMTP. Spamassassin then passes mail over to Dovecot for # storage in the user's mailbox. # # Postfix also listens on port 587 (SMTP+STARTLS) for # connections from users who can authenticate and then sends # their email out to the outside world. Postfix queries Dovecot # to authenticate users. # # Address validation, alias rewriting, and user authentication # is configured in a separate setup script mail-users.sh # because of the overlap of this part with the Dovecot # configuration. source setup/functions.sh # load our functions source /etc/mailinabox.conf # load global vars # ### Install packages. # Install postfix's packages. # # * `postfix`: The SMTP server. # * `postfix-pcre`: Enables header filtering. # * `postgrey`: A mail policy service that soft-rejects mail the first time # it is received. Spammers don't usually try agian. Legitimate mail # always will. # * `ca-certificates`: A trust store used to squelch postfix warnings about # untrusted opportunistically-encrypted connections. apt_install postfix postfix-pcre postgrey ca-certificates # ### Basic Settings # Set some basic settings... # # * Have postfix listen on all network interfaces. # * Set our name (the Debian default seems to be "localhost" but make it our hostname). # * Set the name of the local machine to localhost, which means xxx@localhost is delivered locally, although we don't use it. # * Set the SMTP banner (which must have the hostname first, then anything). tools/editconf.py /etc/postfix/main.cf \ inet_interfaces=all \ myhostname=$PRIMARY_HOSTNAME\ smtpd_banner="\$myhostname ESMTP Hi, I'm a Mail-in-a-Box (Ubuntu/Postfix; see https://mailinabox.email/)" \ mydestination=localhost # ### Outgoing Mail # Enable the 'submission' port 587 smtpd server and tweak its settings. # # * Require the best ciphers for incoming connections per http://baldric.net/2013/12/07/tls-ciphers-in-postfix-and-dovecot/. # but without affecting opportunistic TLS on incoming mail, which will allow any cipher (it's better than none). # * Give it a different name in syslog to distinguish it from the port 25 smtpd server. # * Add a new cleanup service specific to the submission service ('authclean') # that filters out privacy-sensitive headers on mail being sent out by # authenticated users. tools/editconf.py /etc/postfix/master.cf -s -w \ "submission=inet n - - - - smtpd -o syslog_name=postfix/submission -o smtpd_tls_ciphers=high -o smtpd_tls_protocols=!SSLv2,!SSLv3 -o cleanup_service_name=authclean" \ "authclean=unix n - - - 0 cleanup -o header_checks=pcre:/etc/postfix/outgoing_mail_header_filters" # Install the `outgoing_mail_header_filters` file required by the new 'authclean' service. cp conf/postfix_outgoing_mail_header_filters /etc/postfix/outgoing_mail_header_filters # Enable TLS on these and all other connections (i.e. ports 25 *and* 587) and # require TLS before a user is allowed to authenticate. This also makes # opportunistic TLS available on *incoming* mail. # Set stronger DH parameters, which via openssl tend to default to 1024 bits # (see ssl.sh). tools/editconf.py /etc/postfix/main.cf \ smtpd_tls_security_level=may\ smtpd_tls_auth_only=yes \ smtpd_tls_cert_file=$STORAGE_ROOT/ssl/ssl_certificate.pem \ smtpd_tls_key_file=$STORAGE_ROOT/ssl/ssl_private_key.pem \ smtpd_tls_dh1024_param_file=$STORAGE_ROOT/ssl/dh2048.pem \ smtpd_tls_received_header=yes # Prevent non-authenticated users from sending mail that requires being # relayed elsewhere. We don't want to be an "open relay". On outbound # mail, require one of: # # * permit_sasl_authenticated: Authenticated users (i.e. on port 587). # * permit_mynetworks: Mail that originates locally. # * reject_unauth_destination: No one else. (Permits mail whose destination is local and rejects other mail.) tools/editconf.py /etc/postfix/main.cf \ smtpd_relay_restrictions=permit_sasl_authenticated,permit_mynetworks,reject_unauth_destination # ### DANE # When connecting to remote SMTP servers, prefer TLS and use DANE if available. # # Prefering ("opportunistic") TLS means Postfix will use TLS if the remote end # offers it, otherwise it will transmit the message in the clear. Postfix will # accept whatever SSL certificate the remote end provides. Opportunistic TLS # protects against passive easvesdropping (but not man-in-the-middle attacks). # DANE takes this a step further: # # Postfix queries DNS for the TLSA record on the destination MX host. If no TLSA records are found, # then opportunistic TLS is used. Otherwise the server certificate must match the TLSA records # or else the mail bounces. TLSA also requires DNSSEC on the MX host. Postfix doesn't do DNSSEC # itself but assumes the system's nameserver does and reports DNSSEC status. Thus this also # relies on our local bind9 server being present and `smtp_dns_support_level=dnssec`. # # The `smtp_tls_CAfile` is superflous, but it eliminates warnings in the logs about untrusted certs, # which we don't care about seeing because Postfix is doing opportunistic TLS anyway. Better to encrypt, # even if we don't know if it's to the right party, than to not encrypt at all. Instead we'll # now see notices about trusted certs. The CA file is provided by the package `ca-certificates`. tools/editconf.py /etc/postfix/main.cf \ smtp_tls_security_level=dane \ smtp_dns_support_level=dnssec \ smtp_tls_CAfile=/etc/ssl/certs/ca-certificates.crt \ smtp_tls_loglevel=2 # ### Incoming Mail # Pass any incoming mail over to a local delivery agent. Spamassassin # will act as the LDA agent at first. It is listening on port 10025 # with LMTP. Spamassassin will pass the mail over to Dovecot after. # # In a basic setup we would pass mail directly to Dovecot by setting # virtual_transport to `lmtp:unix:private/dovecot-lmtp`. # tools/editconf.py /etc/postfix/main.cf virtual_transport=lmtp:[127.0.0.1]:10025 # Who can send mail to us? Some basic filters. # # * reject_non_fqdn_sender: Reject not-nice-looking return paths. # * reject_unknown_sender_domain: Reject return paths with invalid domains. # * reject_rhsbl_sender: Reject return paths that use blacklisted domains. # * permit_sasl_authenticated: Authenticated users (i.e. on port 587) can skip further checks. # * permit_mynetworks: Mail that originates locally can skip further checks. # * reject_rbl_client: Reject connections from IP addresses blacklisted in zen.spamhaus.org # * reject_unlisted_recipient: Although Postfix will reject mail to unknown recipients, it's nicer to reject such mail ahead of greylisting rather than after. # * check_policy_service: Apply greylisting using postgrey. # # Notes: # permit_dnswl_client can pass through mail from whitelisted IP addresses, which would be good to put before greylisting # so these IPs get mail delivered quickly. But when an IP is not listed in the permit_dnswl_client list (i.e. it is not # whitelisted) then postfix does a DEFER_IF_REJECT, which results in all "unknown user" sorts of messages turning into # "450 4.7.1 Client host rejected: Service unavailable". This is a retry code, so the mail doesn't properly bounce. tools/editconf.py /etc/postfix/main.cf \ smtpd_sender_restrictions="reject_non_fqdn_sender,reject_unknown_sender_domain,reject_rhsbl_sender dbl.spamhaus.org" \ smtpd_recipient_restrictions=permit_sasl_authenticated,permit_mynetworks,"reject_rbl_client zen.spamhaus.org",reject_unlisted_recipient,"check_policy_service inet:127.0.0.1:10023" # Increase the message size limit from 10MB to 128MB. tools/editconf.py /etc/postfix/main.cf \ message_size_limit=134217728 # Allow the two SMTP ports in the firewall. ufw_allow smtp ufw_allow submission # Restart services restart_service postfix