diff --git a/src/content/docs/ispmail-trixie/180-relaying.mdx b/src/content/docs/ispmail-trixie/180-relaying.mdx index db2161a..a50fee9 100644 --- a/src/content/docs/ispmail-trixie/180-relaying.mdx +++ b/src/content/docs/ispmail-trixie/180-relaying.mdx @@ -31,7 +31,7 @@ Put simply, a mail server will accept an email if… I have created two illustrations to explain it. -### Outgoing email (without authentication) +### Without authentication = denied `john@example.org` is on the internet somewhere and wants to send an email to `lisa@example.com` (which is another domain). Your mail server is not responsible for the _example.com_ domain so it receives John’s email and would have to @@ -62,7 +62,7 @@ Towards the end of the output you will find: <~* 554 5.7.1 : Recipient address rejected: Access denied ``` -### Outgoing email (with authentication) +### With authentication = accepted So how does John prove his identity? He needs to use _authenticated_ SMTP. This is similar to the previous case but John’s email program will also send his username and password. @@ -99,8 +99,7 @@ username and password. Postfix just needs some extra configuration. Run these co ``` postconf smtpd_sasl_type=dovecot -postconf smtpd_sasl_path=private/auth -postconf smtpd_sasl_auth_enable=yes +postconf smtpd_sasl_path=private/dovecot-auth ``` This enables SMTP authentication and tells Postfix that it can talk to Dovecot through a socket file located at @@ -113,45 +112,38 @@ TODO: collapsible on anti-spoofing ## Enable encryption -TODO: move this section to a separate page on TLS encryption - We certainly do not want our users to send their password over the wire wihtout encryption. So the following commands enable encryption, set the key and certificate paths for Postfix. Just run these commands: ``` +postconf smtp_tls_security_level=may postconf smtpd_tls_security_level=may -postconf smtpd_tls_auth_only=yes postconf smtpd_tls_cert_file=/etc/letsencrypt/live/mail.example.org/fullchain.pem postconf smtpd_tls_key_file=/etc/letsencrypt/live/mail.example.org/privkey.pem -postconf smtp_tls_security_level=may ``` +What it means: + +- **smtp_tls_security_level**: \ + Allow encrypted _outgoing_ SMTP connections but do not enforce it. +- **smtpd_tls_security_level**: \ + Allow encrypted _incoming_ SMTP connections but do not enforce it. +- **smtpd_tls_cert_file** and **smtpd_tls_key_file**: \ + Where to find the private key and certificate for encryption. + +Please note that configuration in `main.cf` applies to all Postfix services. So the above configuration enabled +encryption for both server-to-server and human-to-server (`submission`) services. The `smtpd_sasl_auth_enable` will only +be set for the `submission` service to enable authentication there.[^1] + - -The above settings allow encrypted incoming (smtpd\_) and outgoing (smtp\_) connections. But they do not enforce it. If -a remote mail server does not have encryption enabled we will still accept their emails. You may be tempted to enforce -encryption but that would reject email communication with servers who have been configured without encryption. - -However the `smtpd_tls_auth_only=yes` setting makes sure that the user’s authentication information (email address and -password) are always encrypted between the user and your mail server. - -
- Click here to learn how SMTP authentication works… + Click here if you want to learn how SMTP authentication works… Are you curious how SMTP authentication looks on a low level? You probably are not. But let’s do it anyway. Just once, so that you get the idea. @@ -203,7 +195,7 @@ Let me briefly explain what these lines mean: The remote server is allowed to send emails up to 10 MB large. This has long been a common maximum size for emails. However nowadays 40 MB or even more are more common sizes because emails have grown larger. Some mail servers even allow 100 MB. - VRFY - Allows remote servers to verify a given name or email address. For example the remote server could send “VRFY john” and + Allows remote servers to verify a given name or email address. For example the remote server could send "VRFY john" and your server might respond `250 John Doe `. It can be used to verify that a certain recipient email address is deliverable - ETRN @@ -212,15 +204,15 @@ Let me briefly explain what these lines mean: your server start sending outstanding emails for that domain. It is rarely used. - STARTTLS This tells the remote system that it might start switching from this unencrypted to an encrypted connection by sending - the “STARTTLS” command. It will then start negotiating a TLS-encrypted connection. You could compare it to an HTTP connection + the "STARTTLS" command. It will then start negotiating a TLS-encrypted connection. You could compare it to an HTTP connection that suddenly switches over to an encrypted HTTPS connection. The advantage is that you can start talking SMTP on TCP port - 25 and don’t have to open up a second TCP port like 465 which is the “SSMTP” (secure SMTP) port and only accepts encrypted + 25 and don’t have to open up a second TCP port like 465 which is the "SSMTP" (secure SMTP) port and only accepts encrypted connections. - ENHANCEDSTATUSCODES This enables more three-digit return codes for various conditions. See the [RFC2034](http://tools.ietf.org/html/rfc2034) if you are curious. - 8BITMIME - In ancient times SMTP only processed 7-bit characters. You couldn’t transfer special characters like “Ä” or “ß” without + In ancient times SMTP only processed 7-bit characters. You couldn’t transfer special characters like "Ä" or "ß" without special encoding. 8BITMIME allows a transmission of emails using 8-bit characters. Still many emails are specially encoded using ISO8859-1 or UTF-8. - DSN @@ -254,7 +246,7 @@ And the server replies: ``` However now it’s getting weird because you would have to speak TLS encryption which is not a language that humans speak. -So let’s quit this using the “QUIT” command and do that differently: +So let’s quit this using the "QUIT" command and do that differently: ``` QUIT @@ -293,7 +285,7 @@ string with a Base64-encoded password: AUTH PLAIN AGpvaG5AZXhhbXBsZS5vcmcAc3VtbWVyc3Vu ``` -Unless you have changed John’s password to something else than “summersun” the server should accept that authentication: +Unless you have changed John’s password to something else than "summersun" the server should accept that authentication: ``` 235 2.7.0 Authentication successful @@ -330,7 +322,7 @@ So for John’s case you can easily create the Base64 string using: printf '\0john@example.org\0summersun' | base64 ``` -As a result you will get the exact same string you used above with “AUTH PLAIN”. +As a result you will get the exact same string you used above with "AUTH PLAIN".
@@ -338,50 +330,49 @@ As a result you will get the exact same string you used above with “AUTH PLAIN Please note that there are two different TCP ports on a mail server that speak SMTP: -| Service name | SMTP | Submission | -| -------------- | -------------------------- | --------------------- | -| TCP Port | 25 | 587 | -| Encryption | Optional | Mandatory | -| Authentication | Optional | Mandatory | -| Meant for | Servers | Humans | -| Used by | Mail transport agent (MTA) | Mail user agent (MUA) | -| Your home ISP | may block this port | will allows this port | +| Service name | SMTP | Submission | Submission TLS | +| -------------------- | ------------------------------------ | ------------------------------------------- | --------------------------- | +| TCP Port | 25 | 587 | 465 | +| Transport encryption | **Off** at first. STARTTLS optional. | **Off**. Then switch to TLS using STARTTLS. | **On** right from the start | +| Authentication | Optional [^1] | Mandatory | Mandatory | +| Meant for | Servers | Humans | Humans | +| Used by | Mail transport agent (MTA) | Mail user agent (MUA) | Mail user agent (MUA) | +| Your home ISP | may block this port | will allows this port | will allow this port | -I hope that makes the distinction a bit clearer. Our human users will have to use the _submission_ service. +[^1]: + In previous guides I recommended to set `smtpd_sasl_auth_enable=yes` globally in the `main.cf`. That would indeed + enable authentication on port 25. For a clearer separation I now rather suggest to leave it out (default is `no`) + and just enable authentication on the `submission` service in the `master.cf` file. A possible scenario that + required this setting to be `on` would be if your server were a relay (_smarthost_) for other servers using + authentication.. -All Postfix services are declared in the `/etc/postfix/master.cf` file. Please edit the file and look for the -_submission_ section that is commented out by default. Turn that section into the following. Basically I removed the ‘#’ -character on all lines of this section and removed the lines with the mua\_\* variables. +I hope that makes the distinction a bit clearer. Our human users will have to use the _submission_ service either on 587 +or 465. + +## Configure the submission service + +All Postfix services are declared in the `/etc/postfix/master.cf` file. Most of them are used by Postfix internally, +like `pickup`, `cleanup`, `rewrite` or `bounce`. But let's another section anywhere in there: ``` -submission inet n - y - - smtpd +submission inet n - y - - 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= - -o smtpd_helo_restrictions= - -o smtpd_sender_restrictions= - -o smtpd_relay_restrictions= -o smtpd_recipient_restrictions=permit_sasl_authenticated,reject - -o milter_macro_daemon_name=ORIGINATING ``` -Make sure to start the first line in the first column and indent the following lines. +Some notes on that: -This service uses the “smtpd” daemon (see the last word of the first line) which is the piece of software that responds -if you open an SMTP connection on TCP port 25. But it gets a few extra options set… - -- in the /var/log/mail.log log file you will see the connections to the submission port as “postfix/submission” (the - _syslog_name_ setting above) -- enforce encryption on this port (_smtpd_tls_security_level_) -- enable authentication (_smtpd_sasl_auth_enable_) -- enforce encryption during authentication (_smtpd_tls_auth_only_) -- allow sending emails to recipients outside of this mail server (_smtpd_reject_unlisted_recipient_) -- remove special restrictions (_smtpd\_\*\_restrictions_) -- allow relaying if the sender was authenticated (_smtpd_recipient_restrictions_) -- send the string _ORIGINATING_ to milter services (_milter_macro_daemon_name_) – you can just leave it like that +- Lines with options (`-o`) need to be indented. +- The word `submission` refers to the definition of the service in the `/etc/services` file. So it translates to TCP + port 587. +- The `smtpd` at the end tells Postfix that connections should be handled as incoming SMTP connections. +- **syslog_name** is adding `postfix/submission` to log files so that you can distinguish it from other connections. +- **smtpd_sasl_auth_enable** enables authentication on this port. SASL is short for _Simple Authentication and Security + Layer_. +- **smtpd_recipient_restrictions** define that only authenticated connections are permitted. Without a username and a + password your connection will be rejected. Restart the Postfix server: @@ -396,30 +387,37 @@ Feel free to send a test mail using the submission port and encryption. You will package first, so that SWAKS can speak TLS: ``` -apt install -y libnet-ssleay-perl -swaks --server localhost --to john@example.org \ +swaks --server localhost:587 \ + --from john@example.org \ + --to lisa@example.com \ --port 587 -tls \ --auth-user john@example.org \ --auth-password summersun ``` -