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diff --git a/src/blog/a/mail_server_with_postfix.md b/src/blog/a/mail_server_with_postfix.md new file mode 100644 index 0000000..63bf564 --- /dev/null +++ b/src/blog/a/mail_server_with_postfix.md @@ -0,0 +1,517 @@ +title: Create a mail server with Postfix, Dovecot, SpamAssassin and OpenDKIM +author: David Luévano +lang: en +summary: How to create mail server using Postfix, Dovecot, SpamAssassin and OpenDKIM. This is a follow up on post about creating a website with Nginx and Certbot. +tags: server + tools + tutorial + english + +The entry is going to be long because it's a *tedious* process. This is also based on [Luke Smith's script](https://github.com/LukeSmithxyz/emailwiz), but adapted to Arch Linux (his script works on debian-based distributions). This entry is mostly so I can record all the notes required while I'm in the process of installing/configuring the mail server on a new VPS of mine; also I'm going to be writing a script that does everything in one go (for Arch Linux), that will be hosted [here](https://git.luevano.xyz/server_scripts.git). + +This configuration works for local users (users that appear in `/etc/passwd`), and does not use any type of SQL Database. And note that most if not all commands executed here are run with root privileges. + +## Prerequisites + +Basically the same as with the [website with Nginx and Certbot](https://blog.luevano.xyz/a/website_with_nginx.html), with the extras: + +- You will need a **CNAME** for "mail" and (optionally) "www.mail", or whatever you want to call the sub-domains (although the [RFC 2181](https://tools.ietf.org/html/rfc2181#section-10.3) states that it NEEDS to be an **A** record, fuck the police). +- An SSL certificate. You can use the SSL certificate obtained following my last post using `certbot` (just create a `mail.conf` and run `certbot --nginx` again). +- Ports 25, 587 (SMTP), 465 (SMTPS), 143 (IMAP) and 993 (IMAPS) open on the firewall. + +## Postfix + +[Postfix](https://wiki.archlinux.org/title/postfix) is a "mail transfer agent" which is the component of the mail server that receives and sends emails via SMTP. + +Install the `postfix` package: + +```sh +pacman -S postfix +``` + +We have two main files to configure (inside `/etc/postfix`): `master.cf` ([master(5)](https://man.archlinux.org/man/master.5)) and `main.cf` ([postconf(5)](https://man.archlinux.org/man/postconf.5)). We're going to edit `main.cf` first either by using the command `postconf -e 'setting'` or by editing the file itself (I prefer to edit the file). + +Note that the default file itself has a lot of comments with description on what each thing does (or you can look up the manual, linked above), I used what Luke's script did plus some other settings that worked for me. + +Now, first locate where your website cert is, mine is at the default location `/etc/letsencrypt/live/`, so my `certdir` is `/etc/letsencrypt/live/luevano.xyz`. Given this information, change `{yourcertdir}` on the corresponding lines. The configuration described below has to be appended in the `main.cf` configuration file. + +Certificates and ciphers to use for authentication and security: + +```apache +smtpd_tls_key_file = {yourcertdir}/privkey.pem +smtpd_tls_cert_file = {yourcertdir}/fullchain.pem +smtpd_use_tls = yes +smtpd_tls_auth_only = yes +smtp_tls_security_level = may +smtp_tls_loglevel = 1 +smtp_tls_CAfile = {yourcertdir}/cert.pem +smtpd_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1 +smtp_tls_mandatory_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1 +smtpd_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1 +smtp_tls_protocols = !SSLv2, !SSLv3, !TLSv1, !TLSv1.1 +tls_preempt_cipherlist = yes +smtpd_tls_exclude_ciphers = aNULL, LOW, EXP, MEDIUM, ADH, AECDH, MD5, + DSS, ECDSA, CAMELLIA128, 3DES, CAMELLIA256, + RSA+AES, eNULL + +smtp_tls_CApath = /etc/ssl/certs +smtpd_tls_CApath = /etc/ssl/certs + +smtpd_relay_restrictions = permit_sasl_authenticated, permit_mynetworks, defer_unauth_destination +``` + +Also, for the *connection* with `dovecot`, append the next few lines (telling postfix that `dovecot` will use user/password for authentication): + +```apache +smtpd_sasl_auth_enable = yes +smtpd_sasl_type = dovecot +smtpd_sasl_path = private/auth +smtpd_sasl_security_options = noanonymous, noplaintext +smtpd_sasl_tls_security_options = noanonymous +``` + +Specify the mailbox home (this is going to be a directory inside your user's home containing the actual mail files): + +```apache +home_mailbox = Mail/Inbox/ +``` + +Pre-configuration to work seamlessly with `dovecot` and `opendkim`: + +```apache +myhostname = {yourdomainname} +mydomain = localdomain +mydestination = $myhostname, localhost.$mydomain, localhost + +milter_default_action = accept +milter_protocol = 6 +smtpd_milters = inet:127.0.0.1:8891 +non_smtpd_milters = inet:127.0.0.1:8891 +mailbox_command = /usr/lib/dovecot/deliver +``` + +Where `{yourdomainname}` is `luevano.xyz` in my case, or if you have `localhost` configured to your domain, then use `localhost` for `myhostname` (`myhostname = localhost`). + +Lastly, if you don't want the sender's IP and user agent (application used to send the mail), add the following line: + +```apache +smtp_header_checks = regexp:/etc/postfix/smtp_header_checks +``` + +And create the `/etc/postfix/smtp_header_checks` file with the following content: + +```coffee +/^Received: .*/ IGNORE +/^User-Agent: .*/ IGNORE +``` + +That's it for `main.cf`, now we have to configure `master.cf`. This one is a bit more tricky. + +First look up lines (they're uncommented) `smtp inet n - n - - smtpd`, `smtp unix - - n - - smtp` and `-o syslog_name=postfix/$service_name` and either delete or uncomment them... or just run `sed -i "/^\s*-o/d;/^\s*submission/d;/\s*smtp/d" /etc/postfix/master.cf` as stated in Luke's script. + +Lastly, append the following lines to complete postfix setup and pre-configure for `spamassassin`. + +```txt +smtp unix - - n - - smtp +smtp inet n - y - - smtpd + -o content_filter=spamassassin +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 +smtps inet n - y - - smtpd + -o syslog_name=postfix/smtps + -o smtpd_tls_wrappermode=yes + -o smtpd_sasl_auth_enable=yes +spamassassin unix - n n - - pipe + user=spamd argv=/usr/bin/vendor_perl/spamc -f -e /usr/sbin/sendmail -oi -f \${sender} \${recipient} +``` + +Now, I ran into some problems with postfix, one being [smtps: Servname not supported for ai_socktype](https://www.faqforge.com/linux/fix-for-opensuse-error-postfixmaster-fatal-0-0-0-0smtps-servname-not-supported-for-ai_socktype/), to fix it, as *Till* posted in that site, edit `/etc/services` and add: + +```apache +smtps 465/tcp +smtps 465/udp +``` + +Before starting the `postfix` service, you need to run `newaliases` first, but you can do a bit of configuration beforehand editing the file `/etc/postfix/aliases`. I only change the `root: you` line (where `you` is the account that will be receiving "root" mail). After you're done, run: + +```sh +postalias /etc/postfix/aliases +newaliases +``` + +At this point you're done configuring `postfix` and you can already start/enable the `postfix` service: + +```sh +systemctl start postfix.service +systemctl enable postfix.service +``` + +## Dovecot + +[Dovecot](https://wiki.archlinux.org/title/Dovecot) is an IMAP and POP3 server, which is what lets an email application retrieve the mail. + +Install the `dovecot` and `pigeonhole` (sieve for `dovecot`) packages: + +```sh +pacman -S dovecot pigeonhole +``` + +On arch, by default, there is no `/etc/dovecot` directory with default configurations set in place, but the package does provide the example configuration files. Create the `dovecot` directory under `/etc` and, optionally, copy the `dovecot.conf` file and `conf.d` directory under the just created `dovecot` directory: + +```sh +mkdir /etc/dovecot +cp /usr/share/doc/dovecot/example-config/dovecot.conf /etc/dovecot/dovecot.conf +cp -r /usr/share/doc/dovecot/example-config/conf.d /etc/dovecot +``` + +As Luke stated, `dovecot` comes with a lot of "modules" (under `/etc/dovecot/conf.d/` if you copied that folder) for all sorts of configurations that you can include, but I do as he does and just edit/create the whole `dovecot.conf` file; although, I would like to check each of the separate configuration files `dovecot` provides I think the options Luke provides are more than good enough. + +I'm working with an empty `dovecot.conf` file. Add the following lines for SSL and login configuration (also replace `{yourcertdir}` with the same certificate directory described in the Postfix section above, note that the `<` is required): + +```apache +ssl = required +ssl_cert = <{yourcertdir}/fullchain.pem +ssl_key = <{yourcertdir}/privkey.pem +ssl_min_protocol = TLSv1.2 +ssl_cipher_list = ALL:!RSA:!CAMELLIA:!aNULL:!eNULL:!LOW:!3DES:!MD5:!EXP:!PSK:!SRP:!DSS:!RC4:!SHA1:!SHA256:!SHA384:!LOW@STRENGTH +ssl_prefer_server_ciphers = yes +ssl_dh = </etc/dovecot/dh.pem + +auth_mechanisms = plain login +auth_username_format = %n +protocols = $protocols imap +``` + +You may notice we specify a file we don't have under `/etc/dovecot`: `dh.pem`. We need to create it with `openssl` (you should already have it installed if you've been following this entry and the one for `nginx`). Just run (might take a few minutes): + +```sh +openssl dhparam -out /etc/dovecot/dh.pem 4096 +``` + +After that, the next lines define what a "valid user is" (really just sets the database for users and passwords to be the local users with their password): + +```apache +userdb { + driver = passwd +} + +passdb { + driver = pam +} +``` + +Next, comes the mail directory structure (has to match the one described in the Postfix section). Here, the `LAYOUT` option is important so the boxes are `.Sent` instead of `Sent`. Add the next lines (plus any you like): + +```apache +mail_location = maildir:~/Mail:INBOX=~/Mail/Inbox:LAYOUT=fs +namespace inbox { + inbox = yes + + mailbox Drafts { + special_use = \Drafts + auto = subscribe + } + + mailbox Junk { + special_use = \Junk + auto = subscribe + autoexpunge = 30d + } + + mailbox Sent { + special_use = \Sent + auto = subscribe + } + + mailbox Trash { + special_use = \Trash + } + + mailbox Archive { + special_use = \Archive + } +} +``` + +Also include this so Postfix can use Dovecot's authentication system: + +```apache +service auth { + unix_listener /var/spool/postfix/private/auth { + mode = 0660 + user = postfix + group = postfix + } +} +``` + +Lastly (for Dovecot at least), the plugin configuration for `sieve` (`pigeonhole`): + +```apache +protocol lda { + mail_plugins = $mail_plugins sieve +} + +protocol lmtp { + mail_plugins = $mail_plugins sieve +} + +plugin { + sieve = ~/.dovecot.sieve + sieve_default = /var/lib/dovecot/sieve/default.sieve + sieve_dir = ~/.sieve + sieve_global_dir = /var/lib/dovecot/sieve/ +``` + +Where `/var/lib/dovecot/sieve/default.sieve` doesn't exist yet. Create the folders: + +```sh +mkdir -p /var/lib/dovecot/sieve +``` + +And create the file `default.sieve` inside that just created folder with the content: + +```nginx +require ["fileinto", "mailbox"]; +if header :contains "X-Spam-Flag" "YES" { + fileinto "Junk"; +} +``` + +Now, if you don't have a `vmail` (virtual mail) user, create one and change the ownership of the `/var/lib/dovecot` directory to this user: + +```sh +grep -q "^vmail:" /etc/passwd || useradd -m vmail -s /usr/bin/nologin +chown -R vmail:vmail /var/lib/dovecot +``` + +Note that I also changed the shell for `vmail` to be `/usr/bin/nologin`. After that, to compile the configuration file run: + +```sh +sievec /var/lib/dovecot/sieve/default.sieve +``` + +A `default.svbin` file will be created next to `default.sieve`. + +Next, add the following lines to `/etc/pam.d/dovecot` if not already present (shouldn't be there if you've been following these notes): + +```txt +auth required pam_unix.so nullok +account required pam_unix.so +``` + +That's it for Dovecot, at this point you can start/enable the `dovecot` service: + +```sh +systemctl start dovecot.service +systemctl enable dovecot.service +``` + +## OpenDKIM + +[OpenDKIM](https://wiki.archlinux.org/title/OpenDKIM) is needed so services like G\*\*gle (we don't mention that name here \[\[\[this is a meme\]\]\]) don't throw the mail to the trash. DKIM stands for "DomainKeys Identified Mail". + +Install the `opendkim` package: + +```sh +pacman -S opendkim +``` + +Generate the keys for your domain: + +```sh +opendkim-genkey -D /etc/opendkim -d {yourdomain} -s {yoursubdomain} -r -b 2048 +``` + +Where you need to change `{yourdomain}` and `{yoursubdomain}` (doesn't really need to be the sub-domain, could be anything that describes your key) accordingly, for me it's `luevano.xyz` and `mail`, respectively. After that, we need to create some files inside the `/etc/opendkim` directory. First, create the file `KeyTable` with the content: + +```txt +{yoursubdomain}._domainkey.{yourdomain} {yourdomain}:{yoursubdomain}:/etc/opendkim/{yoursubdomain}.private +``` + +So, for me it would be: + +```txt +mail._domainkey.luevano.xyz luevano.xyz:mail:/etc/opendkim/mail.private +``` + +Next, create the file `SigningTable` with the content: + +```txt +*@{yourdomain} {yoursubdomain}._domainkey.{yourdomain} +``` + +Again, for me it would be: + +```txt +*@luevano.xyz mail._domainkey.luevano.xyz +``` + +And, lastly create the file `TrustedHosts` with the content: + +```txt +127.0.0.1 +::1 +10.1.0.0/16 +1.2.3.4/24 +localhost +{yourserverip} +... +``` + +And more, make sure to include your server IP and something like `subdomain.domainname`. + +Next, edit `/etc/opendkim/opendkim.conf` to reflect the changes (or rather, addition) of these files, as well as some other configuration. You can look up the example configuration file located at `/usr/share/doc/opendkim/opendkim.conf.sample`, but I'm creating a blank one with the contents: + +```apache +Domain {yourdomain} +Selector {yoursubdomain} + +Syslog Yes +UserID opendkim + +KeyFile /etc/opendkim/{yoursubdomain}.private +Socket inet:8891@localhost +``` + +Now, change the permissions for all the files inside `/etc/opendkim`: + +```sh +chown -R root:opendkim /etc/opendkim +chmod g+r /etc/postfix/dkim/* +``` + +I'm using `root:opendkim` so `opendkim` doesn't complain about the `{yoursubdomani}.private` being insecure (you can change that by using the option `RequireSafeKeys False` in the `opendkim.conf` file, as stated [here](http://lists.opendkim.org/archive/opendkim/users/2014/12/3331.html)). + +That's it for the general configuration, but you could go more in depth and be more secure with some extra configuration. + +Now, just start/enable the `opendkim` service: + +```sh +systemctl start opendkim.service +systemctl enable opendkim.service +``` + +And don't forget to add the following **TXT** records on your domain registrar (these examples are for Epik): + +1. *DKIM* entry: look up your `{yoursubdomain}.txt` file, it should look something like: + +```txt +{yoursubdomain}._domainkey IN TXT ( "v=DKIM1; k=rsa; s=email; " + "p=..." + "..." ) ; ----- DKIM key mail for {yourdomain} +``` + +In the TXT record you will place `{yoursubdomain}._domainkey` as the "Host" and `"v=DKIM1; k=rsa; s=email; " "p=..." "..."` in the "TXT Value" (replace the dots with the actual value you see in your file). + +2. *DMARC* entry: just `_dmarc.{yourdomain}` as the "Host" and `"v=DMARC1; p=reject; rua=mailto:dmarc@{yourdomain}; fo=1"` as the "TXT Value". + +3. *SPF* entry: just `@` as the "Host" and `"v=spf1 mx a:{yoursubdomain}.{yourdomain} - all"` as the "TXT Value". + +And at this point you could test your mail for spoofing and more. + +## SpamAssassin + +[SpamAssassin](https://wiki.archlinux.org/title/SpamAssassin) is just *a mail filter to identify spam*. + +Install the `spamassassin` package (which will install a bunch of ugly `perl` packages...): + +```sh +pacman -S spamassassin +``` + +For some reason, the permissions on all `spamassassin` stuff are all over the place. First, change owner of the executables, and directories: + +```sh +chown spamd:spamd /usr/bin/vendor_perl/sa-* +chown spamd:spamd /usr/bin/vendor_perl/spam* +chwown -R spamd:spamd /etc/mail/spamassassin +``` + +Then, you can edit `local.cf` (located in `/etc/mail/spamassassin`) to fit your needs (I only uncommented the `rewrite_header Subject ...` line). And then you can run the following command to update the patterns and compile them: + +```sh +sudo -u spamd sa-update +sudo -u spamd sa-compile +``` + +And since this should be run periodically, create the service `spamassassin-update.service` under `/etc/systemd/system` with the following content: + +```ini +[Unit] +Description=SpamAssassin housekeeping +After=network.target + +[Service] +User=spamd +Group=spamd +Type=oneshot + +ExecStart=/usr/bin/vendor_perl/sa-update --allowplugins +SuccessExitStatus=1 +ExecStart=/usr/bin/vendor_perl/sa-compile +ExecStart=/usr/bin/systemctl -q --no-block try-restart spamassassin.service +``` + +And you could also execute `sa-learn` to train `spamassassin`'s bayes filter, but this works for me. Then create the timer `spamassassin-update.timer` under the same directory, with the content: + +```ini +[Unit] +Description=SpamAssassin housekeeping + +[Timer] +OnCalendar=daily +Persistent=true + +[Install] +WantedBy=timers.target +``` + +You can now start/enable the `spamassassin-update` timer: + +```sh +systemctl start spamassassin-update.timer +systemctl enable spamassassin-update.timer +``` + +Next, you may want to edit the `spamassassin` service before starting and enabling it, because by default, it could [spawn a lot of "childs"](https://rimuhosting.com/howto/memory.jsp) eating a lot of resources and you really only need one child. Append `--max-children=1` to the line `ExecStart=...` in `/usr/bin/systemd/system/spamassassin.service`: + +```ini +... +ExecStart=/usr/bin/vendor_perl/spamd -x -u spamd -g spamd --listen=/run/spamd/spamd.sock --listen=localhost --max-children=1 +... +``` + +Finally, start and enable the `spamassassin` service: + +```sh +systemctl start spamassassin.service +systemctl enable spamassassin.service +``` + +## Wrapping up + +We should have a working mail server by now. Before continuing check your journal logs (`journalctl -xe --unit={unit}`, where `{unit}` could be `spamassassin.service` for example) to see if there was any error whatsoever and try to debug it, it should be a typo somewhere (the logs are generally really descriptive) because all the settings and steps detailed here just (literally just finished doing everything on a new server as of the writing of this text) worked *(((it just werks on my machine)))*. + +Now, to actually use the mail service: first of all, you need a *normal* account (don't use root) that belongs to the `mail` group (`gpasswd -a user group` to add a user `user` to group `group`) and that has a password. + +Next, to actually login into a mail app/program/whateveryouwanttocallit, you will use the following settings, at least for `thunderdbird`(I tested in windows default mail app and you don't need a lot of settings): + +- \* server: subdomain.domain (mail.luevano.xyz in my case) +- **SMTP** port: 587 +- **SMTPS** port: 465 (I use this one) +- **IMAP** port: 143 +- **IMAPS** port: 993 (again, I use this one) +- Connection/security: SSL/TLS +- Authentication method: Normal password +- Username: just your `user`, not the whole email (`david` in my case) +- Password: your `user` password (as in the password you use to login to the server with that user) + +All that's left to do is test your mail server for spoofing, and to see if everything is setup correctly. Go to [DKIM Test](https://www.appmaildev.com/en/dkim) and follow the instructions (basically click next, and send an email with whatever content to the email that they provide). After you send the email, you should see something like: + + + +Finally, that's actually it for this entry, if you have any problem whatsoever you can [contact me](https://luevano.xyz/contact.html). |