A Fully-Virtual, Host-Mode Version of Poste.io

poste.io is a pretty cool email server implementation for docker. Unfortunately, when used with host-mode networking (the poste.io recommended configuration) it doesn't play well with other mail servers on the same machine. (Which makes it hard to e.g., have both a development and production instance, or to provide service to multiple clients on one machine.)

Specifically, in host mode networking, poste.io binds its outward-facing services to every IP address of the machine, and binds several of its internal services to localhost ports (6379, 11332-11334, 11380, 11381, and 13001), which can conflict with things besides mail servers or other poste.io instances.

As a result, poste.io not only doesn't play well with other mail servers (including other instances of itself), it also doesn't play well with being used on a server that does anything else. (It almost might as well not be a docker container at all, in such a setup!) And last, but not least, it sends email out on any old IP address as well, with no way to choose which IP you actually want to send things on.

So, this image fixes these issues by adding support for two environment variables and a configuration file, that let you not only control which IPs poste will listen on, but also which addresses poste will send mail on, optionally on a per-domain basis. (Plus, it patches poste.io's default configuration so that all its internal services use unix domain sockets inside the docker container, instead of tying up localhost ports on the main server.)

The first variable it adds is LISTEN_ON, which can be set to either a list of specific IP addresses to listen on, host (to listen only on addresses bound to the container's hostname), or * (for poste's default behavior of listening on every available interface).

The second variable is SEND_ON, which can also be set to a list of IP addresses, host, or *. (If unset or empty, it defaults to the value set in LISTEN_ON.) By default, mail will be sent from the first IP address in the resulting list, unless it's *, in which case the operating system will pick the IP. If there's only one sending IP, all mail will be sent from the default IP. Otherwise, a configuration file will be used to pick an IP address from the list, based on the domain the mail is being sent from.

Basic Usage

To use this image, just replace analogic/poste.io in your config with dirtsimple/poste.io. For example, you might use something like this as your docker-compose.yml, replacing mail.example.com with a suitable hostname for your installation:

version: "2.3"
services:
  poste:
    image: dirtsimple/poste.io
    restart: always
    network_mode: host  # <-- a must-have for poste

    # serve everything on `mail.example.com`, which will be the default HELO as well:
    hostname: mail.example.com

    volumes:
      - ./data:/data
      - /etc/localtime:/etc/localtime:ro

    # ==== Optional settings below: you don't need any environment vars by default ====

    environment:
      # Whitespace-separated list of IP addresses to listen on. If this variable
      # is set to "host" (which is also the default if it's empty or unset), the
      # container will listen on all the IPs (v4 and v6) found in DNS or /etc/hosts
      # for the container's hostname.  Or it can be set to "*", to listen on ALL
      # available addresses (the way the standard poste.io image does).
      - "LISTEN_ON=1.2.3.4 5.6.7.8 90a:11:12::13"

      # Whitespace-separated list of IP addresses mail can be sent from; the first
      # one in the list will be the default.  Like LISTEN_ON, it can be set to '*'
      # for "any available address" or 'host' for "any IP (v4 or v6) attached to
      # the container hostname".  If the list expands to only one address, it
      # will be used for all outgoing mail.  Otherwise, data/outbound-hosts.yml
      # is read to determine the outgoing IP for each domain, and the result is
      # validated against this list.  If this variable is empty or unset, it defaults
      # to whatever LISTEN_ON was set to.
      - "SEND_ON=9.10.11.12"

      # Other standard poste.io vars can also be used, e.g. HTTPS_PORT, etc.

Take note of the following, however:

• You must configure the container with a fully-qualified hostname (e.g. mail.example.com above), with at least one IP address listed in the public DNS system
• The hostname's IP addresses (or those listed in LISTEN_ON) must be public IPs attached to the server hosting the container
• The listening IPs must not have any other services listening on ports 25, 80, 110, 143, 443, 466, 587, 993, 995, or 4190. (Though you can change or disable some of those ports using poste.io's environment variables.)
• You should be using host-mode networking (network_mode: host as shown above), since in any other networking mode, this image will behave roughly the same as the original analogic/poste.io image, and have the same limitations and caveats. (Specifically, using any other networking mode means putting specific IP addresses into LISTEN_ON, SEND_ON, or outbound-hosts.yml will not do anything useful!)
• By default, outgoing email to other mail servers will be sent via the first IP address found in LISTEN_ON or returned by running hostname -i in the container. If you need to override this behavior, configure the container with SEND_ON set to the specific IP address to be used, OR create a /data/outbound-hosts.yml file as described in Managing Sender IPs below.

Notice, by the way, that there are no port mappings used in this example, because the container uses host-mode networking and thus has direct access to all of the server's network interfaces. This means that the IP addresses to be used by the container must be explicitly defined (either by the DNS address(es) of the hostname, or by setting the LISTEN_ON variable to the exact IP addresses) so that the container doesn't take over every IP address on the server. (Unless that's what you want, in which case you can set LISTEN_ON to *.)

Managing Hostnames and IP Addresses

In the simplest cases, an installation of this image would only need to use one hostname and IP, and:

• The hostname would be set as the MX record of any domains to be hosted on the instance
• Reverse DNS for the IP would point to the hostname
• The default TLS certificate generated by the image would suffice
• Users would log into webmail and admin using the single, primary hostname

In more complex setups, you may wish to use multiple IPs or hostnames, for example to give each domain its own mail.somedomain.com website and/or MX, or to separate the sending reputation of different domains, while keeping to a single container. These scenarios can be done, but note that it is not possible to 100% hide the fact that all the domains are being served by the same container, as the TLS certificate used for both the web interface and SMTP will list all the hostnames sharing the container. (So if you need truly private instances, you will need to create separate containers.)

But, if all you need is to give users domain-specific hostnames, or separate sender IP reputation for different domains, you can accomplish that with a single shared container.

Vanity or Private-Label Logins

Let's say you want to give each domain its own mail.mydomain.com address for users to put into their mail clients, log into on the web, use as an MX entry, etc. You don't need multiple IP addresses to do this, just multiple hostnames. All that's needed is to:

• Have each vanity/private-label hostname resolve to one of the IP addresses the container listens on (e.g. by being a CNAME of the primary hostname)
• Add each such hostname to the "Alternative names" of your TLS certificate in the "Mailserver settings" of the primary admin interface

You must, however, still pick one primary hostname for the container, as that's what you'll use to boot up the container and access the admin interface to create the TLS certificate. The primary hostname will be the primary name on that certificate, with the vanity hostnames added as alternative names, once they're resolving correctly via public DNS, and the container is listening on the corresponding IP(s).

Separate IPs for Different Domains

If you want to give different domains their own IPs as well as separate hostnames, the steps are the same, except that each private-label hostname would have A or AAAA records pointing to the relevant IP address, instead of a CNAME pointing to the primary hostname. If you want these IPs to be used for outgoing mail as well, you'll also need to configure an outbound-hosts.yml file, as described in the next section. (And if needed, add them to the SEND_ON variable.)

You will, of course, still need to configure the container to listen on all these IPs, either by explicitly putting them in LISTEN_ON, or by adding them as A or AAAA records for the primary hostname. Or, if you're dedicating the entire server to a single poste instance, you can use LISTEN_ON=* to listen on every IP the box has.

(Note, however, that since poste.io only supports using a single TLS certificate for all functions, it will still be possible for clients connecting to the container to see all the hostnames it serves, so if that isn't acceptable for your setup, then you will need to create separate instances instead, each serving separate IPs.)

Managing Sender IPs

In some environments, you may wish to use different sending IP addresses for different origin domains. To support this use case, you can add a file named outbound-hosts.yml to the /data volume, laid out like this:

# This info will be used for domains that don't have an entry of their own
default:
  helo: poste.mygenericdomain.com
  ip: 1.2.3.4

exampledomain.com:
  helo: mx.exampledomain.com
  ip: 5.6.7.8

With the above configuration, mails sent from exampledomain.com will be sent with a HELO of mx.exampledomain.com, using an outbound IP of 5.6.7.8, and mail for any other domain will use the defaults. (Assuming, of course, that 5.6.7.8 is one of the addresses the container listens on.)

Note that the information in this file is not validated against DNS or checked for security (aside from a basic check that the IP is included in the expansion of SEND_ON). It is your responsibility to ensure that all helo hostnames exist in DNS with the matching ip , and that all listed IP addresses are actually valid for the network interfaces on your server.

In addition, for best deliverability, you should also:

• Ensure that SPF will pass for a given domain + helo/ip combination
• Ensure that the reverse DNS for the given ip values has a reasonable result (preferably the same as the helo)
• Ensure that each helo address used as an MX is listed in the "Alternative names" of your TLS certificate in the "Mailserver settings" of the poste admin interface, and that its corresponding ip is an address the container listens on.

And of course, you will need to update all of this information whenever any of the configuration changes! If you control DNS for all the relevant domains yourself, you may be able to generate this file automatically from your domain list and DNS: e.g. by looking up MX records and their corresponding addresses. (But you shouldn't trust the DNS for domains you don't control, as that would effectively let your clients pick their own sending IPs.)

Can I use these changes with poste.io's PRO version?

I don't know, but you can find out by cloning this repo, changing the FROM in the Dockerfile, and trying to run the resulting build. It might work, since the main difference between the two versions is some admin interface code left out of the free version. But if that left-out code contains hardcoded references to localhost or 127.0.0.1, then those admin features will probably break, as they won't have been patched to use unix-domain sockets instead.

If they do break, and you can figure out what to patch (most likely, PHP code in /opt/admin/src/ProBundle/), let me know. (Or if it works fine, I'd love to know that, too.)