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Lua Authentication

Introduction

Dovecot has the ability to let users create their own custom user provisioning and authentication providers in Lua. This allows any data source that can be approached from Lua to be used for authentication, including web servers. It is possible to do more with Dovecot and Lua, but other use cases fall outside of the scope of this documentation page.

Community contributed guide

Dovecot authentication via Lua scripting is not officially supported in DMS. No assistance will be provided should you encounter any issues.

DMS provides the required packages to support this guide. Note that these packages will be removed should they introduce any future maintenance burden.

The example in this guide relies on the current way in which DMS works with Dovecot configuration files. Changes to this to accommodate new authentication methods such as OpenID Connect will likely break this example in the future. This guide is updated on a best-effort base.

Dovecot's Lua support can be used for user provisioning (userdb functionality) and/or password verification (passdb functionality). Consider using other userdb and passdb options before considering Lua, since Lua does require the use of additional (unsupported) program code that might require maintenance when updating DMS.

Each implementation of Lua-based authentication is custom. Therefore it is impossible to write documentation that covers every scenario. Instead, this page describes a single example scenario. If that scenario is followed, you will learn vital aspects that are necessary to kickstart your own Lua development:

  • How to override Dovecot's default configuration to disable parts that conflict with your scenario.
  • How to make Dovecot use your Lua script.
  • How to add your own Lua script and any libraries it uses.
  • How to debug your Lua script.

The example scenario

This scenario starts with DMS being configured to use LDAP for mailbox identification, user authorization and user authentication. In this scenario, Nextcloud is also a service that uses the same LDAP server for user identification, authorization and authentication.

The goal of this scenario is to have Dovecot not authenticate the user against LDAP, but against Nextcloud using an application password. The idea behind this is that a compromised mailbox password does not compromise the user's account entirely. To make this work, Nextcloud is configured to deny the use of account passwords by clients and to disable account password reset through mail verification.

If the application password is configured correctly, an adversary can only use it to access the user's mailbox on DMS, and CalDAV and CardDAV data on Nextcloud. File access through WebDAV can be disabled for the application password used to access mail. Having CalDAV and CardDAV compromised by the same password is a minor setback. If an adversary gets access to a Nextcloud application password through a device of the user, it is likely that the adversary also gets access to the user's calendars and contact lists anyway (locally or through the same account settings used for mail and CalDAV/CardDAV synchronization). The user's stored files in Nextcloud, the LDAP account password and any other services that rely on it would still be protected. A bonus is that a user is able to revoke and renew the mailbox password in Nextcloud for whatever reason, through a friendly user interface with all the security measures with which the Nextcloud instance is configured (e.g. verification of the current account password).

A drawback of this method is that any (compromised) Nextcloud application password can be used to access the user's mailbox. This introduces a risk that a Nextcloud application password used for something else (e.g. WebDAV file access) is compromised and used to access the user's mailbox. Discussion of that risk and possible mitigations fall outside of the scope of this scenario.

To answer the questions asked earlier for this specific scenario:

  1. Do I want to use Lua to identify mailboxes and verify that users are are authorized to use mail services? No. Provisioning is done through LDAP.
  2. Do I want to use Lua to verify passwords that users authenticate with for IMAP/POP3/SMTP in their mail clients? Yes. Password authentication is done through Lua against Nextcloud.
  3. If the answer is 'yes' to question 1 or 2: are there other methods that better facilitate my use case instead of custom scripts which rely on me being a developer and not just a user? No. Only HTTP can be used to authenticate against Nextcloud, which is not supported natively by Dovecot or DMS.

While it is possible to extend the authentication methods which Nextcloud can facilitate with Nextcloud apps, there is currently a mismatch between what DMS supports and what Nextcloud applications can provide. This might change in the future. For now, Lua will be used to bridge the gap between DMS and Nextcloud for authentication only (Dovecot passdb), while LDAP will still be used to identify mailboxes and verify authorization (Dovecot userdb).

Modify Dovecot's configuration

Add to DMS volumes in compose.yaml 
    # All new volumes are marked :ro to configure them as read-only, since their contents are not changed from inside the container
    volumes:
      # Configuration override to disable LDAP authentication
      - ./docker-data/dms/config/dovecot/auth-ldap.conf.ext:/etc/dovecot/conf.d/auth-ldap.conf.ext:ro
      # Configuration addition to enable Lua authentication
      - ./docker-data/dms/config/dovecot/auth-lua-httpbasic.conf:/etc/dovecot/conf.d/auth-lua-httpbasic.conf:ro
      # Directory containing Lua scripts
      - ./docker-data/dms/config/dovecot/lua/:/etc/dovecot/lua/:ro

Create a directory for Lua scripts: 

mkdir -p ./docker-data/dms/config/dovecot/lua

Create configuration file ./docker-data/dms/config/dovecot/auth-ldap.conf.ext for LDAP user provisioning: 

userdb {
  driver = ldap
  args = /etc/dovecot/dovecot-ldap.conf.ext
}

Create configuration file ./docker-data/dms/config/dovecot/auth-lua-httpbasic.conf for Lua user authentication: 

passdb {
  driver = lua
  args = file=/etc/dovecot/lua/auth-httpbasic.lua blocking=yes
}

That is all for configuring Dovecot.

Create the Lua script

Create Lua file ./docker-data/dms/config/dovecot/lua/auth-httpbasic.lua with contents:

local http_url = "https://nextcloud.example.com/remote.php/dav/"
local http_method = "PROPFIND"
local http_status_ok = 207
local http_status_failure = 401
local http_header_forwarded_for = "X-Forwarded-For"

package.path = package.path .. ";/etc/dovecot/lua/?.lua"
local base64 = require("base64")

local http_client = dovecot.http.client {
  timeout = 1000;
  max_attempts = 1;
  debug = false;
}

function script_init()
  return 0
end

function script_deinit()
end

function auth_passdb_lookup(req)
  local auth_request = http_client:request {
    url = http_url;
    method = http_method;
  }
  auth_request:add_header("Authorization", "Basic " .. (base64.encode(req.user .. ":" .. req.password)))
  auth_request:add_header(http_header_forwarded_for, req.remote_ip)
  local auth_response = auth_request:submit()
  local resp_status = auth_response:status()
  local reason = auth_response:reason()

  local returnStatus = dovecot.auth.PASSDB_RESULT_INTERNAL_FAILURE
  local returnDesc = http_method .. " - " .. http_url .. " - " .. resp_status .. " " .. reason
  if resp_status == http_status_ok
  then
    returnStatus = dovecot.auth.PASSDB_RESULT_OK
    returnDesc = "nopassword=y"
  elseif resp_status == http_status_failure
  then
    returnStatus = dovecot.auth.PASSDB_RESULT_PASSWORD_MISMATCH
    returnDesc = ""
  end
  return returnStatus, returnDesc
end

Replace the hostname in the URL to the actual hostname of Nextcloud.

Dovecot provides an HTTP client for use in Lua. Aside of that, Lua by itself is pretty barebones. It chooses library compactness over included functionality. You can see that in that a separate library is referenced to add support for Base64 encoding, which is required for HTTP basic access authentication. This library (also a Lua script) is not included. It must be downloaded and stored in the same directory:

cd ./docker-data/dms/config/dovecot/lua
curl -JLO https://raw.githubusercontent.com/iskolbin/lbase64/master/base64.lua

Only use native (pure Lua) libraries as dependencies if possible, such as base64.lua from the example. This ensures maximum compatibility. Performance is less of an issue since Lua scripts written for Dovecot probably won't be long or complex, and there won't be a lot of data processing by Lua itself.

Debugging a Lua script

To see which Lua version is used by Dovecot if you plan to do something that is version dependent, run:

docker exec CONTAINER_NAME strings /usr/lib/dovecot/libdovecot-lua.so|grep '^LUA_'

While Dovecot logs the status of authentication attempts for any passdb backend, Dovecot will also log Lua scripting errors and messages sent to Dovecot's Lua API log functions. The combined DMS log (including that of Dovecot) can be viewed using docker logs CONTAINER_NAME. If the log is too noisy (due to other processes in the container also logging to it), docker exec CONTAINER_NAME cat /var/log/mail/mail.log can be used to view the log of Dovecot and Postfix specifically.

If working with HTTP in Lua, setting debug = true; when initiating dovecot.http.client will create debug log messages for every HTTP request and response.

Note that Lua runs compiled bytecode, and that scripts will be compiled when they are initially started. Once compiled, the bytecode is cached and changes in the Lua script will not be processed automatically. Dovecot will reload its configuration and clear its cached Lua bytecode when running docker exec CONTAINER_NAME dovecot reload. A (changed) Lua script will be compiled to bytecode the next time it is executed after running the Dovecot reload command.