This document refers to Schleuder version 3.2 To read about older versions of Schleuder please see the older docs.

Disclaimer: this is work in progress. To suggest improvements see Feedback.


Schleuder is an email hub for groups. Subscribers can communicate encryptedly and pseudonymously among themselves, receive emails from non-subscribers and send emails to non-subscribers via the list. Schleuder takes care of all de- and encryption, stripping of headers, formatting conversions, etc. Further Schleuder can send out its own public key and receive administrative commands by email.

A “wanted man-in-the-middle”

Basically Schleuder is a “wanted man-in-the-middle”.

Each list has its own keypair. Schleuder decrypts every incoming email and verifies its signature with the keys from the list’s keyring. Then Schleuder loops over the list of subscribers, creates for each a stripped down copy of the message, encrypts it with the subscriber’s key and signs it with its own key, and sends it out.

Schleuder inserts some lines of metadata into the top of the email, containing a (configurable) copy of some of the original headers and the result of the decryption and verification of the incoming email. Here’s an example:

From: Bob <>
Date: Tue, 6 Apr 2010 17:28:46 +0200
Enc: encrypted
Sig: Good signature from 12345678DEADBEEF Bob <>

The consequence of this approach is that you need to really trust the provider that runs Schleuder: they could store and decrypt all emails that pass the lists, if they wanted.

An email hub for groups

Schleuder can receive and send emails to non-subscribers, which makes it a useful email hub for groups.

Incoming emails from non-subscribers are (re)encrypted and distributed among the subscribers, which can communicate internally as well as answer to the outside person via the list.

The outside person only sees an email address with a public key, but has no information about who is behind it.

Here’s a simple picture of a message that is sent to a non-subscriber (“Zacharias”). It illustrates how Alice, Bob, Claire and David are invisible to Zacharias (click image to enlarge it).


Technical details

Schleuder behaves like an email-filter: it reads email from standard-input, and reports errors to standard-error. If all goes well Schleuder closes the initial connection to the Mail Transport Agent (MTA) only after it sent out all outgoing emails.

In case of an error the MTA includes Schleuder’s error message into a bounce-email that is sent back to the sender.

The keyrings for each list are standard GnuPG keyrings and sit in the filesystem under $lists_dir/$hostname/$listname/ ($lists_dir is read from schleuder.yml, see Configuration). They can be used manually using gpg2. Please be careful to maintain proper file permissions if you touch the files.

In the list-directory there’s also a list specific log-file (might be missing if the log-level is high and no error occurred yet).

Other logging is sent to syslog. Where that ends up depends on the operating system and the system administration.

All other list-related data is stored in the SQL-database. Most data is unserialized, only some values are JSON-encoded.



You can install schleuder either from Linux distribution packages or rubygems. Currently there are supported distribution packages for Debian (“stretch-backports” and “buster”) and CentOS 7. If you use one of the directly supported platforms, you should choose the packages over the gems.

Don’t use the packages provided by Ubuntu in all releases up to and including 17.10, they are severely outdated. On Ubuntu 18.04 only use the package if it has at least version 3.2.2.

Besides schleuder you should also install at least one of schleuder-cli (the command line tool to manage Schleuder lists), and schleuder-web (the web interface to manage and maintain Schleuder lists).

Additionally we recommend running an entropy source such as haveged. This ensures Schleuder won’t be blocked by lacking entropy, which otherwise might happen e.g. during key generation.


All steps need root privileges.

We maintain schleuder and schleuder-cli in “stretch-backports” and “buster”. (For production usage we recommend Debian “stretch”.)

First, add the backports to your APT sources list directory:

echo "deb stretch-backports main" > /etc/apt/sources.list.d/stretch-backports.list

After this, update the APT package cache:

apt-get update

Finally, install schleuder and schleuder-cli via:

apt-get install -t stretch-backports schleuder schleuder-cli

The package will finalize the setup of Schleuder, too.


For CentOS 7 there is a maintained copr-repository using Software Collections.

All steps need root privileges

Install the repository & the SCL repository

yum install centos-release-scl
curl -o /etc/yum.repos.d/schleuder-schleuder-epel-7.repo

Now you are ready to install schleuder and schleuder-cli

yum install schleuder schleuder-cli

Afterwards run schleuder install to finalize the setup of Schleuder. This creates necessary directories, copies example configs, etc. If you see errors about missing write permissions please follow the advice given.

The copr-repository also provides you with a package for schleuder-web. Please read the documentation of schleuder-web on how to get it up and running.

From Ruby-Gem

For instructions on how to install from rubygems please see the README of Schleuder.


There is a schleuder puppet module to automate the installation and configuration of schleuder, schleuder-cli, and schleuder-web, and the creation and deletion of schleuder lists. Have a look at its README on how to use it. Currenty it works for CentOS 7, but we would like to make it work for Debian as well - help would be highly appreciated.


Schleuder reads its basic settings from a file that it by default expects at /etc/schleuder/schleuder.yml. To make Schleuder read a different file set the environment variable SCHLEUDER_CONFIG to the path to your file when running schleuder. E.g.:

SCHLEUDER_CONFIG=/usr/local/etc/schleuder.yml /path/to/bin/schleuder ...

For explanations of the possible settings read the default config file (also available in the repository).

The default settings for new lists are read from another config file. By default Schleuder looks at /etc/schleuder/list-defaults.yml. To make Schleuder read a different file set the environment variable SCHLEUDER_LIST_DEFAULTS analogous to above. The possible settings are explained in the default config file, which is also available in the repository.

Once a list is created, it is not affected by these configuration files any more. Existing lists have their configuration stored in the database. The settings in the database can be shown and set via the schleuder API, available through schleuder-web or schleuder-cli. Run schleuder-cli lists help and schleuder-cli lists list-options for more information on the latter.

Hook into Mail Transport Agent

In “work”-mode, Schleuder expects the list’s email-address as second argument (first one is “work”) and the incoming email on standard-input.

To enable Schleuder to receive emails, your Mail Transport Agent must be configured accordingly. How to do this with Postfix is documented in detail below.


This section describes only those parts of a Postfix-setup that are relevant to Schleuder. We assume that you have a sensible and tested Postfix-setup already.

Firstly, to hook Schleuder into Postfix adapt these lines (path and maybe user) and add them to

schleuder  unix  -       n       n       -       -       pipe
  flags=DRhu user=schleuder argv=/path/to/bin/schleuder work ${recipient}

Then you have to chose how postfix should decide if a message should be delivered to Schleuder. There are two options:

  1. Configure it for each list individually. That’s the way to go if you don’t run many lists, or use the respective domain also for a varying number of email accounts or aliases.
  2. Dedicate a whole domain to Schleuder. That’s the way to go if you run more lists than email accounts or aliases on that domain.

To configure each list individually, add these lines to

schleuder_destination_recipient_limit = 1
transport_maps = hash:/etc/postfix/transport_schleuder

Now adapt the following lines for each list and add them to /etc/postfix/transport_schleuder:          schleuder:  schleuder:    schleuder:   schleuder:  schleuder:

Afterwards run postmap /etc/postfix/transport_schleuder and restart postfix. Remember to repeat this also for newly created lists later.

Another way to tell postfix which domain and list can be piped to schleuder is to get that information out of the sqlite database. A requirement for that is the postfix-sqlite package, which isn’t in the standard repositories of CentOS, but Debian.

To dedicate a whole domain to Schleuder, add these lines to

schleuder_destination_recipient_limit = 1
virtual_mailbox_domains = sqlite:/etc/postfix/
virtual_transport       = schleuder
virtual_alias_maps      = hash:/etc/postfix/virtual_aliases
virtual_mailbox_maps    = sqlite:/etc/postfix/

Then adapt and add at least the following exceptions from the All-to-Schleuder-rule to /etc/postfix/virtual_aliases:    root@anotherdomain         root@anotherdomain root@anotherdomain          root@anotherdomain

Afterwards run postmap /etc/postfix/virtual_aliases.

The file can ask the schleuder sqlite database (this will delegate the whole domain to schleuder):

dbpath = /var/lib/schleuder/db.sqlite
query = select distinct substr(email, instr(email, '@') + 1) from lists
        where email like '%%%s'

And the file can also get the information from the schleuder sqlite database:

dbpath = /var/lib/schleuder/db.sqlite
query = select 'present' from lists
          where email = '%s'
          or    email = replace('%s', '-bounce@', '@')
          or    email = replace('%s', '-owner@', '@')
          or    email = replace('%s', '-request@', '@')
          or    email = replace('%s', '-sendkey@', '@')

From now on each Schleuder-list will instantly be reachable by email once it was created.


This section describes only those parts of a Exim-setup that are relevant to Schleuder. We assume that you have a sensible and tested Exim-setup already.

As with any exim email routing, we need to configure essentially a router that accepts and directs a mail to a transport, which knows how to hand-over an email to schleuder.

Within the begin routers section of your exim.conf you can add the following router:

  driver = accept
  condition = ${lookup {$local_part@$domain}lsearch{/etc/exim/schleuder-lists} {yes}{no}}
  local_part_suffix = +* : -bounce : -sendkey : -request : -owner
  transport = mlschleuder_transport_local

/etc/exim/schleuder-lists is a simple textfile containing one list-address per line. You can for example create it by executing schleuder-cli lists list > /etc/exim/schleuder-lists after creating or deleting any lists. In more advanced setups you might have different conditions depending on how you manage the inventory of your schleuder lists and decide to accept a mail for a list.

Within the begin transports section of your exim.conf you then configure the transport:

   driver = pipe
   user = schleuder
   group = schleuder
   # schleuders generates nice log messages for some of the problems
   return_fail_output = true
   home_directory = /var/lib/schleuder/lists/$domain/$local_part
   command = "/usr/bin/schleuder work $local_part$local_part_suffix@$domain"
   message_size_limit = 10M

Please note that we keep the $local_part_suffix when handing the mail over to schleuder, so schleuder can e.g. detect bounces or sendkey emails properly.

Restart exim and you have your working schleuder+exim setup.

Remember to repeat dumping the list of schleuder-lists to /etc/exim/schleuder-lists also for newly created lists later.

Schleuder API

The Schleuder API is provided by schleuder-api-daemon. Configuration clients (schleuder-web, schleuder-cli) use it to access information about lists, subscriptions, and keys. As you probably want to at least use schleuder-cli from localhost, setting up schleuder-api-daemon is useful even without remote clients.

Schleuder does not use schleuder-api-daemon to process emails. You can stop schleuder-api-daemon at any time without breaking the email flow.

To run schleuder-api-daemon, depending on the type of operating system and the setup you are using, you can either start the systemd-unit-file:

systemctl start schleuder-api-daemon

Or you can run it manually in a shell:


Please take care to run schleuder-api-daemon as the user that owns the directory of schleuder lists (by default /var/lib/schleuder/lists) to avoid running into file permission problems!

Transport encryption

schleuder-api-daemon uses transport encyrption (TLS) for all connections. The required TLS-certifcates should have been generated during the setup (schleuder install). You can generate new ones at any time by executing:

schleuder cert generate

If the file systems permissions allow it, Schleuder will write the certificate and the key directly into the correct files (paths are read from the configuration file). Otherwise you might have to move them. Please read the output of the above command for possible instructions.

In case you already have a suitable certificate you can use that, too. Its hostnames do not matter. Just copy it to the paths specified in the configuration file, or change those paths.

In order to verify the connection, each client needs to know the fingerprint of the API-certificate. The fingerprint will be shown when generating the certificates. Later you can always have it show again by executing this:

schleuder cert fingerprint

Use secure channels to transport this information!


The Schleuder API uses API-keys to authenticate clients.

You can generate new API-keys by executing:

schleuder new_api_key

To enable the client to connect, their API-key must be added to the section valid_api_keys in Schleuder’s configuration file.

Provide each client with their own API-key, and use secure channels to transport this information!

There is no authorization of clients, yet. Each client is allowed every action. So be wary who to give an API-key to. schleuder-web does its own authorization, but schleuder-cli does not!

Managing a list

To create and manage lists you have two options: schleuder-web and schleuder-cli.

Both require a running schleuder-api-daemon. Please see the previous section on how to set that up.


To create lists with schleuder-web log in as root@localhost. Managing lists is allowed to each list-admin.


To use schleuder-cli please see the output of

schleuder-cli help

Using a list

Everything you send to foo@hostname will be send to all subscribers, but they will see only certain headers and the body of your email. The selection of these headers can be configured for each list individually by the list-admins.

Getting a list’s public key

Each Schleuder-list replies with its public key to any email sent to foo-sendkey@hostname. E.g. to receive the key for our contact address write an email to

Special keywords

Schleuder knows some special keywords that trigger different behaviour. You can e.g. subscribe someone, or resend an email to a non-subscriber using keywords. See a list of available keywords below.

Keywords require that:

Keywords can be repeated within one email at will. Letter case doesn’t matter.

There are two types of keywords: those to enhance messages sent over the list (“list-keywords”), and those to request something from Schleuder (“request-keywords”).


You must always provide this keyword once per email. Without it, no other keyword will be considered but you will receive an error message. It helps to mitigate replay-attacks.


The resending-keywords must be included in messages sent to the normal list-address: foo@hostname.

Send the message to the given address, encrypted if possible, otherwise in the clear.
Send the message to the given address only if it could be encrypted. Can be abbreviated to x-resend-enc.
Send the message to the given address without encrypting it. You can use this keyword to make schleuder skip looking for a matching key for this address and enforce sending the email out in the clear.
Send one message to all of the given addresses in Cc, so they get to know of each other (encrypted if possible, otherwise in the clear).
Send one message to all of the given addresses in Cc, so they get to know of each other, only if it could be encrypted to all of those addresses. Can be abbreviated to x-resend-cc-enc.
Send one unencrypted message to all of the given addresses in Cc, so they get to know of each other. We skip looking for any key and will just send out the email in the clear.

Subscription and key management

These keywords must be send to foo-request@hostname. They are used to get information about the list, its subscribers and keys, or to change that information.

List all subscriptions.
x-subscribe: 0x12345678DEADBEEF12345678DEADBEEF12345678
Subscribe the given address and assign it the given OpenPGP-fingerprint.
Unsubscribe the given address.
x-set-fingerprint: 0x12345678DEADBEEF12345678DEADBEEF12345678
Assign the key with the given fingerprint to your subscription.
x-set-fingerprint: 0x12345678DEADBEEF12345678DEADBEEF12345678
Assign the key with the given fingerprint to the given subscription. This variant of this command may only be used by list-admins.
Lists all public keys known to the list.
Import the attachments or the rest of the email-body into the list’s keyring. Only ascii-armored keys are supported.
x-delete-key: 0x12345678DEADBEEF12345678DEADBEEF12345678
Delete the key with the given fingerprint from the list’s keyring.
x-get-key: 0x12345678DEADBEEF12345678DEADBEEF12345678
Export the key with the given fingerprint from the list’s keyring.
x-fetch-key: 0x12345678DEADBEEF12345678DEADBEEF12345678
Fetch the key with the given fingerprint from a keyserver and import it into the list’s keyring. (This works only if a keyserver has been configured by the provider.)

This keyword must be send to the normal list-address: foo@hostname.

Attachs the public key of the list. Probably most useful in combination with x-resend.


These must also be sent to the request-address: foo-request@hostname.

Sends the logfile of the list.
Returns the version of schleuder.
Sign the remaining contents of the email body or the attachments with the list’s key. Use this e.g. to provide GnuPG-signatures for things you are publishing.

Contact list-owner

Write to foo-owner@hostname to contact the list-owner(s) even if you don’t know who they are. Use the list’s key to encrypt the email!


Please take care to have the following commands run by the user that owns the directory of schleuder lists (by default /var/lib/schleuder/lists) to avoid running into file permission problems!

Schleuder can check all keys that are present in the list’s keyrings for (upcoming) expiration dates, revocation, or other reasons for not being usable.

Call this command weekly from cron to automate the check and have the results sent to the respective list-admins:

schleuder check_keys

Schleuder can also refresh all keys in the same manner. Each key of each list will be refreshed from a keyserver one by one. If you’re using gpg 2.1, it’s possible to configure a TOR onion service to be used as keyserver! See the config for an example.

Call this command weekly from cron to automate the check and have the results sent to the respective list-admins:

schleuder refresh_keys

If there is no key assigned, schleuder will try to select a key from the list’s keyring that distinctly matches the subscription’s email address.

This feature should be used with care. It’s easy for a malicious (or inexperienced) person to inject additional user-IDs into the list’s keyring. This can lead to situations in which people suddenly receive emails that are encrypted to a key they don’t own.

You should better not run this command automatedly, and you should always examine the output closely to check for unintended consequences.

schleuder pin_keys

Changes to the previous version of Schleuder

This list describes changes that users or list-admins of Schleuder should be aware of. For a more technical changelog, please see the repository.

User-relevant changes in version 3.2 compared to version 3.1:


To suggest changes to these documents or if you found any errors in them, please use the issue tracker (or contact us by email).

Changes to the documentation

We use a public git-repository to maintain these documents, thus you can browse the list of changes.