Use SSL with PennMUSH

Revised: 04 Jan 2018


As of version 1.7.7p17, PennMUSH supports SSL connections when linked with the OpenSSL library ( As of version 1.8.4p9, OpenSSL is a requirement. OpenBSD’s LibreSSL also works.

The following features are supported:

An SSL overview

When an SSL client connects to an SSL server, it performs a “handshake” that looks something like this:

Client says hello, offers a menu of cipher options
Server says hello, selects a cipher.
Server presents its certificate, requests a client certificate
Client presents a certificate (or not)
Client and server exchange cryptographic session keys

The server is identified to the client by a certificate, an encoded text that gives the server’s name and other attributes and is signed by a certifying authority (CA), like Verisign. The client checks that the signature is by a CA that it trusts, and may perform other validation on the certificate (e.g., checking that the hostname in the certificate matches the hostname it’s trying to connect to).

If the client chooses to present a certificate (or is required to by the server), the server will likewise attempt to validate it against its list of trusted CAs, and may perform other verification.

Once session keys have been exchanged, the client and server can communicate secure from eavesdropping.

Compiling with OpenSSL

What to install

The configure script distributed with PennMUSH automatically detects the OpenSSL libraries (libssl and libcrypto) and sets up the proper compiler flags to use them.

If you install it through your operating system’s package management system, you need shared libraries and development headers. (Packages with names like openssl, libssl, and -dev or -shlibs suffixes are common. Exact names vary from OS to OS. You want at least verison 0.9.7.) If OpenSSL gets installed in a place that isn’t checked by default, you can invoke configure with ./configure --with-ssl=/path/to (The path must be the root directory of where OpenSSL’s include/ and lib/ directories are.)

OpenSSL can also be compiled on Windows, and you could add its libraries to the PennMUSH project file and link it in that way. It’s easier to use something like MSYS2 though - see the README files in win32/.

Persistent SSL connections

Normally, encrypted connections will be booted when a running game restarts, because the internal OpenSSL state cannot be saved to disc and restored later. To allow persistant ssl connections via a proxy process, pass –enable-ssl_slave to configure when you run it. This does not work on Windows.

It requires the libevent 2.X library and headers to be installed. Get it through your OS’s package system or at If libevent is detected, the ssl slave is automatically enabled.

To use the ssl_slave, a full @shutdown and restart of the mush has to take place after the mush source has been configured to enable it, and compiled. Then players can connect via SSL capable clients exactly as they did before, with the only change being that they won’t get booted on a @shutdown/reboot. It’s transparent to the player and the game.

MUSH configuration overview

mush.cnf includes a number of directives that control SSL configuration:


selects the port number for the MUSH to listen for SSL connections. Any port number other than the MUSH’s ordinary listening port can be chosen (subject, of course, to other system restrictions on choosing port numbers).

If left blank, SSL connections will not be enabled. However, other parts of the mush, such as the digest() softcode function, and password encryption, will still use OpenSSL library routines.


Controls the IP address to listen on. When in doubt, leave blank.


Specifies the name of the file (relative to the game/ directory if it’s not an absolute path) that contains the MUSH server’s certificate and private key. See section IV below.


The name of the file that contains certificates of trusted certificate authorities. OpenSSL distributes a file containing the best known CAs that is suitable for use here. If you comment this out, client certificate checking will not be performed.

Defaults to /etc/ssl/certs/ca-certifcates.crt


A directory containing multiple certificate files.

Defaults to /etc/ssl/certs


A boolean option that controls whether the MUSH server will require clients to present valid (that is, signed by a CA for which ssl_ca_file holds a certificate) certificates in order to connect. As no mud clients currently do this, you probably want it off. See section V below.


The path to a file to use as a unix domain socket used for talking to the optional SSL connection proxy.

Installing a server certificate

SSL support requires that the MUSH present a server certificate (except as discussed below). You must create a file containing the certificate and the associated private key (stripped of any passphrase protection) and point the ssl_private_key_file directive at this file. This file should only be readable by the MUSH account!

How do you get such a certificate and private key? Here are the steps you can use with openssl’s command-line tool:

  1. Generate a certificate signing request (mymush.csr) and a private key (temp.key). You will be asked to answer several questions. Be sure the Common Name you request is your MUSH’s hostname:

     $ openssl req -new -out mymush.csr -keyout temp.key -passin pass:foobar
  2. Strip the passphrase off of your private key, leaving you with an unpassworded mymush.key file:

     $ openssl rsa -in temp.key -out mymush.key -passin pass:foobar
     $ rm temp.key
  3. Send the certificate signing request to a certifying authority to have it signed. If the CA needs the private key, send the passphrased one. The CA will send you back a certificate which you save to a file (mymush.crt)

  4. Concatenate the certificate with the unpassworded private key and use this as the ssl_private_key_file:

     $ cat mymush.key >> mymush.crt

Commercial CAs like Verisign sign certificates for a yearly or two-yearly fee that is probably too steep for most MUSH Gods. Instead of using a commercial CA, you can generate a self-signed certificate by changing step 1 above to:

$ openssl req -new -x509 -days 3650 -out mymush.crt -keyout temp.key -passin pass:foobar

A self-signed certificate is free, but clients that attempt to validate certificates will fail to validate a self-signed certificate unless the user manually installs the certificate in their client and configures it to be trusted. How to do that is beyond the scope of this document, and highly client-dependent.

Another option is to skip the use of a certificate altogether. If you don’t provide an ssl_private_key_file, the server will only accept connections from clients that are willing to use the anonymous Diffie-Hellman cipher; it is unknown which clients are configured to offer this. This provides clients with no security that they are actually connecting to your server, and exposes them to a man-in-the-middle attack, but requires no work on your part at all.

Hosting providers or other parties may one day provide CA service to PennMUSHes for free. When they do, you’ll have to install those CAs’ certificates in your client as trusted in order to have the server’s certificate validate, but if a few CAs certify many MUSHes, this is efficient.

Using client certificates for authentication

If you provide PennMUSH with a file containing the certificates of trusted CAs (using the ssl_ca_file directive in mush.cnf), it will, by default, request that clients present certificates when they connect. Clients that do not present certificates will still be allowed to connect (unless ssl_require_client_cert is enabled).

Clients that do present certificates must present certificates signed by a trusted CA, or they will be disconnected. Both valid and invalid certificates are logged (to connect.log and netmush.log, respectively).

If you were really serious about this, you probably would issue your own certs and not allow Verisign, etc. certs. You’d probably want to have the server validate extra attributes on each client cert, which should probably include the player’s dbref and creation time. This is left as an exercise for the reader for now.