ProFTPD module mod_tls

ProFTPD module `mod_tls_shmcache`

The mod_tls_shmcache submodule is contained in the mod_tls_shmcache.c file, and is not compiled by default. Installation instructions are discussed here.

This submodule provides a SysV shared memory-based implementation of an external SSL session cache for use by the mod_tls module's TLSSessionCache directive. The module also implements a SysV shared memory-based implementation of an external OCSP response cache for the TLSStaplingCache directive.

This product includes software developed by the OpenSSL Project for use in the OpenSSL Toolkit (http://www.openssl.org/).

This product includes cryptographic software written by Eric Young (eay@cryptsoft.com).

Author

Please contact TJ Saunders <tj at castaglia.org> with any questions, concerns, or suggestions regarding this module.

The mod_tls_shmcache module supports the "shm" string for the type parameter of the TLSSessionCache configuration directive. The info parameter for mod_tls_shmcache must be formatted like:

  /file=/path/to/cache/file[&size=bytes]

The configured path is used for synchronizing access to the shared memory segment among the various server processes. The default shared memory segment size allocated is 1.5MB; use the optional size key to configure a different size, in bytes. Note that the configured size must be able to hold at least one cached session; if a too-small size is configured, that size will be ignored and the default size will be used.

The mod_tls_shmcache module also supports the "shm" string for the type parameter of the TLStaplingCache configuration directive. The info parameter for mod_tls_shmcache must be formatted like:

  /file=/path/to/cache/file[&size=bytes]

The configured path is used for synchronizing access to the shared memory segment among the various server processes. The default shared memory segment size allocated is 1.5MB; use the optional size key to configure a different size, in bytes. Note that the configured size must be able to hold at least one cached OCSP response; if a too-small size is configured, that size will be ignored and the default size will be used.

Examples

Use the default shared memory segment size and timeout:

  <IfModule mod_tls.c>
    ...

    <IfModule mod_tls_shmcache.c>
      TLSSessionCache shm:/file=/var/ftpd/sess_cache
      TLSStaplingCache shm:/file=/var/ftpd/ocsp_pcache
    </IfModule>
  </IfModule>

Use a larger shared memory segment size:

  <IfModule mod_tls.c>
    ...

    <IfModule mod_tls_shmcache.c>
      TLSSessionCache shm:/file=/var/ftpd/sess_cache&size=2097152
      TLSStaplingCache shm:/file=/var/ftpd/ocsp_cache&size=2097152
    </IfModule>
  </IfModule>

Use a smaller shared memory size, and a shorter timeout:

  <IfModule mod_tls.c>
    ...

    <IfModule mod_tls_shmcache.c>
      TLSSessionCache shm:/file=/var/ftpd/sess_cache&size=512000 600

      # Note that TLSStaplingCache does not use a timeout
      TLSStaplingCache shm:/file=/var/ftpd/ocsp_cache&size=512000
    </IfModule>
  </IfModule>

Installation

The mod_tls_shmcache module is distributed with the ProFTPD source code. Simply follow the normal steps for using third-party modules in ProFTPD, being sure to include the mod_tls module (on which mod_tls_shmcache depends):

  $ ./configure --with-modules=mod_tls:mod_tls_shmcache
  $ make
  $ make install

Alternatively, if your proftpd was compiled with DSO support, you can use the prxs tool to build mod_tls_shmcache as a shared module:

  $ prxs -c -i -d mod_tls_shmcache.c

Note: If using mod_tls_shmcache as a shared module, make sure that this module is loaded after the mod_tls module, i.e.:

  # Load mod_tls first
  LoadModule mod_tls.c
 
  # Then load any SSL caching modules
  LoadModule mod_tls_shmcache.c

Usage

Logging
The mod_tls_shmcache module supports trace logging, via the module-specific log channels:

tls.shmcache

Thus for trace logging, to aid in debugging, you would use the following in your proftpd.conf:

  TraceLog /path/to/ftpd/trace.log
  Trace tls.shmcache:20

This trace logging can generate large files; it is intended for debugging use only, and should be removed from any production configuration.

Frequently Asked Questions

Question: I configured a TLSSessionCache file, but it is empty. Is mod_tls_shmcache not working properly?
Answer: Yes, mod_tls_shmcache is working properly. The actual cache of SSL/TLS session data is stored in shared memory, not on the filesystem. Storing data in system shared memory requires a unique key; the mod_tls_shmcache uses the configured file to create this unique key. The module also uses the configured TLSSessionCache file for locking, as when handling a ftpdctl request to clear the cache.

Question: I am trying to use mod_tls_shmcache as a shared module, but my proftpd server fails to start up, failing with this error:

  proftpd: symbol lookup error: /usr/local/libexec/mod_tls_shmcache.so: undefined symbol: tls_sess_cache_register

Answer: This happens when your configuration is loading the mod_tls_shmcache module before the mod_tls has been loaded. The fix, then, is to make sure your config looks something like this:

  LoadModule mod_tls.c
  LoadModule mod_tls_shmcache.c