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What is Memcache?
Memcache (or "memcached") is an open-source, high performance memory object caching system. A simple (and effective) key/value store accessible, efficiently, over the network.

How Can Memcache Be Useful for ProFTPD?
Like any high-performance object store, memcached offers several possibilities to a server like proftpd. Many sites use memcached for caching; it can also be used as an efficient shared storage mechanism, for sharing data among many different servers. And for ProFTPD specifically, the shared storage aspect is what is most useful. Things like SSL/TLS sessions can be cached and shared across a pool of proftpd servers, as can ban lists for badly-behaved clients.

Enabling Memcache Support for ProFTPD
OK, so you are interested enough in the possibilities that memcached offers that you want to try it out. Excellent! To do this, you will first need to make sure to build your proftpd executable using the --enable-memcache configure option. The --enable-memcache configure option automatically adds the mod_memcache module to your proftpd build.

The mod_memcache module uses the libmemcached library for talking to memcached servers. If your libmemcached library is installed in a non-standard location, you may need to tell the ProFTPD build system where to find the libmemcached header files and libraries using the --with-includes and --with-libraries configure options.

There are other modules which make use of memcached support when available, such as mod_tls_memcache. Thus to take advantage of modules like this, putting everything together, your configure command might look like this:

  $ ./configure --enable-memcache \
    --with-modules=...:mod_tls_memcache:... \
    --with-includes=/path/to/libmemcached/include \
    --with-libraries=/path/to/libmemcached/lib

Configuring mod_memcache
Now that you have compiled proftpd with the mod_memcache module, you need to add the necessary mod_memcache directives to your proftpd.conf. The following example demonstrates this:

  <IfModule mod_memcache.c>
    # Enable mod_memcache
    MemcacheEngine on

    # Tell mod_memcache where to log its messages
    MemcacheLog /path/to/proftpd/memcache.log

    # Tell mod_memcache where to find the memcached servers
    MemcacheServers 192.168.0.10:11211 192.168.0.11:11211
  </IfModule>

  TraceLog /path/to/proftpd/trace.log
  Trace DEFAULT:10 memcache:20

Using Memcache for Shared Storage
You have now compiled support for memcached into ProFTPD, and you have told the mod_memcache module where to find your memcached servers. Is that all you need to do? No. Now you need to tell proftpd modules which bits of data to store in your memcached servers.

Currently, only two modules can take advantage of memcached support: mod_ban and mod_tls_memcache.

First, let us examine mod_ban and how it would use memcached. The mod_ban module manages ban lists, lists of clients/users which have been banned for various reasons. These lists are stored in shared memory by default; this works for a single proftpd server, but if a badly behaved client is banned by one proftpd server in pool of servers, that client can then connect to a different server which might not have a ban for that client -- and the client then gets another chance to be naughty. To configure mod_ban so that it stores its ban lists in memcached, simply use the following in your proftpd.conf:

  <IfModule mod_ban.c>
    BanEngine on

    # ...other mod_ban directives...

    # Tell mod_ban to store its ban lists using memcache
    BanCache memcache
  </IfModule>

The mod_tls_memcache module uses memcached servers for storing SSL/TLS sessions; SSL/TLS session caching can greatly improve SSL/TLS session handshake times, particularly for data transfers using SSL/TLS. If you have a pool of proftpd servers, and you have FTPS clients which may connect to a different node every time, caching the SSL/TLS session data in a shared storage mechanism like memcached can be quite beneficial.

To use memcached for SSL/TLS session caching, then, you use the TLSSessionCache directive of the mod_tls module, using something like this in your proftpd.conf:

  <IfModule mod_tls.c>
    TLSEngine on

    # ...other mod_tls directives...

    <IfModule mod_tls_memcache.c>
      # Tell mod_tls to cache sessions using memcached
      TLSSessionCache memcache:
    </IfModule>
  </IfModule>

Frequently Asked Questions
Question: If I don't use memcache, are there other ways for sharing data (such as ban lists) among different proftpd instances?
Answer: It might be possible using mod_sql and some SQLLogInfo directives, but that would only work for very specific information. For sharing things like ban lists and SSL/TLS sessions across a cluster of proftpd servers, Memcache (or Redis) support is recommended.

Question: Can I use mod_memcache to cache frequently accessed files, similar to nginx+memcache?
Answer: No. And in reality, caching of files like that will probably not give you the same performance gain for FTP transfers as it can for HTTP transfers.

Why not? Many HTTP transfers are for dynamically generated pages; the cost of generating each page is expensive, and the generated content may not change that frequently (relative to the rate of requests). FTP transfers, by contrast, are for static files; FTP servers do not (usually) dynamically generate the bytes of the files being downloaded. The cost of reading files from disk is probably less than reading files from memcached over the network, even a LAN.

Now the above may not be true in all cases -- there may be FTP servers serving files from network-mounted filesystems (e.g. NFS, CIFS et al). And for these very specific cases, having a cache of frequently access files on closer storage such as local disk (or memcached) could make a big difference; please contact the ProFTPD Project if you find yourself in this situation, and we will see what can be done to help.

Question: Why do I see the following error when proftpd starts up?

  mod_tls_memcache/0.1: notice: unable to register 'memcache' SSL session cache: Memcache support not enabled

The above is not a fatal or worrisome error; it is merely pointing out that some of your modules want to use a feature that was not enabled.