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
executable using the
--enable-memcache configure option. The
--enable-memcache configure option automatically adds the
mod_memcache module to
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
There are other modules which make use of memcached support when available,
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
Now that you have compiled
proftpd with the
mod_memcache module, you need to add the necessary
mod_memcache directives to your
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>If you wish to see more detailed logging, at least while you are setting up your memcached servers for ProFTPD, you can enable trace logging for the
memcachetrace channel using e.g.:
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
servers. Is that all you need to do? No. Now you need to tell
proftpd modules which bits of data to store in your
Currently, only two modules can take advantage of
First, let us examine
mod_ban and how it would use
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
that it stores its ban lists in
memcached, simply use the following
<IfModule mod_ban.c> BanEngine on # ...other mod_ban directives... # Tell mod_ban to store its ban lists using memcache BanCache memcache </IfModule>With this,
memcached(as well as shared memory) for reading/writing its ban lists. And this, in turn, means that other
mod_banmodules can see those bans, and reject the badly behaved clients across the pool/cluster.
mod_tls_memcache module uses
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
can be quite beneficial.
memcached for SSL/TLS session caching, then, you use the
TLSSessionCache directive of the
mod_tls module, using something like this
<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>That's it. The
mod_tlsmodule now knows to give the SSL/TLS session data to
mod_tls_memcacheknows how to talk to the
Frequently Asked Questions
Question: If I don't use memcache, are there other ways for sharing data (such as ban lists) among different
Answer: It might be possible using
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
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
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
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 enabledAnswer: This message means that your
mod_tls_memcachebuilt and loaded, but your
proftpdserver was not built with memcache support (i.e. the
--enable-memcacheconfigure option was not used when compiling
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.