Name

SSLRandomSeed

Synopsis

SSLRandomSeed context source [bytes]
Apache v2 only

This configures one or more sources for seeding the PRNG in OpenSSL at startup time (context is 'startup') and/or just before a new SSL connection is established (context is 'connect'). This directive can only be used in the global server context because the PRNG is a global facility.

Specifying the builtin value for source indicates the built-in seeding source. The source used for seeding the PRNG consists of the current time, the current process id, and (when applicable) a randomly chosen 1KB extract of the interprocess scoreboard structure of Apache. However, this is not a strong source, and at startup time (where the scoreboard is not available) it produces only a few bytes of entropy.

So if you are seeding at startup, you should use an additional seeding source of the form:

file:/path/to/source

This variant uses an external file /path/to/source as the source for seeding the PRNG. When bytes is specified, only the first bytes number of bytes of the file form the entropy (and bytes is given to /path/to/source as the first argument). When bytes is not specified, the whole file forms the entropy (and 0 is given to /path/to/source as the first argument). Use this especially at startup time, for instance with /dev/random and/or /dev/urandom devices (which usually exist on modern Unix derivatives like FreeBSD and Linux).

Note

Although /dev/random provides better quality data, it may not have the number of bytes available that you have requested. On some systems the read waits until the requested number of bytes becomes available — which could be annoying; on others you get however many bytes it actually has available — which may not be enough.

Using /dev/urandom may be better, because it never blocks and reliably gives the amount of requested data. The drawback is just that the quality of the data may not be the best.

On some platforms like FreeBSD one can control how the entropy is generated. See man rndcontrol(8). Alternatively, you can use tools like EGD (Entropy Gathering Daemon) and run its client program with the exec:/path/to/program/ variant (see later) or use egd:/path/to/egd-socket (see later).

You can also use an external executable as the source for seeding:

exec:/path/to/program

This variant uses an external executable /path/to/program as the source for seeding the PRNG. When bytes is specified, only the first bytes number of bytes of stdout form the entropy. When bytes is not specified, all the data on stdout forms the entropy. Use this only at startup time when you need a very strong seeding with the help of an external program. But using this in the connection context slows the server down dramatically.

The final variant for source uses the Unix domain socket of the external Entropy Gathering Daemon (EGD):

egd:/path/to/egd-socket (Unix only)

This variant uses the Unix domain socket of the EGD (see http://www.lothar.com/tech/crypto/) to seed the PRNG. Use this if no random device exists on your platform.

Examples

SSLRandomSeed startup builtin
SSLRandomSeed startup file:/dev/random
SSLRandomSeed startup file:/dev/urandom 1024
SSLRandomSeed startup exec:/usr/local/bin/truerand 16
SSLRandomSeed connect builtin
SSLRandomSeed connect file:/dev/random
SSLRandomSeed connect file:/dev/urandom 1024