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The prng (Pseudo Random Number Generator) VMOD provides a
mechanism to generate synthetic responses made of pseudo-random bytes.
The functions prng.fast_random_backend() and
prng.zeros_backend() both return backend objects which can be
assigned to the bereq.backend variable, and the result is that a
synthetic response is generated in place of fetching a response from
an external source.
For now, these functions simply take a size as an argument, and the result is a backend which returns that exact number of bytes of either pseudo random or “null” bytes. In the future, there might be other algorithms for creating pseudo random bytes.
Note that, if the size is very large, trying to cache the object will cause other objects to be evicted and potentially even crash Varnish. For this reason you should not cache large fetches and also enable the transit buffer.
If a randomized size is needed, use utils.fast_random_int() to
select the size, for example like this:
set bereq.backend = prng.fast_random_backend(100 + utils.fast_random_int(100));
The example above will make the size be in the range from 100 to 199 (inclusive).
BACKEND fast_random_backend(INT size)
Returns a backend which will synthesize size psudo random bytes in
a response.
A cryptographically weak pseudo random generator is used to generate
the bytes. Currently it is using the same algorithm as
utils.fast_random(), but this can be changed without notice at any
time. Do not, under any circumstances, use this as a source of
randomness for security applications.
The size parameter must be non-negative. If size is zero,
then an empty response with a status code of 200 is produced.
Arguments:
size accepts type INTType: Function
Returns: Backend
BACKEND zeros_backend(INT size)
Returns a backend which will synthesize size “null” bytes in a
response.
The size parameter must be non-negative. If size is zero,
then an empty response with a status code of 200 is produced.
Arguments:
size accepts type INTType: Function
Returns: Backend
The first version of this VMOD was made available in Varnish Enterprise 6.0.12r7