Is 256 a reasonable length for a Diffie-Hellman safe prime? There's a program that uses the following OpenSSL function...

DH_generate_parameters(256, 5, NULL, NULL)


...to create DH parameters which are later used for hundreds of key exchanges (and the one-time keys used to encrypt login details sent over Internet).

The manual page says 256 is prime_len in bits. How secure is this value? (I have a bad feeling.)

Also, if I'm reading http://www.keylength.com/, does this correspond to "discrete logarithm key" or "discrete logarithm group" or am I looking in the entirely wrong place?
I'm guessing whoever made the program was confused about key lengths. A 256-bit DH modulus will not give 256-bits of security. It will provide far less.

If you're reading http://www.keylength.com, the value here corresponds to the "discrete logarithm group". These days you really want a minimum of 2048 bits.