Well, socat has a git repo at git://repo.or.cz/socat.git so you can see the history of the prime number. In particular, it was upgraded from a 512 bit prime to

Well, socat has a git repo at git://repo.or.cz/socat.git so you can see the history of the prime number. In particular, it was upgraded from a 512 bit prime to a 1024 bit "prime" in commit 281d1bd on Jan 23, 2015. Neither the code then, or the new code in this patch, have comments indicating how the prime was generated. (It is only mentioned in the advisory that openssl dhparams was used for the recent patch)