Pathname to a file holding DH/DSA parameters (in PEM format).
This file holds parameters for an ephemeral DH key exchange. In
most cases, the default RSA authentication does not use this con-
figuration. However, it is possible to set up RSA to use an
ephemeral DH key exchange. In addition, ciphers with DSA keys
always use ephemeral DH keys. This can be used to achieve for-
ward secrecy. If the dh_file is in DSA parameters format, it
will be automatically converted into DH parameters.