How to correctly set Diffie Hellman prime bits?

Hello again list,

i am continuing experimenting with GNUTLS. I have written a client and a
server that perform anonymous (ANON-DH) TLS negotiation.

I successfully connected to a gnutls-serv --http --priority "NORMAL:
+ANON-DH" instance.

When i tried to connect to my own server (which is mostly an example
from the documentation) i got the following error:

> GNUTLS ERROR: The Diffie Hellman prime sent by the server is not
> acceptable (not long enough).

So i manually set the Diffie Hellman prime bits in the server to 1024
and in the client to 1023 (gnutls_dh_set_prime_bits (session, DH_BITS))
- With no effect. Still the same error. I also tried to set the DH prime
bits in the server to 2048. The server needed longer to start up after
this change so i guess that took effect.

I then set the DH prime bits in the client to 0 and in the server to
1024. Now i can connect:

Output of server:
> [lennart <at> sundaysister Debug]$ ./GNUTLSTest-Server 
> Server ready. Listening to port '5556'.
>
> - Anonymous DH using prime of -50 bits
> - connection from 112.93.99.0, port 50879
> - Handshake was completed