lib: Fix DH group not being set for TLS connections.

This meant ciphers using DHE couldn't be used.

Closes #1925. Closes #1476. Thanks to Jonathan Sönnerup, Valerii
Demeshko, and sectokia.

ChangeLog.txt

@@ -2,6 +2,7 @@ Broker:
 - Fix crash on Windows if loading a plugin fails. Closes #1866.
 - Fix DH group not being set for TLS connections, which meant ciphers using
  DHE couldn't be used. Closes #1925. Closes #1476.
+
 - Fix listener not being reassociated with client when reloading a persistence
  file and `per_listener_settings true` is set and the client did not set a
  username. Closes #1891.
@@ -10,6 +11,8 @@ Broker:
 
 Client library:
 - Fix build on Mac Big Sur. Closes #1905.
+- Fix DH group not being set for TLS connections, which meant ciphers using
+ DHE couldn't be used. Closes #1925. Closes #1476.
 
 Clients:
 - mosquitto_sub will now quit with an error if the %U option is used on

lib/net_mosq.c

@@ -648,6 +648,10 @@ static int net__init_ssl_ctx(struct mosquitto *mosq)
  return MOSQ_ERR_INVAL;
  }
 
+#if OPENSSL_VERSION_NUMBER >= 0x10100000L
+ /* Allow use of DHE ciphers */
+ SSL_CTX_set_dh_auto(mosq->ssl_ctx, 1);
+#endif
  /* Disable compression */
  SSL_CTX_set_options(mosq->ssl_ctx, SSL_OP_NO_COMPRESSION);