lib-ssl-iostream: Make DH parameters optional

Since a lot of connections use elliptic curve
Diffie-Hellman these days, make it possible to
use dovecot without providing Diffie-Hellman
parameters. This reduces setup cost as the
parameters do not need to be generated, which
can be a time consuming task.

src/lib-master/master-service-ssl-settings.c

@@ -112,10 +112,6 @@ master_service_ssl_settings_check(void *_set, pool_t pool ATTR_UNUSED,
 		*error_r = "ssl enabled, but ssl_key not set";
 		return FALSE;
 	}
-	if (*set->ssl_dh == '\0') {
-		*error_r = "ssl enabled, but ssl_dh not set";
-		return FALSE;
-	}
 #endif
 	if (set->ssl_verify_client_cert && *set->ssl_ca == '\0') {
 		*error_r = "ssl_verify_client_cert set, but ssl_ca not";

src/lib-ssl-iostream/iostream-openssl-context.c

@@ -47,17 +47,11 @@ static RSA *ssl_gen_rsa_key(SSL *ssl ATTR_UNUSED,
 }
 
 static DH *ssl_tmp_dh_callback(SSL *ssl ATTR_UNUSED,
-			       int is_export, int keylength)
+			       int is_export ATTR_UNUSED, int keylength ATTR_UNUSED)
 {
-	struct ssl_iostream *ssl_io;
-
-	ssl_io = SSL_get_ex_data(ssl, dovecot_ssl_extdata_index);
-	/* Well, I'm not exactly sure why the logic in here is this.
-	   It's the same as in Postfix, so it can't be too wrong. */
-	if (is_export != 0 && keylength == 512 && ssl_io->ctx->dh_512 != NULL)
-		return ssl_io->ctx->dh_512;
-	else
-		return ssl_io->ctx->dh_default;
+	i_error("Diffie-Hellman key exchange requested, "
+		"but no DH parameters provided. Set ssh_dh=</path/to/dh.pem");
+	return NULL;
 }
 
 static int
@@ -168,7 +162,9 @@ ssl_iostream_ctx_use_dh(struct ssl_iostream_context *ctx,
 {
 	DH *dh;
 	int ret = 0;
-
+	if (*set->dh == '\0') {
+		return 0;
+	}
 	if (openssl_iostream_load_dh(set, &dh, error_r) < 0)
 		return -1;
 	if (SSL_CTX_set_tmp_dh(ctx->ssl_ctx, dh) == 0) {
@@ -506,7 +502,7 @@ ssl_proxy_ctx_get_pkey_ec_curve_name(const struct ssl_iostream_settings *set,
 
 static int
 ssl_proxy_ctx_set_crypto_params(SSL_CTX *ssl_ctx,
-				const struct ssl_iostream_settings *set ATTR_UNUSED,
+				const struct ssl_iostream_settings *set,
 				const char **error_r ATTR_UNUSED)
 {
 #if defined(HAVE_ECDH) && !defined(SSL_CTX_set_ecdh_auto)
@@ -516,7 +512,8 @@ ssl_proxy_ctx_set_crypto_params(SSL_CTX *ssl_ctx,
 #endif
 	if (SSL_CTX_need_tmp_RSA(ssl_ctx) != 0)
 		SSL_CTX_set_tmp_rsa_callback(ssl_ctx, ssl_gen_rsa_key);
-	SSL_CTX_set_tmp_dh_callback(ssl_ctx, ssl_tmp_dh_callback);
+	if (set->dh == NULL || *set->dh == '\0')
+		SSL_CTX_set_tmp_dh_callback(ssl_ctx, ssl_tmp_dh_callback);
 #ifdef HAVE_ECDH
 	/* In the non-recommended situation where ECDH cipher suites are being
 	   used instead of ECDHE, do not reuse the same ECDH key pair for