Set server preference of ciphers.

If the server supports weak ciphers, a client can still use a weak encryption by only offering the weakest cipher a server supports. Thus, a server should only support strong ciphers. If the server only supports strong ciphers, it does not matter if the server or the client preference is used.

Still, tests for the server security seem to insist on using the server preference (see #347). There are no disadvantages in doing so. So it is set as default now.

src/civetweb.c

@@ -1015,6 +1015,7 @@ typedef struct SSL_CTX SSL_CTX;
 #include <openssl/pem.h>
 #include <openssl/engine.h>
 #include <openssl/conf.h>
+#include <openssl/dh.h>
 #else
 /* SSL loaded dynamically from DLL.
  * I put the prototypes here to be independent from OpenSSL source
@@ -1045,6 +1046,8 @@ typedef struct x509 X509;
 #define SSL_OP_NO_TLSv1_2 (0x08000000L)
 #define SSL_OP_NO_TLSv1_1 (0x10000000L)
 #define SSL_OP_SINGLE_DH_USE (0x00100000L)
+#define SSL_OP_CIPHER_SERVER_PREFERENCE (0x00400000L)
+#define SSL_OP_NO_SESSION_RESUMPTION_ON_RENEGOTIATION (0x00010000L)
 
 #define SSL_ERROR_NONE (0)
 #define SSL_ERROR_SSL (1)
@@ -11687,6 +11690,7 @@ set_ssl_option(struct mg_context *ctx)
 	protocol_ver = atoi(ctx->config[SSL_PROTOCOL_VERSION]);
 	SSL_CTX_set_options(ctx->ssl_ctx, ssl_get_protocol(protocol_ver));
 	SSL_CTX_set_options(ctx->ssl_ctx, SSL_OP_SINGLE_DH_USE);
+	SSL_CTX_set_options(ctx->ssl_ctx, SSL_OP_CIPHER_SERVER_PREFERENCE);
 	SSL_CTX_set_ecdh_auto(ctx->ssl_ctx, 1);
 
 	/* If a callback has been specified, call it. */