bind to outgoing ip only when specifically requested

we experienced that when binding to 127.0.0.1, no outgoing connections
could be made, and rather than blacklisting all possible ip addresses
that need bind turned off in order for the proxy to work, we instead
add a new command line switch so the user can turn bind mode on when
he sees the need to do so.

the bind functionality was introduced in this commit's parent.

server.c

@@ -49,7 +49,7 @@ int server_setup(struct server *server, const char* listenip, unsigned short por
 		return -3;
 	}
 	server->fd = listenfd;
-	if(strcmp(listenip, "0.0.0.0") && !resolve(listenip, 0, &ainfo)) {
+	if(!resolve(listenip, 0, &ainfo)) {
 		server->bindaddrsz = ainfo->ai_addrlen;
 		memcpy(&server->bindaddr, ainfo->ai_addr, ainfo->ai_addrlen);
 		freeaddrinfo(ainfo);

sockssrv.c

@@ -39,6 +39,7 @@ static const char* auth_pass;
 static sblist* auth_ips;
 static pthread_mutex_t auth_ips_mutex = PTHREAD_MUTEX_INITIALIZER;
 static const struct server* server;
+static int bind_mode;
 
 enum socksstate {
 	SS_1_CONNECTED,
@@ -140,7 +141,7 @@ static int connect_socks_target(unsigned char *buf, size_t n, struct client *cli
 			return -EC_GENERAL_FAILURE;
 		}
 	}
-	if(server_bindtoip(server, fd) == -1)
+	if(bind_mode && server_bindtoip(server, fd) == -1)
 		goto eval_errno;
 	if(connect(fd, remote->ai_addr, remote->ai_addrlen) == -1)
 		goto eval_errno;
@@ -333,9 +334,10 @@ static int usage(void) {
 	dprintf(2,
 		"MicroSocks SOCKS5 Server\n"
 		"------------------------\n"
-		"usage: microsocks -1 -i listenip -p port -u user -P password\n"
+		"usage: microsocks -1 -b -i listenip -p port -u user -P password\n"
 		"all arguments are optional.\n"
 		"by default listenip is 0.0.0.0 and port 1080.\n\n"
+		"option -b forces outgoing connections to be bound to the ip specified with -i\n"
 		"option -1 activates auth_once mode: once a specific ip address\n"
 		"authed successfully with user/pass, it is added to a whitelist\n"
 		"and may use the proxy without auth.\n"
@@ -356,11 +358,14 @@ int main(int argc, char** argv) {
 	int c;
 	const char *listenip = "0.0.0.0";
 	unsigned port = 1080;
-	while((c = getopt(argc, argv, ":1i:p:u:P:")) != -1) {
+	while((c = getopt(argc, argv, ":1bi:p:u:P:")) != -1) {
 		switch(c) {
 			case '1':
 				auth_ips = sblist_new(sizeof(union sockaddr_union), 8);
 				break;
+			case 'b':
+				bind_mode = 1;
+				break;
 			case 'u':
 				auth_user = strdup(optarg);
 				zero_arg(optarg);