selftests: mptcp: use IPPROTO_MPTCP for getaddrinfo

mptcp_connect.c is a startup tutorial of MPTCP programming, however
there is a lack of ai_protocol(IPPROTO_MPTCP) usage. Add comment for
getaddrinfo MPTCP support.

This patch first uses IPPROTO_MPTCP to get addrinfo, and if glibc
version is too old, it falls back to using IPPROTO_TCP.

v2:
 - use IPPROTO_MPTCP for getaddrinfo

Co-developed-by: Geliang Tang <[email protected]>
Signed-off-by: Geliang Tang <[email protected]>
Signed-off-by: zhenwei pi <[email protected]>

Author: pizhenwei

tools/testing/selftests/net/mptcp/mptcp_connect.c

@@ -180,13 +180,26 @@ static void xgetnameinfo(const struct sockaddr *addr, socklen_t addrlen,
 }
 
 static void xgetaddrinfo(const char *node, const char *service,
-			 const struct addrinfo *hints,
+			 struct addrinfo *hints,
 			 struct addrinfo **res)
 {
+again:
 	int err = getaddrinfo(node, service, hints, res);
 
 	if (err) {
-		const char *errstr = getxinfo_strerr(err);
+		const char *errstr;
+
+		/* glibc starts to support MPTCP since v2.42.
+		 * For older versions, use IPPROTO_TCP to resolve,
+		 * and use TCP/MPTCP to create socket.
+		 * Link: https://sourceware.org/git/?p=glibc.git;a=commit;h=a8e9022e0f82
+		 */
+		if (err == EAI_SOCKTYPE) {
+			hints->ai_protocol = IPPROTO_TCP;
+			goto again;
+		}
+
+		errstr = getxinfo_strerr(err);
 
 		fprintf(stderr, "Fatal: getaddrinfo(%s:%s): %s\n",
 			node ? node : "", service ? service : "", errstr);
@@ -292,7 +305,7 @@ static int sock_listen_mptcp(const char * const listenaddr,
 {
 	int sock = -1;
 	struct addrinfo hints = {
-		.ai_protocol = IPPROTO_TCP,
+		.ai_protocol = IPPROTO_MPTCP,
 		.ai_socktype = SOCK_STREAM,
 		.ai_flags = AI_PASSIVE | AI_NUMERICHOST
 	};
@@ -356,7 +369,7 @@ static int sock_connect_mptcp(const char * const remoteaddr,
 			      int infd, struct wstate *winfo)
 {
 	struct addrinfo hints = {
-		.ai_protocol = IPPROTO_TCP,
+		.ai_protocol = IPPROTO_MPTCP,
 		.ai_socktype = SOCK_STREAM,
 	};
 	struct addrinfo *a, *addr;