MPTCP: use it be default on the server side

configure.ac

@@ -2644,38 +2644,6 @@ if test ! -z "$check_for_openpty_ctty_bug"; then
 fi
 
 
-# Testing MPTCP Support
-# Does the OS support MPTCP?
-# We don't use this at the moment
-# but I am holding it in resrve -cjr 04/04/2025
-	AC_MSG_CHECKING([whether the OS supports MPTCP])
-	AC_RUN_IFELSE(
-		[AC_LANG_PROGRAM([[
-		#include <stdio.h>
-		#include <stdlib.h>
-		#include <unistd.h>
-		#include <sys/types.h>
-		#include <sys/socket.h>
-		#include <stdbool.h>
-		#include <netinet/in.h>
-
-		]], [[
-		int sock = -1;
-		sock = socket(AF_INET, SOCK_STREAM, IPPROTO_MPTCP);
-		if (sock < 0) {
-		   exit(1);
-		   }
-		]])],
-		[
-			AC_MSG_RESULT([yes])
-			AC_DEFINE([HAVE_MPTCP], [1],
-			    [OS Supports MPTCP])
-		],
-		[
-			AC_MSG_RESULT([no])
-		]
-	)
-
 if test "x$ac_cv_func_getaddrinfo" = "xyes" && \
     test "x$check_for_hpux_broken_getaddrinfo" = "x1"; then
 	AC_MSG_CHECKING([if getaddrinfo seems to work])

defines.h

@@ -941,10 +941,6 @@ struct winsize {
 # define SSH_IOBUFSZ (32*1024)
 #endif
 
-#ifndef IPPROTO_MPTCP
-#define IPPROTO_MPTCP 262
-#endif
-
 /*
  * We want functions in openbsd-compat, if enabled, to override system ones.
  * We no-op out the weak symbol definition rather than remove it to reduce

hpnsshd_config.5

@@ -2067,9 +2067,10 @@ directives.
 .It Cm UseMPTCP
 If set to
 .Cm yes ,
-this will enable Multipath TCP (MPTCP) instead of TCP (this only works on Linux).
+this will enable Multipath TCP (MPTCP) support. This only works on Linux. MPTCP
+will be used if requested by clients. If not, TCP will be used as before.
 The default is
-.Cm no .
+.Cm yes .
 .It Cm UsePAM
 Enables the Pluggable Authentication Module interface.
 If set to

servconf.c

@@ -487,7 +487,7 @@ fill_default_server_options(ServerOptions *options)
 	if (options->hpn_disabled == -1)
 		options->hpn_disabled = 0;
 	if (options->use_mptcp == -1)
-		options->use_mptcp = 0;
+		options->use_mptcp = 1;
 	if (options->ip_qos_interactive == -1)
 		options->ip_qos_interactive = IPTOS_DSCP_EF;
 	if (options->ip_qos_bulk == -1)

sshconnect.c

@@ -355,7 +355,11 @@ ssh_create_socket(struct addrinfo *ai)
 
 	/* user request for Multipath TCP */
 	if (options.use_mptcp)
+#ifdef IPPROTO_MPTCP
 		sock = socket(ai->ai_family, ai->ai_socktype, IPPROTO_MPTCP);
+#else
+		sock = -1;
+#endif
 	else
 		sock = socket(ai->ai_family, ai->ai_socktype, ai->ai_protocol);
 

sshd.c

@@ -827,10 +827,15 @@ listen_on_addrs(struct listenaddr *la)
 			continue;
 		}
 		/* Create socket for listening. */
+		listen_sock = -1;
+#ifdef IPPROTO_MPTCP
 		if (options.use_mptcp)
 			listen_sock = socket(ai->ai_family, ai->ai_socktype,
 			    IPPROTO_MPTCP);
-		else
+#endif
+
+		/* Fallback to "plain" TCP if MPTCP is not available */
+		if (listen_sock == -1)
 			listen_sock = socket(ai->ai_family, ai->ai_socktype,
 			    ai->ai_protocol);
 

sshd_config

@@ -103,7 +103,7 @@ AuthorizedKeysFile	.ssh/authorized_keys
 #PermitTunnel no
 #ChrootDirectory none
 #VersionAddendum none
-#UseMPTCP no
+#UseMPTCP yes
 
 # no default banner path
 #Banner none