diff -up openssh-7.4p1/channels.c.x11max openssh-7.4p1/channels.c

--- openssh-7.4p1/channels.c.x11max	2017-02-09 12:49:04.690996627 +0100

+++ openssh-7.4p1/channels.c	2017-02-09 12:49:04.744996547 +0100

@@ -152,8 +152,8 @@ static int all_opens_permitted = 0;

 /* -- X11 forwarding */

-/* Maximum number of fake X11 displays to try. */

-#define MAX_DISPLAYS  1000

+/* Minimum port number for X11 forwarding */

+#define X11_PORT_MIN 6000

 /* Saved X11 local (client) display. */

 static char *x11_saved_display = NULL;

@@ -4228,7 +4228,8 @@ channel_send_window_changes(void)

  */

 int

 x11_create_display_inet(int x11_display_offset, int x11_use_localhost,

-    int single_connection, u_int *display_numberp, int **chanids)

+    int x11_max_displays, int single_connection, u_int *display_numberp,

+    int **chanids)

 {

 	Channel *nc = NULL;

 	int display_number, sock;

@@ -4240,10 +4241,15 @@ x11_create_display_inet(int x11_display_

 	if (chanids == NULL)

 		return -1;

+	/* Try to bind ports starting at 6000+X11DisplayOffset */

+	x11_max_displays = x11_max_displays + x11_display_offset;

+

 	for (display_number = x11_display_offset;

-	    display_number < MAX_DISPLAYS;

+	    display_number < x11_max_displays;

 	    display_number++) {

-		port = 6000 + display_number;

+		port = X11_PORT_MIN + display_number;

+		if (port < X11_PORT_MIN) /* overflow */

+			break;

 		memset(&hints, 0, sizeof(hints));

 		hints.ai_family = IPv4or6;

 		hints.ai_flags = x11_use_localhost ? 0: AI_PASSIVE;

@@ -4295,7 +4301,7 @@ x11_create_display_inet(int x11_display_

 		if (num_socks > 0)

 			break;

 	}

-	if (display_number >= MAX_DISPLAYS) {

+	if (display_number >= x11_max_displays || port < X11_PORT_MIN ) {

 		error("Failed to allocate internet-domain X11 display socket.");

 		return -1;

 	}

@@ -4441,7 +4447,7 @@ x11_connect_display(void)

 	memset(&hints, 0, sizeof(hints));

 	hints.ai_family = IPv4or6;

 	hints.ai_socktype = SOCK_STREAM;

-	snprintf(strport, sizeof strport, "%u", 6000 + display_number);

+	snprintf(strport, sizeof strport, "%u", X11_PORT_MIN + display_number);

 	if ((gaierr = getaddrinfo(buf, strport, &hints, &aitop)) != 0) {

 		error("%.100s: unknown host. (%s)", buf,

 		ssh_gai_strerror(gaierr));

@@ -4457,7 +4463,7 @@ x11_connect_display(void)

 		/* Connect it to the display. */

 		if (connect(sock, ai->ai_addr, ai->ai_addrlen) < 0) {

 			debug2("connect %.100s port %u: %.100s", buf,

-			    6000 + display_number, strerror(errno));

+			    X11_PORT_MIN + display_number, strerror(errno));

 			close(sock);

 			continue;

 		}

@@ -4466,8 +4472,8 @@ x11_connect_display(void)

 	}

 	freeaddrinfo(aitop);

 	if (!ai) {

-		error("connect %.100s port %u: %.100s", buf, 6000 + display_number,

-		    strerror(errno));

+		error("connect %.100s port %u: %.100s", buf,

+		    X11_PORT_MIN + display_number, strerror(errno));

 		return -1;

 	}

 	set_nodelay(sock);

diff -up openssh-7.4p1/channels.h.x11max openssh-7.4p1/channels.h

--- openssh-7.4p1/channels.h.x11max	2017-02-09 12:49:04.744996547 +0100

+++ openssh-7.4p1/channels.h	2017-02-09 12:49:50.230929693 +0100

@@ -293,7 +293,7 @@ int	 permitopen_port(const char *);

 void	 channel_set_x11_refuse_time(u_int);

 int	 x11_connect_display(void);

-int	 x11_create_display_inet(int, int, int, u_int *, int **);

+int	 x11_create_display_inet(int, int, int, int, u_int *, int **);

 int      x11_input_open(int, u_int32_t, void *);

 void	 x11_request_forwarding_with_spoofing(int, const char *, const char *,

 	     const char *, int);

diff -up openssh-7.4p1/servconf.c.x11max openssh-7.4p1/servconf.c

--- openssh-7.4p1/servconf.c.x11max	2017-02-09 12:49:04.741996552 +0100

+++ openssh-7.4p1/servconf.c	2017-02-09 12:51:03.167822492 +0100

@@ -94,6 +94,7 @@ initialize_server_options(ServerOptions

 	options->print_lastlog = -1;

 	options->x11_forwarding = -1;

 	options->x11_display_offset = -1;

+	options->x11_max_displays = -1;

 	options->x11_use_localhost = -1;

 	options->permit_tty = -1;

 	options->permit_user_rc = -1;

@@ -242,6 +243,8 @@ fill_default_server_options(ServerOption

 		options->x11_forwarding = 0;

 	if (options->x11_display_offset == -1)

 		options->x11_display_offset = 10;

+	if (options->x11_max_displays == -1)

+		options->x11_max_displays = DEFAULT_MAX_DISPLAYS;

 	if (options->x11_use_localhost == -1)

 		options->x11_use_localhost = 1;

 	if (options->xauth_location == NULL)

@@ -416,7 +419,7 @@ typedef enum {

 	sPasswordAuthentication, sKbdInteractiveAuthentication,

 	sListenAddress, sAddressFamily,

 	sPrintMotd, sPrintLastLog, sIgnoreRhosts,

-	sX11Forwarding, sX11DisplayOffset, sX11UseLocalhost,

+	sX11Forwarding, sX11DisplayOffset, sX11MaxDisplays, sX11UseLocalhost,

 	sPermitTTY, sStrictModes, sEmptyPasswd, sTCPKeepAlive,

 	sPermitUserEnvironment, sAllowTcpForwarding, sCompression,

 	sRekeyLimit, sAllowUsers, sDenyUsers, sAllowGroups, sDenyGroups,

@@ -537,6 +540,7 @@ static struct {

 	{ "ignoreuserknownhosts", sIgnoreUserKnownHosts, SSHCFG_GLOBAL },

 	{ "x11forwarding", sX11Forwarding, SSHCFG_ALL },

 	{ "x11displayoffset", sX11DisplayOffset, SSHCFG_ALL },

+	{ "x11maxdisplays", sX11MaxDisplays, SSHCFG_ALL },

 	{ "x11uselocalhost", sX11UseLocalhost, SSHCFG_ALL },

 	{ "xauthlocation", sXAuthLocation, SSHCFG_GLOBAL },

 	{ "strictmodes", sStrictModes, SSHCFG_GLOBAL },

@@ -1313,6 +1317,10 @@ process_server_config_line(ServerOptions

 			*intptr = value;

 		break;

+	case sX11MaxDisplays:

+		intptr = &options->x11_max_displays;

+		goto parse_int;

+

 	case sX11UseLocalhost:

 		intptr = &options->x11_use_localhost;

 		goto parse_flag;

@@ -2060,6 +2068,7 @@ copy_set_server_options(ServerOptions *d

 	M_CP_INTOPT(fwd_opts.streamlocal_bind_unlink);

 	M_CP_INTOPT(x11_display_offset);

 	M_CP_INTOPT(x11_forwarding);

+	M_CP_INTOPT(x11_max_displays);

 	M_CP_INTOPT(x11_use_localhost);

 	M_CP_INTOPT(permit_tty);

 	M_CP_INTOPT(permit_user_rc);

@@ -2312,6 +2321,7 @@ dump_config(ServerOptions *o)

 #endif

 	dump_cfg_int(sLoginGraceTime, o->login_grace_time);

 	dump_cfg_int(sX11DisplayOffset, o->x11_display_offset);

+	dump_cfg_int(sX11MaxDisplays, o->x11_max_displays);

 	dump_cfg_int(sMaxAuthTries, o->max_authtries);

 	dump_cfg_int(sMaxSessions, o->max_sessions);

 	dump_cfg_int(sClientAliveInterval, o->client_alive_interval);

diff -up openssh-7.4p1/servconf.h.x11max openssh-7.4p1/servconf.h

--- openssh-7.4p1/servconf.h.x11max	2017-02-09 12:49:04.741996552 +0100

+++ openssh-7.4p1/servconf.h	2017-02-09 12:49:04.744996547 +0100

@@ -55,6 +55,7 @@

 #define DEFAULT_AUTH_FAIL_MAX	6	/* Default for MaxAuthTries */

 #define DEFAULT_SESSIONS_MAX	10	/* Default for MaxSessions */

+#define DEFAULT_MAX_DISPLAYS	1000 /* Maximum number of fake X11 displays to try. */

 /* Magic name for internal sftp-server */

 #define INTERNAL_SFTP_NAME	"internal-sftp"

@@ -85,6 +86,7 @@ typedef struct {

 	int     x11_forwarding;	/* If true, permit inet (spoofing) X11 fwd. */

 	int     x11_display_offset;	/* What DISPLAY number to start

 					 * searching at */

+	int 	x11_max_displays; /* Number of displays to search */

 	int     x11_use_localhost;	/* If true, use localhost for fake X11 server. */

 	char   *xauth_location;	/* Location of xauth program */

 	int	permit_tty;	/* If false, deny pty allocation */

diff -up openssh-7.4p1/session.c.x11max openssh-7.4p1/session.c

--- openssh-7.4p1/session.c.x11max	2017-02-09 12:49:04.742996550 +0100

+++ openssh-7.4p1/session.c	2017-02-09 12:49:04.745996546 +0100

@@ -2502,8 +2502,9 @@ session_setup_x11fwd(Session *s)

 		return 0;

 	}

 	if (x11_create_display_inet(options.x11_display_offset,

-	    options.x11_use_localhost, s->single_connection,

-	    &s->display_number, &s->x11_chanids) == -1) {

+	    options.x11_use_localhost, options.x11_max_displays,

+	    s->single_connection, &s->display_number,

+	    &s->x11_chanids) == -1) {

 		debug("x11_create_display_inet failed.");

 		return 0;

 	}

diff -up openssh-7.4p1/sshd_config.5.x11max openssh-7.4p1/sshd_config.5

--- openssh-7.4p1/sshd_config.5.x11max	2017-02-09 12:49:04.742996550 +0100

+++ openssh-7.4p1/sshd_config.5	2017-02-09 12:51:24.656790909 +0100

@@ -1137,6 +1137,7 @@ Available keywords are

 .Cm StreamLocalBindUnlink ,

 .Cm TrustedUserCAKeys ,

 .Cm X11DisplayOffset ,

+.Cm X11MaxDisplays ,

 .Cm X11Forwarding

 and

 .Cm X11UseLocalHost .

@@ -1563,6 +1564,12 @@ Specifies the first display number avail

 X11 forwarding.

 This prevents sshd from interfering with real X11 servers.

 The default is 10.

+.It Cm X11MaxDisplays

+Specifies the maximum number of displays available for

+.Xr sshd 8 Ns 's

+X11 forwarding.

+This prevents sshd from exhausting local ports.

+The default is 1000.

 .It Cm X11Forwarding

 Specifies whether X11 forwarding is permitted.

 The argument must be