diff -up openssh-7.4p1/gss-genr.c.gsskexalg openssh-7.4p1/gss-genr.c

--- openssh-7.4p1/gss-genr.c.gsskexalg	2017-02-09 10:46:50.417893132 +0100

+++ openssh-7.4p1/gss-genr.c	2017-02-09 10:46:50.448893107 +0100

@@ -77,7 +77,8 @@ ssh_gssapi_oid_table_ok() {

  */

 char *

-ssh_gssapi_client_mechanisms(const char *host, const char *client) {

+ssh_gssapi_client_mechanisms(const char *host, const char *client,

+    const char *kex) {

 	gss_OID_set gss_supported;

 	OM_uint32 min_status;

@@ -85,12 +86,12 @@ ssh_gssapi_client_mechanisms(const char

 		return NULL;

 	return(ssh_gssapi_kex_mechs(gss_supported, ssh_gssapi_check_mechanism,

-	    host, client));

+	    host, client, kex));

 }

 char *

 ssh_gssapi_kex_mechs(gss_OID_set gss_supported, ssh_gssapi_check_fn *check,

-    const char *host, const char *client) {

+    const char *host, const char *client, const char *kex) {

 	Buffer buf;

 	size_t i;

 	int oidpos, enclen;

@@ -99,6 +100,7 @@ ssh_gssapi_kex_mechs(gss_OID_set gss_sup

 	char deroid[2];

 	const EVP_MD *evp_md = EVP_md5();

 	EVP_MD_CTX md;

+	char *s, *cp, *p;

 	if (gss_enc2oid != NULL) {

 		for (i = 0; gss_enc2oid[i].encoded != NULL; i++)

@@ -112,6 +114,7 @@ ssh_gssapi_kex_mechs(gss_OID_set gss_sup

 	buffer_init(&buf;;

 	oidpos = 0;

+	s = cp = strdup(kex);

 	for (i = 0; i < gss_supported->count; i++) {

 		if (gss_supported->elements[i].length < 128 &&

 		    (*check)(NULL, &(gss_supported->elements[i]), host, client)) {

@@ -130,26 +133,22 @@ ssh_gssapi_kex_mechs(gss_OID_set gss_sup

 			enclen = __b64_ntop(digest, EVP_MD_size(evp_md),

 			    encoded, EVP_MD_size(evp_md) * 2);

-			if (oidpos != 0)

-				buffer_put_char(&buf, ',');

-

-			buffer_append(&buf, KEX_GSS_GEX_SHA1_ID,

-			    sizeof(KEX_GSS_GEX_SHA1_ID) - 1);

-			buffer_append(&buf, encoded, enclen);

-			buffer_put_char(&buf, ',');

-			buffer_append(&buf, KEX_GSS_GRP1_SHA1_ID, 

-			    sizeof(KEX_GSS_GRP1_SHA1_ID) - 1);

-			buffer_append(&buf, encoded, enclen);

-			buffer_put_char(&buf, ',');

-			buffer_append(&buf, KEX_GSS_GRP14_SHA1_ID,

-			    sizeof(KEX_GSS_GRP14_SHA1_ID) - 1);

-			buffer_append(&buf, encoded, enclen);

+			cp = strncpy(s, kex, strlen(kex));

+			for ((p = strsep(&cp, ",")); p && *p != '\0';

+				(p = strsep(&cp, ","))) {

+				if (buffer_len(&buf) != 0)

+					buffer_put_char(&buf, ',');

+				buffer_append(&buf, p,

+				    strlen(p));

+				buffer_append(&buf, encoded, enclen);

+			}

 			gss_enc2oid[oidpos].oid = &(gss_supported->elements[i]);

 			gss_enc2oid[oidpos].encoded = encoded;

 			oidpos++;

 		}

 	}

+	free(s);

 	gss_enc2oid[oidpos].oid = NULL;

 	gss_enc2oid[oidpos].encoded = NULL;

diff -up openssh-7.4p1/gss-serv.c.gsskexalg openssh-7.4p1/gss-serv.c

--- openssh-7.4p1/gss-serv.c.gsskexalg	2017-02-09 10:46:50.449893106 +0100

+++ openssh-7.4p1/gss-serv.c	2017-02-09 10:55:12.189422901 +0100

@@ -149,7 +149,7 @@ ssh_gssapi_server_mechanisms() {

 	if (supported_oids == NULL)

 		ssh_gssapi_prepare_supported_oids();

 	return (ssh_gssapi_kex_mechs(supported_oids,

-	    &ssh_gssapi_server_check_mech, NULL, NULL));

+	    &ssh_gssapi_server_check_mech, NULL, NULL, options.gss_kex_algorithms));

 }

 /* Unprivileged */

diff -up openssh-7.4p1/kex.c.gsskexalg openssh-7.4p1/kex.c

--- openssh-7.4p1/kex.c.gsskexalg	2017-02-09 10:46:50.449893106 +0100

+++ openssh-7.4p1/kex.c	2017-02-09 10:55:44.008393539 +0100

@@ -248,6 +248,29 @@ kex_assemble_names(const char *def, char

 	return 0;

 }

+/* Validate GSS KEX method name list */

+int

+gss_kex_names_valid(const char *names)

+{

+	char *s, *cp, *p;

+

+	if (names == NULL || *names == '\0')

+		return 0;

+	s = cp = strdup(names);

+	for ((p = strsep(&cp, ",")); p && *p != '\0';

+	    (p = strsep(&cp, ","))) {

+		if (strncmp(p, "gss-", 4) != 0

+		  || kex_alg_by_name(p) == NULL) {

+			error("Unsupported KEX algorithm \"%.100s\"", p);

+			free(s);

+			return 0;

+		}

+	}

+	debug3("gss kex names ok: [%s]", names);

+	free(s);

+	return 1;

+}

+

 /* put algorithm proposal into buffer */

 int

 kex_prop2buf(struct sshbuf *b, char *proposal[PROPOSAL_MAX])

diff -up openssh-7.4p1/kex.h.gsskexalg openssh-7.4p1/kex.h

--- openssh-7.4p1/kex.h.gsskexalg	2017-02-09 10:46:50.452893104 +0100

+++ openssh-7.4p1/kex.h	2017-02-09 11:02:35.313012903 +0100

@@ -179,6 +179,7 @@ struct kex {

 char	*kex_alg_list(char);

 char	*kex_names_cat(const char *, const char *);

 int	 kex_assemble_names(const char *, char **);

+int	 gss_kex_names_valid(const char *);

 int	 kex_new(struct ssh *, char *[PROPOSAL_MAX], struct kex **);

 int	 kex_setup(struct ssh *, char *[PROPOSAL_MAX]);

diff -up openssh-7.4p1/readconf.c.gsskexalg openssh-7.4p1/readconf.c

--- openssh-7.4p1/readconf.c.gsskexalg	2017-02-09 10:46:50.420893129 +0100

+++ openssh-7.4p1/readconf.c	2017-02-09 10:56:06.759372540 +0100

@@ -64,6 +64,7 @@

 #include "uidswap.h"

 #include "myproposal.h"

 #include "digest.h"

+#include "ssh-gss.h"

 /* Format of the configuration file:

@@ -161,7 +162,7 @@ typedef enum {

 	oEnableSSHKeysign, oRekeyLimit, oVerifyHostKeyDNS, oConnectTimeout,

 	oAddressFamily, oGssAuthentication, oGssDelegateCreds,

 	oGssTrustDns, oGssKeyEx, oGssClientIdentity, oGssRenewalRekey,

-	oGssServerIdentity, 

+	oGssServerIdentity, oGssKexAlgorithms,

 	oServerAliveInterval, oServerAliveCountMax, oIdentitiesOnly,

 	oSendEnv, oControlPath, oControlMaster, oControlPersist,

 	oHashKnownHosts,

@@ -213,6 +214,7 @@ static struct {

 	{ "gssapiclientidentity", oGssClientIdentity },

 	{ "gssapiserveridentity", oGssServerIdentity },

 	{ "gssapirenewalforcesrekey", oGssRenewalRekey },

+	{ "gssapikexalgorithms", oGssKexAlgorithms },

 #else

 	{ "gssapiauthentication", oUnsupported },

 	{ "gssapikeyexchange", oUnsupported },

@@ -220,6 +222,7 @@ static struct {

 	{ "gssapitrustdns", oUnsupported },

 	{ "gssapiclientidentity", oUnsupported },

 	{ "gssapirenewalforcesrekey", oUnsupported },

+	{ "gssapikexalgorithms", oUnsupported },

 #endif

 	{ "fallbacktorsh", oDeprecated },

 	{ "usersh", oDeprecated },

@@ -996,6 +999,18 @@ parse_time:

 		intptr = &options->gss_renewal_rekey;

 		goto parse_flag;

+	case oGssKexAlgorithms:

+		arg = strdelim(&s);

+		if (!arg || *arg == '\0')

+			fatal("%.200s line %d: Missing argument.",

+			    filename, linenum);

+		if (!gss_kex_names_valid(arg))

+			fatal("%.200s line %d: Bad GSSAPI KexAlgorithms '%s'.",

+			    filename, linenum, arg ? arg : "<NONE>");

+		if (*activep && options->gss_kex_algorithms == NULL)

+			options->gss_kex_algorithms = strdup(arg);

+		break;

+

 	case oBatchMode:

 		intptr = &options->batch_mode;

 		goto parse_flag;

@@ -1813,6 +1828,7 @@ initialize_options(Options * options)

 	options->gss_renewal_rekey = -1;

 	options->gss_client_identity = NULL;

 	options->gss_server_identity = NULL;

+	options->gss_kex_algorithms = NULL;

 	options->password_authentication = -1;

 	options->kbd_interactive_authentication = -1;

 	options->kbd_interactive_devices = NULL;

@@ -1964,6 +1980,10 @@ fill_default_options(Options * options)

 		options->gss_trust_dns = 0;

 	if (options->gss_renewal_rekey == -1)

 		options->gss_renewal_rekey = 0;

+#ifdef GSSAPI

+	if (options->gss_kex_algorithms == NULL)

+		options->gss_kex_algorithms = strdup(GSS_KEX_DEFAULT_KEX);

+#endif

 	if (options->password_authentication == -1)

 		options->password_authentication = 1;

 	if (options->kbd_interactive_authentication == -1)

diff -up openssh-7.4p1/readconf.h.gsskexalg openssh-7.4p1/readconf.h

--- openssh-7.4p1/readconf.h.gsskexalg	2017-02-09 10:46:50.420893129 +0100

+++ openssh-7.4p1/readconf.h	2017-02-09 10:46:50.450893106 +0100

@@ -51,6 +51,7 @@ typedef struct {

 	int	gss_renewal_rekey;	/* Credential renewal forces rekey */

 	char    *gss_client_identity;   /* Principal to initiate GSSAPI with */

 	char    *gss_server_identity;   /* GSSAPI target principal */

+	char   *gss_kex_algorithms;	/* GSSAPI kex methods to be offered by client. */

 	int     password_authentication;	/* Try password

 						 * authentication. */

 	int     kbd_interactive_authentication; /* Try keyboard-interactive auth. */

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

--- openssh-7.4p1/servconf.c.gsskexalg	2017-02-09 10:46:50.446893109 +0100

+++ openssh-7.4p1/servconf.c	2017-02-09 10:57:15.784309297 +0100

@@ -57,6 +57,7 @@

 #include "auth.h"

 #include "myproposal.h"

 #include "digest.h"

+#include "ssh-gss.h"

 static void add_listen_addr(ServerOptions *, char *, int);

 static void add_one_listen_addr(ServerOptions *, char *, int);

@@ -117,6 +117,7 @@ initialize_server_options(ServerOptions

 	options->gss_cleanup_creds = -1;

 	options->gss_strict_acceptor = -1;

 	options->gss_store_rekey = -1;

+	options->gss_kex_algorithms = NULL;

 	options->password_authentication = -1;

 	options->kbd_interactive_authentication = -1;

 	options->challenge_response_authentication = -1;

@@ -280,6 +281,10 @@ fill_default_server_options(ServerOption

 		options->gss_strict_acceptor = 0;

 	if (options->gss_store_rekey == -1)

 		options->gss_store_rekey = 0;

+#ifdef GSSAPI

+	if (options->gss_kex_algorithms == NULL)

+		options->gss_kex_algorithms = strdup(GSS_KEX_DEFAULT_KEX);

+#endif

 	if (options->password_authentication == -1)

 		options->password_authentication = 1;

 	if (options->kbd_interactive_authentication == -1)

@@ -422,7 +425,7 @@ typedef enum {

 	sHostKeyAlgorithms,

 	sClientAliveInterval, sClientAliveCountMax, sAuthorizedKeysFile,

 	sGssAuthentication, sGssCleanupCreds, sGssEnablek5users, sGssStrictAcceptor,

-	sGssKeyEx, sGssStoreRekey, sAcceptEnv, sPermitTunnel,

+	sGssKeyEx, sGssStoreRekey, sGssKexAlgorithms, sAcceptEnv, sPermitTunnel,

 	sMatch, sPermitOpen, sForceCommand, sChrootDirectory,

 	sUsePrivilegeSeparation, sAllowAgentForwarding,

 	sHostCertificate,

@@ -501,6 +504,7 @@ static struct {

 	{ "gssapikeyexchange", sGssKeyEx, SSHCFG_GLOBAL },

 	{ "gssapistorecredentialsonrekey", sGssStoreRekey, SSHCFG_GLOBAL },

 	{ "gssapienablek5users", sGssEnablek5users, SSHCFG_ALL },

+	{ "gssapikexalgorithms", sGssKexAlgorithms, SSHCFG_GLOBAL },

 #else

 	{ "gssapiauthentication", sUnsupported, SSHCFG_ALL },

 	{ "gssapicleanupcredentials", sUnsupported, SSHCFG_GLOBAL },

@@ -508,6 +512,7 @@ static struct {

 	{ "gssapikeyexchange", sUnsupported, SSHCFG_GLOBAL },

 	{ "gssapistorecredentialsonrekey", sUnsupported, SSHCFG_GLOBAL },

 	{ "gssapienablek5users", sUnsupported, SSHCFG_ALL },

+	{ "gssapikexalgorithms", sUnsupported, SSHCFG_GLOBAL },

 #endif

 	{ "gssusesessionccache", sUnsupported, SSHCFG_GLOBAL },

 	{ "gssapiusesessioncredcache", sUnsupported, SSHCFG_GLOBAL },

@@ -1249,6 +1254,18 @@ process_server_config_line(ServerOptions

 		intptr = &options->gss_store_rekey;

 		goto parse_flag;

+	case sGssKexAlgorithms:

+		arg = strdelim(&cp;;

+		if (!arg || *arg == '\0')

+			fatal("%.200s line %d: Missing argument.",

+			    filename, linenum);

+		if (!gss_kex_names_valid(arg))

+			fatal("%.200s line %d: Bad GSSAPI KexAlgorithms '%s'.",

+			    filename, linenum, arg ? arg : "<NONE>");

+		if (*activep && options->gss_kex_algorithms == NULL)

+			options->gss_kex_algorithms = strdup(arg);

+		break;

+

 	case sPasswordAuthentication:

 		intptr = &options->password_authentication;

 		goto parse_flag;

@@ -2304,6 +2321,7 @@ dump_config(ServerOptions *o)

 	dump_cfg_fmtint(sGssKeyEx, o->gss_keyex);

 	dump_cfg_fmtint(sGssStrictAcceptor, o->gss_strict_acceptor);

 	dump_cfg_fmtint(sGssStoreRekey, o->gss_store_rekey);

+	dump_cfg_string(sGssKexAlgorithms, o->gss_kex_algorithms);

 #endif

 	dump_cfg_fmtint(sPasswordAuthentication, o->password_authentication);

 	dump_cfg_fmtint(sKbdInteractiveAuthentication,

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

--- openssh-7.4p1/servconf.h.gsskexalg	2017-02-09 10:46:50.450893106 +0100

+++ openssh-7.4p1/servconf.h	2017-02-09 10:57:33.717292870 +0100

@@ -116,6 +116,7 @@ typedef struct {

 	int     gss_cleanup_creds;	/* If true, destroy cred cache on logout */

 	int     gss_strict_acceptor;	/* If true, restrict the GSSAPI acceptor name */

 	int 	gss_store_rekey;

+	char   *gss_kex_algorithms;	/* GSSAPI kex methods to be offered by client. */

 	int     password_authentication;	/* If true, permit password

 						 * authentication. */

 	int     kbd_interactive_authentication;	/* If true, permit */

diff -up openssh-7.4p1/ssh.1.gsskexalg openssh-7.4p1/ssh.1

--- openssh-7.4p1/ssh.1.gsskexalg	2017-02-09 10:46:50.443893111 +0100

+++ openssh-7.4p1/ssh.1	2017-02-09 10:46:50.451893105 +0100

@@ -517,6 +517,7 @@ For full details of the options listed b

 .It GSSAPIDelegateCredentials

 .It GSSAPIRenewalForcesRekey

 .It GSSAPITrustDns

+.It GSSAPIKexAlgorithms

 .It HashKnownHosts

 .It Host

 .It HostbasedAuthentication

diff -up openssh-7.4p1/ssh_config.5.gsskexalg openssh-7.4p1/ssh_config.5

--- openssh-7.4p1/ssh_config.5.gsskexalg	2017-02-09 10:46:50.452893104 +0100

+++ openssh-7.4p1/ssh_config.5	2017-02-09 11:00:39.053122745 +0100

@@ -782,6 +782,18 @@ the name of the host being connected to.

 command line will be passed untouched to the GSSAPI library.

 The default is

 .Dq no .

+.It Cm GSSAPIKexAlgorithms

+The list of key exchange algorithms that are offered for GSSAPI

+key exchange. Possible values are

+.Bd -literal -offset 3n

+gss-gex-sha1-,

+gss-group1-sha1-,

+gss-group14-sha1-

+.Ed

+.Pp

+The default is

+.Dq gss-gex-sha1-,gss-group1-sha1-,gss-group14-sha1- .

+This option only applies to protocol version 2 connections using GSSAPI.

 .It Cm HashKnownHosts

 Indicates that

 .Xr ssh 1

diff -up openssh-7.4p1/sshconnect2.c.gsskexalg openssh-7.4p1/sshconnect2.c

--- openssh-7.4p1/sshconnect2.c.gsskexalg	2017-02-09 10:46:50.451893105 +0100

+++ openssh-7.4p1/sshconnect2.c	2017-02-09 10:58:08.533260973 +0100

@@ -181,7 +181,8 @@ ssh_kex2(char *host, struct sockaddr *ho

 		else

 			gss_host = host;

-		gss = ssh_gssapi_client_mechanisms(gss_host, options.gss_client_identity);

+		gss = ssh_gssapi_client_mechanisms(gss_host,

+		    options.gss_client_identity, options.gss_kex_algorithms);

 		if (gss) {

 			debug("Offering GSSAPI proposal: %s", gss);

 			xasprintf(&options.kex_algorithms,

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

--- openssh-7.4p1/sshd_config.5.gsskexalg	2017-02-09 10:46:50.452893104 +0100

+++ openssh-7.4p1/sshd_config.5	2017-02-09 11:01:55.141050861 +0100

@@ -666,6 +666,18 @@ Controls whether the user's GSSAPI crede

 successful connection rekeying. This option can be used to accepted renewed 

 or updated credentials from a compatible client. The default is

 .Dq no .

+.It Cm GSSAPIKexAlgorithms

+The list of key exchange algorithms that are accepted by GSSAPI

+key exchange. Possible values are

+.Bd -literal -offset 3n

+gss-gex-sha1-,

+gss-group1-sha1-,

+gss-group14-sha1-

+.Ed

+.Pp

+The default is

+.Dq gss-gex-sha1-,gss-group1-sha1-,gss-group14-sha1- .

+This option only applies to protocol version 2 connections using GSSAPI.

 .It Cm HostbasedAcceptedKeyTypes

 Specifies the key types that will be accepted for hostbased authentication

 as a comma-separated pattern list.

diff -up openssh-7.4p1/ssh-gss.h.gsskexalg openssh-7.4p1/ssh-gss.h

--- openssh-7.4p1/ssh-gss.h.gsskexalg	2017-02-09 10:46:50.425893125 +0100

+++ openssh-7.4p1/ssh-gss.h	2017-02-09 10:46:50.451893105 +0100

@@ -76,6 +76,11 @@ extern char **k5users_allowed_cmds;

 #define KEX_GSS_GRP14_SHA1_ID				"gss-group14-sha1-"

 #define KEX_GSS_GEX_SHA1_ID				"gss-gex-sha1-"

+#define        GSS_KEX_DEFAULT_KEX \

+	KEX_GSS_GEX_SHA1_ID "," \

+	KEX_GSS_GRP1_SHA1_ID "," \

+	KEX_GSS_GRP14_SHA1_ID

+

 typedef struct {

 	char *filename;

 	char *envvar;

@@ -147,9 +152,9 @@ int ssh_gssapi_credentials_updated(Gssct

 /* In the server */

 typedef int ssh_gssapi_check_fn(Gssctxt **, gss_OID, const char *, 

     const char *);

-char *ssh_gssapi_client_mechanisms(const char *, const char *);

+char *ssh_gssapi_client_mechanisms(const char *, const char *, const char *);

 char *ssh_gssapi_kex_mechs(gss_OID_set, ssh_gssapi_check_fn *, const char *,

-    const char *);

+    const char *, const char *);

 gss_OID ssh_gssapi_id_kex(Gssctxt *, char *, int);

 int ssh_gssapi_server_check_mech(Gssctxt **,gss_OID, const char *, 

     const char *);