openssh/openssh-7.2p2-disable_short_DH_parameters.patch

# HG changeset patch
# Parent  c40dce555117c740f3df867e9fc2b07b64b3ad96

Raise minimal size of DH group parameters to 2048 bits like upstream did in
7.2. 1024b values are believed to be in breaking range for state adversaries
and the default moduli shipped with openssh have been around long enough to
make it more likely for them to be broken.

Also provide an option that allows the client to accept shorter (RFC4419
compliant) parameters.

CVE-2015-4000 (LOGJAM)
bsc#932483

diff --git a/openssh-7.2p2/dh.c b/openssh-7.2p2/dh.c
--- a/openssh-7.2p2/dh.c
+++ b/openssh-7.2p2/dh.c
@@ -37,16 +37,18 @@
 #include <limits.h>
 
 #include "dh.h"
 #include "pathnames.h"
 #include "log.h"
 #include "misc.h"
 #include "ssherr.h"
 
+int dh_grp_min = DH_GRP_MIN;
+
 static int
 parse_prime(int linenum, char *line, struct dhgroup *dhg)
 {
 	char *cp, *arg;
 	char *strsize, *gen, *prime;
 	const char *errstr = NULL;
 	long long n;
 
diff --git a/openssh-7.2p2/dh.h b/openssh-7.2p2/dh.h
--- a/openssh-7.2p2/dh.h
+++ b/openssh-7.2p2/dh.h
@@ -43,16 +43,17 @@ int	 dh_gen_key(DH *, int);
 int	 dh_pub_is_valid(DH *, BIGNUM *);
 
 u_int	 dh_estimate(int);
 
 /*
  * Max value from RFC4419.
  * Miniumum increased in light of DH precomputation attacks.
  */
+#define DH_GRP_MIN_RFC	1024
 #define DH_GRP_MIN	2048
 #define DH_GRP_MAX	8192
 
 /*
  * Values for "type" field of moduli(5)
  * Specifies the internal structure of the prime modulus.
  */
 #define MODULI_TYPE_UNKNOWN		(0)
diff --git a/openssh-7.2p2/kexgexc.c b/openssh-7.2p2/kexgexc.c
--- a/openssh-7.2p2/kexgexc.c
+++ b/openssh-7.2p2/kexgexc.c
@@ -46,29 +46,32 @@
 #include "packet.h"
 #include "dh.h"
 #include "ssh2.h"
 #include "compat.h"
 #include "dispatch.h"
 #include "ssherr.h"
 #include "sshbuf.h"
 
+/* import from dh.c */
+extern int dh_grp_min;
+
 static int input_kex_dh_gex_group(int, u_int32_t, void *);
 static int input_kex_dh_gex_reply(int, u_int32_t, void *);
 
 int
 kexgex_client(struct ssh *ssh)
 {
 	struct kex *kex = ssh->kex;
 	int r;
 	u_int nbits;
 
 	nbits = dh_estimate(kex->dh_need * 8);
 
-	kex->min = DH_GRP_MIN;
+	kex->min = dh_grp_min;
 	kex->max = DH_GRP_MAX;
 	kex->nbits = nbits;
 	if (datafellows & SSH_BUG_DHGEX_LARGE)
 		kex->nbits = MIN(kex->nbits, 4096);
 	/* New GEX request */
 	if ((r = sshpkt_start(ssh, SSH2_MSG_KEX_DH_GEX_REQUEST)) != 0 ||
 	    (r = sshpkt_put_u32(ssh, kex->min)) != 0 ||
 	    (r = sshpkt_put_u32(ssh, kex->nbits)) != 0 ||
@@ -104,16 +107,22 @@ input_kex_dh_gex_group(int type, u_int32
 		goto out;
 	}
 	if ((r = sshpkt_get_bignum2(ssh, p)) != 0 ||
 	    (r = sshpkt_get_bignum2(ssh, g)) != 0 ||
 	    (r = sshpkt_get_end(ssh)) != 0)
 		goto out;
 	if ((bits = BN_num_bits(p)) < 0 ||
 	    (u_int)bits < kex->min || (u_int)bits > kex->max) {
+		if ((u_int)bits < kex->min && (u_int)bits >= DH_GRP_MIN_RFC)
+			logit("DH parameter offered by the server (%d bits) "
+			    "is considered insecure. "
+			    "You can lower the accepted the minimum "
+			    "via the KexDHMin option.",
+			    bits);
 		r = SSH_ERR_DH_GEX_OUT_OF_RANGE;
 		goto out;
 	}
 	if ((kex->dh = dh_new_group(g, p)) == NULL) {
 		r = SSH_ERR_ALLOC_FAIL;
 		goto out;
 	}
 	p = g = NULL; /* belong to kex->dh now */
diff --git a/openssh-7.2p2/kexgexs.c b/openssh-7.2p2/kexgexs.c
--- a/openssh-7.2p2/kexgexs.c
+++ b/openssh-7.2p2/kexgexs.c
@@ -49,16 +49,19 @@
 #ifdef GSSAPI
 #include "ssh-gss.h"
 #endif
 #include "monitor_wrap.h"
 #include "dispatch.h"
 #include "ssherr.h"
 #include "sshbuf.h"
 
+/* import from dh.c */
+extern int dh_grp_min;
+
 static int input_kex_dh_gex_request(int, u_int32_t, void *);
 static int input_kex_dh_gex_init(int, u_int32_t, void *);
 
 int
 kexgex_server(struct ssh *ssh)
 {
 	ssh_dispatch_set(ssh, SSH2_MSG_KEX_DH_GEX_REQUEST,
 	    &input_kex_dh_gex_request);
@@ -78,23 +81,29 @@ input_kex_dh_gex_request(int type, u_int
 	if ((r = sshpkt_get_u32(ssh, &min)) != 0 ||
 	    (r = sshpkt_get_u32(ssh, &nbits)) != 0 ||
 	    (r = sshpkt_get_u32(ssh, &max)) != 0 ||
 	    (r = sshpkt_get_end(ssh)) != 0)
 		goto out;
 	kex->nbits = nbits;
 	kex->min = min;
 	kex->max = max;
-	min = MAX(DH_GRP_MIN, min);
+	min = MAX(dh_grp_min, min);
 	max = MIN(DH_GRP_MAX, max);
-	nbits = MAX(DH_GRP_MIN, nbits);
+	nbits = MAX(dh_grp_min, nbits);
 	nbits = MIN(DH_GRP_MAX, nbits);
 
 	if (kex->max < kex->min || kex->nbits < kex->min ||
 	    kex->max < kex->nbits) {
+		if (kex->nbits < kex->min && kex->nbits >= DH_GRP_MIN_RFC)
+			logit("DH parameter requested by the client (%d bits) "
+			    "is considered insecure. "
+			    "You can lower the accepted minimum "
+			    "via the KexDHMin option.",
+			    kex->nbits);
 		r = SSH_ERR_DH_GEX_OUT_OF_RANGE;
 		goto out;
 	}
 
 	/* Contact privileged parent */
 	kex->dh = PRIVSEP(choose_dh(min, nbits, max));
 	if (kex->dh == NULL) {
 		sshpkt_disconnect(ssh, "no matching DH grp found");
diff --git a/openssh-7.2p2/readconf.c b/openssh-7.2p2/readconf.c
--- a/openssh-7.2p2/readconf.c
+++ b/openssh-7.2p2/readconf.c
@@ -56,16 +56,17 @@
 #include "misc.h"
 #include "readconf.h"
 #include "match.h"
 #include "kex.h"
 #include "mac.h"
 #include "uidswap.h"
 #include "myproposal.h"
 #include "digest.h"
+#include "dh.h"
 
 /* Format of the configuration file:
 
    # Configuration data is parsed as follows:
    #  1. command line options
    #  2. user-specific file
    #  3. system-wide file
    # Any configuration value is only changed the first time it is set.
@@ -148,17 +149,18 @@ typedef enum {
 	oClearAllForwardings, oNoHostAuthenticationForLocalhost,
 	oEnableSSHKeysign, oRekeyLimit, oVerifyHostKeyDNS, oConnectTimeout,
 	oAddressFamily, oGssAuthentication, oGssDelegateCreds,
 	oServerAliveInterval, oServerAliveCountMax, oIdentitiesOnly,
 	oSendEnv, oControlPath, oControlMaster, oControlPersist,
 	oHashKnownHosts,
 	oTunnel, oTunnelDevice, oLocalCommand, oPermitLocalCommand,
 	oVisualHostKey,
-	oKexAlgorithms, oIPQoS, oRequestTTY, oIgnoreUnknown, oProxyUseFdpass,
+	oKexAlgorithms, oKexDHMin,
+	oIPQoS, oRequestTTY, oIgnoreUnknown, oProxyUseFdpass,
 	oCanonicalDomains, oCanonicalizeHostname, oCanonicalizeMaxDots,
 	oCanonicalizeFallbackLocal, oCanonicalizePermittedCNAMEs,
 	oStreamLocalBindMask, oStreamLocalBindUnlink, oRevokedHostKeys,
 	oFingerprintHash, oUpdateHostkeys, oHostbasedKeyTypes,
 	oPubkeyAcceptedKeyTypes,
 	oIgnoredUnknownOption, oDeprecated, oUnsupported
 } OpCodes;
 
@@ -260,16 +262,17 @@ static struct {
 	{ "hashknownhosts", oHashKnownHosts },
 	{ "tunnel", oTunnel },
 	{ "tunneldevice", oTunnelDevice },
 	{ "localcommand", oLocalCommand },
 	{ "permitlocalcommand", oPermitLocalCommand },
 	{ "visualhostkey", oVisualHostKey },
 	{ "useroaming", oDeprecated },
 	{ "kexalgorithms", oKexAlgorithms },
+	{ "kexdhmin", oKexDHMin },
 	{ "ipqos", oIPQoS },
 	{ "requesttty", oRequestTTY },
 	{ "proxyusefdpass", oProxyUseFdpass },
 	{ "canonicaldomains", oCanonicalDomains },
 	{ "canonicalizefallbacklocal", oCanonicalizeFallbackLocal },
 	{ "canonicalizehostname", oCanonicalizeHostname },
 	{ "canonicalizemaxdots", oCanonicalizeMaxDots },
 	{ "canonicalizepermittedcnames", oCanonicalizePermittedCNAMEs },
@@ -280,16 +283,19 @@ static struct {
 	{ "updatehostkeys", oUpdateHostkeys },
 	{ "hostbasedkeytypes", oHostbasedKeyTypes },
 	{ "pubkeyacceptedkeytypes", oPubkeyAcceptedKeyTypes },
 	{ "ignoreunknown", oIgnoreUnknown },
 
 	{ NULL, oBadOption }
 };
 
+/* import from dh.c */
+extern int dh_grp_min;
+
 /*
  * Adds a local TCP/IP port forward to options.  Never returns if there is an
  * error.
  */
 
 void
 add_local_forward(Options *options, const struct Forward *newfwd)
 {
@@ -1157,16 +1163,20 @@ parse_int:
 			    filename, linenum);
 		if (!kex_names_valid(*arg == '+' ? arg + 1 : arg))
 			fatal("%.200s line %d: Bad SSH2 KexAlgorithms '%s'.",
 			    filename, linenum, arg ? arg : "<NONE>");
 		if (*activep && options->kex_algorithms == NULL)
 			options->kex_algorithms = xstrdup(arg);
 		break;
 
+	case oKexDHMin:
+		intptr = &options->kex_dhmin;
+		goto parse_int;
+
 	case oHostKeyAlgorithms:
 		charptr = &options->hostkeyalgorithms;
 parse_keytypes:
 		arg = strdelim(&s);
 		if (!arg || *arg == '\0')
 			fatal("%.200s line %d: Missing argument.",
 			    filename, linenum);
 		if (!sshkey_names_valid2(*arg == '+' ? arg + 1 : arg, 1))
@@ -1664,16 +1674,17 @@ initialize_options(Options * options)
 	options->address_family = -1;
 	options->connection_attempts = -1;
 	options->connection_timeout = -1;
 	options->number_of_password_prompts = -1;
 	options->cipher = -1;
 	options->ciphers = NULL;
 	options->macs = NULL;
 	options->kex_algorithms = NULL;
+	options->kex_dhmin = -1;
 	options->hostkeyalgorithms = NULL;
 	options->protocol = SSH_PROTO_UNKNOWN;
 	options->num_identity_files = 0;
 	options->num_certificate_files = 0;
 	options->hostname = NULL;
 	options->host_key_alias = NULL;
 	options->proxy_command = NULL;
 	options->user = NULL;
@@ -1805,16 +1816,23 @@ fill_default_options(Options * options)
 		options->address_family = AF_UNSPEC;
 	if (options->connection_attempts == -1)
 		options->connection_attempts = 1;
 	if (options->number_of_password_prompts == -1)
 		options->number_of_password_prompts = 3;
 	/* Selected in ssh_login(). */
 	if (options->cipher == -1)
 		options->cipher = SSH_CIPHER_NOT_SET;
+	if (options->kex_dhmin == -1)
+		options->kex_dhmin = DH_GRP_MIN_RFC;
+	else {
+		options->kex_dhmin = MAX(options->kex_dhmin, DH_GRP_MIN_RFC);
+		options->kex_dhmin = MIN(options->kex_dhmin, DH_GRP_MAX);
+	}
+	dh_grp_min = options->kex_dhmin;
 	/* options->hostkeyalgorithms, default set in myproposals.h */
 	if (options->protocol == SSH_PROTO_UNKNOWN)
 		options->protocol = SSH_PROTO_2;
 	if (options->add_keys_to_agent == -1)
 		options->add_keys_to_agent = 0;
 	if (options->num_identity_files == 0) {
 		if (options->protocol & SSH_PROTO_1) {
 			add_identity_file(options, "~/",
diff --git a/openssh-7.2p2/readconf.h b/openssh-7.2p2/readconf.h
--- a/openssh-7.2p2/readconf.h
+++ b/openssh-7.2p2/readconf.h
@@ -69,16 +69,17 @@ typedef struct {
 					 * aborting connection attempt */
 	int     number_of_password_prompts;	/* Max number of password
 						 * prompts. */
 	int     cipher;		/* Cipher to use. */
 	char   *ciphers;	/* SSH2 ciphers in order of preference. */
 	char   *macs;		/* SSH2 macs in order of preference. */
 	char   *hostkeyalgorithms;	/* SSH2 server key types in order of preference. */
 	char   *kex_algorithms;	/* SSH2 kex methods in order of preference. */
+	int     kex_dhmin;	/* minimum bit length of the DH group parameter */
 	int	protocol;	/* Protocol in order of preference. */
 	char   *hostname;	/* Real host to connect. */
 	char   *host_key_alias;	/* hostname alias for .ssh/known_hosts */
 	char   *proxy_command;	/* Proxy command for connecting the host. */
 	char   *user;		/* User to log in as. */
 	int     escape_char;	/* Escape character; -2 = none */
 
 	u_int	num_system_hostfiles;	/* Paths for /etc/ssh/ssh_known_hosts */
diff --git a/openssh-7.2p2/servconf.c b/openssh-7.2p2/servconf.c
--- a/openssh-7.2p2/servconf.c
+++ b/openssh-7.2p2/servconf.c
@@ -52,16 +52,20 @@
 #include "channels.h"
 #include "groupaccess.h"
 #include "canohost.h"
 #include "packet.h"
 #include "hostfile.h"
 #include "auth.h"
 #include "myproposal.h"
 #include "digest.h"
+#include "dh.h"
+
+/* import from dh.c */
+extern int dh_grp_min;
 
 static void add_listen_addr(ServerOptions *, char *, int);
 static void add_one_listen_addr(ServerOptions *, char *, int);
 
 /* Use of privilege separation or not */
 extern int use_privsep;
 extern Buffer cfg;
 
@@ -134,16 +138,17 @@ initialize_server_options(ServerOptions 
 	options->allow_agent_forwarding = -1;
 	options->num_allow_users = 0;
 	options->num_deny_users = 0;
 	options->num_allow_groups = 0;
 	options->num_deny_groups = 0;
 	options->ciphers = NULL;
 	options->macs = NULL;
 	options->kex_algorithms = NULL;
+	options->kex_dhmin = -1;
 	options->protocol = SSH_PROTO_UNKNOWN;
 	options->fwd_opts.gateway_ports = -1;
 	options->fwd_opts.streamlocal_bind_mask = (mode_t)-1;
 	options->fwd_opts.streamlocal_bind_unlink = -1;
 	options->num_subsystems = 0;
 	options->max_startups_begin = -1;
 	options->max_startups_rate = -1;
 	options->max_startups = -1;
@@ -199,16 +204,23 @@ fill_default_server_options(ServerOption
 	int i;
 
 	/* Portable-specific options */
 	if (options->use_pam == -1)
 		options->use_pam = 0;
 	if (options->use_pam_check_locks == -1)
 		options->use_pam_check_locks = 0;
 
+	if (options->kex_dhmin == -1)
+		options->kex_dhmin = DH_GRP_MIN_RFC;
+	else {
+		options->kex_dhmin = MAX(options->kex_dhmin, DH_GRP_MIN_RFC);
+		options->kex_dhmin = MIN(options->kex_dhmin, DH_GRP_MAX);
+	}
+	dh_grp_min = options->kex_dhmin;
 	/* Standard Options */
 	if (options->protocol == SSH_PROTO_UNKNOWN)
 		options->protocol = SSH_PROTO_2;
 	if (options->num_host_key_files == 0) {
 		/* fill default hostkeys for protocols */
 		if (options->protocol & SSH_PROTO_1)
 			options->host_key_files[options->num_host_key_files++] =
 			    _PATH_HOST_KEY_FILE;
@@ -423,17 +435,18 @@ typedef enum {
 	sClientAliveInterval, sClientAliveCountMax, sAuthorizedKeysFile,
 	sGssAuthentication, sGssCleanupCreds, sGssStrictAcceptor,
 	sAcceptEnv, sPermitTunnel,
 	sMatch, sPermitOpen, sForceCommand, sChrootDirectory,
 	sUsePrivilegeSeparation, sAllowAgentForwarding,
 	sHostCertificate,
 	sRevokedKeys, sTrustedUserCAKeys, sAuthorizedPrincipalsFile,
 	sAuthorizedPrincipalsCommand, sAuthorizedPrincipalsCommandUser,
-	sKexAlgorithms, sIPQoS, sVersionAddendum,
+	sKexAlgorithms, sKexDHMin,
+	sIPQoS, sVersionAddendum,
 	sAuthorizedKeysCommand, sAuthorizedKeysCommandUser,
 	sAuthenticationMethods, sHostKeyAgent, sPermitUserRC,
 	sStreamLocalBindMask, sStreamLocalBindUnlink,
 	sAllowStreamLocalForwarding, sFingerprintHash,
 	sDeprecated, sUnsupported
 } ServerOpCodes;
 
 #define SSHCFG_GLOBAL	0x01	/* allowed in main section of sshd_config */
@@ -561,16 +574,17 @@ static struct {
 	{ "permitopen", sPermitOpen, SSHCFG_ALL },
 	{ "forcecommand", sForceCommand, SSHCFG_ALL },
 	{ "chrootdirectory", sChrootDirectory, SSHCFG_ALL },
 	{ "hostcertificate", sHostCertificate, SSHCFG_GLOBAL },
 	{ "revokedkeys", sRevokedKeys, SSHCFG_ALL },
 	{ "trustedusercakeys", sTrustedUserCAKeys, SSHCFG_ALL },
 	{ "authorizedprincipalsfile", sAuthorizedPrincipalsFile, SSHCFG_ALL },
 	{ "kexalgorithms", sKexAlgorithms, SSHCFG_GLOBAL },
+	{ "kexdhmin", sKexDHMin },
 	{ "ipqos", sIPQoS, SSHCFG_ALL },
 	{ "authorizedkeyscommand", sAuthorizedKeysCommand, SSHCFG_ALL },
 	{ "authorizedkeyscommanduser", sAuthorizedKeysCommandUser, SSHCFG_ALL },
 	{ "authorizedprincipalscommand", sAuthorizedPrincipalsCommand, SSHCFG_ALL },
 	{ "authorizedprincipalscommanduser", sAuthorizedPrincipalsCommandUser, SSHCFG_ALL },
 	{ "versionaddendum", sVersionAddendum, SSHCFG_GLOBAL },
 	{ "authenticationmethods", sAuthenticationMethods, SSHCFG_ALL },
 	{ "streamlocalbindmask", sStreamLocalBindMask, SSHCFG_ALL },
@@ -1481,16 +1495,20 @@ process_server_config_line(ServerOptions
 			    filename, linenum);
 		if (!kex_names_valid(*arg == '+' ? arg + 1 : arg))
 			fatal("%s line %d: Bad SSH2 KexAlgorithms '%s'.",
 			    filename, linenum, arg ? arg : "<NONE>");
 		if (options->kex_algorithms == NULL)
 			options->kex_algorithms = xstrdup(arg);
 		break;
 
+	case sKexDHMin:
+		intptr = &options->kex_dhmin;
+		goto parse_int;
+
 	case sProtocol:
 		intptr = &options->protocol;
 		arg = strdelim(&cp);
 		if (!arg || *arg == '\0')
 			fatal("%s line %d: Missing argument.", filename, linenum);
 		value = proto_spec(arg);
 		if (value == SSH_PROTO_UNKNOWN)
 			fatal("%s line %d: Bad protocol spec '%s'.",
@@ -2247,16 +2265,17 @@ dump_config(ServerOptions *o)
 	dump_cfg_int(sLoginGraceTime, o->login_grace_time);
 	dump_cfg_int(sKeyRegenerationTime, o->key_regeneration_time);
 	dump_cfg_int(sX11DisplayOffset, o->x11_display_offset);
 	dump_cfg_int(sMaxAuthTries, o->max_authtries);
 	dump_cfg_int(sMaxSessions, o->max_sessions);
 	dump_cfg_int(sClientAliveInterval, o->client_alive_interval);
 	dump_cfg_int(sClientAliveCountMax, o->client_alive_count_max);
 	dump_cfg_oct(sStreamLocalBindMask, o->fwd_opts.streamlocal_bind_mask);
+	dump_cfg_int(sKexDHMin, o->kex_dhmin);
 
 	/* formatted integer arguments */
 	dump_cfg_fmtint(sPermitRootLogin, o->permit_root_login);
 	dump_cfg_fmtint(sIgnoreRhosts, o->ignore_rhosts);
 	dump_cfg_fmtint(sIgnoreUserKnownHosts, o->ignore_user_known_hosts);
 	dump_cfg_fmtint(sRhostsRSAAuthentication, o->rhosts_rsa_authentication);
 	dump_cfg_fmtint(sHostbasedAuthentication, o->hostbased_authentication);
 	dump_cfg_fmtint(sHostbasedUsesNameFromPacketOnly,
diff --git a/openssh-7.2p2/servconf.h b/openssh-7.2p2/servconf.h
--- a/openssh-7.2p2/servconf.h
+++ b/openssh-7.2p2/servconf.h
@@ -88,16 +88,17 @@ typedef struct {
 	int	permit_user_rc;	/* If false, deny ~/.ssh/rc execution */
 	int     strict_modes;	/* If true, require string home dir modes. */
 	int     tcp_keep_alive;	/* If true, set SO_KEEPALIVE. */
 	int	ip_qos_interactive;	/* IP ToS/DSCP/class for interactive */
 	int	ip_qos_bulk;		/* IP ToS/DSCP/class for bulk traffic */
 	char   *ciphers;	/* Supported SSH2 ciphers. */
 	char   *macs;		/* Supported SSH2 macs. */
 	char   *kex_algorithms;	/* SSH2 kex methods in order of preference. */
+	int     kex_dhmin;	/* minimum bit length of the DH group parameter */
 	int	protocol;	/* Supported protocol versions. */
 	struct ForwardOptions fwd_opts;	/* forwarding options */
 	SyslogFacility log_facility;	/* Facility for system logging. */
 	LogLevel log_level;	/* Level for system logging. */
 	int     rhosts_rsa_authentication;	/* If true, permit rhosts RSA
 						 * authentication. */
 	int     hostbased_authentication;	/* If true, permit ssh2 hostbased auth */
 	int     hostbased_uses_name_from_packet_only; /* experimental */
diff --git a/openssh-7.2p2/ssh_config b/openssh-7.2p2/ssh_config
--- a/openssh-7.2p2/ssh_config
+++ b/openssh-7.2p2/ssh_config
@@ -12,16 +12,21 @@
 # Any configuration value is only changed the first time it is set.
 # Thus, host-specific definitions should be at the beginning of the
 # configuration file, and defaults at the end.
 
 # Site-wide defaults for some commonly used options.  For a comprehensive
 # list of available options, their meanings and defaults, please see the
 # ssh_config(5) man page.
 
+# Minimum accepted size of the DH parameter p. By default this is set to 1024
+# to maintain compatibility with RFC4419, but should be set higher.
+# Upstream default is identical to setting this to 2048.
+#KexDHMin 1024
+
 Host *
 #   ForwardAgent no
 #   ForwardX11 no
 
 # If you do not trust your remote host (or its administrator), you
 # should not forward X11 connections to your local X11-display for
 # security reasons: Someone stealing the authentification data on the
 # remote side (the "spoofed" X-server by the remote sshd) can read your
diff --git a/openssh-7.2p2/ssh_config.0 b/openssh-7.2p2/ssh_config.0
--- a/openssh-7.2p2/ssh_config.0
+++ b/openssh-7.2p2/ssh_config.0
@@ -606,16 +606,33 @@ DESCRIPTION
                    ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
                    diffie-hellman-group-exchange-sha256,
                    diffie-hellman-group-exchange-sha1,
                    diffie-hellman-group14-sha1
 
              The list of available key exchange algorithms may also be
              obtained using the -Q option of ssh(1) with an argument of M-bM-^@M-^\kexM-bM-^@M-^].
 
+     KexDHMin
+             Specifies the minimum accepted bit length of the DH group
+             parameter p.
+
+             As per RFC4419, this is 1024 bits, however this has increasingly
+             been seen as insecure, which prompted the change to 2048 bits.
+             Setting this option allows the client to accept parameters shorter
+             than the current minimum, down to the RFC specified 1024 bits.
+             Using this option may be needed when connecting to servers that
+             only know short DH group parameters.
+
+             Note, that while by default this option is set to 1024 to maintain
+             maximum backward compatibility, using it can severly impact
+             security and thus should be viewed as a temporary fix of last
+             resort and all efforts should be made to fix the (broken)
+             counterparty.
+
      LocalCommand
              Specifies a command to execute on the local machine after
              successfully connecting to the server.  The command string
              extends to the end of the line, and is executed with the user's
              shell.  The following escape character substitutions will be
              performed: M-bM-^@M-^X%dM-bM-^@M-^Y (local user's home directory), M-bM-^@M-^X%hM-bM-^@M-^Y (remote host
              name), M-bM-^@M-^X%lM-bM-^@M-^Y (local host name), M-bM-^@M-^X%nM-bM-^@M-^Y (host name as provided on the
              command line), M-bM-^@M-^X%pM-bM-^@M-^Y (remote port), M-bM-^@M-^X%rM-bM-^@M-^Y (remote user name) or
diff --git a/openssh-7.2p2/ssh_config.5 b/openssh-7.2p2/ssh_config.5
--- a/openssh-7.2p2/ssh_config.5
+++ b/openssh-7.2p2/ssh_config.5
@@ -1092,16 +1092,32 @@ diffie-hellman-group14-sha1
 .Ed
 .Pp
 The list of available key exchange algorithms may also be obtained using the
 .Fl Q
 option of
 .Xr ssh 1
 with an argument of
 .Dq kex .
+.It Cm KexDHMin
+Specifies the minimum accepted bit length of the DH group
+parameter p.
+.Pp
+As per RFC4419, this is 1024 bits, however this has increasingly
+been seen as insecure, which prompted the change to 2048 bits.
+Setting this option allows the client to accept parameters shorter
+than the current minimum, down to the RFC specified 1024 bits.
+Using this option may be needed when connecting to servers that
+only know short DH group parameters.
+.Pp
+Note, that while by default this option is set to 1024 to maintain
+maximum backward compatibility, using it can severly impact
+security and thus should be viewed as a temporary fix of last
+resort and all efforts should be made to fix the (broken)
+counterparty.
 .It Cm LocalCommand
 Specifies a command to execute on the local machine after successfully
 connecting to the server.
 The command string extends to the end of the line, and is executed with
 the user's shell.
 The following escape character substitutions will be performed:
 .Ql %d
 (local user's home directory),
diff --git a/openssh-7.2p2/sshd_config b/openssh-7.2p2/sshd_config
--- a/openssh-7.2p2/sshd_config
+++ b/openssh-7.2p2/sshd_config
@@ -21,16 +21,21 @@
 # HostKey for protocol version 1
 #HostKey /etc/ssh/ssh_host_key
 # HostKeys for protocol version 2
 #HostKey /etc/ssh/ssh_host_rsa_key
 #HostKey /etc/ssh/ssh_host_dsa_key
 #HostKey /etc/ssh/ssh_host_ecdsa_key
 #HostKey /etc/ssh/ssh_host_ed25519_key
 
+# Minimum accepted size of the DH parameter p. By default this is set to 1024
+# to maintain compatibility with RFC4419, but should be set higher.
+# Upstream default is identical to setting this to 2048.
+#KexDHMin 1024
+
 # Lifetime and size of ephemeral version 1 server key
 #KeyRegenerationInterval 1h
 #ServerKeyBits 1024
 
 # Ciphers and keying
 #RekeyLimit default none
 
 # Logging
diff --git a/openssh-7.2p2/sshd_config.0 b/openssh-7.2p2/sshd_config.0
--- a/openssh-7.2p2/sshd_config.0
+++ b/openssh-7.2p2/sshd_config.0
@@ -539,16 +539,33 @@ DESCRIPTION
                    curve25519-sha256@libssh.org,
                    ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
                    diffie-hellman-group-exchange-sha256,
                    diffie-hellman-group14-sha1
 
              The list of available key exchange algorithms may also be
              obtained using the -Q option of ssh(1) with an argument of M-bM-^@M-^\kexM-bM-^@M-^].
 
+     KexDHMin
+             Specifies the minimum accepted bit length of the DH group
+             parameter p.
+
+             As per RFC4419, this is 1024 bits, however this has increasingly
+             been seen as insecure, which prompted the change to 2048 bits.
+             Setting this option allows the server to accept parameters shorter
+             than the current minimum, down to the RFC specified 1024 bits.
+             Using this option may be needed when some of the connectiong
+             clients only know short DH group parameters.
+
+             Note, that while by default this option is set to 1024 to maintain
+             maximum backward compatibility, using it can severly impact
+             security and thus should be viewed as a temporary fix of last
+             resort and all efforts should be made to fix the (broken)
+             counterparty.
+
      KeyRegenerationInterval
              In protocol version 1, the ephemeral server key is automatically
              regenerated after this many seconds (if it has been used).  The
              purpose of regeneration is to prevent decrypting captured
              sessions by later breaking into the machine and stealing the
              keys.  The key is never stored anywhere.  If the value is 0, the
              key is never regenerated.  The default is 3600 (seconds).
 
diff --git a/openssh-7.2p2/sshd_config.5 b/openssh-7.2p2/sshd_config.5
--- a/openssh-7.2p2/sshd_config.5
+++ b/openssh-7.2p2/sshd_config.5
@@ -895,16 +895,32 @@ diffie-hellman-group14-sha1
 .Ed
 .Pp
 The list of available key exchange algorithms may also be obtained using the
 .Fl Q
 option of
 .Xr ssh 1
 with an argument of
 .Dq kex .
+.It Cm KexDHMin
+Specifies the minimum accepted bit length of the DH group
+parameter p.
+.Pp
+As per RFC4419, this is 1024 bits, however this has increasingly
+been seen as insecure, which prompted the change to 2048 bits.
+Setting this option allows the server to accept parameters shorter
+than the current minimum, down to the RFC specified 1024 bits.
+Using this option may be needed when some of the connectiong
+clients only know short DH group parameters.
+.Pp
+Note, that while by default this option is set to 1024 to maintain
+maximum backward compatibility, using it can severly impact
+security and thus should be viewed as a temporary fix of last
+resort and all efforts should be made to fix the (broken)
+counterparty.
 .It Cm KeyRegenerationInterval
 In protocol version 1, the ephemeral server key is automatically regenerated
 after this many seconds (if it has been used).
 The purpose of regeneration is to prevent
 decrypting captured sessions by later breaking into the machine and
 stealing the keys.
 The key is never stored anywhere.
 If the value is 0, the key is never regenerated.