https://bugs.gentoo.org/165444
https://bugzilla.mindrot.org/show_bug.cgi?id=1008

--- a/auth.c
+++ b/auth.c
@@ -728,120 +728,6 @@ fakepw(void)
 	return (&fake);
 }
 
-/*
- * Returns the remote DNS hostname as a string. The returned string must not
- * be freed. NB. this will usually trigger a DNS query the first time it is
- * called.
- * This function does additional checks on the hostname to mitigate some
- * attacks on legacy rhosts-style authentication.
- * XXX is RhostsRSAAuthentication vulnerable to these?
- * XXX Can we remove these checks? (or if not, remove RhostsRSAAuthentication?)
- */
-
-static char *
-remote_hostname(struct ssh *ssh)
-{
-	struct sockaddr_storage from;
-	socklen_t fromlen;
-	struct addrinfo hints, *ai, *aitop;
-	char name[NI_MAXHOST], ntop2[NI_MAXHOST];
-	const char *ntop = ssh_remote_ipaddr(ssh);
-
-	/* Get IP address of client. */
-	fromlen = sizeof(from);
-	memset(&from, 0, sizeof(from));
-	if (getpeername(ssh_packet_get_connection_in(ssh),
-	    (struct sockaddr *)&from, &fromlen) < 0) {
-		debug("getpeername failed: %.100s", strerror(errno));
-		return strdup(ntop);
-	}
-
-	ipv64_normalise_mapped(&from, &fromlen);
-	if (from.ss_family == AF_INET6)
-		fromlen = sizeof(struct sockaddr_in6);
-
-	debug3("Trying to reverse map address %.100s.", ntop);
-	/* Map the IP address to a host name. */
-	if (getnameinfo((struct sockaddr *)&from, fromlen, name, sizeof(name),
-	    NULL, 0, NI_NAMEREQD) != 0) {
-		/* Host name not found.  Use ip address. */
-		return strdup(ntop);
-	}
-
-	/*
-	 * if reverse lookup result looks like a numeric hostname,
-	 * someone is trying to trick us by PTR record like following:
-	 *	1.1.1.10.in-addr.arpa.	IN PTR	2.3.4.5
-	 */
-	memset(&hints, 0, sizeof(hints));
-	hints.ai_socktype = SOCK_DGRAM;	/*dummy*/
-	hints.ai_flags = AI_NUMERICHOST;
-	if (getaddrinfo(name, NULL, &hints, &ai) == 0) {
-		logit("Nasty PTR record \"%s\" is set up for %s, ignoring",
-		    name, ntop);
-		freeaddrinfo(ai);
-		return strdup(ntop);
-	}
-
-	/* Names are stored in lowercase. */
-	lowercase(name);
-
-	/*
-	 * Map it back to an IP address and check that the given
-	 * address actually is an address of this host.  This is
-	 * necessary because anyone with access to a name server can
-	 * define arbitrary names for an IP address. Mapping from
-	 * name to IP address can be trusted better (but can still be
-	 * fooled if the intruder has access to the name server of
-	 * the domain).
-	 */
-	memset(&hints, 0, sizeof(hints));
-	hints.ai_family = from.ss_family;
-	hints.ai_socktype = SOCK_STREAM;
-	if (getaddrinfo(name, NULL, &hints, &aitop) != 0) {
-		logit("reverse mapping checking getaddrinfo for %.700s "
-		    "[%s] failed.", name, ntop);
-		return strdup(ntop);
-	}
-	/* Look for the address from the list of addresses. */
-	for (ai = aitop; ai; ai = ai->ai_next) {
-		if (getnameinfo(ai->ai_addr, ai->ai_addrlen, ntop2,
-		    sizeof(ntop2), NULL, 0, NI_NUMERICHOST) == 0 &&
-		    (strcmp(ntop, ntop2) == 0))
-				break;
-	}
-	freeaddrinfo(aitop);
-	/* If we reached the end of the list, the address was not there. */
-	if (ai == NULL) {
-		/* Address not found for the host name. */
-		logit("Address %.100s maps to %.600s, but this does not "
-		    "map back to the address.", ntop, name);
-		return strdup(ntop);
-	}
-	return strdup(name);
-}
-
-/*
- * Return the canonical name of the host in the other side of the current
- * connection.  The host name is cached, so it is efficient to call this
- * several times.
- */
-
-const char *
-auth_get_canonical_hostname(struct ssh *ssh, int use_dns)
-{
-	static char *dnsname;
-
-	if (!use_dns)
-		return ssh_remote_ipaddr(ssh);
-	else if (dnsname != NULL)
-		return dnsname;
-	else {
-		dnsname = remote_hostname(ssh);
-		return dnsname;
-	}
-}
-
 /*
  * Runs command in a subprocess wuth a minimal environment.
  * Returns pid on success, 0 on failure.
--- a/canohost.c
+++ b/canohost.c
@@ -202,3 +202,117 @@ get_local_port(int sock)
 {
 	return get_sock_port(sock, 1);
 }
+
+/*
+ * Returns the remote DNS hostname as a string. The returned string must not
+ * be freed. NB. this will usually trigger a DNS query the first time it is
+ * called.
+ * This function does additional checks on the hostname to mitigate some
+ * attacks on legacy rhosts-style authentication.
+ * XXX is RhostsRSAAuthentication vulnerable to these?
+ * XXX Can we remove these checks? (or if not, remove RhostsRSAAuthentication?)
+ */
+
+static char *
+remote_hostname(struct ssh *ssh)
+{
+	struct sockaddr_storage from;
+	socklen_t fromlen;
+	struct addrinfo hints, *ai, *aitop;
+	char name[NI_MAXHOST], ntop2[NI_MAXHOST];
+	const char *ntop = ssh_remote_ipaddr(ssh);
+
+	/* Get IP address of client. */
+	fromlen = sizeof(from);
+	memset(&from, 0, sizeof(from));
+	if (getpeername(ssh_packet_get_connection_in(ssh),
+	    (struct sockaddr *)&from, &fromlen) < 0) {
+		debug("getpeername failed: %.100s", strerror(errno));
+		return strdup(ntop);
+	}
+
+	ipv64_normalise_mapped(&from, &fromlen);
+	if (from.ss_family == AF_INET6)
+		fromlen = sizeof(struct sockaddr_in6);
+
+	debug3("Trying to reverse map address %.100s.", ntop);
+	/* Map the IP address to a host name. */
+	if (getnameinfo((struct sockaddr *)&from, fromlen, name, sizeof(name),
+	    NULL, 0, NI_NAMEREQD) != 0) {
+		/* Host name not found.  Use ip address. */
+		return strdup(ntop);
+	}
+
+	/*
+	 * if reverse lookup result looks like a numeric hostname,
+	 * someone is trying to trick us by PTR record like following:
+	 *	1.1.1.10.in-addr.arpa.	IN PTR	2.3.4.5
+	 */
+	memset(&hints, 0, sizeof(hints));
+	hints.ai_socktype = SOCK_DGRAM;	/*dummy*/
+	hints.ai_flags = AI_NUMERICHOST;
+	if (getaddrinfo(name, NULL, &hints, &ai) == 0) {
+		logit("Nasty PTR record \"%s\" is set up for %s, ignoring",
+		    name, ntop);
+		freeaddrinfo(ai);
+		return strdup(ntop);
+	}
+
+	/* Names are stored in lowercase. */
+	lowercase(name);
+
+	/*
+	 * Map it back to an IP address and check that the given
+	 * address actually is an address of this host.  This is
+	 * necessary because anyone with access to a name server can
+	 * define arbitrary names for an IP address. Mapping from
+	 * name to IP address can be trusted better (but can still be
+	 * fooled if the intruder has access to the name server of
+	 * the domain).
+	 */
+	memset(&hints, 0, sizeof(hints));
+	hints.ai_family = from.ss_family;
+	hints.ai_socktype = SOCK_STREAM;
+	if (getaddrinfo(name, NULL, &hints, &aitop) != 0) {
+		logit("reverse mapping checking getaddrinfo for %.700s "
+		    "[%s] failed.", name, ntop);
+		return strdup(ntop);
+	}
+	/* Look for the address from the list of addresses. */
+	for (ai = aitop; ai; ai = ai->ai_next) {
+		if (getnameinfo(ai->ai_addr, ai->ai_addrlen, ntop2,
+		    sizeof(ntop2), NULL, 0, NI_NUMERICHOST) == 0 &&
+		    (strcmp(ntop, ntop2) == 0))
+				break;
+	}
+	freeaddrinfo(aitop);
+	/* If we reached the end of the list, the address was not there. */
+	if (ai == NULL) {
+		/* Address not found for the host name. */
+		logit("Address %.100s maps to %.600s, but this does not "
+		    "map back to the address.", ntop, name);
+		return strdup(ntop);
+	}
+	return strdup(name);
+}
+
+/*
+ * Return the canonical name of the host in the other side of the current
+ * connection.  The host name is cached, so it is efficient to call this
+ * several times.
+ */
+
+const char *
+auth_get_canonical_hostname(struct ssh *ssh, int use_dns)
+{
+	static char *dnsname;
+
+	if (!use_dns)
+		return ssh_remote_ipaddr(ssh);
+	else if (dnsname != NULL)
+		return dnsname;
+	else {
+		dnsname = remote_hostname(ssh);
+		return dnsname;
+	}
+}
--- a/readconf.c
+++ b/readconf.c
@@ -160,6 +160,7 @@ typedef enum {
 	oClearAllForwardings, oNoHostAuthenticationForLocalhost,
 	oEnableSSHKeysign, oRekeyLimit, oVerifyHostKeyDNS, oConnectTimeout,
 	oAddressFamily, oGssAuthentication, oGssDelegateCreds,
+	oGssTrustDns,
 	oServerAliveInterval, oServerAliveCountMax, oIdentitiesOnly,
 	oSendEnv, oControlPath, oControlMaster, oControlPersist,
 	oHashKnownHosts,
@@ -200,9 +201,11 @@ static struct {
 #if defined(GSSAPI)
 	{ "gssapiauthentication", oGssAuthentication },
 	{ "gssapidelegatecredentials", oGssDelegateCreds },
+	{ "gssapitrustdns", oGssTrustDns },
 # else
 	{ "gssapiauthentication", oUnsupported },
 	{ "gssapidelegatecredentials", oUnsupported },
+	{ "gssapitrustdns", oUnsupported },
 #endif
 #ifdef ENABLE_PKCS11
 	{ "smartcarddevice", oPKCS11Provider },
@@ -954,6 +957,10 @@ parse_time:
 		intptr = &options->gss_deleg_creds;
 		goto parse_flag;
 
+	case oGssTrustDns:
+		intptr = &options->gss_trust_dns;
+		goto parse_flag;
+
 	case oBatchMode:
 		intptr = &options->batch_mode;
 		goto parse_flag;
@@ -1766,6 +1773,7 @@ initialize_options(Options * options)
 	options->challenge_response_authentication = -1;
 	options->gss_authentication = -1;
 	options->gss_deleg_creds = -1;
+	options->gss_trust_dns = -1;
 	options->password_authentication = -1;
 	options->kbd_interactive_authentication = -1;
 	options->kbd_interactive_devices = NULL;
@@ -1908,6 +1916,8 @@ fill_default_options(Options * options)
 		options->gss_authentication = 0;
 	if (options->gss_deleg_creds == -1)
 		options->gss_deleg_creds = 0;
+	if (options->gss_trust_dns == -1)
+		options->gss_trust_dns = 0;
 	if (options->password_authentication == -1)
 		options->password_authentication = 1;
 	if (options->kbd_interactive_authentication == -1)
--- a/readconf.h
+++ b/readconf.h
@@ -43,6 +43,7 @@ typedef struct {
 					/* Try S/Key or TIS, authentication. */
 	int     gss_authentication;	/* Try GSS authentication */
 	int     gss_deleg_creds;	/* Delegate GSS credentials */
+	int	gss_trust_dns;		/* Trust DNS for GSS canonicalization */
 	int     password_authentication;	/* Try password
 						 * authentication. */
 	int     kbd_interactive_authentication; /* Try keyboard-interactive auth. */
--- a/ssh_config.5
+++ b/ssh_config.5
@@ -731,6 +731,16 @@ The default is
 Forward (delegate) credentials to the server.
 The default is
 .Cm no .
+Note that this option applies to protocol version 2 connections using GSSAPI.
+.It Cm GSSAPITrustDns
+Set to
+.Dq yes to indicate that the DNS is trusted to securely canonicalize
+the name of the host being connected to. If
+.Dq no, the hostname entered on the
+command line will be passed untouched to the GSSAPI library.
+The default is
+.Dq no .
+This option only applies to protocol version 2 connections using GSSAPI.
 .It Cm HashKnownHosts
 Indicates that
 .Xr ssh 1
--- a/sshconnect2.c
+++ b/sshconnect2.c
@@ -643,6 +643,13 @@ userauth_gssapi(Authctxt *authctxt)
 	static u_int mech = 0;
 	OM_uint32 min;
 	int ok = 0;
+	const char *gss_host;
+
+	if (options.gss_trust_dns) {
+		extern const char *auth_get_canonical_hostname(struct ssh *ssh, int use_dns);
+		gss_host = auth_get_canonical_hostname(active_state, 1);
+	} else
+		gss_host = authctxt->host;
 
 	/* Try one GSSAPI method at a time, rather than sending them all at
 	 * once. */
@@ -655,7 +662,7 @@ userauth_gssapi(Authctxt *authctxt)
 		/* My DER encoding requires length<128 */
 		if (gss_supported->elements[mech].length < 128 &&
 		    ssh_gssapi_check_mechanism(&gssctxt, 
-		    &gss_supported->elements[mech], authctxt->host)) {
+		    &gss_supported->elements[mech], gss_host)) {
 			ok = 1; /* Mechanism works */
 		} else {
 			mech++;
--