add ssh-agent(1) support to sshd(8); allows encrypted hostkeys,

or hostkeys on smartcards; most of the work by Zev Weiss; bz #1974
ok djm@

11 files changed, 144 insertions, 42 deletions

diff --git a/usr.bin/ssh/auth.h b/usr.bin/ssh/auth.h
index 2ce518a17df..cb0db428172 100644
--- a/usr.bin/ssh/auth.h
+++ b/usr.bin/ssh/auth.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: auth.h,v 1.75 2013/06/21 00:34:49 djm Exp $ */
+/* $OpenBSD: auth.h,v 1.76 2013/07/19 07:37:48 markus Exp $ */
 
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
@@ -174,10 +174,12 @@ check_key_in_hostfiles(struct passwd *, Key *, const char *,
 
 /* hostkey handling */
 Key	*get_hostkey_by_index(int);
+Key	*get_hostkey_public_by_index(int);
 Key	*get_hostkey_public_by_type(int);
 Key	*get_hostkey_private_by_type(int);
 int	 get_hostkey_index(Key *);
 int	 ssh1_session_key(BIGNUM *);
+void	 sshd_hostkey_sign(Key *, Key *, u_char **, u_int *, u_char *, u_int);
 
 /* debug messages during authentication */
 void	 auth_debug_add(const char *fmt,...) __attribute__((format(printf, 1, 2)));
diff --git a/usr.bin/ssh/kex.h b/usr.bin/ssh/kex.h
index 28e15c8520b..6d8252c1290 100644
--- a/usr.bin/ssh/kex.h
+++ b/usr.bin/ssh/kex.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: kex.h,v 1.55 2013/04/19 01:06:50 djm Exp $ */
+/* $OpenBSD: kex.h,v 1.56 2013/07/19 07:37:48 markus Exp $ */
 
 /*
  * Copyright (c) 2000, 2001 Markus Friedl.  All rights reserved.
@@ -136,6 +136,7 @@ struct Kex {
 	Key	*(*load_host_public_key)(int);
 	Key	*(*load_host_private_key)(int);
 	int	(*host_key_index)(Key *);
+	void    (*sign)(Key *, Key *, u_char **, u_int *, u_char *, u_int);
 	void	(*kex[KEX_MAX])(Kex *);
 };
 
diff --git a/usr.bin/ssh/kexdhs.c b/usr.bin/ssh/kexdhs.c
index abbd54465bc..de085ba155b 100644
--- a/usr.bin/ssh/kexdhs.c
+++ b/usr.bin/ssh/kexdhs.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kexdhs.c,v 1.13 2013/05/17 00:13:13 djm Exp $ */
+/* $OpenBSD: kexdhs.c,v 1.14 2013/07/19 07:37:48 markus Exp $ */
 /*
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
  *
@@ -77,9 +77,6 @@ kexdh_server(Kex *kex)
 	if (server_host_public == NULL)
 		fatal("Unsupported hostkey type %d", kex->hostkey_type);
 	server_host_private = kex->load_host_private_key(kex->hostkey_type);
-	if (server_host_private == NULL)
-		fatal("Missing private key for hostkey type %d",
-		    kex->hostkey_type);
 
 	/* key, cert */
 	if ((dh_client_pub = BN_new()) == NULL)
@@ -141,9 +138,8 @@ kexdh_server(Kex *kex)
 	}
 
 	/* sign H */
-	if (PRIVSEP(key_sign(server_host_private, &signature, &slen, hash,
-	    hashlen)) < 0)
-		fatal("kexdh_server: key_sign failed");
+	kex->sign(server_host_private, server_host_public, &signature, &slen,
+	    hash, hashlen);
 
 	/* destroy_sensitive_data(); */
 
diff --git a/usr.bin/ssh/kexecdhs.c b/usr.bin/ssh/kexecdhs.c
index 5a3307e5152..fbca7ea7b32 100644
--- a/usr.bin/ssh/kexecdhs.c
+++ b/usr.bin/ssh/kexecdhs.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kexecdhs.c,v 1.4 2013/05/17 00:13:13 djm Exp $ */
+/* $OpenBSD: kexecdhs.c,v 1.5 2013/07/19 07:37:48 markus Exp $ */
 /*
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
  * Copyright (c) 2010 Damien Miller.  All rights reserved.
@@ -74,9 +74,6 @@ kexecdh_server(Kex *kex)
 	if (server_host_public == NULL)
 		fatal("Unsupported hostkey type %d", kex->hostkey_type);
 	server_host_private = kex->load_host_private_key(kex->hostkey_type);
-	if (server_host_private == NULL)
-		fatal("Missing private key for hostkey type %d",
-		    kex->hostkey_type);
 
 	debug("expecting SSH2_MSG_KEX_ECDH_INIT");
 	packet_read_expect(SSH2_MSG_KEX_ECDH_INIT);
@@ -135,9 +132,8 @@ kexecdh_server(Kex *kex)
 	}
 
 	/* sign H */
-	if (PRIVSEP(key_sign(server_host_private, &signature, &slen,
-	    hash, hashlen)) < 0)
-		fatal("kexdh_server: key_sign failed");
+	kex->sign(server_host_private, server_host_public, &signature, &slen,
+	    hash, hashlen);
 
 	/* destroy_sensitive_data(); */
 
diff --git a/usr.bin/ssh/kexgexs.c b/usr.bin/ssh/kexgexs.c
index cabe3b5c813..1df2243c398 100644
--- a/usr.bin/ssh/kexgexs.c
+++ b/usr.bin/ssh/kexgexs.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: kexgexs.c,v 1.15 2013/05/17 00:13:13 djm Exp $ */
+/* $OpenBSD: kexgexs.c,v 1.16 2013/07/19 07:37:48 markus Exp $ */
 /*
  * Copyright (c) 2000 Niels Provos.  All rights reserved.
  * Copyright (c) 2001 Markus Friedl.  All rights reserved.
@@ -65,10 +65,6 @@ kexgex_server(Kex *kex)
 	if (server_host_public == NULL)
 		fatal("Unsupported hostkey type %d", kex->hostkey_type);
 	server_host_private = kex->load_host_private_key(kex->hostkey_type);
-	if (server_host_private == NULL)
-		fatal("Missing private key for hostkey type %d",
-		    kex->hostkey_type);
-
 
 	type = packet_read();
 	switch (type) {
@@ -184,9 +180,8 @@ kexgex_server(Kex *kex)
 	}
 
 	/* sign H */
-	if (PRIVSEP(key_sign(server_host_private, &signature, &slen, hash,
-	    hashlen)) < 0)
-		fatal("kexgex_server: key_sign failed");
+	kex->sign(server_host_private, server_host_public, &signature, &slen,
+	    hash, hashlen);
 
 	/* destroy_sensitive_data(); */
 
diff --git a/usr.bin/ssh/monitor.c b/usr.bin/ssh/monitor.c
index 4fad8eee5ee..9c03bc98d45 100644
--- a/usr.bin/ssh/monitor.c
+++ b/usr.bin/ssh/monitor.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: monitor.c,v 1.126 2013/06/21 00:34:49 djm Exp $ */
+/* $OpenBSD: monitor.c,v 1.127 2013/07/19 07:37:48 markus Exp $ */
 /*
  * Copyright 2002 Niels Provos <provos@citi.umich.edu>
  * Copyright 2002 Markus Friedl <markus@openbsd.org>
@@ -75,6 +75,7 @@
 #include "ssh2.h"
 #include "jpake.h"
 #include "roaming.h"
+#include "authfd.h"
 
 #ifdef GSSAPI
 static Gssctxt *gsscontext = NULL;
@@ -594,6 +595,8 @@ mm_answer_moduli(int sock, Buffer *m)
 	return (0);
 }
 
+extern AuthenticationConnection *auth_conn;
+
 int
 mm_answer_sign(int sock, Buffer *m)
 {
@@ -622,10 +625,16 @@ mm_answer_sign(int sock, Buffer *m)
 		memcpy(session_id2, p, session_id2_len);
 	}
 
-	if ((key = get_hostkey_by_index(keyid)) == NULL)
+	if ((key = get_hostkey_by_index(keyid)) != NULL) {
+		if (key_sign(key, &signature, &siglen, p, datlen) < 0)
+			fatal("%s: key_sign failed", __func__);
+	} else if ((key = get_hostkey_public_by_index(keyid)) != NULL &&
+	    auth_conn != NULL) {
+		if (ssh_agent_sign(auth_conn, key, &signature, &siglen, p,
+		    datlen) < 0)
+			fatal("%s: ssh_agent_sign failed", __func__);
+	} else
 		fatal("%s: no hostkey from index %d", __func__, keyid);
-	if (key_sign(key, &signature, &siglen, p, datlen) < 0)
-		fatal("%s: key_sign failed", __func__);
 
 	debug3("%s: signature %p(%u)", __func__, signature, siglen);
 
@@ -1533,6 +1542,7 @@ mm_get_kex(Buffer *m)
 	kex->load_host_public_key=&get_hostkey_public_by_type;
 	kex->load_host_private_key=&get_hostkey_private_by_type;
 	kex->host_key_index=&get_hostkey_index;
+	kex->sign = sshd_hostkey_sign;
 
 	return (kex);
 }
diff --git a/usr.bin/ssh/servconf.c b/usr.bin/ssh/servconf.c
index 518d86e7547..5ee9bb9db95 100644
--- a/usr.bin/ssh/servconf.c
+++ b/usr.bin/ssh/servconf.c
@@ -1,5 +1,5 @@
 
-/* $OpenBSD: servconf.c,v 1.239 2013/05/17 00:13:14 djm Exp $ */
+/* $OpenBSD: servconf.c,v 1.240 2013/07/19 07:37:48 markus Exp $ */
 /*
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
  *                    All rights reserved
@@ -70,6 +70,7 @@ initialize_server_options(ServerOptions *options)
 	options->address_family = -1;
 	options->num_host_key_files = 0;
 	options->num_host_cert_files = 0;
+	options->host_key_agent = NULL;
 	options->pid_file = NULL;
 	options->server_key_bits = -1;
 	options->login_grace_time = -1;
@@ -315,7 +316,7 @@ typedef enum {
 	sRevokedKeys, sTrustedUserCAKeys, sAuthorizedPrincipalsFile,
 	sKexAlgorithms, sIPQoS, sVersionAddendum,
 	sAuthorizedKeysCommand, sAuthorizedKeysCommandUser,
-	sAuthenticationMethods,
+	sAuthenticationMethods, sHostKeyAgent,
 	sDeprecated, sUnsupported
 } ServerOpCodes;
 
@@ -332,6 +333,7 @@ static struct {
 	{ "port", sPort, SSHCFG_GLOBAL },
 	{ "hostkey", sHostKeyFile, SSHCFG_GLOBAL },
 	{ "hostdsakey", sHostKeyFile, SSHCFG_GLOBAL },		/* alias */
+	{ "hostkeyagent", sHostKeyAgent, SSHCFG_GLOBAL },
 	{ "pidfile", sPidFile, SSHCFG_GLOBAL },
 	{ "serverkeybits", sServerKeyBits, SSHCFG_GLOBAL },
 	{ "logingracetime", sLoginGraceTime, SSHCFG_GLOBAL },
@@ -931,6 +933,17 @@ process_server_config_line(ServerOptions *options, char *line,
 		}
 		break;
 
+	case sHostKeyAgent:
+		charptr = &options->host_key_agent;
+		arg = strdelim(&cp);
+		if (!arg || *arg == '\0')
+			fatal("%s line %d: missing socket name.",
+			    filename, linenum);
+		if (*activep && *charptr == NULL)
+			*charptr = !strcmp(arg, SSH_AUTHSOCKET_ENV_NAME) ?
+			    xstrdup(arg) : derelativise_path(arg);
+		break;
+
 	case sHostCertificate:
 		intptr = &options->num_host_cert_files;
 		if (*intptr >= MAX_HOSTKEYS)
@@ -1959,6 +1972,7 @@ dump_config(ServerOptions *o)
 	dump_cfg_string(sVersionAddendum, o->version_addendum);
 	dump_cfg_string(sAuthorizedKeysCommand, o->authorized_keys_command);
 	dump_cfg_string(sAuthorizedKeysCommandUser, o->authorized_keys_command_user);
+	dump_cfg_string(sHostKeyAgent, o->host_key_agent);
 
 	/* string arguments requiring a lookup */
 	dump_cfg_string(sLogLevel, log_level_name(o->log_level));
diff --git a/usr.bin/ssh/servconf.h b/usr.bin/ssh/servconf.h
index d32f2a141fe..fd7c7151ba9 100644
--- a/usr.bin/ssh/servconf.h
+++ b/usr.bin/ssh/servconf.h
@@ -1,4 +1,4 @@
-/* $OpenBSD: servconf.h,v 1.108 2013/05/16 04:09:14 dtucker Exp $ */
+/* $OpenBSD: servconf.h,v 1.109 2013/07/19 07:37:48 markus Exp $ */
 
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
@@ -65,6 +65,7 @@ typedef struct {
 	int     num_host_key_files;     /* Number of files for host keys. */
 	char   *host_cert_files[MAX_HOSTCERTS];	/* Files containing host certs. */
 	int     num_host_cert_files;     /* Number of files for host certs. */
+	char   *host_key_agent;		 /* ssh-agent socket for host keys. */
 	char   *pid_file;	/* Where to put our pid */
 	int     server_key_bits;/* Size of the server key. */
 	int     login_grace_time;	/* Disconnect if no auth in this time
diff --git a/usr.bin/ssh/session.c b/usr.bin/ssh/session.c
index cf04d33df6a..ef54cbc0b86 100644
--- a/usr.bin/ssh/session.c
+++ b/usr.bin/ssh/session.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: session.c,v 1.265 2013/05/17 00:13:14 djm Exp $ */
+/* $OpenBSD: session.c,v 1.266 2013/07/19 07:37:48 markus Exp $ */
 /*
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
  *                    All rights reserved
@@ -70,6 +70,7 @@
 #include "hostfile.h"
 #include "auth.h"
 #include "auth-options.h"
+#include "authfd.h"
 #include "pathnames.h"
 #include "log.h"
 #include "servconf.h"
@@ -1261,6 +1262,13 @@ launch_login(struct passwd *pw, const char *hostname)
 static void
 child_close_fds(void)
 {
+	extern AuthenticationConnection *auth_conn;
+
+	if (auth_conn) {
+		ssh_close_authentication_connection(auth_conn);
+		auth_conn = NULL;
+	}
+
 	if (packet_get_connection_in() == packet_get_connection_out())
 		close(packet_get_connection_in());
 	else {
diff --git a/usr.bin/ssh/sshd.c b/usr.bin/ssh/sshd.c
index 76b7f2535be..c0b09330224 100644
--- a/usr.bin/ssh/sshd.c
+++ b/usr.bin/ssh/sshd.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: sshd.c,v 1.403 2013/06/05 02:27:50 dtucker Exp $ */
+/* $OpenBSD: sshd.c,v 1.404 2013/07/19 07:37:48 markus Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -90,6 +90,7 @@
 #include "canohost.h"
 #include "hostfile.h"
 #include "auth.h"
+#include "authfd.h"
 #include "misc.h"
 #include "msg.h"
 #include "dispatch.h"
@@ -177,6 +178,10 @@ char *server_version_string = NULL;
 /* for rekeying XXX fixme */
 Kex *xxx_kex;
 
+/* Daemon's agent connection */
+AuthenticationConnection *auth_conn = NULL;
+int have_agent = 0;
+
 /*
  * Any really sensitive data in the application is contained in this
  * structure. The idea is that this structure could be locked into memory so
@@ -189,6 +194,7 @@ struct {
 	Key	*server_key;		/* ephemeral server key */
 	Key	*ssh1_host_key;		/* ssh1 host key */
 	Key	**host_keys;		/* all private host keys */
+	Key	**host_pubkeys;		/* all public host keys */
 	Key	**host_certificates;	/* all public host certificates */
 	int	have_ssh1_key;
 	int	have_ssh2_key;
@@ -640,6 +646,8 @@ privsep_preauth(Authctxt *authctxt)
 		debug2("Network child is on pid %ld", (long)pid);
 
 		pmonitor->m_pid = pid;
+		if (have_agent)
+			auth_conn = ssh_get_authentication_connection();
 		if (box != NULL)
 			ssh_sandbox_parent_preauth(box, pid);
 		monitor_child_preauth(authctxt, pmonitor);
@@ -750,6 +758,8 @@ list_hostkey_types(void)
 	for (i = 0; i < options.num_host_key_files; i++) {
 		key = sensitive_data.host_keys[i];
 		if (key == NULL)
+			key = sensitive_data.host_pubkeys[i];
+		if (key == NULL)
 			continue;
 		switch (key->type) {
 		case KEY_RSA:
@@ -802,6 +812,8 @@ get_hostkey_by_type(int type, int need_private)
 			break;
 		default:
 			key = sensitive_data.host_keys[i];
+			if (key == NULL && !need_private)
+				key = sensitive_data.host_pubkeys[i];
 			break;
 		}
 		if (key != NULL && key->type == type)
@@ -831,6 +843,14 @@ get_hostkey_by_index(int ind)
 	return (sensitive_data.host_keys[ind]);
 }
 
+Key *
+get_hostkey_public_by_index(int ind)
+{
+	if (ind < 0 || ind >= options.num_host_key_files)
+		return (NULL);
+	return (sensitive_data.host_pubkeys[ind]);
+}
+
 int
 get_hostkey_index(Key *key)
 {
@@ -843,6 +863,8 @@ get_hostkey_index(Key *key)
 		} else {
 			if (key == sensitive_data.host_keys[i])
 				return (i);
+			if (key == sensitive_data.host_pubkeys[i])
+				return (i);
 		}
 	}
 	return (-1);
@@ -1310,6 +1332,8 @@ main(int ac, char **av)
 	u_int64_t ibytes, obytes;
 	mode_t new_umask;
 	Key *key;
+	Key *pubkey;
+	int keytype;
 	Authctxt *authctxt;
 	struct connection_info *connection_info = get_connection_info(0, 0);
 
@@ -1542,22 +1566,45 @@ main(int ac, char **av)
 	debug("sshd version %s, %s", SSH_VERSION,
 	    SSLeay_version(SSLEAY_VERSION));
 
-	/* load private host keys */
+	/* load host keys */
 	sensitive_data.host_keys = xcalloc(options.num_host_key_files,
 	    sizeof(Key *));
-	for (i = 0; i < options.num_host_key_files; i++)
+	sensitive_data.host_pubkeys = xcalloc(options.num_host_key_files,
+	    sizeof(Key *));
+	for (i = 0; i < options.num_host_key_files; i++) {
 		sensitive_data.host_keys[i] = NULL;
+		sensitive_data.host_pubkeys[i] = NULL;
+	}
+
+	if (options.host_key_agent) {
+		if (strcmp(options.host_key_agent, SSH_AUTHSOCKET_ENV_NAME))
+			setenv(SSH_AUTHSOCKET_ENV_NAME,
+			    options.host_key_agent, 1);
+		have_agent = ssh_agent_present();
+	}
 
 	for (i = 0; i < options.num_host_key_files; i++) {
 		key = key_load_private(options.host_key_files[i], "", NULL);
+		pubkey = key_load_public(options.host_key_files[i], NULL);
 		sensitive_data.host_keys[i] = key;
-		if (key == NULL) {
+		sensitive_data.host_pubkeys[i] = pubkey;
+
+		if (key == NULL && pubkey != NULL && pubkey->type != KEY_RSA1 &&
+		    have_agent) {
+			debug("will rely on agent for hostkey %s",
+			    options.host_key_files[i]);
+			keytype = pubkey->type;
+		} else if (key != NULL) {
+			keytype = key->type;
+		} else {
 			error("Could not load host key: %s",
 			    options.host_key_files[i]);
 			sensitive_data.host_keys[i] = NULL;
+			sensitive_data.host_pubkeys[i] = NULL;
 			continue;
 		}
-		switch (key->type) {
+
+		switch (keytype) {
 		case KEY_RSA1:
 			sensitive_data.ssh1_host_key = key;
 			sensitive_data.have_ssh1_key = 1;
@@ -1568,8 +1615,8 @@ main(int ac, char **av)
 			sensitive_data.have_ssh2_key = 1;
 			break;
 		}
-		debug("private host key: #%d type %d %s", i, key->type,
-		    key_type(key));
+		debug("private host key: #%d type %d %s", i, keytype,
+		    key_type(key ? key : pubkey));
 	}
 	if ((options.protocol & SSH_PROTO_1) && !sensitive_data.have_ssh1_key) {
 		logit("Disabling protocol version 1. Could not load host key");
@@ -1906,9 +1953,11 @@ main(int ac, char **av)
 	buffer_init(&loginmsg);
 	auth_debug_reset();
 
-	if (use_privsep)
+	if (use_privsep) {
 		if (privsep_preauth(authctxt) == 1)
 			goto authenticated;
+	} else if (compat20 && have_agent)
+		auth_conn = ssh_get_authentication_connection();
 
 	/* perform the key exchange */
 	/* authenticate user and start session */
@@ -2194,6 +2243,23 @@ do_ssh1_kex(void)
 	packet_write_wait();
 }
 
+void
+sshd_hostkey_sign(Key *privkey, Key *pubkey, u_char **signature, u_int *slen,
+    u_char *data, u_int dlen)
+{
+	if (privkey) {
+		if (PRIVSEP(key_sign(privkey, signature, slen, data, dlen) < 0))
+			fatal("%s: key_sign failed", __func__);
+	} else if (use_privsep) {
+		if (mm_key_sign(pubkey, signature, slen, data, dlen) < 0)
+			fatal("%s: pubkey_sign failed", __func__);
+	} else {
+		if (ssh_agent_sign(auth_conn, pubkey, signature, slen, data,
+		    dlen))
+			fatal("%s: ssh_agent_sign failed", __func__);
+	}
+}
+
 /*
  * SSH2 key exchange: diffie-hellman-group1-sha1
  */
@@ -2244,6 +2310,7 @@ do_ssh2_kex(void)
 	kex->load_host_public_key=&get_hostkey_public_by_type;
 	kex->load_host_private_key=&get_hostkey_private_by_type;
 	kex->host_key_index=&get_hostkey_index;
+	kex->sign = sshd_hostkey_sign;
 
 	xxx_kex = kex;
 
diff --git a/usr.bin/ssh/sshd_config.5 b/usr.bin/ssh/sshd_config.5
index 23970b8c391..f6870a2b581 100644
--- a/usr.bin/ssh/sshd_config.5
+++ b/usr.bin/ssh/sshd_config.5
@@ -33,8 +33,8 @@
 .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 .\"
-.\" $OpenBSD: sshd_config.5,v 1.161 2013/06/27 14:05:37 jmc Exp $
-.Dd $Mdocdate: June 27 2013 $
+.\" $OpenBSD: sshd_config.5,v 1.162 2013/07/19 07:37:48 markus Exp $
+.Dd $Mdocdate: July 19 2013 $
 .Dt SSHD_CONFIG 5
 .Os
 .Sh NAME
@@ -547,6 +547,18 @@ keys are used for version 1 and
 or
 .Dq rsa
 are used for version 2 of the SSH protocol.
+It is also possible to specify public host key files instead.
+In this case operations on the private key will be delegated
+to an
+.Xr ssh-agent 1 .
+.It Cm HostKeyAgent
+Identifies the UNIX-domain socket used to communicate
+with an agent that has access to the private host keys.
+If
+.Dq SSH_AUTH_SOCK
+is specified, the location of the socket will be read from the
+.Ev SSH_AUTH_SOCK
+environment variable.
 .It Cm IgnoreRhosts
 Specifies that
 .Pa .rhosts