upstream commit

Allow ssh_config and sshd_config kex parameters options be
 prefixed by a '+' to indicate that the specified items be appended to the
 default rather than replacing it.

approach suggested by dtucker@, feedback dlg@, ok markus@

Upstream-ID: 0f901137298fc17095d5756ff1561a7028e8882a

kex.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: kex.c,v 1.108 2015/07/29 08:34:54 djm Exp $ */
+/* $OpenBSD: kex.c,v 1.109 2015/07/30 00:01:34 djm Exp $ */
 /*
  * Copyright (c) 2000, 2001 Markus Friedl.  All rights reserved.
  *
@@ -155,6 +155,68 @@ kex_names_valid(const char *names)
 	return 1;
 }
 
+/*
+ * Concatenate algorithm names, avoiding duplicates in the process.
+ * Caller must free returned string.
+ */
+char *
+kex_names_cat(const char *a, const char *b)
+{
+	char *ret = NULL, *tmp = NULL, *cp, *p;
+	size_t len;
+
+	if (a == NULL || *a == '\0')
+		return NULL;
+	if (b == NULL || *b == '\0')
+		return strdup(a);
+	if (strlen(b) > 1024*1024)
+		return NULL;
+	len = strlen(a) + strlen(b) + 2;
+	if ((tmp = cp = strdup(b)) == NULL ||
+	    (ret = calloc(1, len)) == NULL) {
+		free(tmp);
+		return NULL;
+	}
+	strlcpy(ret, a, len);
+	for ((p = strsep(&cp, ",")); p && *p != '\0'; (p = strsep(&cp, ","))) {
+		if (match_list(ret, p, NULL) != NULL)
+			continue; /* Algorithm already present */
+		if (strlcat(ret, ",", len) >= len ||
+		    strlcat(ret, p, len) >= len) {
+			free(tmp);
+			free(ret);
+			return NULL; /* Shouldn't happen */
+		}
+	}
+	free(tmp);
+	return ret;
+}
+
+/*
+ * Assemble a list of algorithms from a default list and a string from a
+ * configuration file. The user-provided string may begin with '+' to
+ * indicate that it should be appended to the default.
+ */
+int
+kex_assemble_names(const char *def, char **list)
+{
+	char *ret;
+
+	if (list == NULL || *list == NULL || **list == '\0') {
+		*list = strdup(def);
+		return 0;
+	}
+	if (**list != '+') {
+		return 0;
+	}
+
+	if ((ret = kex_names_cat(def, *list + 1)) == NULL)
+		return SSH_ERR_ALLOC_FAIL;
+	free(*list);
+	*list = ret;
+	return 0;
+}
+
 /* put algorithm proposal into buffer */
 int
 kex_prop2buf(struct sshbuf *b, char *proposal[PROPOSAL_MAX])

kex.h

@@ -1,4 +1,4 @@
-/* $OpenBSD: kex.h,v 1.72 2015/07/29 04:43:06 djm Exp $ */
+/* $OpenBSD: kex.h,v 1.73 2015/07/30 00:01:34 djm Exp $ */
 
 /*
  * Copyright (c) 2000, 2001 Markus Friedl.  All rights reserved.
@@ -160,6 +160,8 @@ struct kex {
 
 int	 kex_names_valid(const char *);
 char	*kex_alg_list(char);
+char	*kex_names_cat(const char *, const char *);
+int	 kex_assemble_names(const char *, char **);
 
 int	 kex_new(struct ssh *, char *[PROPOSAL_MAX], struct kex **);
 int	 kex_setup(struct ssh *, char *[PROPOSAL_MAX]);

readconf.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: readconf.c,v 1.238 2015/07/10 06:21:53 markus Exp $ */
+/* $OpenBSD: readconf.c,v 1.239 2015/07/30 00:01:34 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -1086,7 +1086,7 @@ parse_int:
 		arg = strdelim(&s);
 		if (!arg || *arg == '\0')
 			fatal("%.200s line %d: Missing argument.", filename, linenum);
-		if (!ciphers_valid(arg))
+		if (!ciphers_valid(*arg == '+' ? arg + 1 : arg))
 			fatal("%.200s line %d: Bad SSH2 cipher spec '%s'.",
 			    filename, linenum, arg ? arg : "<NONE>");
 		if (*activep && options->ciphers == NULL)
@@ -1097,7 +1097,7 @@ parse_int:
 		arg = strdelim(&s);
 		if (!arg || *arg == '\0')
 			fatal("%.200s line %d: Missing argument.", filename, linenum);
-		if (!mac_valid(arg))
+		if (!mac_valid(*arg == '+' ? arg + 1 : arg))
 			fatal("%.200s line %d: Bad SSH2 Mac spec '%s'.",
 			    filename, linenum, arg ? arg : "<NONE>");
 		if (*activep && options->macs == NULL)
@@ -1109,7 +1109,7 @@ parse_int:
 		if (!arg || *arg == '\0')
 			fatal("%.200s line %d: Missing argument.",
 			    filename, linenum);
-		if (!kex_names_valid(arg))
+		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)
@@ -1123,7 +1123,7 @@ parse_keytypes:
 		if (!arg || *arg == '\0')
 			fatal("%.200s line %d: Missing argument.",
 			    filename, linenum);
-		if (!sshkey_names_valid2(arg, 1))
+		if (!sshkey_names_valid2(*arg == '+' ? arg + 1 : arg, 1))
 			fatal("%s line %d: Bad key types '%s'.",
 				filename, linenum, arg ? arg : "<NONE>");
 		if (*activep && *charptr == NULL)
@@ -1762,9 +1762,6 @@ fill_default_options(Options * options)
 	/* Selected in ssh_login(). */
 	if (options->cipher == -1)
 		options->cipher = SSH_CIPHER_NOT_SET;
-	/* options->ciphers, default set in myproposals.h */
-	/* options->macs, default set in myproposals.h */
-	/* options->kex_algorithms, default set in myproposals.h */
 	/* options->hostkeyalgorithms, default set in myproposals.h */
 	if (options->protocol == SSH_PROTO_UNKNOWN)
 		options->protocol = SSH_PROTO_2;
@@ -1858,10 +1855,14 @@ fill_default_options(Options * options)
 		options->fingerprint_hash = SSH_FP_HASH_DEFAULT;
 	if (options->update_hostkeys == -1)
 		options->update_hostkeys = 0;
-	if (options->hostbased_key_types == NULL)
+	if (kex_assemble_names(KEX_CLIENT_ENCRYPT, &options->ciphers) != 0 ||
-		options->hostbased_key_types = xstrdup(KEX_DEFAULT_PK_ALG);
+	    kex_assemble_names(KEX_CLIENT_MAC, &options->macs) != 0 ||
-	if (options->pubkey_key_types == NULL)
+	    kex_assemble_names(KEX_CLIENT_KEX, &options->kex_algorithms) != 0 ||
-		options->pubkey_key_types = xstrdup(KEX_DEFAULT_PK_ALG);
+	    kex_assemble_names(KEX_DEFAULT_PK_ALG,
+	    &options->hostbased_key_types) != 0 ||
+	    kex_assemble_names(KEX_DEFAULT_PK_ALG,
+	    &options->pubkey_key_types) != 0)
+		fatal("%s: kex_assemble_names failed", __func__);
 
 #define CLEAR_ON_NONE(v) \
 	do { \

servconf.c

@@ -1,5 +1,5 @@
 
-/* $OpenBSD: servconf.c,v 1.276 2015/07/10 06:21:53 markus Exp $ */
+/* $OpenBSD: servconf.c,v 1.277 2015/07/30 00:01:34 djm Exp $ */
 /*
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
  *                    All rights reserved
@@ -259,16 +259,12 @@ fill_default_server_options(ServerOptions *options)
 		options->hostbased_authentication = 0;
 	if (options->hostbased_uses_name_from_packet_only == -1)
 		options->hostbased_uses_name_from_packet_only = 0;
-	if (options->hostbased_key_types == NULL)
-		options->hostbased_key_types = xstrdup(KEX_DEFAULT_PK_ALG);
 	if (options->hostkeyalgorithms == NULL)
 		options->hostkeyalgorithms = xstrdup(KEX_DEFAULT_PK_ALG);
 	if (options->rsa_authentication == -1)
 		options->rsa_authentication = 1;
 	if (options->pubkey_authentication == -1)
 		options->pubkey_authentication = 1;
-	if (options->pubkey_key_types == NULL)
-		options->pubkey_key_types = xstrdup(KEX_DEFAULT_PK_ALG);
 	if (options->kerberos_authentication == -1)
 		options->kerberos_authentication = 0;
 	if (options->kerberos_or_local_passwd == -1)
@@ -345,6 +341,16 @@ fill_default_server_options(ServerOptions *options)
 		options->fwd_opts.streamlocal_bind_unlink = 0;
 	if (options->fingerprint_hash == -1)
 		options->fingerprint_hash = SSH_FP_HASH_DEFAULT;
+
+	if (kex_assemble_names(KEX_SERVER_ENCRYPT, &options->ciphers) != 0 ||
+	    kex_assemble_names(KEX_SERVER_MAC, &options->macs) != 0 ||
+	    kex_assemble_names(KEX_SERVER_KEX, &options->kex_algorithms) != 0 ||
+	    kex_assemble_names(KEX_DEFAULT_PK_ALG,
+	    &options->hostbased_key_types) != 0 ||
+	    kex_assemble_names(KEX_DEFAULT_PK_ALG,
+	    &options->pubkey_key_types) != 0)
+		fatal("%s: kex_assemble_names failed", __func__);
+
 	/* Turn privilege separation on by default */
 	if (use_privsep == -1)
 		use_privsep = PRIVSEP_NOSANDBOX;
@@ -1181,7 +1187,7 @@ process_server_config_line(ServerOptions *options, char *line,
 		if (!arg || *arg == '\0')
 			fatal("%s line %d: Missing argument.",
 			    filename, linenum);
-		if (!sshkey_names_valid2(arg, 1))
+		if (!sshkey_names_valid2(*arg == '+' ? arg + 1 : arg, 1))
 			fatal("%s line %d: Bad key types '%s'.",
 			    filename, linenum, arg ? arg : "<NONE>");
 		if (*activep && *charptr == NULL)
@@ -1434,7 +1440,7 @@ process_server_config_line(ServerOptions *options, char *line,
 		arg = strdelim(&cp);
 		if (!arg || *arg == '\0')
 			fatal("%s line %d: Missing argument.", filename, linenum);
-		if (!ciphers_valid(arg))
+		if (!ciphers_valid(*arg == '+' ? arg + 1 : arg))
 			fatal("%s line %d: Bad SSH2 cipher spec '%s'.",
 			    filename, linenum, arg ? arg : "<NONE>");
 		if (options->ciphers == NULL)
@@ -1445,7 +1451,7 @@ process_server_config_line(ServerOptions *options, char *line,
 		arg = strdelim(&cp);
 		if (!arg || *arg == '\0')
 			fatal("%s line %d: Missing argument.", filename, linenum);
-		if (!mac_valid(arg))
+		if (!mac_valid(*arg == '+' ? arg + 1 : arg))
 			fatal("%s line %d: Bad SSH2 mac spec '%s'.",
 			    filename, linenum, arg ? arg : "<NONE>");
 		if (options->macs == NULL)
@@ -1457,7 +1463,7 @@ process_server_config_line(ServerOptions *options, char *line,
 		if (!arg || *arg == '\0')
 			fatal("%s line %d: Missing argument.",
 			    filename, linenum);
-		if (!kex_names_valid(arg))
+		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)

ssh.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh.c,v 1.419 2015/07/20 18:42:35 millert Exp $ */
+/* $OpenBSD: ssh.c,v 1.420 2015/07/30 00:01:34 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -108,6 +108,7 @@
 #include "roaming.h"
 #include "version.h"
 #include "ssherr.h"
+#include "myproposal.h"
 
 #ifdef ENABLE_PKCS11
 #include "ssh-pkcs11.h"
@@ -794,26 +795,26 @@ main(int ac, char **av)
 			}
 			break;
 		case 'c':
-			if (ciphers_valid(optarg)) {
+			if (ciphers_valid(*optarg == '+' ?
+			    optarg + 1 : optarg)) {
 				/* SSH2 only */
 				options.ciphers = xstrdup(optarg);
 				options.cipher = SSH_CIPHER_INVALID;
-			} else {
+				break;
-				/* SSH1 only */
-				options.cipher = cipher_number(optarg);
-				if (options.cipher == -1) {
-					fprintf(stderr,
-					    "Unknown cipher type '%s'\n",
-					    optarg);
-					exit(255);
-				}
-				if (options.cipher == SSH_CIPHER_3DES)
-					options.ciphers = "3des-cbc";
-				else if (options.cipher == SSH_CIPHER_BLOWFISH)
-					options.ciphers = "blowfish-cbc";
-				else
-					options.ciphers = (char *)-1;
 			}
+			/* SSH1 only */
+			options.cipher = cipher_number(optarg);
+			if (options.cipher == -1) {
+				fprintf(stderr, "Unknown cipher type '%s'\n",
+				    optarg);
+				exit(255);
+			}
+			if (options.cipher == SSH_CIPHER_3DES)
+				options.ciphers = xstrdup("3des-cbc");
+			else if (options.cipher == SSH_CIPHER_BLOWFISH)
+				options.ciphers = xstrdup("blowfish-cbc");
+			else
+				options.ciphers = xstrdup(KEX_CLIENT_ENCRYPT);
 			break;
 		case 'm':
 			if (mac_valid(optarg))

ssh_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: ssh_config.5,v 1.213 2015/07/10 06:21:53 markus Exp $
+.\" $OpenBSD: ssh_config.5,v 1.214 2015/07/30 00:01:34 djm Exp $
-.Dd $Mdocdate: July 10 2015 $
+.Dd $Mdocdate: July 30 2015 $
 .Dt SSH_CONFIG 5
 .Os
 .Sh NAME
@@ -373,6 +373,11 @@ The default is
 Specifies the ciphers allowed for protocol version 2
 in order of preference.
 Multiple ciphers must be comma-separated.
+If the specified value begins with a
+.Sq +
+character, then the specified ciphers will be appended to the default set
+instead of replacing them.
+.Pp
 The supported ciphers are:
 .Pp
 .Bl -item -compact -offset indent
@@ -781,6 +786,10 @@ is similar to
 .It Cm HostbasedKeyTypes
 Specifies the key types that will be used for hostbased authentication
 as a comma-separated pattern list.
+Alternately if the specified value begins with a
+.Sq +
+character, then the specified key types will be appended to the default set
+instead of replacing them.
 The default for this option is:
 .Bd -literal -offset 3n
 ecdsa-sha2-nistp256-cert-v01@openssh.com,
@@ -800,6 +809,10 @@ may be used to list supported key types.
 .It Cm HostKeyAlgorithms
 Specifies the protocol version 2 host key algorithms
 that the client wants to use in order of preference.
+Alternately if the specified value begins with a
+.Sq +
+character, then the specified key types will be appended to the default set
+instead of replacing them.
 The default for this option is:
 .Bd -literal -offset 3n
 ecdsa-sha2-nistp256-cert-v01@openssh.com,
@@ -981,6 +994,10 @@ and
 .It Cm KexAlgorithms
 Specifies the available KEX (Key Exchange) algorithms.
 Multiple algorithms must be comma-separated.
+Alternately if the specified value begins with a
+.Sq +
+character, then the specified methods will be appended to the default set
+instead of replacing them.
 The default is:
 .Bd -literal -offset indent
 curve25519-sha256@libssh.org,
@@ -1069,10 +1086,16 @@ in order of preference.
 The MAC algorithm is used in protocol version 2
 for data integrity protection.
 Multiple algorithms must be comma-separated.
+If the specified value begins with a
+.Sq +
+character, then the specified algorithms will be appended to the default set
+instead of replacing them.
+.Pp
 The algorithms that contain
 .Dq -etm
 calculate the MAC after encryption (encrypt-then-mac).
 These are considered safer and their use recommended.
+.Pp
 The default is:
 .Bd -literal -offset indent
 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
@@ -1216,6 +1239,10 @@ The default is
 .It Cm PubkeyAcceptedKeyTypes
 Specifies the key types that will be used for public key authentication
 as a comma-separated pattern list.
+Alternately if the specified value begins with a
+.Sq +
+character, then the key types after it will be appended to the default
+instead of replacing it.
 The default for this option is:
 .Bd -literal -offset 3n
 ecdsa-sha2-nistp256-cert-v01@openssh.com,

sshconnect2.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: sshconnect2.c,v 1.225 2015/07/10 06:21:53 markus Exp $ */
+/* $OpenBSD: sshconnect2.c,v 1.226 2015/07/30 00:01:34 djm Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  * Copyright (c) 2008 Damien Miller.  All rights reserved.
@@ -163,18 +163,12 @@ ssh_kex2(char *host, struct sockaddr *hostaddr, u_short port)
 	xxx_host = host;
 	xxx_hostaddr = hostaddr;
 
-	if (options.ciphers == (char *)-1) {
+	myproposal[PROPOSAL_KEX_ALGS] = compat_kex_proposal(
-		logit("No valid ciphers for protocol version 2 given, using defaults.");
+	    options.kex_algorithms);
-		options.ciphers = NULL;
-	}
-	if (options.ciphers != NULL) {
-		myproposal[PROPOSAL_ENC_ALGS_CTOS] =
-		myproposal[PROPOSAL_ENC_ALGS_STOC] = options.ciphers;
-	}
 	myproposal[PROPOSAL_ENC_ALGS_CTOS] =
-	    compat_cipher_proposal(myproposal[PROPOSAL_ENC_ALGS_CTOS]);
+	    compat_cipher_proposal(options.ciphers);
 	myproposal[PROPOSAL_ENC_ALGS_STOC] =
-	    compat_cipher_proposal(myproposal[PROPOSAL_ENC_ALGS_STOC]);
+	    compat_cipher_proposal(options.ciphers);
 	if (options.compression) {
 		myproposal[PROPOSAL_COMP_ALGS_CTOS] =
 		myproposal[PROPOSAL_COMP_ALGS_STOC] = "zlib@openssh.com,zlib,none";
@@ -182,14 +176,15 @@ ssh_kex2(char *host, struct sockaddr *hostaddr, u_short port)
 		myproposal[PROPOSAL_COMP_ALGS_CTOS] =
 		myproposal[PROPOSAL_COMP_ALGS_STOC] = "none,zlib@openssh.com,zlib";
 	}
-	if (options.macs != NULL) {
+	myproposal[PROPOSAL_MAC_ALGS_CTOS] =
-		myproposal[PROPOSAL_MAC_ALGS_CTOS] =
+	    myproposal[PROPOSAL_MAC_ALGS_STOC] = options.macs;
-		myproposal[PROPOSAL_MAC_ALGS_STOC] = options.macs;
+	if (options.hostkeyalgorithms != NULL) {
-	}
+		if (kex_assemble_names(KEX_DEFAULT_PK_ALG,
-	if (options.hostkeyalgorithms != NULL)
+		    &options.hostkeyalgorithms) != 0)
+			fatal("%s: kex_assemble_namelist", __func__);
 		myproposal[PROPOSAL_SERVER_HOST_KEY_ALGS] =
 		    compat_pkalg_proposal(options.hostkeyalgorithms);
-	else {
+	} else {
 		/* Enforce default */
 		options.hostkeyalgorithms = xstrdup(KEX_DEFAULT_PK_ALG);
 		/* Prefer algorithms that we already have keys for */
@@ -197,10 +192,6 @@ ssh_kex2(char *host, struct sockaddr *hostaddr, u_short port)
 		    compat_pkalg_proposal(
 		    order_hostkeyalgs(host, hostaddr, port));
 	}
-	if (options.kex_algorithms != NULL)
-		myproposal[PROPOSAL_KEX_ALGS] = options.kex_algorithms;
-	myproposal[PROPOSAL_KEX_ALGS] = compat_kex_proposal(
-	    myproposal[PROPOSAL_KEX_ALGS]);
 
 	if (options.rekey_limit || options.rekey_interval)
 		packet_set_rekey_limits((u_int32_t)options.rekey_limit,

sshd.c

@@ -1,4 +1,4 @@
-/* $OpenBSD: sshd.c,v 1.456 2015/07/17 02:47:45 djm Exp $ */
+/* $OpenBSD: sshd.c,v 1.457 2015/07/30 00:01:34 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -2539,19 +2539,15 @@ do_ssh2_kex(void)
 	struct kex *kex;
 	int r;
 
-	if (options.ciphers != NULL) {
+	myproposal[PROPOSAL_KEX_ALGS] = compat_kex_proposal(
-		myproposal[PROPOSAL_ENC_ALGS_CTOS] =
+	    options.kex_algorithms);
-		myproposal[PROPOSAL_ENC_ALGS_STOC] = options.ciphers;
+	myproposal[PROPOSAL_ENC_ALGS_CTOS] = compat_cipher_proposal(
-	}
+	    options.ciphers);
-	myproposal[PROPOSAL_ENC_ALGS_CTOS] =
+	myproposal[PROPOSAL_ENC_ALGS_STOC] = compat_cipher_proposal(
-	    compat_cipher_proposal(myproposal[PROPOSAL_ENC_ALGS_CTOS]);
+	    options.ciphers);
-	myproposal[PROPOSAL_ENC_ALGS_STOC] =
+	myproposal[PROPOSAL_MAC_ALGS_CTOS] =
-	    compat_cipher_proposal(myproposal[PROPOSAL_ENC_ALGS_STOC]);
+	    myproposal[PROPOSAL_MAC_ALGS_STOC] = options.macs;
 
-	if (options.macs != NULL) {
-		myproposal[PROPOSAL_MAC_ALGS_CTOS] =
-		myproposal[PROPOSAL_MAC_ALGS_STOC] = options.macs;
-	}
 	if (options.compression == COMP_NONE) {
 		myproposal[PROPOSAL_COMP_ALGS_CTOS] =
 		myproposal[PROPOSAL_COMP_ALGS_STOC] = "none";
@@ -2559,11 +2555,6 @@ do_ssh2_kex(void)
 		myproposal[PROPOSAL_COMP_ALGS_CTOS] =
 		myproposal[PROPOSAL_COMP_ALGS_STOC] = "none,zlib@openssh.com";
 	}
-	if (options.kex_algorithms != NULL)
-		myproposal[PROPOSAL_KEX_ALGS] = options.kex_algorithms;
-
-	myproposal[PROPOSAL_KEX_ALGS] = compat_kex_proposal(
-	    myproposal[PROPOSAL_KEX_ALGS]);
 
 	if (options.rekey_limit || options.rekey_interval)
 		packet_set_rekey_limits((u_int32_t)options.rekey_limit,

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 1.207 2015/07/20 00:30:01 djm Exp $
+.\" $OpenBSD: sshd_config.5,v 1.208 2015/07/30 00:01:34 djm Exp $
-.Dd $Mdocdate: July 20 2015 $
+.Dd $Mdocdate: July 30 2015 $
 .Dt SSHD_CONFIG 5
 .Os
 .Sh NAME
@@ -434,6 +434,11 @@ The default is not to
 .It Cm Ciphers
 Specifies the ciphers allowed for protocol version 2.
 Multiple ciphers must be comma-separated.
+If the specified value begins with a
+.Sq +
+character, then the specified ciphers will be appended to the default set
+instead of replacing them.
+.Pp
 The supported ciphers are:
 .Pp
 .Bl -item -compact -offset indent
@@ -640,6 +645,10 @@ The default is
 .It Cm HostbasedAcceptedKeyTypes
 Specifies the key types that will be accepted for hostbased authentication
 as a comma-separated pattern list.
+Alternately if the specified value begins with a
+.Sq +
+character, then the specified key types will be appended to the default set
+instead of replacing them.
 The default for this option is:
 .Bd -literal -offset 3n
 ecdsa-sha2-nistp256-cert-v01@openssh.com,
@@ -855,6 +864,10 @@ The default is
 .It Cm KexAlgorithms
 Specifies the available KEX (Key Exchange) algorithms.
 Multiple algorithms must be comma-separated.
+Alternately if the specified value begins with a
+.Sq +
+character, then the specified methods will be appended to the default set
+instead of replacing them.
 The supported algorithms are:
 .Pp
 .Bl -item -compact -offset indent
@@ -953,6 +966,11 @@ Specifies the available MAC (message authentication code) algorithms.
 The MAC algorithm is used in protocol version 2
 for data integrity protection.
 Multiple algorithms must be comma-separated.
+If the specified value begins with a
+.Sq +
+character, then the specified algorithms will be appended to the default set
+instead of replacing them.
+.Pp
 The algorithms that contain
 .Dq -etm
 calculate the MAC after encryption (encrypt-then-mac).
@@ -1313,6 +1331,10 @@ is identical to
 .It Cm PubkeyAcceptedKeyTypes
 Specifies the key types that will be accepted for public key authentication
 as a comma-separated pattern list.
+Alternately if the specified value begins with a
+.Sq +
+character, then the specified key types will be appended to the default set
+instead of replacing them.
 The default for this option is:
 .Bd -literal -offset 3n
 ecdsa-sha2-nistp256-cert-v01@openssh.com,