]> asedeno.scripts.mit.edu Git - PuTTY.git/blobdiff - ssh.c
first pass
[PuTTY.git] / ssh.c
diff --git a/ssh.c b/ssh.c
index 9429724804485dea33c388c0094a2996c1e50368..4de741c7b6d0640d916bafe830841fc621e00de4 100644 (file)
--- a/ssh.c
+++ b/ssh.c
@@ -10,7 +10,9 @@
 #include <signal.h>
 
 #include "putty.h"
+#include "pageant.h" /* for AGENT_MAX_MSGLEN */
 #include "tree234.h"
+#include "storage.h"
 #include "ssh.h"
 #ifndef NO_GSSAPI
 #include "sshgssc.h"
@@ -88,7 +90,7 @@ static const char *const ssh2_disconnect_reasons[] = {
  * This list is derived from RFC 4254 and
  * SSH-1 RFC-1.2.31.
  */
-static const struct {
+static const struct ssh_ttymode {
     const char* const mode;
     int opcode;
     enum { TTY_OP_CHAR, TTY_OP_BOOL } type;
@@ -124,6 +126,7 @@ static const struct {
     { "IXANY",      39, TTY_OP_BOOL },
     { "IXOFF",      40, TTY_OP_BOOL },
     { "IMAXBEL",     41, TTY_OP_BOOL },
+    { "IUTF8",       42, TTY_OP_BOOL },
     { "ISIG",       50, TTY_OP_BOOL },
     { "ICANON",             51, TTY_OP_BOOL },
     { "XCASE",      52, TTY_OP_BOOL },
@@ -362,8 +365,13 @@ static int do_ssh1_login(Ssh ssh, const unsigned char *in, int inlen,
                         struct Packet *pktin);
 static void do_ssh2_authconn(Ssh ssh, const unsigned char *in, int inlen,
                             struct Packet *pktin);
+static void ssh_channel_init(struct ssh_channel *c);
+static struct ssh_channel *ssh_channel_msg(Ssh ssh, struct Packet *pktin);
+static void ssh_channel_got_eof(struct ssh_channel *c);
 static void ssh2_channel_check_close(struct ssh_channel *c);
+static void ssh_channel_close_local(struct ssh_channel *c, char const *reason);
 static void ssh_channel_destroy(struct ssh_channel *c);
+static void ssh_channel_unthrottle(struct ssh_channel *c, int bufsize);
 static void ssh2_msg_something_unimplemented(Ssh ssh, struct Packet *pktin);
 
 /*
@@ -383,7 +391,7 @@ static void ssh2_msg_something_unimplemented(Ssh ssh, struct Packet *pktin);
  *    ensure that the server never has any need to throttle its end
  *    of the connection), so we set this high as well.
  * 
- *  - OUR_V2_WINSIZE is the maximum window size we present on SSH-2
+ *  - OUR_V2_WINSIZE is the default window size we present on SSH-2
  *    channels.
  *
  *  - OUR_V2_BIGWIN is the window size we advertise for the only
@@ -407,16 +415,23 @@ static void ssh2_msg_something_unimplemented(Ssh ssh, struct Packet *pktin);
 #define OUR_V2_MAXPKT 0x4000UL
 #define OUR_V2_PACKETLIMIT 0x9000UL
 
-const static struct ssh_signkey *hostkey_algs[] = {
-    &ssh_ecdsa_ed25519,
-    &ssh_ecdsa_nistp256, &ssh_ecdsa_nistp384, &ssh_ecdsa_nistp521,
-    &ssh_rsa, &ssh_dss
+struct ssh_signkey_with_user_pref_id {
+    const struct ssh_signkey *alg;
+    int id;
+};
+const static struct ssh_signkey_with_user_pref_id hostkey_algs[] = {
+    { &ssh_ecdsa_ed25519, HK_ED25519 },
+    { &ssh_ecdsa_nistp256, HK_ECDSA },
+    { &ssh_ecdsa_nistp384, HK_ECDSA },
+    { &ssh_ecdsa_nistp521, HK_ECDSA },
+    { &ssh_dss, HK_DSA },
+    { &ssh_rsa, HK_RSA },
 };
 
-const static struct ssh_mac *macs[] = {
+const static struct ssh_mac *const macs[] = {
     &ssh_hmac_sha256, &ssh_hmac_sha1, &ssh_hmac_sha1_96, &ssh_hmac_md5
 };
-const static struct ssh_mac *buggymacs[] = {
+const static struct ssh_mac *const buggymacs[] = {
     &ssh_hmac_sha1_buggy, &ssh_hmac_sha1_96_buggy, &ssh_hmac_md5
 };
 
@@ -443,7 +458,7 @@ const static struct ssh_compress ssh_comp_none = {
     ssh_comp_none_disable, NULL
 };
 extern const struct ssh_compress ssh_zlib;
-const static struct ssh_compress *compressions[] = {
+const static struct ssh_compress *const compressions[] = {
     &ssh_zlib, &ssh_comp_none
 };
 
@@ -452,7 +467,6 @@ enum {                                     /* channel types */
     CHAN_X11,
     CHAN_AGENT,
     CHAN_SOCKDATA,
-    CHAN_SOCKDATA_DORMANT,            /* one the remote hasn't confirmed */
     /*
      * CHAN_SHARING indicates a channel which is tracked here on
      * behalf of a connection-sharing downstream. We do almost nothing
@@ -560,10 +574,8 @@ struct ssh_channel {
     } v;
     union {
        struct ssh_agent_channel {
-           unsigned char *message;
-           unsigned char msglen[4];
-           unsigned lensofar, totallen;
-            int outstanding_requests;
+            bufchain inbuffer;
+            agent_pending_query *pending;
        } a;
        struct ssh_x11_channel {
            struct X11Connection *xconn;
@@ -700,8 +712,8 @@ static void ssh2_bare_connection_protocol_setup(Ssh ssh);
 static void ssh_size(void *handle, int width, int height);
 static void ssh_special(void *handle, Telnet_Special);
 static int ssh2_try_send(struct ssh_channel *c);
-static void ssh2_add_channel_data(struct ssh_channel *c,
-                                  const char *buf, int len);
+static int ssh_send_channel_data(struct ssh_channel *c,
+                                const char *buf, int len);
 static void ssh_throttle_all(Ssh ssh, int enable, int bufsize);
 static void ssh2_set_window(struct ssh_channel *c, int newwin);
 static int ssh_sendbuffer(void *handle);
@@ -954,8 +966,41 @@ struct ssh_tag {
      */
     struct ssh_gss_liblist *gsslibs;
 #endif
+
+    /*
+     * The last list returned from get_specials.
+     */
+    struct telnet_special *specials;
+
+    /*
+     * List of host key algorithms for which we _don't_ have a stored
+     * host key. These are indices into the main hostkey_algs[] array
+     */
+    int uncert_hostkeys[lenof(hostkey_algs)];
+    int n_uncert_hostkeys;
+
+    /*
+     * Flag indicating that the current rekey is intended to finish
+     * with a newly cross-certified host key.
+     */
+    int cross_certifying;
+
+    /*
+     * Any asynchronous query to our SSH agent that we might have in
+     * flight from the main authentication loop. (Queries from
+     * agent-forwarding channels live in their channel structure.)
+     */
+    agent_pending_query *auth_agent_query;
 };
 
+static const char *ssh_pkt_type(Ssh ssh, int type)
+{
+    if (ssh->version == 1)
+       return ssh1_pkt_type(type);
+    else
+       return ssh2_pkt_type(ssh->pkt_kctx, ssh->pkt_actx, type);
+}
+
 #define logevent(s) logevent(ssh->frontend, s)
 
 /* logevent, only printf-formatted. */
@@ -983,27 +1028,37 @@ static void bomb_out(Ssh ssh, char *text)
 
 /* Helper function for common bits of parsing ttymodes. */
 static void parse_ttymodes(Ssh ssh,
-                          void (*do_mode)(void *data, char *mode, char *val),
+                           void (*do_mode)(void *data,
+                                           const struct ssh_ttymode *mode,
+                                           char *val),
                           void *data)
 {
-    char *key, *val;
+    int i;
+    const struct ssh_ttymode *mode;
+    char *val;
+    char default_val[2];
+
+    strcpy(default_val, "A");
+
+    for (i = 0; i < lenof(ssh_ttymodes); i++) {
+        mode = ssh_ttymodes + i;
+        val = conf_get_str_str_opt(ssh->conf, CONF_ttymodes, mode->mode);
+        if (!val)
+            val = default_val;
 
-    for (val = conf_get_str_strs(ssh->conf, CONF_ttymodes, NULL, &key);
-        val != NULL;
-        val = conf_get_str_strs(ssh->conf, CONF_ttymodes, key, &key)) {
        /*
         * val[0] is either 'V', indicating that an explicit value
         * follows it, or 'A' indicating that we should pass the
         * value through from the local environment via get_ttymode.
         */
        if (val[0] == 'A') {
-           val = get_ttymode(ssh->frontend, key);
+           val = get_ttymode(ssh->frontend, mode->mode);
            if (val) {
-               do_mode(data, key, val);
+               do_mode(data, mode, val);
                sfree(val);
            }
        } else
-           do_mode(data, key, val + 1);               /* skip the 'V' */
+            do_mode(data, mode, val + 1);              /* skip the 'V' */
     }
 }
 
@@ -2495,13 +2550,15 @@ static void ssh_pkt_defersend(Ssh ssh)
     if (backlog > SSH_MAX_BACKLOG)
        ssh_throttle_all(ssh, 1, backlog);
 
-    ssh->outgoing_data_size += ssh->deferred_data_size;
-    if (!ssh->kex_in_progress &&
-        !ssh->bare_connection &&
-       ssh->max_data_size != 0 &&
-       ssh->outgoing_data_size > ssh->max_data_size)
-       do_ssh2_transport(ssh, "too much data sent", -1, NULL);
-    ssh->deferred_data_size = 0;
+    if (ssh->version == 2) {
+       ssh->outgoing_data_size += ssh->deferred_data_size;
+       ssh->deferred_data_size = 0;
+       if (!ssh->kex_in_progress &&
+           !ssh->bare_connection &&
+           ssh->max_data_size != 0 &&
+           ssh->outgoing_data_size > ssh->max_data_size)
+           do_ssh2_transport(ssh, "too much data sent", -1, NULL);
+    }
 }
 
 /*
@@ -3112,15 +3169,21 @@ static int do_ssh_init(Ssh ssh, unsigned char c)
     /* Anything greater or equal to "1.99" means protocol 2 is supported. */
     s->proto2 = ssh_versioncmp(s->version, "1.99") >= 0;
 
-    if (conf_get_int(ssh->conf, CONF_sshprot) == 0 && !s->proto1) {
-       bombout(("SSH protocol version 1 required by configuration but "
-                "not provided by server"));
-       crStop(0);
-    }
-    if (conf_get_int(ssh->conf, CONF_sshprot) == 3 && !s->proto2) {
-       bombout(("SSH protocol version 2 required by configuration but "
-                "not provided by server"));
-       crStop(0);
+    if (conf_get_int(ssh->conf, CONF_sshprot) == 0) {
+       if (!s->proto1) {
+           bombout(("SSH protocol version 1 required by our configuration "
+                    "but not provided by server"));
+           crStop(0);
+       }
+    } else if (conf_get_int(ssh->conf, CONF_sshprot) == 3) {
+       if (!s->proto2) {
+           bombout(("SSH protocol version 2 required by our configuration "
+                    "but server only provides (old, insecure) SSH-1"));
+           crStop(0);
+       }
+    } else {
+       /* No longer support values 1 or 2 for CONF_sshprot */
+       assert(!"Unexpected value for CONF_sshprot");
     }
 
     if (s->proto2 && (conf_get_int(ssh->conf, CONF_sshprot) >= 2 || !s->proto1))
@@ -3408,15 +3471,7 @@ static int ssh_do_close(Ssh ssh, int notify_exit)
      */
     if (ssh->channels) {
        while (NULL != (c = index234(ssh->channels, 0))) {
-           switch (c->type) {
-             case CHAN_X11:
-               x11_close(c->u.x11.xconn);
-               break;
-             case CHAN_SOCKDATA:
-             case CHAN_SOCKDATA_DORMANT:
-               pfd_close(c->u.pfd.pf);
-               break;
-           }
+           ssh_channel_close_local(c, NULL);
            del234(ssh->channels, c); /* moving next one to index 0 */
            if (ssh->version == 2)
                bufchain_clear(&c->v.v2.outbuffer);
@@ -3682,13 +3737,17 @@ static const char *connect_to_host(Ssh ssh, const char *host, int port,
     }
 
     /*
-     * If the SSH version number's fixed, set it now, and if it's SSH-2,
-     * send the version string too.
+     * The SSH version number is always fixed (since we no longer support
+     * fallback between versions), so set it now, and if it's SSH-2,
+     * send the version string now too.
      */
     sshprot = conf_get_int(ssh->conf, CONF_sshprot);
+    assert(sshprot == 0 || sshprot == 3);
     if (sshprot == 0)
+       /* SSH-1 only */
        ssh->version = 1;
     if (sshprot == 3 && !ssh->bare_connection) {
+       /* SSH-2 only */
        ssh->version = 2;
        ssh_send_verstring(ssh, "SSH-", NULL);
     }
@@ -3719,6 +3778,8 @@ static void ssh_throttle_conn(Ssh ssh, int adjust)
     }
 }
 
+static void ssh_agentf_try_forward(struct ssh_channel *c);
+
 /*
  * Throttle or unthrottle _all_ local data streams (for when sends
  * on the SSH connection itself back up).
@@ -3745,7 +3806,12 @@ static void ssh_throttle_all(Ssh ssh, int enable, int bufsize)
            x11_override_throttle(c->u.x11.xconn, enable);
            break;
          case CHAN_AGENT:
-           /* Agent channels require no buffer management. */
+           /* Agent forwarding channels are buffer-managed by
+             * checking ssh->throttled_all in ssh_agentf_try_forward.
+             * So at the moment we _un_throttle again, we must make an
+             * attempt to do something. */
+            if (!enable)
+                ssh_agentf_try_forward(c);
            break;
          case CHAN_SOCKDATA:
            pfd_override_throttle(c->u.pfd.pf, enable);
@@ -3758,6 +3824,8 @@ static void ssh_agent_callback(void *sshv, void *reply, int replylen)
 {
     Ssh ssh = (Ssh) sshv;
 
+    ssh->auth_agent_query = NULL;
+
     ssh->agent_response = reply;
     ssh->agent_response_len = replylen;
 
@@ -3785,38 +3853,140 @@ static void ssh_dialog_callback(void *sshv, int ret)
     ssh_process_queued_incoming_data(ssh);
 }
 
-static void ssh_agentf_callback(void *cv, void *reply, int replylen)
+static void ssh_agentf_got_response(struct ssh_channel *c,
+                                    void *reply, int replylen)
 {
-    struct ssh_channel *c = (struct ssh_channel *)cv;
-    Ssh ssh = c->ssh;
-    const void *sentreply = reply;
+    c->u.a.pending = NULL;
+
+    assert(!(c->closes & CLOSES_SENT_EOF));
 
-    c->u.a.outstanding_requests--;
-    if (!sentreply) {
-       /* Fake SSH_AGENT_FAILURE. */
-       sentreply = "\0\0\0\1\5";
+    if (!reply) {
+       /* The real agent didn't send any kind of reply at all for
+         * some reason, so fake an SSH_AGENT_FAILURE. */
+       reply = "\0\0\0\1\5";
        replylen = 5;
     }
-    if (ssh->version == 2) {
-       ssh2_add_channel_data(c, sentreply, replylen);
-       ssh2_try_send(c);
-    } else {
-       send_packet(ssh, SSH1_MSG_CHANNEL_DATA,
-                   PKT_INT, c->remoteid,
-                   PKT_INT, replylen,
-                   PKT_DATA, sentreply, replylen,
-                   PKT_END);
+
+    ssh_send_channel_data(c, reply, replylen);
+}
+
+static void ssh_agentf_callback(void *cv, void *reply, int replylen);
+
+static void ssh_agentf_try_forward(struct ssh_channel *c)
+{
+    unsigned datalen, lengthfield, messagelen;
+    unsigned char *message;
+    unsigned char msglen[4];
+    void *reply;
+    int replylen;
+
+    /*
+     * Don't try to parallelise agent requests. Wait for each one to
+     * return before attempting the next.
+     */
+    if (c->u.a.pending)
+        return;
+
+    /*
+     * If the outgoing side of the channel connection is currently
+     * throttled (for any reason, either that channel's window size or
+     * the entire SSH connection being throttled), don't submit any
+     * new forwarded requests to the real agent. This causes the input
+     * side of the agent forwarding not to be emptied, exerting the
+     * required back-pressure on the remote client, and encouraging it
+     * to read our responses before sending too many more requests.
+     */
+    if (c->ssh->throttled_all ||
+        (c->ssh->version == 2 && c->v.v2.remwindow == 0))
+        return;
+
+    if (c->closes & CLOSES_SENT_EOF) {
+        /*
+         * If we've already sent outgoing EOF, there's nothing we can
+         * do with incoming data except consume it and throw it away.
+         */
+        bufchain_clear(&c->u.a.inbuffer);
+        return;
     }
-    if (reply)
-       sfree(reply);
+
+    while (1) {
+        /*
+         * Try to extract a complete message from the input buffer.
+         */
+        datalen = bufchain_size(&c->u.a.inbuffer);
+        if (datalen < 4)
+            break;         /* not even a length field available yet */
+
+        bufchain_fetch(&c->u.a.inbuffer, msglen, 4);
+        lengthfield = GET_32BIT(msglen);
+
+        if (lengthfield > AGENT_MAX_MSGLEN) {
+            /*
+             * If the remote has sent a message that's just _too_
+             * long, we should reject it in advance of seeing the rest
+             * of the incoming message, and also close the connection
+             * for good measure (which avoids us having to faff about
+             * with carefully ignoring just the right number of bytes
+             * from the overlong message).
+             */
+            ssh_agentf_got_response(c, NULL, 0);
+            sshfwd_write_eof(c);
+            return;
+        }
+
+        if (lengthfield > datalen - 4)
+            break;          /* a whole message is not yet available */
+
+        messagelen = lengthfield + 4;
+
+        message = snewn(messagelen, unsigned char);
+        bufchain_fetch(&c->u.a.inbuffer, message, messagelen);
+        bufchain_consume(&c->u.a.inbuffer, messagelen);
+        c->u.a.pending = agent_query(
+            message, messagelen, &reply, &replylen, ssh_agentf_callback, c);
+        sfree(message);
+
+        if (c->u.a.pending)
+            return;   /* agent_query promised to reply in due course */
+
+        /*
+         * If the agent gave us an answer immediately, pass it
+         * straight on and go round this loop again.
+         */
+        ssh_agentf_got_response(c, reply, replylen);
+        sfree(reply);
+    }
+
     /*
-     * If we've already seen an incoming EOF but haven't sent an
-     * outgoing one, this may be the moment to send it.
+     * If we get here (i.e. we left the above while loop via 'break'
+     * rather than 'return'), that means we've determined that the
+     * input buffer for the agent forwarding connection doesn't
+     * contain a complete request.
+     *
+     * So if there's potentially more data to come, we can return now,
+     * and wait for the remote client to send it. But if the remote
+     * has sent EOF, it would be a mistake to do that, because we'd be
+     * waiting a long time. So this is the moment to check for EOF,
+     * and respond appropriately.
      */
-    if (c->u.a.outstanding_requests == 0 && (c->closes & CLOSES_RCVD_EOF))
+    if (c->closes & CLOSES_RCVD_EOF)
         sshfwd_write_eof(c);
 }
 
+static void ssh_agentf_callback(void *cv, void *reply, int replylen)
+{
+    struct ssh_channel *c = (struct ssh_channel *)cv;
+
+    ssh_agentf_got_response(c, reply, replylen);
+    sfree(reply);
+
+    /*
+     * Now try to extract and send further messages from the channel's
+     * input-side buffer.
+     */
+    ssh_agentf_try_forward(c);
+}
+
 /*
  * Client-initiated disconnection. Send a DISCONNECT if `wire_reason'
  * non-NULL, otherwise just close the connection. `client_reason' == NULL
@@ -4319,8 +4489,9 @@ static int do_ssh1_login(Ssh ssh, const unsigned char *in, int inlen,
            /* Request the keys held by the agent. */
            PUT_32BIT(s->request, 1);
            s->request[4] = SSH1_AGENTC_REQUEST_RSA_IDENTITIES;
-           if (!agent_query(s->request, 5, &r, &s->responselen,
-                            ssh_agent_callback, ssh)) {
+            ssh->auth_agent_query = agent_query(
+                s->request, 5, &r, &s->responselen, ssh_agent_callback, ssh);
+           if (ssh->auth_agent_query) {
                do {
                    crReturn(0);
                    if (pktin) {
@@ -4425,8 +4596,10 @@ static int do_ssh1_login(Ssh ssh, const unsigned char *in, int inlen,
                        memcpy(q, s->session_id, 16);
                        q += 16;
                        PUT_32BIT(q, 1);        /* response format */
-                       if (!agent_query(agentreq, len + 4, &vret, &retlen,
-                                        ssh_agent_callback, ssh)) {
+                        ssh->auth_agent_query = agent_query(
+                            agentreq, len + 4, &vret, &retlen,
+                            ssh_agent_callback, ssh);
+                       if (ssh->auth_agent_query) {
                            sfree(agentreq);
                            do {
                                crReturn(0);
@@ -4953,23 +5126,14 @@ void sshfwd_write_eof(struct ssh_channel *c)
 void sshfwd_unclean_close(struct ssh_channel *c, const char *err)
 {
     Ssh ssh = c->ssh;
+    char *reason;
 
     if (ssh->state == SSH_STATE_CLOSED)
        return;
 
-    switch (c->type) {
-      case CHAN_X11:
-        x11_close(c->u.x11.xconn);
-        logeventf(ssh, "Forwarded X11 connection terminated due to local "
-                  "error: %s", err);
-        break;
-      case CHAN_SOCKDATA:
-      case CHAN_SOCKDATA_DORMANT:
-        pfd_close(c->u.pfd.pf);
-        logeventf(ssh, "Forwarded port closed due to local error: %s", err);
-        break;
-    }
-    c->type = CHAN_ZOMBIE;
+    reason = dupprintf("due to local error: %s", err);
+    ssh_channel_close_local(c, reason);
+    sfree(reason);
     c->pending_eof = FALSE;   /* this will confuse a zombie channel */
 
     ssh2_channel_check_close(c);
@@ -4982,43 +5146,17 @@ int sshfwd_write(struct ssh_channel *c, char *buf, int len)
     if (ssh->state == SSH_STATE_CLOSED)
        return 0;
 
-    if (ssh->version == 1) {
-       send_packet(ssh, SSH1_MSG_CHANNEL_DATA,
-                   PKT_INT, c->remoteid,
-                   PKT_INT, len, PKT_DATA, buf, len,
-                   PKT_END);
-       /*
-        * In SSH-1 we can return 0 here - implying that forwarded
-        * connections are never individually throttled - because
-        * the only circumstance that can cause throttling will be
-        * the whole SSH connection backing up, in which case
-        * _everything_ will be throttled as a whole.
-        */
-       return 0;
-    } else {
-       ssh2_add_channel_data(c, buf, len);
-       return ssh2_try_send(c);
-    }
+    return ssh_send_channel_data(c, buf, len);
 }
 
 void sshfwd_unthrottle(struct ssh_channel *c, int bufsize)
 {
     Ssh ssh = c->ssh;
-    int buflimit;
 
     if (ssh->state == SSH_STATE_CLOSED)
        return;
 
-    if (ssh->version == 1) {
-       buflimit = SSH1_BUFFER_LIMIT;
-    } else {
-       buflimit = c->v.v2.locmaxwin;
-       ssh2_set_window(c, bufsize < buflimit ? buflimit - bufsize : 0);
-    }
-    if (c->throttling_conn && bufsize <= buflimit) {
-       c->throttling_conn = 0;
-       ssh_throttle_conn(ssh, -1);
-    }
+    ssh_channel_unthrottle(c, bufsize);
 }
 
 static void ssh_queueing_handler(Ssh ssh, struct Packet *pktin)
@@ -5501,15 +5639,11 @@ static void ssh1_smsg_x11_open(Ssh ssh, struct Packet *pktin)
        c = snew(struct ssh_channel);
        c->ssh = ssh;
 
+       ssh_channel_init(c);
        c->u.x11.xconn = x11_init(ssh->x11authtree, c, NULL, -1);
         c->remoteid = remoteid;
         c->halfopen = FALSE;
-        c->localid = alloc_channel_id(ssh);
-        c->closes = 0;
-        c->pending_eof = FALSE;
-        c->throttling_conn = 0;
         c->type = CHAN_X11;    /* identify channel type */
-        add234(ssh->channels, c);
         send_packet(ssh, SSH1_MSG_CHANNEL_OPEN_CONFIRMATION,
                     PKT_INT, c->remoteid, PKT_INT,
                     c->localid, PKT_END);
@@ -5531,17 +5665,12 @@ static void ssh1_smsg_agent_open(Ssh ssh, struct Packet *pktin)
     } else {
        c = snew(struct ssh_channel);
        c->ssh = ssh;
+       ssh_channel_init(c);
        c->remoteid = remoteid;
        c->halfopen = FALSE;
-       c->localid = alloc_channel_id(ssh);
-       c->closes = 0;
-       c->pending_eof = FALSE;
-       c->throttling_conn = 0;
        c->type = CHAN_AGENT;   /* identify channel type */
-       c->u.a.lensofar = 0;
-       c->u.a.message = NULL;
-       c->u.a.outstanding_requests = 0;
-       add234(ssh->channels, c);
+       c->u.a.pending = NULL;
+        bufchain_init(&c->u.a.inbuffer);
        send_packet(ssh, SSH1_MSG_CHANNEL_OPEN_CONFIRMATION,
                    PKT_INT, c->remoteid, PKT_INT, c->localid,
                    PKT_END);
@@ -5562,7 +5691,7 @@ static void ssh1_msg_port_open(Ssh ssh, struct Packet *pktin)
     ssh_pkt_getstring(pktin, &host, &hostsize);
     port = ssh_pkt_getuint32(pktin);
 
-    pf.dhost = dupprintf("%.*s", hostsize, host);
+    pf.dhost = dupprintf("%.*s", hostsize, NULLTOEMPTY(host));
     pf.dport = port;
     pfp = find234(ssh->rportfwds, &pf, NULL);
 
@@ -5586,14 +5715,10 @@ static void ssh1_msg_port_open(Ssh ssh, struct Packet *pktin)
            send_packet(ssh, SSH1_MSG_CHANNEL_OPEN_FAILURE,
                        PKT_INT, remoteid, PKT_END);
        } else {
+           ssh_channel_init(c);
            c->remoteid = remoteid;
            c->halfopen = FALSE;
-           c->localid = alloc_channel_id(ssh);
-           c->closes = 0;
-           c->pending_eof = FALSE;
-           c->throttling_conn = 0;
            c->type = CHAN_SOCKDATA;    /* identify channel type */
-           add234(ssh->channels, c);
            send_packet(ssh, SSH1_MSG_CHANNEL_OPEN_CONFIRMATION,
                        PKT_INT, c->remoteid, PKT_INT,
                        c->localid, PKT_END);
@@ -5606,15 +5731,12 @@ static void ssh1_msg_port_open(Ssh ssh, struct Packet *pktin)
 
 static void ssh1_msg_channel_open_confirmation(Ssh ssh, struct Packet *pktin)
 {
-    unsigned int remoteid = ssh_pkt_getuint32(pktin);
-    unsigned int localid = ssh_pkt_getuint32(pktin);
     struct ssh_channel *c;
 
-    c = find234(ssh->channels, &remoteid, ssh_channelfind);
-    if (c && c->type == CHAN_SOCKDATA_DORMANT) {
-       c->remoteid = localid;
+    c = ssh_channel_msg(ssh, pktin);
+    if (c && c->type == CHAN_SOCKDATA) {
+       c->remoteid = ssh_pkt_getuint32(pktin);
        c->halfopen = FALSE;
-       c->type = CHAN_SOCKDATA;
        c->throttling_conn = 0;
        pfd_confirm(c->u.pfd.pf);
     }
@@ -5632,11 +5754,10 @@ static void ssh1_msg_channel_open_confirmation(Ssh ssh, struct Packet *pktin)
 
 static void ssh1_msg_channel_open_failure(Ssh ssh, struct Packet *pktin)
 {
-    unsigned int remoteid = ssh_pkt_getuint32(pktin);
     struct ssh_channel *c;
 
-    c = find234(ssh->channels, &remoteid, ssh_channelfind);
-    if (c && c->type == CHAN_SOCKDATA_DORMANT) {
+    c = ssh_channel_msg(ssh, pktin);
+    if (c && c->type == CHAN_SOCKDATA) {
        logevent("Forwarded connection refused by server");
        pfd_close(c->u.pfd.pf);
        del234(ssh->channels, c);
@@ -5647,52 +5768,26 @@ static void ssh1_msg_channel_open_failure(Ssh ssh, struct Packet *pktin)
 static void ssh1_msg_channel_close(Ssh ssh, struct Packet *pktin)
 {
     /* Remote side closes a channel. */
-    unsigned i = ssh_pkt_getuint32(pktin);
     struct ssh_channel *c;
-    c = find234(ssh->channels, &i, ssh_channelfind);
-    if (c && !c->halfopen) {
 
-        if (pktin->type == SSH1_MSG_CHANNEL_CLOSE &&
-            !(c->closes & CLOSES_RCVD_EOF)) {
+    c = ssh_channel_msg(ssh, pktin);
+    if (c) {
+
+        if (pktin->type == SSH1_MSG_CHANNEL_CLOSE) {
             /*
              * Received CHANNEL_CLOSE, which we translate into
              * outgoing EOF.
              */
-            int send_close = FALSE;
-
-            c->closes |= CLOSES_RCVD_EOF;
-
-            switch (c->type) {
-              case CHAN_X11:
-                if (c->u.x11.xconn)
-                    x11_send_eof(c->u.x11.xconn);
-                else
-                    send_close = TRUE;
-               break;
-              case CHAN_SOCKDATA:
-                if (c->u.pfd.pf)
-                    pfd_send_eof(c->u.pfd.pf);
-                else
-                    send_close = TRUE;
-               break;
-              case CHAN_AGENT:
-                send_close = TRUE;
-               break;
-            }
-
-            if (send_close && !(c->closes & CLOSES_SENT_EOF)) {
-                send_packet(ssh, SSH1_MSG_CHANNEL_CLOSE, PKT_INT, c->remoteid,
-                            PKT_END);
-                c->closes |= CLOSES_SENT_EOF;
-            }
+           ssh_channel_got_eof(c);
         }
 
         if (pktin->type == SSH1_MSG_CHANNEL_CLOSE_CONFIRMATION &&
             !(c->closes & CLOSES_RCVD_CLOSE)) {
 
             if (!(c->closes & CLOSES_SENT_EOF)) {
-                bombout(("Received CHANNEL_CLOSE_CONFIRMATION for channel %d"
-                         " for which we never sent CHANNEL_CLOSE\n", i));
+                bombout(("Received CHANNEL_CLOSE_CONFIRMATION for channel %u"
+                         " for which we never sent CHANNEL_CLOSE\n",
+                        c->localid));
             }
 
             c->closes |= CLOSES_RCVD_CLOSE;
@@ -5707,78 +5802,57 @@ static void ssh1_msg_channel_close(Ssh ssh, struct Packet *pktin)
 
        if (!((CLOSES_SENT_CLOSE | CLOSES_RCVD_CLOSE) & ~c->closes))
             ssh_channel_destroy(c);
-    } else {
-       bombout(("Received CHANNEL_CLOSE%s for %s channel %d\n",
-                pktin->type == SSH1_MSG_CHANNEL_CLOSE ? "" :
-                "_CONFIRMATION", c ? "half-open" : "nonexistent",
-                i));
     }
 }
 
+/*
+ * Handle incoming data on an SSH-1 or SSH-2 agent-forwarding channel.
+ */
+static int ssh_agent_channel_data(struct ssh_channel *c, char *data,
+                                 int length)
+{
+    bufchain_add(&c->u.a.inbuffer, data, length);
+    ssh_agentf_try_forward(c);
+
+    /*
+     * We exert back-pressure on an agent forwarding client if and
+     * only if we're waiting for the response to an asynchronous agent
+     * request. This prevents the client running out of window while
+     * receiving the _first_ message, but means that if any message
+     * takes time to process, the client will be discouraged from
+     * sending an endless stream of further ones after it.
+     */
+    return (c->u.a.pending ? bufchain_size(&c->u.a.inbuffer) : 0);
+}
+
+static int ssh_channel_data(struct ssh_channel *c, int is_stderr,
+                           char *data,  int length)
+{
+    switch (c->type) {
+      case CHAN_MAINSESSION:
+       return from_backend(c->ssh->frontend, is_stderr, data, length);
+      case CHAN_X11:
+       return x11_send(c->u.x11.xconn, data, length);
+      case CHAN_SOCKDATA:
+       return pfd_send(c->u.pfd.pf, data, length);
+      case CHAN_AGENT:
+       return ssh_agent_channel_data(c, data, length);
+    }
+    return 0;
+}
+
 static void ssh1_msg_channel_data(Ssh ssh, struct Packet *pktin)
 {
     /* Data sent down one of our channels. */
-    int i = ssh_pkt_getuint32(pktin);
     char *p;
     int len;
     struct ssh_channel *c;
 
+    c = ssh_channel_msg(ssh, pktin);
     ssh_pkt_getstring(pktin, &p, &len);
 
-    c = find234(ssh->channels, &i, ssh_channelfind);
     if (c) {
-       int bufsize = 0;
-       switch (c->type) {
-         case CHAN_X11:
-           bufsize = x11_send(c->u.x11.xconn, p, len);
-           break;
-         case CHAN_SOCKDATA:
-           bufsize = pfd_send(c->u.pfd.pf, p, len);
-           break;
-         case CHAN_AGENT:
-           /* Data for an agent message. Buffer it. */
-           while (len > 0) {
-               if (c->u.a.lensofar < 4) {
-                   unsigned int l = min(4 - c->u.a.lensofar, (unsigned)len);
-                   memcpy(c->u.a.msglen + c->u.a.lensofar, p,
-                          l);
-                   p += l;
-                   len -= l;
-                   c->u.a.lensofar += l;
-               }
-               if (c->u.a.lensofar == 4) {
-                   c->u.a.totallen =
-                       4 + GET_32BIT(c->u.a.msglen);
-                   c->u.a.message = snewn(c->u.a.totallen,
-                                          unsigned char);
-                   memcpy(c->u.a.message, c->u.a.msglen, 4);
-               }
-               if (c->u.a.lensofar >= 4 && len > 0) {
-                   unsigned int l =
-                       min(c->u.a.totallen - c->u.a.lensofar,
-                           (unsigned)len);
-                   memcpy(c->u.a.message + c->u.a.lensofar, p,
-                          l);
-                   p += l;
-                   len -= l;
-                   c->u.a.lensofar += l;
-               }
-               if (c->u.a.lensofar == c->u.a.totallen) {
-                   void *reply;
-                   int replylen;
-                    c->u.a.outstanding_requests++;
-                   if (agent_query(c->u.a.message,
-                                   c->u.a.totallen,
-                                   &reply, &replylen,
-                                   ssh_agentf_callback, c))
-                       ssh_agentf_callback(c, reply, replylen);
-                   sfree(c->u.a.message);
-                   c->u.a.lensofar = 0;
-               }
-           }
-           bufsize = 0;   /* agent channels never back up */
-           break;
-       }
+       int bufsize = ssh_channel_data(c, FALSE, p, len);
        if (!c->throttling_conn && bufsize > SSH1_BUFFER_LIMIT) {
            c->throttling_conn = 1;
            ssh_throttle_conn(ssh, +1);
@@ -5802,14 +5876,13 @@ static void ssh1_smsg_exit_status(Ssh ssh, struct Packet *pktin)
 }
 
 /* Helper function to deal with sending tty modes for REQUEST_PTY */
-static void ssh1_send_ttymode(void *data, char *mode, char *val)
+static void ssh1_send_ttymode(void *data,
+                              const struct ssh_ttymode *mode, char *val)
 {
     struct Packet *pktout = (struct Packet *)data;
-    int i = 0;
     unsigned int arg = 0;
-    while (strcmp(mode, ssh_ttymodes[i].mode) != 0) i++;
-    if (i == lenof(ssh_ttymodes)) return;
-    switch (ssh_ttymodes[i].type) {
+
+    switch (mode->type) {
       case TTY_OP_CHAR:
        arg = ssh_tty_parse_specchar(val);
        break;
@@ -5817,7 +5890,7 @@ static void ssh1_send_ttymode(void *data, char *mode, char *val)
        arg = ssh_tty_parse_boolean(val);
        break;
     }
-    ssh2_pkt_addbyte(pktout, ssh_ttymodes[i].opcode);
+    ssh2_pkt_addbyte(pktout, mode->opcode);
     ssh2_pkt_addbyte(pktout, arg);
 }
 
@@ -6045,7 +6118,7 @@ static void ssh1_msg_debug(Ssh ssh, struct Packet *pktin)
     int msglen;
 
     ssh_pkt_getstring(pktin, &msg, &msglen);
-    logeventf(ssh, "Remote debug message: %.*s", msglen, msg);
+    logeventf(ssh, "Remote debug message: %.*s", msglen, NULLTOEMPTY(msg));
 }
 
 static void ssh1_msg_disconnect(Ssh ssh, struct Packet *pktin)
@@ -6055,7 +6128,8 @@ static void ssh1_msg_disconnect(Ssh ssh, struct Packet *pktin)
     int msglen;
 
     ssh_pkt_getstring(pktin, &msg, &msglen);
-    bombout(("Server sent disconnect message:\n\"%.*s\"", msglen, msg));
+    bombout(("Server sent disconnect message:\n\"%.*s\"",
+             msglen, NULLTOEMPTY(msg)));
 }
 
 static void ssh_msg_ignore(Ssh ssh, struct Packet *pktin)
@@ -6222,7 +6296,10 @@ struct kexinit_algorithm {
            const struct ssh_kex *kex;
            int warn;
        } kex;
-       const struct ssh_signkey *hostkey;
+       struct {
+            const struct ssh_signkey *hostkey;
+            int warn;
+        } hk;
        struct {
            const struct ssh2_cipher *cipher;
            int warn;
@@ -6277,12 +6354,12 @@ static void do_ssh2_transport(Ssh ssh, const void *vin, int inlen,
        "server-to-client compression method" };
     struct do_ssh2_transport_state {
        int crLine;
-       int nbits, pbits, warn_kex, warn_cscipher, warn_sccipher;
+       int nbits, pbits, warn_kex, warn_hk, warn_cscipher, warn_sccipher;
        Bignum p, g, e, f, K;
        void *our_kexinit;
        int our_kexinitlen;
        int kex_init_value, kex_reply_value;
-       const struct ssh_mac **maclist;
+       const struct ssh_mac *const *maclist;
        int nmacs;
        const struct ssh2_cipher *cscipher_tobe;
        const struct ssh2_cipher *sccipher_tobe;
@@ -6299,6 +6376,8 @@ static void do_ssh2_transport(Ssh ssh, const void *vin, int inlen,
        unsigned char exchange_hash[SSH2_KEX_MAX_HASH_LEN];
        int n_preferred_kex;
        const struct ssh_kexes *preferred_kex[KEX_MAX];
+       int n_preferred_hk;
+       int preferred_hk[HK_MAX];
        int n_preferred_ciphers;
        const struct ssh2_ciphers *preferred_ciphers[CIPHER_MAX];
        const struct ssh_compress *preferred_comp;
@@ -6314,6 +6393,7 @@ static void do_ssh2_transport(Ssh ssh, const void *vin, int inlen,
     crState(do_ssh2_transport_state);
 
     assert(!ssh->bare_connection);
+    assert(ssh->version == 2);
 
     crBeginState;
 
@@ -6375,6 +6455,20 @@ static void do_ssh2_transport(Ssh ssh, const void *vin, int inlen,
            }
        }
 
+       /*
+        * Set up the preferred host key types. These are just the ids
+        * in the enum in putty.h, so 'warn below here' is indicated
+        * by HK_WARN.
+        */
+       s->n_preferred_hk = 0;
+       for (i = 0; i < HK_MAX; i++) {
+            int id = conf_get_int_int(ssh->conf, CONF_ssh_hklist, i);
+            /* As above, don't bother with HK_WARN if it's last in the
+             * list */
+           if (id != HK_WARN || i < HK_MAX - 1)
+                s->preferred_hk[s->n_preferred_hk++] = id;
+       }
+
        /*
         * Set up the preferred ciphers. (NULL => warn below here)
         */
@@ -6451,20 +6545,43 @@ static void do_ssh2_transport(Ssh ssh, const void *vin, int inlen,
              * In the first key exchange, we list all the algorithms
              * we're prepared to cope with, but prefer those algorithms
             * for which we have a host key for this host.
+             *
+             * If the host key algorithm is below the warning
+             * threshold, we warn even if we did already have a key
+             * for it, on the basis that if the user has just
+             * reconfigured that host key type to be warned about,
+             * they surely _do_ want to be alerted that a server
+             * they're actually connecting to is using it.
              */
-            for (i = 0; i < lenof(hostkey_algs); i++) {
-               if (have_ssh_host_key(ssh->savedhost, ssh->savedport,
-                                     hostkey_algs[i]->keytype)) {
-                   alg = ssh2_kexinit_addalg(s->kexlists[KEXLIST_HOSTKEY],
-                                             hostkey_algs[i]->name);
-                   alg->u.hostkey = hostkey_algs[i];
-               }
-           }
-            for (i = 0; i < lenof(hostkey_algs); i++) {
-               alg = ssh2_kexinit_addalg(s->kexlists[KEXLIST_HOSTKEY],
-                                         hostkey_algs[i]->name);
-               alg->u.hostkey = hostkey_algs[i];
+            warn = FALSE;
+            for (i = 0; i < s->n_preferred_hk; i++) {
+                if (s->preferred_hk[i] == HK_WARN)
+                    warn = TRUE;
+                for (j = 0; j < lenof(hostkey_algs); j++) {
+                    if (hostkey_algs[j].id != s->preferred_hk[i])
+                        continue;
+                    if (have_ssh_host_key(ssh->savedhost, ssh->savedport,
+                                          hostkey_algs[j].alg->keytype)) {
+                        alg = ssh2_kexinit_addalg(s->kexlists[KEXLIST_HOSTKEY],
+                                                  hostkey_algs[j].alg->name);
+                        alg->u.hk.hostkey = hostkey_algs[j].alg;
+                        alg->u.hk.warn = warn;
+                    }
+                }
            }
+            warn = FALSE;
+            for (i = 0; i < s->n_preferred_hk; i++) {
+                if (s->preferred_hk[i] == HK_WARN)
+                    warn = TRUE;
+                for (j = 0; j < lenof(hostkey_algs); j++) {
+                    if (hostkey_algs[j].id != s->preferred_hk[i])
+                        continue;
+                    alg = ssh2_kexinit_addalg(s->kexlists[KEXLIST_HOSTKEY],
+                                              hostkey_algs[j].alg->name);
+                    alg->u.hk.hostkey = hostkey_algs[j].alg;
+                    alg->u.hk.warn = warn;
+                }
+            }
         } else {
             /*
              * In subsequent key exchanges, we list only the kex
@@ -6476,7 +6593,8 @@ static void do_ssh2_transport(Ssh ssh, const void *vin, int inlen,
             assert(ssh->kex);
            alg = ssh2_kexinit_addalg(s->kexlists[KEXLIST_HOSTKEY],
                                      ssh->hostkey->name);
-           alg->u.hostkey = ssh->hostkey;
+           alg->u.hk.hostkey = ssh->hostkey;
+            alg->u.hk.warn = FALSE;
         }
        /* List encryption algorithms (client->server then server->client). */
        for (k = KEXLIST_CSCIPHER; k <= KEXLIST_SCCIPHER; k++) {
@@ -6597,7 +6715,8 @@ static void do_ssh2_transport(Ssh ssh, const void *vin, int inlen,
        s->scmac_tobe = NULL;
        s->cscomp_tobe = NULL;
        s->sccomp_tobe = NULL;
-       s->warn_kex = s->warn_cscipher = s->warn_sccipher = FALSE;
+       s->warn_kex = s->warn_hk = FALSE;
+        s->warn_cscipher = s->warn_sccipher = FALSE;
 
        pktin->savedpos += 16;          /* skip garbage cookie */
 
@@ -6641,7 +6760,8 @@ static void do_ssh2_transport(Ssh ssh, const void *vin, int inlen,
                        ssh->kex = alg->u.kex.kex;
                        s->warn_kex = alg->u.kex.warn;
                    } else if (i == KEXLIST_HOSTKEY) {
-                       ssh->hostkey = alg->u.hostkey;
+                       ssh->hostkey = alg->u.hk.hostkey;
+                        s->warn_hk = alg->u.hk.warn;
                    } else if (i == KEXLIST_CSCIPHER) {
                        s->cscipher_tobe = alg->u.cipher.cipher;
                        s->warn_cscipher = alg->u.cipher.warn;
@@ -6665,10 +6785,37 @@ static void do_ssh2_transport(Ssh ssh, const void *vin, int inlen,
                    in_commasep_string(alg->u.comp->delayed_name, str, len))
                    s->pending_compression = TRUE;  /* try this later */
            }
-           bombout(("Couldn't agree a %s ((available: %.*s)",
+           bombout(("Couldn't agree a %s (available: %.*s)",
                     kexlist_descr[i], len, str));
            crStopV;
          matched:;
+
+            if (i == KEXLIST_HOSTKEY) {
+                int j;
+
+                /*
+                 * In addition to deciding which host key we're
+                 * actually going to use, we should make a list of the
+                 * host keys offered by the server which we _don't_
+                 * have cached. These will be offered as cross-
+                 * certification options by ssh_get_specials.
+                 *
+                 * We also count the key we're currently using for KEX
+                 * as one we've already got, because by the time this
+                 * menu becomes visible, it will be.
+                 */
+                ssh->n_uncert_hostkeys = 0;
+
+                for (j = 0; j < lenof(hostkey_algs); j++) {
+                    if (hostkey_algs[j].alg != ssh->hostkey &&
+                        in_commasep_string(hostkey_algs[j].alg->name,
+                                           str, len) &&
+                        !have_ssh_host_key(ssh->savedhost, ssh->savedport,
+                                           hostkey_algs[j].alg->keytype)) {
+                        ssh->uncert_hostkeys[ssh->n_uncert_hostkeys++] = j;
+                    }
+                }
+            }
        }
 
        if (s->pending_compression) {
@@ -6713,6 +6860,73 @@ static void do_ssh2_transport(Ssh ssh, const void *vin, int inlen,
            }
        }
 
+       if (s->warn_hk) {
+            int j, k;
+            char *betteralgs;
+
+           ssh_set_frozen(ssh, 1);
+
+            /*
+             * Change warning box wording depending on why we chose a
+             * warning-level host key algorithm. If it's because
+             * that's all we have *cached*, use the askhk mechanism,
+             * and list the host keys we could usefully cross-certify.
+             * Otherwise, use askalg for the standard wording.
+             */
+            betteralgs = NULL;
+            for (j = 0; j < ssh->n_uncert_hostkeys; j++) {
+                const struct ssh_signkey_with_user_pref_id *hktype =
+                    &hostkey_algs[ssh->uncert_hostkeys[j]];
+                int better = FALSE;
+                for (k = 0; k < HK_MAX; k++) {
+                    int id = conf_get_int_int(ssh->conf, CONF_ssh_hklist, k);
+                    if (id == HK_WARN) {
+                        break;
+                    } else if (id == hktype->id) {
+                        better = TRUE;
+                        break;
+                    }
+                }
+                if (better) {
+                    if (betteralgs) {
+                        char *old_ba = betteralgs;
+                        betteralgs = dupcat(betteralgs, ",",
+                                            hktype->alg->name,
+                                            (const char *)NULL);
+                        sfree(old_ba);
+                    } else {
+                        betteralgs = dupstr(hktype->alg->name);
+                    }
+                }
+            }
+            if (betteralgs) {
+                s->dlgret = askhk(ssh->frontend, ssh->hostkey->name,
+                                  betteralgs, ssh_dialog_callback, ssh);
+                sfree(betteralgs);
+            } else {
+                s->dlgret = askalg(ssh->frontend, "host key type",
+                                   ssh->hostkey->name,
+                                   ssh_dialog_callback, ssh);
+            }
+           if (s->dlgret < 0) {
+               do {
+                   crReturnV;
+                   if (pktin) {
+                       bombout(("Unexpected data from server while"
+                                " waiting for user response"));
+                       crStopV;
+                   }
+               } while (pktin || inlen > 0);
+               s->dlgret = ssh->user_response;
+           }
+           ssh_set_frozen(ssh, 0);
+           if (s->dlgret == 0) {
+               ssh_disconnect(ssh, "User aborted at host key warning", NULL,
+                              0, TRUE);
+               crStopV;
+           }
+       }
+
        if (s->warn_cscipher) {
            ssh_set_frozen(ssh, 1);
            s->dlgret = askalg(ssh->frontend,
@@ -7129,6 +7343,42 @@ static void do_ssh2_transport(Ssh ssh, const void *vin, int inlen,
 
     s->keystr = ssh->hostkey->fmtkey(s->hkey);
     if (!s->got_session_id) {
+       /*
+        * Make a note of any other host key formats that are available.
+        */
+       {
+           int i, j, nkeys = 0;
+           char *list = NULL;
+           for (i = 0; i < lenof(hostkey_algs); i++) {
+               if (hostkey_algs[i].alg == ssh->hostkey)
+                   continue;
+
+                for (j = 0; j < ssh->n_uncert_hostkeys; j++)
+                    if (ssh->uncert_hostkeys[j] == i)
+                        break;
+
+                if (j < ssh->n_uncert_hostkeys) {
+                   char *newlist;
+                   if (list)
+                       newlist = dupprintf("%s/%s", list,
+                                           hostkey_algs[i].alg->name);
+                   else
+                       newlist = dupprintf("%s", hostkey_algs[i].alg->name);
+                   sfree(list);
+                   list = newlist;
+                   nkeys++;
+               }
+           }
+           if (list) {
+               logeventf(ssh,
+                         "Server also has %s host key%s, but we "
+                         "don't know %s", list,
+                         nkeys > 1 ? "s" : "",
+                         nkeys > 1 ? "any of them" : "it");
+               sfree(list);
+           }
+       }
+
         /*
          * Authenticate remote host: verify host key. (We've already
          * checked the signature of the exchange hash.)
@@ -7176,6 +7426,19 @@ static void do_ssh2_transport(Ssh ssh, const void *vin, int inlen,
          * subsequent rekeys.
          */
         ssh->hostkey_str = s->keystr;
+    } else if (ssh->cross_certifying) {
+        s->fingerprint = ssh2_fingerprint(ssh->hostkey, s->hkey);
+        logevent("Storing additional host key for this host:");
+        logevent(s->fingerprint);
+        sfree(s->fingerprint);
+        store_host_key(ssh->savedhost, ssh->savedport,
+                       ssh->hostkey->keytype, s->keystr);
+        ssh->cross_certifying = FALSE;
+        /*
+         * Don't forget to store the new key as the one we'll be
+         * re-checking in future normal rekeys.
+         */
+        ssh->hostkey_str = s->keystr;
     } else {
         /*
          * In a rekey, we never present an interactive host key
@@ -7362,6 +7625,12 @@ static void do_ssh2_transport(Ssh ssh, const void *vin, int inlen,
      */
     freebn(s->K);
 
+    /*
+     * Update the specials menu to list the remaining uncertified host
+     * keys.
+     */
+    update_specials_menu(ssh->frontend);
+
     /*
      * Key exchange is over. Loop straight back round if we have a
      * deferred rekey reason.
@@ -7470,12 +7739,30 @@ static void do_ssh2_transport(Ssh ssh, const void *vin, int inlen,
 }
 
 /*
- * Add data to an SSH-2 channel output buffer.
+ * Send data on an SSH channel.  In SSH-2, this involves buffering it
+ * first.
  */
-static void ssh2_add_channel_data(struct ssh_channel *c, const char *buf,
-                                 int len)
+static int ssh_send_channel_data(struct ssh_channel *c, const char *buf,
+                                  int len)
 {
-    bufchain_add(&c->v.v2.outbuffer, buf, len);
+    if (c->ssh->version == 2) {
+       bufchain_add(&c->v.v2.outbuffer, buf, len);
+       return ssh2_try_send(c);
+    } else {
+       send_packet(c->ssh, SSH1_MSG_CHANNEL_DATA,
+                   PKT_INT, c->remoteid,
+                   PKT_INT, len,
+                   PKT_DATA, buf, len,
+                   PKT_END);
+       /*
+        * In SSH-1 we can return 0 here - implying that channels are
+        * never individually throttled - because the only
+        * circumstance that can cause throttling will be the whole
+        * SSH connection backing up, in which case _everything_ will
+        * be throttled as a whole.
+        */
+       return 0;
+    }
 }
 
 /*
@@ -7536,8 +7823,9 @@ static void ssh2_try_send_and_unthrottle(Ssh ssh, struct ssh_channel *c)
            x11_unthrottle(c->u.x11.xconn);
            break;
          case CHAN_AGENT:
-           /* agent sockets are request/response and need no
-            * buffer management */
+            /* Now that we've successfully sent all the outgoing
+             * replies we had, try to process more incoming data. */
+            ssh_agentf_try_forward(c);
            break;
          case CHAN_SOCKDATA:
            pfd_unthrottle(c->u.pfd.pf);
@@ -7560,20 +7848,23 @@ static int ssh_is_simple(Ssh ssh)
 }
 
 /*
- * Set up most of a new ssh_channel for SSH-2.
+ * Set up most of a new ssh_channel.
  */
-static void ssh2_channel_init(struct ssh_channel *c)
+static void ssh_channel_init(struct ssh_channel *c)
 {
     Ssh ssh = c->ssh;
     c->localid = alloc_channel_id(ssh);
     c->closes = 0;
     c->pending_eof = FALSE;
     c->throttling_conn = FALSE;
-    c->v.v2.locwindow = c->v.v2.locmaxwin = c->v.v2.remlocwin =
-       ssh_is_simple(ssh) ? OUR_V2_BIGWIN : OUR_V2_WINSIZE;
-    c->v.v2.chanreq_head = NULL;
-    c->v.v2.throttle_state = UNTHROTTLED;
-    bufchain_init(&c->v.v2.outbuffer);
+    if (ssh->version == 2) {
+       c->v.v2.locwindow = c->v.v2.locmaxwin = c->v.v2.remlocwin =
+           ssh_is_simple(ssh) ? OUR_V2_BIGWIN : OUR_V2_WINSIZE;
+       c->v.v2.chanreq_head = NULL;
+       c->v.v2.throttle_state = UNTHROTTLED;
+       bufchain_init(&c->v.v2.outbuffer);
+    }
+    add234(ssh->channels, c);
 }
 
 /*
@@ -7643,6 +7934,27 @@ static struct Packet *ssh2_chanreq_init(struct ssh_channel *c,
     return pktout;
 }
 
+static void ssh_channel_unthrottle(struct ssh_channel *c, int bufsize)
+{
+    Ssh ssh = c->ssh;
+    int buflimit;
+
+    if (ssh->version == 1) {
+       buflimit = SSH1_BUFFER_LIMIT;
+    } else {
+       if (ssh_is_simple(ssh))
+           buflimit = 0;
+       else
+           buflimit = c->v.v2.locmaxwin;
+       if (bufsize < buflimit)
+           ssh2_set_window(c, buflimit - bufsize);
+    }
+    if (c->throttling_conn && bufsize <= buflimit) {
+       c->throttling_conn = 0;
+       ssh_throttle_conn(ssh, -1);
+    }
+}
+
 /*
  * Potentially enlarge the window on an SSH-2 channel.
  */
@@ -7724,25 +8036,38 @@ static void ssh2_set_window(struct ssh_channel *c, int newwin)
 /*
  * Find the channel associated with a message.  If there's no channel,
  * or it's not properly open, make a noise about it and return NULL.
+ * If the channel is shared, pass the message on to downstream and
+ * also return NULL (meaning the caller should ignore this message).
  */
-static struct ssh_channel *ssh2_channel_msg(Ssh ssh, struct Packet *pktin)
+static struct ssh_channel *ssh_channel_msg(Ssh ssh, struct Packet *pktin)
 {
     unsigned localid = ssh_pkt_getuint32(pktin);
     struct ssh_channel *c;
+    int halfopen_ok;
 
+    /* Is this message OK on a half-open connection? */
+    if (ssh->version == 1)
+       halfopen_ok = (pktin->type == SSH1_MSG_CHANNEL_OPEN_CONFIRMATION ||
+                      pktin->type == SSH1_MSG_CHANNEL_OPEN_FAILURE);
+    else
+       halfopen_ok = (pktin->type == SSH2_MSG_CHANNEL_OPEN_CONFIRMATION ||
+                      pktin->type == SSH2_MSG_CHANNEL_OPEN_FAILURE);
     c = find234(ssh->channels, &localid, ssh_channelfind);
-    if (!c ||
-       (c->type != CHAN_SHARING && c->halfopen &&
-         pktin->type != SSH2_MSG_CHANNEL_OPEN_CONFIRMATION &&
-        pktin->type != SSH2_MSG_CHANNEL_OPEN_FAILURE)) {
+    if (!c || (c->type != CHAN_SHARING && (c->halfopen != halfopen_ok))) {
        char *buf = dupprintf("Received %s for %s channel %u",
-                             ssh2_pkt_type(ssh->pkt_kctx, ssh->pkt_actx,
-                                           pktin->type),
-                             c ? "half-open" : "nonexistent", localid);
+                             ssh_pkt_type(ssh, pktin->type),
+                             !c ? "nonexistent" :
+                             c->halfopen ? "half-open" : "open",
+                             localid);
        ssh_disconnect(ssh, NULL, buf, SSH2_DISCONNECT_PROTOCOL_ERROR, FALSE);
        sfree(buf);
        return NULL;
     }
+    if (c->type == CHAN_SHARING) {
+        share_got_pkt_from_server(c->u.sharing.ctx, pktin->type,
+                                  pktin->body, pktin->length);
+        return NULL;
+    }
     return c;
 }
 
@@ -7772,15 +8097,10 @@ static void ssh2_handle_winadj_response(struct ssh_channel *c,
 
 static void ssh2_msg_channel_response(Ssh ssh, struct Packet *pktin)
 {
-    struct ssh_channel *c = ssh2_channel_msg(ssh, pktin);
+    struct ssh_channel *c = ssh_channel_msg(ssh, pktin);
     struct outstanding_channel_request *ocr;
 
     if (!c) return;
-    if (c->type == CHAN_SHARING) {
-        share_got_pkt_from_server(c->u.sharing.ctx, pktin->type,
-                                  pktin->body, pktin->length);
-        return;
-    }
     ocr = c->v.v2.chanreq_head;
     if (!ocr) {
        ssh2_msg_unexpected(ssh, pktin);
@@ -7800,14 +8120,9 @@ static void ssh2_msg_channel_response(Ssh ssh, struct Packet *pktin)
 static void ssh2_msg_channel_window_adjust(Ssh ssh, struct Packet *pktin)
 {
     struct ssh_channel *c;
-    c = ssh2_channel_msg(ssh, pktin);
+    c = ssh_channel_msg(ssh, pktin);
     if (!c)
        return;
-    if (c->type == CHAN_SHARING) {
-        share_got_pkt_from_server(c->u.sharing.ctx, pktin->type,
-                                  pktin->body, pktin->length);
-        return;
-    }
     if (!(c->closes & CLOSES_SENT_EOF)) {
        c->v.v2.remwindow += ssh_pkt_getuint32(pktin);
        ssh2_try_send_and_unthrottle(ssh, c);
@@ -7818,81 +8133,22 @@ static void ssh2_msg_channel_data(Ssh ssh, struct Packet *pktin)
 {
     char *data;
     int length;
+    unsigned ext_type = 0; /* 0 means not extended */
     struct ssh_channel *c;
-    c = ssh2_channel_msg(ssh, pktin);
+    c = ssh_channel_msg(ssh, pktin);
     if (!c)
        return;
-    if (c->type == CHAN_SHARING) {
-        share_got_pkt_from_server(c->u.sharing.ctx, pktin->type,
-                                  pktin->body, pktin->length);
-        return;
-    }
-    if (pktin->type == SSH2_MSG_CHANNEL_EXTENDED_DATA &&
-       ssh_pkt_getuint32(pktin) != SSH2_EXTENDED_DATA_STDERR)
-       return;                        /* extended but not stderr */
+    if (pktin->type == SSH2_MSG_CHANNEL_EXTENDED_DATA)
+       ext_type = ssh_pkt_getuint32(pktin);
     ssh_pkt_getstring(pktin, &data, &length);
     if (data) {
-       int bufsize = 0;
+       int bufsize;
        c->v.v2.locwindow -= length;
        c->v.v2.remlocwin -= length;
-       switch (c->type) {
-         case CHAN_MAINSESSION:
-           bufsize =
-               from_backend(ssh->frontend, pktin->type ==
-                            SSH2_MSG_CHANNEL_EXTENDED_DATA,
-                            data, length);
-           break;
-         case CHAN_X11:
-           bufsize = x11_send(c->u.x11.xconn, data, length);
-           break;
-         case CHAN_SOCKDATA:
-           bufsize = pfd_send(c->u.pfd.pf, data, length);
-           break;
-         case CHAN_AGENT:
-           while (length > 0) {
-               if (c->u.a.lensofar < 4) {
-                   unsigned int l = min(4 - c->u.a.lensofar,
-                                        (unsigned)length);
-                   memcpy(c->u.a.msglen + c->u.a.lensofar,
-                          data, l);
-                   data += l;
-                   length -= l;
-                   c->u.a.lensofar += l;
-               }
-               if (c->u.a.lensofar == 4) {
-                   c->u.a.totallen =
-                       4 + GET_32BIT(c->u.a.msglen);
-                   c->u.a.message = snewn(c->u.a.totallen,
-                                          unsigned char);
-                   memcpy(c->u.a.message, c->u.a.msglen, 4);
-               }
-               if (c->u.a.lensofar >= 4 && length > 0) {
-                   unsigned int l =
-                       min(c->u.a.totallen - c->u.a.lensofar,
-                           (unsigned)length);
-                   memcpy(c->u.a.message + c->u.a.lensofar,
-                          data, l);
-                   data += l;
-                   length -= l;
-                   c->u.a.lensofar += l;
-               }
-               if (c->u.a.lensofar == c->u.a.totallen) {
-                   void *reply;
-                   int replylen;
-                    c->u.a.outstanding_requests++;
-                   if (agent_query(c->u.a.message,
-                                   c->u.a.totallen,
-                                   &reply, &replylen,
-                                   ssh_agentf_callback, c))
-                       ssh_agentf_callback(c, reply, replylen);
-                   sfree(c->u.a.message);
-                    c->u.a.message = NULL;
-                   c->u.a.lensofar = 0;
-               }
-           }
-           bufsize = 0;
-           break;
-       }
+       if (ext_type != 0 && ext_type != SSH2_EXTENDED_DATA_STDERR)
+           length = 0; /* Don't do anything with unknown extended data. */
+       bufsize = ssh_channel_data(c, ext_type == SSH2_EXTENDED_DATA_STDERR,
+                                  data, length);
        /*
         * If it looks like the remote end hit the end of its window,
         * and we didn't want it to do that, think about using a
@@ -7908,8 +8164,8 @@ static void ssh2_msg_channel_data(Ssh ssh, struct Packet *pktin)
         * need to adjust the window if the server's
         * sent excess data.
         */
-       ssh2_set_window(c, bufsize < c->v.v2.locmaxwin ?
-                       c->v.v2.locmaxwin - bufsize : 0);
+       if (bufsize < c->v.v2.locmaxwin)
+           ssh2_set_window(c, c->v.v2.locmaxwin - bufsize);
        /*
         * If we're either buffering way too much data, or if we're
         * buffering anything at all and we're in "simple" mode,
@@ -7973,9 +8229,14 @@ void ssh_sharing_logf(Ssh ssh, unsigned id, const char *logfmt, ...)
     sfree(buf);
 }
 
-static void ssh_channel_destroy(struct ssh_channel *c)
+/*
+ * Close any local socket and free any local resources associated with
+ * a channel.  This converts the channel into a CHAN_ZOMBIE.
+ */
+static void ssh_channel_close_local(struct ssh_channel *c, char const *reason)
 {
     Ssh ssh = c->ssh;
+    char const *msg = NULL;
 
     switch (c->type) {
       case CHAN_MAINSESSION:
@@ -7983,19 +8244,36 @@ static void ssh_channel_destroy(struct ssh_channel *c)
         update_specials_menu(ssh->frontend);
         break;
       case CHAN_X11:
-        if (c->u.x11.xconn != NULL)
-            x11_close(c->u.x11.xconn);
-        logevent("Forwarded X11 connection terminated");
+        assert(c->u.x11.xconn != NULL);
+       x11_close(c->u.x11.xconn);
+        msg = "Forwarded X11 connection terminated";
         break;
       case CHAN_AGENT:
-        sfree(c->u.a.message);
+        if (c->u.a.pending)
+            agent_cancel_query(c->u.a.pending);
+        bufchain_clear(&c->u.a.inbuffer);
+       msg = "Agent-forwarding connection closed";
         break;
       case CHAN_SOCKDATA:
-        if (c->u.pfd.pf != NULL)
-            pfd_close(c->u.pfd.pf);
-        logevent("Forwarded port closed");
+        assert(c->u.pfd.pf != NULL);
+       pfd_close(c->u.pfd.pf);
+       msg = "Forwarded port closed";
         break;
     }
+    c->type = CHAN_ZOMBIE;
+    if (msg != NULL) {
+       if (reason != NULL)
+           logeventf(ssh, "%s %s", msg, reason);
+       else
+           logevent(msg);
+    }
+}
+
+static void ssh_channel_destroy(struct ssh_channel *c)
+{
+    Ssh ssh = c->ssh;
+
+    ssh_channel_close_local(c, NULL);
 
     del234(ssh->channels, c);
     if (ssh->version == 2) {
@@ -8016,6 +8294,7 @@ static void ssh2_channel_check_close(struct ssh_channel *c)
     Ssh ssh = c->ssh;
     struct Packet *pktout;
 
+    assert(ssh->version == 2);
     if (c->halfopen) {
         /*
          * If we've sent out our own CHANNEL_OPEN but not yet seen
@@ -8051,20 +8330,22 @@ static void ssh2_channel_check_close(struct ssh_channel *c)
     }
 }
 
-static void ssh2_channel_got_eof(struct ssh_channel *c)
+static void ssh_channel_got_eof(struct ssh_channel *c)
 {
     if (c->closes & CLOSES_RCVD_EOF)
         return;                        /* already seen EOF */
     c->closes |= CLOSES_RCVD_EOF;
 
     if (c->type == CHAN_X11) {
+       assert(c->u.x11.xconn != NULL);
        x11_send_eof(c->u.x11.xconn);
     } else if (c->type == CHAN_AGENT) {
-        if (c->u.a.outstanding_requests == 0) {
-            /* Manufacture an outgoing EOF in response to the incoming one. */
-            sshfwd_write_eof(c);
-        }
+        /* Just call try_forward, which will respond to the EOF now if
+         * appropriate, or wait until the queue of outstanding
+         * requests is dealt with if not */
+        ssh_agentf_try_forward(c);
     } else if (c->type == CHAN_SOCKDATA) {
+       assert(c->u.pfd.pf != NULL);
        pfd_send_eof(c->u.pfd.pf);
     } else if (c->type == CHAN_MAINSESSION) {
         Ssh ssh = c->ssh;
@@ -8083,43 +8364,32 @@ static void ssh2_channel_got_eof(struct ssh_channel *c)
         }
         ssh->sent_console_eof = TRUE;
     }
-
-    ssh2_channel_check_close(c);
 }
 
 static void ssh2_msg_channel_eof(Ssh ssh, struct Packet *pktin)
 {
     struct ssh_channel *c;
 
-    c = ssh2_channel_msg(ssh, pktin);
+    c = ssh_channel_msg(ssh, pktin);
     if (!c)
        return;
-    if (c->type == CHAN_SHARING) {
-        share_got_pkt_from_server(c->u.sharing.ctx, pktin->type,
-                                  pktin->body, pktin->length);
-        return;
-    }
-    ssh2_channel_got_eof(c);
+    ssh_channel_got_eof(c);
+    ssh2_channel_check_close(c);
 }
 
 static void ssh2_msg_channel_close(Ssh ssh, struct Packet *pktin)
 {
     struct ssh_channel *c;
 
-    c = ssh2_channel_msg(ssh, pktin);
+    c = ssh_channel_msg(ssh, pktin);
     if (!c)
        return;
-    if (c->type == CHAN_SHARING) {
-        share_got_pkt_from_server(c->u.sharing.ctx, pktin->type,
-                                  pktin->body, pktin->length);
-        return;
-    }
 
     /*
      * When we receive CLOSE on a channel, we assume it comes with an
      * implied EOF if we haven't seen EOF yet.
      */
-    ssh2_channel_got_eof(c);
+    ssh_channel_got_eof(c);
 
     if (!(ssh->remote_bugs & BUG_SENDS_LATE_REQUEST_REPLY)) {
         /*
@@ -8188,24 +8458,18 @@ static void ssh2_msg_channel_open_confirmation(Ssh ssh, struct Packet *pktin)
 {
     struct ssh_channel *c;
 
-    c = ssh2_channel_msg(ssh, pktin);
+    c = ssh_channel_msg(ssh, pktin);
     if (!c)
        return;
-    if (c->type == CHAN_SHARING) {
-        share_got_pkt_from_server(c->u.sharing.ctx, pktin->type,
-                                  pktin->body, pktin->length);
-        return;
-    }
-    assert(c->halfopen); /* ssh2_channel_msg will have enforced this */
+    assert(c->halfopen); /* ssh_channel_msg will have enforced this */
     c->remoteid = ssh_pkt_getuint32(pktin);
     c->halfopen = FALSE;
     c->v.v2.remwindow = ssh_pkt_getuint32(pktin);
     c->v.v2.remmaxpkt = ssh_pkt_getuint32(pktin);
 
-    if (c->type == CHAN_SOCKDATA_DORMANT) {
-        c->type = CHAN_SOCKDATA;
-        if (c->u.pfd.pf)
-            pfd_confirm(c->u.pfd.pf);
+    if (c->type == CHAN_SOCKDATA) {
+       assert(c->u.pfd.pf != NULL);
+       pfd_confirm(c->u.pfd.pf);
     } else if (c->type == CHAN_ZOMBIE) {
         /*
          * This case can occur if a local socket error occurred
@@ -8248,23 +8512,19 @@ static void ssh2_msg_channel_open_failure(Ssh ssh, struct Packet *pktin)
     int reason_length;
     struct ssh_channel *c;
 
-    c = ssh2_channel_msg(ssh, pktin);
+    c = ssh_channel_msg(ssh, pktin);
     if (!c)
        return;
-    if (c->type == CHAN_SHARING) {
-        share_got_pkt_from_server(c->u.sharing.ctx, pktin->type,
-                                  pktin->body, pktin->length);
-        return;
-    }
-    assert(c->halfopen); /* ssh2_channel_msg will have enforced this */
+    assert(c->halfopen); /* ssh_channel_msg will have enforced this */
 
-    if (c->type == CHAN_SOCKDATA_DORMANT) {
+    if (c->type == CHAN_SOCKDATA) {
         reason_code = ssh_pkt_getuint32(pktin);
         if (reason_code >= lenof(reasons))
             reason_code = 0; /* ensure reasons[reason_code] in range */
         ssh_pkt_getstring(pktin, &reason_string, &reason_length);
         logeventf(ssh, "Forwarded connection refused by server: %s [%.*s]",
-                  reasons[reason_code], reason_length, reason_string);
+                  reasons[reason_code], reason_length,
+                  NULLTOEMPTY(reason_string));
 
         pfd_close(c->u.pfd.pf);
     } else if (c->type == CHAN_ZOMBIE) {
@@ -8300,14 +8560,9 @@ static void ssh2_msg_channel_request(Ssh ssh, struct Packet *pktin)
     struct ssh_channel *c;
     struct Packet *pktout;
 
-    c = ssh2_channel_msg(ssh, pktin);
+    c = ssh_channel_msg(ssh, pktin);
     if (!c)
        return;
-    if (c->type == CHAN_SHARING) {
-        share_got_pkt_from_server(c->u.sharing.ctx, pktin->type,
-                                  pktin->body, pktin->length);
-        return;
-    }
     ssh_pkt_getstring(pktin, &type, &typelen);
     want_reply = ssh2_pkt_getbool(pktin);
 
@@ -8560,9 +8815,7 @@ static void ssh2_msg_channel_open(Ssh ssh, struct Packet *pktin)
        char *addrstr;
 
        ssh_pkt_getstring(pktin, &peeraddr, &peeraddrlen);
-       addrstr = snewn(peeraddrlen+1, char);
-       memcpy(addrstr, peeraddr, peeraddrlen);
-       addrstr[peeraddrlen] = '\0';
+       addrstr = dupprintf("%.*s", peeraddrlen, NULLTOEMPTY(peeraddr));
        peerport = ssh_pkt_getuint32(pktin);
 
        logeventf(ssh, "Received X11 connect request from %s:%d",
@@ -8597,13 +8850,14 @@ static void ssh2_msg_channel_open(Ssh ssh, struct Packet *pktin)
        char *shost;
        int shostlen;
        ssh_pkt_getstring(pktin, &shost, &shostlen);/* skip address */
-        pf.shost = dupprintf("%.*s", shostlen, shost);
+        pf.shost = dupprintf("%.*s", shostlen, NULLTOEMPTY(shost));
        pf.sport = ssh_pkt_getuint32(pktin);
        ssh_pkt_getstring(pktin, &peeraddr, &peeraddrlen);
        peerport = ssh_pkt_getuint32(pktin);
        realpf = find234(ssh->rportfwds, &pf, NULL);
        logeventf(ssh, "Received remote port %s:%d open request "
-                 "from %s:%d", pf.shost, pf.sport, peeraddr, peerport);
+                 "from %.*s:%d", pf.shost, pf.sport,
+                  peeraddrlen, NULLTOEMPTY(peeraddr), peerport);
         sfree(pf.shost);
 
        if (realpf == NULL) {
@@ -8643,9 +8897,8 @@ static void ssh2_msg_channel_open(Ssh ssh, struct Packet *pktin)
            error = "Agent forwarding is not enabled";
        else {
            c->type = CHAN_AGENT;       /* identify channel type */
-           c->u.a.lensofar = 0;
-            c->u.a.message = NULL;
-            c->u.a.outstanding_requests = 0;
+            bufchain_init(&c->u.a.inbuffer);
+            c->u.a.pending = NULL;
        }
     } else {
        error = "Unsupported channel type requested";
@@ -8663,14 +8916,13 @@ static void ssh2_msg_channel_open(Ssh ssh, struct Packet *pktin)
        logeventf(ssh, "Rejected channel open: %s", error);
        sfree(c);
     } else {
-       ssh2_channel_init(c);
+       ssh_channel_init(c);
        c->v.v2.remwindow = winsize;
        c->v.v2.remmaxpkt = pktsize;
         if (our_winsize_override) {
             c->v.v2.locwindow = c->v.v2.locmaxwin = c->v.v2.remlocwin =
                 our_winsize_override;
         }
-       add234(ssh->channels, c);
        pktout = ssh2_pkt_init(SSH2_MSG_CHANNEL_OPEN_CONFIRMATION);
        ssh2_pkt_adduint32(pktout, c->remoteid);
        ssh2_pkt_adduint32(pktout, c->localid);
@@ -8735,14 +8987,13 @@ static void ssh2_msg_userauth_banner(Ssh ssh, struct Packet *pktin)
 }
 
 /* Helper function to deal with sending tty modes for "pty-req" */
-static void ssh2_send_ttymode(void *data, char *mode, char *val)
+static void ssh2_send_ttymode(void *data,
+                              const struct ssh_ttymode *mode, char *val)
 {
     struct Packet *pktout = (struct Packet *)data;
-    int i = 0;
     unsigned int arg = 0;
-    while (strcmp(mode, ssh_ttymodes[i].mode) != 0) i++;
-    if (i == lenof(ssh_ttymodes)) return;
-    switch (ssh_ttymodes[i].type) {
+
+    switch (mode->type) {
       case TTY_OP_CHAR:
        arg = ssh_tty_parse_specchar(val);
        break;
@@ -8750,7 +9001,7 @@ static void ssh2_send_ttymode(void *data, char *mode, char *val)
        arg = ssh_tty_parse_boolean(val);
        break;
     }
-    ssh2_pkt_addbyte(pktout, ssh_ttymodes[i].opcode);
+    ssh2_pkt_addbyte(pktout, mode->opcode);
     ssh2_pkt_adduint32(pktout, arg);
 }
 
@@ -8997,7 +9248,6 @@ static void do_ssh2_authconn(Ssh ssh, const unsigned char *in, int inlen,
        int pklen, alglen, commentlen;
        int siglen, retlen, len;
        char *q, *agentreq, *ret;
-       int try_send;
        struct Packet *pktout;
        Filename *keyfile;
 #ifndef NO_GSSAPI
@@ -9151,8 +9401,10 @@ static void do_ssh2_authconn(Ssh ssh, const unsigned char *in, int inlen,
            /* Request the keys held by the agent. */
            PUT_32BIT(s->agent_request, 1);
            s->agent_request[4] = SSH2_AGENTC_REQUEST_IDENTITIES;
-           if (!agent_query(s->agent_request, 5, &r, &s->agent_responselen,
-                            ssh_agent_callback, ssh)) {
+            ssh->auth_agent_query = agent_query(
+                s->agent_request, 5, &r, &s->agent_responselen,
+                ssh_agent_callback, ssh);
+           if (ssh->auth_agent_query) {
                do {
                    crReturnV;
                    if (pktin) {
@@ -9193,21 +9445,25 @@ static void do_ssh2_authconn(Ssh ssh, const unsigned char *in, int inlen,
                             goto done_agent_query;
                         }
                         bloblen = toint(GET_32BIT(q));
+                        lenleft -= 4;
+                        q += 4;
                         if (bloblen < 0 || bloblen > lenleft) {
                             logeventf(ssh, "Pageant response was truncated");
                             s->nkeys = 0;
                             goto done_agent_query;
                         }
-                        lenleft -= 4 + bloblen;
-                        q += 4 + bloblen;
+                        lenleft -= bloblen;
+                        q += bloblen;
                         commentlen = toint(GET_32BIT(q));
+                        lenleft -= 4;
+                        q += 4;
                         if (commentlen < 0 || commentlen > lenleft) {
                             logeventf(ssh, "Pageant response was truncated");
                             s->nkeys = 0;
                             goto done_agent_query;
                         }
-                        lenleft -= 4 + commentlen;
-                        q += 4 + commentlen;
+                        lenleft -= commentlen;
+                        q += commentlen;
                     }
                 }
 
@@ -9590,9 +9846,10 @@ static void do_ssh2_authconn(Ssh ssh, const unsigned char *in, int inlen,
                    s->q += s->pktout->length - 5;
                    /* And finally the (zero) flags word. */
                    PUT_32BIT(s->q, 0);
-                   if (!agent_query(s->agentreq, s->len + 4,
-                                    &vret, &s->retlen,
-                                    ssh_agent_callback, ssh)) {
+                    ssh->auth_agent_query = agent_query(
+                        s->agentreq, s->len + 4, &vret, &s->retlen,
+                        ssh_agent_callback, ssh);
+                    if (ssh->auth_agent_query) {
                        do {
                            crReturnV;
                            if (pktin) {
@@ -10267,7 +10524,7 @@ static void do_ssh2_authconn(Ssh ssh, const unsigned char *in, int inlen,
                    s->cur_prompt->to_server = TRUE;
                    s->cur_prompt->name = dupstr("New SSH password");
                    s->cur_prompt->instruction =
-                       dupprintf("%.*s", prompt_len, prompt);
+                       dupprintf("%.*s", prompt_len, NULLTOEMPTY(prompt));
                    s->cur_prompt->instr_reqd = TRUE;
                    /*
                     * There's no explicit requirement in the protocol
@@ -10453,7 +10710,7 @@ static void do_ssh2_authconn(Ssh ssh, const unsigned char *in, int inlen,
     } else {
        ssh->mainchan = snew(struct ssh_channel);
        ssh->mainchan->ssh = ssh;
-       ssh2_channel_init(ssh->mainchan);
+       ssh_channel_init(ssh->mainchan);
 
        if (*conf_get_str(ssh->conf, CONF_ssh_nc_host)) {
            /*
@@ -10486,7 +10743,6 @@ static void do_ssh2_authconn(Ssh ssh, const unsigned char *in, int inlen,
        ssh->mainchan->type = CHAN_MAINSESSION;
        ssh->mainchan->v.v2.remwindow = ssh_pkt_getuint32(pktin);
        ssh->mainchan->v.v2.remmaxpkt = ssh_pkt_getuint32(pktin);
-       add234(ssh->channels, ssh->mainchan);
        update_specials_menu(ssh->frontend);
        logevent("Opened main channel");
     }
@@ -10650,7 +10906,6 @@ static void do_ssh2_authconn(Ssh ssh, const unsigned char *in, int inlen,
        ssh->send_ok = 1;
     while (1) {
        crReturnV;
-       s->try_send = FALSE;
        if (pktin) {
 
            /*
@@ -10665,18 +10920,7 @@ static void do_ssh2_authconn(Ssh ssh, const unsigned char *in, int inlen,
            /*
             * We have spare data. Add it to the channel buffer.
             */
-           ssh2_add_channel_data(ssh->mainchan, (char *)in, inlen);
-           s->try_send = TRUE;
-       }
-       if (s->try_send) {
-           int i;
-           struct ssh_channel *c;
-           /*
-            * Try to send data on all channels if we can.
-            */
-           for (i = 0; NULL != (c = index234(ssh->channels, i)); i++)
-                if (c->type != CHAN_SHARING)
-                    ssh2_try_send_and_unthrottle(ssh, c);
+           ssh_send_channel_data(ssh->mainchan, (char *)in, inlen);
        }
     }
 
@@ -10705,13 +10949,13 @@ static void ssh2_msg_disconnect(Ssh ssh, struct Packet *pktin)
     logevent(buf);
     sfree(buf);
     buf = dupprintf("Disconnection message text: %.*s",
-                   msglen, msg);
+                   msglen, NULLTOEMPTY(msg));
     logevent(buf);
     bombout(("Server sent disconnect message\ntype %d (%s):\n\"%.*s\"",
             reason,
             (reason > 0 && reason < lenof(ssh2_disconnect_reasons)) ?
             ssh2_disconnect_reasons[reason] : "unknown",
-            msglen, msg));
+            msglen, NULLTOEMPTY(msg)));
     sfree(buf);
 }
 
@@ -10725,7 +10969,7 @@ static void ssh2_msg_debug(Ssh ssh, struct Packet *pktin)
     ssh2_pkt_getbool(pktin);
     ssh_pkt_getstring(pktin, &msg, &msglen);
 
-    logeventf(ssh, "Remote debug message: %.*s", msglen, msg);
+    logeventf(ssh, "Remote debug message: %.*s", msglen, NULLTOEMPTY(msg));
 }
 
 static void ssh2_msg_transport(Ssh ssh, struct Packet *pktin)
@@ -11003,6 +11247,9 @@ static const char *ssh_init(void *frontend_handle, void **backend_handle,
     ssh->connshare = NULL;
     ssh->attempting_connshare = FALSE;
     ssh->session_started = FALSE;
+    ssh->specials = NULL;
+    ssh->n_uncert_hostkeys = 0;
+    ssh->cross_certifying = FALSE;
 
     *backend_handle = ssh;
 
@@ -11038,6 +11285,8 @@ static const char *ssh_init(void *frontend_handle, void **backend_handle,
                                                      CONF_ssh_rekey_data));
     ssh->kex_in_progress = FALSE;
 
+    ssh->auth_agent_query = NULL;
+
 #ifndef NO_GSSAPI
     ssh->gsslibs = NULL;
 #endif
@@ -11099,17 +11348,7 @@ static void ssh_free(void *handle)
 
     if (ssh->channels) {
        while ((c = delpos234(ssh->channels, 0)) != NULL) {
-           switch (c->type) {
-             case CHAN_X11:
-               if (c->u.x11.xconn != NULL)
-                   x11_close(c->u.x11.xconn);
-               break;
-             case CHAN_SOCKDATA:
-             case CHAN_SOCKDATA_DORMANT:
-               if (c->u.pfd.pf != NULL)
-                   pfd_close(c->u.pfd.pf);
-               break;
-           }
+           ssh_channel_close_local(c, NULL);
            if (ssh->version == 2) {
                struct outstanding_channel_request *ocr, *nocr;
                ocr = c->v.v2.chanreq_head;
@@ -11150,6 +11389,7 @@ static void ssh_free(void *handle)
     sfree(ssh->v_s);
     sfree(ssh->fullhostname);
     sfree(ssh->hostkey_str);
+    sfree(ssh->specials);
     if (ssh->crcda_ctx) {
        crcda_free_context(ssh->crcda_ctx);
        ssh->crcda_ctx = NULL;
@@ -11162,6 +11402,10 @@ static void ssh_free(void *handle)
     bufchain_clear(&ssh->queued_incoming_data);
     sfree(ssh->username);
     conf_free(ssh->conf);
+
+    if (ssh->auth_agent_query)
+        agent_cancel_query(ssh->auth_agent_query);
+
 #ifndef NO_GSSAPI
     if (ssh->gsslibs)
        ssh_gss_cleanup(ssh->gsslibs);
@@ -11363,19 +11607,24 @@ static const struct telnet_special *ssh_get_specials(void *handle)
     static const struct telnet_special specials_end[] = {
        {NULL, TS_EXITMENU}
     };
-    /* XXX review this length for any changes: */
-    static struct telnet_special ssh_specials[lenof(ssh2_ignore_special) +
-                                             lenof(ssh2_rekey_special) +
-                                             lenof(ssh2_session_specials) +
-                                             lenof(specials_end)];
+
+    struct telnet_special *specials = NULL;
+    int nspecials = 0, specialsize = 0;
+
     Ssh ssh = (Ssh) handle;
-    int i = 0;
-#define ADD_SPECIALS(name) \
-    do { \
-       assert((i + lenof(name)) <= lenof(ssh_specials)); \
-       memcpy(&ssh_specials[i], name, sizeof name); \
-       i += lenof(name); \
-    } while(0)
+
+    sfree(ssh->specials);
+
+#define ADD_SPECIALS(name) do                                           \
+    {                                                                   \
+        int len = lenof(name);                                          \
+        if (nspecials + len > specialsize) {                            \
+            specialsize = (nspecials + len) * 5 / 4 + 32;               \
+            specials = sresize(specials, specialsize, struct telnet_special); \
+        }                                                               \
+       memcpy(specials+nspecials, name, len*sizeof(struct telnet_special)); \
+        nspecials += len;                                               \
+    } while (0)
 
     if (ssh->version == 1) {
        /* Don't bother offering IGNORE if we've decided the remote
@@ -11390,11 +11639,37 @@ static const struct telnet_special *ssh_get_specials(void *handle)
            ADD_SPECIALS(ssh2_rekey_special);
        if (ssh->mainchan)
            ADD_SPECIALS(ssh2_session_specials);
+
+        if (ssh->n_uncert_hostkeys) {
+            static const struct telnet_special uncert_start[] = {
+                {NULL, TS_SEP},
+                {"Cache new host key type", TS_SUBMENU},
+            };
+            static const struct telnet_special uncert_end[] = {
+                {NULL, TS_EXITMENU},
+            };
+            int i;
+
+            ADD_SPECIALS(uncert_start);
+            for (i = 0; i < ssh->n_uncert_hostkeys; i++) {
+                struct telnet_special uncert[1];
+                const struct ssh_signkey *alg =
+                    hostkey_algs[ssh->uncert_hostkeys[i]].alg;
+                uncert[0].name = alg->name;
+                uncert[0].code = TS_LOCALSTART + ssh->uncert_hostkeys[i];
+                ADD_SPECIALS(uncert);
+            }
+            ADD_SPECIALS(uncert_end);
+        }
     } /* else we're not ready yet */
 
-    if (i) {
+    if (nspecials)
        ADD_SPECIALS(specials_end);
-       return ssh_specials;
+
+    ssh->specials = specials;
+
+    if (nspecials) {
+        return specials;
     } else {
        return NULL;
     }
@@ -11446,6 +11721,13 @@ static void ssh_special(void *handle, Telnet_Special code)
             ssh->version == 2) {
            do_ssh2_transport(ssh, "at user request", -1, NULL);
        }
+    } else if (code >= TS_LOCALSTART) {
+        ssh->hostkey = hostkey_algs[code - TS_LOCALSTART].alg;
+        ssh->cross_certifying = TRUE;
+       if (!ssh->kex_in_progress && !ssh->bare_connection &&
+            ssh->version == 2) {
+           do_ssh2_transport(ssh, "cross-certifying new host key", -1, NULL);
+       }
     } else if (code == TS_BRK) {
        if (ssh->state == SSH_STATE_CLOSED
            || ssh->state == SSH_STATE_PREPACKET) return;
@@ -11495,11 +11777,10 @@ void *new_sock_channel(void *handle, struct PortForwarding *pf)
     c = snew(struct ssh_channel);
 
     c->ssh = ssh;
-    ssh2_channel_init(c);
+    ssh_channel_init(c);
     c->halfopen = TRUE;
-    c->type = CHAN_SOCKDATA_DORMANT;/* identify channel type */
+    c->type = CHAN_SOCKDATA;/* identify channel type */
     c->u.pfd.pf = pf;
-    add234(ssh->channels, c);
     return c;
 }
 
@@ -11509,10 +11790,9 @@ unsigned ssh_alloc_sharing_channel(Ssh ssh, void *sharing_ctx)
     c = snew(struct ssh_channel);
 
     c->ssh = ssh;
-    ssh2_channel_init(c);
+    ssh_channel_init(c);
     c->type = CHAN_SHARING;
     c->u.sharing.ctx = sharing_ctx;
-    add234(ssh->channels, c);
     return c->localid;
 }
 
@@ -11545,7 +11825,6 @@ void ssh_send_packet_from_downstream(Ssh ssh, unsigned id, int type,
 static void ssh_unthrottle(void *handle, int bufsize)
 {
     Ssh ssh = (Ssh) handle;
-    int buflimit;
 
     if (ssh->version == 1) {
        if (ssh->v1_stdout_throttling && bufsize < SSH1_BUFFER_LIMIT) {
@@ -11553,19 +11832,8 @@ static void ssh_unthrottle(void *handle, int bufsize)
            ssh_throttle_conn(ssh, -1);
        }
     } else {
-       if (ssh->mainchan) {
-           ssh2_set_window(ssh->mainchan,
-                           bufsize < ssh->mainchan->v.v2.locmaxwin ?
-                           ssh->mainchan->v.v2.locmaxwin - bufsize : 0);
-           if (ssh_is_simple(ssh))
-               buflimit = 0;
-           else
-               buflimit = ssh->mainchan->v.v2.locmaxwin;
-           if (ssh->mainchan->throttling_conn && bufsize <= buflimit) {
-               ssh->mainchan->throttling_conn = 0;
-               ssh_throttle_conn(ssh, -1);
-           }
-       }
+       if (ssh->mainchan)
+           ssh_channel_unthrottle(ssh->mainchan, bufsize);
     }
 
     /*