Added plink callback.c sshshare.c winsecur.c

1 files changed, 2103 insertions, 0 deletions

diff --git a/tools/plink/sshshare.c b/tools/plink/sshshare.c
new file mode 100644
index 000000000..bd4602b5b
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+++ b/tools/plink/sshshare.c
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+/*
+ * Support for SSH connection sharing, i.e. permitting one PuTTY to
+ * open its own channels over the SSH session being run by another.
+ */
+
+/*
+ * Discussion and technical documentation
+ * ======================================
+ *
+ * The basic strategy for PuTTY's implementation of SSH connection
+ * sharing is to have a single 'upstream' PuTTY process, which manages
+ * the real SSH connection and all the cryptography, and then zero or
+ * more 'downstream' PuTTYs, which never talk to the real host but
+ * only talk to the upstream through local IPC (Unix-domain sockets or
+ * Windows named pipes).
+ *
+ * The downstreams communicate with the upstream using a protocol
+ * derived from SSH itself, which I'll document in detail below. In
+ * brief, though: the downstream->upstream protocol uses a trivial
+ * binary packet protocol (just length/type/data) to encapsulate
+ * unencrypted SSH messages, and downstreams talk to the upstream more
+ * or less as if it was an SSH server itself. (So downstreams can
+ * themselves open multiple SSH channels, for example, by sending
+ * multiple SSH2_MSG_CHANNEL_OPENs; they can send CHANNEL_REQUESTs of
+ * their choice within each channel, and they handle their own
+ * WINDOW_ADJUST messages.)
+ *
+ * The upstream would ideally handle these downstreams by just putting
+ * their messages into the queue for proper SSH-2 encapsulation and
+ * encryption and sending them straight on to the server. However,
+ * that's not quite feasible as written, because client-side channel
+ * IDs could easily conflict (between multiple downstreams, or between
+ * a downstream and the upstream). To protect against that, the
+ * upstream rewrites the client-side channel IDs in messages it passes
+ * on to the server, so that it's performing what you might describe
+ * as 'channel-number NAT'. Then the upstream remembers which of its
+ * own channel IDs are channels it's managing itself, and which are
+ * placeholders associated with a particular downstream, so that when
+ * replies come in from the server they can be sent on to the relevant
+ * downstream (after un-NATting the channel number, of course).
+ *
+ * Global requests from downstreams are only accepted if the upstream
+ * knows what to do about them; currently the only such requests are
+ * the ones having to do with remote-to-local port forwarding (in
+ * which, again, the upstream remembers that some of the forwardings
+ * it's asked the server to set up were on behalf of particular
+ * downstreams, and sends the incoming CHANNEL_OPENs to those
+ * downstreams when connections come in).
+ *
+ * Other fiddly pieces of this mechanism are X forwarding and
+ * (OpenSSH-style) agent forwarding. Both of these have a fundamental
+ * problem arising from the protocol design: that the CHANNEL_OPEN
+ * from the server introducing a forwarded connection does not carry
+ * any indication of which session channel gave rise to it; so if
+ * session channels from multiple downstreams enable those forwarding
+ * methods, it's hard for the upstream to know which downstream to
+ * send the resulting connections back to.
+ *
+ * For X forwarding, we can work around this in a really painful way
+ * by using the fake X11 authorisation data sent to the server as part
+ * of the forwarding setup: upstream ensures that every X forwarding
+ * request carries distinguishable fake auth data, and then when X
+ * connections come in it waits to see the auth data in the X11 setup
+ * message before it decides which downstream to pass the connection
+ * on to.
+ *
+ * For agent forwarding, that workaround is unavailable. As a result,
+ * this system (and, as far as I can think of, any other system too)
+ * has the fundamental constraint that it can only forward one SSH
+ * agent - it can't forward two agents to different session channels.
+ * So downstreams can request agent forwarding if they like, but if
+ * they do, they'll get whatever SSH agent is known to the upstream
+ * (if any) forwarded to their sessions.
+ *
+ * Downstream-to-upstream protocol
+ * -------------------------------
+ *
+ * Here I document in detail the protocol spoken between PuTTY
+ * downstreams and upstreams over local IPC. The IPC mechanism can
+ * vary between host platforms, but the protocol is the same.
+ *
+ * The protocol commences with a version exchange which is exactly
+ * like the SSH-2 one, in that each side sends a single line of text
+ * of the form
+ *
+ *   <protocol>-<version>-<softwareversion> [comments] \r\n
+ *
+ * The only difference is that in real SSH-2, <protocol> is the string
+ * "SSH", whereas in this protocol the string is
+ * "SSHCONNECTION@putty.projects.tartarus.org".
+ *
+ * (The SSH RFCs allow many protocol-level identifier namespaces to be
+ * extended by implementors without central standardisation as long as
+ * they suffix "@" and a domain name they control to their new ids.
+ * RFC 4253 does not define this particular name to be changeable at
+ * all, but I like to think this is obviously how it would have done
+ * so if the working group had foreseen the need :-)
+ *
+ * Thereafter, all data exchanged consists of a sequence of binary
+ * packets concatenated end-to-end, each of which is of the form
+ *
+ *     uint32     length of packet, N
+ *     byte[N]    N bytes of packet data
+ *
+ * and, since these are SSH-2 messages, the first data byte is taken
+ * to be the packet type code.
+ *
+ * These messages are interpreted as those of an SSH connection, after
+ * userauth completes, and without any repeat key exchange.
+ * Specifically, any message from the SSH Connection Protocol is
+ * permitted, and also SSH_MSG_IGNORE, SSH_MSG_DEBUG,
+ * SSH_MSG_DISCONNECT and SSH_MSG_UNIMPLEMENTED from the SSH Transport
+ * Protocol.
+ *
+ * This protocol imposes a few additional requirements, over and above
+ * those of the standard SSH Connection Protocol:
+ *
+ * Message sizes are not permitted to exceed 0x4010 (16400) bytes,
+ * including their length header.
+ *
+ * When the server (i.e. really the PuTTY upstream) sends
+ * SSH_MSG_CHANNEL_OPEN with channel type "x11", and the client
+ * (downstream) responds with SSH_MSG_CHANNEL_OPEN_CONFIRMATION, that
+ * confirmation message MUST include an initial window size of at
+ * least 256. (Rationale: this is a bit of a fudge which makes it
+ * easier, by eliminating the possibility of nasty edge cases, for an
+ * upstream to arrange not to pass the CHANNEL_OPEN on to downstream
+ * until after it's seen the X11 auth data to decide which downstream
+ * it needs to go to.)
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <assert.h>
+#include <limits.h>
+
+#include "putty.h"
+#include "tree234.h"
+#include "ssh.h"
+
+struct ssh_sharing_state {
+    const struct plug_function_table *fn;
+    /* the above variable absolutely *must* be the first in this structure */
+
+    char *sockname;                  /* the socket name, kept for cleanup */
+    Socket listensock;               /* the master listening Socket */
+    tree234 *connections;            /* holds ssh_sharing_connstates */
+    unsigned nextid;                 /* preferred id for next connstate */
+    Ssh ssh;                         /* instance of the ssh backend */
+    char *server_verstring;          /* server version string after "SSH-" */
+};
+
+struct share_globreq;
+
+struct ssh_sharing_connstate {
+    const struct plug_function_table *fn;
+    /* the above variable absolutely *must* be the first in this structure */
+
+    unsigned id;    /* used to identify this downstream in log messages */
+
+    Socket sock;                     /* the Socket for this connection */
+    struct ssh_sharing_state *parent;
+
+    int crLine;                        /* coroutine state for share_receive */
+
+    int sent_verstring, got_verstring, curr_packetlen;
+
+    unsigned char recvbuf[0x4010];
+    int recvlen;
+
+    /*
+     * Assorted state we have to remember about this downstream, so
+     * that we can clean it up appropriately when the downstream goes
+     * away.
+     */
+
+    /* Channels which don't have a downstream id, i.e. we've passed a
+     * CHANNEL_OPEN down from the server but not had an
+     * OPEN_CONFIRMATION or OPEN_FAILURE back. If downstream goes
+     * away, we respond to all of these with OPEN_FAILURE. */
+    tree234 *halfchannels;         /* stores 'struct share_halfchannel' */
+
+    /* Channels which do have a downstream id. We need to index these
+     * by both server id and upstream id, so we can find a channel
+     * when handling either an upward or a downward message referring
+     * to it. */
+    tree234 *channels_by_us;       /* stores 'struct share_channel' */
+    tree234 *channels_by_server;   /* stores 'struct share_channel' */
+
+    /* Another class of channel which doesn't have a downstream id.
+     * The difference between these and halfchannels is that xchannels
+     * do have an *upstream* id, because upstream has already accepted
+     * the channel request from the server. This arises in the case of
+     * X forwarding, where we have to accept the request and read the
+     * X authorisation data before we know whether the channel needs
+     * to be forwarded to a downstream. */
+    tree234 *xchannels_by_us;     /* stores 'struct share_xchannel' */
+    tree234 *xchannels_by_server; /* stores 'struct share_xchannel' */
+
+    /* Remote port forwarding requests in force. */
+    tree234 *forwardings;          /* stores 'struct share_forwarding' */
+
+    /* Global requests we've sent on to the server, pending replies. */
+    struct share_globreq *globreq_head, *globreq_tail;
+};
+
+struct share_halfchannel {
+    unsigned server_id;
+};
+
+/* States of a share_channel. */
+enum {
+    OPEN,
+    SENT_CLOSE,
+    RCVD_CLOSE,
+    /* Downstream has sent CHANNEL_OPEN but server hasn't replied yet.
+     * If downstream goes away when a channel is in this state, we
+     * must wait for the server's response before starting to send
+     * CLOSE. Channels in this state are also not held in
+     * channels_by_server, because their server_id field is
+     * meaningless. */
+    UNACKNOWLEDGED
+};
+
+struct share_channel {
+    unsigned downstream_id, upstream_id, server_id;
+    int downstream_maxpkt;
+    int state;
+    /*
+     * Some channels (specifically, channels on which downstream has
+     * sent "x11-req") have the additional function of storing a set
+     * of downstream X authorisation data and a handle to an upstream
+     * fake set.
+     */
+    struct X11FakeAuth *x11_auth_upstream;
+    int x11_auth_proto;
+    char *x11_auth_data;
+    int x11_auth_datalen;
+    int x11_one_shot;
+};
+
+struct share_forwarding {
+    char *host;
+    int port;
+    int active;             /* has the server sent REQUEST_SUCCESS? */
+};
+
+struct share_xchannel_message {
+    struct share_xchannel_message *next;
+    int type;
+    unsigned char *data;
+    int datalen;
+};
+
+struct share_xchannel {
+    unsigned upstream_id, server_id;
+
+    /*
+     * xchannels come in two flavours: live and dead. Live ones are
+     * waiting for an OPEN_CONFIRMATION or OPEN_FAILURE from
+     * downstream; dead ones have had an OPEN_FAILURE, so they only
+     * exist as a means of letting us conveniently respond to further
+     * channel messages from the server until such time as the server
+     * sends us CHANNEL_CLOSE.
+     */
+    int live;
+
+    /*
+     * When we receive OPEN_CONFIRMATION, we will need to send a
+     * WINDOW_ADJUST to the server to synchronise the windows. For
+     * this purpose we need to know what window we have so far offered
+     * the server. We record this as exactly the value in the
+     * OPEN_CONFIRMATION that upstream sent us, adjusted by the amount
+     * by which the two X greetings differed in length.
+     */
+    int window;
+
+    /*
+     * Linked list of SSH messages from the server relating to this
+     * channel, which we queue up until downstream sends us an
+     * OPEN_CONFIRMATION and we can belatedly send them all on.
+     */
+    struct share_xchannel_message *msghead, *msgtail;
+};
+
+enum {
+    GLOBREQ_TCPIP_FORWARD,
+    GLOBREQ_CANCEL_TCPIP_FORWARD
+};
+
+struct share_globreq {
+    struct share_globreq *next;
+    int type;
+    int want_reply;
+    struct share_forwarding *fwd;
+};
+
+static int share_connstate_cmp(void *av, void *bv)
+{
+    const struct ssh_sharing_connstate *a =
+        (const struct ssh_sharing_connstate *)av;
+    const struct ssh_sharing_connstate *b =
+        (const struct ssh_sharing_connstate *)bv;
+
+    if (a->id < b->id)
+        return -1;
+    else if (a->id > b->id)
+        return +1;
+    else
+        return 0;
+}
+
+static unsigned share_find_unused_id
+(struct ssh_sharing_state *sharestate, unsigned first)
+{
+    int low_orig, low, mid, high, high_orig;
+    struct ssh_sharing_connstate *cs;
+    unsigned ret;
+
+    /*
+     * Find the lowest unused downstream ID greater or equal to
+     * 'first'.
+     *
+     * Begin by seeing if 'first' itself is available. If it is, we'll
+     * just return it; if it's already in the tree, we'll find the
+     * tree index where it appears and use that for the next stage.
+     */
+    {
+        struct ssh_sharing_connstate dummy;
+        dummy.id = first;
+        cs = findrelpos234(sharestate->connections, &dummy, NULL,
+                           REL234_GE, &low_orig);
+        if (!cs)
+            return first;
+    }
+
+    /*
+     * Now binary-search using the counted B-tree, to find the largest
+     * ID which is in a contiguous sequence from the beginning of that
+     * range.
+     */
+    low = low_orig;
+    high = high_orig = count234(sharestate->connections);
+    while (high - low > 1) {
+	mid = (high + low) / 2;
+	cs = index234(sharestate->connections, mid);
+	if (cs->id == first + (mid - low_orig))
+	    low = mid;		       /* this one is still in the sequence */
+	else
+	    high = mid;		       /* this one is past the end */
+    }
+
+    /*
+     * Now low is the tree index of the largest ID in the initial
+     * sequence. So the return value is one more than low's id, and we
+     * know low's id is given by the formula in the binary search loop
+     * above.
+     *
+     * (If an SSH connection went on for _enormously_ long, we might
+     * reach a point where all ids from 'first' to UINT_MAX were in
+     * use. In that situation the formula below would wrap round by
+     * one and return zero, which is conveniently the right way to
+     * signal 'no id available' from this function.)
+     */
+    ret = first + (low - low_orig) + 1;
+    {
+        struct ssh_sharing_connstate dummy;
+        dummy.id = ret;
+	assert(NULL == find234(sharestate->connections, &dummy, NULL));
+    }
+    return ret;
+}
+
+static int share_halfchannel_cmp(void *av, void *bv)
+{
+    const struct share_halfchannel *a = (const struct share_halfchannel *)av;
+    const struct share_halfchannel *b = (const struct share_halfchannel *)bv;
+
+    if (a->server_id < b->server_id)
+        return -1;
+    else if (a->server_id > b->server_id)
+        return +1;
+    else
+        return 0;
+}
+
+static int share_channel_us_cmp(void *av, void *bv)
+{
+    const struct share_channel *a = (const struct share_channel *)av;
+    const struct share_channel *b = (const struct share_channel *)bv;
+
+    if (a->upstream_id < b->upstream_id)
+        return -1;
+    else if (a->upstream_id > b->upstream_id)
+        return +1;
+    else
+        return 0;
+}
+
+static int share_channel_server_cmp(void *av, void *bv)
+{
+    const struct share_channel *a = (const struct share_channel *)av;
+    const struct share_channel *b = (const struct share_channel *)bv;
+
+    if (a->server_id < b->server_id)
+        return -1;
+    else if (a->server_id > b->server_id)
+        return +1;
+    else
+        return 0;
+}
+
+static int share_xchannel_us_cmp(void *av, void *bv)
+{
+    const struct share_xchannel *a = (const struct share_xchannel *)av;
+    const struct share_xchannel *b = (const struct share_xchannel *)bv;
+
+    if (a->upstream_id < b->upstream_id)
+        return -1;
+    else if (a->upstream_id > b->upstream_id)
+        return +1;
+    else
+        return 0;
+}
+
+static int share_xchannel_server_cmp(void *av, void *bv)
+{
+    const struct share_xchannel *a = (const struct share_xchannel *)av;
+    const struct share_xchannel *b = (const struct share_xchannel *)bv;
+
+    if (a->server_id < b->server_id)
+        return -1;
+    else if (a->server_id > b->server_id)
+        return +1;
+    else
+        return 0;
+}
+
+static int share_forwarding_cmp(void *av, void *bv)
+{
+    const struct share_forwarding *a = (const struct share_forwarding *)av;
+    const struct share_forwarding *b = (const struct share_forwarding *)bv;
+    int i;
+
+    if ((i = strcmp(a->host, b->host)) != 0)
+        return i;
+    else if (a->port < b->port)
+        return -1;
+    else if (a->port > b->port)
+        return +1;
+    else
+        return 0;
+}
+
+static void share_xchannel_free(struct share_xchannel *xc)
+{
+    while (xc->msghead) {
+        struct share_xchannel_message *tmp = xc->msghead;
+        xc->msghead = tmp->next;
+        sfree(tmp);
+    }
+    sfree(xc);
+}
+
+static void share_connstate_free(struct ssh_sharing_connstate *cs)
+{
+    struct share_halfchannel *hc;
+    struct share_xchannel *xc;
+    struct share_channel *chan;
+    struct share_forwarding *fwd;
+
+    while ((hc = (struct share_halfchannel *)
+            delpos234(cs->halfchannels, 0)) != NULL)
+        sfree(hc);
+    freetree234(cs->halfchannels);
+
+    /* All channels live in 'channels_by_us' but only some in
+     * 'channels_by_server', so we use the former to find the list of
+     * ones to free */
+    freetree234(cs->channels_by_server);
+    while ((chan = (struct share_channel *)
+            delpos234(cs->channels_by_us, 0)) != NULL)
+        sfree(chan);
+    freetree234(cs->channels_by_us);
+
+    /* But every xchannel is in both trees, so it doesn't matter which
+     * we use to free them. */
+    while ((xc = (struct share_xchannel *)
+            delpos234(cs->xchannels_by_us, 0)) != NULL)
+        share_xchannel_free(xc);
+    freetree234(cs->xchannels_by_us);
+    freetree234(cs->xchannels_by_server);
+
+    while ((fwd = (struct share_forwarding *)
+            delpos234(cs->forwardings, 0)) != NULL)
+        sfree(fwd);
+    freetree234(cs->forwardings);
+
+    while (cs->globreq_head) {
+        struct share_globreq *globreq = cs->globreq_head;
+        cs->globreq_head = cs->globreq_head->next;
+        sfree(globreq);
+    }
+
+    sfree(cs);
+}
+
+void sharestate_free(void *v)
+{
+    struct ssh_sharing_state *sharestate = (struct ssh_sharing_state *)v;
+    struct ssh_sharing_connstate *cs;
+
+    platform_ssh_share_cleanup(sharestate->sockname);
+
+    while ((cs = (struct ssh_sharing_connstate *)
+            delpos234(sharestate->connections, 0)) != NULL) {
+        share_connstate_free(cs);
+    }
+    freetree234(sharestate->connections);
+    sfree(sharestate->server_verstring);
+    sfree(sharestate->sockname);
+    sfree(sharestate);
+}
+
+static struct share_halfchannel *share_add_halfchannel
+    (struct ssh_sharing_connstate *cs, unsigned server_id)
+{
+    struct share_halfchannel *hc = snew(struct share_halfchannel);
+    hc->server_id = server_id;
+    if (add234(cs->halfchannels, hc) != hc) {
+        /* Duplicate?! */
+        sfree(hc);
+        return NULL;
+    } else {
+        return hc;
+    }
+}
+
+static struct share_halfchannel *share_find_halfchannel
+    (struct ssh_sharing_connstate *cs, unsigned server_id)
+{
+    struct share_halfchannel dummyhc;
+    dummyhc.server_id = server_id;
+    return find234(cs->halfchannels, &dummyhc, NULL);
+}
+
+static void share_remove_halfchannel(struct ssh_sharing_connstate *cs,
+                                     struct share_halfchannel *hc)
+{
+    del234(cs->halfchannels, hc);
+    sfree(hc);
+}
+
+static struct share_channel *share_add_channel
+    (struct ssh_sharing_connstate *cs, unsigned downstream_id,
+     unsigned upstream_id, unsigned server_id, int state, int maxpkt)
+{
+    struct share_channel *chan = snew(struct share_channel);
+    chan->downstream_id = downstream_id;
+    chan->upstream_id = upstream_id;
+    chan->server_id = server_id;
+    chan->state = state;
+    chan->downstream_maxpkt = maxpkt;
+    chan->x11_auth_upstream = NULL;
+    chan->x11_auth_data = NULL;
+    chan->x11_auth_proto = -1;
+    chan->x11_auth_datalen = 0;
+    chan->x11_one_shot = 0;
+    if (add234(cs->channels_by_us, chan) != chan) {
+        sfree(chan);
+        return NULL;
+    }
+    if (chan->state != UNACKNOWLEDGED) {
+        if (add234(cs->channels_by_server, chan) != chan) {
+            del234(cs->channels_by_us, chan);
+            sfree(chan);            
+            return NULL;
+        }
+    }
+    return chan;
+}
+
+static void share_channel_set_server_id(struct ssh_sharing_connstate *cs,
+                                        struct share_channel *chan,
+                                        unsigned server_id, int newstate)
+{
+    chan->server_id = server_id;
+    chan->state = newstate;
+    assert(newstate != UNACKNOWLEDGED);
+    add234(cs->channels_by_server, chan);
+}
+
+static struct share_channel *share_find_channel_by_upstream
+    (struct ssh_sharing_connstate *cs, unsigned upstream_id)
+{
+    struct share_channel dummychan;
+    dummychan.upstream_id = upstream_id;
+    return find234(cs->channels_by_us, &dummychan, NULL);
+}
+
+static struct share_channel *share_find_channel_by_server
+    (struct ssh_sharing_connstate *cs, unsigned server_id)
+{
+    struct share_channel dummychan;
+    dummychan.server_id = server_id;
+    return find234(cs->channels_by_server, &dummychan, NULL);
+}
+
+static void share_remove_channel(struct ssh_sharing_connstate *cs,
+                                 struct share_channel *chan)
+{
+    del234(cs->channels_by_us, chan);
+    del234(cs->channels_by_server, chan);
+    if (chan->x11_auth_upstream)
+        ssh_sharing_remove_x11_display(cs->parent->ssh,
+                                       chan->x11_auth_upstream);
+    sfree(chan->x11_auth_data);
+    sfree(chan);
+}
+
+static struct share_xchannel *share_add_xchannel
+    (struct ssh_sharing_connstate *cs,
+     unsigned upstream_id, unsigned server_id)
+{
+    struct share_xchannel *xc = snew(struct share_xchannel);
+    xc->upstream_id = upstream_id;
+    xc->server_id = server_id;
+    xc->live = TRUE;
+    xc->msghead = xc->msgtail = NULL;
+    if (add234(cs->xchannels_by_us, xc) != xc) {
+        sfree(xc);
+        return NULL;
+    }
+    if (add234(cs->xchannels_by_server, xc) != xc) {
+        del234(cs->xchannels_by_us, xc);
+        sfree(xc);
+        return NULL;
+    }
+    return xc;
+}
+
+static struct share_xchannel *share_find_xchannel_by_upstream
+    (struct ssh_sharing_connstate *cs, unsigned upstream_id)
+{
+    struct share_xchannel dummyxc;
+    dummyxc.upstream_id = upstream_id;
+    return find234(cs->xchannels_by_us, &dummyxc, NULL);
+}
+
+static struct share_xchannel *share_find_xchannel_by_server
+    (struct ssh_sharing_connstate *cs, unsigned server_id)
+{
+    struct share_xchannel dummyxc;
+    dummyxc.server_id = server_id;
+    return find234(cs->xchannels_by_server, &dummyxc, NULL);
+}
+
+static void share_remove_xchannel(struct ssh_sharing_connstate *cs,
+                                 struct share_xchannel *xc)
+{
+    del234(cs->xchannels_by_us, xc);
+    del234(cs->xchannels_by_server, xc);
+    share_xchannel_free(xc);
+}
+
+static struct share_forwarding *share_add_forwarding
+    (struct ssh_sharing_connstate *cs,
+     const char *host, int port)
+{
+    struct share_forwarding *fwd = snew(struct share_forwarding);
+    fwd->host = dupstr(host);
+    fwd->port = port;
+    fwd->active = FALSE;
+    if (add234(cs->forwardings, fwd) != fwd) {
+        /* Duplicate?! */
+        sfree(fwd);
+        return NULL;
+    }
+    return fwd;
+}
+
+static struct share_forwarding *share_find_forwarding
+    (struct ssh_sharing_connstate *cs, const char *host, int port)
+{
+    struct share_forwarding dummyfwd, *ret;
+    dummyfwd.host = dupstr(host);
+    dummyfwd.port = port;
+    ret = find234(cs->forwardings, &dummyfwd, NULL);
+    sfree(dummyfwd.host);
+    return ret;
+}
+
+static void share_remove_forwarding(struct ssh_sharing_connstate *cs,
+                                    struct share_forwarding *fwd)
+{
+    del234(cs->forwardings, fwd);
+    sfree(fwd);
+}
+
+static void send_packet_to_downstream(struct ssh_sharing_connstate *cs,
+                                      int type, const void *pkt, int pktlen,
+                                      struct share_channel *chan)
+{
+    if (!cs->sock) /* throw away all packets destined for a dead downstream */
+        return;
+
+    if (type == SSH2_MSG_CHANNEL_DATA) {
+        /*
+         * Special case which we take care of at a low level, so as to
+         * be sure to apply it in all cases. On rare occasions we
+         * might find that we have a channel for which the
+         * downstream's maximum packet size exceeds the max packet
+         * size we presented to the server on its behalf. (This can
+         * occur in X11 forwarding, where we have to send _our_
+         * CHANNEL_OPEN_CONFIRMATION before we discover which if any
+         * downstream the channel is destined for, so if that
+         * downstream turns out to present a smaller max packet size
+         * then we're in this situation.)
+         *
+         * If that happens, we just chop up the packet into pieces and
+         * send them as separate CHANNEL_DATA packets.
+         */
+        const char *upkt = (const char *)pkt;
+        char header[13]; /* 4 length + 1 type + 4 channel id + 4 string len */
+
+        int len = toint(GET_32BIT(upkt + 4));
+        upkt += 8;                /* skip channel id + length field */
+
+        if (len < 0 || len > pktlen - 8)
+            len = pktlen - 8;
+
+        do {
+            int this_len = (len > chan->downstream_maxpkt ?
+                            chan->downstream_maxpkt : len);
+            PUT_32BIT(header, this_len + 9);
+            header[4] = type;
+            PUT_32BIT(header + 5, chan->downstream_id);
+            PUT_32BIT(header + 9, this_len);
+            sk_write(cs->sock, header, 13);
+            sk_write(cs->sock, upkt, this_len);
+            len -= this_len;
+            upkt += this_len;
+        } while (len > 0);
+    } else {
+        /*
+         * Just do the obvious thing.
+         */
+        char header[9];
+
+        PUT_32BIT(header, pktlen + 1);
+        header[4] = type;
+        sk_write(cs->sock, header, 5);
+        sk_write(cs->sock, pkt, pktlen);
+    }
+}
+
+static void share_try_cleanup(struct ssh_sharing_connstate *cs)
+{
+    int i;
+    struct share_halfchannel *hc;
+    struct share_channel *chan;
+    struct share_forwarding *fwd;
+
+    /*
+     * Any half-open channels, i.e. those for which we'd received
+     * CHANNEL_OPEN from the server but not passed back a response
+     * from downstream, should be responded to with OPEN_FAILURE.
+     */
+    while ((hc = (struct share_halfchannel *)
+            index234(cs->halfchannels, 0)) != NULL) {
+        static const char reason[] = "PuTTY downstream no longer available";
+        static const char lang[] = "en";
+        unsigned char packet[256];
+        int pos = 0;
+
+        PUT_32BIT(packet + pos, hc->server_id); pos += 4;
+        PUT_32BIT(packet + pos, SSH2_OPEN_CONNECT_FAILED); pos += 4;
+        PUT_32BIT(packet + pos, strlen(reason)); pos += 4;
+        memcpy(packet + pos, reason, strlen(reason)); pos += strlen(reason);
+        PUT_32BIT(packet + pos, strlen(lang)); pos += 4;
+        memcpy(packet + pos, lang, strlen(lang)); pos += strlen(lang);
+        ssh_send_packet_from_downstream(cs->parent->ssh, cs->id,
+                                        SSH2_MSG_CHANNEL_OPEN_FAILURE,
+                                        packet, pos, "cleanup after"
+                                        " downstream went away");
+
+        share_remove_halfchannel(cs, hc);
+    }
+
+    /*
+     * Any actually open channels should have a CHANNEL_CLOSE sent for
+     * them, unless we've already done so. We won't be able to
+     * actually clean them up until CHANNEL_CLOSE comes back from the
+     * server, though (unless the server happens to have sent a CLOSE
+     * already).
+     *
+     * Another annoying exception is UNACKNOWLEDGED channels, i.e.
+     * we've _sent_ a CHANNEL_OPEN to the server but not received an
+     * OPEN_CONFIRMATION or OPEN_FAILURE. We must wait for a reply
+     * before closing the channel, because until we see that reply we
+     * won't have the server's channel id to put in the close message.
+     */
+    for (i = 0; (chan = (struct share_channel *)
+                 index234(cs->channels_by_us, i)) != NULL; i++) {
+        unsigned char packet[256];
+        int pos = 0;
+
+        if (chan->state != SENT_CLOSE && chan->state != UNACKNOWLEDGED) {
+            PUT_32BIT(packet + pos, chan->server_id); pos += 4;
+            ssh_send_packet_from_downstream(cs->parent->ssh, cs->id,
+                                            SSH2_MSG_CHANNEL_CLOSE,
+                                            packet, pos, "cleanup after"
+                                            " downstream went away");
+            if (chan->state != RCVD_CLOSE) {
+                chan->state = SENT_CLOSE;
+            } else {
+                /* In this case, we _can_ clear up the channel now. */
+                ssh_delete_sharing_channel(cs->parent->ssh, chan->upstream_id);
+                share_remove_channel(cs, chan);
+                i--;    /* don't accidentally skip one as a result */
+            }
+        }
+    }
+
+    /*
+     * Any remote port forwardings we're managing on behalf of this
+     * downstream should be cancelled. Again, we must defer those for
+     * which we haven't yet seen REQUEST_SUCCESS/FAILURE.
+     *
+     * We take a fire-and-forget approach during cleanup, not
+     * bothering to set want_reply.
+     */
+    for (i = 0; (fwd = (struct share_forwarding *)
+                 index234(cs->forwardings, i)) != NULL; i++) {
+        if (fwd->active) {
+            static const char request[] = "cancel-tcpip-forward";
+            char *packet = snewn(256 + strlen(fwd->host), char);
+            int pos = 0;
+
+            PUT_32BIT(packet + pos, strlen(request)); pos += 4;
+            memcpy(packet + pos, request, strlen(request));
+            pos += strlen(request);
+
+            packet[pos++] = 0;         /* !want_reply */
+
+            PUT_32BIT(packet + pos, strlen(fwd->host)); pos += 4;
+            memcpy(packet + pos, fwd->host, strlen(fwd->host));
+            pos += strlen(fwd->host);
+
+            PUT_32BIT(packet + pos, fwd->port); pos += 4;
+
+            ssh_send_packet_from_downstream(cs->parent->ssh, cs->id,
+                                            SSH2_MSG_GLOBAL_REQUEST,
+                                            packet, pos, "cleanup after"
+                                            " downstream went away");
+
+            share_remove_forwarding(cs, fwd);
+            i--;    /* don't accidentally skip one as a result */
+        }
+    }
+
+    if (count234(cs->halfchannels) == 0 &&
+        count234(cs->channels_by_us) == 0 &&
+        count234(cs->forwardings) == 0) {
+        /*
+         * Now we're _really_ done, so we can get rid of cs completely.
+         */
+        del234(cs->parent->connections, cs);
+        ssh_sharing_downstream_disconnected(cs->parent->ssh, cs->id);
+        share_connstate_free(cs);
+    }
+}
+
+static void share_begin_cleanup(struct ssh_sharing_connstate *cs)
+{
+
+    sk_close(cs->sock);
+    cs->sock = NULL;
+
+    share_try_cleanup(cs);
+}
+
+static void share_disconnect(struct ssh_sharing_connstate *cs,
+                             const char *message)
+{
+    static const char lang[] = "en";
+    int msglen = strlen(message);
+    char *packet = snewn(msglen + 256, char);
+    int pos = 0;
+
+    PUT_32BIT(packet + pos, SSH2_DISCONNECT_PROTOCOL_ERROR); pos += 4;
+
+    PUT_32BIT(packet + pos, msglen); pos += 4;
+    memcpy(packet + pos, message, msglen);
+    pos += msglen;
+
+    PUT_32BIT(packet + pos, strlen(lang)); pos += 4;
+    memcpy(packet + pos, lang, strlen(lang)); pos += strlen(lang);
+
+    send_packet_to_downstream(cs, SSH2_MSG_DISCONNECT, packet, pos, NULL);
+
+    share_begin_cleanup(cs);
+}
+
+static int share_closing(Plug plug, const char *error_msg, int error_code,
+                         int calling_back)
+{
+    struct ssh_sharing_connstate *cs = (struct ssh_sharing_connstate *)plug;
+    if (error_msg)
+        ssh_sharing_logf(cs->parent->ssh, cs->id, "%s", error_msg);
+    share_begin_cleanup(cs);
+    return 1;
+}
+
+static int getstring_inner(const void *vdata, int datalen,
+                           char **out, int *outlen)
+{
+    const unsigned char *data = (const unsigned char *)vdata;
+    int len;
+
+    if (datalen < 4)
+        return FALSE;
+
+    len = toint(GET_32BIT(data));
+    if (len < 0 || len > datalen - 4)
+        return FALSE;
+
+    if (outlen)
+        *outlen = len + 4;         /* total size including length field */
+    if (out)
+        *out = dupprintf("%.*s", len, (char *)data + 4);
+    return TRUE;
+}
+
+static char *getstring(const void *data, int datalen)
+{
+    char *ret;
+    if (getstring_inner(data, datalen, &ret, NULL))
+        return ret;
+    else
+        return NULL;
+}
+
+static int getstring_size(const void *data, int datalen)
+{
+    int ret;
+    if (getstring_inner(data, datalen, NULL, &ret))
+        return ret;
+    else
+        return -1;
+}
+
+/*
+ * Append a message to the end of an xchannel's queue, with the length
+ * and type code filled in and the data block allocated but
+ * uninitialised.
+ */
+struct share_xchannel_message *share_xchannel_add_message
+(struct share_xchannel *xc, int type, int len)
+{
+    unsigned char *block;
+    struct share_xchannel_message *msg;
+
+    /*
+     * Be a little tricksy here by allocating a single memory block
+     * containing both the 'struct share_xchannel_message' and the
+     * actual data. Simplifies freeing it later.
+     */
+    block = smalloc(sizeof(struct share_xchannel_message) + len);
+    msg = (struct share_xchannel_message *)block;
+    msg->data = block + sizeof(struct share_xchannel_message);
+    msg->datalen = len;
+    msg->type = type;
+
+    /*
+     * Queue it in the xchannel.
+     */
+    if (xc->msgtail)
+        xc->msgtail->next = msg;
+    else
+        xc->msghead = msg;
+    msg->next = NULL;
+    xc->msgtail = msg;
+
+    return msg;
+}
+
+void share_dead_xchannel_respond(struct ssh_sharing_connstate *cs,
+                                 struct share_xchannel *xc)
+{
+    /*
+     * Handle queued incoming messages from the server destined for an
+     * xchannel which is dead (i.e. downstream sent OPEN_FAILURE).
+     */
+    int delete = FALSE;
+    while (xc->msghead) {
+        struct share_xchannel_message *msg = xc->msghead;
+        xc->msghead = msg->next;
+
+        if (msg->type == SSH2_MSG_CHANNEL_REQUEST && msg->datalen > 4) {
+            /*
+             * A CHANNEL_REQUEST is responded to by sending
+             * CHANNEL_FAILURE, if it has want_reply set.
+             */
+            int wantreplypos = getstring_size(msg->data, msg->datalen);
+            if (wantreplypos > 0 && wantreplypos < msg->datalen &&
+                msg->data[wantreplypos] != 0) {
+                unsigned char id[4];
+                PUT_32BIT(id, xc->server_id);
+                ssh_send_packet_from_downstream
+                    (cs->parent->ssh, cs->id, SSH2_MSG_CHANNEL_FAILURE, id, 4,
+                     "downstream refused X channel open");
+            }
+        } else if (msg->type == SSH2_MSG_CHANNEL_CLOSE) {
+            /*
+             * On CHANNEL_CLOSE we can discard the channel completely.
+             */
+            delete = TRUE;
+        }
+
+        sfree(msg);
+    }
+    xc->msgtail = NULL;
+    if (delete) {
+        ssh_delete_sharing_channel(cs->parent->ssh, xc->upstream_id);
+        share_remove_xchannel(cs, xc);
+    }
+}
+
+void share_xchannel_confirmation(struct ssh_sharing_connstate *cs,
+                                 struct share_xchannel *xc,
+                                 struct share_channel *chan,
+                                 unsigned downstream_window)
+{
+    unsigned char window_adjust[8];
+
+    /*
+     * Send all the queued messages downstream.
+     */
+    while (xc->msghead) {
+        struct share_xchannel_message *msg = xc->msghead;
+        xc->msghead = msg->next;
+
+        if (msg->datalen >= 4)
+            PUT_32BIT(msg->data, chan->downstream_id);
+        send_packet_to_downstream(cs, msg->type,
+                                  msg->data, msg->datalen, chan);
+
+        sfree(msg);
+    }
+
+    /*
+     * Send a WINDOW_ADJUST back upstream, to synchronise the window
+     * size downstream thinks it's presented with the one we've
+     * actually presented.
+     */
+    PUT_32BIT(window_adjust, xc->server_id);
+    PUT_32BIT(window_adjust + 4, downstream_window - xc->window);
+    ssh_send_packet_from_downstream(cs->parent->ssh, cs->id,
+                                    SSH2_MSG_CHANNEL_WINDOW_ADJUST,
+                                    window_adjust, 8, "window adjustment after"
+                                    " downstream accepted X channel");
+}
+
+void share_xchannel_failure(struct ssh_sharing_connstate *cs,
+                            struct share_xchannel *xc)
+{
+    /*
+     * If downstream refuses to open our X channel at all for some
+     * reason, we must respond by sending an emergency CLOSE upstream.
+     */
+    unsigned char id[4];
+    PUT_32BIT(id, xc->server_id);
+    ssh_send_packet_from_downstream
+        (cs->parent->ssh, cs->id, SSH2_MSG_CHANNEL_CLOSE, id, 4,
+         "downstream refused X channel open");
+
+    /*
+     * Now mark the xchannel as dead, and respond to anything sent on
+     * it until we see CLOSE for it in turn.
+     */
+    xc->live = FALSE;
+    share_dead_xchannel_respond(cs, xc);
+}
+
+void share_setup_x11_channel(void *csv, void *chanv,
+                             unsigned upstream_id, unsigned server_id,
+                             unsigned server_currwin, unsigned server_maxpkt,
+                             unsigned client_adjusted_window,
+                             const char *peer_addr, int peer_port, int endian,
+                             int protomajor, int protominor,
+                             const void *initial_data, int initial_len)
+{
+    struct ssh_sharing_connstate *cs = (struct ssh_sharing_connstate *)csv;
+    struct share_channel *chan = (struct share_channel *)chanv;
+    struct share_xchannel *xc;
+    struct share_xchannel_message *msg;
+    void *greeting;
+    int greeting_len;
+    unsigned char *pkt;
+    int pktlen;
+
+    /*
+     * Create an xchannel containing data we've already received from
+     * the X client, and preload it with a CHANNEL_DATA message
+     * containing our own made-up authorisation greeting and any
+     * additional data sent from the server so far.
+     */
+    xc = share_add_xchannel(cs, upstream_id, server_id);
+    greeting = x11_make_greeting(endian, protomajor, protominor,
+                                 chan->x11_auth_proto,
+                                 chan->x11_auth_data, chan->x11_auth_datalen,
+                                 peer_addr, peer_port, &greeting_len);
+    msg = share_xchannel_add_message(xc, SSH2_MSG_CHANNEL_DATA,
+                                     8 + greeting_len + initial_len);
+    /* leave the channel id field unfilled - we don't know the
+     * downstream id yet, of course */
+    PUT_32BIT(msg->data + 4, greeting_len + initial_len);
+    memcpy(msg->data + 8, greeting, greeting_len);
+    memcpy(msg->data + 8 + greeting_len, initial_data, initial_len);
+    sfree(greeting);
+
+    xc->window = client_adjusted_window + greeting_len;
+
+    /*
+     * Send on a CHANNEL_OPEN to downstream.
+     */
+    pktlen = 27 + strlen(peer_addr);
+    pkt = snewn(pktlen, unsigned char);
+    PUT_32BIT(pkt, 3);                 /* strlen("x11") */
+    memcpy(pkt+4, "x11", 3);
+    PUT_32BIT(pkt+7, server_id);
+    PUT_32BIT(pkt+11, server_currwin);
+    PUT_32BIT(pkt+15, server_maxpkt);
+    PUT_32BIT(pkt+19, strlen(peer_addr));
+    memcpy(pkt+23, peer_addr, strlen(peer_addr));
+    PUT_32BIT(pkt+23+strlen(peer_addr), peer_port);
+    send_packet_to_downstream(cs, SSH2_MSG_CHANNEL_OPEN, pkt, pktlen, NULL);
+    sfree(pkt);
+
+    /*
+     * If this was a once-only X forwarding, clean it up now.
+     */
+    if (chan->x11_one_shot) {
+        ssh_sharing_remove_x11_display(cs->parent->ssh,
+                                       chan->x11_auth_upstream);
+        chan->x11_auth_upstream = NULL;
+        sfree(chan->x11_auth_data);
+        chan->x11_auth_proto = -1;
+        chan->x11_auth_datalen = 0;
+        chan->x11_one_shot = 0;
+    }
+}
+
+void share_got_pkt_from_server(void *csv, int type,
+                               unsigned char *pkt, int pktlen)
+{
+    struct ssh_sharing_connstate *cs = (struct ssh_sharing_connstate *)csv;
+    struct share_globreq *globreq;
+    int id_pos;
+    unsigned upstream_id, server_id;
+    struct share_channel *chan;
+    struct share_xchannel *xc;
+
+    switch (type) {
+      case SSH2_MSG_REQUEST_SUCCESS:
+      case SSH2_MSG_REQUEST_FAILURE:
+        globreq = cs->globreq_head;
+        if (globreq->type == GLOBREQ_TCPIP_FORWARD) {
+            if (type == SSH2_MSG_REQUEST_FAILURE) {
+                share_remove_forwarding(cs, globreq->fwd);
+            } else {
+                globreq->fwd->active = TRUE;
+            }
+        } else if (globreq->type == GLOBREQ_CANCEL_TCPIP_FORWARD) {
+            if (type == SSH2_MSG_REQUEST_SUCCESS) {
+                share_remove_forwarding(cs, globreq->fwd);
+            }
+        }
+        if (globreq->want_reply) {
+            send_packet_to_downstream(cs, type, pkt, pktlen, NULL);
+        }
+        cs->globreq_head = globreq->next;
+        sfree(globreq);
+        if (cs->globreq_head == NULL)
+            cs->globreq_tail = NULL;
+
+        if (!cs->sock) {
+            /* Retry cleaning up this connection, in case that reply
+             * was the last thing we were waiting for. */
+            share_try_cleanup(cs);
+        }
+
+        break;
+
+      case SSH2_MSG_CHANNEL_OPEN:
+        id_pos = getstring_size(pkt, pktlen);
+        assert(id_pos >= 0);
+        server_id = GET_32BIT(pkt + id_pos);
+        share_add_halfchannel(cs, server_id);
+
+        send_packet_to_downstream(cs, type, pkt, pktlen, NULL);
+        break;
+
+      case SSH2_MSG_CHANNEL_OPEN_CONFIRMATION:
+      case SSH2_MSG_CHANNEL_OPEN_FAILURE:
+      case SSH2_MSG_CHANNEL_CLOSE:
+      case SSH2_MSG_CHANNEL_WINDOW_ADJUST:
+      case SSH2_MSG_CHANNEL_DATA:
+      case SSH2_MSG_CHANNEL_EXTENDED_DATA:
+      case SSH2_MSG_CHANNEL_EOF:
+      case SSH2_MSG_CHANNEL_REQUEST:
+      case SSH2_MSG_CHANNEL_SUCCESS:
+      case SSH2_MSG_CHANNEL_FAILURE:
+        /*
+         * All these messages have the recipient channel id as the
+         * first uint32 field in the packet. Substitute the downstream
+         * channel id for our one and pass the packet downstream.
+         */
+        assert(pktlen >= 4);
+        upstream_id = GET_32BIT(pkt);
+        if ((chan = share_find_channel_by_upstream(cs, upstream_id)) != NULL) {
+            /*
+             * The normal case: this id refers to an open channel.
+             */
+            PUT_32BIT(pkt, chan->downstream_id);
+            send_packet_to_downstream(cs, type, pkt, pktlen, chan);
+
+            /*
+             * Update the channel state, for messages that need it.
+             */
+            if (type == SSH2_MSG_CHANNEL_OPEN_CONFIRMATION) {
+                if (chan->state == UNACKNOWLEDGED && pktlen >= 8) {
+                    share_channel_set_server_id(cs, chan, GET_32BIT(pkt+4),
+                                                OPEN);
+                    if (!cs->sock) {
+                        /* Retry cleaning up this connection, so that we
+                         * can send an immediate CLOSE on this channel for
+                         * which we now know the server id. */
+                        share_try_cleanup(cs);
+                    }
+                }
+            } else if (type == SSH2_MSG_CHANNEL_OPEN_FAILURE) {
+                ssh_delete_sharing_channel(cs->parent->ssh, chan->upstream_id);
+                share_remove_channel(cs, chan);
+            } else if (type == SSH2_MSG_CHANNEL_CLOSE) {
+                if (chan->state == SENT_CLOSE) {
+                    ssh_delete_sharing_channel(cs->parent->ssh,
+                                               chan->upstream_id);
+                    share_remove_channel(cs, chan);
+                    if (!cs->sock) {
+                        /* Retry cleaning up this connection, in case this
+                         * channel closure was the last thing we were
+                         * waiting for. */
+                        share_try_cleanup(cs);
+                    }
+                } else {
+                    chan->state = RCVD_CLOSE;
+                }
+            }
+        } else if ((xc = share_find_xchannel_by_upstream(cs, upstream_id))
+                   != NULL) {
+            /*
+             * The unusual case: this id refers to an xchannel. Add it
+             * to the xchannel's queue.
+             */
+            struct share_xchannel_message *msg;
+
+            msg = share_xchannel_add_message(xc, type, pktlen);
+            memcpy(msg->data, pkt, pktlen);
+
+            /* If the xchannel is dead, then also respond to it (which
+             * may involve deleting the channel). */
+            if (!xc->live)
+                share_dead_xchannel_respond(cs, xc);
+        }
+        break;
+
+      default:
+        assert(!"This packet type should never have come from ssh.c");
+        break;
+    }
+}
+
+static void share_got_pkt_from_downstream(struct ssh_sharing_connstate *cs,
+                                          int type,
+                                          unsigned char *pkt, int pktlen)
+{
+    char *request_name;
+    struct share_forwarding *fwd;
+    int id_pos;
+    unsigned old_id, new_id, server_id;
+    struct share_globreq *globreq;
+    struct share_channel *chan;
+    struct share_halfchannel *hc;
+    struct share_xchannel *xc;
+    char *err = NULL;
+
+    switch (type) {
+      case SSH2_MSG_DISCONNECT:
+        /*
+         * This message stops here: if downstream is disconnecting
+         * from us, that doesn't mean we want to disconnect from the
+         * SSH server. Close the downstream connection and start
+         * cleanup.
+         */
+        share_begin_cleanup(cs);
+        break;
+
+      case SSH2_MSG_GLOBAL_REQUEST:
+        /*
+         * The only global requests we understand are "tcpip-forward"
+         * and "cancel-tcpip-forward". Since those require us to
+         * maintain state, we must assume that other global requests
+         * will probably require that too, and so we don't forward on
+         * any request we don't understand.
+         */
+        request_name = getstring(pkt, pktlen);
+        if (request_name == NULL) {
+            err = dupprintf("Truncated GLOBAL_REQUEST packet");
+            goto confused;
+        }
+
+        if (!strcmp(request_name, "tcpip-forward")) {
+            int wantreplypos, orig_wantreply, port, ret;
+            char *host;
+
+            sfree(request_name);
+
+            /*
+             * Pick the packet apart to find the want_reply field and
+             * the host/port we're going to ask to listen on.
+             */
+            wantreplypos = getstring_size(pkt, pktlen);
+            if (wantreplypos < 0 || wantreplypos >= pktlen) {
+                err = dupprintf("Truncated GLOBAL_REQUEST packet");
+                goto confused;
+            }
+            orig_wantreply = pkt[wantreplypos];
+            port = getstring_size(pkt + (wantreplypos + 1),
+                                  pktlen - (wantreplypos + 1));
+            port += (wantreplypos + 1);
+            if (port < 0 || port > pktlen - 4) {
+                err = dupprintf("Truncated GLOBAL_REQUEST packet");
+                goto confused;
+            }
+            host = getstring(pkt + (wantreplypos + 1),
+                             pktlen - (wantreplypos + 1));
+            assert(host != NULL);
+            port = GET_32BIT(pkt + port);
+
+            /*
+             * See if we can allocate space in ssh.c's tree of remote
+             * port forwardings. If we can't, it's because another
+             * client sharing this connection has already allocated
+             * the identical port forwarding, so we take it on
+             * ourselves to manufacture a failure packet and send it
+             * back to downstream.
+             */
+            ret = ssh_alloc_sharing_rportfwd(cs->parent->ssh, host, port, cs);
+            if (!ret) {
+                if (orig_wantreply) {
+                    send_packet_to_downstream(cs, SSH2_MSG_REQUEST_FAILURE,
+                                              "", 0, NULL);
+                }
+            } else {
+                /*
+                 * We've managed to make space for this forwarding
+                 * locally. Pass the request on to the SSH server, but
+                 * set want_reply even if it wasn't originally set, so
+                 * that we know whether this forwarding needs to be
+                 * cleaned up if downstream goes away.
+                 */
+                int old_wantreply = pkt[wantreplypos];
+                pkt[wantreplypos] = 1;
+                ssh_send_packet_from_downstream
+                    (cs->parent->ssh, cs->id, type, pkt, pktlen,
+                     old_wantreply ? NULL : "upstream added want_reply flag");
+                fwd = share_add_forwarding(cs, host, port);
+                ssh_sharing_queue_global_request(cs->parent->ssh, cs);
+
+                if (fwd) {
+                    globreq = snew(struct share_globreq);
+                    globreq->next = NULL;
+                    if (cs->globreq_tail)
+                        cs->globreq_tail->next = globreq;
+                    else
+                        cs->globreq_head = globreq;
+                    globreq->fwd = fwd;
+                    globreq->want_reply = orig_wantreply;
+                    globreq->type = GLOBREQ_TCPIP_FORWARD;
+                }
+            }
+
+            sfree(host);
+        } else if (!strcmp(request_name, "cancel-tcpip-forward")) {
+            int wantreplypos, orig_wantreply, port;
+            char *host;
+            struct share_forwarding *fwd;
+
+            sfree(request_name);
+
+            /*
+             * Pick the packet apart to find the want_reply field and
+             * the host/port we're going to ask to listen on.
+             */
+            wantreplypos = getstring_size(pkt, pktlen);
+            if (wantreplypos < 0 || wantreplypos >= pktlen) {
+                err = dupprintf("Truncated GLOBAL_REQUEST packet");
+                goto confused;
+            }
+            orig_wantreply = pkt[wantreplypos];
+            port = getstring_size(pkt + (wantreplypos + 1),
+                                  pktlen - (wantreplypos + 1));
+            port += (wantreplypos + 1);
+            if (port < 0 || port > pktlen - 4) {
+                err = dupprintf("Truncated GLOBAL_REQUEST packet");
+                goto confused;
+            }
+            host = getstring(pkt + (wantreplypos + 1),
+                             pktlen - (wantreplypos + 1));
+            assert(host != NULL);
+            port = GET_32BIT(pkt + port);
+
+            /*
+             * Look up the existing forwarding with these details.
+             */
+            fwd = share_find_forwarding(cs, host, port);
+            if (!fwd) {
+                if (orig_wantreply) {
+                    send_packet_to_downstream(cs, SSH2_MSG_REQUEST_FAILURE,
+                                              "", 0, NULL);
+                }
+            } else {
+                /*
+                 * Pass the cancel request on to the SSH server, but
+                 * set want_reply even if it wasn't originally set, so
+                 * that _we_ know whether the forwarding has been
+                 * deleted even if downstream doesn't want to know.
+                 */
+                int old_wantreply = pkt[wantreplypos];
+                pkt[wantreplypos] = 1;
+                ssh_send_packet_from_downstream
+                    (cs->parent->ssh, cs->id, type, pkt, pktlen,
+                     old_wantreply ? NULL : "upstream added want_reply flag");
+                ssh_sharing_queue_global_request(cs->parent->ssh, cs);
+            }
+
+            sfree(host);
+        } else {
+            /*
+             * Request we don't understand. Manufacture a failure
+             * message if an answer was required.
+             */
+            int wantreplypos;
+
+            sfree(request_name);
+
+            wantreplypos = getstring_size(pkt, pktlen);
+            if (wantreplypos < 0 || wantreplypos >= pktlen) {
+                err = dupprintf("Truncated GLOBAL_REQUEST packet");
+                goto confused;
+            }
+            if (pkt[wantreplypos])
+                send_packet_to_downstream(cs, SSH2_MSG_REQUEST_FAILURE,
+                                          "", 0, NULL);
+        }
+        break;
+
+      case SSH2_MSG_CHANNEL_OPEN:
+        /* Sender channel id comes after the channel type string */
+        id_pos = getstring_size(pkt, pktlen);
+        if (id_pos < 0 || id_pos > pktlen - 12) {
+            err = dupprintf("Truncated CHANNEL_OPEN packet");
+            goto confused;
+        }
+
+        old_id = GET_32BIT(pkt + id_pos);
+        new_id = ssh_alloc_sharing_channel(cs->parent->ssh, cs);
+        share_add_channel(cs, old_id, new_id, 0, UNACKNOWLEDGED,
+                          GET_32BIT(pkt + id_pos + 8));
+        PUT_32BIT(pkt + id_pos, new_id);
+        ssh_send_packet_from_downstream(cs->parent->ssh, cs->id,
+                                        type, pkt, pktlen, NULL);
+        break;
+
+      case SSH2_MSG_CHANNEL_OPEN_CONFIRMATION:
+        if (pktlen < 16) {
+            err = dupprintf("Truncated CHANNEL_OPEN_CONFIRMATION packet");
+            goto confused;
+        }
+
+        id_pos = 4;  /* sender channel id is 2nd uint32 field in packet */
+        old_id = GET_32BIT(pkt + id_pos);
+
+        server_id = GET_32BIT(pkt);
+        /* This server id may refer to either a halfchannel or an xchannel. */
+        hc = NULL, xc = NULL;          /* placate optimiser */
+        if ((hc = share_find_halfchannel(cs, server_id)) != NULL) {
+            new_id = ssh_alloc_sharing_channel(cs->parent->ssh, cs);
+        } else if ((xc = share_find_xchannel_by_server(cs, server_id))
+                   != NULL) {
+            new_id = xc->upstream_id;
+        } else {
+            err = dupprintf("CHANNEL_OPEN_CONFIRMATION packet cited unknown channel %u", (unsigned)server_id);
+            goto confused;
+        }
+            
+        PUT_32BIT(pkt + id_pos, new_id);
+
+        chan = share_add_channel(cs, old_id, new_id, server_id, OPEN,
+                                 GET_32BIT(pkt + 12));
+
+        if (hc) {
+            ssh_send_packet_from_downstream(cs->parent->ssh, cs->id,
+                                            type, pkt, pktlen, NULL);
+            share_remove_halfchannel(cs, hc);
+        } else if (xc) {
+            unsigned downstream_window = GET_32BIT(pkt + 8);
+            if (downstream_window < 256) {
+                err = dupprintf("Initial window size for x11 channel must be at least 256 (got %u)", downstream_window);
+                goto confused;
+            }
+            share_xchannel_confirmation(cs, xc, chan, downstream_window);
+            share_remove_xchannel(cs, xc);
+        }
+
+        break;
+
+      case SSH2_MSG_CHANNEL_OPEN_FAILURE:
+        if (pktlen < 4) {
+            err = dupprintf("Truncated CHANNEL_OPEN_FAILURE packet");
+            goto confused;
+        }
+
+        server_id = GET_32BIT(pkt);
+        /* This server id may refer to either a halfchannel or an xchannel. */
+        if ((hc = share_find_halfchannel(cs, server_id)) != NULL) {
+            ssh_send_packet_from_downstream(cs->parent->ssh, cs->id,
+                                            type, pkt, pktlen, NULL);
+            share_remove_halfchannel(cs, hc);
+        } else if ((xc = share_find_xchannel_by_server(cs, server_id))
+                   != NULL) {
+            share_xchannel_failure(cs, xc);
+        } else {
+            err = dupprintf("CHANNEL_OPEN_FAILURE packet cited unknown channel %u", (unsigned)server_id);
+            goto confused;
+        }
+
+        break;
+
+      case SSH2_MSG_CHANNEL_WINDOW_ADJUST:
+      case SSH2_MSG_CHANNEL_DATA:
+      case SSH2_MSG_CHANNEL_EXTENDED_DATA:
+      case SSH2_MSG_CHANNEL_EOF:
+      case SSH2_MSG_CHANNEL_CLOSE:
+      case SSH2_MSG_CHANNEL_REQUEST:
+      case SSH2_MSG_CHANNEL_SUCCESS:
+      case SSH2_MSG_CHANNEL_FAILURE:
+      case SSH2_MSG_IGNORE:
+      case SSH2_MSG_DEBUG:
+        if (type == SSH2_MSG_CHANNEL_REQUEST &&
+            (request_name = getstring(pkt + 4, pktlen - 4)) != NULL) {
+            /*
+             * Agent forwarding requests from downstream are treated
+             * specially. Because OpenSSHD doesn't let us enable agent
+             * forwarding independently per session channel, and in
+             * particular because the OpenSSH-defined agent forwarding
+             * protocol does not mark agent-channel requests with the
+             * id of the session channel they originate from, the only
+             * way we can implement agent forwarding in a
+             * connection-shared PuTTY is to forward the _upstream_
+             * agent. Hence, we unilaterally deny agent forwarding
+             * requests from downstreams if we aren't prepared to
+             * forward an agent ourselves.
+             *
+             * (If we are, then we dutifully pass agent forwarding
+             * requests upstream. OpenSSHD has the curious behaviour
+             * that all but the first such request will be rejected,
+             * but all session channels opened after the first request
+             * get agent forwarding enabled whether they ask for it or
+             * not; but that's not our concern, since other SSH
+             * servers supporting the same piece of protocol might in
+             * principle at least manage to enable agent forwarding on
+             * precisely the channels that requested it, even if the
+             * subsequent CHANNEL_OPENs still can't be associated with
+             * a parent session channel.)
+             */
+            if (!strcmp(request_name, "auth-agent-req@openssh.com") &&
+                !ssh_agent_forwarding_permitted(cs->parent->ssh)) {
+                unsigned server_id = GET_32BIT(pkt);
+                unsigned char recipient_id[4];
+                chan = share_find_channel_by_server(cs, server_id);
+                if (chan) {
+                    PUT_32BIT(recipient_id, chan->downstream_id);
+                    send_packet_to_downstream(cs, SSH2_MSG_CHANNEL_FAILURE,
+                                              recipient_id, 4, NULL);
+                } else {
+                    char *buf = dupprintf("Agent forwarding request for "
+                                          "unrecognised channel %u", server_id);
+                    share_disconnect(cs, buf);
+                    sfree(buf);
+                    return;
+                }
+                break;
+            }
+
+            /*
+             * Another thing we treat specially is X11 forwarding
+             * requests. For these, we have to make up another set of
+             * X11 auth data, and enter it into our SSH connection's
+             * list of possible X11 authorisation credentials so that
+             * when we see an X11 channel open request we can know
+             * whether it's one to handle locally or one to pass on to
+             * a downstream, and if the latter, which one.
+             */
+            if (!strcmp(request_name, "x11-req")) {
+                unsigned server_id = GET_32BIT(pkt);
+                int want_reply, single_connection, screen;
+                char *auth_proto_str, *auth_data;
+                int auth_proto, protolen, datalen;
+                int pos;
+
+                chan = share_find_channel_by_server(cs, server_id);
+                if (!chan) {
+                    char *buf = dupprintf("X11 forwarding request for "
+                                          "unrecognised channel %u", server_id);
+                    share_disconnect(cs, buf);
+                    sfree(buf);
+                    return;
+                }
+
+                /*
+                 * Pick apart the whole message to find the downstream
+                 * auth details.
+                 */
+                /* we have already seen: 4 bytes channel id, 4+7 request name */
+                if (pktlen < 17) {
+                    err = dupprintf("Truncated CHANNEL_REQUEST(\"x11\") packet");
+                    goto confused;
+                }
+                want_reply = pkt[15] != 0;
+                single_connection = pkt[16] != 0;
+                auth_proto_str = getstring(pkt+17, pktlen-17);
+                pos = 17 + getstring_size(pkt+17, pktlen-17);
+                auth_data = getstring(pkt+pos, pktlen-pos);
+                pos += getstring_size(pkt+pos, pktlen-pos);
+                if (pktlen < pos+4) {
+                    err = dupprintf("Truncated CHANNEL_REQUEST(\"x11\") packet");
+                    goto confused;
+                }
+                screen = GET_32BIT(pkt+pos);
+
+                auth_proto = x11_identify_auth_proto(auth_proto_str);
+                if (auth_proto < 0) {
+                    /* Reject due to not understanding downstream's
+                     * requested authorisation method. */
+                    unsigned char recipient_id[4];
+                    PUT_32BIT(recipient_id, chan->downstream_id);
+                    send_packet_to_downstream(cs, SSH2_MSG_CHANNEL_FAILURE,
+                                              recipient_id, 4, NULL);
+                }
+
+                chan->x11_auth_proto = auth_proto;
+                chan->x11_auth_data = x11_dehexify(auth_data,
+                                                   &chan->x11_auth_datalen);
+                chan->x11_auth_upstream =
+                    ssh_sharing_add_x11_display(cs->parent->ssh, auth_proto,
+                                                cs, chan);
+                chan->x11_one_shot = single_connection;
+
+                /*
+                 * Now construct a replacement X forwarding request,
+                 * containing our own auth data, and send that to the
+                 * server.
+                 */
+                protolen = strlen(chan->x11_auth_upstream->protoname);
+                datalen = strlen(chan->x11_auth_upstream->datastring);
+                pktlen = 29+protolen+datalen;
+                pkt = snewn(pktlen, unsigned char);
+                PUT_32BIT(pkt, server_id);
+                PUT_32BIT(pkt+4, 7);   /* strlen("x11-req") */
+                memcpy(pkt+8, "x11-req", 7);
+                pkt[15] = want_reply;
+                pkt[16] = single_connection;
+                PUT_32BIT(pkt+17, protolen);
+                memcpy(pkt+21, chan->x11_auth_upstream->protoname, protolen);
+                PUT_32BIT(pkt+21+protolen, datalen);
+                memcpy(pkt+25+protolen, chan->x11_auth_upstream->datastring,
+                       datalen);
+                PUT_32BIT(pkt+25+protolen+datalen, screen);
+                ssh_send_packet_from_downstream(cs->parent->ssh, cs->id,
+                                                SSH2_MSG_CHANNEL_REQUEST,
+                                                pkt, pktlen, NULL);
+                sfree(pkt);
+
+                break;
+            }
+        }
+
+        ssh_send_packet_from_downstream(cs->parent->ssh, cs->id,
+                                        type, pkt, pktlen, NULL);
+        if (type == SSH2_MSG_CHANNEL_CLOSE && pktlen >= 4) {
+            server_id = GET_32BIT(pkt);
+            chan = share_find_channel_by_server(cs, server_id);
+            if (chan) {
+                if (chan->state == RCVD_CLOSE) {
+                    ssh_delete_sharing_channel(cs->parent->ssh,
+                                               chan->upstream_id);
+                    share_remove_channel(cs, chan);
+                } else {
+                    chan->state = SENT_CLOSE;
+                }
+            }
+        }
+        break;
+
+      default:
+        err = dupprintf("Unexpected packet type %d\n", type);
+        goto confused;
+
+        /*
+         * Any other packet type is unexpected. In particular, we
+         * never pass GLOBAL_REQUESTs downstream, so we never expect
+         * to see SSH2_MSG_REQUEST_{SUCCESS,FAILURE}.
+         */
+      confused:
+        assert(err != NULL);
+        share_disconnect(cs, err);
+        sfree(err);
+        break;
+    }
+}
+
+/*
+ * Coroutine macros similar to, but simplified from, those in ssh.c.
+ */
+#define crBegin(v)	{ int *crLine = &v; switch(v) { case 0:;
+#define crFinish(z)	} *crLine = 0; return (z); }
+#define crGetChar(c) do                                         \
+    {                                                           \
+        while (len == 0) {                                      \
+            *crLine =__LINE__; return 1; case __LINE__:;        \
+        }                                                       \
+        len--;                                                  \
+        (c) = (unsigned char)*data++;                           \
+    } while (0)
+
+static int share_receive(Plug plug, int urgent, char *data, int len)
+{
+    struct ssh_sharing_connstate *cs = (struct ssh_sharing_connstate *)plug;
+    static const char expected_verstring_prefix[] =
+        "SSHCONNECTION@putty.projects.tartarus.org-2.0-";
+    unsigned char c;
+
+    crBegin(cs->crLine);
+
+    /*
+     * First read the version string from downstream.
+     */
+    cs->recvlen = 0;
+    while (1) {
+        crGetChar(c);
+        if (c == '\012')
+            break;
+        if (cs->recvlen > sizeof(cs->recvbuf)) {
+            char *buf = dupprintf("Version string far too long\n");
+            share_disconnect(cs, buf);
+            sfree(buf);
+            goto dead;
+        }
+        cs->recvbuf[cs->recvlen++] = c;
+    }
+
+    /*
+     * Now parse the version string to make sure it's at least vaguely
+     * sensible, and log it.
+     */
+    if (cs->recvlen < sizeof(expected_verstring_prefix)-1 ||
+        memcmp(cs->recvbuf, expected_verstring_prefix,
+               sizeof(expected_verstring_prefix) - 1)) {
+        char *buf = dupprintf("Version string did not have expected prefix\n");
+        share_disconnect(cs, buf);
+        sfree(buf);
+        goto dead;
+    }
+    if (cs->recvlen > 0 && cs->recvbuf[cs->recvlen-1] == '\015')
+        cs->recvlen--;                 /* trim off \r before \n */
+    ssh_sharing_logf(cs->parent->ssh, cs->id,
+                     "Downstream version string: %.*s",
+                     cs->recvlen, cs->recvbuf);
+
+    /*
+     * Loop round reading packets.
+     */
+    while (1) {
+        cs->recvlen = 0;
+        while (cs->recvlen < 4) {
+            crGetChar(c);
+            cs->recvbuf[cs->recvlen++] = c;
+        }
+        cs->curr_packetlen = toint(GET_32BIT(cs->recvbuf) + 4);
+        if (cs->curr_packetlen < 5 ||
+            cs->curr_packetlen > sizeof(cs->recvbuf)) {
+            char *buf = dupprintf("Bad packet length %u\n",
+                                  (unsigned)cs->curr_packetlen);
+            share_disconnect(cs, buf);
+            sfree(buf);
+            goto dead;
+        }
+        while (cs->recvlen < cs->curr_packetlen) {
+            crGetChar(c);
+            cs->recvbuf[cs->recvlen++] = c;
+        }
+
+        share_got_pkt_from_downstream(cs, cs->recvbuf[4],
+                                      cs->recvbuf + 5, cs->recvlen - 5);
+    }
+
+  dead:;
+    crFinish(1);
+}
+
+static void share_sent(Plug plug, int bufsize)
+{
+    /* struct ssh_sharing_connstate *cs = (struct ssh_sharing_connstate *)plug; */
+
+    /*
+     * We do nothing here, because we expect that there won't be a
+     * need to throttle and unthrottle the connection to a downstream.
+     * It should automatically throttle itself: if the SSH server
+     * sends huge amounts of data on all channels then it'll run out
+     * of window until our downstream sends it back some
+     * WINDOW_ADJUSTs.
+     */
+}
+
+static int share_listen_closing(Plug plug, const char *error_msg,
+                                int error_code, int calling_back)
+{
+    struct ssh_sharing_state *sharestate = (struct ssh_sharing_state *)plug;
+    if (error_msg)
+        ssh_sharing_logf(sharestate->ssh, 0,
+                         "listening socket: %s", error_msg);
+    sk_close(sharestate->listensock);
+    return 1;
+}
+
+static void share_send_verstring(struct ssh_sharing_connstate *cs)
+{
+    char *fullstring = dupcat("SSHCONNECTION@putty.projects.tartarus.org-2.0-",
+                              cs->parent->server_verstring, "\015\012", NULL);
+    sk_write(cs->sock, fullstring, strlen(fullstring));
+    sfree(fullstring);
+
+    cs->sent_verstring = TRUE;
+}
+
+int share_ndownstreams(void *state)
+{
+    struct ssh_sharing_state *sharestate = (struct ssh_sharing_state *)state;
+    return count234(sharestate->connections);
+}
+
+void share_activate(void *state, const char *server_verstring)
+{
+    /*
+     * Indication from ssh.c that we are now ready to begin serving
+     * any downstreams that have already connected to us.
+     */
+    struct ssh_sharing_state *sharestate = (struct ssh_sharing_state *)state;
+    struct ssh_sharing_connstate *cs;
+    int i;
+
+    /*
+     * Trim the server's version string down to just the software
+     * version component, removing "SSH-2.0-" or whatever at the
+     * front.
+     */
+    for (i = 0; i < 2; i++) {
+        server_verstring += strcspn(server_verstring, "-");
+        if (*server_verstring)
+            server_verstring++;
+    }
+
+    sharestate->server_verstring = dupstr(server_verstring);
+
+    for (i = 0; (cs = (struct ssh_sharing_connstate *)
+                 index234(sharestate->connections, i)) != NULL; i++) {
+        assert(!cs->sent_verstring);
+        share_send_verstring(cs);
+    }
+}
+
+static int share_listen_accepting(Plug plug,
+                                  accept_fn_t constructor, accept_ctx_t ctx)
+{
+    static const struct plug_function_table connection_fn_table = {
+	NULL, /* no log function, because that's for outgoing connections */
+	share_closing,
+        share_receive,
+        share_sent,
+	NULL /* no accepting function, because we've already done it */
+    };
+    struct ssh_sharing_state *sharestate = (struct ssh_sharing_state *)plug;
+    struct ssh_sharing_connstate *cs;
+    const char *err;
+
+    /*
+     * A new downstream has connected to us.
+     */
+    cs = snew(struct ssh_sharing_connstate);
+    cs->fn = &connection_fn_table;
+    cs->parent = sharestate;
+
+    if ((cs->id = share_find_unused_id(sharestate, sharestate->nextid)) == 0 &&
+        (cs->id = share_find_unused_id(sharestate, 1)) == 0) {
+        sfree(cs);
+        return 1;
+    }
+    sharestate->nextid = cs->id + 1;
+    if (sharestate->nextid == 0)
+        sharestate->nextid++; /* only happens in VERY long-running upstreams */
+
+    cs->sock = constructor(ctx, (Plug) cs);
+    if ((err = sk_socket_error(cs->sock)) != NULL) {
+        sfree(cs);
+	return err != NULL;
+    }
+
+    sk_set_frozen(cs->sock, 0);
+
+    add234(cs->parent->connections, cs);
+
+    cs->sent_verstring = FALSE;
+    if (sharestate->server_verstring)
+        share_send_verstring(cs);
+
+    cs->got_verstring = FALSE;
+    cs->recvlen = 0;
+    cs->crLine = 0;
+    cs->halfchannels = newtree234(share_halfchannel_cmp);
+    cs->channels_by_us = newtree234(share_channel_us_cmp);
+    cs->channels_by_server = newtree234(share_channel_server_cmp);
+    cs->xchannels_by_us = newtree234(share_xchannel_us_cmp);
+    cs->xchannels_by_server = newtree234(share_xchannel_server_cmp);
+    cs->forwardings = newtree234(share_forwarding_cmp);
+    cs->globreq_head = cs->globreq_tail = NULL;
+
+    ssh_sharing_downstream_connected(sharestate->ssh, cs->id);
+
+    return 0;
+}
+
+/* Per-application overrides for what roles we can take (e.g. pscp
+ * will never be an upstream) */
+extern const int share_can_be_downstream;
+extern const int share_can_be_upstream;
+
+/*
+ * Init function for connection sharing. We either open a listening
+ * socket and become an upstream, or connect to an existing one and
+ * become a downstream, or do neither. We are responsible for deciding
+ * which of these to do (including checking the Conf to see if
+ * connection sharing is even enabled in the first place). If we
+ * become a downstream, we return the Socket with which we connected
+ * to the upstream; otherwise (whether or not we have established an
+ * upstream) we return NULL.
+ */
+Socket ssh_connection_sharing_init(const char *host, int port,
+                                   Conf *conf, Ssh ssh, void **state)
+{
+    static const struct plug_function_table listen_fn_table = {
+	NULL, /* no log function, because that's for outgoing connections */
+	share_listen_closing,
+        NULL, /* no receive function on a listening socket */
+        NULL, /* no sent function on a listening socket */
+	share_listen_accepting
+    };
+
+    int result, can_upstream, can_downstream;
+    char *logtext, *ds_err, *us_err;
+    char *sockname;
+    Socket sock;
+    struct ssh_sharing_state *sharestate;
+
+    if (!conf_get_int(conf, CONF_ssh_connection_sharing))
+        return NULL;                   /* do not share anything */
+    can_upstream = share_can_be_upstream &&
+        conf_get_int(conf, CONF_ssh_connection_sharing_upstream);
+    can_downstream = share_can_be_downstream &&
+        conf_get_int(conf, CONF_ssh_connection_sharing_downstream);
+    if (!can_upstream && !can_downstream)
+        return NULL;
+
+    /*
+     * Decide on the string used to identify the connection point
+     * between upstream and downstream (be it a Windows named pipe or
+     * a Unix-domain socket or whatever else).
+     *
+     * I wondered about making this a SHA hash of all sorts of pieces
+     * of the PuTTY configuration - essentially everything PuTTY uses
+     * to know where and how to make a connection, including all the
+     * proxy details (or rather, all the _relevant_ ones - only
+     * including settings that other settings didn't prevent from
+     * having any effect), plus the username. However, I think it's
+     * better to keep it really simple: the connection point
+     * identifier is derived from the hostname and port used to index
+     * the host-key cache (not necessarily where we _physically_
+     * connected to, in cases involving proxies or CONF_loghost), plus
+     * the username if one is specified.
+     */
+    {
+        char *username = get_remote_username(conf);
+
+        if (port == 22) {
+            if (username)
+                sockname = dupprintf("%s@%s", username, host);
+            else
+                sockname = dupprintf("%s", host);
+        } else {
+            if (username)
+                sockname = dupprintf("%s@%s:%d", username, host, port);
+            else
+                sockname = dupprintf("%s:%d", host, port);
+        }
+
+        sfree(username);
+
+        /*
+         * The platform-specific code may transform this further in
+         * order to conform to local namespace conventions (e.g. not
+         * using slashes in filenames), but that's its job and not
+         * ours.
+         */
+    }
+
+    /*
+     * Create a data structure for the listening plug if we turn out
+     * to be an upstream.
+     */
+    sharestate = snew(struct ssh_sharing_state);
+    sharestate->fn = &listen_fn_table;
+    sharestate->listensock = NULL;
+
+    /*
+     * Now hand off to a per-platform routine that either connects to
+     * an existing upstream (using 'ssh' as the plug), establishes our
+     * own upstream (using 'sharestate' as the plug), or forks off a
+     * separate upstream and then connects to that. It will return a
+     * code telling us which kind of socket it put in 'sock'.
+     */
+    sock = NULL;
+    logtext = ds_err = us_err = NULL;
+    result = platform_ssh_share(sockname, conf, (Plug)ssh,
+                                (Plug)sharestate, &sock, &logtext, &ds_err,
+                                &us_err, can_upstream, can_downstream);
+    ssh_connshare_log(ssh, result, logtext, ds_err, us_err);
+    sfree(logtext);
+    sfree(ds_err);
+    sfree(us_err);
+    switch (result) {
+      case SHARE_NONE:
+        /*
+         * We aren't sharing our connection at all (e.g. something
+         * went wrong setting the socket up). Free the upstream
+         * structure and return NULL.
+         */
+        assert(sock == NULL);
+        *state = NULL;
+        sfree(sharestate);
+        sfree(sockname);
+        return NULL;
+
+      case SHARE_DOWNSTREAM:
+        /*
+         * We are downstream, so free sharestate which it turns out we
+         * don't need after all, and return the downstream socket as a
+         * replacement for an ordinary SSH connection.
+         */
+        *state = NULL;
+        sfree(sharestate);
+        sfree(sockname);
+        return sock;
+
+      case SHARE_UPSTREAM:
+        /*
+         * We are upstream. Set up sharestate properly and pass a copy
+         * to the caller; return NULL, to tell ssh.c that it has to
+         * make an ordinary connection after all.
+         */
+        *state = sharestate;
+        sharestate->listensock = sock;
+        sharestate->connections = newtree234(share_connstate_cmp);
+        sharestate->ssh = ssh;
+        sharestate->server_verstring = NULL;
+        sharestate->sockname = dupstr(sockname);
+        sharestate->nextid = 1;
+        return NULL;
+    }
+
+    return NULL;
+}