Wrong length was causing spurious \0 to be sent during SSH negotiation with

[u/mdw/putty] / proxy.c

/*
 * Network proxy abstraction in PuTTY
 *
 * A proxy layer, if necessary, wedges itself between the network
 * code and the higher level backend.
 */

#include <assert.h>
#include <ctype.h>
#include <string.h>

#define DEFINE_PLUG_METHOD_MACROS
#include "putty.h"
#include "network.h"
#include "proxy.h"

/*
 * Call this when proxy negotiation is complete, so that this
 * socket can begin working normally.
 */
void proxy_activate (Proxy_Socket p)
{
    void *data;
    int len;
    long output_before, output_after;
    
    p->state = PROXY_STATE_ACTIVE;

    /* we want to ignore new receive events until we have sent
     * all of our buffered receive data.
     */
    sk_set_frozen(p->sub_socket, 1);

    /* how many bytes of output have we buffered? */
    output_before = bufchain_size(&p->pending_oob_output_data) +
        bufchain_size(&p->pending_output_data);
    /* and keep track of how many bytes do not get sent. */
    output_after = 0;
    
    /* send buffered OOB writes */
    while (bufchain_size(&p->pending_oob_output_data) > 0) {
        bufchain_prefix(&p->pending_oob_output_data, &data, &len);
        output_after += sk_write_oob(p->sub_socket, data, len);
        bufchain_consume(&p->pending_oob_output_data, len);
    }

    /* send buffered normal writes */
    while (bufchain_size(&p->pending_output_data) > 0) {
        bufchain_prefix(&p->pending_output_data, &data, &len);
        output_after += sk_write(p->sub_socket, data, len);
        bufchain_consume(&p->pending_output_data, len);
    }

    /* if we managed to send any data, let the higher levels know. */
    if (output_after < output_before)
        plug_sent(p->plug, output_after);

    /* if we were asked to flush the output during
     * the proxy negotiation process, do so now.
     */
    if (p->pending_flush) sk_flush(p->sub_socket);

    /* if the backend wanted the socket unfrozen, try to unfreeze.
     * our set_frozen handler will flush buffered receive data before
     * unfreezing the actual underlying socket.
     */
    if (!p->freeze)
        sk_set_frozen((Socket)p, 0);
}

/* basic proxy socket functions */

static Plug sk_proxy_plug (Socket s, Plug p)
{
    Proxy_Socket ps = (Proxy_Socket) s;
    Plug ret = ps->plug;
    if (p)
        ps->plug = p;
    return ret;
}

static void sk_proxy_close (Socket s)
{
    Proxy_Socket ps = (Proxy_Socket) s;

    sk_close(ps->sub_socket);
    sfree(ps);
}

static int sk_proxy_write (Socket s, char *data, int len)
{
    Proxy_Socket ps = (Proxy_Socket) s;

    if (ps->state != PROXY_STATE_ACTIVE) {
        bufchain_add(&ps->pending_output_data, data, len);
        return bufchain_size(&ps->pending_output_data);
    }
    return sk_write(ps->sub_socket, data, len);
}

static int sk_proxy_write_oob (Socket s, char *data, int len)
{
    Proxy_Socket ps = (Proxy_Socket) s;

    if (ps->state != PROXY_STATE_ACTIVE) {
        bufchain_clear(&ps->pending_output_data);
        bufchain_clear(&ps->pending_oob_output_data);
        bufchain_add(&ps->pending_oob_output_data, data, len);
        return len;
    }
    return sk_write_oob(ps->sub_socket, data, len);
}

static void sk_proxy_flush (Socket s)
{
    Proxy_Socket ps = (Proxy_Socket) s;

    if (ps->state != PROXY_STATE_ACTIVE) {
        ps->pending_flush = 1;
        return;
    }
    sk_flush(ps->sub_socket);
}

static void sk_proxy_set_private_ptr (Socket s, void *ptr)
{
    Proxy_Socket ps = (Proxy_Socket) s;
    sk_set_private_ptr(ps->sub_socket, ptr);
}

static void * sk_proxy_get_private_ptr (Socket s)
{
    Proxy_Socket ps = (Proxy_Socket) s;
    return sk_get_private_ptr(ps->sub_socket);
}

static void sk_proxy_set_frozen (Socket s, int is_frozen)
{
    Proxy_Socket ps = (Proxy_Socket) s;

    if (ps->state != PROXY_STATE_ACTIVE) {
        ps->freeze = is_frozen;
        return;
    }
    
    /* handle any remaining buffered recv data first */
    if (bufchain_size(&ps->pending_input_data) > 0) {
        ps->freeze = is_frozen;

        /* loop while we still have buffered data, and while we are
         * unfrozen. the plug_receive call in the loop could result 
         * in a call back into this function refreezing the socket, 
         * so we have to check each time.
         */
        while (!ps->freeze && bufchain_size(&ps->pending_input_data) > 0) {
            void *data;
            int len;
            bufchain_prefix(&ps->pending_input_data, &data, &len);
            plug_receive(ps->plug, 0, data, len);
            bufchain_consume(&ps->pending_input_data, len);
        }

        /* if we're still frozen, we'll have to wait for another
         * call from the backend to finish unbuffering the data.
         */
        if (ps->freeze) return;
    }
    
    sk_set_frozen(ps->sub_socket, is_frozen);
}

static char * sk_proxy_socket_error (Socket s)
{
    Proxy_Socket ps = (Proxy_Socket) s;
    if (ps->error != NULL || ps->sub_socket == NULL) {
        return ps->error;
    }
    return sk_socket_error(ps->sub_socket);
}

/* basic proxy plug functions */

static int plug_proxy_closing (Plug p, char *error_msg,
                               int error_code, int calling_back)
{
    Proxy_Plug pp = (Proxy_Plug) p;
    Proxy_Socket ps = pp->proxy_socket;

    if (ps->state != PROXY_STATE_ACTIVE) {
        ps->closing_error_msg = error_msg;
        ps->closing_error_code = error_code;
        ps->closing_calling_back = calling_back;
        return ps->negotiate(ps, PROXY_CHANGE_CLOSING);
    }
    return plug_closing(ps->plug, error_msg,
                        error_code, calling_back);
}

static int plug_proxy_receive (Plug p, int urgent, char *data, int len)
{
    Proxy_Plug pp = (Proxy_Plug) p;
    Proxy_Socket ps = pp->proxy_socket;

    if (ps->state != PROXY_STATE_ACTIVE) {
        /* we will lose the urgentness of this data, but since most,
         * if not all, of this data will be consumed by the negotiation
         * process, hopefully it won't affect the protocol above us
         */
        bufchain_add(&ps->pending_input_data, data, len);
        ps->receive_urgent = urgent;
        ps->receive_data = data;
        ps->receive_len = len;
        return ps->negotiate(ps, PROXY_CHANGE_RECEIVE);
    }
    return plug_receive(ps->plug, urgent, data, len);
}

static void plug_proxy_sent (Plug p, int bufsize)
{
    Proxy_Plug pp = (Proxy_Plug) p;
    Proxy_Socket ps = pp->proxy_socket;

    if (ps->state != PROXY_STATE_ACTIVE) {
        ps->sent_bufsize = bufsize;
        ps->negotiate(ps, PROXY_CHANGE_SENT);
        return;
    }
    plug_sent(ps->plug, bufsize);
}

static int plug_proxy_accepting (Plug p, void *sock)
{
    Proxy_Plug pp = (Proxy_Plug) p;
    Proxy_Socket ps = pp->proxy_socket;

    if (ps->state != PROXY_STATE_ACTIVE) {
        ps->accepting_sock = sock;
        return ps->negotiate(ps, PROXY_CHANGE_ACCEPTING);
    }
    return plug_accepting(ps->plug, sock);
}

static int proxy_for_destination (SockAddr addr, char * hostname, int port)
{
    int s = 0, e = 0;
    char hostip[64];
    int hostip_len, hostname_len;
    char * exclude_list;

    /* we want a string representation of the IP address for comparisons */
    sk_getaddr(addr, hostip, 64);

    hostip_len = strlen(hostip);
    hostname_len = strlen(hostname);

    exclude_list = cfg.proxy_exclude_list;

    /* now parse the exclude list, and see if either our IP
     * or hostname matches anything in it.
     */

    while (exclude_list[s]) {
        while (exclude_list[s] &&
               (isspace(exclude_list[s]) ||
                exclude_list[s] == ',')) s++;

        if (!exclude_list[s]) break;

        e = s;

        while (exclude_list[e] &&
               (isalnum(exclude_list[e]) ||
                exclude_list[e] == '-' ||
                exclude_list[e] == '.' ||
                exclude_list[e] == '*')) e++;

        if (exclude_list[s] == '*') {
            /* wildcard at beginning of entry */

            if (strnicmp(hostip + hostip_len - (e - s - 1),
                         exclude_list + s + 1, e - s - 1) == 0 ||
                strnicmp(hostname + hostname_len - (e - s - 1),
                         exclude_list + s + 1, e - s - 1) == 0)
                return 0; /* IP/hostname range excluded. do not use proxy. */

        } else if (exclude_list[e-1] == '*') {
            /* wildcard at end of entry */

            if (strnicmp(hostip, exclude_list + s, e - s - 1) == 0 ||
                strnicmp(hostname, exclude_list + s, e - s - 1) == 0)
                return 0; /* IP/hostname range excluded. do not use proxy. */

        } else {
            /* no wildcard at either end, so let's try an absolute
             * match (ie. a specific IP)
             */

            if (stricmp(hostip, exclude_list + s) == 0)
                return 0; /* IP/hostname excluded. do not use proxy. */
            if (stricmp(hostname, exclude_list + s) == 0)
                return 0; /* IP/hostname excluded. do not use proxy. */
        }

        s = e;
    }

    /* no matches in the exclude list, so use the proxy */
    return 1;
}

Socket new_connection(SockAddr addr, char *hostname,
                      int port, int privport,
                      int oobinline, int nodelay, Plug plug)
{
    static const struct socket_function_table socket_fn_table = {
        sk_proxy_plug,
        sk_proxy_close,
        sk_proxy_write,
        sk_proxy_write_oob,
        sk_proxy_flush,
        sk_proxy_set_private_ptr,
        sk_proxy_get_private_ptr,
        sk_proxy_set_frozen,
        sk_proxy_socket_error
    };

    static const struct plug_function_table plug_fn_table = {
        plug_proxy_closing,
        plug_proxy_receive,
        plug_proxy_sent,
        plug_proxy_accepting
    };

    if (cfg.proxy_type != PROXY_NONE &&
        proxy_for_destination(addr, hostname, port))
    {
        Proxy_Socket ret;
        Proxy_Plug pplug;
        SockAddr proxy_addr;
        char * proxy_canonical_name;

        ret = smalloc(sizeof(struct Socket_proxy_tag));
        ret->fn = &socket_fn_table;
        ret->plug = plug;
        ret->remote_addr = addr;
        ret->remote_port = port;

        ret->error = NULL;
        ret->pending_flush = 0;
        ret->freeze = 0;

        bufchain_init(&ret->pending_input_data);
        bufchain_init(&ret->pending_output_data);
        bufchain_init(&ret->pending_oob_output_data);

        ret->sub_socket = NULL;
        ret->state = PROXY_STATE_NEW;
        ret->negotiate = NULL;
        
        if (cfg.proxy_type == PROXY_HTTP) {
            ret->negotiate = proxy_http_negotiate;
        } else if (cfg.proxy_type == PROXY_SOCKS) {
            if (cfg.proxy_socks_version == 4)
                ret->negotiate = proxy_socks4_negotiate;
            else
                ret->negotiate = proxy_socks5_negotiate;
        } else if (cfg.proxy_type == PROXY_TELNET) {
            ret->negotiate = proxy_telnet_negotiate;
        } else {
            ret->error = "Proxy error: Unknown proxy method";
            return (Socket) ret;
        }

        /* create the proxy plug to map calls from the actual
         * socket into our proxy socket layer */
        pplug = smalloc(sizeof(struct Plug_proxy_tag));
        pplug->fn = &plug_fn_table;
        pplug->proxy_socket = ret;

        /* look-up proxy */
        proxy_addr = sk_namelookup(cfg.proxy_host,
                                   &proxy_canonical_name);
        sfree(proxy_canonical_name);

        /* create the actual socket we will be using,
         * connected to our proxy server and port.
         */
        ret->sub_socket = sk_new(proxy_addr, cfg.proxy_port,
                                 privport, oobinline,
                                 nodelay, (Plug) pplug);
        if (sk_socket_error(ret->sub_socket) != NULL)
            return (Socket) ret;

        sk_addr_free(proxy_addr);

        /* start the proxy negotiation process... */
        sk_set_frozen(ret->sub_socket, 0);
        ret->negotiate(ret, PROXY_CHANGE_NEW);

        return (Socket) ret;
    }

    /* no proxy, so just return the direct socket */
    return sk_new(addr, port, privport, oobinline, nodelay, plug);
}

Socket new_listener(int port, Plug plug, int local_host_only)
{
    /* TODO: SOCKS (and potentially others) support inbound
     * TODO: connections via the proxy. support them.
     */

    return sk_newlistener(port, plug, local_host_only);
}

/* ----------------------------------------------------------------------
 * HTTP CONNECT proxy type.
 */

static int get_line_end (char * data, int len)
{
    int off = 0;

    while (off < len)
    {
        if (data[off] == '\n') {
            /* we have a newline */
            off++;

            /* is that the only thing on this line? */
            if (off <= 2) return off;

            /* if not, then there is the possibility that this header
             * continues onto the next line, if it starts with a space
             * or a tab.
             */

            if (off + 1 < len &&
                data[off+1] != ' ' &&
                data[off+1] != '\t') return off;

            /* the line does continue, so we have to keep going
             * until we see an the header's "real" end of line.
             */
            off++;
        }

        off++;
    }

    return -1;
}

int proxy_http_negotiate (Proxy_Socket p, int change)
{
    if (p->state == PROXY_STATE_NEW) {
        /* we are just beginning the proxy negotiate process,
         * so we'll send off the initial bits of the request.
         * for this proxy method, it's just a simple HTTP
         * request
         */
        char *buf, dest[64];

        sk_getaddr(p->remote_addr, dest, 64);

        buf = dupprintf("CONNECT %s:%i HTTP/1.1\r\nHost: %s:%i\r\n",
                        dest, p->remote_port, dest, p->remote_port);
        sk_write(p->sub_socket, buf, strlen(buf));
        sfree(buf);

        if (cfg.proxy_username[0] || cfg.proxy_password[0]) {
            char buf[sizeof(cfg.proxy_username)+sizeof(cfg.proxy_password)];
            char buf2[sizeof(buf)*4/3 + 100];
            int i, j, len;
            sprintf(buf, "%s:%s", cfg.proxy_username, cfg.proxy_password);
            len = strlen(buf);
            sprintf(buf2, "Proxy-Authorization: basic ");
            for (i = 0, j = strlen(buf2); i < len; i += 3, j += 4)
                base64_encode_atom(buf+i, (len-i > 3 ? 3 : len-i), buf2+j);
            strcpy(buf2+j, "\r\n");
            sk_write(p->sub_socket, buf2, strlen(buf2));
        }

        sk_write(p->sub_socket, "\r\n", 2);

        p->state = 1;
        return 0;
    }

    if (change == PROXY_CHANGE_CLOSING) {
        /* if our proxy negotiation process involves closing and opening
         * new sockets, then we would want to intercept this closing
         * callback when we were expecting it. if we aren't anticipating
         * a socket close, then some error must have occurred. we'll
         * just pass those errors up to the backend.
         */
        return plug_closing(p->plug, p->closing_error_msg,
                            p->closing_error_code,
                            p->closing_calling_back);
    }

    if (change == PROXY_CHANGE_SENT) {
        /* some (or all) of what we wrote to the proxy was sent.
         * we don't do anything new, however, until we receive the
         * proxy's response. we might want to set a timer so we can
         * timeout the proxy negotiation after a while...
         */
        return 0;
    }

    if (change == PROXY_CHANGE_ACCEPTING) {
        /* we should _never_ see this, as we are using our socket to
         * connect to a proxy, not accepting inbound connections.
         * what should we do? close the socket with an appropriate
         * error message?
         */
        return plug_accepting(p->plug, p->accepting_sock);
    }

    if (change == PROXY_CHANGE_RECEIVE) {
        /* we have received data from the underlying socket, which
         * we'll need to parse, process, and respond to appropriately.
         */

        char *data, *datap;
        int len;
        int eol;

        if (p->state == 1) {

            int min_ver, maj_ver, status;

            /* get the status line */
            len = bufchain_size(&p->pending_input_data);
            assert(len > 0);           /* or we wouldn't be here */
            data = smalloc(len);
            bufchain_fetch(&p->pending_input_data, data, len);

            eol = get_line_end(data, len);
            if (eol < 0) {
                sfree(data);
                return 1;
            }

            status = -1;
            /* We can't rely on whether the %n incremented the sscanf return */
            if (sscanf((char *)data, "HTTP/%i.%i %n",
                       &maj_ver, &min_ver, &status) < 2 || status == -1) {
                plug_closing(p->plug, "Proxy error: HTTP response was absent",
                             PROXY_ERROR_GENERAL, 0);
                sfree(data);
                return 1;
            }

            /* remove the status line from the input buffer. */
            bufchain_consume(&p->pending_input_data, eol);
            if (data[status] != '2') {
                /* error */
                char *buf;
                data[eol] = '\0';
                while (eol > status &&
                       (data[eol-1] == '\r' || data[eol-1] == '\n'))
                    data[--eol] = '\0';
                buf = dupprintf("Proxy error: %s", data+status);
                plug_closing(p->plug, buf, PROXY_ERROR_GENERAL, 0);
                sfree(buf);
                sfree(data);
                return 1;
            }

            sfree(data);

            p->state = 2;
        }

        if (p->state == 2) {

            /* get headers. we're done when we get a
             * header of length 2, (ie. just "\r\n")
             */

            len = bufchain_size(&p->pending_input_data);
            assert(len > 0);           /* or we wouldn't be here */
            data = smalloc(len);
            datap = data;
            bufchain_fetch(&p->pending_input_data, data, len);

            eol = get_line_end(datap, len);
            if (eol < 0) {
                sfree(data);
                return 1;
            }
            while (eol > 2)
            {
                bufchain_consume(&p->pending_input_data, eol);
                datap += eol;
                len   -= eol;
                eol = get_line_end(datap, len);
            }

            if (eol == 2) {
                /* we're done */
                bufchain_consume(&p->pending_input_data, 2);
                proxy_activate(p);
                /* proxy activate will have dealt with
                 * whatever is left of the buffer */
                sfree(data);
                return 1;
            }

            sfree(data);
            return 1;
        }
    }

    plug_closing(p->plug, "Proxy error: unexpected proxy error",
                 PROXY_ERROR_UNEXPECTED, 0);
    return 1;
}

/* ----------------------------------------------------------------------
 * SOCKS proxy type.
 */

/* SOCKS version 4 */
int proxy_socks4_negotiate (Proxy_Socket p, int change)
{
    if (p->state == PROXY_CHANGE_NEW) {

        /* request format:
         *  version number (1 byte) = 4
         *  command code (1 byte)
         *    1 = CONNECT
         *    2 = BIND
         *  dest. port (2 bytes) [network order]
         *  dest. address (4 bytes)
         *  user ID (variable length, null terminated string)
         */

        int length;
        char * command;

        if (sk_addrtype(p->remote_addr) != ADDRTYPE_IPV4) {
            plug_closing(p->plug, "Proxy error: SOCKS version 4 does"
                         " not support IPv6", PROXY_ERROR_GENERAL, 0);
            return 1;
        }

        length = strlen(cfg.proxy_username) + 9;
        command = (char*) smalloc(length);
        strcpy(command + 8, cfg.proxy_username);

        command[0] = 4; /* version 4 */
        command[1] = 1; /* CONNECT command */

        /* port */
        command[2] = (char) (p->remote_port >> 8) & 0xff;
        command[3] = (char) p->remote_port & 0xff;

        /* address */
        sk_addrcopy(p->remote_addr, command + 4);

        sk_write(p->sub_socket, command, length);
        sfree(command);

        p->state = 1;
        return 0;
    }

    if (change == PROXY_CHANGE_CLOSING) {
        /* if our proxy negotiation process involves closing and opening
         * new sockets, then we would want to intercept this closing
         * callback when we were expecting it. if we aren't anticipating
         * a socket close, then some error must have occurred. we'll
         * just pass those errors up to the backend.
         */
        return plug_closing(p->plug, p->closing_error_msg,
                            p->closing_error_code,
                            p->closing_calling_back);
    }

    if (change == PROXY_CHANGE_SENT) {
        /* some (or all) of what we wrote to the proxy was sent.
         * we don't do anything new, however, until we receive the
         * proxy's response. we might want to set a timer so we can
         * timeout the proxy negotiation after a while...
         */
        return 0;
    }

    if (change == PROXY_CHANGE_ACCEPTING) {
        /* we should _never_ see this, as we are using our socket to
         * connect to a proxy, not accepting inbound connections.
         * what should we do? close the socket with an appropriate
         * error message?
         */
        return plug_accepting(p->plug, p->accepting_sock);
    }

    if (change == PROXY_CHANGE_RECEIVE) {
        /* we have received data from the underlying socket, which
         * we'll need to parse, process, and respond to appropriately.
         */

        if (p->state == 1) {
            /* response format:
             *  version number (1 byte) = 4
             *  reply code (1 byte)
             *    90 = request granted
             *    91 = request rejected or failed
             *    92 = request rejected due to lack of IDENTD on client
             *    93 = request rejected due to difference in user ID 
             *         (what we sent vs. what IDENTD said)
             *  dest. port (2 bytes)
             *  dest. address (4 bytes)
             */

            char data[8];

            if (bufchain_size(&p->pending_input_data) < 8)
                return 1;              /* not got anything yet */
            
            /* get the response */
            bufchain_fetch(&p->pending_input_data, data, 8);

            if (data[0] != 0) {
                plug_closing(p->plug, "Proxy error: SOCKS proxy responded with "
                                      "unexpected reply code version",
                             PROXY_ERROR_GENERAL, 0);
                return 1;
            }

            if (data[1] != 90) {

                switch (data[1]) {
                  case 92:
                    plug_closing(p->plug, "Proxy error: SOCKS server wanted IDENTD on client",
                                 PROXY_ERROR_GENERAL, 0);
                    break;
                  case 93:
                    plug_closing(p->plug, "Proxy error: Username and IDENTD on client don't agree",
                                 PROXY_ERROR_GENERAL, 0);
                    break;
                  case 91:
                  default:
                    plug_closing(p->plug, "Proxy error: Error while communicating with proxy",
                                 PROXY_ERROR_GENERAL, 0);
                    break;
                }

                return 1;
            }
            bufchain_consume(&p->pending_input_data, 8);

            /* we're done */
            proxy_activate(p);
            /* proxy activate will have dealt with
             * whatever is left of the buffer */
            return 1;
        }
    }

    plug_closing(p->plug, "Proxy error: unexpected proxy error",
                 PROXY_ERROR_UNEXPECTED, 0);
    return 1;
}

/* SOCKS version 5 */
int proxy_socks5_negotiate (Proxy_Socket p, int change)
{
    if (p->state == PROXY_CHANGE_NEW) {

        /* initial command:
         *  version number (1 byte) = 5
         *  number of available authentication methods (1 byte)
         *  available authentication methods (1 byte * previous value)
         *    authentication methods:
         *     0x00 = no authentication
         *     0x01 = GSSAPI
         *     0x02 = username/password
         *     0x03 = CHAP
         */

        char command[4];
        int len;

        command[0] = 5; /* version 5 */
        if (cfg.proxy_username[0] || cfg.proxy_password[0]) {
            command[1] = 2;            /* two methods supported: */
            command[2] = 0x00;         /* no authentication */
            command[3] = 0x02;         /* username/password */
            len = 4;
        } else {
            command[1] = 1;            /* one methods supported: */
            command[2] = 0x00;         /* no authentication */
            len = 3;
        }

        sk_write(p->sub_socket, command, len);

        p->state = 1;
        return 0;
    }

    if (change == PROXY_CHANGE_CLOSING) {
        /* if our proxy negotiation process involves closing and opening
         * new sockets, then we would want to intercept this closing
         * callback when we were expecting it. if we aren't anticipating
         * a socket close, then some error must have occurred. we'll
         * just pass those errors up to the backend.
         */
        return plug_closing(p->plug, p->closing_error_msg,
                            p->closing_error_code,
                            p->closing_calling_back);
    }

    if (change == PROXY_CHANGE_SENT) {
        /* some (or all) of what we wrote to the proxy was sent.
         * we don't do anything new, however, until we receive the
         * proxy's response. we might want to set a timer so we can
         * timeout the proxy negotiation after a while...
         */
        return 0;
    }

    if (change == PROXY_CHANGE_ACCEPTING) {
        /* we should _never_ see this, as we are using our socket to
         * connect to a proxy, not accepting inbound connections.
         * what should we do? close the socket with an appropriate
         * error message?
         */
        return plug_accepting(p->plug, p->accepting_sock);
    }

    if (change == PROXY_CHANGE_RECEIVE) {
        /* we have received data from the underlying socket, which
         * we'll need to parse, process, and respond to appropriately.
         */

        if (p->state == 1) {

            /* initial response:
             *  version number (1 byte) = 5
             *  authentication method (1 byte)
             *    authentication methods:
             *     0x00 = no authentication
             *     0x01 = GSSAPI
             *     0x02 = username/password 
             *     0x03 = CHAP
             *     0xff = no acceptable methods
             */
            char data[2];

            if (bufchain_size(&p->pending_input_data) < 2)
                return 1;              /* not got anything yet */

            /* get the response */
            bufchain_fetch(&p->pending_input_data, data, 2);

            if (data[0] != 5) {
                plug_closing(p->plug, "Proxy error: SOCKS proxy returned unexpected version",
                             PROXY_ERROR_GENERAL, 0);
                return 1;
            }

            if (data[1] == 0x00) p->state = 2; /* no authentication needed */
            else if (data[1] == 0x01) p->state = 4; /* GSSAPI authentication */
            else if (data[1] == 0x02) p->state = 5; /* username/password authentication */
            else if (data[1] == 0x03) p->state = 6; /* CHAP authentication */
            else {
                plug_closing(p->plug, "Proxy error: SOCKS proxy did not accept our authentication",
                             PROXY_ERROR_GENERAL, 0);
                return 1;
            }
            bufchain_consume(&p->pending_input_data, 2);
        }

        if (p->state == 7) {

            /* password authentication reply format:
             *  version number (1 bytes) = 1
             *  reply code (1 byte)
             *    0 = succeeded
             *    >0 = failed
             */
            char data[2];

            if (bufchain_size(&p->pending_input_data) < 2)
                return 1;              /* not got anything yet */

            /* get the response */
            bufchain_fetch(&p->pending_input_data, data, 2);

            if (data[0] != 1) {
                plug_closing(p->plug, "Proxy error: SOCKS password "
                             "subnegotiation contained wrong version number",
                             PROXY_ERROR_GENERAL, 0);
                return 1;
            }

            if (data[1] != 0) {

                plug_closing(p->plug, "Proxy error: SOCKS proxy refused"
                             " password authentication",
                             PROXY_ERROR_GENERAL, 0);
                return 1;
            }

            bufchain_consume(&p->pending_input_data, 2);
            p->state = 2;              /* now proceed as authenticated */
        }

        if (p->state == 2) {

            /* request format:
             *  version number (1 byte) = 5
             *  command code (1 byte)
             *    1 = CONNECT
             *    2 = BIND
             *    3 = UDP ASSOCIATE
             *  reserved (1 byte) = 0x00
             *  address type (1 byte)
             *    1 = IPv4
             *    3 = domainname (first byte has length, no terminating null)
             *    4 = IPv6
             *  dest. address (variable)
             *  dest. port (2 bytes) [network order]
             */

            char command[22];
            int len;

            if (sk_addrtype(p->remote_addr) == ADDRTYPE_IPV6) {
                len = 10;
                command[3] = 1; /* IPv4 */
            } else {
                len = 22;
                command[3] = 4; /* IPv6 */
            }

            command[0] = 5; /* version 5 */
            command[1] = 1; /* CONNECT command */
            command[2] = 0x00;

            /* address */
            sk_addrcopy(p->remote_addr, command+4);

            /* port */
            command[len-2] = (char) (p->remote_port >> 8) & 0xff;
            command[len-1] = (char) p->remote_port & 0xff;

            sk_write(p->sub_socket, command, len);

            p->state = 3;
            return 1;
        }

        if (p->state == 3) {

            /* reply format:
             *  version number (1 bytes) = 5
             *  reply code (1 byte)
             *    0 = succeeded
             *    1 = general SOCKS server failure
             *    2 = connection not allowed by ruleset
             *    3 = network unreachable
             *    4 = host unreachable
             *    5 = connection refused
             *    6 = TTL expired
             *    7 = command not supported
             *    8 = address type not supported
             * reserved (1 byte) = x00
             * address type (1 byte)
             *    1 = IPv4
             *    3 = domainname (first byte has length, no terminating null)
             *    4 = IPv6
             * server bound address (variable)
             * server bound port (2 bytes) [network order]
             */
            char data[5];
            int len;

            /* First 5 bytes of packet are enough to tell its length. */ 
            if (bufchain_size(&p->pending_input_data) < 5)
                return 1;              /* not got anything yet */

            /* get the response */
            bufchain_fetch(&p->pending_input_data, data, 5);

            if (data[0] != 5) {
                plug_closing(p->plug, "Proxy error: SOCKS proxy returned wrong version number",
                             PROXY_ERROR_GENERAL, 0);
                return 1;
            }

            if (data[1] != 0) {
                char buf[256];

                strcpy(buf, "Proxy error: ");

                switch (data[1]) {
                  case 1: strcat(buf, "General SOCKS server failure"); break;
                  case 2: strcat(buf, "Connection not allowed by ruleset"); break;
                  case 3: strcat(buf, "Network unreachable"); break;
                  case 4: strcat(buf, "Host unreachable"); break;
                  case 5: strcat(buf, "Connection refused"); break;
                  case 6: strcat(buf, "TTL expired"); break;
                  case 7: strcat(buf, "Command not supported"); break;
                  case 8: strcat(buf, "Address type not supported"); break;
                  default: sprintf(buf+strlen(buf),
                                   "Unrecognised SOCKS error code %d",
                                   data[1]);
                    break;
                }
                plug_closing(p->plug, buf, PROXY_ERROR_GENERAL, 0);

                return 1;
            }

            /*
             * Eat the rest of the reply packet.
             */
            len = 6;                   /* first 4 bytes, last 2 */
            switch (data[3]) {
              case 1: len += 4; break; /* IPv4 address */
              case 4: len += 16; break;/* IPv6 address */
              case 3: len += (unsigned char)data[4]; break; /* domain name */
              default:
                plug_closing(p->plug, "Proxy error: SOCKS proxy returned "
                             "unrecognised address format",
                             PROXY_ERROR_GENERAL, 0);
                return 1;
            }
            if (bufchain_size(&p->pending_input_data) < len)
                return 1;              /* not got whole reply yet */
            bufchain_consume(&p->pending_input_data, len);

            /* we're done */
            proxy_activate(p);
            return 1;
        }

        if (p->state == 4) {
            /* TODO: Handle GSSAPI authentication */
            plug_closing(p->plug, "Proxy error: We don't support GSSAPI authentication",
                         PROXY_ERROR_GENERAL, 0);
            return 1;
        }

        if (p->state == 5) {
            if (cfg.proxy_username[0] || cfg.proxy_password[0]) {
                char userpwbuf[514];
                int ulen, plen;
                ulen = strlen(cfg.proxy_username);
                if (ulen > 255) ulen = 255; if (ulen < 1) ulen = 1;
                plen = strlen(cfg.proxy_password);
                if (plen > 255) plen = 255; if (plen < 1) plen = 1;
                userpwbuf[0] = 1;      /* version number of subnegotiation */
                userpwbuf[1] = ulen;
                memcpy(userpwbuf+2, cfg.proxy_username, ulen);
                userpwbuf[ulen+2] = plen;
                memcpy(userpwbuf+ulen+3, cfg.proxy_password, plen);
                sk_write(p->sub_socket, userpwbuf, ulen + plen + 3);
                p->state = 7;
            } else 
                plug_closing(p->plug, "Proxy error: Server chose "
                             "username/password authentication but we "
                             "didn't offer it!",
                         PROXY_ERROR_GENERAL, 0);
            return 1;
        }

        if (p->state == 6) {
            /* TODO: Handle CHAP authentication */
            plug_closing(p->plug, "Proxy error: We don't support CHAP authentication",
                         PROXY_ERROR_GENERAL, 0);
            return 1;
        }

    }

    plug_closing(p->plug, "Proxy error: Unexpected proxy error",
                 PROXY_ERROR_UNEXPECTED, 0);
    return 1;
}

/* ----------------------------------------------------------------------
 * `Telnet' proxy type.
 *
 * (This is for ad-hoc proxies where you connect to the proxy's
 * telnet port and send a command such as `connect host port'. The
 * command is configurable, since this proxy type is typically not
 * standardised or at all well-defined.)
 */

int proxy_telnet_negotiate (Proxy_Socket p, int change)
{
    if (p->state == PROXY_CHANGE_NEW) {

        int so = 0, eo = 0;

        /* we need to escape \\, \%, \r, \n, \t, \x??, \0???, 
         * %%, %host, %port, %user, and %pass
         */

        while (cfg.proxy_telnet_command[eo] != 0) {

            /* scan forward until we hit end-of-line, 
             * or an escape character (\ or %) */
            while (cfg.proxy_telnet_command[eo] != 0 &&
                   cfg.proxy_telnet_command[eo] != '%' &&
                   cfg.proxy_telnet_command[eo] != '\\') eo++;

            /* if we hit eol, break out of our escaping loop */
            if (cfg.proxy_telnet_command[eo] == 0) break;

            /* if there was any unescaped text before the escape
             * character, send that now */
            if (eo != so) {
                sk_write(p->sub_socket, 
                         cfg.proxy_telnet_command + so, eo - so);
            }

            so = eo++;

            /* if the escape character was the last character of
             * the line, we'll just stop and send it. */
            if (cfg.proxy_telnet_command[eo] == 0) break;

            if (cfg.proxy_telnet_command[so] == '\\') {

                /* we recognize \\, \%, \r, \n, \t, \x??. 
                 * anything else, we just send unescaped (including the \). 
                 */

                switch (cfg.proxy_telnet_command[eo]) {

                  case '\\':
                    sk_write(p->sub_socket, "\\", 1);
                    eo++;
                    break;

                  case '%':
                    sk_write(p->sub_socket, "%%", 1);
                    eo++;
                    break;

                  case 'r':
                    sk_write(p->sub_socket, "\r", 1);
                    eo++;
                    break;

                  case 'n':
                    sk_write(p->sub_socket, "\n", 1);
                    eo++;
                    break;

                  case 't':
                    sk_write(p->sub_socket, "\t", 1);
                    eo++;
                    break;

                  case 'x':
                  case 'X':
                    {
                    /* escaped hexadecimal value (ie. \xff) */
                    unsigned char v = 0;
                    int i = 0;

                    for (;;) {
                        eo++;
                        if (cfg.proxy_telnet_command[eo] >= '0' &&
                            cfg.proxy_telnet_command[eo] <= '9')
                            v += cfg.proxy_telnet_command[eo] - '0';
                        else if (cfg.proxy_telnet_command[eo] >= 'a' &&
                                 cfg.proxy_telnet_command[eo] <= 'f')
                            v += cfg.proxy_telnet_command[eo] - 'a' + 10;
                        else if (cfg.proxy_telnet_command[eo] >= 'A' &&
                                 cfg.proxy_telnet_command[eo] <= 'F')
                            v += cfg.proxy_telnet_command[eo] - 'A' + 10;
                        else {
                            /* non hex character, so we abort and just
                             * send the whole thing unescaped (including \x)
                             */
                            sk_write(p->sub_socket, "\\", 1);
                            eo = so + 1;
                            break;
                        }

                        /* we only extract two hex characters */
                        if (i == 1) {
                            sk_write(p->sub_socket, &v, 1);
                            eo++;
                            break;
                        }

                        i++;
                        v <<= 4;
                    }
                    }
                    break;

                  default:
                    sk_write(p->sub_socket, 
                             cfg.proxy_telnet_command + so, 2);
                    eo++;
                    break;
                }
            } else {

                /* % escape. we recognize %%, %host, %port, %user, %pass. 
                 * anything else, we just send unescaped (including the %). 
                 */

                if (cfg.proxy_telnet_command[eo] == '%') {
                    sk_write(p->sub_socket, "%", 1);
                    eo++;
                }
                else if (strnicmp(cfg.proxy_telnet_command + eo,
                                  "host", 4) == 0) {
                    char dest[64];
                    sk_getaddr(p->remote_addr, dest, 64);
                    sk_write(p->sub_socket, dest, strlen(dest));
                    eo += 4;
                }
                else if (strnicmp(cfg.proxy_telnet_command + eo,
                                  "port", 4) == 0) {
                    char port[8];
                    sprintf(port, "%i", p->remote_port);
                    sk_write(p->sub_socket, port, strlen(port));
                    eo += 4;
                }
                else if (strnicmp(cfg.proxy_telnet_command + eo,
                                  "user", 4) == 0) {
                    sk_write(p->sub_socket, cfg.proxy_username, 
                        strlen(cfg.proxy_username));
                    eo += 4;
                }
                else if (strnicmp(cfg.proxy_telnet_command + eo,
                                  "pass", 4) == 0) {
                    sk_write(p->sub_socket, cfg.proxy_password, 
                        strlen(cfg.proxy_password));
                    eo += 4;
                }
                else {
                    /* we don't escape this, so send the % now, and
                     * don't advance eo, so that we'll consider the
                     * text immediately following the % as unescaped.
                     */
                    sk_write(p->sub_socket, "%", 1);
                }
            }

            /* resume scanning for additional escapes after this one. */
            so = eo;
        }

        /* if there is any unescaped text at the end of the line, send it */
        if (eo != so) {
            sk_write(p->sub_socket, cfg.proxy_telnet_command + so, eo - so);
        }

        p->state = 1;
        return 0;
    }

    if (change == PROXY_CHANGE_CLOSING) {
        /* if our proxy negotiation process involves closing and opening
         * new sockets, then we would want to intercept this closing
         * callback when we were expecting it. if we aren't anticipating
         * a socket close, then some error must have occurred. we'll
         * just pass those errors up to the backend.
         */
        return plug_closing(p->plug, p->closing_error_msg,
                            p->closing_error_code,
                            p->closing_calling_back);
    }

    if (change == PROXY_CHANGE_SENT) {
        /* some (or all) of what we wrote to the proxy was sent.
         * we don't do anything new, however, until we receive the
         * proxy's response. we might want to set a timer so we can
         * timeout the proxy negotiation after a while...
         */
        return 0;
    }

    if (change == PROXY_CHANGE_ACCEPTING) {
        /* we should _never_ see this, as we are using our socket to
         * connect to a proxy, not accepting inbound connections.
         * what should we do? close the socket with an appropriate
         * error message?
         */
        return plug_accepting(p->plug, p->accepting_sock);
    }

    if (change == PROXY_CHANGE_RECEIVE) {
        /* we have received data from the underlying socket, which
         * we'll need to parse, process, and respond to appropriately.
         */

        /* we're done */
        proxy_activate(p);
        /* proxy activate will have dealt with
         * whatever is left of the buffer */
        return 1;
    }

    plug_closing(p->plug, "Proxy error: Unexpected proxy error",
                 PROXY_ERROR_UNEXPECTED, 0);
    return 1;
}