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>

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

#define do_proxy_dns(conf) \
    (conf_get_int(conf, CONF_proxy_dns) == FORCE_ON || \
         (conf_get_int(conf, CONF_proxy_dns) == AUTO && \
              conf_get_int(conf, CONF_proxy_type) != PROXY_SOCKS4))

/*
 * Call this when proxy negotiation is complete, so that this
 * socket can begin working normally.
 */
void proxy_activate (ProxySocket *p)
{
    size_t 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, true);

    /* 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) {
        ptrlen data = bufchain_prefix(&p->pending_oob_output_data);
        output_after += sk_write_oob(p->sub_socket, data.ptr, data.len);
        bufchain_consume(&p->pending_oob_output_data, data.len);
    }

    /* send buffered normal writes */
    while (bufchain_size(&p->pending_output_data) > 0) {
        ptrlen data = bufchain_prefix(&p->pending_output_data);
        output_after += sk_write(p->sub_socket, data.ptr, data.len);
        bufchain_consume(&p->pending_output_data, 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 have a pending EOF to send, send it */
    if (p->pending_eof) sk_write_eof(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(&p->sock, false);
}

/* basic proxy socket functions */

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

static void sk_proxy_close (Socket *s)
{
    ProxySocket *ps = container_of(s, ProxySocket, sock);

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

static size_t sk_proxy_write (Socket *s, const void *data, size_t len)
{
    ProxySocket *ps = container_of(s, ProxySocket, sock);

    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 size_t sk_proxy_write_oob (Socket *s, const void *data, size_t len)
{
    ProxySocket *ps = container_of(s, ProxySocket, sock);

    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_write_eof (Socket *s)
{
    ProxySocket *ps = container_of(s, ProxySocket, sock);

    if (ps->state != PROXY_STATE_ACTIVE) {
        ps->pending_eof = true;
        return;
    }
    sk_write_eof(ps->sub_socket);
}

static void sk_proxy_set_frozen (Socket *s, bool is_frozen)
{
    ProxySocket *ps = container_of(s, ProxySocket, sock);

    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) {
            char databuf[512];
            ptrlen data = bufchain_prefix(&ps->pending_input_data);
            if (data.len > lenof(databuf))
                data.len = lenof(databuf);
            memcpy(databuf, data.ptr, data.len);
            bufchain_consume(&ps->pending_input_data, data.len);
            plug_receive(ps->plug, 0, databuf, 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 const char * sk_proxy_socket_error (Socket *s)
{
    ProxySocket *ps = container_of(s, ProxySocket, sock);
    if (ps->error != NULL || ps->sub_socket == NULL) {
        return ps->error;
    }
    return sk_socket_error(ps->sub_socket);
}

/* basic proxy plug functions */

static void plug_proxy_log(Plug *plug, PlugLogType type, SockAddr *addr,
                           int port, const char *error_msg, int error_code)
{
    ProxySocket *ps = container_of(plug, ProxySocket, plugimpl);

    plug_log(ps->plug, type, addr, port, error_msg, error_code);
}

static void plug_proxy_closing (Plug *p, const char *error_msg,
                                int error_code, bool calling_back)
{
    ProxySocket *ps = container_of(p, ProxySocket, plugimpl);

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

static void plug_proxy_receive(
    Plug *p, int urgent, const char *data, size_t len)
{
    ProxySocket *ps = container_of(p, ProxySocket, plugimpl);

    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 != 0);
        ps->receive_data = data;
        ps->receive_len = len;
        ps->negotiate(ps, PROXY_CHANGE_RECEIVE);
    } else {
        plug_receive(ps->plug, urgent, data, len);
    }
}

static void plug_proxy_sent (Plug *p, size_t bufsize)
{
    ProxySocket *ps = container_of(p, ProxySocket, plugimpl);

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

static int plug_proxy_accepting(Plug *p,
                                accept_fn_t constructor, accept_ctx_t ctx)
{
    ProxySocket *ps = container_of(p, ProxySocket, plugimpl);

    if (ps->state != PROXY_STATE_ACTIVE) {
        ps->accepting_constructor = constructor;
        ps->accepting_ctx = ctx;
        return ps->negotiate(ps, PROXY_CHANGE_ACCEPTING);
    }
    return plug_accepting(ps->plug, constructor, ctx);
}

/*
 * This function can accept a NULL pointer as `addr', in which case
 * it will only check the host name.
 */
static bool proxy_for_destination(SockAddr *addr, const char *hostname,
                                  int port, Conf *conf)
{
    int s = 0, e = 0;
    char hostip[64];
    int hostip_len, hostname_len;
    const char *exclude_list;

    /*
     * Special local connections such as Unix-domain sockets
     * unconditionally cannot be proxied, even in proxy-localhost
     * mode. There just isn't any way to ask any known proxy type for
     * them.
     */
    if (addr && sk_address_is_special_local(addr))
        return false;                  /* do not proxy */

    /*
     * Check the host name and IP against the hard-coded
     * representations of `localhost'.
     */
    if (!conf_get_bool(conf, CONF_even_proxy_localhost) &&
        (sk_hostname_is_local(hostname) ||
         (addr && sk_address_is_local(addr))))
        return false;                  /* do not proxy */

    /* we want a string representation of the IP address for comparisons */
    if (addr) {
        sk_getaddr(addr, hostip, 64);
        hostip_len = strlen(hostip);
    } else
        hostip_len = 0;                /* placate gcc; shouldn't be required */

    hostname_len = strlen(hostname);

    exclude_list = conf_get_str(conf, CONF_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((unsigned char)exclude_list[s]) ||
                exclude_list[s] == ',')) s++;

        if (!exclude_list[s]) break;

        e = s;

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

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

            if ((addr && 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) {
                /* IP/hostname range excluded. do not use proxy. */
                return false;
            }
        } else if (exclude_list[e-1] == '*') {
            /* wildcard at end of entry */

            if ((addr && strnicmp(hostip, exclude_list + s, e - s - 1) == 0) ||
                strnicmp(hostname, exclude_list + s, e - s - 1) == 0) {
                /* IP/hostname range excluded. do not use proxy. */
                return false;
            }
        } else {
            /* no wildcard at either end, so let's try an absolute
             * match (ie. a specific IP)
             */

            if (addr && strnicmp(hostip, exclude_list + s, e - s) == 0)
                return false; /* IP/hostname excluded. do not use proxy. */
            if (strnicmp(hostname, exclude_list + s, e - s) == 0)
                return false; /* IP/hostname excluded. do not use proxy. */
        }

        s = e;

        /* Make sure we really have reached the next comma or end-of-string */
        while (exclude_list[s] &&
               !isspace((unsigned char)exclude_list[s]) &&
               exclude_list[s] != ',') s++;
    }

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

static char *dns_log_msg(const char *host, int addressfamily,
                         const char *reason)
{
    return dupprintf("Looking up host \"%s\"%s for %s", host,
                     (addressfamily == ADDRTYPE_IPV4 ? " (IPv4)" :
                      addressfamily == ADDRTYPE_IPV6 ? " (IPv6)" :
                      ""), reason);
}

SockAddr *name_lookup(const char *host, int port, char **canonicalname,
                     Conf *conf, int addressfamily, LogContext *logctx,
                     const char *reason)
{
    if (conf_get_int(conf, CONF_proxy_type) != PROXY_NONE &&
        do_proxy_dns(conf) &&
        proxy_for_destination(NULL, host, port, conf)) {

        if (logctx)
            logeventf(logctx, "Leaving host lookup to proxy of \"%s\""
                      " (for %s)", host, reason);

        *canonicalname = dupstr(host);
        return sk_nonamelookup(host);
    } else {
        if (logctx)
            logevent_and_free(
                logctx, dns_log_msg(host, addressfamily, reason));

        return sk_namelookup(host, canonicalname, addressfamily);
    }
}

static const SocketVtable ProxySocket_sockvt = {
    .plug = sk_proxy_plug,
    .close = sk_proxy_close,
    .write = sk_proxy_write,
    .write_oob = sk_proxy_write_oob,
    .write_eof = sk_proxy_write_eof,
    .set_frozen = sk_proxy_set_frozen,
    .socket_error = sk_proxy_socket_error,
    .peer_info = NULL,
};

static const PlugVtable ProxySocket_plugvt = {
    .log = plug_proxy_log,
    .closing = plug_proxy_closing,
    .receive = plug_proxy_receive,
    .sent = plug_proxy_sent,
    .accepting = plug_proxy_accepting
};

Socket *new_connection(SockAddr *addr, const char *hostname,
                       int port, bool privport,
                       bool oobinline, bool nodelay, bool keepalive,
                       Plug *plug, Conf *conf)
{
    if (conf_get_int(conf, CONF_proxy_type) != PROXY_NONE &&
        proxy_for_destination(addr, hostname, port, conf))
    {
        ProxySocket *ret;
        SockAddr *proxy_addr;
        char *proxy_canonical_name;
        const char *proxy_type;
        Socket *sret;
        int type;

        if ((sret = platform_new_connection(addr, hostname, port, privport,
                                            oobinline, nodelay, keepalive,
                                            plug, conf)) != NULL)
            return sret;

        ret = snew(ProxySocket);
        ret->sock.vt = &ProxySocket_sockvt;
        ret->plugimpl.vt = &ProxySocket_plugvt;
        ret->conf = conf_copy(conf);
        ret->plug = plug;
        ret->remote_addr = addr;       /* will need to be freed on close */
        ret->remote_port = port;

        ret->error = NULL;
        ret->pending_eof = false;
        ret->freeze = false;

        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;

        type = conf_get_int(conf, CONF_proxy_type);
        if (type == PROXY_HTTP) {
            ret->negotiate = proxy_http_negotiate;
            proxy_type = "HTTP";
        } else if (type == PROXY_SOCKS4) {
            ret->negotiate = proxy_socks4_negotiate;
            proxy_type = "SOCKS 4";
        } else if (type == PROXY_SOCKS5) {
            ret->negotiate = proxy_socks5_negotiate;
            proxy_type = "SOCKS 5";
        } else if (type == PROXY_TELNET) {
            ret->negotiate = proxy_telnet_negotiate;
            proxy_type = "Telnet";
        } else {
            ret->error = "Proxy error: Unknown proxy method";
            return &ret->sock;
        }

        {
            char *logmsg = dupprintf("Will use %s proxy at %s:%d to connect"
                                      " to %s:%d", proxy_type,
                                      conf_get_str(conf, CONF_proxy_host),
                                      conf_get_int(conf, CONF_proxy_port),
                                      hostname, port);
            plug_log(plug, PLUGLOG_PROXY_MSG, NULL, 0, logmsg, 0);
            sfree(logmsg);
        }

        {
            char *logmsg = dns_log_msg(conf_get_str(conf, CONF_proxy_host),
                                       conf_get_int(conf, CONF_addressfamily),
                                       "proxy");
            plug_log(plug, PLUGLOG_PROXY_MSG, NULL, 0, logmsg, 0);
            sfree(logmsg);
        }

        /* look-up proxy */
        proxy_addr = sk_namelookup(conf_get_str(conf, CONF_proxy_host),
                                   &proxy_canonical_name,
                                   conf_get_int(conf, CONF_addressfamily));
        if (sk_addr_error(proxy_addr) != NULL) {
            ret->error = "Proxy error: Unable to resolve proxy host name";
            sk_addr_free(proxy_addr);
            return &ret->sock;
        }
        sfree(proxy_canonical_name);

        {
            char addrbuf[256], *logmsg;
            sk_getaddr(proxy_addr, addrbuf, lenof(addrbuf));
            logmsg = dupprintf("Connecting to %s proxy at %s port %d",
                               proxy_type, addrbuf,
                               conf_get_int(conf, CONF_proxy_port));
            plug_log(plug, PLUGLOG_PROXY_MSG, NULL, 0, logmsg, 0);
            sfree(logmsg);
        }

        /* create the actual socket we will be using,
         * connected to our proxy server and port.
         */
        ret->sub_socket = sk_new(proxy_addr,
                                 conf_get_int(conf, CONF_proxy_port),
                                 privport, oobinline,
                                 nodelay, keepalive, &ret->plugimpl);
        if (sk_socket_error(ret->sub_socket) != NULL)
            return &ret->sock;

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

        return &ret->sock;
    }

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

Socket *new_listener(const char *srcaddr, int port, Plug *plug,
                     bool local_host_only, Conf *conf, int addressfamily)
{
    /* TODO: SOCKS (and potentially others) support inbound
     * TODO: connections via the proxy. support them.
     */

    return sk_newlistener(srcaddr, port, plug, local_host_only, addressfamily);
}

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

static bool get_line_end(char *data, size_t len, size_t *out)
{
    size_t 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) {
                *out = off;
                return true;
            }

            /* 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') {
                *out = off;
                return true;
            }

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

        off++;
    }

    return false;
}

int proxy_http_negotiate (ProxySocket *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[512];
        char *username, *password;

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

        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);

        username = conf_get_str(p->conf, CONF_proxy_username);
        password = conf_get_str(p->conf, CONF_proxy_password);
        if (username[0] || password[0]) {
            char *buf, *buf2;
            int i, j, len;
            buf = dupprintf("%s:%s", username, password);
            len = strlen(buf);
            buf2 = snewn(len * 4 / 3 + 100, char);
            sprintf(buf2, "Proxy-Authorization: Basic ");
            for (i = 0, j = strlen(buf2); i < len; i += 3, j += 4)
                base64_encode_atom((unsigned char *)(buf+i),
                                   (len-i > 3 ? 3 : len-i), buf2+j);
            strcpy(buf2+j, "\r\n");
            sk_write(p->sub_socket, buf2, strlen(buf2));
            sfree(buf);
            sfree(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.
         */
        plug_closing(p->plug, p->closing_error_msg, p->closing_error_code,
                     p->closing_calling_back);
        return 0; /* ignored */
    }

    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_constructor, p->accepting_ctx);
    }

    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;
        size_t len, 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 = snewn(len+1, char);
            bufchain_fetch(&p->pending_input_data, data, len);
            /*
             * We must NUL-terminate this data, because Windows
             * sscanf appears to require a NUL at the end of the
             * string because it strlens it _first_. Sigh.
             */
            data[len] = '\0';

            if (!get_line_end(data, len, &eol)) {
                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 = snewn(len, char);
            datap = data;
            bufchain_fetch(&p->pending_input_data, data, len);

            if (!get_line_end(datap, len, &eol)) {
                sfree(data);
                return 1;
            }
            while (eol > 2) {
                bufchain_consume(&p->pending_input_data, eol);
                datap += eol;
                len   -= eol;
                if (!get_line_end(datap, len, &eol))
                    eol = 0;           /* terminate the loop */
            }

            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 (ProxySocket *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)
         */

        strbuf *command = strbuf_new();
        char hostname[512];
        bool write_hostname = false;

        put_byte(command, 4);          /* SOCKS version 4 */
        put_byte(command, 1);          /* CONNECT command */
        put_uint16(command, p->remote_port);

        switch (sk_addrtype(p->remote_addr)) {
          case ADDRTYPE_IPV4: {
            char addr[4];
            sk_addrcopy(p->remote_addr, addr);
            put_data(command, addr, 4);
            break;
          }
          case ADDRTYPE_NAME:
            sk_getaddr(p->remote_addr, hostname, lenof(hostname));
            put_uint32(command, 1);
            write_hostname = true;
            break;
          case ADDRTYPE_IPV6:
            p->error = "Proxy error: SOCKS version 4 does not support IPv6";
            strbuf_free(command);
            return 1;
        }

        put_asciz(command, conf_get_str(p->conf, CONF_proxy_username));
        if (write_hostname)
            put_asciz(command, hostname);
        sk_write(p->sub_socket, command->s, command->len);
        strbuf_free(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.
         */
        plug_closing(p->plug, p->closing_error_msg, p->closing_error_code,
                     p->closing_calling_back);
        return 0; /* ignored */
    }

    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_constructor, p->accepting_ctx);
    }

    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 (ProxySocket *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
         */

        strbuf *command;
        char *username, *password;
        int method_count_offset, methods_start;

        command = strbuf_new();
        put_byte(command, 5);          /* SOCKS version 5 */
        username = conf_get_str(p->conf, CONF_proxy_username);
        password = conf_get_str(p->conf, CONF_proxy_password);

        method_count_offset = command->len;
        put_byte(command, 0);
        methods_start = command->len;

        put_byte(command, 0x00);       /* no authentication */

        if (username[0] || password[0]) {
            proxy_socks5_offerencryptedauth(BinarySink_UPCAST(command));
            put_byte(command, 0x02);    /* username/password */
        }

        command->u[method_count_offset] = command->len - methods_start;

        sk_write(p->sub_socket, command->s, command->len);
        strbuf_free(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.
         */
        plug_closing(p->plug, p->closing_error_msg, p->closing_error_code,
                     p->closing_calling_back);
        return 0; /* ignored */
    }

    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_constructor, p->accepting_ctx);
    }

    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;
            }