tor/src/or/proto_socks.c

/* Copyright (c) 2001 Matej Pfajfar.
 * Copyright (c) 2001-2004, Roger Dingledine.
 * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
 * Copyright (c) 2007-2017, The Tor Project, Inc. */
/* See LICENSE for licensing information */

#include "or.h"
#include "addressmap.h"
#include "buffers.h"
#include "control.h"
#include "config.h"
#include "ext_orport.h"
#include "proto_socks.h"
#include "reasons.h"

static void socks_request_set_socks5_error(socks_request_t *req,
                              socks5_reply_status_t reason);

static int parse_socks(const char *data, size_t datalen, socks_request_t *req,
                       int log_sockstype, int safe_socks, ssize_t *drain_out,
                       size_t *want_length_out);
static int parse_socks_client(const uint8_t *data, size_t datalen,
                              int state, char **reason,
                              ssize_t *drain_out);
/**
 * Wait this many seconds before warning the user about using SOCKS unsafely
 * again. */
#define SOCKS_WARN_INTERVAL 5

/** Warn that the user application has made an unsafe socks request using
 * protocol <b>socks_protocol</b> on port <b>port</b>.  Don't warn more than
 * once per SOCKS_WARN_INTERVAL, unless <b>safe_socks</b> is set. */
static void
log_unsafe_socks_warning(int socks_protocol, const char *address,
                         uint16_t port, int safe_socks)
{
  static ratelim_t socks_ratelim = RATELIM_INIT(SOCKS_WARN_INTERVAL);

  if (safe_socks) {
    log_fn_ratelim(&socks_ratelim, LOG_WARN, LD_APP,
             "Your application (using socks%d to port %d) is giving "
             "Tor only an IP address. Applications that do DNS resolves "
             "themselves may leak information. Consider using Socks4A "
             "(e.g. via privoxy or socat) instead. For more information, "
             "please see https://wiki.torproject.org/TheOnionRouter/"
             "TorFAQ#SOCKSAndDNS.%s",
             socks_protocol,
             (int)port,
             safe_socks ? " Rejecting." : "");
  }
  control_event_client_status(LOG_WARN,
                              "DANGEROUS_SOCKS PROTOCOL=SOCKS%d ADDRESS=%s:%d",
                              socks_protocol, address, (int)port);
}

/** Do not attempt to parse socks messages longer than this.  This value is
 * actually significantly higher than the longest possible socks message. */
#define MAX_SOCKS_MESSAGE_LEN 512

/** Return a new socks_request_t. */
socks_request_t *
socks_request_new(void)
{
  return tor_malloc_zero(sizeof(socks_request_t));
}

/** Free all storage held in the socks_request_t <b>req</b>. */
void
socks_request_free(socks_request_t *req)
{
  if (!req)
    return;
  if (req->username) {
    memwipe(req->username, 0x10, req->usernamelen);
    tor_free(req->username);
  }
  if (req->password) {
    memwipe(req->password, 0x04, req->passwordlen);
    tor_free(req->password);
  }
  memwipe(req, 0xCC, sizeof(socks_request_t));
  tor_free(req);
}

/** There is a (possibly incomplete) socks handshake on <b>buf</b>, of one
 * of the forms
 *  - socks4: "socksheader username\\0"
 *  - socks4a: "socksheader username\\0 destaddr\\0"
 *  - socks5 phase one: "version #methods methods"
 *  - socks5 phase two: "version command 0 addresstype..."
 * If it's a complete and valid handshake, and destaddr fits in
 *   MAX_SOCKS_ADDR_LEN bytes, then pull the handshake off the buf,
 *   assign to <b>req</b>, and return 1.
 *
 * If it's invalid or too big, return -1.
 *
 * Else it's not all there yet, leave buf alone and return 0.
 *
 * If you want to specify the socks reply, write it into <b>req->reply</b>
 *   and set <b>req->replylen</b>, else leave <b>req->replylen</b> alone.
 *
 * If <b>log_sockstype</b> is non-zero, then do a notice-level log of whether
 * the connection is possibly leaking DNS requests locally or not.
 *
 * If <b>safe_socks</b> is true, then reject unsafe socks protocols.
 *
 * If returning 0 or -1, <b>req->address</b> and <b>req->port</b> are
 * undefined.
 */
int
fetch_from_buf_socks(buf_t *buf, socks_request_t *req,
                     int log_sockstype, int safe_socks)
{
  int res;
  ssize_t n_drain;
  size_t want_length = 128;
  const char *head = NULL;
  size_t datalen = 0;

  if (buf_datalen(buf) < 2) /* version and another byte */
    return 0;

  do {
    n_drain = 0;
    buf_pullup(buf, want_length, &head, &datalen);
    tor_assert(head && datalen >= 2);
    want_length = 0;

    res = parse_socks(head, datalen, req, log_sockstype,
                      safe_socks, &n_drain, &want_length);

    if (n_drain < 0)
      buf_clear(buf);
    else if (n_drain > 0)
      buf_remove_from_front(buf, n_drain);

  } while (res == 0 && head && want_length < buf_datalen(buf) &&
           buf_datalen(buf) >= 2);

  return res;
}

/** The size of the header of an Extended ORPort message: 2 bytes for
 *  COMMAND, 2 bytes for BODYLEN */
#define EXT_OR_CMD_HEADER_SIZE 4

/** Read <b>buf</b>, which should contain an Extended ORPort message
 *  from a transport proxy. If well-formed, create and populate
 *  <b>out</b> with the Extended ORport message. Return 0 if the
 *  buffer was incomplete, 1 if it was well-formed and -1 if we
 *  encountered an error while parsing it.  */
int
fetch_ext_or_command_from_buf(buf_t *buf, ext_or_cmd_t **out)
{
  char hdr[EXT_OR_CMD_HEADER_SIZE];
  uint16_t len;

  if (buf_datalen(buf) < EXT_OR_CMD_HEADER_SIZE)
    return 0;
  peek_from_buf(hdr, sizeof(hdr), buf);
  len = ntohs(get_uint16(hdr+2));
  if (buf_datalen(buf) < (unsigned)len + EXT_OR_CMD_HEADER_SIZE)
    return 0;
  *out = ext_or_cmd_new(len);
  (*out)->cmd = ntohs(get_uint16(hdr));
  (*out)->len = len;
  buf_remove_from_front(buf, EXT_OR_CMD_HEADER_SIZE);
  fetch_from_buf((*out)->body, len, buf);
  return 1;
}

/** Create a SOCKS5 reply message with <b>reason</b> in its REP field and
 * have Tor send it as error response to <b>req</b>.
 */
static void
socks_request_set_socks5_error(socks_request_t *req,
                  socks5_reply_status_t reason)
{
   req->replylen = 10;
   memset(req->reply,0,10);

   req->reply[0] = 0x05;   // VER field.
   req->reply[1] = reason; // REP field.
   req->reply[3] = 0x01;   // ATYP field.
}

static const char SOCKS_PROXY_IS_NOT_AN_HTTP_PROXY_MSG[] =
  "HTTP/1.0 501 Tor is not an HTTP Proxy\r\n"
  "Content-Type: text/html; charset=iso-8859-1\r\n\r\n"
  "<html>\n"
  "<head>\n"
  "<title>Tor is not an HTTP Proxy</title>\n"
  "</head>\n"
  "<body>\n"
  "<h1>Tor is not an HTTP Proxy</h1>\n"
  "<p>\n"
  "It appears you have configured your web browser to use Tor as "
  "an HTTP proxy.\n\n"
  "This is not correct: Tor is a SOCKS proxy, not an HTTP proxy.\n"
  "Please configure your client accordingly.\n"
  "</p>\n"
  "<p>\n"
  "See <a href=\"https://www.torproject.org/documentation.html\">"
  "https://www.torproject.org/documentation.html</a> for more "
  "information.\n"
  "<!-- Plus this comment, to make the body response more than 512 bytes, so "
  "     IE will be willing to display it. Comment comment comment comment "
  "     comment comment comment comment comment comment comment comment.-->\n"
  "</p>\n"
  "</body>\n"
  "</html>\n";

/** Implementation helper to implement fetch_from_*_socks.  Instead of looking
 * at a buffer's contents, we look at the <b>datalen</b> bytes of data in
 * <b>data</b>. Instead of removing data from the buffer, we set
 * <b>drain_out</b> to the amount of data that should be removed (or -1 if the
 * buffer should be cleared).  Instead of pulling more data into the first
 * chunk of the buffer, we set *<b>want_length_out</b> to the number of bytes
 * we'd like to see in the input buffer, if they're available. */
static int
parse_socks(const char *data, size_t datalen, socks_request_t *req,
            int log_sockstype, int safe_socks, ssize_t *drain_out,
            size_t *want_length_out)
{
  unsigned int len;
  char tmpbuf[TOR_ADDR_BUF_LEN+1];
  tor_addr_t destaddr;
  uint32_t destip;
  uint8_t socksver;
  char *next, *startaddr;
  unsigned char usernamelen, passlen;
  struct in_addr in;

  if (datalen < 2) {
    /* We always need at least 2 bytes. */
    *want_length_out = 2;
    return 0;
  }

  if (req->socks_version == 5 && !req->got_auth) {
    /* See if we have received authentication.  Strictly speaking, we should
       also check whether we actually negotiated username/password
       authentication.  But some broken clients will send us authentication
       even if we negotiated SOCKS_NO_AUTH. */
    if (*data == 1) { /* username/pass version 1 */
      /* Format is: authversion [1 byte] == 1
                    usernamelen [1 byte]
                    username    [usernamelen bytes]
                    passlen     [1 byte]
                    password    [passlen bytes] */
      usernamelen = (unsigned char)*(data + 1);
      if (datalen < 2u + usernamelen + 1u) {
        *want_length_out = 2u + usernamelen + 1u;
        return 0;
      }
      passlen = (unsigned char)*(data + 2u + usernamelen);
      if (datalen < 2u + usernamelen + 1u + passlen) {
        *want_length_out = 2u + usernamelen + 1u + passlen;
        return 0;
      }
      req->replylen = 2; /* 2 bytes of response */
      req->reply[0] = 1; /* authversion == 1 */
      req->reply[1] = 0; /* authentication successful */
      log_debug(LD_APP,
               "socks5: Accepted username/password without checking.");
      if (usernamelen) {
        req->username = tor_memdup(data+2u, usernamelen);
        req->usernamelen = usernamelen;
      }
      if (passlen) {
        req->password = tor_memdup(data+3u+usernamelen, passlen);
        req->passwordlen = passlen;
      }
      *drain_out = 2u + usernamelen + 1u + passlen;
      req->got_auth = 1;
      *want_length_out = 7; /* Minimal socks5 command. */
      return 0;
    } else if (req->auth_type == SOCKS_USER_PASS) {
      /* unknown version byte */
      log_warn(LD_APP, "Socks5 username/password version %d not recognized; "
               "rejecting.", (int)*data);
      return -1;
    }
  }

  socksver = *data;

  switch (socksver) { /* which version of socks? */
    case 5: /* socks5 */

      if (req->socks_version != 5) { /* we need to negotiate a method */
        unsigned char nummethods = (unsigned char)*(data+1);
        int have_user_pass, have_no_auth;
        int r=0;
        tor_assert(!req->socks_version);
        if (datalen < 2u+nummethods) {
          *want_length_out = 2u+nummethods;
          return 0;
        }
        if (!nummethods)
          return -1;
        req->replylen = 2; /* 2 bytes of response */
        req->reply[0] = 5; /* socks5 reply */
        have_user_pass = (memchr(data+2, SOCKS_USER_PASS, nummethods) !=NULL);
        have_no_auth   = (memchr(data+2, SOCKS_NO_AUTH,   nummethods) !=NULL);
        if (have_user_pass && !(have_no_auth && req->socks_prefer_no_auth)) {
          req->auth_type = SOCKS_USER_PASS;
          req->reply[1] = SOCKS_USER_PASS; /* tell client to use "user/pass"
                                              auth method */
          req->socks_version = 5; /* remember we've already negotiated auth */
          log_debug(LD_APP,"socks5: accepted method 2 (username/password)");
          r=0;
        } else if (have_no_auth) {
          req->reply[1] = SOCKS_NO_AUTH; /* tell client to use "none" auth
                                            method */
          req->socks_version = 5; /* remember we've already negotiated auth */
          log_debug(LD_APP,"socks5: accepted method 0 (no authentication)");
          r=0;
        } else {
          log_warn(LD_APP,
                    "socks5: offered methods don't include 'no auth' or "
                    "username/password. Rejecting.");
          req->reply[1] = '\xFF'; /* reject all methods */
          r=-1;
        }
        /* Remove packet from buf. Some SOCKS clients will have sent extra
         * junk at this point; let's hope it's an authentication message. */
        *drain_out = 2u + nummethods;

        return r;
      }
      if (req->auth_type != SOCKS_NO_AUTH && !req->got_auth) {
        log_warn(LD_APP,
                 "socks5: negotiated authentication, but none provided");
        return -1;
      }
      /* we know the method; read in the request */
      log_debug(LD_APP,"socks5: checking request");
      if (datalen < 7) {/* basic info plus >=1 for addr plus 2 for port */
        *want_length_out = 7;
        return 0; /* not yet */
      }
      req->command = (unsigned char) *(data+1);
      if (req->command != SOCKS_COMMAND_CONNECT &&
          req->command != SOCKS_COMMAND_RESOLVE &&
          req->command != SOCKS_COMMAND_RESOLVE_PTR) {
        /* not a connect or resolve or a resolve_ptr? we don't support it. */
        socks_request_set_socks5_error(req,SOCKS5_COMMAND_NOT_SUPPORTED);

        log_warn(LD_APP,"socks5: command %d not recognized. Rejecting.",
                 req->command);
        return -1;
      }
      switch (*(data+3)) { /* address type */
        case 1: /* IPv4 address */
        case 4: /* IPv6 address */ {
          const int is_v6 = *(data+3) == 4;
          const unsigned addrlen = is_v6 ? 16 : 4;
          log_debug(LD_APP,"socks5: ipv4 address type");
          if (datalen < 6+addrlen) {/* ip/port there? */
            *want_length_out = 6+addrlen;
            return 0; /* not yet */
          }

          if (is_v6)
            tor_addr_from_ipv6_bytes(&destaddr, data+4);
          else
            tor_addr_from_ipv4n(&destaddr, get_uint32(data+4));

          tor_addr_to_str(tmpbuf, &destaddr, sizeof(tmpbuf), 1);

          if (strlen(tmpbuf)+1 > MAX_SOCKS_ADDR_LEN) {
            socks_request_set_socks5_error(req, SOCKS5_GENERAL_ERROR);
            log_warn(LD_APP,
                     "socks5 IP takes %d bytes, which doesn't fit in %d. "
                     "Rejecting.",
                     (int)strlen(tmpbuf)+1,(int)MAX_SOCKS_ADDR_LEN);
            return -1;
          }
          strlcpy(req->address,tmpbuf,sizeof(req->address));
          req->port = ntohs(get_uint16(data+4+addrlen));
          *drain_out = 6+addrlen;
          if (req->command != SOCKS_COMMAND_RESOLVE_PTR &&
              !addressmap_have_mapping(req->address,0)) {
            log_unsafe_socks_warning(5, req->address, req->port, safe_socks);
            if (safe_socks) {
              socks_request_set_socks5_error(req, SOCKS5_NOT_ALLOWED);
              return -1;
            }
          }
          return 1;
        }
        case 3: /* fqdn */
          log_debug(LD_APP,"socks5: fqdn address type");
          if (req->command == SOCKS_COMMAND_RESOLVE_PTR) {
            socks_request_set_socks5_error(req,
                                           SOCKS5_ADDRESS_TYPE_NOT_SUPPORTED);
            log_warn(LD_APP, "socks5 received RESOLVE_PTR command with "
                     "hostname type. Rejecting.");
            return -1;
          }
          len = (unsigned char)*(data+4);
          if (datalen < 7+len) { /* addr/port there? */
            *want_length_out = 7+len;
            return 0; /* not yet */
          }
          if (len+1 > MAX_SOCKS_ADDR_LEN) {
            socks_request_set_socks5_error(req, SOCKS5_GENERAL_ERROR);
            log_warn(LD_APP,
                     "socks5 hostname is %d bytes, which doesn't fit in "
                     "%d. Rejecting.", len+1,MAX_SOCKS_ADDR_LEN);
            return -1;
          }
          memcpy(req->address,data+5,len);
          req->address[len] = 0;
          req->port = ntohs(get_uint16(data+5+len));
          *drain_out = 5+len+2;

          if (!string_is_valid_hostname(req->address)) {
            socks_request_set_socks5_error(req, SOCKS5_GENERAL_ERROR);

            log_warn(LD_PROTOCOL,
                     "Your application (using socks5 to port %d) gave Tor "
                     "a malformed hostname: %s. Rejecting the connection.",
                     req->port, escaped_safe_str_client(req->address));
            return -1;
          }
          if (log_sockstype)
            log_notice(LD_APP,
                  "Your application (using socks5 to port %d) instructed "
                  "Tor to take care of the DNS resolution itself if "
                  "necessary. This is good.", req->port);
          return 1;
        default: /* unsupported */
          socks_request_set_socks5_error(req,
                                         SOCKS5_ADDRESS_TYPE_NOT_SUPPORTED);
          log_warn(LD_APP,"socks5: unsupported address type %d. Rejecting.",
                   (int) *(data+3));
          return -1;
      }
      tor_assert(0);
      break;
    case 4: { /* socks4 */
      enum {socks4, socks4a} socks4_prot = socks4a;
      const char *authstart, *authend;
      /* http://ss5.sourceforge.net/socks4.protocol.txt */
      /* http://ss5.sourceforge.net/socks4A.protocol.txt */

      req->socks_version = 4;
      if (datalen < SOCKS4_NETWORK_LEN) {/* basic info available? */
        *want_length_out = SOCKS4_NETWORK_LEN;
        return 0; /* not yet */
      }
      // buf_pullup(buf, 1280);
      req->command = (unsigned char) *(data+1);
      if (req->command != SOCKS_COMMAND_CONNECT &&
          req->command != SOCKS_COMMAND_RESOLVE) {
        /* not a connect or resolve? we don't support it. (No resolve_ptr with
         * socks4.) */
        log_warn(LD_APP,"socks4: command %d not recognized. Rejecting.",
                 req->command);
        return -1;
      }

      req->port = ntohs(get_uint16(data+2));
      destip = ntohl(get_uint32(data+4));
      if ((!req->port && req->command!=SOCKS_COMMAND_RESOLVE) || !destip) {
        log_warn(LD_APP,"socks4: Port or DestIP is zero. Rejecting.");
        return -1;
      }
      if (destip >> 8) {
        log_debug(LD_APP,"socks4: destip not in form 0.0.0.x.");
        in.s_addr = htonl(destip);
        tor_inet_ntoa(&in,tmpbuf,sizeof(tmpbuf));
        if (strlen(tmpbuf)+1 > MAX_SOCKS_ADDR_LEN) {
          log_debug(LD_APP,"socks4 addr (%d bytes) too long. Rejecting.",
                    (int)strlen(tmpbuf));
          return -1;
        }
        log_debug(LD_APP,
                  "socks4: successfully read destip (%s)",
                  safe_str_client(tmpbuf));
        socks4_prot = socks4;
      }

      authstart = data + SOCKS4_NETWORK_LEN;
      next = memchr(authstart, 0,
                    datalen-SOCKS4_NETWORK_LEN);
      if (!next) {
        if (datalen >= 1024) {
          log_debug(LD_APP, "Socks4 user name too long; rejecting.");
          return -1;
        }
        log_debug(LD_APP,"socks4: Username not here yet.");
        *want_length_out = datalen+1024; /* More than we need, but safe */
        return 0;
      }
      authend = next;
      tor_assert(next < data+datalen);

      startaddr = NULL;
      if (socks4_prot != socks4a &&
          !addressmap_have_mapping(tmpbuf,0)) {
        log_unsafe_socks_warning(4, tmpbuf, req->port, safe_socks);

        if (safe_socks)
          return -1;
      }
      if (socks4_prot == socks4a) {
        if (next+1 == data+datalen) {
          log_debug(LD_APP,"socks4: No part of destaddr here yet.");
          *want_length_out = datalen + 1024; /* More than we need, but safe */
          return 0;
        }
        startaddr = next+1;
        next = memchr(startaddr, 0, data + datalen - startaddr);
        if (!next) {
          if (datalen >= 1024) {
            log_debug(LD_APP,"socks4: Destaddr too long.");
            return -1;
          }
          log_debug(LD_APP,"socks4: Destaddr not all here yet.");
          *want_length_out = datalen + 1024; /* More than we need, but safe */
          return 0;
        }
        if (MAX_SOCKS_ADDR_LEN <= next-startaddr) {
          log_warn(LD_APP,"socks4: Destaddr too long. Rejecting.");
          return -1;
        }
        // tor_assert(next < buf->cur+buf_datalen(buf));

        if (log_sockstype)
          log_notice(LD_APP,
                     "Your application (using socks4a to port %d) instructed "
                     "Tor to take care of the DNS resolution itself if "
                     "necessary. This is good.", req->port);
      }
      log_debug(LD_APP,"socks4: Everything is here. Success.");
      strlcpy(req->address, startaddr ? startaddr : tmpbuf,
              sizeof(req->address));
      if (!string_is_valid_hostname(req->address)) {
        log_warn(LD_PROTOCOL,
                 "Your application (using socks4 to port %d) gave Tor "
                 "a malformed hostname: %s. Rejecting the connection.",
                 req->port, escaped_safe_str_client(req->address));
        return -1;
      }
      if (authend != authstart) {
        req->got_auth = 1;
        req->usernamelen = authend - authstart;
        req->username = tor_memdup(authstart, authend - authstart);
      }
      /* next points to the final \0 on inbuf */
      *drain_out = next - data + 1;
      return 1;
    }
    case 'G': /* get */
    case 'H': /* head */
    case 'P': /* put/post */
    case 'C': /* connect */
      strlcpy((char*)req->reply, SOCKS_PROXY_IS_NOT_AN_HTTP_PROXY_MSG,
              MAX_SOCKS_REPLY_LEN);
      req->replylen = strlen((char*)req->reply)+1;
      /* fall through */
    default: /* version is not socks4 or socks5 */
      log_warn(LD_APP,
               "Socks version %d not recognized. (Tor is not an http proxy.)",
               *(data));
      {
        /* Tell the controller the first 8 bytes. */
        char *tmp = tor_strndup(data, datalen < 8 ? datalen : 8);
        control_event_client_status(LOG_WARN,
                                    "SOCKS_UNKNOWN_PROTOCOL DATA=\"%s\"",
                                    escaped(tmp));
        tor_free(tmp);
      }
      return -1;
  }
}

/** Inspect a reply from SOCKS server stored in <b>buf</b> according
 * to <b>state</b>, removing the protocol data upon success. Return 0 on
 * incomplete response, 1 on success and -1 on error, in which case
 * <b>reason</b> is set to a descriptive message (free() when finished
 * with it).
 *
 * As a special case, 2 is returned when user/pass is required
 * during SOCKS5 handshake and user/pass is configured.
 */
int
fetch_from_buf_socks_client(buf_t *buf, int state, char **reason)
{
  ssize_t drain = 0;
  int r;
  const char *head = NULL;
  size_t datalen = 0;

  if (buf_datalen(buf) < 2)
    return 0;

  buf_pullup(buf, MAX_SOCKS_MESSAGE_LEN, &head, &datalen);
  tor_assert(head && datalen >= 2);

  r = parse_socks_client((uint8_t*)head, datalen,
                         state, reason, &drain);
  if (drain > 0)
    buf_remove_from_front(buf, drain);
  else if (drain < 0)
    buf_clear(buf);

  return r;
}

/** Implementation logic for fetch_from_*_socks_client. */
static int
parse_socks_client(const uint8_t *data, size_t datalen,
                   int state, char **reason,
                   ssize_t *drain_out)
{
  unsigned int addrlen;
  *drain_out = 0;
  if (datalen < 2)
    return 0;

  switch (state) {
    case PROXY_SOCKS4_WANT_CONNECT_OK:
      /* Wait for the complete response */
      if (datalen < 8)
        return 0;

      if (data[1] != 0x5a) {
        *reason = tor_strdup(socks4_response_code_to_string(data[1]));
        return -1;
      }

      /* Success */
      *drain_out = 8;
      return 1;

    case PROXY_SOCKS5_WANT_AUTH_METHOD_NONE:
      /* we don't have any credentials */
      if (data[1] != 0x00) {
        *reason = tor_strdup("server doesn't support any of our "
                             "available authentication methods");
        return -1;
      }

      log_info(LD_NET, "SOCKS 5 client: continuing without authentication");
      *drain_out = -1;
      return 1;

    case PROXY_SOCKS5_WANT_AUTH_METHOD_RFC1929:
      /* we have a username and password. return 1 if we can proceed without
       * providing authentication, or 2 otherwise. */
      switch (data[1]) {
        case 0x00:
          log_info(LD_NET, "SOCKS 5 client: we have auth details but server "
                            "doesn't require authentication.");
          *drain_out = -1;
          return 1;
        case 0x02:
          log_info(LD_NET, "SOCKS 5 client: need authentication.");
          *drain_out = -1;
          return 2;
        /* fall through */
      }

      *reason = tor_strdup("server doesn't support any of our available "
                           "authentication methods");
      return -1;

    case PROXY_SOCKS5_WANT_AUTH_RFC1929_OK:
      /* handle server reply to rfc1929 authentication */
      if (data[1] != 0x00) {
        *reason = tor_strdup("authentication failed");
        return -1;
      }

      log_info(LD_NET, "SOCKS 5 client: authentication successful.");
      *drain_out = -1;
      return 1;

    case PROXY_SOCKS5_WANT_CONNECT_OK:
      /* response is variable length. BND.ADDR, etc, isn't needed
       * (don't bother with buf_pullup()), but make sure to eat all
       * the data used */

      /* wait for address type field to arrive */
      if (datalen < 4)
        return 0;

      switch (data[3]) {
        case 0x01: /* ip4 */
          addrlen = 4;
          break;
        case 0x04: /* ip6 */
          addrlen = 16;
          break;
        case 0x03: /* fqdn (can this happen here?) */
          if (datalen < 5)
            return 0;
          addrlen = 1 + data[4];
          break;
        default:
          *reason = tor_strdup("invalid response to connect request");
          return -1;
      }

      /* wait for address and port */
      if (datalen < 6 + addrlen)
        return 0;

      if (data[1] != 0x00) {
        *reason = tor_strdup(socks5_response_code_to_string(data[1]));
        return -1;
      }

      *drain_out = 6 + addrlen;
      return 1;
  }

  /* shouldn't get here... */
  tor_assert(0);

  return -1;
}