x11fwd.c

   1 /*
   2  * Platform-independent bits of X11 forwarding.
   3  */
   4
   5 #include <stdio.h>
   6 #include <stdlib.h>
   7 #include <assert.h>
   8 #include <time.h>
   9
  10 #include "putty.h"
  11 #include "ssh.h"
  12 #include "tree234.h"
  13
  14 #define GET_16BIT(endian, cp) \
  15   (endian=='B' ? GET_16BIT_MSB_FIRST(cp) : GET_16BIT_LSB_FIRST(cp))
  16
  17 #define PUT_16BIT(endian, cp, val) \
  18   (endian=='B' ? PUT_16BIT_MSB_FIRST(cp, val) : PUT_16BIT_LSB_FIRST(cp, val))
  19
  20 const char *const x11_authnames[] = {
  21     "", "MIT-MAGIC-COOKIE-1", "XDM-AUTHORIZATION-1"
  22 };
  23
  24 struct XDMSeen {
  25     unsigned int time;
  26     unsigned char clientid[6];
  27 };
  28
  29 struct X11Private {
  30     const struct plug_function_table *fn;
  31     /* the above variable absolutely *must* be the first in this structure */
  32     unsigned char firstpkt[12];        /* first X data packet */
  33     struct X11Display *disp;
  34     char *auth_protocol;
  35     unsigned char *auth_data;
  36     int data_read, auth_plen, auth_psize, auth_dlen, auth_dsize;
  37     int verified;
  38     int throttled, throttle_override;
  39     unsigned long peer_ip;
  40     int peer_port;
  41     void *c;                           /* data used by ssh.c */
  42     Socket s;
  43 };
  44
  45 static int xdmseen_cmp(void *a, void *b)
  46 {
  47     struct XDMSeen *sa = a, *sb = b;
  48     return sa->time > sb->time ? 1 :
  49            sa->time < sb->time ? -1 :
  50            memcmp(sa->clientid, sb->clientid, sizeof(sa->clientid));
  51 }
  52
  53 /* Do-nothing "plug" implementation, used by x11_setup_display() when it
  54  * creates a trial connection (and then immediately closes it).
  55  * XXX: bit out of place here, could in principle live in a platform-
  56  *      independent network.c or something */
  57 static void dummy_plug_log(Plug p, int type, SockAddr addr, int port,
  58                            const char *error_msg, int error_code) { }
  59 static int dummy_plug_closing
  60      (Plug p, const char *error_msg, int error_code, int calling_back)
  61 { return 1; }
  62 static int dummy_plug_receive(Plug p, int urgent, char *data, int len)
  63 { return 1; }
  64 static void dummy_plug_sent(Plug p, int bufsize) { }
  65 static int dummy_plug_accepting(Plug p, OSSocket sock) { return 1; }
  66 static const struct plug_function_table dummy_plug = {
  67     dummy_plug_log, dummy_plug_closing, dummy_plug_receive,
  68     dummy_plug_sent, dummy_plug_accepting
  69 };
  70
  71 struct X11Display *x11_setup_display(char *display, int authtype,
  72                                      const Config *cfg)
  73 {
  74     struct X11Display *disp = snew(struct X11Display);
  75     char *localcopy;
  76     int i;
  77
  78     if (!display || !*display) {
  79         localcopy = platform_get_x_display();
  80         if (!localcopy || !*localcopy) {
  81             sfree(localcopy);
  82             localcopy = dupstr(":0");  /* plausible default for any platform */
  83         }
  84     } else
  85         localcopy = dupstr(display);
  86
  87     /*
  88      * Parse the display name.
  89      *
  90      * We expect this to have one of the following forms:
  91      *
  92      *  - the standard X format which looks like
  93      *    [ [ protocol '/' ] host ] ':' displaynumber [ '.' screennumber ]
  94      *    (X11 also permits a double colon to indicate DECnet, but
  95      *    that's not our problem, thankfully!)
  96      *
  97      *  - only seen in the wild on MacOS (so far): a pathname to a
  98      *    Unix-domain socket, which will typically and confusingly
  99      *    end in ":0", and which I'm currently distinguishing from
 100      *    the standard scheme by noting that it starts with '/'.
 101      */
 102     if (localcopy[0] == '/') {
 103         disp->unixsocketpath = localcopy;
 104         disp->unixdomain = TRUE;
 105         disp->hostname = NULL;
 106         disp->displaynum = -1;
 107         disp->screennum = 0;
 108         disp->addr = NULL;
 109     } else {
 110         char *colon, *dot, *slash;
 111         char *protocol, *hostname;
 112
 113         colon = strrchr(localcopy, ':');
 114         if (!colon) {
 115             sfree(disp);
 116             sfree(localcopy);
 117             return NULL;               /* FIXME: report a specific error? */
 118         }
 119
 120         *colon++ = '\0';
 121         dot = strchr(colon, '.');
 122         if (dot)
 123             *dot++ = '\0';
 124
 125         disp->displaynum = atoi(colon);
 126         if (dot)
 127             disp->screennum = atoi(dot);
 128         else
 129             disp->screennum = 0;
 130
 131         protocol = NULL;
 132         hostname = localcopy;
 133         if (colon > localcopy) {
 134             slash = strchr(localcopy, '/');
 135             if (slash) {
 136                 *slash++ = '\0';
 137                 protocol = localcopy;
 138                 hostname = slash;
 139             }
 140         }
 141
 142         disp->hostname = *hostname ? dupstr(hostname) : NULL;
 143
 144         if (protocol)
 145             disp->unixdomain = (!strcmp(protocol, "local") ||
 146                                 !strcmp(protocol, "unix"));
 147         else if (!*hostname || !strcmp(hostname, "unix"))
 148             disp->unixdomain = platform_uses_x11_unix_by_default;
 149         else
 150             disp->unixdomain = FALSE;
 151
 152         if (!disp->hostname && !disp->unixdomain)
 153             disp->hostname = dupstr("localhost");
 154
 155         disp->unixsocketpath = NULL;
 156         disp->addr = NULL;
 157
 158         sfree(localcopy);
 159     }
 160
 161     /*
 162      * Look up the display hostname, if we need to.
 163      */
 164     if (!disp->unixdomain) {
 165         const char *err;
 166
 167         disp->port = 6000 + disp->displaynum;
 168         disp->addr = name_lookup(disp->hostname, disp->port,
 169                                  &disp->realhost, cfg, ADDRTYPE_UNSPEC);
 170
 171         if ((err = sk_addr_error(disp->addr)) != NULL) {
 172             sk_addr_free(disp->addr);
 173             sfree(disp->hostname);
 174             sfree(disp->unixsocketpath);
 175             return NULL;               /* FIXME: report an error */
 176         }
 177     }
 178
 179     /*
 180      * Try upgrading an IP-style localhost display to a Unix-socket
 181      * display (as the standard X connection libraries do).
 182      */
 183     if (!disp->unixdomain && sk_address_is_local(disp->addr)) {
 184         SockAddr ux = platform_get_x11_unix_address(NULL, disp->displaynum);
 185         const char *err = sk_addr_error(ux);
 186         if (!err) {
 187             /* Create trial connection to see if there is a useful Unix-domain
 188              * socket */
 189             const struct plug_function_table *dummy = &dummy_plug;
 190             Socket s = sk_new(sk_addr_dup(ux), 0, 0, 0, 0, 0, (Plug)&dummy);
 191             err = sk_socket_error(s);
 192             sk_close(s);
 193         }
 194         if (err) {
 195             sk_addr_free(ux);
 196         } else {
 197             sk_addr_free(disp->addr);
 198             disp->unixdomain = TRUE;
 199             disp->addr = ux;
 200             /* Fill in the rest in a moment */
 201         }
 202     }
 203
 204     if (disp->unixdomain) {
 205         if (!disp->addr)
 206             disp->addr = platform_get_x11_unix_address(disp->unixsocketpath,
 207                                                        disp->displaynum);
 208         if (disp->unixsocketpath)
 209             disp->realhost = dupstr(disp->unixsocketpath);
 210         else
 211             disp->realhost = dupprintf("unix:%d", disp->displaynum);
 212         disp->port = 0;
 213     }
 214
 215     /*
 216      * Invent the remote authorisation details.
 217      */
 218     if (authtype == X11_MIT) {
 219         disp->remoteauthproto = X11_MIT;
 220
 221         /* MIT-MAGIC-COOKIE-1. Cookie size is 128 bits (16 bytes). */
 222         disp->remoteauthdata = snewn(16, unsigned char);
 223         for (i = 0; i < 16; i++)
 224             disp->remoteauthdata[i] = random_byte();
 225         disp->remoteauthdatalen = 16;
 226
 227         disp->xdmseen = NULL;
 228     } else {
 229         assert(authtype == X11_XDM);
 230         disp->remoteauthproto = X11_XDM;
 231
 232         /* XDM-AUTHORIZATION-1. Cookie size is 16 bytes; byte 8 is zero. */
 233         disp->remoteauthdata = snewn(16, unsigned char);
 234         for (i = 0; i < 16; i++)
 235             disp->remoteauthdata[i] = (i == 8 ? 0 : random_byte());
 236         disp->remoteauthdatalen = 16;
 237
 238         disp->xdmseen = newtree234(xdmseen_cmp);
 239     }
 240     disp->remoteauthprotoname = dupstr(x11_authnames[disp->remoteauthproto]);
 241     disp->remoteauthdatastring = snewn(disp->remoteauthdatalen * 2 + 1, char);
 242     for (i = 0; i < disp->remoteauthdatalen; i++)
 243         sprintf(disp->remoteauthdatastring + i*2, "%02x",
 244                 disp->remoteauthdata[i]);
 245
 246     /*
 247      * Fetch the local authorisation details.
 248      */
 249     disp->localauthproto = X11_NO_AUTH;
 250     disp->localauthdata = NULL;
 251     disp->localauthdatalen = 0;
 252     platform_get_x11_auth(disp, cfg);
 253
 254     return disp;
 255 }
 256
 257 void x11_free_display(struct X11Display *disp)
 258 {
 259     if (disp->xdmseen != NULL) {
 260         struct XDMSeen *seen;
 261         while ((seen = delpos234(disp->xdmseen, 0)) != NULL)
 262             sfree(seen);
 263         freetree234(disp->xdmseen);
 264     }
 265     sfree(disp->hostname);
 266     sfree(disp->unixsocketpath);
 267     if (disp->localauthdata)
 268         memset(disp->localauthdata, 0, disp->localauthdatalen);
 269     sfree(disp->localauthdata);
 270     if (disp->remoteauthdata)
 271         memset(disp->remoteauthdata, 0, disp->remoteauthdatalen);
 272     sfree(disp->remoteauthdata);
 273     sfree(disp->remoteauthprotoname);
 274     sfree(disp->remoteauthdatastring);
 275     sk_addr_free(disp->addr);
 276     sfree(disp);
 277 }
 278
 279 #define XDM_MAXSKEW 20*60      /* 20 minute clock skew should be OK */
 280
 281 static char *x11_verify(unsigned long peer_ip, int peer_port,
 282                         struct X11Display *disp, char *proto,
 283                         unsigned char *data, int dlen)
 284 {
 285     if (strcmp(proto, x11_authnames[disp->remoteauthproto]) != 0)
 286         return "wrong authorisation protocol attempted";
 287     if (disp->remoteauthproto == X11_MIT) {
 288         if (dlen != disp->remoteauthdatalen)
 289             return "MIT-MAGIC-COOKIE-1 data was wrong length";
 290         if (memcmp(disp->remoteauthdata, data, dlen) != 0)
 291             return "MIT-MAGIC-COOKIE-1 data did not match";
 292     }
 293     if (disp->remoteauthproto == X11_XDM) {
 294         unsigned long t;
 295         time_t tim;
 296         int i;
 297         struct XDMSeen *seen, *ret;
 298
 299         if (dlen != 24)
 300             return "XDM-AUTHORIZATION-1 data was wrong length";
 301         if (peer_port == -1)
 302             return "cannot do XDM-AUTHORIZATION-1 without remote address data";
 303         des_decrypt_xdmauth(disp->remoteauthdata+9, data, 24);
 304         if (memcmp(disp->remoteauthdata, data, 8) != 0)
 305             return "XDM-AUTHORIZATION-1 data failed check"; /* cookie wrong */
 306         if (GET_32BIT_MSB_FIRST(data+8) != peer_ip)
 307             return "XDM-AUTHORIZATION-1 data failed check";   /* IP wrong */
 308         if ((int)GET_16BIT_MSB_FIRST(data+12) != peer_port)
 309             return "XDM-AUTHORIZATION-1 data failed check";   /* port wrong */
 310         t = GET_32BIT_MSB_FIRST(data+14);
 311         for (i = 18; i < 24; i++)
 312             if (data[i] != 0)          /* zero padding wrong */
 313                 return "XDM-AUTHORIZATION-1 data failed check";
 314         tim = time(NULL);
 315         if (abs(t - tim) > XDM_MAXSKEW)
 316             return "XDM-AUTHORIZATION-1 time stamp was too far out";
 317         seen = snew(struct XDMSeen);
 318         seen->time = t;
 319         memcpy(seen->clientid, data+8, 6);
 320         assert(disp->xdmseen != NULL);
 321         ret = add234(disp->xdmseen, seen);
 322         if (ret != seen) {
 323             sfree(seen);
 324             return "XDM-AUTHORIZATION-1 data replayed";
 325         }
 326         /* While we're here, purge entries too old to be replayed. */
 327         for (;;) {
 328             seen = index234(disp->xdmseen, 0);
 329             assert(seen != NULL);
 330             if (t - seen->time <= XDM_MAXSKEW)
 331                 break;
 332             sfree(delpos234(disp->xdmseen, 0));
 333         }
 334     }
 335     /* implement other protocols here if ever required */
 336     return NULL;
 337 }
 338
 339 void x11_get_auth_from_authfile(struct X11Display *disp,
 340                                 const char *authfilename)
 341 {
 342     FILE *authfp;
 343     char *buf, *ptr, *str[4];
 344     int len[4];
 345     int family, protocol;
 346     int ideal_match = FALSE;
 347     char *ourhostname = get_hostname();
 348
 349     /*
 350      * Normally we should look for precisely the details specified in
 351      * `disp'. However, there's an oddity when the display is local:
 352      * displays like "localhost:0" usually have their details stored
 353      * in a Unix-domain-socket record (even if there isn't actually a
 354      * real Unix-domain socket available, as with OpenSSH's proxy X11
 355      * server).
 356      *
 357      * This is apparently a fudge to get round the meaninglessness of
 358      * "localhost" in a shared-home-directory context -- xauth entries
 359      * for Unix-domain sockets already disambiguate this by storing
 360      * the *local* hostname in the conveniently-blank hostname field,
 361      * but IP "localhost" records couldn't do this. So, typically, an
 362      * IP "localhost" entry in the auth database isn't present and if
 363      * it were it would be ignored.
 364      *
 365      * However, we don't entirely trust that (say) Windows X servers
 366      * won't rely on a straight "localhost" entry, bad idea though
 367      * that is; so if we can't find a Unix-domain-socket entry we'll
 368      * fall back to an IP-based entry if we can find one.
 369      */
 370     int localhost = !disp->unixdomain && sk_address_is_local(disp->addr);
 371
 372     authfp = fopen(authfilename, "rb");
 373     if (!authfp)
 374         return;
 375
 376     /* Records in .Xauthority contain four strings of up to 64K each */
 377     buf = snewn(65537 * 4, char);
 378
 379     while (!ideal_match) {
 380         int c, i, j, match = FALSE;
 381
 382 #define GET do { c = fgetc(authfp); if (c == EOF) goto done; c = (unsigned char)c; } while (0)
 383         /* Expect a big-endian 2-byte number giving address family */
 384         GET; family = c;
 385         GET; family = (family << 8) | c;
 386         /* Then expect four strings, each composed of a big-endian 2-byte
 387          * length field followed by that many bytes of data */
 388         ptr = buf;
 389         for (i = 0; i < 4; i++) {
 390             GET; len[i] = c;
 391             GET; len[i] = (len[i] << 8) | c;
 392             str[i] = ptr;
 393             for (j = 0; j < len[i]; j++) {
 394                 GET; *ptr++ = c;
 395             }
 396             *ptr++ = '\0';
 397         }
 398 #undef GET
 399
 400         /*
 401          * Now we have a full X authority record in memory. See
 402          * whether it matches the display we're trying to
 403          * authenticate to.
 404          *
 405          * The details we've just read should be interpreted as
 406          * follows:
 407          *
 408          *  - 'family' is the network address family used to
 409          *    connect to the display. 0 means IPv4; 6 means IPv6;
 410          *    256 means Unix-domain sockets.
 411          *
 412          *  - str[0] is the network address itself. For IPv4 and
 413          *    IPv6, this is a string of binary data of the
 414          *    appropriate length (respectively 4 and 16 bytes)
 415          *    representing the address in big-endian format, e.g.
 416          *    7F 00 00 01 means IPv4 localhost. For Unix-domain
 417          *    sockets, this is the host name of the machine on
 418          *    which the Unix-domain display resides (so that an
 419          *    .Xauthority file on a shared file system can contain
 420          *    authority entries for Unix-domain displays on
 421          *    several machines without them clashing).
 422          *
 423          *  - str[1] is the display number. I've no idea why
 424          *    .Xauthority stores this as a string when it has a
 425          *    perfectly good integer format, but there we go.
 426          *
 427          *  - str[2] is the authorisation method, encoded as its
 428          *    canonical string name (i.e. "MIT-MAGIC-COOKIE-1",
 429          *    "XDM-AUTHORIZATION-1" or something we don't
 430          *    recognise).
 431          *
 432          *  - str[3] is the actual authorisation data, stored in
 433          *    binary form.
 434          */
 435
 436         if (disp->displaynum < 0 || disp->displaynum != atoi(str[1]))
 437             continue;                  /* not the one */
 438
 439         for (protocol = 1; protocol < lenof(x11_authnames); protocol++)
 440             if (!strcmp(str[2], x11_authnames[protocol]))
 441                 break;
 442         if (protocol == lenof(x11_authnames))
 443             continue;  /* don't recognise this protocol, look for another */
 444
 445         switch (family) {
 446           case 0:   /* IPv4 */
 447             if (!disp->unixdomain &&
 448                 sk_addrtype(disp->addr) == ADDRTYPE_IPV4) {
 449                 char buf[4];
 450                 sk_addrcopy(disp->addr, buf);
 451                 if (len[0] == 4 && !memcmp(str[0], buf, 4)) {
 452                     match = TRUE;
 453                     /* If this is a "localhost" entry, note it down
 454                      * but carry on looking for a Unix-domain entry. */
 455                     ideal_match = !localhost;
 456                 }
 457             }
 458             break;
 459           case 6:   /* IPv6 */
 460             if (!disp->unixdomain &&
 461                 sk_addrtype(disp->addr) == ADDRTYPE_IPV6) {
 462                 char buf[16];
 463                 sk_addrcopy(disp->addr, buf);
 464                 if (len[0] == 16 && !memcmp(str[0], buf, 16)) {
 465                     match = TRUE;
 466                     ideal_match = !localhost;
 467                 }
 468             }
 469             break;
 470           case 256: /* Unix-domain / localhost */
 471             if ((disp->unixdomain || localhost)
 472                 && ourhostname && !strcmp(ourhostname, str[0]))
 473                 /* A matching Unix-domain socket is always the best
 474                  * match. */
 475                 match = ideal_match = TRUE;
 476             break;
 477         }
 478
 479         if (match) {
 480             /* Current best guess -- may be overridden if !ideal_match */
 481             disp->localauthproto = protocol;
 482             sfree(disp->localauthdata); /* free previous guess, if any */
 483             disp->localauthdata = snewn(len[3], unsigned char);
 484             memcpy(disp->localauthdata, str[3], len[3]);
 485             disp->localauthdatalen = len[3];
 486         }
 487     }
 488
 489     done:
 490     fclose(authfp);
 491     memset(buf, 0, 65537 * 4);
 492     sfree(buf);
 493     sfree(ourhostname);
 494 }
 495
 496 static void x11_log(Plug p, int type, SockAddr addr, int port,
 497                     const char *error_msg, int error_code)
 498 {
 499     /* We have no interface to the logging module here, so we drop these. */
 500 }
 501
 502 static int x11_closing(Plug plug, const char *error_msg, int error_code,
 503                        int calling_back)
 504 {
 505     struct X11Private *pr = (struct X11Private *) plug;
 506
 507     /*
 508      * We have no way to communicate down the forwarded connection,
 509      * so if an error occurred on the socket, we just ignore it
 510      * and treat it like a proper close.
 511      */
 512     sshfwd_close(pr->c);
 513     x11_close(pr->s);
 514     return 1;
 515 }
 516
 517 static int x11_receive(Plug plug, int urgent, char *data, int len)
 518 {
 519     struct X11Private *pr = (struct X11Private *) plug;
 520
 521     if (sshfwd_write(pr->c, data, len) > 0) {
 522         pr->throttled = 1;
 523         sk_set_frozen(pr->s, 1);
 524     }
 525
 526     return 1;
 527 }
 528
 529 static void x11_sent(Plug plug, int bufsize)
 530 {
 531     struct X11Private *pr = (struct X11Private *) plug;
 532
 533     sshfwd_unthrottle(pr->c, bufsize);
 534 }
 535
 536 /*
 537  * When setting up X forwarding, we should send the screen number
 538  * from the specified local display. This function extracts it from
 539  * the display string.
 540  */
 541 int x11_get_screen_number(char *display)
 542 {
 543     int n;
 544
 545     n = strcspn(display, ":");
 546     if (!display[n])
 547         return 0;
 548     n = strcspn(display, ".");
 549     if (!display[n])
 550         return 0;
 551     return atoi(display + n + 1);
 552 }
 553
 554 /*
 555  * Called to set up the raw connection.
 556  *
 557  * Returns an error message, or NULL on success.
 558  * also, fills the SocketsStructure
 559  */
 560 extern const char *x11_init(Socket *s, struct X11Display *disp, void *c,
 561                             const char *peeraddr, int peerport,
 562                             const Config *cfg)
 563 {
 564     static const struct plug_function_table fn_table = {
 565         x11_log,
 566         x11_closing,
 567         x11_receive,
 568         x11_sent,
 569         NULL
 570     };
 571
 572     const char *err;
 573     struct X11Private *pr;
 574
 575     /*
 576      * Open socket.
 577      */
 578     pr = snew(struct X11Private);
 579     pr->fn = &fn_table;
 580     pr->auth_protocol = NULL;
 581     pr->disp = disp;
 582     pr->verified = 0;
 583     pr->data_read = 0;
 584     pr->throttled = pr->throttle_override = 0;
 585     pr->c = c;
 586
 587     pr->s = *s = new_connection(sk_addr_dup(disp->addr),
 588                                 disp->realhost, disp->port,
 589                                 0, 1, 0, 0, (Plug) pr, cfg);
 590     if ((err = sk_socket_error(*s)) != NULL) {
 591         sfree(pr);
 592         return err;
 593     }
 594
 595     /*
 596      * See if we can make sense of the peer address we were given.
 597      */
 598     {
 599         int i[4];
 600         if (peeraddr &&
 601             4 == sscanf(peeraddr, "%d.%d.%d.%d", i+0, i+1, i+2, i+3)) {
 602             pr->peer_ip = (i[0] << 24) | (i[1] << 16) | (i[2] << 8) | i[3];
 603             pr->peer_port = peerport;
 604         } else {
 605             pr->peer_ip = 0;
 606             pr->peer_port = -1;
 607         }
 608     }
 609
 610     sk_set_private_ptr(*s, pr);
 611     return NULL;
 612 }
 613
 614 void x11_close(Socket s)
 615 {
 616     struct X11Private *pr;
 617     if (!s)
 618         return;
 619     pr = (struct X11Private *) sk_get_private_ptr(s);
 620     if (pr->auth_protocol) {
 621         sfree(pr->auth_protocol);
 622         sfree(pr->auth_data);
 623     }
 624
 625     sfree(pr);
 626
 627     sk_close(s);
 628 }
 629
 630 void x11_unthrottle(Socket s)
 631 {
 632     struct X11Private *pr;
 633     if (!s)
 634         return;
 635     pr = (struct X11Private *) sk_get_private_ptr(s);
 636
 637     pr->throttled = 0;
 638     sk_set_frozen(s, pr->throttled || pr->throttle_override);
 639 }
 640
 641 void x11_override_throttle(Socket s, int enable)
 642 {
 643     struct X11Private *pr;
 644     if (!s)
 645         return;
 646     pr = (struct X11Private *) sk_get_private_ptr(s);
 647
 648     pr->throttle_override = enable;
 649     sk_set_frozen(s, pr->throttled || pr->throttle_override);
 650 }
 651
 652 /*
 653  * Called to send data down the raw connection.
 654  */
 655 int x11_send(Socket s, char *data, int len)
 656 {
 657     struct X11Private *pr;
 658     if (!s)
 659         return 0;
 660     pr = (struct X11Private *) sk_get_private_ptr(s);
 661
 662     /*
 663      * Read the first packet.
 664      */
 665     while (len > 0 && pr->data_read < 12)
 666         pr->firstpkt[pr->data_read++] = (unsigned char) (len--, *data++);
 667     if (pr->data_read < 12)
 668         return 0;
 669
 670     /*
 671      * If we have not allocated the auth_protocol and auth_data
 672      * strings, do so now.
 673      */
 674     if (!pr->auth_protocol) {
 675         pr->auth_plen = GET_16BIT(pr->firstpkt[0], pr->firstpkt + 6);
 676         pr->auth_dlen = GET_16BIT(pr->firstpkt[0], pr->firstpkt + 8);
 677         pr->auth_psize = (pr->auth_plen + 3) & ~3;
 678         pr->auth_dsize = (pr->auth_dlen + 3) & ~3;
 679         /* Leave room for a terminating zero, to make our lives easier. */
 680         pr->auth_protocol = snewn(pr->auth_psize + 1, char);
 681         pr->auth_data = snewn(pr->auth_dsize, unsigned char);
 682     }
 683
 684     /*
 685      * Read the auth_protocol and auth_data strings.
 686      */
 687     while (len > 0 && pr->data_read < 12 + pr->auth_psize)
 688         pr->auth_protocol[pr->data_read++ - 12] = (len--, *data++);
 689     while (len > 0 && pr->data_read < 12 + pr->auth_psize + pr->auth_dsize)
 690         pr->auth_data[pr->data_read++ - 12 -
 691                       pr->auth_psize] = (unsigned char) (len--, *data++);
 692     if (pr->data_read < 12 + pr->auth_psize + pr->auth_dsize)
 693         return 0;
 694
 695     /*
 696      * If we haven't verified the authorisation, do so now.
 697      */
 698     if (!pr->verified) {
 699         char *err;
 700
 701         pr->auth_protocol[pr->auth_plen] = '\0';        /* ASCIZ */
 702         err = x11_verify(pr->peer_ip, pr->peer_port,
 703                          pr->disp, pr->auth_protocol,
 704                          pr->auth_data, pr->auth_dlen);
 705
 706         /*
 707          * If authorisation failed, construct and send an error
 708          * packet, then terminate the connection.
 709          */
 710         if (err) {
 711             char *message;
 712             int msglen, msgsize;
 713             unsigned char *reply;
 714
 715             message = dupprintf("%s X11 proxy: %s", appname, err);
 716             msglen = strlen(message);
 717             reply = snewn(8 + msglen+1 + 4, unsigned char); /* include zero */
 718             msgsize = (msglen + 3) & ~3;
 719             reply[0] = 0;              /* failure */
 720             reply[1] = msglen;         /* length of reason string */
 721             memcpy(reply + 2, pr->firstpkt + 2, 4);     /* major/minor proto vsn */
 722             PUT_16BIT(pr->firstpkt[0], reply + 6, msgsize >> 2);/* data len */
 723             memset(reply + 8, 0, msgsize);
 724             memcpy(reply + 8, message, msglen);
 725             sshfwd_write(pr->c, (char *)reply, 8 + msgsize);
 726             sshfwd_close(pr->c);
 727             x11_close(s);
 728             sfree(reply);
 729             sfree(message);
 730             return 0;
 731         }
 732
 733         /*
 734          * Now we know we're going to accept the connection. Strip
 735          * the fake auth data, and optionally put real auth data in
 736          * instead.
 737          */
 738         {
 739             char realauthdata[64];
 740             int realauthlen = 0;
 741             int authstrlen = strlen(x11_authnames[pr->disp->localauthproto]);
 742             int buflen = 0;            /* initialise to placate optimiser */
 743             static const char zeroes[4] = { 0,0,0,0 };
 744             void *buf;
 745
 746             if (pr->disp->localauthproto == X11_MIT) {
 747                 assert(pr->disp->localauthdatalen <= lenof(realauthdata));
 748                 realauthlen = pr->disp->localauthdatalen;
 749                 memcpy(realauthdata, pr->disp->localauthdata, realauthlen);
 750             } else if (pr->disp->localauthproto == X11_XDM &&
 751                        pr->disp->localauthdatalen == 16 &&
 752                        ((buf = sk_getxdmdata(s, &buflen))!=0)) {
 753                 time_t t;
 754                 realauthlen = (buflen+12+7) & ~7;
 755                 assert(realauthlen <= lenof(realauthdata));
 756                 memset(realauthdata, 0, realauthlen);
 757                 memcpy(realauthdata, pr->disp->localauthdata, 8);
 758                 memcpy(realauthdata+8, buf, buflen);
 759                 t = time(NULL);
 760                 PUT_32BIT_MSB_FIRST(realauthdata+8+buflen, t);
 761                 des_encrypt_xdmauth(pr->disp->localauthdata+9,
 762                                     (unsigned char *)realauthdata,
 763                                     realauthlen);
 764                 sfree(buf);
 765             }
 766             /* implement other auth methods here if required */
 767
 768             PUT_16BIT(pr->firstpkt[0], pr->firstpkt + 6, authstrlen);
 769             PUT_16BIT(pr->firstpkt[0], pr->firstpkt + 8, realauthlen);
 770
 771             sk_write(s, (char *)pr->firstpkt, 12);
 772
 773             if (authstrlen) {
 774                 sk_write(s, x11_authnames[pr->disp->localauthproto],
 775                          authstrlen);
 776                 sk_write(s, zeroes, 3 & (-authstrlen));
 777             }
 778             if (realauthlen) {
 779                 sk_write(s, realauthdata, realauthlen);
 780                 sk_write(s, zeroes, 3 & (-realauthlen));
 781             }
 782         }
 783         pr->verified = 1;
 784     }
 785
 786     /*
 787      * After initialisation, just copy data simply.
 788      */
 789
 790     return sk_write(s, data, len);
 791 }