The Tcpdump Group git mirrors - tcpdump/blob - print-tcp.c

   1 /*
   2  * Copyright (c) 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997
   3  *      The Regents of the University of California.  All rights reserved.
   4  *
   5  * Redistribution and use in source and binary forms, with or without
   6  * modification, are permitted provided that: (1) source code distributions
   7  * retain the above copyright notice and this paragraph in its entirety, (2)
   8  * distributions including binary code include the above copyright notice and
   9  * this paragraph in its entirety in the documentation or other materials
  10  * provided with the distribution, and (3) all advertising materials mentioning
  11  * features or use of this software display the following acknowledgement:
  12  * ``This product includes software developed by the University of California,
  13  * Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
  14  * the University nor the names of its contributors may be used to endorse
  15  * or promote products derived from this software without specific prior
  16  * written permission.
  17  * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
  18  * WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
  19  * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
  20  */
  21
  22
  23 #ifdef HAVE_CONFIG_H
  24 #include "config.h"
  25 #endif
  26
  27 #include <tcpdump-stdinc.h>
  28
  29 #include <rpc/rpc.h>
  30
  31 #include <stdio.h>
  32 #include <stdlib.h>
  33 #include <string.h>
  34
  35 #include "interface.h"
  36 #include "addrtoname.h"
  37 #include "extract.h"
  38
  39 #ifndef lint
  40 static const char rcsid[] _U_ =
  41     "@(#) $Header: /tcpdump/master/tcpdump/print-tcp.c,v 1.107.2.1 2003-11-15 22:29:11 guy Exp $ (LBL)";
  42 #endif
  43 #include "tcp.h"
  44
  45 #include "ip.h"
  46 #ifdef INET6
  47 #include "ip6.h"
  48 #endif
  49 #include "ipproto.h"
  50
  51 #include "nameser.h"
  52
  53 static void print_tcp_rst_data(register const u_char *sp, u_int length);
  54
  55 #define MAX_RST_DATA_LEN        30
  56
  57
  58 struct tha {
  59 #ifndef INET6
  60         struct in_addr src;
  61         struct in_addr dst;
  62 #else
  63         struct in6_addr src;
  64         struct in6_addr dst;
  65 #endif /*INET6*/
  66         u_int port;
  67 };
  68
  69 struct tcp_seq_hash {
  70         struct tcp_seq_hash *nxt;
  71         struct tha addr;
  72         tcp_seq seq;
  73         tcp_seq ack;
  74 };
  75
  76 #define TSEQ_HASHSIZE 919
  77
  78 /* These tcp optinos do not have the size octet */
  79 #define ZEROLENOPT(o) ((o) == TCPOPT_EOL || (o) == TCPOPT_NOP)
  80
  81 static struct tcp_seq_hash tcp_seq_hash[TSEQ_HASHSIZE];
  82
  83
  84 #ifndef TELNET_PORT
  85 #define TELNET_PORT     23
  86 #endif
  87 #ifndef BGP_PORT
  88 #define BGP_PORT        179
  89 #endif
  90 #define NETBIOS_SSN_PORT 139
  91 #ifndef PPTP_PORT
  92 #define PPTP_PORT       1723
  93 #endif
  94 #define BEEP_PORT        10288
  95 #ifndef NFS_PORT
  96 #define NFS_PORT        2049
  97 #endif
  98 #define MSDP_PORT       639
  99 #define LDP_PORT        646
 100
 101 static int tcp_cksum(register const struct ip *ip,
 102                      register const struct tcphdr *tp,
 103                      register int len)
 104 {
 105         union phu {
 106                 struct phdr {
 107                         u_int32_t src;
 108                         u_int32_t dst;
 109                         u_char mbz;
 110                         u_char proto;
 111                         u_int16_t len;
 112                 } ph;
 113                 u_int16_t pa[6];
 114         } phu;
 115         const u_int16_t *sp;
 116
 117         /* pseudo-header.. */
 118         phu.ph.len = htons(len);        /* XXX */
 119         phu.ph.mbz = 0;
 120         phu.ph.proto = IPPROTO_TCP;
 121         memcpy(&phu.ph.src, &ip->ip_src.s_addr, sizeof(u_int32_t));
 122         if (IP_HL(ip) == 5)
 123                 memcpy(&phu.ph.dst, &ip->ip_dst.s_addr, sizeof(u_int32_t));
 124         else
 125                 phu.ph.dst = ip_finddst(ip);
 126
 127         sp = &phu.pa[0];
 128         return in_cksum((u_short *)tp, len,
 129                         sp[0]+sp[1]+sp[2]+sp[3]+sp[4]+sp[5]);
 130 }
 131
 132 #ifdef INET6
 133 static int tcp6_cksum(const struct ip6_hdr *ip6, const struct tcphdr *tp,
 134         int len)
 135 {
 136         size_t i, tlen;
 137         register const u_int16_t *sp;
 138         u_int32_t sum;
 139         union {
 140                 struct {
 141                         struct in6_addr ph_src;
 142                         struct in6_addr ph_dst;
 143                         u_int32_t       ph_len;
 144                         u_int8_t        ph_zero[3];
 145                         u_int8_t        ph_nxt;
 146                 } ph;
 147                 u_int16_t pa[20];
 148         } phu;
 149
 150         tlen = EXTRACT_16BITS(&ip6->ip6_plen) + sizeof(struct ip6_hdr) -
 151             ((const char *)tp - (const char*)ip6);
 152
 153         /* pseudo-header */
 154         memset(&phu, 0, sizeof(phu));
 155         phu.ph.ph_src = ip6->ip6_src;
 156         phu.ph.ph_dst = ip6->ip6_dst;
 157         phu.ph.ph_len = htonl(tlen);
 158         phu.ph.ph_nxt = IPPROTO_TCP;
 159
 160         sum = 0;
 161         for (i = 0; i < sizeof(phu.pa) / sizeof(phu.pa[0]); i++)
 162                 sum += phu.pa[i];
 163
 164         sp = (const u_int16_t *)tp;
 165
 166         for (i = 0; i < (tlen & ~1); i += 2)
 167                 sum += *sp++;
 168
 169         if (tlen & 1)
 170                 sum += htons((*(const u_int8_t *)sp) << 8);
 171
 172         while (sum > 0xffff)
 173                 sum = (sum & 0xffff) + (sum >> 16);
 174         sum = ~sum & 0xffff;
 175
 176         return (sum);
 177 }
 178 #endif
 179
 180 void
 181 tcp_print(register const u_char *bp, register u_int length,
 182           register const u_char *bp2, int fragmented)
 183 {
 184         register const struct tcphdr *tp;
 185         register const struct ip *ip;
 186         register u_char flags;
 187         register u_int hlen;
 188         register char ch;
 189         u_int16_t sport, dport, win, urp;
 190         u_int32_t seq, ack, thseq, thack;
 191         int threv;
 192 #ifdef INET6
 193         register const struct ip6_hdr *ip6;
 194 #endif
 195
 196         tp = (struct tcphdr *)bp;
 197         ip = (struct ip *)bp2;
 198 #ifdef INET6
 199         if (IP_V(ip) == 6)
 200                 ip6 = (struct ip6_hdr *)bp2;
 201         else
 202                 ip6 = NULL;
 203 #endif /*INET6*/
 204         ch = '\0';
 205         if (!TTEST(tp->th_dport)) {
 206                 (void)printf("%s > %s: [|tcp]",
 207                         ipaddr_string(&ip->ip_src),
 208                         ipaddr_string(&ip->ip_dst));
 209                 return;
 210         }
 211
 212         sport = EXTRACT_16BITS(&tp->th_sport);
 213         dport = EXTRACT_16BITS(&tp->th_dport);
 214
 215         hlen = TH_OFF(tp) * 4;
 216
 217         /*
 218          * If data present and NFS port used, assume NFS.
 219          * Pass offset of data plus 4 bytes for RPC TCP msg length
 220          * to NFS print routines.
 221          */
 222         if (!qflag) {
 223                 if ((u_char *)tp + 4 + sizeof(struct rpc_msg) <= snapend &&
 224                     dport == NFS_PORT) {
 225                         nfsreq_print((u_char *)tp + hlen + 4, length - hlen,
 226                                      (u_char *)ip);
 227                         return;
 228                 } else if ((u_char *)tp + 4 + sizeof(struct rpc_msg)
 229                            <= snapend &&
 230                            sport == NFS_PORT) {
 231                         nfsreply_print((u_char *)tp + hlen + 4, length - hlen,
 232                                        (u_char *)ip);
 233                         return;
 234                 }
 235         }
 236 #ifdef INET6
 237         if (ip6) {
 238                 if (ip6->ip6_nxt == IPPROTO_TCP) {
 239                         (void)printf("%s.%s > %s.%s: ",
 240                                 ip6addr_string(&ip6->ip6_src),
 241                                 tcpport_string(sport),
 242                                 ip6addr_string(&ip6->ip6_dst),
 243                                 tcpport_string(dport));
 244                 } else {
 245                         (void)printf("%s > %s: ",
 246                                 tcpport_string(sport), tcpport_string(dport));
 247                 }
 248         } else
 249 #endif /*INET6*/
 250         {
 251                 if (ip->ip_p == IPPROTO_TCP) {
 252                         (void)printf("%s.%s > %s.%s: ",
 253                                 ipaddr_string(&ip->ip_src),
 254                                 tcpport_string(sport),
 255                                 ipaddr_string(&ip->ip_dst),
 256                                 tcpport_string(dport));
 257                 } else {
 258                         (void)printf("%s > %s: ",
 259                                 tcpport_string(sport), tcpport_string(dport));
 260                 }
 261         }
 262
 263         TCHECK(*tp);
 264
 265         seq = EXTRACT_32BITS(&tp->th_seq);
 266         ack = EXTRACT_32BITS(&tp->th_ack);
 267         win = EXTRACT_16BITS(&tp->th_win);
 268         urp = EXTRACT_16BITS(&tp->th_urp);
 269
 270         if (qflag) {
 271                 (void)printf("tcp %d", length - TH_OFF(tp) * 4);
 272                 return;
 273         }
 274         if ((flags = tp->th_flags) & (TH_SYN|TH_FIN|TH_RST|TH_PUSH|
 275                                       TH_ECNECHO|TH_CWR)) {
 276                 if (flags & TH_SYN)
 277                         putchar('S');
 278                 if (flags & TH_FIN)
 279                         putchar('F');
 280                 if (flags & TH_RST)
 281                         putchar('R');
 282                 if (flags & TH_PUSH)
 283                         putchar('P');
 284                 if (flags & TH_CWR)
 285                         putchar('W');   /* congestion _W_indow reduced (ECN) */
 286                 if (flags & TH_ECNECHO)
 287                         putchar('E');   /* ecn _E_cho sent (ECN) */
 288         } else
 289                 putchar('.');
 290
 291         if (!Sflag && (flags & TH_ACK)) {
 292                 register struct tcp_seq_hash *th;
 293                 const void *src, *dst;
 294                 register int rev;
 295                 struct tha tha;
 296                 /*
 297                  * Find (or record) the initial sequence numbers for
 298                  * this conversation.  (we pick an arbitrary
 299                  * collating order so there's only one entry for
 300                  * both directions).
 301                  */
 302 #ifdef INET6
 303                 memset(&tha, 0, sizeof(tha));
 304                 rev = 0;
 305                 if (ip6) {
 306                         src = &ip6->ip6_src;
 307                         dst = &ip6->ip6_dst;
 308                         if (sport > dport)
 309                                 rev = 1;
 310                         else if (sport == dport) {
 311                                 if (memcmp(src, dst, sizeof ip6->ip6_dst) > 0)
 312                                         rev = 1;
 313                         }
 314                         if (rev) {
 315                                 memcpy(&tha.src, dst, sizeof ip6->ip6_dst);
 316                                 memcpy(&tha.dst, src, sizeof ip6->ip6_src);
 317                                 tha.port = dport << 16 | sport;
 318                         } else {
 319                                 memcpy(&tha.dst, dst, sizeof ip6->ip6_dst);
 320                                 memcpy(&tha.src, src, sizeof ip6->ip6_src);
 321                                 tha.port = sport << 16 | dport;
 322                         }
 323                 } else {
 324                         src = &ip->ip_src;
 325                         dst = &ip->ip_dst;
 326                         if (sport > dport)
 327                                 rev = 1;
 328                         else if (sport == dport) {
 329                                 if (memcmp(src, dst, sizeof ip->ip_dst) > 0)
 330                                         rev = 1;
 331                         }
 332                         if (rev) {
 333                                 memcpy(&tha.src, dst, sizeof ip->ip_dst);
 334                                 memcpy(&tha.dst, src, sizeof ip->ip_src);
 335                                 tha.port = dport << 16 | sport;
 336                         } else {
 337                                 memcpy(&tha.dst, dst, sizeof ip->ip_dst);
 338                                 memcpy(&tha.src, src, sizeof ip->ip_src);
 339                                 tha.port = sport << 16 | dport;
 340                         }
 341                 }
 342 #else
 343                 rev = 0;
 344                 src = &ip->ip_src;
 345                 dst = &ip->ip_dst;
 346                 if (sport > dport)
 347                         rev = 1;
 348                 else if (sport == dport) {
 349                         if (memcmp(src, dst, sizeof ip->ip_dst) > 0)
 350                                 rev = 1;
 351                 }
 352                 if (rev) {
 353                         memcpy(&tha.src, dst, sizeof ip->ip_dst);
 354                         memcpy(&tha.dst, src, sizeof ip->ip_src);
 355                         tha.port = dport << 16 | sport;
 356                 } else {
 357                         memcpy(&tha.dst, dst, sizeof ip->ip_dst);
 358                         memcpy(&tha.src, src, sizeof ip->ip_src);
 359                         tha.port = sport << 16 | dport;
 360                 }
 361 #endif
 362
 363                 threv = rev;
 364                 for (th = &tcp_seq_hash[tha.port % TSEQ_HASHSIZE];
 365                      th->nxt; th = th->nxt)
 366                         if (memcmp((char *)&tha, (char *)&th->addr,
 367                                   sizeof(th->addr)) == 0)
 368                                 break;
 369
 370                 if (!th->nxt || (flags & TH_SYN)) {
 371                         /* didn't find it or new conversation */
 372                         if (th->nxt == NULL) {
 373                                 th->nxt = (struct tcp_seq_hash *)
 374                                         calloc(1, sizeof(*th));
 375                                 if (th->nxt == NULL)
 376                                         error("tcp_print: calloc");
 377                         }
 378                         th->addr = tha;
 379                         if (rev)
 380                                 th->ack = seq, th->seq = ack - 1;
 381                         else
 382                                 th->seq = seq, th->ack = ack - 1;
 383                 } else {
 384                         if (rev)
 385                                 seq -= th->ack, ack -= th->seq;
 386                         else
 387                                 seq -= th->seq, ack -= th->ack;
 388                 }
 389
 390                 thseq = th->seq;
 391                 thack = th->ack;
 392         } else {
 393                 /*fool gcc*/
 394                 thseq = thack = threv = 0;
 395         }
 396         if (hlen > length) {
 397                 (void)printf(" [bad hdr length]");
 398                 return;
 399         }
 400
 401         if (IP_V(ip) == 4 && vflag && !fragmented) {
 402                 u_int16_t sum, tcp_sum;
 403                 if (TTEST2(tp->th_sport, length)) {
 404                         sum = tcp_cksum(ip, tp, length);
 405                         if (sum != 0) {
 406                                 tcp_sum = EXTRACT_16BITS(&tp->th_sum);
 407                                 (void)printf(" [bad tcp cksum %x (->%x)!]",
 408                                     tcp_sum, in_cksum_shouldbe(tcp_sum, sum));
 409                         } else
 410                                 (void)printf(" [tcp sum ok]");
 411                 }
 412         }
 413 #ifdef INET6
 414         if (IP_V(ip) == 6 && ip6->ip6_plen && vflag && !fragmented) {
 415                 int sum;
 416                 if (TTEST2(tp->th_sport, length)) {
 417                         sum = tcp6_cksum(ip6, tp, length);
 418                         if (sum != 0)
 419                                 (void)printf(" [bad tcp cksum %x!]", sum);
 420                         else
 421                                 (void)printf(" [tcp sum ok]");
 422                 }
 423         }
 424 #endif
 425
 426         length -= hlen;
 427         if (vflag > 1 || length > 0 || flags & (TH_SYN | TH_FIN | TH_RST))
 428                 (void)printf(" %u:%u(%u)", seq, seq + length, length);
 429         if (flags & TH_ACK)
 430                 (void)printf(" ack %u", ack);
 431
 432         (void)printf(" win %d", win);
 433
 434         if (flags & TH_URG)
 435                 (void)printf(" urg %d", urp);
 436         /*
 437          * Handle any options.
 438          */
 439         if (hlen > sizeof(*tp)) {
 440                 register const u_char *cp;
 441                 register u_int i, opt, datalen;
 442                 register u_int len;
 443
 444                 hlen -= sizeof(*tp);
 445                 cp = (const u_char *)tp + sizeof(*tp);
 446                 putchar(' ');
 447                 ch = '<';
 448                 while (hlen > 0) {
 449                         putchar(ch);
 450                         TCHECK(*cp);
 451                         opt = *cp++;
 452                         if (ZEROLENOPT(opt))
 453                                 len = 1;
 454                         else {
 455                                 TCHECK(*cp);
 456                                 len = *cp++;    /* total including type, len */
 457                                 if (len < 2 || len > hlen)
 458                                         goto bad;
 459                                 --hlen;         /* account for length byte */
 460                         }
 461                         --hlen;                 /* account for type byte */
 462                         datalen = 0;
 463
 464 /* Bail if "l" bytes of data are not left or were not captured  */
 465 #define LENCHECK(l) { if ((l) > hlen) goto bad; TCHECK2(*cp, l); }
 466
 467                         switch (opt) {
 468
 469                         case TCPOPT_MAXSEG:
 470                                 (void)printf("mss");
 471                                 datalen = 2;
 472                                 LENCHECK(datalen);
 473                                 (void)printf(" %u", EXTRACT_16BITS(cp));
 474
 475                                 break;
 476
 477                         case TCPOPT_EOL:
 478                                 (void)printf("eol");
 479                                 break;
 480
 481                         case TCPOPT_NOP:
 482                                 (void)printf("nop");
 483                                 break;
 484
 485                         case TCPOPT_WSCALE:
 486                                 (void)printf("wscale");
 487                                 datalen = 1;
 488                                 LENCHECK(datalen);
 489                                 (void)printf(" %u", *cp);
 490                                 break;
 491
 492                         case TCPOPT_SACKOK:
 493                                 (void)printf("sackOK");
 494                                 break;
 495
 496                         case TCPOPT_SACK:
 497                                 (void)printf("sack");
 498                                 datalen = len - 2;
 499                                 if (datalen % 8 != 0) {
 500                                         (void)printf(" malformed sack ");
 501                                 } else {
 502                                         u_int32_t s, e;
 503
 504                                         (void)printf(" sack %d ", datalen / 8);
 505                                         for (i = 0; i < datalen; i += 8) {
 506                                                 LENCHECK(i + 4);
 507                                                 s = EXTRACT_32BITS(cp + i);
 508                                                 LENCHECK(i + 8);
 509                                                 e = EXTRACT_32BITS(cp + i + 4);
 510                                                 if (threv) {
 511                                                         s -= thseq;
 512                                                         e -= thseq;
 513                                                 } else {
 514                                                         s -= thack;
 515                                                         e -= thack;
 516                                                 }
 517                                                 (void)printf("{%u:%u}", s, e);
 518                                         }
 519                                         (void)printf(" ");
 520                                 }
 521                                 break;
 522
 523                         case TCPOPT_ECHO:
 524                                 (void)printf("echo");
 525                                 datalen = 4;
 526                                 LENCHECK(datalen);
 527                                 (void)printf(" %u", EXTRACT_32BITS(cp));
 528                                 break;
 529
 530                         case TCPOPT_ECHOREPLY:
 531                                 (void)printf("echoreply");
 532                                 datalen = 4;
 533                                 LENCHECK(datalen);
 534                                 (void)printf(" %u", EXTRACT_32BITS(cp));
 535                                 break;
 536
 537                         case TCPOPT_TIMESTAMP:
 538                                 (void)printf("timestamp");
 539                                 datalen = 8;
 540                                 LENCHECK(4);
 541                                 (void)printf(" %u", EXTRACT_32BITS(cp));
 542                                 LENCHECK(datalen);
 543                                 (void)printf(" %u", EXTRACT_32BITS(cp + 4));
 544                                 break;
 545
 546                         case TCPOPT_CC:
 547                                 (void)printf("cc");
 548                                 datalen = 4;
 549                                 LENCHECK(datalen);
 550                                 (void)printf(" %u", EXTRACT_32BITS(cp));
 551                                 break;
 552
 553                         case TCPOPT_CCNEW:
 554                                 (void)printf("ccnew");
 555                                 datalen = 4;
 556                                 LENCHECK(datalen);
 557                                 (void)printf(" %u", EXTRACT_32BITS(cp));
 558                                 break;
 559
 560                         case TCPOPT_CCECHO:
 561                                 (void)printf("ccecho");
 562                                 datalen = 4;
 563                                 LENCHECK(datalen);
 564                                 (void)printf(" %u", EXTRACT_32BITS(cp));
 565                                 break;
 566
 567                         default:
 568                                 (void)printf("opt-%u:", opt);
 569                                 datalen = len - 2;
 570                                 for (i = 0; i < datalen; ++i) {
 571                                         LENCHECK(i);
 572                                         (void)printf("%02x", cp[i]);
 573                                 }
 574                                 break;
 575                         }
 576
 577                         /* Account for data printed */
 578                         cp += datalen;
 579                         hlen -= datalen;
 580
 581                         /* Check specification against observed length */
 582                         ++datalen;                      /* option octet */
 583                         if (!ZEROLENOPT(opt))
 584                                 ++datalen;              /* size octet */
 585                         if (datalen != len)
 586                                 (void)printf("[len %d]", len);
 587                         ch = ',';
 588                         if (opt == TCPOPT_EOL)
 589                                 break;
 590                 }
 591                 putchar('>');
 592         }
 593
 594         if (length <= 0)
 595                 return;
 596
 597         /*
 598          * Decode payload if necessary.
 599          */
 600         bp += TH_OFF(tp) * 4;
 601         if (flags & TH_RST) {
 602                 if (vflag)
 603                         print_tcp_rst_data(bp, length);
 604         } else {
 605                 if (sport == TELNET_PORT || dport == TELNET_PORT) {
 606                         if (!qflag && vflag)
 607                                 telnet_print(bp, length);
 608                 } else if (sport == BGP_PORT || dport == BGP_PORT)
 609                         bgp_print(bp, length);
 610                 else if (sport == PPTP_PORT || dport == PPTP_PORT)
 611                         pptp_print(bp);
 612 #ifdef TCPDUMP_DO_SMB
 613                 else if (sport == NETBIOS_SSN_PORT || dport == NETBIOS_SSN_PORT)
 614                         nbt_tcp_print(bp, length);
 615 #endif
 616                 else if (sport == BEEP_PORT || dport == BEEP_PORT)
 617                         beep_print(bp, length);
 618                 else if (length > 2 &&
 619                     (sport == NAMESERVER_PORT || dport == NAMESERVER_PORT ||
 620                      sport == MULTICASTDNS_PORT || dport == MULTICASTDNS_PORT)) {
 621                         /*
 622                          * TCP DNS query has 2byte length at the head.
 623                          * XXX packet could be unaligned, it can go strange
 624                          */
 625                         ns_print(bp + 2, length - 2, 0);
 626                 } else if (sport == MSDP_PORT || dport == MSDP_PORT) {
 627                         msdp_print(bp, length);
 628                 }
 629                 else if (sport == LDP_PORT || dport == LDP_PORT)
 630                         printf(": LDP, length: %u", length);
 631         }
 632         return;
 633 bad:
 634         fputs("[bad opt]", stdout);
 635         if (ch != '\0')
 636                 putchar('>');
 637         return;
 638 trunc:
 639         fputs("[|tcp]", stdout);
 640         if (ch != '\0')
 641                 putchar('>');
 642 }
 643
 644 /*
 645  * RFC1122 says the following on data in RST segments:
 646  *
 647  *         4.2.2.12  RST Segment: RFC-793 Section 3.4
 648  *
 649  *            A TCP SHOULD allow a received RST segment to include data.
 650  *
 651  *            DISCUSSION
 652  *                 It has been suggested that a RST segment could contain
 653  *                 ASCII text that encoded and explained the cause of the
 654  *                 RST.  No standard has yet been established for such
 655  *                 data.
 656  *
 657  */
 658
 659 static void
 660 print_tcp_rst_data(register const u_char *sp, u_int length)
 661 {
 662         int c;
 663
 664         if (TTEST2(*sp, length))
 665                 printf(" [RST");
 666         else
 667                 printf(" [!RST");
 668         if (length > MAX_RST_DATA_LEN) {
 669                 length = MAX_RST_DATA_LEN;      /* can use -X for longer */
 670                 putchar('+');                   /* indicate we truncate */
 671         }
 672         putchar(' ');
 673         while (length-- && sp <= snapend) {
 674                 c = *sp++;
 675                 safeputchar(c);
 676         }
 677         putchar(']');
 678 }