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 #ifndef lint
  23 static const char rcsid[] =
  24     "@(#) $Header: /tcpdump/master/tcpdump/print-tcp.c,v 1.74 2000-09-23 08:54:41 guy Exp $ (LBL)";
  25 #endif
  26
  27 #ifdef HAVE_CONFIG_H
  28 #include "config.h"
  29 #endif
  30
  31 #include <sys/param.h>
  32 #include <sys/time.h>
  33
  34 #include <rpc/rpc.h>
  35
  36 #include <netinet/in.h>
  37 #include <netinet/in_systm.h>
  38
  39 #include <stdio.h>
  40 #include <stdlib.h>
  41 #include <string.h>
  42 #include <ctype.h>
  43 #include <unistd.h>
  44
  45 #ifdef INET6
  46 #include <netinet/ip6.h>
  47 #endif
  48
  49 #include "interface.h"
  50 #include "addrtoname.h"
  51 #include "extract.h"
  52
  53 #include "tcp.h"
  54
  55 #include "ip.h"
  56
  57 static void print_tcp_rst_data(register const u_char *sp, u_int length);
  58
  59 #define MAX_RST_DATA_LEN        30
  60
  61 /* Compatibility */
  62 #ifndef TCPOPT_WSCALE
  63 #define TCPOPT_WSCALE           3       /* window scale factor (rfc1072) */
  64 #endif
  65 #ifndef TCPOPT_SACKOK
  66 #define TCPOPT_SACKOK           4       /* selective ack ok (rfc1072) */
  67 #endif
  68 #ifndef TCPOPT_SACK
  69 #define TCPOPT_SACK             5       /* selective ack (rfc1072) */
  70 #endif
  71 #ifndef TCPOPT_ECHO
  72 #define TCPOPT_ECHO             6       /* echo (rfc1072) */
  73 #endif
  74 #ifndef TCPOPT_ECHOREPLY
  75 #define TCPOPT_ECHOREPLY        7       /* echo (rfc1072) */
  76 #endif
  77 #ifndef TCPOPT_TIMESTAMP
  78 #define TCPOPT_TIMESTAMP        8       /* timestamps (rfc1323) */
  79 #endif
  80 #ifndef TCPOPT_CC
  81 #define TCPOPT_CC               11      /* T/TCP CC options (rfc1644) */
  82 #endif
  83 #ifndef TCPOPT_CCNEW
  84 #define TCPOPT_CCNEW            12      /* T/TCP CC options (rfc1644) */
  85 #endif
  86 #ifndef TCPOPT_CCECHO
  87 #define TCPOPT_CCECHO           13      /* T/TCP CC options (rfc1644) */
  88 #endif
  89
  90 /*
  91  * Definitions required for ECN
  92  * for use if the OS running tcpdump does not have ECN
  93  */
  94 #ifndef TH_ECNECHO
  95 #define TH_ECNECHO              0x40    /* ECN Echo in tcp header */
  96 #endif
  97 #ifndef TH_CWR
  98 #define TH_CWR                  0x80    /* ECN Cwnd Reduced in tcp header*/
  99 #endif
 100
 101 struct tha {
 102 #ifndef INET6
 103         struct in_addr src;
 104         struct in_addr dst;
 105 #else
 106         struct in6_addr src;
 107         struct in6_addr dst;
 108 #endif /*INET6*/
 109         u_int port;
 110 };
 111
 112 struct tcp_seq_hash {
 113         struct tcp_seq_hash *nxt;
 114         struct tha addr;
 115         tcp_seq seq;
 116         tcp_seq ack;
 117 };
 118
 119 #define TSEQ_HASHSIZE 919
 120
 121 /* These tcp optinos do not have the size octet */
 122 #define ZEROLENOPT(o) ((o) == TCPOPT_EOL || (o) == TCPOPT_NOP)
 123
 124 static struct tcp_seq_hash tcp_seq_hash[TSEQ_HASHSIZE];
 125
 126
 127 #ifndef TELNET_PORT
 128 #define TELNET_PORT     23
 129 #endif
 130 #ifndef BGP_PORT
 131 #define BGP_PORT        179
 132 #endif
 133 #define NETBIOS_SSN_PORT 139
 134 #ifndef NFS_PORT
 135 #define NFS_PORT        2049
 136 #endif
 137
 138 static int tcp_cksum(register const struct ip *ip,
 139                      register const struct tcphdr *tp,
 140                      register int len)
 141 {
 142         int i, tlen;
 143         union phu {
 144                 struct phdr {
 145                         u_int32_t src;
 146                         u_int32_t dst;
 147                         u_char mbz;
 148                         u_char proto;
 149                         u_int16_t len;
 150                 } ph;
 151                 u_int16_t pa[6];
 152         } phu;
 153         register const u_int16_t *sp;
 154         u_int32_t sum;
 155         tlen = ntohs(ip->ip_len) - ((const char *)tp-(const char*)ip);
 156
 157         /* pseudo-header.. */
 158         phu.ph.len = htons(tlen);
 159         phu.ph.mbz = 0;
 160         phu.ph.proto = IPPROTO_TCP;
 161         memcpy(&phu.ph.src, &ip->ip_src.s_addr, sizeof(u_int32_t));
 162         memcpy(&phu.ph.dst, &ip->ip_dst.s_addr, sizeof(u_int32_t));
 163
 164         sp = &phu.pa[0];
 165         sum = sp[0]+sp[1]+sp[2]+sp[3]+sp[4]+sp[5];
 166
 167         sp = (const u_int16_t *)tp;
 168
 169         for (i=0; i<(tlen&~1); i+= 2)
 170                 sum += *sp++;
 171
 172         if (tlen & 1) {
 173                 sum += htons( (*(const char *)sp) << 8);
 174         }
 175
 176         while (sum > 0xffff)
 177                 sum = (sum & 0xffff) + (sum >> 16);
 178         sum = ~sum & 0xffff;
 179
 180         return (sum);
 181 }
 182
 183 #ifdef INET6
 184 static int tcp6_cksum(const struct ip6_hdr *ip6, const struct tcphdr *tp,
 185         int len)
 186 {
 187         int i, tlen;
 188         register const u_int16_t *sp;
 189         u_int32_t sum;
 190         union {
 191                 struct {
 192                         struct in6_addr ph_src;
 193                         struct in6_addr ph_dst;
 194                         u_int32_t       ph_len;
 195                         u_int8_t        ph_zero[3];
 196                         u_int8_t        ph_nxt;
 197                 } ph;
 198                 u_int16_t pa[20];
 199         } phu;
 200
 201         tlen = ntohs(ip6->ip6_plen) + sizeof(struct ip6_hdr) -
 202             ((const char *)tp - (const char*)ip6);
 203
 204         /* pseudo-header */
 205         memset(&phu, 0, sizeof(phu));
 206         phu.ph.ph_src = ip6->ip6_src;
 207         phu.ph.ph_dst = ip6->ip6_dst;
 208         phu.ph.ph_len = htonl(tlen);
 209         phu.ph.ph_nxt = IPPROTO_TCP;
 210
 211         sum = 0;
 212         for (i = 0; i < sizeof(phu.pa) / sizeof(phu.pa[0]); i++)
 213                 sum += phu.pa[i];
 214
 215         sp = (const u_int16_t *)tp;
 216
 217         for (i = 0; i < (tlen & ~1); i += 2)
 218                 sum += *sp++;
 219
 220         if (tlen & 1)
 221                 sum += htons((*(const char *)sp) << 8);
 222
 223         while (sum > 0xffff)
 224                 sum = (sum & 0xffff) + (sum >> 16);
 225         sum = ~sum & 0xffff;
 226
 227         return (sum);
 228 }
 229 #endif
 230
 231 void
 232 tcp_print(register const u_char *bp, register u_int length,
 233           register const u_char *bp2)
 234 {
 235         register const struct tcphdr *tp;
 236         register const struct ip *ip;
 237         register u_char flags;
 238         register int hlen;
 239         register char ch;
 240         u_int16_t sport, dport, win, urp;
 241         u_int32_t seq, ack, thseq, thack;
 242         int threv;
 243 #ifdef INET6
 244         register const struct ip6_hdr *ip6;
 245 #endif
 246
 247         tp = (struct tcphdr *)bp;
 248         ip = (struct ip *)bp2;
 249 #ifdef INET6
 250         if (ip->ip_v == 6)
 251                 ip6 = (struct ip6_hdr *)bp2;
 252         else
 253                 ip6 = NULL;
 254 #endif /*INET6*/
 255         ch = '\0';
 256         if (!TTEST(tp->th_dport)) {
 257                 (void)printf("%s > %s: [|tcp]",
 258                         ipaddr_string(&ip->ip_src),
 259                         ipaddr_string(&ip->ip_dst));
 260                 return;
 261         }
 262
 263         sport = ntohs(tp->th_sport);
 264         dport = ntohs(tp->th_dport);
 265
 266
 267         hlen = tp->th_off * 4;
 268
 269         /*
 270          * If data present and NFS port used, assume NFS.
 271          * Pass offset of data plus 4 bytes for RPC TCP msg length
 272          * to NFS print routines.
 273          */
 274         if (!qflag) {
 275                 if ((u_char *)tp + 4 + sizeof(struct rpc_msg) <= snapend &&
 276                     dport == NFS_PORT) {
 277                         nfsreq_print((u_char *)tp + hlen + 4, length-hlen,
 278                                      (u_char *)ip);
 279                         return;
 280                 } else if ((u_char *)tp + 4 + sizeof(struct rpc_msg)
 281                            <= snapend &&
 282                            sport == NFS_PORT) {
 283                         nfsreply_print((u_char *)tp + hlen + 4,length-hlen,
 284                                        (u_char *)ip);
 285                         return;
 286                 }
 287         }
 288 #ifdef INET6
 289         if (ip6) {
 290                 if (ip6->ip6_nxt == IPPROTO_TCP) {
 291                         (void)printf("%s.%s > %s.%s: ",
 292                                 ip6addr_string(&ip6->ip6_src),
 293                                 tcpport_string(sport),
 294                                 ip6addr_string(&ip6->ip6_dst),
 295                                 tcpport_string(dport));
 296                 } else {
 297                         (void)printf("%s > %s: ",
 298                                 tcpport_string(sport), tcpport_string(dport));
 299                 }
 300         } else
 301 #endif /*INET6*/
 302         {
 303                 if (ip->ip_p == IPPROTO_TCP) {
 304                         (void)printf("%s.%s > %s.%s: ",
 305                                 ipaddr_string(&ip->ip_src),
 306                                 tcpport_string(sport),
 307                                 ipaddr_string(&ip->ip_dst),
 308                                 tcpport_string(dport));
 309                 } else {
 310                         (void)printf("%s > %s: ",
 311                                 tcpport_string(sport), tcpport_string(dport));
 312                 }
 313         }
 314
 315         TCHECK(*tp);
 316
 317         seq = (u_int32_t)ntohl(tp->th_seq);
 318         ack = (u_int32_t)ntohl(tp->th_ack);
 319         win = ntohs(tp->th_win);
 320         urp = ntohs(tp->th_urp);
 321
 322         if (qflag) {
 323                 (void)printf("tcp %d", length - tp->th_off * 4);
 324                 return;
 325         }
 326         if ((flags = tp->th_flags) & (TH_SYN|TH_FIN|TH_RST|TH_PUSH|
 327                                       TH_ECNECHO|TH_CWR)) {
 328                 if (flags & TH_SYN)
 329                         putchar('S');
 330                 if (flags & TH_FIN)
 331                         putchar('F');
 332                 if (flags & TH_RST)
 333                         putchar('R');
 334                 if (flags & TH_PUSH)
 335                         putchar('P');
 336                 if (flags & TH_CWR)
 337                         putchar('W');   /* congestion _W_indow reduced (ECN) */
 338                 if (flags & TH_ECNECHO)
 339                         putchar('E');   /* ecn _E_cho sent (ECN) */
 340         } else
 341                 putchar('.');
 342
 343         if (!Sflag && (flags & TH_ACK)) {
 344                 register struct tcp_seq_hash *th;
 345                 register int rev;
 346                 struct tha tha;
 347                 /*
 348                  * Find (or record) the initial sequence numbers for
 349                  * this conversation.  (we pick an arbitrary
 350                  * collating order so there's only one entry for
 351                  * both directions).
 352                  */
 353 #ifdef INET6
 354                 memset(&tha, 0, sizeof(tha));
 355                 rev = 0;
 356                 if (ip6) {
 357                         if (sport > dport) {
 358                                 rev = 1;
 359                         } else if (sport == dport) {
 360                             int i;
 361
 362                             for (i = 0; i < 4; i++) {
 363                                 if (((u_int32_t *)(&ip6->ip6_src))[i] >
 364                                     ((u_int32_t *)(&ip6->ip6_dst))[i]) {
 365                                         rev = 1;
 366                                         break;
 367                                 }
 368                             }
 369                         }
 370                         if (rev) {
 371                                 tha.src = ip6->ip6_dst;
 372                                 tha.dst = ip6->ip6_src;
 373                                 tha.port = dport << 16 | sport;
 374                         } else {
 375                                 tha.dst = ip6->ip6_dst;
 376                                 tha.src = ip6->ip6_src;
 377                                 tha.port = sport << 16 | dport;
 378                         }
 379                 } else {
 380                         if (sport > dport ||
 381                             (sport == dport &&
 382                              ip->ip_src.s_addr > ip->ip_dst.s_addr)) {
 383                                 rev = 1;
 384                         }
 385                         if (rev) {
 386                                 *(struct in_addr *)&tha.src = ip->ip_dst;
 387                                 *(struct in_addr *)&tha.dst = ip->ip_src;
 388                                 tha.port = dport << 16 | sport;
 389                         } else {
 390                                 *(struct in_addr *)&tha.dst = ip->ip_dst;
 391                                 *(struct in_addr *)&tha.src = ip->ip_src;
 392                                 tha.port = sport << 16 | dport;
 393                         }
 394                 }
 395 #else
 396                 if (sport < dport ||
 397                     (sport == dport &&
 398                      ip->ip_src.s_addr < ip->ip_dst.s_addr)) {
 399                         tha.src = ip->ip_src, tha.dst = ip->ip_dst;
 400                         tha.port = sport << 16 | dport;
 401                         rev = 0;
 402                 } else {
 403                         tha.src = ip->ip_dst, tha.dst = ip->ip_src;
 404                         tha.port = dport << 16 | sport;
 405                         rev = 1;
 406                 }
 407 #endif
 408
 409                 threv = rev;
 410                 for (th = &tcp_seq_hash[tha.port % TSEQ_HASHSIZE];
 411                      th->nxt; th = th->nxt)
 412                         if (!memcmp((char *)&tha, (char *)&th->addr,
 413                                   sizeof(th->addr)))
 414                                 break;
 415
 416                 if (!th->nxt || (flags & TH_SYN)) {
 417                         /* didn't find it or new conversation */
 418                         if (th->nxt == NULL) {
 419                                 th->nxt = (struct tcp_seq_hash *)
 420                                         calloc(1, sizeof(*th));
 421                                 if (th->nxt == NULL)
 422                                         error("tcp_print: calloc");
 423                         }
 424                         th->addr = tha;
 425                         if (rev)
 426                                 th->ack = seq, th->seq = ack - 1;
 427                         else
 428                                 th->seq = seq, th->ack = ack - 1;
 429
 430                 } else {
 431                         if (rev)
 432                                 seq -= th->ack, ack -= th->seq;
 433                         else
 434                                 seq -= th->seq, ack -= th->ack;
 435                 }
 436
 437                 thseq = th->seq;
 438                 thack = th->ack;
 439         } else {
 440                 /*fool gcc*/
 441                 thseq = thack = threv = 0;
 442         }
 443         if (hlen > length) {
 444                 (void)printf(" [bad hdr length]");
 445                 return;
 446         }
 447
 448         if (ip->ip_v == 4 && vflag) {
 449                 int sum;
 450                 if (TTEST2(tp->th_sport, length)) {
 451                         sum = tcp_cksum(ip, tp, length);
 452                         if (sum != 0)
 453                                 (void)printf(" [bad tcp cksum %x!]", sum);
 454                         else
 455                                 (void)printf(" [tcp sum ok]");
 456                 }
 457         }
 458 #ifdef INET6
 459         if (ip->ip_v == 6 && ip6->ip6_plen && vflag) {
 460                 int sum;
 461                 if (TTEST2(tp->th_sport, length)) {
 462                         sum = tcp6_cksum(ip6, tp, length);
 463                         if (sum != 0)
 464                                 (void)printf(" [bad tcp cksum %x!]", sum);
 465                         else
 466                                 (void)printf(" [tcp sum ok]");
 467                 }
 468         }
 469 #endif
 470
 471         length -= hlen;
 472         if (vflag > 1 || length > 0 || flags & (TH_SYN | TH_FIN | TH_RST))
 473                 (void)printf(" %u:%u(%d)", seq, seq + length, length);
 474         if (flags & TH_ACK)
 475                 (void)printf(" ack %u", ack);
 476
 477         (void)printf(" win %d", win);
 478
 479         if (flags & TH_URG)
 480                 (void)printf(" urg %d", urp);
 481         /*
 482          * Handle any options.
 483          */
 484         if ((hlen -= sizeof(*tp)) > 0) {
 485                 register const u_char *cp;
 486                 register int i, opt, len, datalen;
 487
 488                 cp = (const u_char *)tp + sizeof(*tp);
 489                 putchar(' ');
 490                 ch = '<';
 491                 while (hlen > 0) {
 492                         putchar(ch);
 493                         TCHECK(*cp);
 494                         opt = *cp++;
 495                         if (ZEROLENOPT(opt))
 496                                 len = 1;
 497                         else {
 498                                 TCHECK(*cp);
 499                                 len = *cp++;    /* total including type, len */
 500                                 if (len < 2 || len > hlen)
 501                                         goto bad;
 502                                 --hlen;         /* account for length byte */
 503                         }
 504                         --hlen;                 /* account for type byte */
 505                         datalen = 0;
 506
 507 /* Bail if "l" bytes of data are not left or were not captured  */
 508 #define LENCHECK(l) { if ((l) > hlen) goto bad; TCHECK2(*cp, l); }
 509
 510                         switch (opt) {
 511
 512                         case TCPOPT_MAXSEG:
 513                                 (void)printf("mss");
 514                                 datalen = 2;
 515                                 LENCHECK(datalen);
 516                                 (void)printf(" %u", EXTRACT_16BITS(cp));
 517
 518                                 break;
 519
 520                         case TCPOPT_EOL:
 521                                 (void)printf("eol");
 522                                 break;
 523
 524                         case TCPOPT_NOP:
 525                                 (void)printf("nop");
 526                                 break;
 527
 528                         case TCPOPT_WSCALE:
 529                                 (void)printf("wscale");
 530                                 datalen = 1;
 531                                 LENCHECK(datalen);
 532                                 (void)printf(" %u", *cp);
 533                                 break;
 534
 535                         case TCPOPT_SACKOK:
 536                                 (void)printf("sackOK");
 537                                 break;
 538
 539                         case TCPOPT_SACK:
 540                                 (void)printf("sack");
 541                                 datalen = len - 2;
 542                                 if (datalen % 8 != 0) {
 543                                         (void)printf(" malformed sack ");
 544                                 } else {
 545                                         u_int32_t s, e;
 546
 547                                         (void)printf(" sack %d ", datalen / 8);
 548                                         for (i = 0; i < datalen; i += 8) {
 549                                                 LENCHECK(i + 4);
 550                                                 s = EXTRACT_32BITS(cp + i);
 551                                                 LENCHECK(i + 8);
 552                                                 e = EXTRACT_32BITS(cp + i + 4);
 553                                                 if (threv) {
 554                                                         s -= thseq;
 555                                                         e -= thseq;
 556                                                 } else {
 557                                                         s -= thack;
 558                                                         e -= thack;
 559                                                 }
 560                                                 (void)printf("{%u:%u}", s, e);
 561                                         }
 562                                         (void)printf(" ");
 563                                 }
 564                                 break;
 565
 566                         case TCPOPT_ECHO:
 567                                 (void)printf("echo");
 568                                 datalen = 4;
 569                                 LENCHECK(datalen);
 570                                 (void)printf(" %u", EXTRACT_32BITS(cp));
 571                                 break;
 572
 573                         case TCPOPT_ECHOREPLY:
 574                                 (void)printf("echoreply");
 575                                 datalen = 4;
 576                                 LENCHECK(datalen);
 577                                 (void)printf(" %u", EXTRACT_32BITS(cp));
 578                                 break;
 579
 580                         case TCPOPT_TIMESTAMP:
 581                                 (void)printf("timestamp");
 582                                 datalen = 8;
 583                                 LENCHECK(4);
 584                                 (void)printf(" %u", EXTRACT_32BITS(cp));
 585                                 LENCHECK(datalen);
 586                                 (void)printf(" %u", EXTRACT_32BITS(cp + 4));
 587                                 break;
 588
 589                         case TCPOPT_CC:
 590                                 (void)printf("cc");
 591                                 datalen = 4;
 592                                 LENCHECK(datalen);
 593                                 (void)printf(" %u", EXTRACT_32BITS(cp));
 594                                 break;
 595
 596                         case TCPOPT_CCNEW:
 597                                 (void)printf("ccnew");
 598                                 datalen = 4;
 599                                 LENCHECK(datalen);
 600                                 (void)printf(" %u", EXTRACT_32BITS(cp));
 601                                 break;
 602
 603                         case TCPOPT_CCECHO:
 604                                 (void)printf("ccecho");
 605                                 datalen = 4;
 606                                 LENCHECK(datalen);
 607                                 (void)printf(" %u", EXTRACT_32BITS(cp));
 608                                 break;
 609
 610                         default:
 611                                 (void)printf("opt-%d:", opt);
 612                                 datalen = len - 2;
 613                                 for (i = 0; i < datalen; ++i) {
 614                                         LENCHECK(i);
 615                                         (void)printf("%02x", cp[i]);
 616                                 }
 617                                 break;
 618                         }
 619
 620                         /* Account for data printed */
 621                         cp += datalen;
 622                         hlen -= datalen;
 623
 624                         /* Check specification against observed length */
 625                         ++datalen;                      /* option octet */
 626                         if (!ZEROLENOPT(opt))
 627                                 ++datalen;              /* size octet */
 628                         if (datalen != len)
 629                                 (void)printf("[len %d]", len);
 630                         ch = ',';
 631                         if (opt == TCPOPT_EOL)
 632                                 break;
 633                 }
 634                 putchar('>');
 635         }
 636
 637         if (length <= 0)
 638                 return;
 639
 640         /*
 641          * Decode payload if necessary.
 642          */
 643         bp += (tp->th_off * 4);
 644         if (flags & TH_RST) {
 645                 if (vflag)
 646                         print_tcp_rst_data(bp, length);
 647         } else {
 648                 if (sport == TELNET_PORT || dport == TELNET_PORT) {
 649                         if (!qflag && vflag)
 650                                 telnet_print(bp, length);
 651                 } else if (sport == BGP_PORT || dport == BGP_PORT)
 652                         bgp_print(bp, length);
 653                 else if (sport == NETBIOS_SSN_PORT || dport == NETBIOS_SSN_PORT)
 654                         nbt_tcp_print(bp, length);
 655         }
 656         return;
 657 bad:
 658         fputs("[bad opt]", stdout);
 659         if (ch != '\0')
 660                 putchar('>');
 661         return;
 662 trunc:
 663         fputs("[|tcp]", stdout);
 664         if (ch != '\0')
 665                 putchar('>');
 666 }
 667
 668 /*
 669  * RFC1122 says the following on data in RST segments:
 670  *
 671  *         4.2.2.12  RST Segment: RFC-793 Section 3.4
 672  *
 673  *            A TCP SHOULD allow a received RST segment to include data.
 674  *
 675  *            DISCUSSION
 676  *                 It has been suggested that a RST segment could contain
 677  *                 ASCII text that encoded and explained the cause of the
 678  *                 RST.  No standard has yet been established for such
 679  *                 data.
 680  *
 681  */
 682
 683 static void
 684 print_tcp_rst_data(register const u_char *sp, u_int length)
 685 {
 686         int c;
 687
 688         if (TTEST2(*sp, length))
 689                 printf(" [RST");
 690         else
 691                 printf(" [!RST");
 692         if (length > MAX_RST_DATA_LEN) {
 693                 length = MAX_RST_DATA_LEN;      /* can use -X for longer */
 694                 putchar('+');                   /* indicate we truncate */
 695         }
 696         putchar(' ');
 697         while (length-- && sp <= snapend) {
 698                 c = *sp++;
 699                 if (isprint(c))
 700                         putchar(c);
 701                 else
 702                         putchar('.');
 703         }
 704         putchar(']');
 705 }