lib/packets.c

   1 /*
   2  * Copyright (c) 2009, 2010, 2011, 2012 Nicira, Inc.
   3  *
   4  * Licensed under the Apache License, Version 2.0 (the "License");
   5  * you may not use this file except in compliance with the License.
   6  * You may obtain a copy of the License at:
   7  *
   8  *     http://www.apache.org/licenses/LICENSE-2.0
   9  *
  10  * Unless required by applicable law or agreed to in writing, software
  11  * distributed under the License is distributed on an "AS IS" BASIS,
  12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  13  * See the License for the specific language governing permissions and
  14  * limitations under the License.
  15  */
  16
  17 #include <config.h>
  18 #include "packets.h"
  19 #include <assert.h>
  20 #include <arpa/inet.h>
  21 #include <sys/socket.h>
  22 #include <netinet/in.h>
  23 #include <stdlib.h>
  24 #include "byte-order.h"
  25 #include "csum.h"
  26 #include "flow.h"
  27 #include "hmap.h"
  28 #include "dynamic-string.h"
  29 #include "ofpbuf.h"
  30
  31 const struct in6_addr in6addr_exact = IN6ADDR_EXACT_INIT;
  32
  33 /* Parses 's' as a 16-digit hexadecimal number representing a datapath ID.  On
  34  * success stores the dpid into '*dpidp' and returns true, on failure stores 0
  35  * into '*dpidp' and returns false.
  36  *
  37  * Rejects an all-zeros dpid as invalid. */
  38 bool
  39 dpid_from_string(const char *s, uint64_t *dpidp)
  40 {
  41     *dpidp = (strlen(s) == 16 && strspn(s, "0123456789abcdefABCDEF") == 16
  42               ? strtoull(s, NULL, 16)
  43               : 0);
  44     return *dpidp != 0;
  45 }
  46
  47 /* Returns true if 'ea' is a reserved address, that a bridge must never
  48  * forward, false otherwise.
  49  *
  50  * If you change this function's behavior, please update corresponding
  51  * documentation in vswitch.xml at the same time. */
  52 bool
  53 eth_addr_is_reserved(const uint8_t ea[ETH_ADDR_LEN])
  54 {
  55     struct eth_addr_node {
  56         struct hmap_node hmap_node;
  57         uint64_t ea64;
  58     };
  59
  60     static struct eth_addr_node nodes[] = {
  61         /* STP, IEEE pause frames, and other reserved protocols. */
  62         { HMAP_NODE_NULL_INITIALIZER, 0x0108c2000000ULL },
  63         { HMAP_NODE_NULL_INITIALIZER, 0x0108c2000001ULL },
  64         { HMAP_NODE_NULL_INITIALIZER, 0x0108c2000002ULL },
  65         { HMAP_NODE_NULL_INITIALIZER, 0x0108c2000003ULL },
  66         { HMAP_NODE_NULL_INITIALIZER, 0x0108c2000004ULL },
  67         { HMAP_NODE_NULL_INITIALIZER, 0x0108c2000005ULL },
  68         { HMAP_NODE_NULL_INITIALIZER, 0x0108c2000006ULL },
  69         { HMAP_NODE_NULL_INITIALIZER, 0x0108c2000007ULL },
  70         { HMAP_NODE_NULL_INITIALIZER, 0x0108c2000008ULL },
  71         { HMAP_NODE_NULL_INITIALIZER, 0x0108c2000009ULL },
  72         { HMAP_NODE_NULL_INITIALIZER, 0x0108c200000aULL },
  73         { HMAP_NODE_NULL_INITIALIZER, 0x0108c200000bULL },
  74         { HMAP_NODE_NULL_INITIALIZER, 0x0108c200000cULL },
  75         { HMAP_NODE_NULL_INITIALIZER, 0x0108c200000dULL },
  76         { HMAP_NODE_NULL_INITIALIZER, 0x0108c200000eULL },
  77         { HMAP_NODE_NULL_INITIALIZER, 0x0108c200000fULL },
  78
  79         /* Extreme protocols. */
  80         { HMAP_NODE_NULL_INITIALIZER, 0x00e02b000000ULL }, /* EDP. */
  81         { HMAP_NODE_NULL_INITIALIZER, 0x00e02b000004ULL }, /* EAPS. */
  82         { HMAP_NODE_NULL_INITIALIZER, 0x00e02b000006ULL }, /* EAPS. */
  83
  84         /* Cisco protocols. */
  85         { HMAP_NODE_NULL_INITIALIZER, 0x01000c000000ULL }, /* ISL. */
  86         { HMAP_NODE_NULL_INITIALIZER, 0x01000cccccccULL }, /* PAgP, UDLD, CDP,
  87                                                             * DTP, VTP. */
  88         { HMAP_NODE_NULL_INITIALIZER, 0x01000ccccccdULL }, /* PVST+. */
  89         { HMAP_NODE_NULL_INITIALIZER, 0x01000ccdcdcdULL }, /* STP Uplink Fast,
  90                                                             * FlexLink. */
  91
  92         /* Cisco CFM. */
  93         { HMAP_NODE_NULL_INITIALIZER, 0x01000cccccc0ULL },
  94         { HMAP_NODE_NULL_INITIALIZER, 0x01000cccccc1ULL },
  95         { HMAP_NODE_NULL_INITIALIZER, 0x01000cccccc2ULL },
  96         { HMAP_NODE_NULL_INITIALIZER, 0x01000cccccc3ULL },
  97         { HMAP_NODE_NULL_INITIALIZER, 0x01000cccccc4ULL },
  98         { HMAP_NODE_NULL_INITIALIZER, 0x01000cccccc5ULL },
  99         { HMAP_NODE_NULL_INITIALIZER, 0x01000cccccc6ULL },
 100         { HMAP_NODE_NULL_INITIALIZER, 0x01000cccccc7ULL },
 101     };
 102
 103     static struct hmap addrs = HMAP_INITIALIZER(&addrs);
 104     struct eth_addr_node *node;
 105     uint64_t ea64;
 106
 107     if (hmap_is_empty(&addrs)) {
 108         for (node = nodes; node < &nodes[ARRAY_SIZE(nodes)]; node++) {
 109             hmap_insert(&addrs, &node->hmap_node,
 110                         hash_2words(node->ea64, node->ea64 >> 32));
 111         }
 112     }
 113
 114     ea64 = eth_addr_to_uint64(ea);
 115     HMAP_FOR_EACH_IN_BUCKET (node, hmap_node, hash_2words(ea64, ea64 >> 32),
 116                              &addrs) {
 117         if (node->ea64 == ea64) {
 118             return true;
 119         }
 120     }
 121     return false;
 122 }
 123
 124 bool
 125 eth_addr_from_string(const char *s, uint8_t ea[ETH_ADDR_LEN])
 126 {
 127     if (sscanf(s, ETH_ADDR_SCAN_FMT, ETH_ADDR_SCAN_ARGS(ea))
 128         == ETH_ADDR_SCAN_COUNT) {
 129         return true;
 130     } else {
 131         memset(ea, 0, ETH_ADDR_LEN);
 132         return false;
 133     }
 134 }
 135
 136 /* Fills 'b' with a Reverse ARP packet with Ethernet source address 'eth_src'.
 137  * This function is used by Open vSwitch to compose packets in cases where
 138  * context is important but content doesn't (or shouldn't) matter.
 139  *
 140  * The returned packet has enough headroom to insert an 802.1Q VLAN header if
 141  * desired. */
 142 void
 143 compose_rarp(struct ofpbuf *b, const uint8_t eth_src[ETH_ADDR_LEN])
 144 {
 145     struct eth_header *eth;
 146     struct arp_eth_header *arp;
 147
 148     ofpbuf_clear(b);
 149     ofpbuf_prealloc_tailroom(b, ETH_HEADER_LEN + VLAN_HEADER_LEN
 150                              + ARP_ETH_HEADER_LEN);
 151     ofpbuf_reserve(b, VLAN_HEADER_LEN);
 152     eth = ofpbuf_put_uninit(b, sizeof *eth);
 153     memcpy(eth->eth_dst, eth_addr_broadcast, ETH_ADDR_LEN);
 154     memcpy(eth->eth_src, eth_src, ETH_ADDR_LEN);
 155     eth->eth_type = htons(ETH_TYPE_RARP);
 156
 157     arp = ofpbuf_put_uninit(b, sizeof *arp);
 158     arp->ar_hrd = htons(ARP_HRD_ETHERNET);
 159     arp->ar_pro = htons(ARP_PRO_IP);
 160     arp->ar_hln = sizeof arp->ar_sha;
 161     arp->ar_pln = sizeof arp->ar_spa;
 162     arp->ar_op = htons(ARP_OP_RARP);
 163     memcpy(arp->ar_sha, eth_src, ETH_ADDR_LEN);
 164     arp->ar_spa = htonl(0);
 165     memcpy(arp->ar_tha, eth_src, ETH_ADDR_LEN);
 166     arp->ar_tpa = htonl(0);
 167 }
 168
 169 /* Insert VLAN header according to given TCI. Packet passed must be Ethernet
 170  * packet.  Ignores the CFI bit of 'tci' using 0 instead.
 171  *
 172  * Also sets 'packet->l2' to point to the new Ethernet header. */
 173 void
 174 eth_push_vlan(struct ofpbuf *packet, ovs_be16 tci)
 175 {
 176     struct eth_header *eh = packet->data;
 177     struct vlan_eth_header *veh;
 178
 179     /* Insert new 802.1Q header. */
 180     struct vlan_eth_header tmp;
 181     memcpy(tmp.veth_dst, eh->eth_dst, ETH_ADDR_LEN);
 182     memcpy(tmp.veth_src, eh->eth_src, ETH_ADDR_LEN);
 183     tmp.veth_type = htons(ETH_TYPE_VLAN);
 184     tmp.veth_tci = tci & htons(~VLAN_CFI);
 185     tmp.veth_next_type = eh->eth_type;
 186
 187     veh = ofpbuf_push_uninit(packet, VLAN_HEADER_LEN);
 188     memcpy(veh, &tmp, sizeof tmp);
 189
 190     packet->l2 = packet->data;
 191 }
 192
 193 /* Removes outermost VLAN header (if any is present) from 'packet'.
 194  *
 195  * 'packet->l2' must initially point to 'packet''s Ethernet header. */
 196 void
 197 eth_pop_vlan(struct ofpbuf *packet)
 198 {
 199     struct vlan_eth_header *veh = packet->l2;
 200     if (packet->size >= sizeof *veh
 201         && veh->veth_type == htons(ETH_TYPE_VLAN)) {
 202         struct eth_header tmp;
 203
 204         memcpy(tmp.eth_dst, veh->veth_dst, ETH_ADDR_LEN);
 205         memcpy(tmp.eth_src, veh->veth_src, ETH_ADDR_LEN);
 206         tmp.eth_type = veh->veth_next_type;
 207
 208         ofpbuf_pull(packet, VLAN_HEADER_LEN);
 209         packet->l2 = (char*)packet->l2 + VLAN_HEADER_LEN;
 210         memcpy(packet->data, &tmp, sizeof tmp);
 211     }
 212 }
 213
 214 /* Converts hex digits in 'hex' to an Ethernet packet in '*packetp'.  The
 215  * caller must free '*packetp'.  On success, returns NULL.  On failure, returns
 216  * an error message and stores NULL in '*packetp'. */
 217 const char *
 218 eth_from_hex(const char *hex, struct ofpbuf **packetp)
 219 {
 220     struct ofpbuf *packet;
 221
 222     packet = *packetp = ofpbuf_new(strlen(hex) / 2);
 223
 224     if (ofpbuf_put_hex(packet, hex, NULL)[0] != '\0') {
 225         ofpbuf_delete(packet);
 226         *packetp = NULL;
 227         return "Trailing garbage in packet data";
 228     }
 229
 230     if (packet->size < ETH_HEADER_LEN) {
 231         ofpbuf_delete(packet);
 232         *packetp = NULL;
 233         return "Packet data too short for Ethernet";
 234     }
 235
 236     return NULL;
 237 }
 238
 239 void
 240 eth_format_masked(const uint8_t eth[ETH_ADDR_LEN],
 241                   const uint8_t mask[ETH_ADDR_LEN], struct ds *s)
 242 {
 243     ds_put_format(s, ETH_ADDR_FMT, ETH_ADDR_ARGS(eth));
 244     if (mask && !eth_mask_is_exact(mask)) {
 245         ds_put_format(s, "/"ETH_ADDR_FMT, ETH_ADDR_ARGS(mask));
 246     }
 247 }
 248
 249 void
 250 eth_addr_bitand(const uint8_t src[ETH_ADDR_LEN],
 251                 const uint8_t mask[ETH_ADDR_LEN],
 252                 uint8_t dst[ETH_ADDR_LEN])
 253 {
 254     int i;
 255
 256     for (i = 0; i < ETH_ADDR_LEN; i++) {
 257         dst[i] = src[i] & mask[i];
 258     }
 259 }
 260
 261 /* Given the IP netmask 'netmask', returns the number of bits of the IP address
 262  * that it specifies, that is, the number of 1-bits in 'netmask'.
 263  *
 264  * If 'netmask' is not a CIDR netmask (see ip_is_cidr()), the return value will
 265  * still be in the valid range but isn't otherwise meaningful. */
 266 int
 267 ip_count_cidr_bits(ovs_be32 netmask)
 268 {
 269     return 32 - ctz(ntohl(netmask));
 270 }
 271
 272 void
 273 ip_format_masked(ovs_be32 ip, ovs_be32 mask, struct ds *s)
 274 {
 275     ds_put_format(s, IP_FMT, IP_ARGS(&ip));
 276     if (mask != htonl(UINT32_MAX)) {
 277         if (ip_is_cidr(mask)) {
 278             ds_put_format(s, "/%d", ip_count_cidr_bits(mask));
 279         } else {
 280             ds_put_format(s, "/"IP_FMT, IP_ARGS(&mask));
 281         }
 282     }
 283 }
 284
 285
 286 /* Stores the string representation of the IPv6 address 'addr' into the
 287  * character array 'addr_str', which must be at least INET6_ADDRSTRLEN
 288  * bytes long. */
 289 void
 290 format_ipv6_addr(char *addr_str, const struct in6_addr *addr)
 291 {
 292     inet_ntop(AF_INET6, addr, addr_str, INET6_ADDRSTRLEN);
 293 }
 294
 295 void
 296 print_ipv6_addr(struct ds *string, const struct in6_addr *addr)
 297 {
 298     char *dst;
 299
 300     ds_reserve(string, string->length + INET6_ADDRSTRLEN);
 301
 302     dst = string->string + string->length;
 303     format_ipv6_addr(dst, addr);
 304     string->length += strlen(dst);
 305 }
 306
 307 void
 308 print_ipv6_masked(struct ds *s, const struct in6_addr *addr,
 309                   const struct in6_addr *mask)
 310 {
 311     print_ipv6_addr(s, addr);
 312     if (mask && !ipv6_mask_is_exact(mask)) {
 313         if (ipv6_is_cidr(mask)) {
 314             int cidr_bits = ipv6_count_cidr_bits(mask);
 315             ds_put_format(s, "/%d", cidr_bits);
 316         } else {
 317             ds_put_char(s, '/');
 318             print_ipv6_addr(s, mask);
 319         }
 320     }
 321 }
 322
 323 struct in6_addr ipv6_addr_bitand(const struct in6_addr *a,
 324                                  const struct in6_addr *b)
 325 {
 326     int i;
 327     struct in6_addr dst;
 328
 329 #ifdef s6_addr32
 330     for (i=0; i<4; i++) {
 331         dst.s6_addr32[i] = a->s6_addr32[i] & b->s6_addr32[i];
 332     }
 333 #else
 334     for (i=0; i<16; i++) {
 335         dst.s6_addr[i] = a->s6_addr[i] & b->s6_addr[i];
 336     }
 337 #endif
 338
 339     return dst;
 340 }
 341
 342 /* Returns an in6_addr consisting of 'mask' high-order 1-bits and 128-N
 343  * low-order 0-bits. */
 344 struct in6_addr
 345 ipv6_create_mask(int mask)
 346 {
 347     struct in6_addr netmask;
 348     uint8_t *netmaskp = &netmask.s6_addr[0];
 349
 350     memset(&netmask, 0, sizeof netmask);
 351     while (mask > 8) {
 352         *netmaskp = 0xff;
 353         netmaskp++;
 354         mask -= 8;
 355     }
 356
 357     if (mask) {
 358         *netmaskp = 0xff << (8 - mask);
 359     }
 360
 361     return netmask;
 362 }
 363
 364 /* Given the IPv6 netmask 'netmask', returns the number of bits of the IPv6
 365  * address that it specifies, that is, the number of 1-bits in 'netmask'.
 366  * 'netmask' must be a CIDR netmask (see ipv6_is_cidr()).
 367  *
 368  * If 'netmask' is not a CIDR netmask (see ipv6_is_cidr()), the return value
 369  * will still be in the valid range but isn't otherwise meaningful. */
 370 int
 371 ipv6_count_cidr_bits(const struct in6_addr *netmask)
 372 {
 373     int i;
 374     int count = 0;
 375     const uint8_t *netmaskp = &netmask->s6_addr[0];
 376
 377     for (i=0; i<16; i++) {
 378         if (netmaskp[i] == 0xff) {
 379             count += 8;
 380         } else {
 381             uint8_t nm;
 382
 383             for(nm = netmaskp[i]; nm; nm <<= 1) {
 384                 count++;
 385             }
 386             break;
 387         }
 388
 389     }
 390
 391     return count;
 392 }
 393
 394 /* Returns true if 'netmask' is a CIDR netmask, that is, if it consists of N
 395  * high-order 1-bits and 128-N low-order 0-bits. */
 396 bool
 397 ipv6_is_cidr(const struct in6_addr *netmask)
 398 {
 399     const uint8_t *netmaskp = &netmask->s6_addr[0];
 400     int i;
 401
 402     for (i=0; i<16; i++) {
 403         if (netmaskp[i] != 0xff) {
 404             uint8_t x = ~netmaskp[i];
 405             if (x & (x + 1)) {
 406                 return false;
 407             }
 408             while (++i < 16) {
 409                 if (netmaskp[i]) {
 410                     return false;
 411                 }
 412             }
 413         }
 414     }
 415
 416     return true;
 417 }
 418
 419 /* Populates 'b' with an Ethernet II packet headed with the given 'eth_dst',
 420  * 'eth_src' and 'eth_type' parameters.  A payload of 'size' bytes is allocated
 421  * in 'b' and returned.  This payload may be populated with appropriate
 422  * information by the caller.  Sets 'b''s 'l2' and 'l3' pointers to the
 423  * Ethernet header and payload respectively.
 424  *
 425  * The returned packet has enough headroom to insert an 802.1Q VLAN header if
 426  * desired. */
 427 void *
 428 eth_compose(struct ofpbuf *b, const uint8_t eth_dst[ETH_ADDR_LEN],
 429             const uint8_t eth_src[ETH_ADDR_LEN], uint16_t eth_type,
 430             size_t size)
 431 {
 432     void *data;
 433     struct eth_header *eth;
 434
 435     ofpbuf_clear(b);
 436
 437     ofpbuf_prealloc_tailroom(b, ETH_HEADER_LEN + VLAN_HEADER_LEN + size);
 438     ofpbuf_reserve(b, VLAN_HEADER_LEN);
 439     eth = ofpbuf_put_uninit(b, ETH_HEADER_LEN);
 440     data = ofpbuf_put_uninit(b, size);
 441
 442     memcpy(eth->eth_dst, eth_dst, ETH_ADDR_LEN);
 443     memcpy(eth->eth_src, eth_src, ETH_ADDR_LEN);
 444     eth->eth_type = htons(eth_type);
 445
 446     b->l2 = eth;
 447     b->l3 = data;
 448
 449     return data;
 450 }
 451
 452 static void
 453 packet_set_ipv4_addr(struct ofpbuf *packet, ovs_be32 *addr, ovs_be32 new_addr)
 454 {
 455     struct ip_header *nh = packet->l3;
 456
 457     if (nh->ip_proto == IPPROTO_TCP && packet->l7) {
 458         struct tcp_header *th = packet->l4;
 459
 460         th->tcp_csum = recalc_csum32(th->tcp_csum, *addr, new_addr);
 461     } else if (nh->ip_proto == IPPROTO_UDP && packet->l7) {
 462         struct udp_header *uh = packet->l4;
 463
 464         if (uh->udp_csum) {
 465             uh->udp_csum = recalc_csum32(uh->udp_csum, *addr, new_addr);
 466             if (!uh->udp_csum) {
 467                 uh->udp_csum = htons(0xffff);
 468             }
 469         }
 470     }
 471     nh->ip_csum = recalc_csum32(nh->ip_csum, *addr, new_addr);
 472     *addr = new_addr;
 473 }
 474
 475 /* Modifies the IPv4 header fields of 'packet' to be consistent with 'src',
 476  * 'dst', 'tos', and 'ttl'.  Updates 'packet''s L4 checksums as appropriate.
 477  * 'packet' must contain a valid IPv4 packet with correctly populated l[347]
 478  * markers. */
 479 void
 480 packet_set_ipv4(struct ofpbuf *packet, ovs_be32 src, ovs_be32 dst,
 481                 uint8_t tos, uint8_t ttl)
 482 {
 483     struct ip_header *nh = packet->l3;
 484
 485     if (nh->ip_src != src) {
 486         packet_set_ipv4_addr(packet, &nh->ip_src, src);
 487     }
 488
 489     if (nh->ip_dst != dst) {
 490         packet_set_ipv4_addr(packet, &nh->ip_dst, dst);
 491     }
 492
 493     if (nh->ip_tos != tos) {
 494         uint8_t *field = &nh->ip_tos;
 495
 496         nh->ip_csum = recalc_csum16(nh->ip_csum, htons((uint16_t) *field),
 497                                     htons((uint16_t) tos));
 498         *field = tos;
 499     }
 500
 501     if (nh->ip_ttl != ttl) {
 502         uint8_t *field = &nh->ip_ttl;
 503
 504         nh->ip_csum = recalc_csum16(nh->ip_csum, htons(*field << 8),
 505                                     htons(ttl << 8));
 506         *field = ttl;
 507     }
 508 }
 509
 510 static void
 511 packet_set_port(ovs_be16 *port, ovs_be16 new_port, ovs_be16 *csum)
 512 {
 513     if (*port != new_port) {
 514         *csum = recalc_csum16(*csum, *port, new_port);
 515         *port = new_port;
 516     }
 517 }
 518
 519 /* Sets the TCP source and destination port ('src' and 'dst' respectively) of
 520  * the TCP header contained in 'packet'.  'packet' must be a valid TCP packet
 521  * with its l4 marker properly populated. */
 522 void
 523 packet_set_tcp_port(struct ofpbuf *packet, ovs_be16 src, ovs_be16 dst)
 524 {
 525     struct tcp_header *th = packet->l4;
 526
 527     packet_set_port(&th->tcp_src, src, &th->tcp_csum);
 528     packet_set_port(&th->tcp_dst, dst, &th->tcp_csum);
 529 }
 530
 531 /* Sets the UDP source and destination port ('src' and 'dst' respectively) of
 532  * the UDP header contained in 'packet'.  'packet' must be a valid UDP packet
 533  * with its l4 marker properly populated. */
 534 void
 535 packet_set_udp_port(struct ofpbuf *packet, ovs_be16 src, ovs_be16 dst)
 536 {
 537     struct udp_header *uh = packet->l4;
 538
 539     if (uh->udp_csum) {
 540         packet_set_port(&uh->udp_src, src, &uh->udp_csum);
 541         packet_set_port(&uh->udp_dst, dst, &uh->udp_csum);
 542
 543         if (!uh->udp_csum) {
 544             uh->udp_csum = htons(0xffff);
 545         }
 546     } else {
 547         uh->udp_src = src;
 548         uh->udp_dst = dst;
 549     }
 550 }
 551
 552 /* If 'packet' is a TCP packet, returns the TCP flags.  Otherwise, returns 0.
 553  *
 554  * 'flow' must be the flow corresponding to 'packet' and 'packet''s header
 555  * pointers must be properly initialized (e.g. with flow_extract()). */
 556 uint8_t
 557 packet_get_tcp_flags(const struct ofpbuf *packet, const struct flow *flow)
 558 {
 559     if ((flow->dl_type == htons(ETH_TYPE_IP) ||
 560          flow->dl_type == htons(ETH_TYPE_IPV6)) &&
 561         flow->nw_proto == IPPROTO_TCP && packet->l7) {
 562         const struct tcp_header *tcp = packet->l4;
 563         return TCP_FLAGS(tcp->tcp_ctl);
 564     } else {
 565         return 0;
 566     }
 567 }
 568
 569 /* Appends a string representation of the TCP flags value 'tcp_flags'
 570  * (e.g. obtained via packet_get_tcp_flags() or TCP_FLAGS) to 's', in the
 571  * format used by tcpdump. */
 572 void
 573 packet_format_tcp_flags(struct ds *s, uint8_t tcp_flags)
 574 {
 575     if (!tcp_flags) {
 576         ds_put_cstr(s, "none");
 577         return;
 578     }
 579
 580     if (tcp_flags & TCP_SYN) {
 581         ds_put_char(s, 'S');
 582     }
 583     if (tcp_flags & TCP_FIN) {
 584         ds_put_char(s, 'F');
 585     }
 586     if (tcp_flags & TCP_PSH) {
 587         ds_put_char(s, 'P');
 588     }
 589     if (tcp_flags & TCP_RST) {
 590         ds_put_char(s, 'R');
 591     }
 592     if (tcp_flags & TCP_URG) {
 593         ds_put_char(s, 'U');
 594     }
 595     if (tcp_flags & TCP_ACK) {
 596         ds_put_char(s, '.');
 597     }
 598     if (tcp_flags & 0x40) {
 599         ds_put_cstr(s, "[40]");
 600     }
 601     if (tcp_flags & 0x80) {
 602         ds_put_cstr(s, "[80]");
 603     }
 604 }