src/router.c

   1 /**
   2  * Copyright (C) 2012-2013 Steven Barth <steven@midlink.org>
   3  *
   4  * This program is free software; you can redistribute it and/or modify
   5  * it under the terms of the GNU General Public License v2 as published by
   6  * the Free Software Foundation.
   7  *
   8  * This program is distributed in the hope that it will be useful,
   9  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  10  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  11  * GNU General Public License for more details.
  12  *
  13  */
  14
  15 #include <errno.h>
  16 #include <fcntl.h>
  17 #include <signal.h>
  18 #include <resolv.h>
  19 #include <stdio.h>
  20 #include <stdlib.h>
  21 #include <unistd.h>
  22 #include <stdbool.h>
  23 #include <net/route.h>
  24
  25 #include "router.h"
  26 #include "odhcpd.h"
  27
  28
  29 static void forward_router_solicitation(const struct interface *iface);
  30 static void forward_router_advertisement(uint8_t *data, size_t len);
  31
  32 static void handle_icmpv6(void *addr, void *data, size_t len,
  33                 struct interface *iface, void *dest);
  34 static void trigger_router_advert(struct uloop_timeout *event);
  35 static void sigusr1_refresh(int signal);
  36
  37 static struct odhcpd_event router_event = {{.fd = -1}, handle_icmpv6};
  38
  39 static FILE *fp_route = NULL;
  40
  41
  42 int init_router(void)
  43 {
  44         // Open ICMPv6 socket
  45         int sock = socket(AF_INET6, SOCK_RAW | SOCK_CLOEXEC, IPPROTO_ICMPV6);
  46         if (sock < 0 && errno != EAFNOSUPPORT) {
  47                 syslog(LOG_ERR, "Failed to open RAW-socket: %s", strerror(errno));
  48                 return -1;
  49         }
  50
  51         // Let the kernel compute our checksums
  52         int val = 2;
  53         setsockopt(sock, IPPROTO_RAW, IPV6_CHECKSUM, &val, sizeof(val));
  54
  55         // This is required by RFC 4861
  56         val = 255;
  57         setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_HOPS, &val, sizeof(val));
  58         setsockopt(sock, IPPROTO_IPV6, IPV6_UNICAST_HOPS, &val, sizeof(val));
  59
  60         // We need to know the source interface
  61         val = 1;
  62         setsockopt(sock, IPPROTO_IPV6, IPV6_RECVPKTINFO, &val, sizeof(val));
  63         setsockopt(sock, IPPROTO_IPV6, IPV6_RECVHOPLIMIT, &val, sizeof(val));
  64
  65         // Don't loop back
  66         val = 0;
  67         setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_LOOP, &val, sizeof(val));
  68
  69         // Filter ICMPv6 package types
  70         struct icmp6_filter filt;
  71         ICMP6_FILTER_SETBLOCKALL(&filt);
  72         ICMP6_FILTER_SETPASS(ND_ROUTER_ADVERT, &filt);
  73         ICMP6_FILTER_SETPASS(ND_ROUTER_SOLICIT, &filt);
  74         setsockopt(sock, IPPROTO_ICMPV6, ICMP6_FILTER, &filt, sizeof(filt));
  75
  76         // Register socket
  77         router_event.uloop.fd = sock;
  78         odhcpd_register(&router_event);
  79
  80         if (!(fp_route = fopen("/proc/net/ipv6_route", "r")))
  81                 syslog(LOG_ERR, "Failed to open routing table: %s",
  82                                 strerror(errno));
  83
  84         signal(SIGUSR1, sigusr1_refresh);
  85         return 0;
  86 }
  87
  88
  89 int setup_router_interface(struct interface *iface, bool enable)
  90 {
  91         if (!fp_route || router_event.uloop.fd < 0)
  92                 return -1;
  93
  94         struct ipv6_mreq all_nodes = {ALL_IPV6_NODES, iface->ifindex};
  95         struct ipv6_mreq all_routers = {ALL_IPV6_ROUTERS, iface->ifindex};
  96
  97         uloop_timeout_cancel(&iface->timer_rs);
  98         iface->timer_rs.cb = NULL;
  99
 100         setsockopt(router_event.uloop.fd, IPPROTO_IPV6, IPV6_DROP_MEMBERSHIP,
 101                         &all_nodes, sizeof(all_nodes));
 102         setsockopt(router_event.uloop.fd, IPPROTO_IPV6, IPV6_DROP_MEMBERSHIP,
 103                         &all_routers, sizeof(all_routers));
 104
 105         if (!enable) {
 106                 if (iface->ra)
 107                         trigger_router_advert(&iface->timer_rs);
 108         } else {
 109                 void *mreq = &all_routers;
 110
 111                 if (iface->ra == RELAYD_RELAY && iface->master) {
 112                         mreq = &all_nodes;
 113                         forward_router_solicitation(iface);
 114                 } else if (iface->ra == RELAYD_SERVER && !iface->master) {
 115                         iface->timer_rs.cb = trigger_router_advert;
 116                         trigger_router_advert(&iface->timer_rs);
 117                 }
 118
 119                 if (iface->ra == RELAYD_RELAY || (iface->ra == RELAYD_SERVER && !iface->master))
 120                         setsockopt(router_event.uloop.fd, IPPROTO_IPV6,
 121                                         IPV6_ADD_MEMBERSHIP, mreq, sizeof(all_nodes));
 122         }
 123         return 0;
 124 }
 125
 126
 127 // Signal handler to resend all RDs
 128 static void sigusr1_refresh(_unused int signal)
 129 {
 130         struct interface *iface;
 131         list_for_each_entry(iface, &interfaces, head)
 132                 if (iface->ra == RELAYD_SERVER && !iface->master)
 133                         uloop_timeout_set(&iface->timer_rs, 1000);
 134 }
 135
 136 static bool router_icmpv6_valid(struct sockaddr_in6 *source, uint8_t *data, size_t len)
 137 {
 138         struct icmp6_hdr *hdr = (struct icmp6_hdr *)data;
 139         struct icmpv6_opt *opt, *end = (struct icmpv6_opt*)&data[len];
 140
 141         /* Hoplimit is already checked in odhcpd_receive_packets */
 142         if (len < sizeof(*hdr) || hdr->icmp6_code)
 143                 return false;
 144
 145         switch (hdr->icmp6_type) {
 146         case ND_ROUTER_ADVERT:
 147                 if (!IN6_IS_ADDR_LINKLOCAL(&source->sin6_addr))
 148                         return false;
 149
 150                 opt = (struct icmpv6_opt *)((struct nd_router_advert *)data + 1);
 151                 break;
 152
 153         case ND_ROUTER_SOLICIT:
 154                 opt = (struct icmpv6_opt *)((struct nd_router_solicit *)data + 1);
 155                 break;
 156
 157         default:
 158                 return false;
 159         }
 160
 161         icmpv6_for_each_option(opt, opt, end)
 162                 if (opt->type == ND_OPT_SOURCE_LINKADDR &&
 163                                 IN6_IS_ADDR_UNSPECIFIED(&source->sin6_addr) &&
 164                                 hdr->icmp6_type == ND_ROUTER_SOLICIT)
 165                         return false;
 166
 167         // Check all options parsed successfully
 168         return opt == end;
 169 }
 170
 171
 172 // Detect whether a default route exists, also find the source prefixes
 173 static bool parse_routes(struct odhcpd_ipaddr *n, ssize_t len)
 174 {
 175         rewind(fp_route);
 176
 177         char line[512], ifname[16];
 178         bool found_default = false;
 179         struct odhcpd_ipaddr p = {IN6ADDR_ANY_INIT, 0, 0, 0, 0};
 180         while (fgets(line, sizeof(line), fp_route)) {
 181                 uint32_t rflags;
 182                 if (sscanf(line, "00000000000000000000000000000000 00 "
 183                                 "%*s %*s %*s %*s %*s %*s %*s %15s", ifname) &&
 184                                 strcmp(ifname, "lo")) {
 185                         found_default = true;
 186                 } else if (sscanf(line, "%8" SCNx32 "%8" SCNx32 "%*8" SCNx32 "%*8" SCNx32 " %hhx %*s "
 187                                 "%*s 00000000000000000000000000000000 %*s %*s %*s %" SCNx32 " lo",
 188                                 &p.addr.s6_addr32[0], &p.addr.s6_addr32[1], &p.prefix, &rflags) &&
 189                                 p.prefix > 0 && (rflags & RTF_NONEXTHOP) && (rflags & RTF_REJECT)) {
 190                         // Find source prefixes by scanning through unreachable-routes
 191                         p.addr.s6_addr32[0] = htonl(p.addr.s6_addr32[0]);
 192                         p.addr.s6_addr32[1] = htonl(p.addr.s6_addr32[1]);
 193
 194                         for (ssize_t i = 0; i < len; ++i) {
 195                                 if (n[i].prefix <= 64 && n[i].prefix >= p.prefix &&
 196                                                 !odhcpd_bmemcmp(&p.addr, &n[i].addr, p.prefix)) {
 197                                         n[i].dprefix = p.prefix;
 198                                         break;
 199                                 }
 200                         }
 201
 202                 }
 203         }
 204
 205         return found_default;
 206 }
 207
 208
 209 // Router Advert server mode
 210 static uint64_t send_router_advert(struct interface *iface, const struct in6_addr *from)
 211 {
 212         int mtu = odhcpd_get_interface_mtu(iface->ifname);
 213         if (mtu < 0)
 214                 mtu = 1500;
 215
 216         struct {
 217                 struct nd_router_advert h;
 218                 struct icmpv6_opt lladdr;
 219                 struct nd_opt_mtu mtu;
 220                 struct nd_opt_prefix_info prefix[RELAYD_MAX_PREFIXES];
 221         } adv = {
 222                 .h = {{.icmp6_type = ND_ROUTER_ADVERT, .icmp6_code = 0}, 0, 0},
 223                 .lladdr = {ND_OPT_SOURCE_LINKADDR, 1, {0}},
 224                 .mtu = {ND_OPT_MTU, 1, 0, htonl(mtu)},
 225         };
 226
 227         if (iface->dhcpv6)
 228                 adv.h.nd_ra_flags_reserved = ND_RA_FLAG_OTHER;
 229
 230         if (iface->managed >= RELAYD_MANAGED_MFLAG)
 231                 adv.h.nd_ra_flags_reserved |= ND_RA_FLAG_MANAGED;
 232
 233         if (iface->route_preference < 0)
 234                 adv.h.nd_ra_flags_reserved |= ND_RA_PREF_LOW;
 235         else if (iface->route_preference > 0)
 236                 adv.h.nd_ra_flags_reserved |= ND_RA_PREF_HIGH;
 237         odhcpd_get_mac(iface, adv.lladdr.data);
 238
 239         // If not currently shutting down
 240         struct odhcpd_ipaddr addrs[RELAYD_MAX_PREFIXES];
 241         ssize_t ipcnt = 0;
 242         uint64_t maxpreferred = 0;
 243
 244         // If not shutdown
 245         if (iface->timer_rs.cb) {
 246                 ipcnt = odhcpd_get_interface_addresses(iface->ifindex,
 247                                 addrs, ARRAY_SIZE(addrs));
 248
 249                 // Check default route
 250                 if (parse_routes(addrs, ipcnt) || iface->default_router > 1)
 251                         adv.h.nd_ra_router_lifetime =
 252                                         htons(3 * MaxRtrAdvInterval);
 253         }
 254
 255         // Construct Prefix Information options
 256         bool have_public = false;
 257         size_t cnt = 0;
 258
 259         struct in6_addr dns_pref = IN6ADDR_ANY_INIT, *dns_addr = &dns_pref;
 260         uint32_t dns_time = 0;
 261         size_t dns_cnt = 1;
 262
 263         for (ssize_t i = 0; i < ipcnt; ++i) {
 264                 struct odhcpd_ipaddr *addr = &addrs[i];
 265                 if (addr->prefix > 96)
 266                         continue; // Address not suitable
 267
 268                 if (addr->preferred > MaxPreferredTime)
 269                         addr->preferred = MaxPreferredTime;
 270
 271                 if (addr->valid > MaxValidTime)
 272                         addr->valid = MaxValidTime;
 273
 274                 struct nd_opt_prefix_info *p = NULL;
 275                 for (size_t i = 0; i < cnt; ++i) {
 276                         if (addr->prefix == adv.prefix[i].nd_opt_pi_prefix_len &&
 277                                         !odhcpd_bmemcmp(&adv.prefix[i].nd_opt_pi_prefix,
 278                                         &addr->addr, addr->prefix))
 279                                 p = &adv.prefix[i];
 280                 }
 281
 282                 if (!p) {
 283                         if (cnt >= ARRAY_SIZE(adv.prefix))
 284                                 break;
 285
 286                         p = &adv.prefix[cnt++];
 287                 }
 288
 289                 if ((addr->addr.s6_addr[0] & 0xfe) != 0xfc && addr->preferred > 0) {
 290                         have_public = true;
 291
 292                         if (maxpreferred < 1000 * addr->preferred)
 293                                 maxpreferred = 1000 * addr->preferred;
 294                 }
 295
 296                 odhcpd_bmemcpy(&p->nd_opt_pi_prefix, &addr->addr,
 297                                 (iface->ra_advrouter) ? 128 : addr->prefix);
 298                 p->nd_opt_pi_type = ND_OPT_PREFIX_INFORMATION;
 299                 p->nd_opt_pi_len = 4;
 300                 p->nd_opt_pi_prefix_len = (addr->prefix < 64) ? 64 : addr->prefix;
 301                 p->nd_opt_pi_flags_reserved = 0;
 302                 if (!iface->ra_not_onlink)
 303                         p->nd_opt_pi_flags_reserved |= ND_OPT_PI_FLAG_ONLINK;
 304                 if (iface->managed < RELAYD_MANAGED_NO_AFLAG && addr->prefix <= 64)
 305                         p->nd_opt_pi_flags_reserved |= ND_OPT_PI_FLAG_AUTO;
 306                 if (iface->ra_advrouter)
 307                         p->nd_opt_pi_flags_reserved |= ND_OPT_PI_FLAG_RADDR;
 308                 p->nd_opt_pi_valid_time = htonl(addr->valid);
 309                 p->nd_opt_pi_preferred_time = htonl(addr->preferred);
 310
 311                 if (addr->preferred > dns_time) {
 312                         dns_time = addr->preferred;
 313                         dns_pref = addr->addr;
 314                 }
 315         }
 316
 317         if (!have_public && !iface->default_router && adv.h.nd_ra_router_lifetime) {
 318                 syslog(LOG_WARNING, "A default route is present but there is no public prefix "
 319                                 "on %s thus we don't announce a default route!", iface->ifname);
 320                 adv.h.nd_ra_router_lifetime = 0;
 321         }
 322
 323         // DNS Recursive DNS
 324         if (iface->dns_cnt > 0) {
 325                 dns_addr = iface->dns;
 326                 dns_cnt = iface->dns_cnt;
 327                 dns_time = 2 * MaxRtrAdvInterval;
 328         }
 329
 330         if (!dns_addr || IN6_IS_ADDR_UNSPECIFIED(dns_addr))
 331                 dns_cnt = 0;
 332
 333         struct {
 334                 uint8_t type;
 335                 uint8_t len;
 336                 uint8_t pad;
 337                 uint8_t pad2;
 338                 uint32_t lifetime;
 339         } dns = {ND_OPT_RECURSIVE_DNS, (1 + (2 * dns_cnt)), 0, 0, htonl(dns_time)};
 340
 341
 342
 343         // DNS Search options
 344         uint8_t search_buf[256], *search_domain = iface->search;
 345         size_t search_len = iface->search_len, search_padded = 0;
 346
 347         if (!search_domain && !res_init() && _res.dnsrch[0] && _res.dnsrch[0][0]) {
 348                 int len = dn_comp(_res.dnsrch[0], search_buf,
 349                                 sizeof(search_buf), NULL, NULL);
 350                 if (len > 0) {
 351                         search_domain = search_buf;
 352                         search_len = len;
 353                 }
 354         }
 355
 356         if (search_len > 0)
 357                 search_padded = ((search_len + 7) & (~7)) + 8;
 358
 359         struct {
 360                 uint8_t type;
 361                 uint8_t len;
 362                 uint8_t pad;
 363                 uint8_t pad2;
 364                 uint32_t lifetime;
 365                 uint8_t name[];
 366         } *search = alloca(sizeof(*search) + search_padded);
 367
 368         search->type = ND_OPT_DNS_SEARCH;
 369         search->len = search_len ? ((sizeof(*search) + search_padded) / 8) : 0;
 370         search->pad = 0;
 371         search->pad2 = 0;
 372         search->lifetime = htonl(2 * MaxRtrAdvInterval);;
 373         memcpy(search->name, search_domain, search_len);
 374         memset(&search->name[search_len], 0, search_padded - search_len);
 375
 376
 377         size_t routes_cnt = 0;
 378         struct {
 379                 uint8_t type;
 380                 uint8_t len;
 381                 uint8_t prefix;
 382                 uint8_t flags;
 383                 uint32_t lifetime;
 384                 uint32_t addr[4];
 385         } routes[RELAYD_MAX_PREFIXES];
 386
 387         for (ssize_t i = 0; i < ipcnt; ++i) {
 388                 struct odhcpd_ipaddr *addr = &addrs[i];
 389                 if (addr->dprefix > 64 || addr->dprefix == 0 ||
 390                                 (addr->dprefix == 64 && addr->prefix == 64)) {
 391                         continue; // Address not suitable
 392                 } else if (addr->dprefix > 32) {
 393                         addr->addr.s6_addr32[1] &= htonl(~((1U << (64 - addr->dprefix)) - 1));
 394                 } else if (addr->dprefix <= 32) {
 395                         addr->addr.s6_addr32[0] &= htonl(~((1U << (32 - addr->dprefix)) - 1));
 396                         addr->addr.s6_addr32[1] = 0;
 397                 }
 398
 399                 routes[routes_cnt].type = ND_OPT_ROUTE_INFO;
 400                 routes[routes_cnt].len = sizeof(*routes) / 8;
 401                 routes[routes_cnt].prefix = addr->dprefix;
 402                 routes[routes_cnt].flags = 0;
 403                 if (iface->route_preference < 0)
 404                         routes[routes_cnt].flags |= ND_RA_PREF_LOW;
 405                 else if (iface->route_preference > 0)
 406                         routes[routes_cnt].flags |= ND_RA_PREF_HIGH;
 407                 routes[routes_cnt].lifetime = htonl(addr->valid);
 408                 routes[routes_cnt].addr[0] = addr->addr.s6_addr32[0];
 409                 routes[routes_cnt].addr[1] = addr->addr.s6_addr32[1];
 410                 routes[routes_cnt].addr[2] = 0;
 411                 routes[routes_cnt].addr[3] = 0;
 412
 413                 ++routes_cnt;
 414         }
 415
 416         // Calculate periodic transmit
 417         int msecs = 0;
 418         uint32_t maxival = MaxRtrAdvInterval * 1000;
 419         uint32_t minival = MinRtrAdvInterval * 1000;
 420
 421         if (maxpreferred > 0 && maxival > maxpreferred / 2) {
 422                 maxival = maxpreferred / 2;
 423                 if (maxival < 4000)
 424                         maxival = 4000;
 425
 426                 if (maxival >= 9000)
 427                         minival = maxival / 3;
 428                 else
 429                         minival = (maxival * 3) / 4;
 430         }
 431
 432         odhcpd_urandom(&msecs, sizeof(msecs));
 433         msecs = (labs(msecs) % (maxival - minival)) + minival;
 434
 435         struct icmpv6_opt adv_interval = {
 436                 .type = ND_OPT_RTR_ADV_INTERVAL,
 437                 .len = 1,
 438                 .data = {0, 0, maxival >> 24, maxival >> 16, maxival >> 8, maxival}
 439         };
 440
 441         struct iovec iov[] = {{&adv, (uint8_t*)&adv.prefix[cnt] - (uint8_t*)&adv},
 442                         {&routes, routes_cnt * sizeof(*routes)},
 443                         {&dns, (dns_cnt) ? sizeof(dns) : 0},
 444                         {dns_addr, dns_cnt * sizeof(*dns_addr)},
 445                         {search, search->len * 8},
 446                         {&adv_interval, adv_interval.len * 8}};
 447         struct sockaddr_in6 dest = {AF_INET6, 0, 0, ALL_IPV6_NODES, 0};
 448
 449         if (from && !IN6_IS_ADDR_UNSPECIFIED(from))
 450                 dest.sin6_addr = *from;
 451
 452         odhcpd_send(router_event.uloop.fd,
 453                         &dest, iov, ARRAY_SIZE(iov), iface);
 454
 455         return msecs;
 456 }
 457
 458
 459 static void trigger_router_advert(struct uloop_timeout *event)
 460 {
 461         struct interface *iface = container_of(event, struct interface, timer_rs);
 462         int msecs = send_router_advert(iface, NULL);
 463
 464         // Rearm timer if not shut down
 465         if (event->cb)
 466                 uloop_timeout_set(event, msecs);
 467 }
 468
 469
 470 // Event handler for incoming ICMPv6 packets
 471 static void handle_icmpv6(void *addr, void *data, size_t len,
 472                 struct interface *iface, _unused void *dest)
 473 {
 474         struct icmp6_hdr *hdr = data;
 475         struct sockaddr_in6 *from = addr;
 476
 477         if (!router_icmpv6_valid(addr, data, len))
 478                 return;
 479
 480         if ((iface->ra == RELAYD_SERVER && !iface->master)) { // Server mode
 481                 if (hdr->icmp6_type == ND_ROUTER_SOLICIT)
 482                         send_router_advert(iface, &from->sin6_addr);
 483         } else if (iface->ra == RELAYD_RELAY) { // Relay mode
 484                 if (hdr->icmp6_type == ND_ROUTER_ADVERT && iface->master)
 485                         forward_router_advertisement(data, len);
 486                 else if (hdr->icmp6_type == ND_ROUTER_SOLICIT && !iface->master)
 487                         forward_router_solicitation(odhcpd_get_master_interface());
 488         }
 489 }
 490
 491
 492 // Forward router solicitation
 493 static void forward_router_solicitation(const struct interface *iface)
 494 {
 495         if (!iface)
 496                 return;
 497
 498         struct icmp6_hdr rs = {ND_ROUTER_SOLICIT, 0, 0, {{0}}};
 499         struct iovec iov = {&rs, sizeof(rs)};
 500         struct sockaddr_in6 all_routers =
 501                 {AF_INET6, 0, 0, ALL_IPV6_ROUTERS, iface->ifindex};
 502
 503         syslog(LOG_NOTICE, "Sending RS to %s", iface->ifname);
 504         odhcpd_send(router_event.uloop.fd, &all_routers, &iov, 1, iface);
 505 }
 506
 507
 508 // Handler for incoming router solicitations on slave interfaces
 509 static void forward_router_advertisement(uint8_t *data, size_t len)
 510 {
 511         struct nd_router_advert *adv = (struct nd_router_advert *)data;
 512
 513         // Rewrite options
 514         uint8_t *end = data + len;
 515         uint8_t *mac_ptr = NULL;
 516         struct in6_addr *dns_ptr = NULL;
 517         size_t dns_count = 0;
 518
 519         struct icmpv6_opt *opt;
 520         icmpv6_for_each_option(opt, &adv[1], end) {
 521                 if (opt->type == ND_OPT_SOURCE_LINKADDR) {
 522                         // Store address of source MAC-address
 523                         mac_ptr = opt->data;
 524                 } else if (opt->type == ND_OPT_RECURSIVE_DNS && opt->len > 1) {
 525                         // Check if we have to rewrite DNS
 526                         dns_ptr = (struct in6_addr*)&opt->data[6];
 527                         dns_count = (opt->len - 1) / 2;
 528                 }
 529         }
 530
 531         syslog(LOG_NOTICE, "Got a RA");
 532
 533         // Indicate a proxy, however we don't follow the rest of RFC 4389 yet
 534         adv->nd_ra_flags_reserved |= ND_RA_FLAG_PROXY;
 535
 536         // Forward advertisement to all slave interfaces
 537         struct sockaddr_in6 all_nodes = {AF_INET6, 0, 0, ALL_IPV6_NODES, 0};
 538         struct iovec iov = {data, len};
 539
 540         struct interface *iface;
 541         list_for_each_entry(iface, &interfaces, head) {
 542                 if (iface->ra != RELAYD_RELAY || iface->master)
 543                         continue;
 544
 545                 // Fixup source hardware address option
 546                 if (mac_ptr)
 547                         odhcpd_get_mac(iface, mac_ptr);
 548
 549                 // If we have to rewrite DNS entries
 550                 if (iface->always_rewrite_dns && dns_ptr && dns_count > 0) {
 551                         const struct in6_addr *rewrite = iface->dns;
 552                         struct in6_addr addr;
 553                         size_t rewrite_cnt = iface->dns_cnt;
 554
 555                         if (rewrite_cnt == 0) {
 556                                 if (odhcpd_get_preferred_interface_address(iface->ifindex, &addr) < 1)
 557                                         continue; // Unable to comply
 558
 559                                 rewrite = &addr;
 560                                 rewrite_cnt = 1;
 561                         }
 562
 563                         // Copy over any other addresses
 564                         for (size_t i = 0; i < dns_count; ++i) {
 565                                 size_t j = (i < rewrite_cnt) ? i : rewrite_cnt - 1;
 566                                 dns_ptr[i] = rewrite[j];
 567                         }
 568                 }
 569
 570                 odhcpd_send(router_event.uloop.fd, &all_nodes, &iov, 1, iface);
 571         }
 572 }