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 #define RA_IOV_LEN 6
  41
  42 #define TIME_LEFT(t1, now) ((t1) != UINT32_MAX ? (t1) - (now) : UINT32_MAX)
  43
  44 int init_router(void)
  45 {
  46         // Open ICMPv6 socket
  47         int sock = socket(AF_INET6, SOCK_RAW | SOCK_CLOEXEC, IPPROTO_ICMPV6);
  48         if (sock < 0 && errno != EAFNOSUPPORT) {
  49                 syslog(LOG_ERR, "Failed to open RAW-socket: %s", strerror(errno));
  50                 return -1;
  51         }
  52
  53         // Let the kernel compute our checksums
  54         int val = 2;
  55         setsockopt(sock, IPPROTO_RAW, IPV6_CHECKSUM, &val, sizeof(val));
  56
  57         // This is required by RFC 4861
  58         val = 255;
  59         setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_HOPS, &val, sizeof(val));
  60         setsockopt(sock, IPPROTO_IPV6, IPV6_UNICAST_HOPS, &val, sizeof(val));
  61
  62         // We need to know the source interface
  63         val = 1;
  64         setsockopt(sock, IPPROTO_IPV6, IPV6_RECVPKTINFO, &val, sizeof(val));
  65         setsockopt(sock, IPPROTO_IPV6, IPV6_RECVHOPLIMIT, &val, sizeof(val));
  66
  67         // Don't loop back
  68         val = 0;
  69         setsockopt(sock, IPPROTO_IPV6, IPV6_MULTICAST_LOOP, &val, sizeof(val));
  70
  71         // Filter ICMPv6 package types
  72         struct icmp6_filter filt;
  73         ICMP6_FILTER_SETBLOCKALL(&filt);
  74         ICMP6_FILTER_SETPASS(ND_ROUTER_ADVERT, &filt);
  75         ICMP6_FILTER_SETPASS(ND_ROUTER_SOLICIT, &filt);
  76         setsockopt(sock, IPPROTO_ICMPV6, ICMP6_FILTER, &filt, sizeof(filt));
  77
  78         // Register socket
  79         router_event.uloop.fd = sock;
  80         odhcpd_register(&router_event);
  81
  82         if (!(fp_route = fopen("/proc/net/ipv6_route", "r")))
  83                 syslog(LOG_ERR, "Failed to open routing table: %s",
  84                                 strerror(errno));
  85
  86         signal(SIGUSR1, sigusr1_refresh);
  87         return 0;
  88 }
  89
  90
  91 int setup_router_interface(struct interface *iface, bool enable)
  92 {
  93         if (!fp_route || router_event.uloop.fd < 0)
  94                 return -1;
  95
  96         struct ipv6_mreq all_nodes = {ALL_IPV6_NODES, iface->ifindex};
  97         struct ipv6_mreq all_routers = {ALL_IPV6_ROUTERS, iface->ifindex};
  98
  99         uloop_timeout_cancel(&iface->timer_rs);
 100         iface->timer_rs.cb = NULL;
 101
 102         setsockopt(router_event.uloop.fd, IPPROTO_IPV6, IPV6_DROP_MEMBERSHIP,
 103                         &all_nodes, sizeof(all_nodes));
 104         setsockopt(router_event.uloop.fd, IPPROTO_IPV6, IPV6_DROP_MEMBERSHIP,
 105                         &all_routers, sizeof(all_routers));
 106
 107         if (!enable) {
 108                 if (iface->ra)
 109                         trigger_router_advert(&iface->timer_rs);
 110         } else {
 111                 void *mreq = &all_routers;
 112
 113                 if (iface->ra == RELAYD_RELAY && iface->master) {
 114                         mreq = &all_nodes;
 115                         forward_router_solicitation(iface);
 116                 } else if (iface->ra == RELAYD_SERVER && !iface->master) {
 117                         iface->timer_rs.cb = trigger_router_advert;
 118                         uloop_timeout_set(&iface->timer_rs, 1000);
 119                 }
 120
 121                 if (iface->ra == RELAYD_RELAY || (iface->ra == RELAYD_SERVER && !iface->master))
 122                         setsockopt(router_event.uloop.fd, IPPROTO_IPV6,
 123                                         IPV6_ADD_MEMBERSHIP, mreq, sizeof(all_nodes));
 124         }
 125         return 0;
 126 }
 127
 128
 129 // Signal handler to resend all RDs
 130 static void sigusr1_refresh(_unused int signal)
 131 {
 132         struct interface *iface;
 133         list_for_each_entry(iface, &interfaces, head)
 134                 if (iface->ra == RELAYD_SERVER && !iface->master)
 135                         uloop_timeout_set(&iface->timer_rs, 1000);
 136 }
 137
 138 static bool router_icmpv6_valid(struct sockaddr_in6 *source, uint8_t *data, size_t len)
 139 {
 140         struct icmp6_hdr *hdr = (struct icmp6_hdr *)data;
 141         struct icmpv6_opt *opt, *end = (struct icmpv6_opt*)&data[len];
 142
 143         /* Hoplimit is already checked in odhcpd_receive_packets */
 144         if (len < sizeof(*hdr) || hdr->icmp6_code)
 145                 return false;
 146
 147         switch (hdr->icmp6_type) {
 148         case ND_ROUTER_ADVERT:
 149                 if (!IN6_IS_ADDR_LINKLOCAL(&source->sin6_addr))
 150                         return false;
 151
 152                 opt = (struct icmpv6_opt *)((struct nd_router_advert *)data + 1);
 153                 break;
 154
 155         case ND_ROUTER_SOLICIT:
 156                 opt = (struct icmpv6_opt *)((struct nd_router_solicit *)data + 1);
 157                 break;
 158
 159         default:
 160                 return false;
 161         }
 162
 163         icmpv6_for_each_option(opt, opt, end)
 164                 if (opt->type == ND_OPT_SOURCE_LINKADDR &&
 165                                 IN6_IS_ADDR_UNSPECIFIED(&source->sin6_addr) &&
 166                                 hdr->icmp6_type == ND_ROUTER_SOLICIT)
 167                         return false;
 168
 169         // Check all options parsed successfully
 170         return opt == end;
 171 }
 172
 173
 174 // Detect whether a default route exists, also find the source prefixes
 175 static bool parse_routes(struct odhcpd_ipaddr *n, ssize_t len)
 176 {
 177         rewind(fp_route);
 178
 179         char line[512], ifname[16];
 180         bool found_default = false;
 181         struct odhcpd_ipaddr p = {IN6ADDR_ANY_INIT, 0, 0, 0, 0};
 182         while (fgets(line, sizeof(line), fp_route)) {
 183                 uint32_t rflags;
 184                 if (sscanf(line, "00000000000000000000000000000000 00 "
 185                                 "%*s %*s %*s %*s %*s %*s %*s %15s", ifname) &&
 186                                 strcmp(ifname, "lo")) {
 187                         found_default = true;
 188                 } else if (sscanf(line, "%8" SCNx32 "%8" SCNx32 "%*8" SCNx32 "%*8" SCNx32 " %hhx %*s "
 189                                 "%*s 00000000000000000000000000000000 %*s %*s %*s %" SCNx32 " lo",
 190                                 &p.addr.s6_addr32[0], &p.addr.s6_addr32[1], &p.prefix, &rflags) &&
 191                                 p.prefix > 0 && (rflags & RTF_NONEXTHOP) && (rflags & RTF_REJECT)) {
 192                         // Find source prefixes by scanning through unreachable-routes
 193                         p.addr.s6_addr32[0] = htonl(p.addr.s6_addr32[0]);
 194                         p.addr.s6_addr32[1] = htonl(p.addr.s6_addr32[1]);
 195
 196                         for (ssize_t i = 0; i < len; ++i) {
 197                                 if (n[i].prefix <= 64 && n[i].prefix >= p.prefix &&
 198                                                 !odhcpd_bmemcmp(&p.addr, &n[i].addr, p.prefix)) {
 199                                         n[i].dprefix = p.prefix;
 200                                         break;
 201                                 }
 202                         }
 203
 204                 }
 205         }
 206
 207         return found_default;
 208 }
 209
 210 // Router Advert server mode
 211 static uint64_t send_router_advert(struct interface *iface, const struct in6_addr *from)
 212 {
 213         time_t now = odhcpd_time();
 214         int mtu = odhcpd_get_interface_config(iface->ifname, "mtu");
 215         int hlim = odhcpd_get_interface_config(iface->ifname, "hop_limit");
 216
 217         if (mtu < 1280)
 218                 mtu = 1280;
 219
 220         struct {
 221                 struct nd_router_advert h;
 222                 struct icmpv6_opt lladdr;
 223                 struct nd_opt_mtu mtu;
 224                 struct nd_opt_prefix_info prefix[sizeof(iface->ia_addr) / sizeof(*iface->ia_addr)];
 225         } adv = {
 226                 .h = {{.icmp6_type = ND_ROUTER_ADVERT, .icmp6_code = 0}, 0, 0},
 227                 .lladdr = {ND_OPT_SOURCE_LINKADDR, 1, {0}},
 228                 .mtu = {ND_OPT_MTU, 1, 0, htonl(mtu)},
 229         };
 230
 231         if (hlim > 0)
 232                 adv.h.nd_ra_curhoplimit = hlim;
 233
 234         if (iface->dhcpv6)
 235                 adv.h.nd_ra_flags_reserved = ND_RA_FLAG_OTHER;
 236
 237         if (iface->managed >= RELAYD_MANAGED_MFLAG)
 238                 adv.h.nd_ra_flags_reserved |= ND_RA_FLAG_MANAGED;
 239
 240         if (iface->route_preference < 0)
 241                 adv.h.nd_ra_flags_reserved |= ND_RA_PREF_LOW;
 242         else if (iface->route_preference > 0)
 243                 adv.h.nd_ra_flags_reserved |= ND_RA_PREF_HIGH;
 244         odhcpd_get_mac(iface, adv.lladdr.data);
 245
 246         // If not currently shutting down
 247         struct odhcpd_ipaddr addrs[8];
 248         ssize_t ipcnt = 0;
 249         int64_t minvalid = INT64_MAX;
 250
 251         // If not shutdown
 252         if (iface->timer_rs.cb) {
 253                 ipcnt = iface->ia_addr_len;
 254                 memcpy(addrs, iface->ia_addr, ipcnt * sizeof(*addrs));
 255
 256                 // Check default route
 257                 if (parse_routes(addrs, ipcnt))
 258                         adv.h.nd_ra_router_lifetime = htons(1);
 259                 if (iface->default_router > 1)
 260                         adv.h.nd_ra_router_lifetime = htons(iface->default_router);
 261         }
 262
 263         // Construct Prefix Information options
 264         size_t cnt = 0;
 265
 266         struct in6_addr dns_pref, *dns_addr = &dns_pref;
 267         size_t dns_cnt = 1;
 268
 269         odhcpd_get_linklocal_interface_address(iface->ifindex, &dns_pref);
 270
 271         for (ssize_t i = 0; i < ipcnt; ++i) {
 272                 struct odhcpd_ipaddr *addr = &addrs[i];
 273                 if (addr->prefix > 96 || addr->valid <= (uint32_t)now)
 274                         continue; // Address not suitable
 275
 276                 struct nd_opt_prefix_info *p = NULL;
 277                 for (size_t i = 0; i < cnt; ++i) {
 278                         if (addr->prefix == adv.prefix[i].nd_opt_pi_prefix_len &&
 279                                         !odhcpd_bmemcmp(&adv.prefix[i].nd_opt_pi_prefix,
 280                                         &addr->addr, addr->prefix))
 281                                 p = &adv.prefix[i];
 282                 }
 283
 284                 if (!p) {
 285                         if (cnt >= ARRAY_SIZE(adv.prefix))
 286                                 break;
 287
 288                         p = &adv.prefix[cnt++];
 289                 }
 290
 291                 if (addr->preferred > (uint32_t)now &&
 292                                 minvalid > 1000LL * TIME_LEFT(addr->valid, now))
 293                         minvalid = 1000LL * TIME_LEFT(addr->valid, now);
 294
 295                 if (((addr->addr.s6_addr[0] & 0xfe) != 0xfc || iface->default_router)
 296                                 && adv.h.nd_ra_router_lifetime
 297                                 && ntohs(adv.h.nd_ra_router_lifetime) < TIME_LEFT(addr->valid, now))
 298                         adv.h.nd_ra_router_lifetime = htons(TIME_LEFT(addr->valid, now));
 299
 300                 odhcpd_bmemcpy(&p->nd_opt_pi_prefix, &addr->addr,
 301                                 (iface->ra_advrouter) ? 128 : addr->prefix);
 302                 p->nd_opt_pi_type = ND_OPT_PREFIX_INFORMATION;
 303                 p->nd_opt_pi_len = 4;
 304                 p->nd_opt_pi_prefix_len = (addr->prefix < 64) ? 64 : addr->prefix;
 305                 p->nd_opt_pi_flags_reserved = 0;
 306                 if (!iface->ra_not_onlink)
 307                         p->nd_opt_pi_flags_reserved |= ND_OPT_PI_FLAG_ONLINK;
 308                 if (iface->managed < RELAYD_MANAGED_NO_AFLAG && addr->prefix <= 64)
 309                         p->nd_opt_pi_flags_reserved |= ND_OPT_PI_FLAG_AUTO;
 310                 if (iface->ra_advrouter)
 311                         p->nd_opt_pi_flags_reserved |= ND_OPT_PI_FLAG_RADDR;
 312                 p->nd_opt_pi_valid_time = htonl(TIME_LEFT(addr->valid, now));
 313                 if (addr->preferred > (uint32_t)now)
 314                         p->nd_opt_pi_preferred_time = htonl(TIME_LEFT(addr->preferred, now));
 315                 else if (addr->valid - now < 7200)
 316                         p->nd_opt_pi_valid_time = 0;
 317         }
 318
 319         if (!iface->default_router && adv.h.nd_ra_router_lifetime == htons(1)) {
 320                 syslog(LOG_WARNING, "A default route is present but there is no public prefix "
 321                                 "on %s thus we don't announce a default route!", iface->ifname);
 322                 adv.h.nd_ra_router_lifetime = 0;
 323         }
 324
 325         // DNS Recursive DNS
 326         if (iface->dns_cnt > 0) {
 327                 dns_addr = iface->dns;
 328                 dns_cnt = iface->dns_cnt;
 329         }
 330
 331         if (!dns_addr || IN6_IS_ADDR_UNSPECIFIED(dns_addr))
 332                 dns_cnt = 0;
 333
 334         struct {
 335                 uint8_t type;
 336                 uint8_t len;
 337                 uint8_t pad;
 338                 uint8_t pad2;
 339                 uint32_t lifetime;
 340         } dns = {ND_OPT_RECURSIVE_DNS, (1 + (2 * dns_cnt)), 0, 0, 0};
 341
 342
 343
 344         // DNS Search options
 345         uint8_t search_buf[256], *search_domain = iface->search;
 346         size_t search_len = iface->search_len, search_padded = 0;
 347
 348         if (!search_domain && !res_init() && _res.dnsrch[0] && _res.dnsrch[0][0]) {
 349                 int len = dn_comp(_res.dnsrch[0], search_buf,
 350                                 sizeof(search_buf), NULL, NULL);
 351                 if (len > 0) {
 352                         search_domain = search_buf;
 353                         search_len = len;
 354                 }
 355         }
 356
 357         if (search_len > 0)
 358                 search_padded = ((search_len + 7) & (~7)) + 8;
 359
 360         struct {
 361                 uint8_t type;
 362                 uint8_t len;
 363                 uint8_t pad;
 364                 uint8_t pad2;
 365                 uint32_t lifetime;
 366                 uint8_t name[];
 367         } *search = alloca(sizeof(*search) + search_padded);
 368
 369         search->type = ND_OPT_DNS_SEARCH;
 370         search->len = search_len ? ((sizeof(*search) + search_padded) / 8) : 0;
 371         search->pad = 0;
 372         search->pad2 = 0;
 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 || addr->valid <= (uint32_t)now ||
 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(TIME_LEFT(addr->valid, now));
 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 = iface->ra_maxinterval * 1000;
 419         uint32_t minival;
 420
 421         if (maxival < 4000 || maxival > MaxRtrAdvInterval * 1000)
 422                 maxival = MaxRtrAdvInterval * 1000;
 423
 424         if (maxival > minvalid / 3) {
 425                 maxival = minvalid / 3;
 426
 427                 if (maxival < 4000)
 428                         maxival = 4000;
 429         }
 430
 431         minival = (maxival * 3) / 4;
 432
 433         search->lifetime = htonl(maxival / 100);
 434         dns.lifetime = search->lifetime;
 435
 436         odhcpd_urandom(&msecs, sizeof(msecs));
 437         msecs = (labs(msecs) % (maxival - minival)) + minival;
 438
 439         struct icmpv6_opt adv_interval = {
 440                 .type = ND_OPT_RTR_ADV_INTERVAL,
 441                 .len = 1,
 442                 .data = {0, 0, maxival >> 24, maxival >> 16, maxival >> 8, maxival}
 443         };
 444
 445         struct iovec iov[RA_IOV_LEN] = {
 446                         {&adv, (uint8_t*)&adv.prefix[cnt] - (uint8_t*)&adv},
 447                         {&routes, routes_cnt * sizeof(*routes)},
 448                         {&dns, (dns_cnt) ? sizeof(dns) : 0},
 449                         {dns_addr, dns_cnt * sizeof(*dns_addr)},
 450                         {search, search->len * 8},
 451                         {&adv_interval, adv_interval.len * 8}};
 452         struct sockaddr_in6 dest = {AF_INET6, 0, 0, ALL_IPV6_NODES, 0};
 453
 454         if (from && !IN6_IS_ADDR_UNSPECIFIED(from))
 455                 dest.sin6_addr = *from;
 456
 457         odhcpd_send(router_event.uloop.fd,
 458                         &dest, iov, ARRAY_SIZE(iov), iface);
 459
 460         return msecs;
 461 }
 462
 463
 464 static void trigger_router_advert(struct uloop_timeout *event)
 465 {
 466         struct interface *iface = container_of(event, struct interface, timer_rs);
 467         int msecs = send_router_advert(iface, NULL);
 468
 469         // Rearm timer if not shut down
 470         if (event->cb)
 471                 uloop_timeout_set(event, msecs);
 472 }
 473
 474
 475 // Event handler for incoming ICMPv6 packets
 476 static void handle_icmpv6(void *addr, void *data, size_t len,
 477                 struct interface *iface, _unused void *dest)
 478 {
 479         struct icmp6_hdr *hdr = data;
 480         struct sockaddr_in6 *from = addr;
 481
 482         if (!router_icmpv6_valid(addr, data, len))
 483                 return;
 484
 485         if ((iface->ra == RELAYD_SERVER && !iface->master)) { // Server mode
 486                 if (hdr->icmp6_type == ND_ROUTER_SOLICIT)
 487                         send_router_advert(iface, &from->sin6_addr);
 488         } else if (iface->ra == RELAYD_RELAY) { // Relay mode
 489                 if (hdr->icmp6_type == ND_ROUTER_ADVERT && iface->master)
 490                         forward_router_advertisement(data, len);
 491                 else if (hdr->icmp6_type == ND_ROUTER_SOLICIT && !iface->master)
 492                         forward_router_solicitation(odhcpd_get_master_interface());
 493         }
 494 }
 495
 496
 497 // Forward router solicitation
 498 static void forward_router_solicitation(const struct interface *iface)
 499 {
 500         if (!iface)
 501                 return;
 502
 503         struct icmp6_hdr rs = {ND_ROUTER_SOLICIT, 0, 0, {{0}}};
 504         struct iovec iov = {&rs, sizeof(rs)};
 505         struct sockaddr_in6 all_routers =
 506                 {AF_INET6, 0, 0, ALL_IPV6_ROUTERS, iface->ifindex};
 507
 508         syslog(LOG_NOTICE, "Sending RS to %s", iface->ifname);
 509         odhcpd_send(router_event.uloop.fd, &all_routers, &iov, 1, iface);
 510 }
 511
 512
 513 // Handler for incoming router solicitations on slave interfaces
 514 static void forward_router_advertisement(uint8_t *data, size_t len)
 515 {
 516         struct nd_router_advert *adv = (struct nd_router_advert *)data;
 517
 518         // Rewrite options
 519         uint8_t *end = data + len;
 520         uint8_t *mac_ptr = NULL;
 521         struct in6_addr *dns_ptr = NULL;
 522         size_t dns_count = 0;
 523
 524         struct icmpv6_opt *opt;
 525         icmpv6_for_each_option(opt, &adv[1], end) {
 526                 if (opt->type == ND_OPT_SOURCE_LINKADDR) {
 527                         // Store address of source MAC-address
 528                         mac_ptr = opt->data;
 529                 } else if (opt->type == ND_OPT_RECURSIVE_DNS && opt->len > 1) {
 530                         // Check if we have to rewrite DNS
 531                         dns_ptr = (struct in6_addr*)&opt->data[6];
 532                         dns_count = (opt->len - 1) / 2;
 533                 }
 534         }
 535
 536         syslog(LOG_NOTICE, "Got a RA");
 537
 538         // Indicate a proxy, however we don't follow the rest of RFC 4389 yet
 539         adv->nd_ra_flags_reserved |= ND_RA_FLAG_PROXY;
 540
 541         // Forward advertisement to all slave interfaces
 542         struct sockaddr_in6 all_nodes = {AF_INET6, 0, 0, ALL_IPV6_NODES, 0};
 543         struct iovec iov = {data, len};
 544
 545         struct interface *iface;
 546         list_for_each_entry(iface, &interfaces, head) {
 547                 if (iface->ra != RELAYD_RELAY || iface->master)
 548                         continue;
 549
 550                 // Fixup source hardware address option
 551                 if (mac_ptr)
 552                         odhcpd_get_mac(iface, mac_ptr);
 553
 554                 // If we have to rewrite DNS entries
 555                 if (iface->always_rewrite_dns && dns_ptr && dns_count > 0) {
 556                         const struct in6_addr *rewrite = iface->dns;
 557                         struct in6_addr addr;
 558                         size_t rewrite_cnt = iface->dns_cnt;
 559
 560                         if (rewrite_cnt == 0) {
 561                                 if (odhcpd_get_linklocal_interface_address(iface->ifindex, &addr))
 562                                         continue; // Unable to comply
 563
 564                                 rewrite = &addr;
 565                                 rewrite_cnt = 1;
 566                         }
 567
 568                         // Copy over any other addresses
 569                         for (size_t i = 0; i < dns_count; ++i) {
 570                                 size_t j = (i < rewrite_cnt) ? i : rewrite_cnt - 1;
 571                                 dns_ptr[i] = rewrite[j];
 572                         }
 573                 }
 574
 575                 odhcpd_send(router_event.uloop.fd, &all_nodes, &iov, 1, iface);
 576         }
 577 }