/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/ /*** This file is part of systemd. Copyright 2014 Lennart Poettering systemd is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version. systemd is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details. You should have received a copy of the GNU Lesser General Public License along with systemd; If not, see . ***/ #include "alloc-util.h" #include "resolved-dns-server.h" #include "resolved-resolv-conf.h" #include "siphash24.h" #include "string-table.h" #include "string-util.h" /* After how much time to repeat classic DNS requests */ #define DNS_TIMEOUT_MIN_USEC (500 * USEC_PER_MSEC) #define DNS_TIMEOUT_MAX_USEC (5 * USEC_PER_SEC) /* The amount of time to wait before retrying with a full feature set */ #define DNS_SERVER_FEATURE_GRACE_PERIOD_MAX_USEC (6 * USEC_PER_HOUR) #define DNS_SERVER_FEATURE_GRACE_PERIOD_MIN_USEC (5 * USEC_PER_MINUTE) /* The number of times we will attempt a certain feature set before degrading */ #define DNS_SERVER_FEATURE_RETRY_ATTEMPTS 3 int dns_server_new( Manager *m, DnsServer **ret, DnsServerType type, Link *l, int family, const union in_addr_union *in_addr) { DnsServer *s; assert(m); assert((type == DNS_SERVER_LINK) == !!l); assert(in_addr); if (!IN_SET(family, AF_INET, AF_INET6)) return -EAFNOSUPPORT; if (l) { if (l->n_dns_servers >= LINK_DNS_SERVERS_MAX) return -E2BIG; } else { if (m->n_dns_servers >= MANAGER_DNS_SERVERS_MAX) return -E2BIG; } s = new0(DnsServer, 1); if (!s) return -ENOMEM; s->n_ref = 1; s->manager = m; s->verified_feature_level = _DNS_SERVER_FEATURE_LEVEL_INVALID; s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_BEST; s->features_grace_period_usec = DNS_SERVER_FEATURE_GRACE_PERIOD_MIN_USEC; s->received_udp_packet_max = DNS_PACKET_UNICAST_SIZE_MAX; s->type = type; s->family = family; s->address = *in_addr; s->resend_timeout = DNS_TIMEOUT_MIN_USEC; switch (type) { case DNS_SERVER_LINK: s->link = l; LIST_APPEND(servers, l->dns_servers, s); l->n_dns_servers++; break; case DNS_SERVER_SYSTEM: LIST_APPEND(servers, m->dns_servers, s); m->n_dns_servers++; break; case DNS_SERVER_FALLBACK: LIST_APPEND(servers, m->fallback_dns_servers, s); m->n_dns_servers++; break; default: assert_not_reached("Unknown server type"); } s->linked = true; /* A new DNS server that isn't fallback is added and the one * we used so far was a fallback one? Then let's try to pick * the new one */ if (type != DNS_SERVER_FALLBACK && m->current_dns_server && m->current_dns_server->type == DNS_SERVER_FALLBACK) manager_set_dns_server(m, NULL); if (ret) *ret = s; return 0; } DnsServer* dns_server_ref(DnsServer *s) { if (!s) return NULL; assert(s->n_ref > 0); s->n_ref ++; return s; } DnsServer* dns_server_unref(DnsServer *s) { if (!s) return NULL; assert(s->n_ref > 0); s->n_ref --; if (s->n_ref > 0) return NULL; free(s->server_string); free(s); return NULL; } void dns_server_unlink(DnsServer *s) { assert(s); assert(s->manager); /* This removes the specified server from the linked list of * servers, but any server might still stay around if it has * refs, for example from an ongoing transaction. */ if (!s->linked) return; switch (s->type) { case DNS_SERVER_LINK: assert(s->link); assert(s->link->n_dns_servers > 0); LIST_REMOVE(servers, s->link->dns_servers, s); break; case DNS_SERVER_SYSTEM: assert(s->manager->n_dns_servers > 0); LIST_REMOVE(servers, s->manager->dns_servers, s); s->manager->n_dns_servers--; break; case DNS_SERVER_FALLBACK: assert(s->manager->n_dns_servers > 0); LIST_REMOVE(servers, s->manager->fallback_dns_servers, s); s->manager->n_dns_servers--; break; } s->linked = false; if (s->link && s->link->current_dns_server == s) link_set_dns_server(s->link, NULL); if (s->manager->current_dns_server == s) manager_set_dns_server(s->manager, NULL); dns_server_unref(s); } void dns_server_move_back_and_unmark(DnsServer *s) { DnsServer *tail; assert(s); if (!s->marked) return; s->marked = false; if (!s->linked || !s->servers_next) return; /* Move us to the end of the list, so that the order is * strictly kept, if we are not at the end anyway. */ switch (s->type) { case DNS_SERVER_LINK: assert(s->link); LIST_FIND_TAIL(servers, s, tail); LIST_REMOVE(servers, s->link->dns_servers, s); LIST_INSERT_AFTER(servers, s->link->dns_servers, tail, s); break; case DNS_SERVER_SYSTEM: LIST_FIND_TAIL(servers, s, tail); LIST_REMOVE(servers, s->manager->dns_servers, s); LIST_INSERT_AFTER(servers, s->manager->dns_servers, tail, s); break; case DNS_SERVER_FALLBACK: LIST_FIND_TAIL(servers, s, tail); LIST_REMOVE(servers, s->manager->fallback_dns_servers, s); LIST_INSERT_AFTER(servers, s->manager->fallback_dns_servers, tail, s); break; default: assert_not_reached("Unknown server type"); } } static void dns_server_verified(DnsServer *s, DnsServerFeatureLevel level) { assert(s); if (s->verified_feature_level > level) return; if (s->verified_feature_level != level) { log_debug("Verified we get a response at feature level %s from DNS server %s.", dns_server_feature_level_to_string(level), dns_server_string(s)); s->verified_feature_level = level; } assert_se(sd_event_now(s->manager->event, clock_boottime_or_monotonic(), &s->verified_usec) >= 0); } void dns_server_packet_received(DnsServer *s, int protocol, DnsServerFeatureLevel level, usec_t rtt, size_t size) { assert(s); if (protocol == IPPROTO_UDP) { if (s->possible_feature_level == level) s->n_failed_udp = 0; if (level == DNS_SERVER_FEATURE_LEVEL_LARGE) /* Even if we successfully receive a reply to a request announcing support for large packets, that does not mean we can necessarily receive large packets. */ dns_server_verified(s, DNS_SERVER_FEATURE_LEVEL_LARGE - 1); else /* A successful UDP reply, verifies UDP, ENDS0 and DO levels */ dns_server_verified(s, level); } else if (protocol == IPPROTO_TCP) { if (s->possible_feature_level == level) s->n_failed_tcp = 0; /* Successful TCP connections are only useful to verify the TCP feature level. */ dns_server_verified(s, DNS_SERVER_FEATURE_LEVEL_TCP); } /* Remember the size of the largest UDP packet we received from a server, we know that we can always announce support for packets with at least this size. */ if (protocol == IPPROTO_UDP && s->received_udp_packet_max < size) s->received_udp_packet_max = size; if (s->max_rtt < rtt) { s->max_rtt = rtt; s->resend_timeout = CLAMP(s->max_rtt * 2, DNS_TIMEOUT_MIN_USEC, DNS_TIMEOUT_MAX_USEC); } } void dns_server_packet_lost(DnsServer *s, int protocol, DnsServerFeatureLevel level, usec_t usec) { assert(s); assert(s->manager); if (s->possible_feature_level == level) { if (protocol == IPPROTO_UDP) s->n_failed_udp ++; else if (protocol == IPPROTO_TCP) s->n_failed_tcp ++; } if (s->resend_timeout > usec) return; s->resend_timeout = MIN(s->resend_timeout * 2, DNS_TIMEOUT_MAX_USEC); } void dns_server_packet_failed(DnsServer *s, DnsServerFeatureLevel level) { assert(s); /* Invoked whenever we get a FORMERR, SERVFAIL or NOTIMP rcode from a server. */ if (s->possible_feature_level != level) return; s->packet_failed = true; } void dns_server_packet_truncated(DnsServer *s, DnsServerFeatureLevel level) { assert(s); /* Invoked whenever we get a packet with TC bit set. */ if (s->possible_feature_level != level) return; s->packet_truncated = true; } void dns_server_packet_rrsig_missing(DnsServer *s, DnsServerFeatureLevel level) { assert(s); if (level < DNS_SERVER_FEATURE_LEVEL_DO) return; s->packet_rrsig_missing = true; } void dns_server_packet_bad_opt(DnsServer *s, DnsServerFeatureLevel level) { assert(s); if (level < DNS_SERVER_FEATURE_LEVEL_EDNS0) return; s->packet_bad_opt = true; } static bool dns_server_grace_period_expired(DnsServer *s) { usec_t ts; assert(s); assert(s->manager); if (s->verified_usec == 0) return false; assert_se(sd_event_now(s->manager->event, clock_boottime_or_monotonic(), &ts) >= 0); if (s->verified_usec + s->features_grace_period_usec > ts) return false; s->features_grace_period_usec = MIN(s->features_grace_period_usec * 2, DNS_SERVER_FEATURE_GRACE_PERIOD_MAX_USEC); return true; } static void dns_server_reset_counters(DnsServer *s) { assert(s); s->n_failed_udp = 0; s->n_failed_tcp = 0; s->packet_failed = false; s->packet_truncated = false; s->packet_bad_opt = false; s->packet_rrsig_missing = false; s->verified_usec = 0; } DnsServerFeatureLevel dns_server_possible_feature_level(DnsServer *s) { assert(s); if (s->possible_feature_level != DNS_SERVER_FEATURE_LEVEL_BEST && dns_server_grace_period_expired(s)) { s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_BEST; dns_server_reset_counters(s); log_info("Grace period over, resuming full feature set (%s) for DNS server %s.", dns_server_feature_level_to_string(s->possible_feature_level), dns_server_string(s)); } else if (s->possible_feature_level <= s->verified_feature_level) s->possible_feature_level = s->verified_feature_level; else { DnsServerFeatureLevel p = s->possible_feature_level; if (s->n_failed_tcp >= DNS_SERVER_FEATURE_RETRY_ATTEMPTS && s->possible_feature_level == DNS_SERVER_FEATURE_LEVEL_TCP) { /* We are at the TCP (lowest) level, and we tried a couple of TCP connections, and it didn't * work. Upgrade back to UDP again. */ log_debug("Reached maximum number of failed TCP connection attempts, trying UDP again..."); s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_UDP; } else if (s->packet_bad_opt && s->possible_feature_level >= DNS_SERVER_FEATURE_LEVEL_EDNS0) { /* A reply to one of our EDNS0 queries didn't carry a valid OPT RR, then downgrade to below * EDNS0 levels. After all, some records generate different responses with and without OPT RR * in the request. Example: * https://open.nlnetlabs.nl/pipermail/dnssec-trigger/2014-November/000376.html */ log_debug("Server doesn't support EDNS(0) properly, downgrading feature level..."); s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_UDP; } else if (s->packet_rrsig_missing && s->possible_feature_level >= DNS_SERVER_FEATURE_LEVEL_DO) { /* RRSIG data was missing on a EDNS0 packet with DO bit set. This means the server doesn't * augment responses with DNSSEC RRs. If so, let's better not ask the server for it anymore, * after all some servers generate different replies depending if an OPT RR is in the query or * not. */ log_debug("Detected server responses lack RRSIG records, downgrading feature level..."); s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_EDNS0; } else if (s->n_failed_udp >= DNS_SERVER_FEATURE_RETRY_ATTEMPTS && s->possible_feature_level >= DNS_SERVER_FEATURE_LEVEL_UDP) { /* We lost too many UDP packets in a row, and are on a feature level of UDP or higher. If the * packets are lost, maybe the server cannot parse them, hence downgrading sounds like a good * idea. We might downgrade all the way down to TCP this way. */ log_debug("Lost too many UDP packets, downgrading feature level..."); s->possible_feature_level--; } else if (s->packet_failed && s->possible_feature_level > DNS_SERVER_FEATURE_LEVEL_UDP) { /* We got a failure packet, and are at a feature level above UDP. Note that in this case we * downgrade no further than UDP, under the assumption that a failure packet indicates an * incompatible packet contents, but not a problem with the transport. */ log_debug("Got server failure, downgrading feature level..."); s->possible_feature_level--; } else if (s->n_failed_tcp >= DNS_SERVER_FEATURE_RETRY_ATTEMPTS && s->packet_truncated && s->possible_feature_level > DNS_SERVER_FEATURE_LEVEL_UDP) { /* We got too many TCP connection failures in a row, we had at least one truncated packet, and * are on a feature level above UDP. By downgrading things and getting rid of DNSSEC or EDNS0 * data we hope to make the packet smaller, so that it still works via UDP given that TCP * appears not to be a fallback. Note that if we are already at the lowest UDP level, we don't * go further down, since that's TCP, and TCP failed too often after all. */ log_debug("Got too many failed TCP connection failures and truncated UDP packets, downgrading feature level..."); s->possible_feature_level--; } if (p != s->possible_feature_level) { /* We changed the feature level, reset the counting */ dns_server_reset_counters(s); log_warning("Using degraded feature set (%s) for DNS server %s.", dns_server_feature_level_to_string(s->possible_feature_level), dns_server_string(s)); } } return s->possible_feature_level; } int dns_server_adjust_opt(DnsServer *server, DnsPacket *packet, DnsServerFeatureLevel level) { size_t packet_size; bool edns_do; int r; assert(server); assert(packet); assert(packet->protocol == DNS_PROTOCOL_DNS); /* Fix the OPT field in the packet to match our current feature level. */ r = dns_packet_truncate_opt(packet); if (r < 0) return r; if (level < DNS_SERVER_FEATURE_LEVEL_EDNS0) return 0; edns_do = level >= DNS_SERVER_FEATURE_LEVEL_DO; if (level >= DNS_SERVER_FEATURE_LEVEL_LARGE) packet_size = DNS_PACKET_UNICAST_SIZE_LARGE_MAX; else packet_size = server->received_udp_packet_max; return dns_packet_append_opt(packet, packet_size, edns_do, NULL); } const char *dns_server_string(DnsServer *server) { assert(server); if (!server->server_string) (void) in_addr_to_string(server->family, &server->address, &server->server_string); return strna(server->server_string); } bool dns_server_dnssec_supported(DnsServer *server) { assert(server); /* Returns whether the server supports DNSSEC according to what we know about it */ if (server->possible_feature_level < DNS_SERVER_FEATURE_LEVEL_DO) return false; if (server->packet_bad_opt) return false; if (server->packet_rrsig_missing) return false; /* DNSSEC servers need to support TCP properly (see RFC5966), if they don't, we assume DNSSEC is borked too */ if (server->n_failed_tcp >= DNS_SERVER_FEATURE_RETRY_ATTEMPTS) return false; return true; } static void dns_server_hash_func(const void *p, struct siphash *state) { const DnsServer *s = p; assert(s); siphash24_compress(&s->family, sizeof(s->family), state); siphash24_compress(&s->address, FAMILY_ADDRESS_SIZE(s->family), state); } static int dns_server_compare_func(const void *a, const void *b) { const DnsServer *x = a, *y = b; if (x->family < y->family) return -1; if (x->family > y->family) return 1; return memcmp(&x->address, &y->address, FAMILY_ADDRESS_SIZE(x->family)); } const struct hash_ops dns_server_hash_ops = { .hash = dns_server_hash_func, .compare = dns_server_compare_func }; void dns_server_unlink_all(DnsServer *first) { DnsServer *next; if (!first) return; next = first->servers_next; dns_server_unlink(first); dns_server_unlink_all(next); } void dns_server_unlink_marked(DnsServer *first) { DnsServer *next; if (!first) return; next = first->servers_next; if (first->marked) dns_server_unlink(first); dns_server_unlink_marked(next); } void dns_server_mark_all(DnsServer *first) { if (!first) return; first->marked = true; dns_server_mark_all(first->servers_next); } DnsServer *dns_server_find(DnsServer *first, int family, const union in_addr_union *in_addr) { DnsServer *s; LIST_FOREACH(servers, s, first) if (s->family == family && in_addr_equal(family, &s->address, in_addr) > 0) return s; return NULL; } DnsServer *manager_get_first_dns_server(Manager *m, DnsServerType t) { assert(m); switch (t) { case DNS_SERVER_SYSTEM: return m->dns_servers; case DNS_SERVER_FALLBACK: return m->fallback_dns_servers; default: return NULL; } } DnsServer *manager_set_dns_server(Manager *m, DnsServer *s) { assert(m); if (m->current_dns_server == s) return s; if (s) log_info("Switching to system DNS server %s.", dns_server_string(s)); dns_server_unref(m->current_dns_server); m->current_dns_server = dns_server_ref(s); if (m->unicast_scope) dns_cache_flush(&m->unicast_scope->cache); return s; } DnsServer *manager_get_dns_server(Manager *m) { Link *l; assert(m); /* Try to read updates resolv.conf */ manager_read_resolv_conf(m); /* If no DNS server was chose so far, pick the first one */ if (!m->current_dns_server) manager_set_dns_server(m, m->dns_servers); if (!m->current_dns_server) { bool found = false; Iterator i; /* No DNS servers configured, let's see if there are * any on any links. If not, we use the fallback * servers */ HASHMAP_FOREACH(l, m->links, i) if (l->dns_servers) { found = true; break; } if (!found) manager_set_dns_server(m, m->fallback_dns_servers); } return m->current_dns_server; } void manager_next_dns_server(Manager *m) { assert(m); /* If there's currently no DNS server set, then the next * manager_get_dns_server() will find one */ if (!m->current_dns_server) return; /* Change to the next one, but make sure to follow the linked * list only if the server is still linked. */ if (m->current_dns_server->linked && m->current_dns_server->servers_next) { manager_set_dns_server(m, m->current_dns_server->servers_next); return; } /* If there was no next one, then start from the beginning of * the list */ if (m->current_dns_server->type == DNS_SERVER_FALLBACK) manager_set_dns_server(m, m->fallback_dns_servers); else manager_set_dns_server(m, m->dns_servers); } static const char* const dns_server_feature_level_table[_DNS_SERVER_FEATURE_LEVEL_MAX] = { [DNS_SERVER_FEATURE_LEVEL_TCP] = "TCP", [DNS_SERVER_FEATURE_LEVEL_UDP] = "UDP", [DNS_SERVER_FEATURE_LEVEL_EDNS0] = "UDP+EDNS0", [DNS_SERVER_FEATURE_LEVEL_DO] = "UDP+EDNS0+DO", [DNS_SERVER_FEATURE_LEVEL_LARGE] = "UDP+EDNS0+DO+LARGE", }; DEFINE_STRING_TABLE_LOOKUP(dns_server_feature_level, DnsServerFeatureLevel);