From de54e62b4bd7856fb897c9a2ee93cc228adb2135 Mon Sep 17 00:00:00 2001 From: Lennart Poettering Date: Fri, 15 Jan 2016 19:23:51 +0100 Subject: resolved: downgrade server feature level more aggressively when we have reason to This adds logic to downgrade the feature level more aggressively when we have reason to. Specifically: - When we get a response packet that lacks an OPT RR for a query that had it. If so, downgrade immediately to UDP mode, i.e. don't generate EDNS0 packets anymore. - When we get a response which we are sure should be signed, but lacks RRSIG RRs, we downgrade to EDNS0 mode, i.e. below DO mode, since DO is apparently not really supported. This should increase compatibility with servers that generate non-sensical responses if they messages with OPT RRs and suchlike, for example the situation described here: https://open.nlnetlabs.nl/pipermail/dnssec-trigger/2014-November/000376.html This also changes the downgrade code to explain in a debug log message why a specific downgrade happened. --- src/resolve/resolved-dns-server.c | 117 +++++++++++++++++++++++++------------- 1 file changed, 79 insertions(+), 38 deletions(-) (limited to 'src/resolve/resolved-dns-server.c') diff --git a/src/resolve/resolved-dns-server.c b/src/resolve/resolved-dns-server.c index 0969e31e8a..6732030d65 100644 --- a/src/resolve/resolved-dns-server.c +++ b/src/resolve/resolved-dns-server.c @@ -232,7 +232,9 @@ static void dns_server_verified(DnsServer *s, DnsServerFeatureLevel level) { return; if (s->verified_feature_level != level) { - log_debug("Verified feature level %s.", dns_server_feature_level_to_string(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; } @@ -294,7 +296,6 @@ void dns_server_packet_lost(DnsServer *s, int protocol, DnsServerFeatureLevel le void dns_server_packet_failed(DnsServer *s, DnsServerFeatureLevel level) { assert(s); - assert(s->manager); /* Invoked whenever we get a FORMERR, SERVFAIL or NOTIMP rcode from a server. */ @@ -306,7 +307,6 @@ void dns_server_packet_failed(DnsServer *s, DnsServerFeatureLevel level) { void dns_server_packet_truncated(DnsServer *s, DnsServerFeatureLevel level) { assert(s); - assert(s->manager); /* Invoked whenever we get a packet with TC bit set. */ @@ -316,13 +316,22 @@ void dns_server_packet_truncated(DnsServer *s, DnsServerFeatureLevel level) { s->packet_truncated = true; } -void dns_server_packet_rrsig_missing(DnsServer *s) { +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); - assert(s->manager); - log_warning("DNS server %s does not augment replies with RRSIG records, DNSSEC not available.", dns_server_string(s)); + if (level < DNS_SERVER_FEATURE_LEVEL_EDNS0) + return; - s->rrsig_missing = true; + s->packet_bad_opt = true; } static bool dns_server_grace_period_expired(DnsServer *s) { @@ -351,6 +360,8 @@ static void dns_server_reset_counters(DnsServer *s) { 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; } @@ -361,11 +372,9 @@ DnsServerFeatureLevel dns_server_possible_feature_level(DnsServer *s) { dns_server_grace_period_expired(s)) { s->possible_feature_level = DNS_SERVER_FEATURE_LEVEL_BEST; - s->rrsig_missing = false; - dns_server_reset_counters(s); - log_info("Grace period over, resuming full feature set (%s) for DNS server %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)); @@ -375,46 +384,75 @@ DnsServerFeatureLevel dns_server_possible_feature_level(DnsServer *s) { 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) + 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->n_failed_udp >= DNS_SERVER_FEATURE_RETRY_ATTEMPTS && - s->possible_feature_level >= DNS_SERVER_FEATURE_LEVEL_UDP) || - (s->packet_failed && - s->possible_feature_level > DNS_SERVER_FEATURE_LEVEL_UDP) || - (s->n_failed_tcp >= DNS_SERVER_FEATURE_RETRY_ATTEMPTS && - s->packet_truncated && - s->possible_feature_level > DNS_SERVER_FEATURE_LEVEL_UDP)) - - /* Downgrade the feature one level, maybe things will work better then. We do this under any of - * three conditions: - * - * 1. 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. - * - * 2. 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. - * - * 3. 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. - */ + } 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", + 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)); } @@ -468,7 +506,10 @@ bool dns_server_dnssec_supported(DnsServer *server) { if (server->possible_feature_level < DNS_SERVER_FEATURE_LEVEL_DO) return false; - if (server->rrsig_missing) + 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 */ -- cgit v1.2.3-54-g00ecf From b64513580ce627578351b76a502455e7bc62cae4 Mon Sep 17 00:00:00 2001 From: Lennart Poettering Date: Fri, 15 Jan 2016 20:29:56 +0100 Subject: resolved: when we receive an reply which is OPT-less or RRSIG-less, downgrade what we verified If we receive a reply that lacks the OPT RR, then this is reason to downgrade what was verified before, as it's apparently no longer true, and the previous OPT RR we saw was only superficially OK. Similar, if we realize that RRSIGs are not augmented, then also downgrade the feature level that was verified, as DNSSEC is after all not supported. This check is in particular necessary, as we might notice the fact that RRSIG is not augmented only very late, when verifying the root domain. Also, when verifying a successful response, actually take in consideration that it might have been reported already that RRSIG or OPT are missing in the response. --- src/resolve/resolved-dns-server.c | 29 ++++++++++++++++++++++------- 1 file changed, 22 insertions(+), 7 deletions(-) (limited to 'src/resolve/resolved-dns-server.c') diff --git a/src/resolve/resolved-dns-server.c b/src/resolve/resolved-dns-server.c index 6732030d65..949c0d66e1 100644 --- a/src/resolve/resolved-dns-server.c +++ b/src/resolve/resolved-dns-server.c @@ -248,13 +248,18 @@ void dns_server_packet_received(DnsServer *s, int protocol, DnsServerFeatureLeve if (s->possible_feature_level == level) s->n_failed_udp = 0; + /* If the RRSIG data is missing, then we can only validate EDNS0 at max */ + if (s->packet_rrsig_missing && level >= DNS_SERVER_FEATURE_LEVEL_DO) + level = DNS_SERVER_FEATURE_LEVEL_DO - 1; + + /* If the OPT RR got lost, then we can only validate UDP at max */ + if (s->packet_bad_opt && level >= DNS_SERVER_FEATURE_LEVEL_EDNS0) + level = DNS_SERVER_FEATURE_LEVEL_EDNS0 - 1; + + /* 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. */ 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); + level = DNS_SERVER_FEATURE_LEVEL_LARGE - 1; } else if (protocol == IPPROTO_TCP) { @@ -262,9 +267,11 @@ void dns_server_packet_received(DnsServer *s, int protocol, DnsServerFeatureLeve 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); + level = DNS_SERVER_FEATURE_LEVEL_TCP; } + dns_server_verified(s, level); + /* 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. */ @@ -322,6 +329,10 @@ void dns_server_packet_rrsig_missing(DnsServer *s, DnsServerFeatureLevel level) if (level < DNS_SERVER_FEATURE_LEVEL_DO) return; + /* If the RRSIG RRs are missing, we have to downgrade what we previously verified */ + if (s->verified_feature_level >= DNS_SERVER_FEATURE_LEVEL_DO) + s->verified_feature_level = DNS_SERVER_FEATURE_LEVEL_DO-1; + s->packet_rrsig_missing = true; } @@ -331,6 +342,10 @@ void dns_server_packet_bad_opt(DnsServer *s, DnsServerFeatureLevel level) { if (level < DNS_SERVER_FEATURE_LEVEL_EDNS0) return; + /* If the OPT RR got lost, we have to downgrade what we previously verified */ + if (s->verified_feature_level >= DNS_SERVER_FEATURE_LEVEL_EDNS0) + s->verified_feature_level = DNS_SERVER_FEATURE_LEVEL_EDNS0-1; + s->packet_bad_opt = true; } -- cgit v1.2.3-54-g00ecf