From c8b7432cba73a4215538726cd98da23a8de9b426 Mon Sep 17 00:00:00 2001 From: Lennart Poettering Date: Tue, 10 Nov 2020 14:48:13 +0100 Subject: [PATCH] resolved: gracefully handle with packets with too large RR count Apparently, there are plenty routers in place that report an incorrect RR count in the packets: they declare more RRs than are actually included. Let's accept these responses, but let's downgrade them to baseline, i.e. let's suppress OPT in this case: if they don't even get the RR count right, let's operate on the absolute baseline, and not bother with anything fancier such as EDNS. Prompted-by: https://github.com/systemd/systemd/issues/12841#issuecomment-724063973 Fixes: #3980 Most likely fixes: #12841 (cherry picked from commit 18674159ebbf622a9e6e5a45cc36b38f74dae315) (cherry picked from commit fdfffdaf20a18a50c9a6d858359cf4af6d2f4c8b) --- src/resolve/resolved-dns-packet.c | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-) diff --git a/src/resolve/resolved-dns-packet.c b/src/resolve/resolved-dns-packet.c index 90a40244b1..e97d9c6a12 100644 --- a/src/resolve/resolved-dns-packet.c +++ b/src/resolve/resolved-dns-packet.c @@ -2210,6 +2210,18 @@ static int dns_packet_extract_answer(DnsPacket *p, DnsAnswer **ret_answer) { if (r < 0) return r; + if (p->rindex == p->size) { + /* If we reached the end of the packet already, but there are still more RRs + * declared, then that's a corrupt packet. Let's accept the packet anyway, since it's + * apparently a common bug in routers. Let's however suppress OPT support in this + * case, so that we force the rest of the logic into lowest DNS baseline support. Or + * to say this differently: if the DNS server doesn't even get the RR counts right, + * it's highly unlikely it gets EDNS right. */ + log_debug("More resource records declared in packet than included, suppressing OPT."); + bad_opt = true; + break; + } + /* Try to reduce memory usage a bit */ if (previous) dns_resource_key_reduce(&rr->key, &previous->key); @@ -2295,8 +2307,10 @@ static int dns_packet_extract_answer(DnsPacket *p, DnsAnswer **ret_answer) { previous = dns_resource_record_ref(rr); } - if (bad_opt) + if (bad_opt) { p->opt = dns_resource_record_unref(p->opt); + p->opt_start = p->opt_size = SIZE_MAX; + } *ret_answer = TAKE_PTR(answer); -- 2.25.1