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59508e309e
This fixes FS#391 for lede-17.01 Signed-off-by: Baptiste Jonglez <git@bitsofnetworks.org>
131 lines
6.1 KiB
Diff
131 lines
6.1 KiB
Diff
From 68f6312d4bae30b78daafcd6f51dc441b8685b1e Mon Sep 17 00:00:00 2001
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From: Baptiste Jonglez <git@bitsofnetworks.org>
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Date: Mon, 6 Feb 2017 21:09:11 +0000
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Subject: [PATCH] Stop treating SERVFAIL as a successful response from upstream
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servers.
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This effectively reverts most of 51967f9807 ("SERVFAIL is an expected
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error return, don't try all servers.") and 4ace25c5d6 ("Treat REFUSED (not
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SERVFAIL) as an unsuccessful upstream response").
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With the current behaviour, as soon as dnsmasq receives a SERVFAIL from an
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upstream server, it stops trying to resolve the query and simply returns
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SERVFAIL to the client. With this commit, dnsmasq will instead try to
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query other upstream servers upon receiving a SERVFAIL response.
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According to RFC 1034 and 1035, the semantic of SERVFAIL is that of a
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temporary error condition. Recursive resolvers are expected to encounter
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network or resources issues from time to time, and will respond with
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SERVFAIL in this case. Similarly, if a validating DNSSEC resolver [RFC
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4033] encounters issues when checking signatures (unknown signing
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algorithm, missing signatures, expired signatures because of a wrong
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system clock, etc), it will respond with SERVFAIL.
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Note that all those behaviours are entirely different from a negative
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response, which would provide a definite indication that the requested
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name does not exist. In our case, if an upstream server responds with
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SERVFAIL, another upstream server may well provide a positive answer for
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the same query.
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Thus, this commit will increase robustness whenever some upstream servers
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encounter temporary issues or are misconfigured.
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Quoting RFC 1034, Section 4.3.1. "Queries and responses":
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If recursive service is requested and available, the recursive response
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to a query will be one of the following:
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- The answer to the query, possibly preface by one or more CNAME
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RRs that specify aliases encountered on the way to an answer.
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- A name error indicating that the name does not exist. This
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may include CNAME RRs that indicate that the original query
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name was an alias for a name which does not exist.
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- A temporary error indication.
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Here is Section 5.2.3. of RFC 1034, "Temporary failures":
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In a less than perfect world, all resolvers will occasionally be unable
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to resolve a particular request. This condition can be caused by a
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resolver which becomes separated from the rest of the network due to a
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link failure or gateway problem, or less often by coincident failure or
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unavailability of all servers for a particular domain.
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And finally, RFC 1035 specifies RRCODE 2 for this usage, which is now more
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widely known as SERVFAIL (RFC 1035, Section 4.1.1. "Header section format"):
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RCODE Response code - this 4 bit field is set as part of
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responses. The values have the following
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interpretation:
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(...)
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2 Server failure - The name server was
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unable to process this query due to a
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problem with the name server.
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For the DNSSEC-related usage of SERVFAIL, here is RFC 4033
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Section 5. "Scope of the DNSSEC Document Set and Last Hop Issues":
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A validating resolver can determine the following 4 states:
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(...)
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Insecure: The validating resolver has a trust anchor, a chain of
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trust, and, at some delegation point, signed proof of the
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non-existence of a DS record. This indicates that subsequent
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branches in the tree are provably insecure. A validating resolver
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may have a local policy to mark parts of the domain space as
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insecure.
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Bogus: The validating resolver has a trust anchor and a secure
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delegation indicating that subsidiary data is signed, but the
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response fails to validate for some reason: missing signatures,
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expired signatures, signatures with unsupported algorithms, data
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missing that the relevant NSEC RR says should be present, and so
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forth.
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(...)
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This specification only defines how security-aware name servers can
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signal non-validating stub resolvers that data was found to be bogus
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(using RCODE=2, "Server Failure"; see [RFC4035]).
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Notice the difference between a definite negative answer ("Insecure"
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state), and an indefinite error condition ("Bogus" state). The second
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type of error may be specific to a recursive resolver, for instance
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because its system clock has been incorrectly set, or because it does not
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implement newer cryptographic primitives. Another recursive resolver may
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succeed for the same query.
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There are other similar situations in which the specified behaviour is
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similar to the one implemented by this commit.
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For instance, RFC 2136 specifies the behaviour of a "requestor" that wants
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to update a zone using the DNS UPDATE mechanism. The requestor tries to
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contact all authoritative name servers for the zone, with the following
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behaviour specified in RFC 2136, Section 4:
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4.6. If a response is received whose RCODE is SERVFAIL or NOTIMP, or
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if no response is received within an implementation dependent timeout
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period, or if an ICMP error is received indicating that the server's
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port is unreachable, then the requestor will delete the unusable
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server from its internal name server list and try the next one,
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repeating until the name server list is empty. If the requestor runs
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out of servers to try, an appropriate error will be returned to the
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requestor's caller.
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---
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src/forward.c | 3 ++-
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1 file changed, 2 insertions(+), 1 deletion(-)
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--- a/src/forward.c
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+++ b/src/forward.c
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@@ -853,7 +853,8 @@ void reply_query(int fd, int family, tim
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we get a good reply from another server. Kill it when we've
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had replies from all to avoid filling the forwarding table when
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everything is broken */
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- if (forward->forwardall == 0 || --forward->forwardall == 1 || RCODE(header) != REFUSED)
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+ if (forward->forwardall == 0 || --forward->forwardall == 1 ||
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+ (RCODE(header) != REFUSED && RCODE(header) != SERVFAIL))
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{
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int check_rebind = 0, no_cache_dnssec = 0, cache_secure = 0, bogusanswer = 0;
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