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From 122392e0b352507cabb9e982208d35d2e56902e0 Mon Sep 17 00:00:00 2001
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From: Simon Kelley <simon@thekelleys.org.uk>
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Date: Wed, 31 Oct 2018 22:24:02 +0000
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Subject: [PATCH 09/32] Revert 68f6312d4bae30b78daafcd6f51dc441b8685b1e
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The above is intended to increase robustness, but actually does the
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opposite. The problem is that by ignoring SERVFAIL messages and hoping
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for a better answer from another of the servers we've forwarded to,
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we become vulnerable in the case that one or more of the configured
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servers is down or not responding.
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Consider the case that a domain is indeed BOGUS, and we've send the
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query to n servers. With 68f6312d4bae30b78daafcd6f51dc441b8685b1e
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we ignore the first n-1 SERVFAIL replies, and only return the
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final n'th answer to the client. Now, if one of the servers we are
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forwarding to is down, then we won't get all n replies, and the
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client will never get an answer! This is a far more likely scenario
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than a temporary SERVFAIL from only one of a set of notionally identical
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servers, so, on the ground of robustness, we have to believe
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any SERVFAIL answers we get, and return them to the client.
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The client could be using the same recursive servers we are,
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so it should, in theory, retry on SERVFAIL anyway.
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Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
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---
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src/forward.c | 3 +--
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1 file changed, 1 insertion(+), 2 deletions(-)
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--- a/src/forward.c
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+++ b/src/forward.c
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@@ -957,8 +957,7 @@ 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 ||
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- (RCODE(header) != REFUSED && RCODE(header) != SERVFAIL))
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+ if (forward->forwardall == 0 || --forward->forwardall == 1 || RCODE(header) != REFUSED)
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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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