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plugin/cache: cache SERVFAIL when the resolver fails without a response (#8629)

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* plugin/cache: cache SERVFAIL when the resolver fails without a response

The forward plugin returns (SERVFAIL, err) on an upstream timeout without
calling WriteMsg, so the response never reaches the cache ResponseWriter
and is not stored. Every client retry then hits the upstream again and
pays the full forward timeout, while a SERVFAIL sent by the upstream is
cached normally.

Store a SERVFAIL generated from the failure, bounded by the existing
servfail TTL, so identical queries are answered from the cache while it is
valid. A servfail TTL of 0 keeps the previous behaviour.

Signed-off-by: Ahmad Mehrabi <mehrabi@post.com>

* plugin/cache: do not cache SERVFAIL caused by a canceled request

forward also returns SERVFAIL together with an error when the request
context is canceled. DoH propagates the HTTP request context, so caching
that failure would make every subsequent request with the same cache key
fail until the servfail TTL expires, even though the upstream is healthy.

Only insert the synthetic SERVFAIL when the request context is still live,
and add a regression test that cancels the first request and then verifies
that a subsequent healthy request reaches the backend.

Signed-off-by: Ahmad Mehrabi <mehrabi@post.com>

---------

Signed-off-by: Ahmad Mehrabi <mehrabi@post.com>
This commit is contained in:
Ahmad Mehrabi
2026-10-16 05:59:41 +03:30
committed by GitHub
parent 3909cc187c
commit 16b53a46c1
3 changed files with 141 additions and 1 deletions
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13
plugin/cache/handler.go vendored
View File

@@ -45,7 +45,18 @@ func (c *Cache) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg)
refreshState := authenticatedRefreshState(state)
crr := &ResponseWriter{ResponseWriter: w, Cache: c, state: refreshState, server: server, do: do, ad: ad, cd: cd,
nexcept: c.nexcept, pexcept: c.pexcept, wildcardFunc: wildcardFunc(ctx)}
return c.doRefresh(ctx, refreshState, crr)
ret, err := c.doRefresh(ctx, refreshState, crr)
// The next plugin can fail without writing a response, e.g. forward when
// the upstream times out. In that case the failure never reaches
// ResponseWriter.WriteMsg, so it is not cached and every client retry
// hits the upstream again. Store a SERVFAIL bounded by the servfail TTL
// so identical queries are answered from the cache while it is valid.
// A canceled request context (e.g. a canceled DoH request) is not an
// upstream failure, so it must not poison the shared failure cache.
if err != nil && ret == dns.RcodeServerFailure && ctx.Err() == nil && crr.lastResponse == nil && c.failttl > 0 {
crr.cacheServerFailure(refreshState.Req)
}
return ret, err
}
ttl := i.ttl(now)
stale := ttl <= 0