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net/dns/resolver: permit larger max responses, signal truncation

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This raises the maximum DNS response message size from 512 to 4095. This
should be large enough for almost all situations that do not need TCP.
We still do not recognize EDNS, so we will still forward requests that
claim support for a larger response size than 4095 (that will be solved
later). For now, when a response comes back that is too large to fit in
our receive buffer, we now set the truncation flag in the DNS header,
which is an improvement from before but will prompt attempts to use TCP
which isn't supported yet.

On Windows, WSARecvFrom into a buffer that's too small returns an error
in addition to the data. On other OSes, the extra data is silently
discarded. In this case, we prefer the latter so need to catch the error
on Windows.

Partially addresses #1123

Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>
This commit is contained in:
Adrian Dewhurst
2021-06-07 17:16:07 -04:00
committed by Adrian Dewhurst
parent fc5fba0fbf
commit 8b11937eaf
7 changed files with 199 additions and 9 deletions
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6
net/dns/resolver/tsdns.go
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@ -22,8 +22,10 @@ import (
"tailscale.com/wgengine/monitor"
)
// maxResponseBytes is the maximum size of a response from a Resolver.
const maxResponseBytes = 512
// maxResponseBytes is the maximum size of a response from a Resolver. The
// actual buffer size will be one larger than this so that we can detect
// truncation in a platform-agnostic way.
const maxResponseBytes = 4095
// queueSize is the maximal number of DNS requests that can await polling.
// If EnqueueRequest is called when this many requests are already pending,