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Detect client bufsize and truncate/compress the reply accordingly #183

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merged 6 commits into from
Aug 7, 2024
Merged

Detect client bufsize and truncate/compress the reply accordingly #183

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6fcca56
Detect client bufsize and truncate/compress the reply accordingly
mreiger Jul 31, 2024
e789dd6
Declaration instead of assignment to make linter happy
mreiger Jul 31, 2024
68d0b00
Review.
Gerrit91 Aug 6, 2024
3cc7cc2
Explain the reason for the buffer gymnastics in dns proxy
mreiger Aug 6, 2024
ec6cdfc
Truncate before updating the cache so we don't update the cache on in…
mreiger Aug 7, 2024
4fb4d3d
Streamlined debug output a bit
mreiger Aug 7, 2024
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36 changes: 34 additions & 2 deletions pkg/dns/dns_proxy_handler.go
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Original file line number Diff line number Diff line change
Expand Up @@ -58,15 +58,47 @@ func (h *DNSProxyHandler) ServeDNS(w dnsgo.ResponseWriter, request *dnsgo.Msg) {
}()

scopedLog.Info("started processing request")
response, err := h.getDataFromDNS(w.LocalAddr(), request)
serverAddress := w.LocalAddr()
bufsize := getBufSize(serverAddress.Network(), request)
scopedLog.Info("request info", "server", serverAddress, "bufsize", bufsize, "request", request)

response, err := h.getDataFromDNS(serverAddress, request)
if err != nil {
scopedLog.Error(err, "failed to get DNS response")
err = w.WriteMsg(refusedMsg(request))
return
}

go h.updateCache(time.Now(), response)
err = w.WriteMsg(response)

scopedLog.Info("truncating response before sending reply to client", "bufsize", bufsize)
reply := response.Copy()
/*
Why are we truncating the answer?
DNS has a feature where a DNS server can "compress" the names in a message, and will usually do this if the reply will not fit in the maximum payload length for a DNS packet; for more explanation see here: https://datatracker.ietf.org/doc/html/rfc1035#autoid-44
Unfortunately, in dnsgo the compression status of a message is apparently not retained, so if you take a compressed message and write it out again your wind up with a bigger message, without any checks whether the resulting message is too big for the receiver. If this happens, it breaks name resolution.
Therefore we find out the buffer size of the client from the request (with UDP it's 512 bytes by default) and limit the reply to this buffer size before we send it out. The Truncate method will try compression first to fit the message into the buffer size and will truncate the message if necessary.
*/
reply.Truncate(bufsize)

scopedLog.Info("original response and processed reply", "response", response, "reply", reply)
err = w.WriteMsg(reply)
}

func getBufSize(protocol string, request *dnsgo.Msg) int {
if request.Extra != nil {
for _, rr := range request.Extra {
switch r := rr.(type) {
case *dnsgo.OPT:
return int(r.UDPSize())
}
}
}

if protocol == "tcp" {
return dnsgo.MaxMsgSize
}
return dnsgo.MinMsgSize
}

// UpdateDNSServerAddr validates and if successful updates DNS server address
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