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libnetwork: truncate DNS msgs using library method

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(*dns.Msg).Truncate() is more intelligent and standards-compliant about
truncating DNS response messages than our hand-rolled version. Fix a
silly fencepost error the max TCP message size: the limit is
dns.MaxMsgSize (65535), full stop.

Signed-off-by: Cory Snider <csnider@mirantis.com>
This commit is contained in:
Cory Snider 2022-12-15 19:21:11 -05:00
parent 860e83e52f
commit 51cdd7ceac
1 changed files with 16 additions and 38 deletions
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54
libnetwork/resolver.go
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@ -60,15 +60,14 @@ type DNSBackend interface {
}
const (
dnsPort = "53"
ptrIPv4domain = ".in-addr.arpa."
ptrIPv6domain = ".ip6.arpa."
respTTL = 600
maxExtDNS = 3 // max number of external servers to try
extIOTimeout = 4 * time.Second
defaultRespSize = 512
maxConcurrent = 1024
logInterval = 2 * time.Second
dnsPort = "53"
ptrIPv4domain = ".in-addr.arpa."
ptrIPv6domain = ".ip6.arpa."
respTTL = 600
maxExtDNS = 3 // max number of external servers to try
extIOTimeout = 4 * time.Second
maxConcurrent = 1024
logInterval = 2 * time.Second
)
type extDNSEntry struct {
@ -352,23 +351,6 @@ func (r *resolver) handleSRVQuery(query *dns.Msg) (*dns.Msg, error) {
return resp, nil
}
func truncateResp(resp *dns.Msg, maxSize int, isTCP bool) {
if !isTCP {
resp.Truncated = true
}
srv := resp.Question[0].Qtype == dns.TypeSRV
// trim the Answer RRs one by one till the whole message fits
// within the reply size
for resp.Len() > maxSize {
resp.Answer = resp.Answer[:len(resp.Answer)-1]
if srv && len(resp.Extra) > 0 {
resp.Extra = resp.Extra[:len(resp.Extra)-1]
}
}
}
func (r *resolver) ServeDNS(w dns.ResponseWriter, query *dns.Msg) {
var (
resp *dns.Msg
@ -403,23 +385,19 @@ func (r *resolver) ServeDNS(w dns.ResponseWriter, query *dns.Msg) {
}
proto := w.LocalAddr().Network()
maxSize := 0
maxSize := dns.MinMsgSize
if proto == "tcp" {
maxSize = dns.MaxMsgSize - 1
} else if proto == "udp" {
optRR := query.IsEdns0()
if optRR != nil {
maxSize = int(optRR.UDPSize())
}
if maxSize < defaultRespSize {
maxSize = defaultRespSize
maxSize = dns.MaxMsgSize
} else {
if optRR := query.IsEdns0(); optRR != nil {
if udpsize := int(optRR.UDPSize()); udpsize > maxSize {
maxSize = udpsize
}
}
}
if resp != nil {
if resp.Len() > maxSize {
truncateResp(resp, maxSize, proto == "tcp")
}
resp.Truncate(maxSize)
} else if r.proxyDNS {
// If the user sets ndots > 0 explicitly and the query is
// in the root domain don't forward it out. We will return