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package dns
// Truncate ensures the reply message will fit into the requested buffer
// size by removing records that exceed the requested size.
//
// It will first check if the reply fits without compression and then with
// compression. If it won't fit with compression, Truncate then walks the
// record adding as many records as possible without exceeding the
// requested buffer size.
//
// If the message fits within the requested size without compression,
// Truncate will set the message's Compress attribute to false. It is
// the caller's responsibility to set it back to true if they wish to
// compress the payload regardless of size.
//
// The TC bit will be set if any records were excluded from the message.
// If the TC bit is already set on the message it will be retained.
// TC indicates that the client should retry over TCP.
//
// According to RFC 2181, the TC bit should only be set if not all of the
// "required" RRs can be included in the response. Unfortunately, we have
// no way of knowing which RRs are required so we set the TC bit if any RR
// had to be omitted from the response.
//
// The appropriate buffer size can be retrieved from the requests OPT
// record, if present, and is transport specific otherwise. dns.MinMsgSize
// should be used for UDP requests without an OPT record, and
// dns.MaxMsgSize for TCP requests without an OPT record.
func (dns *Msg) Truncate(size int) {
if dns.IsTsig() != nil {
// To simplify this implementation, we don't perform
// truncation on responses with a TSIG record.
return
}
// RFC 6891 mandates that the payload size in an OPT record
// less than 512 (MinMsgSize) bytes must be treated as equal to 512 bytes.
//
// For ease of use, we impose that restriction here.
if size < MinMsgSize {
size = MinMsgSize
}
l := msgLenWithCompressionMap(dns, nil) // uncompressed length
if l <= size {
// Don't waste effort compressing this message.
dns.Compress = false
return
}
dns.Compress = true
edns0 := dns.popEdns0()
if edns0 != nil {
// Account for the OPT record that gets added at the end,
// by subtracting that length from our budget.
//
// The EDNS(0) OPT record must have the root domain and
// it's length is thus unaffected by compression.
size -= Len(edns0)
}
compression := make(map[string]struct{})
l = headerSize
for _, r := range dns.Question {
l += r.len(l, compression)
}
var numAnswer int
if l < size {
l, numAnswer = truncateLoop(dns.Answer, size, l, compression)
}
var numNS int
if l < size {
l, numNS = truncateLoop(dns.Ns, size, l, compression)
}
var numExtra int
if l < size {
_, numExtra = truncateLoop(dns.Extra, size, l, compression)
}
// See the function documentation for when we set this.
dns.Truncated = dns.Truncated || len(dns.Answer) > numAnswer ||
len(dns.Ns) > numNS || len(dns.Extra) > numExtra
dns.Answer = dns.Answer[:numAnswer]
dns.Ns = dns.Ns[:numNS]
dns.Extra = dns.Extra[:numExtra]
if edns0 != nil {
// Add the OPT record back onto the additional section.
dns.Extra = append(dns.Extra, edns0)
}
}
func truncateLoop(rrs []RR, size, l int, compression map[string]struct{}) (int, int) {
for i, r := range rrs {
if r == nil {
continue
}
l += r.len(l, compression)
if l > size {
// Return size, rather than l prior to this record,
// to prevent any further records being added.
return size, i
}
if l == size {
return l, i + 1
}
}
return l, len(rrs)
}