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.
|
//
|
// The TC bit will be set if any records were excluded from the message.
|
// This indicates to 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 bytes must be treated as equal to 512 bytes.
|
//
|
// For ease of use, we impose that restriction here.
|
if size < 512 {
|
size = 512
|
}
|
|
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 {
|
l, numExtra = truncateLoop(dns.Extra, size, l, compression)
|
}
|
|
// See the function documentation for when we set this.
|
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)
|
}
|
