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