ee8fef6743
* Update Truncate doc with compress behaviour This is a documentation update to highlight the behaviour of Truncate, which will reset dns.Compress to false when the message fits in the requested size without truncation, and make it the caller responsibility to set it back to true if they wish to compress, regardless of fitting, uncompressed, in the requested message size in the first place or not. Fixes #1216 * address comments * d/Note that/ * s/reset/set/ * s/caller/caller's/ * removed backticks * regardless of size
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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