feat(application): inital commit

vendor/github.com/miekg/dns/msg_truncate.go

@@ -0,0 +1,117 @@
+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)
+}