refactor: DNS

dns/client_truncate.go

@@ -0,0 +1,29 @@
+package dns
+
+import (
+	"github.com/sagernet/sing/common/buf"
+
+	"github.com/miekg/dns"
+)
+
+func TruncateDNSMessage(request *dns.Msg, response *dns.Msg, headroom int) (*buf.Buffer, error) {
+	maxLen := 512
+	if edns0Option := request.IsEdns0(); edns0Option != nil {
+		if udpSize := int(edns0Option.UDPSize()); udpSize > 512 {
+			maxLen = udpSize
+		}
+	}
+	responseLen := response.Len()
+	if responseLen > maxLen {
+		response.Truncate(maxLen)
+	}
+	buffer := buf.NewSize(headroom*2 + 1 + responseLen)
+	buffer.Resize(headroom, 0)
+	rawMessage, err := response.PackBuffer(buffer.FreeBytes())
+	if err != nil {
+		buffer.Release()
+		return nil, err
+	}
+	buffer.Truncate(len(rawMessage))
+	return buffer, nil
+}