all: set edns0 cookie for shared message

For cached or singleflight messages, the edns0 cookie is currently
shared among all of them, causing mismatch cookie warning from clients.
The ctrld proxy should re-set client cookies for each request
separately, even though they use the same shared answer.

dns.go

@@ -0,0 +1,30 @@
+package ctrld
+
+import (
+	"github.com/miekg/dns"
+)
+
+// SetCacheReply extracts and stores the necessary data from the message for a cached answer.
+func SetCacheReply(answer, msg *dns.Msg, code int) {
+	answer.SetRcode(msg, code)
+	cCookie := getEdns0Cookie(msg.IsEdns0())
+	sCookie := getEdns0Cookie(answer.IsEdns0())
+	if cCookie != nil && sCookie != nil {
+		// Client cookie is fixed size 8 bytes, Server cookie is variable size 8 -> 32 bytes.
+		// See https://datatracker.ietf.org/doc/html/rfc7873#section-4
+		sCookie.Cookie = cCookie.Cookie[:16] + sCookie.Cookie[16:]
+	}
+}
+
+// getEdns0Cookie returns Edns0 cookie from *dns.OPT if present.
+func getEdns0Cookie(opt *dns.OPT) *dns.EDNS0_COOKIE {
+	if opt == nil {
+		return nil
+	}
+	for _, o := range opt.Option {
+		if e, ok := o.(*dns.EDNS0_COOKIE); ok {
+			return e
+		}
+	}
+	return nil
+}