plugin/file: canonicalize escape form in owner names (#8109)

The miekg/dns zone parser preserves whichever text form the input
used for an escaped byte. RFC 1035 §5.1 lets the same byte appear
as \DDD (decimal) or \c (literal character), so a zone file
written with has\046dot.campus.edu. is stored under that literal
string. Incoming queries, by contrast, arrive on the wire and are
unpacked by miekg/dns into the canonical form has\.dot.campus.edu.
The two strings don't compare equal in the tree, so the record is
silently unreachable.

Pack-then-unpack the owner name on Insert so the stored key uses
the same canonical form as anything that comes off the wire. Only
runs when the name contains a backslash, so the common case is a
no-op string compare.

Fixes #8012

Signed-off-by: Charlie Tonneslan <cst0520@gmail.com>

plugin/file/zone.go

@@ -81,7 +81,7 @@ func (z *Zone) CopyWithoutApex() *Zone {
 func (z *Zone) Insert(r dns.RR) error {
 	// r.Header().Name = strings.ToLower(r.Header().Name)
 	if r.Header().Rrtype != dns.TypeSRV {
-		r.Header().Name = strings.ToLower(r.Header().Name)
+		r.Header().Name = strings.ToLower(canonicalEscape(r.Header().Name))
 	}
 
 	switch h := r.Header().Rrtype; h {
@@ -212,3 +212,27 @@ func (z *Zone) getSOA() *dns.SOA {
 	defer z.RUnlock()
 	return z.SOA
 }
+
+// canonicalEscape normalizes the escape representation of name. RFC 1035
+// §5.1 lets the same byte be written as \DDD (decimal) or \c (literal
+// character), and miekg/dns's zone parser preserves whichever form
+// appeared in the input. Wire format has no such freedom, so packing
+// then unpacking yields the canonical text representation that incoming
+// queries use. Without this, an owner name written as has\046dot.example.
+// in a zone file is stored under that literal string and never matches a
+// query for has\.dot.example., which is how the wire form unpacks.
+func canonicalEscape(name string) string {
+	if !strings.ContainsRune(name, '\\') {
+		return name
+	}
+	buf := make([]byte, len(name)+1)
+	off, err := dns.PackDomainName(name, buf, 0, nil, false)
+	if err != nil {
+		return name
+	}
+	out, _, err := dns.UnpackDomainName(buf[:off], 0)
+	if err != nil {
+		return name
+	}
+	return out
+}