plugin/hosts: fix for ipv4-in-ipv6 (#2506)

* fix for ipv4-in-ipv6

* update comment as requested

plugin/hosts/hosts_test.go

@@ -44,6 +44,12 @@ var hostsTestCases = []test.Case{
 			test.A("example.org. 3600	IN	A 10.0.0.1"),
 		},
 	},
+	{
+		Qname: "example.com.", Qtype: dns.TypeA,
+		Answer: []dns.RR{
+			test.A("example.com. 3600	IN	A 10.0.0.2"),
+		},
+	},
 	{
 		Qname: "localhost.", Qtype: dns.TypeAAAA,
 		Answer: []dns.RR{
@@ -56,6 +62,12 @@ var hostsTestCases = []test.Case{
 			test.PTR("1.0.0.10.in-addr.arpa. 3600 PTR example.org."),
 		},
 	},
+	{
+		Qname: "2.0.0.10.in-addr.arpa.", Qtype: dns.TypePTR,
+		Answer: []dns.RR{
+			test.PTR("2.0.0.10.in-addr.arpa. 3600 PTR example.com."),
+		},
+	},
 	{
 		Qname: "1.0.0.127.in-addr.arpa.", Qtype: dns.TypePTR,
 		Answer: []dns.RR{
@@ -76,4 +88,6 @@ var hostsTestCases = []test.Case{
 const hostsExample = `
 127.0.0.1 localhost localhost.domain
 ::1 localhost localhost.domain
-10.0.0.1 example.org`
+10.0.0.1 example.org
+::FFFF:10.0.0.2 example.com
+`

plugin/hosts/hostsfile.go

@@ -185,8 +185,10 @@ func (h *Hostsfile) parse(r io.Reader, override *hostsMap) *hostsMap {
 }
 
 // ipVersion returns what IP version was used textually
+// For why the string is parsed end to start,
+// see IPv4-Compatible IPv6 addresses - RFC 4291 section 2.5.5
 func ipVersion(s string) int {
-	for i := 0; i < len(s); i++ {
+	for i := len(s) - 1; i >= 0; i-- {
 		switch s[i] {
 		case '.':
 			return 4