fix(dns): treat trailing-dot FQDN form of localhost as loopback

RFC 1034: `localhost.` and `localhost` are the same name — the trailing
dot just marks the FQDN form. Without this, an upstream answer of
127.0.0.1 for `localhost.` (or `relay.localhost.`) would get filtered as
poison, breaking user-configured local relays addressed in FQDN form.

amethyst/src/main/java/com/vitorpamplona/amethyst/service/okhttp/SurgeDns.kt

@@ -296,8 +296,10 @@ class SurgeDns(
      * already lowercased.
      */
     private fun isLoopbackHostname(host: String): Boolean {
-        if (host == "localhost" || host.endsWith(".localhost")) return true
-        val literal = parseIpLiteral(host) ?: return false
+        // RFC 1034: a trailing dot is the FQDN form (e.g. `localhost.`), still the same name.
+        val name = host.trimEnd('.')
+        if (name == "localhost" || name.endsWith(".localhost")) return true
+        val literal = parseIpLiteral(name) ?: return false
         return literal.isLoopbackAddress || literal.isAnyLocalAddress
     }
 

amethyst/src/test/java/com/vitorpamplona/amethyst/service/okhttp/SurgeDnsTest.kt

@@ -540,6 +540,15 @@ class SurgeDnsTest {
         assertEquals(listOf(ip("127.0.0.1")), dns.lookup("relay.localhost"))
     }
 
+    @Test
+    fun `trailing-dot FQDN form of localhost keeps loopback answers`() {
+        // RFC 1034: `localhost.` is the same name as `localhost`, just in FQDN form.
+        val upstream = CountingDns(mapOf("localhost." to listOf(ip("127.0.0.1"))))
+        val dns = SurgeDns(delegate = upstream)
+
+        assertEquals(listOf(ip("127.0.0.1")), dns.lookup("localhost."))
+    }
+
     @Test
     fun `ipv4 loopback literal keeps loopback answers`() {
         val upstream = CountingDns(mapOf("127.0.0.1" to listOf(ip("127.0.0.1"))))