Reach the server by address when its name will not resolve

A measurement tool that cannot report from a broken network is useless
exactly when it matters, and a wedged resolver is one of the faults this
app is built to find — it should not also be the thing that stops the
finding being delivered. The profile already carries the server's
addresses; they are now kept and used when the name fails.

Safe because the pin is the trust and the name is not part of it: the
server presents the same certificate however it was reached, and a wrong
address fails the pin like anything else. Only the primaries are cached —
the alternate pair exists for NAT behaviour discovery and does not carry
the control plane, so falling back to one would fail for a second,
unrelated reason.

Substituted only when the name genuinely does not resolve, and only after
checking the candidate answers on the port: on a v4-only network a v6
address would otherwise be chosen and fail slowly, which is the wrong
answer delivered late.

The server had to meet it halfway. Sharing 443 by SNI meant a client
arriving by IP sent no server name and got the Let's Encrypt certificate,
failing the pin. A numeric host — or no SNI at all — now selects the
pinned certificate and routes to the control plane. That is sound because
the admin UI is only ever reached by name: browsers always send SNI, and
nobody bookmarks an IP for a site with a CA-issued certificate.

Verified against fmr: by IP on both families the served pin is the
control one and /v1/profile answers 401, while fmr.echo-lot.app still
serves the Let's Encrypt certificate and the admin UI.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

echolot-app/core-protocol/src/main/kotlin/app/echo_lot/protocol/ControlClient.kt

@@ -35,13 +35,57 @@ class ControlClient(
     private val controlUrl: String,
     pins: Set<String>,
     private val appVersion: String = "",
+    /**
+     * Addresses to fall back to when the server's name will not resolve, learned from its profile.
+     *
+     * A measurement tool that cannot report from a broken network is useless exactly when it
+     * matters, and a wedged resolver is one of the faults it is built to find — it should not also
+     * be the thing that stops the finding being delivered.
+     *
+     * Safe because the pin is the trust and the name is not part of it: the server presents the
+     * same certificate whether it was reached by name or by address, and a wrong address fails the
+     * pin like anything else would.
+     */
+    private val fallbackAddrs: List<String> = emptyList(),
 ) {
 
     private val json = Json { ignoreUnknownKeys = true }
     private val socketFactory = Pinning.sslContext(pins).socketFactory
 
+    /**
+     * The base URL to use, substituting a cached address only when the name genuinely fails.
+     *
+     * Resolved once per client and only on failure, so a working network pays nothing and never
+     * silently drifts onto an address that may be stale.
+     */
+    private val base: String by lazy { resolveBase() }
+
+    private fun resolveBase(): String {
+        if (fallbackAddrs.isEmpty()) return controlUrl
+        val uri = runCatching { java.net.URI(controlUrl) }.getOrNull() ?: return controlUrl
+        val host = uri.host ?: return controlUrl
+        if (runCatching { java.net.InetAddress.getByName(host) }.isSuccess) return controlUrl
+
+        val port = if (uri.port > 0) uri.port else 443
+        for (ip in fallbackAddrs) {
+            // Checked rather than assumed: on a v4-only network a v6 address would otherwise be
+            // chosen and fail slowly, which is the wrong answer delivered late.
+            val reachable = runCatching {
+                java.net.Socket().use { sock ->
+                    sock.connect(java.net.InetSocketAddress(ip, port), 4000)
+                    true
+                }
+            }.getOrDefault(false)
+            if (reachable) {
+                val literal = if (ip.contains(':')) "[$ip]" else ip
+                return uri.scheme + "://" + literal + ":" + port
+            }
+        }
+        return controlUrl
+    }
+
     private fun open(path: String, method: String, credential: String?): HttpsURLConnection {
-        val conn = URL(controlUrl.trimEnd('/') + path).openConnection() as HttpsURLConnection
+        val conn = URL(base.trimEnd('/') + path).openConnection() as HttpsURLConnection
         conn.sslSocketFactory = socketFactory
         conn.setHostnameVerifier { _, _ -> true } // pin is the trust, not the name
         conn.requestMethod = method