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/app/src/main/kotlin/app/echo_lot/app/Account.kt

@@ -118,6 +118,8 @@ class Account(private val settings: Settings) {
         settings.accountName.takeIf { it.isNotBlank() }
     }.getOrNull()
 
-    private fun client() =
-        ControlClient(settings.serverUrl, setOf(settings.serverPin), BuildConfig.APP_SEMVER)
+    private fun client() = ControlClient(
+        settings.serverUrl, setOf(settings.serverPin), BuildConfig.APP_SEMVER,
+        fallbackAddrs = settings.serverAddrList(),
+    )
 }

echolot-app/app/src/main/kotlin/app/echo_lot/app/RunStore.kt

@@ -80,8 +80,16 @@ class RunStore(context: Context, private val settings: Settings) {
 
     private fun client() = ControlClient(
         settings.serverUrl, setOf(settings.serverPin), BuildConfig.APP_SEMVER,
+        fallbackAddrs = settings.serverAddrList(),
     )
 
+    /** Remembers where the server lives, so a later run can reach it without DNS. */
+    private fun rememberAddrs(p: app.echo_lot.protocol.Profile) {
+        val addrs = p.targets.flatMap { listOfNotNull(it.ip4, it.ip6) }
+            .filter { it.isNotBlank() }
+        if (addrs.isNotEmpty()) settings.serverAddrs = addrs.joinToString(",")
+    }
+
     /**
      * Checks the configured server without uploading anything: reachable, pinned, compatible, and
      * willing to accept runs. Lets the user find out in settings rather than from a failed run.
@@ -93,6 +101,7 @@ class RunStore(context: Context, private val settings: Settings) {
             // Learned here so the next run's canary probe knows what to ask for.
             profile.canaryZone.takeIf { it.isNotBlank() }?.let { settings.canaryZone = it }
             settings.serverFacts = describeFacts(profile)
+            rememberAddrs(profile)
             val compat = Compat.check(profile, BuildConfig.APP_SEMVER)
             val head = "${profile.name} · server ${profile.serverVersion} · " +
                 "protocol ${profile.compat.protocolVersion.ifBlank { "unstated" }}"
@@ -156,6 +165,7 @@ class RunStore(context: Context, private val settings: Settings) {
             val enrolled = parsed.redeem(deviceName, BuildConfig.APP_SEMVER)
             val compat = Compat.check(enrolled.profile, BuildConfig.APP_SEMVER)
             settings.serverFacts = describeFacts(enrolled.profile)
+            rememberAddrs(enrolled.profile)
             enrolled.profile.canaryZone.takeIf { it.isNotBlank() }?.let { settings.canaryZone = it }
             settings.serverUrl = enrolled.controlUrl
             settings.serverPublicUrl = enrolled.publicUrl
@@ -188,6 +198,7 @@ class RunStore(context: Context, private val settings: Settings) {
             val profile = client.profile(settings.serverCredential)
             profile.canaryZone.takeIf { it.isNotBlank() }?.let { settings.canaryZone = it }
             settings.serverFacts = describeFacts(profile)
+            rememberAddrs(profile)
 
             // Compatibility before policy: an incompatible server may well advertise an upload
             // policy it would never actually apply to us.

echolot-app/app/src/main/kotlin/app/echo_lot/app/Settings.kt

@@ -119,6 +119,19 @@ class Settings(context: Context) {
         get() = prefs.getString(SERVER_FACTS, "") ?: ""
         set(v) = prefs.edit().putString(SERVER_FACTS, v).apply()
 
+    /**
+     * The server's own addresses, learned from its profile, for reaching it when DNS will not.
+     *
+     * Only the primaries: 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.
+     */
+    var serverAddrs: String
+        get() = prefs.getString(SERVER_ADDRS, "") ?: ""
+        set(v) = prefs.edit().putString(SERVER_ADDRS, v).apply()
+
+    fun serverAddrList(): List<String> =
+        serverAddrs.split(',').map { it.trim() }.filter { it.isNotEmpty() }
+
     var serverCredential: String
         get() = prefs.getString(SERVER_CRED, "") ?: ""
         set(v) = prefs.edit().putString(SERVER_CRED, v.trim()).apply()
@@ -196,6 +209,7 @@ class Settings(context: Context) {
         const val SERVER_CRED = "server_credential"
         const val SERVER_PUBLIC_URL = "server_public_url"
         const val SERVER_FACTS = "server_facts"
+        const val SERVER_ADDRS = "server_addrs"
         const val CANARY_ZONE = "server_canary_zone"
         const val PENDING_VERIFIER = "pending_auth_verifier"
         const val PENDING_STATE = "pending_auth_state"