From 4908bcad47fbe6992b16ae21c96d279f466d3cb4 Mon Sep 17 00:00:00 2001
From: cketti <cketti@thunderbird.net>
Date: Fri, 28 Oct 2022 12:24:22 +0200
Subject: [PATCH] Try all IP addresses when connecting to a POP3 server

---
 .../k9/mail/store/pop3/Pop3Connection.java    | 51 +++++++++++++++----
 .../fsck/k9/mail/store/pop3/Pop3StoreTest.kt  |  7 +--
 2 files changed, 45 insertions(+), 13 deletions(-)

diff --git a/mail/protocols/pop3/src/main/java/com/fsck/k9/mail/store/pop3/Pop3Connection.java b/mail/protocols/pop3/src/main/java/com/fsck/k9/mail/store/pop3/Pop3Connection.java
index aaff29d73..4badc73e5 100644
--- a/mail/protocols/pop3/src/main/java/com/fsck/k9/mail/store/pop3/Pop3Connection.java
+++ b/mail/protocols/pop3/src/main/java/com/fsck/k9/mail/store/pop3/Pop3Connection.java
@@ -5,9 +5,10 @@ import java.io.BufferedInputStream;
 import java.io.BufferedOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
+import java.net.InetAddress;
 import java.net.InetSocketAddress;
 import java.net.Socket;
-import java.net.SocketAddress;
+import java.net.UnknownHostException;
 import java.security.GeneralSecurityException;
 import java.security.KeyManagementException;
 import java.security.MessageDigest;
@@ -61,15 +62,7 @@ class Pop3Connection {
 
     void open() throws MessagingException {
         try {
-            SocketAddress socketAddress = new InetSocketAddress(settings.getHost(), settings.getPort());
-            if (settings.getConnectionSecurity() == ConnectionSecurity.SSL_TLS_REQUIRED) {
-                socket = trustedSocketFactory.createSocket(null, settings.getHost(),
-                        settings.getPort(), settings.getClientCertificateAlias());
-            } else {
-                socket = new Socket();
-            }
-
-            socket.connect(socketAddress, SOCKET_CONNECT_TIMEOUT);
+            socket = connect();
             in = new BufferedInputStream(socket.getInputStream(), 1024);
             out = new BufferedOutputStream(socket.getOutputStream(), 512);
 
@@ -102,6 +95,44 @@ class Pop3Connection {
         }
     }
 
+    private Socket connect()
+            throws IOException, MessagingException, NoSuchAlgorithmException, KeyManagementException {
+        InetAddress[] inetAddresses = InetAddress.getAllByName(settings.getHost());
+
+        IOException connectException = null;
+        for (InetAddress address : inetAddresses) {
+            try {
+                return connectToAddress(address);
+            } catch (IOException e) {
+                Timber.w(e, "Could not connect to %s", address);
+                connectException = e;
+            }
+        }
+
+        throw connectException != null ? connectException : new UnknownHostException();
+    }
+
+    private Socket connectToAddress(InetAddress address)
+            throws IOException, MessagingException, NoSuchAlgorithmException, KeyManagementException {
+        if (K9MailLib.isDebug() && K9MailLib.DEBUG_PROTOCOL_POP3) {
+            Timber.d("Connecting to %s as %s", settings.getHost(), address);
+        }
+
+        InetSocketAddress socketAddress = new InetSocketAddress(address, settings.getPort());
+
+        final Socket socket;
+        if (settings.getConnectionSecurity() == ConnectionSecurity.SSL_TLS_REQUIRED) {
+            socket = trustedSocketFactory.createSocket(null, settings.getHost(), settings.getPort(),
+                    settings.getClientCertificateAlias());
+        } else {
+            socket = new Socket();
+        }
+
+        socket.connect(socketAddress, SOCKET_CONNECT_TIMEOUT);
+
+        return socket;
+    }
+
     /*
      * If STARTTLS is not available throws a CertificateValidationException which in K-9
      * triggers a "Certificate error" notification that takes the user to the incoming
diff --git a/mail/protocols/pop3/src/test/java/com/fsck/k9/mail/store/pop3/Pop3StoreTest.kt b/mail/protocols/pop3/src/test/java/com/fsck/k9/mail/store/pop3/Pop3StoreTest.kt
index 82ab36616..554f76683 100644
--- a/mail/protocols/pop3/src/test/java/com/fsck/k9/mail/store/pop3/Pop3StoreTest.kt
+++ b/mail/protocols/pop3/src/test/java/com/fsck/k9/mail/store/pop3/Pop3StoreTest.kt
@@ -37,7 +37,7 @@ class Pop3StoreTest {
     @Test(expected = MessagingException::class)
     fun `checkSettings() with TrustedSocketFactory throwing should throw MessagingException`() {
         stubbing(trustedSocketFactory) {
-            on { createSocket(null, "server", 12345, null) } doThrow IOException()
+            on { createSocket(null, HOST, 12345, null) } doThrow IOException()
         }
 
         store.checkSettings()
@@ -72,7 +72,7 @@ class Pop3StoreTest {
     private fun createServerSettings(): ServerSettings {
         return ServerSettings(
             type = "pop3",
-            host = "server",
+            host = HOST,
             port = 12345,
             connectionSecurity = ConnectionSecurity.SSL_TLS_REQUIRED,
             authenticationType = AuthType.PLAIN,
@@ -93,13 +93,14 @@ class Pop3StoreTest {
         }
 
         stubbing(trustedSocketFactory) {
-            on { createSocket(null, "server", 12345, null) } doReturn socket
+            on { createSocket(null, HOST, 12345, null) } doReturn socket
         }
 
         return outputStream
     }
 
     companion object {
+        private const val HOST = "127.0.0.1"
         private const val INITIAL_RESPONSE = "+OK POP3 server greeting\r\n"
 
         private const val CAPA_RESPONSE = "+OK Listing of supported mechanisms follows\r\n" +