diff --git a/Changes.rst b/Changes.rst
index 00dd6ed8..8d9d74c1 100644
--- a/Changes.rst
+++ b/Changes.rst
@@ -13,6 +13,9 @@ ChaCha20-Poly1305 cipher support
     Added support for using the ChaCha20-Poly1305 cipher in the OpenVPN data
     channel.
 
+Asynchronous (deferred) authentication support for auth-pam plugin.
+    See src/plugins/auth-pam/README.auth-pam for details.
+
 
 Overview of changes in 2.4
 ==========================
diff --git a/src/plugins/auth-pam/README.auth-pam b/src/plugins/auth-pam/README.auth-pam
index 4d3d4ecc..e604d5a8 100644
--- a/src/plugins/auth-pam/README.auth-pam
+++ b/src/plugins/auth-pam/README.auth-pam
@@ -73,6 +73,41 @@ underlying PAM module.  This is a useful debugging tool to figure
 out which queries a given PAM module is making, so that you can
 craft the appropriate plugin directive to answer it.
 
+Since running OpenVPN with verb 7 is quite verbose, alternatively
+you can put
+
+   verb 3
+   setenv verb 9
+
+in the openvpn config which will only increase logging for this plugin.
+
+
+ASYNCHRONOUS OPERATION
+
+Sometimes PAM modules take very long to complete (for example, a LDAP
+or Radius query might timeout trying to connect an unreachable external
+server).  Normal plugin auth operation will block the whole OpenVPN
+process in this time, that is, all forwarding for all other client stops.
+
+The auth-pam plugin can operate asynchronously ("deferred authentication")
+to remedy this situation.  To enable this, put
+
+  setenv deferred_auth_pam 1
+
+in your openvpn server config.  If set, this will make the "PAM background
+process" fork() and do its job detached from OpenVPN.  When finished, a
+status file is written, which OpenVPN will then pick up and read the
+success/failure result from it.
+
+While the plugin is working in the background, OpenVPN will continue to
+service other clients normally.
+
+Asynchronous operation is recommended for all PAM queries that could
+"take time" (LDAP, Radius, NIS, ...).  If only local files are queried
+(passwd, pam_userdb, ...), synchronous operation has slightly lower
+overhead, so this is still the default mode of operation.
+
+
 CAVEATS
 
 This module will only work on *nix systems which support PAM,
diff --git a/src/plugins/auth-pam/auth-pam.c b/src/plugins/auth-pam/auth-pam.c
index 9a11876d..f537652e 100644
--- a/src/plugins/auth-pam/auth-pam.c
+++ b/src/plugins/auth-pam/auth-pam.c
@@ -62,6 +62,7 @@
 #define RESPONSE_INIT_FAILED      11
 #define RESPONSE_VERIFY_SUCCEEDED 12
 #define RESPONSE_VERIFY_FAILED    13
+#define RESPONSE_DEFER            14
 
 /* Pointers to functions exported from openvpn */
 static plugin_log_t plugin_log = NULL;
@@ -69,7 +70,7 @@ static plugin_secure_memzero_t plugin_secure_memzero = NULL;
 static plugin_base64_decode_t plugin_base64_decode = NULL;
 
 /* module name for plugin_log() */
-static char * MODULE = "AUTH-PAM";
+static char *MODULE = "AUTH-PAM";
 
 /*
  * Plugin state, used by foreground
@@ -524,12 +525,31 @@ openvpn_plugin_func_v1(openvpn_plugin_handle_t handle, const int type, const cha
         const char *password = get_env("password", envp);
         const char *common_name = get_env("common_name", envp) ? get_env("common_name", envp) : "";
 
+        /* should we do deferred auth?
+         *  yes, if there is "auth_control_file" and "deferred_auth_pam" env
+         */
+        const char *auth_control_file = get_env("auth_control_file", envp);
+        const char *deferred_auth_pam = get_env("deferred_auth_pam", envp);
+        if (auth_control_file != NULL && deferred_auth_pam != NULL)
+        {
+            if (DEBUG(context->verb))
+            {
+                plugin_log(PLOG_NOTE, MODULE, "do deferred auth '%s'",
+                           auth_control_file);
+            }
+        }
+        else
+        {
+            auth_control_file = "";
+        }
+
         if (username && strlen(username) > 0 && password)
         {
             if (send_control(context->foreground_fd, COMMAND_VERIFY) == -1
                 || send_string(context->foreground_fd, username) == -1
                 || send_string(context->foreground_fd, password) == -1
-                || send_string(context->foreground_fd, common_name) == -1)
+                || send_string(context->foreground_fd, common_name) == -1
+                || send_string(context->foreground_fd, auth_control_file) == -1)
             {
                 plugin_log(PLOG_ERR|PLOG_ERRNO, MODULE, "Error sending auth info to background process");
             }
@@ -540,6 +560,14 @@ openvpn_plugin_func_v1(openvpn_plugin_handle_t handle, const int type, const cha
                 {
                     return OPENVPN_PLUGIN_FUNC_SUCCESS;
                 }
+                if (status == RESPONSE_DEFER)
+                {
+                    if (DEBUG(context->verb))
+                    {
+                        plugin_log(PLOG_NOTE, MODULE, "deferred authentication");
+                    }
+                    return OPENVPN_PLUGIN_FUNC_DEFERRED;
+                }
                 if (status == -1)
                 {
                     plugin_log(PLOG_ERR|PLOG_ERRNO, MODULE, "Error receiving auth confirmation from background process");
@@ -782,6 +810,80 @@ pam_auth(const char *service, const struct user_pass *up)
     return ret;
 }
 
+/*
+ * deferred auth handler
+ *   - fork() (twice, to avoid the need for async wait / SIGCHLD handling)
+ *   - query PAM stack via pam_auth()
+ *   - send response back to OpenVPN via "ac_file_name"
+ *
+ * parent process returns "0" for "fork() and wait() succeeded",
+ *                        "-1" for "something went wrong, abort program"
+ */
+
+static void
+do_deferred_pam_auth(int fd, const char *ac_file_name,
+                     const char *service, const struct user_pass *up)
+{
+    if (send_control(fd, RESPONSE_DEFER) == -1)
+    {
+        plugin_log(PLOG_ERR|PLOG_ERRNO, MODULE, "BACKGROUND: write error on response socket [4]");
+        return;
+    }
+
+    /* double forking so we do not need to wait() for async auth kids */
+    pid_t p1 = fork();
+
+    if (p1 < 0)
+    {
+        plugin_log(PLOG_ERR|PLOG_ERRNO, MODULE, "BACKGROUND: fork(1) failed");
+        return;
+    }
+    if (p1 != 0)                           /* parent */
+    {
+        waitpid(p1, NULL, 0);
+        return;                            /* parent's job succeeded */
+    }
+
+    /* child */
+    close(fd);                              /* socketpair no longer needed */
+
+    pid_t p2 = fork();
+    if (p2 < 0)
+    {
+        plugin_log(PLOG_ERR|PLOG_ERRNO, MODULE, "BACKGROUND: fork(2) failed");
+        exit(1);
+    }
+
+    if (p2 != 0)                            /* new parent: exit right away */
+    {
+        exit(0);
+    }
+
+    /* grandchild */
+    plugin_log(PLOG_NOTE, MODULE, "BACKGROUND: deferred auth for '%s', pid=%d",
+               up->username, (int) getpid() );
+
+    /* the rest is very simple: do PAM, write status byte to file, done */
+    int ac_fd = open( ac_file_name, O_WRONLY );
+    if (ac_fd < 0)
+    {
+        plugin_log(PLOG_ERR|PLOG_ERRNO, MODULE, "cannot open '%s' for writing",
+                   ac_file_name );
+        exit(1);
+    }
+    int pam_success = pam_auth(service, up);
+
+    if (write( ac_fd, pam_success ? "1" : "0", 1 ) != 1)
+    {
+        plugin_log(PLOG_ERR|PLOG_ERRNO, MODULE, "cannot write to '%s'",
+                   ac_file_name );
+    }
+    close(ac_fd);
+    plugin_log(PLOG_NOTE, MODULE, "BACKGROUND: %s: deferred auth: PAM %s",
+               up->username, pam_success ? "succeeded" : "rejected" );
+    exit(0);
+}
+
 /*
  * Background process -- runs with privilege.
  */
@@ -789,6 +891,7 @@ static void
 pam_server(int fd, const char *service, int verb, const struct name_value_list *name_value_list)
 {
     struct user_pass up;
+    char ac_file_name[PATH_MAX];
     int command;
 #ifdef USE_PAM_DLOPEN
     static const char pam_so[] = "libpam.so";
@@ -847,7 +950,8 @@ pam_server(int fd, const char *service, int verb, const struct name_value_list *
             case COMMAND_VERIFY:
                 if (recv_string(fd, up.username, sizeof(up.username)) == -1
                     || recv_string(fd, up.password, sizeof(up.password)) == -1
-                    || recv_string(fd, up.common_name, sizeof(up.common_name)) == -1)
+                    || recv_string(fd, up.common_name, sizeof(up.common_name)) == -1
+                    || recv_string(fd, ac_file_name, sizeof(ac_file_name)) == -1)
                 {
                     plugin_log(PLOG_ERR|PLOG_ERRNO, MODULE, "BACKGROUND: read error on command channel: code=%d, exiting",
                             command);
@@ -867,6 +971,18 @@ pam_server(int fd, const char *service, int verb, const struct name_value_list *
                 /* If password is of the form SCRV1:base64:base64 split it up */
                 split_scrv1_password(&up);
 
+                /* client wants deferred auth
+                 */
+                if (strlen(ac_file_name) > 0)
+                {
+                    do_deferred_pam_auth(fd, ac_file_name, service, &up);
+                    break;
+                }
+
+
+                /* non-deferred auth: wait for pam result and send
+                 * result back via control socketpair
+                 */
                 if (pam_auth(service, &up)) /* Succeeded */
                 {
                     if (send_control(fd, RESPONSE_VERIFY_SUCCEEDED) == -1)
