[PATCH] Introduce extra delay before closing unauthenticated sessions

[Thomas De Schampheleire]

# HG changeset patch
# User Thomas De Schampheleire <thomas.de_schampheleire at nokia.com>
# Date 1487163184 -3600
#      Wed Feb 15 13:53:04 2017 +0100
# Node ID ef434ebf63f7a935e9530bb2cd2e8d0463a5217a
# Parent  249681d9ecda383b7241b3cc360884093015dede
Introduce extra delay before closing unauthenticated sessions

To make it harder for attackers, introduce a delay to keep an
unauthenticated session open a bit longer, thus blocking a connection
slot until after the delay.

Without this, while there is a limit on the amount of attempts an attacker
can make at the same time (MAX_UNAUTH_PER_IP), the time taken by dropbear to
handle one attempt is still short and thus for each of the allowed parallel
attempts many attempts can be chained one after the other. The attempt rate
is then:
    "MAX_UNAUTH_PER_IP / <process time of one attempt>".

With the delay, this rate becomes:
    "MAX_UNAUTH_PER_IP / UNAUTH_CLOSE_DELAY".

diff --git a/default_options.h b/default_options.h
--- a/default_options.h
+++ b/default_options.h
@@ -256,6 +256,9 @@ Homedir is prepended unless path begins 
 /* -T server option overrides */
 #define MAX_AUTH_TRIES 10
 
+/* Delay introduced before closing an unauthenticated session (seconds) */
+#define UNAUTH_CLOSE_DELAY 30
+
 /* The default file to store the daemon's process ID, for shutdown
    scripts etc. This can be overridden with the -P flag */
 #define DROPBEAR_PIDFILE "/var/run/dropbear.pid"
diff --git a/svr-session.c b/svr-session.c
--- a/svr-session.c
+++ b/svr-session.c
@@ -215,6 +215,7 @@ void svr_dropbear_exit(int exitcode, con
 	char fullmsg[300];
 	char fromaddr[60];
 	int i;
+	int add_delay = 0;
 
 #if DROPBEAR_PLUGIN
         if ((ses.plugin_session != NULL)) {
@@ -247,13 +248,33 @@ void svr_dropbear_exit(int exitcode, con
 		snprintf(fullmsg, sizeof(fullmsg), 
 				"Exit before auth%s: (user '%s', %u fails): %s",
 				fromaddr, ses.authstate.pw_name, ses.authstate.failcount, exitmsg);
+		add_delay = 1;
 	} else {
 		/* before userauth */
 		snprintf(fullmsg, sizeof(fullmsg), "Exit before auth%s: %s", fromaddr, exitmsg);
+		add_delay = 1;
 	}
 
 	dropbear_log(LOG_INFO, "%s", fullmsg);
 
+	/* To make it harder for attackers, introduce a delay to keep an
+	 * unauthenticated session open a bit longer, thus blocking a connection
+	 * slot until after the delay. Without this, while there is a limit on
+	 * the amount of attempts an attacker can make at the same time
+	 * (MAX_UNAUTH_PER_IP), the time taken by dropbear to handle one attempt
+	 * is still short and thus for each of the allowed parallel attempts
+	 * many attempts can be chained one after the other. The attempt rate is
+	 * then:
+	 *     "MAX_UNAUTH_PER_IP / <process time of one attempt>".
+	 * With the delay, this rate becomes:
+	 *     "MAX_UNAUTH_PER_IP / UNAUTH_CLOSE_DELAY".
+	 */
+	if ((add_delay != 0) && (UNAUTH_CLOSE_DELAY > 0)) {
+		TRACE(("svr_dropbear_exit: start delay of %d seconds", UNAUTH_CLOSE_DELAY));
+		sleep(UNAUTH_CLOSE_DELAY);
+		TRACE(("svr_dropbear_exit: end delay of %d seconds", UNAUTH_CLOSE_DELAY));
+	}
+
 #if DROPBEAR_VFORK
 	/* For uclinux only the main server process should cleanup - we don't want
 	 * forked children doing that */