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".

--- a/default_options.h	Thu Dec 10 23:18:48 2020 +0800
+++ b/default_options.h	Wed Feb 15 13:53:04 2017 +0100
@@ -256,6 +256,9 @@
 /* -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"