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".

1 files changed, 3 insertions, 0 deletions

diff --git a/default_options.h b/default_options.h
index 375506d..d6b6d55 100644
--- a/default_options.h
+++ b/default_options.h
@@ -256,6 +256,9 @@ Homedir is prepended unless path begins with / */
 /* -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"