[stunnel-users] Question about behavior with "failover = prio"

[Michael Gebis]

Hello again.

We recently noticed a behavior in stunnel that surprised us, and seems
to be inconsistent with the man page description.  We use "failover =
prio" in our configuration file. To refresh everyone's memory, the man
page describes this option here:

    failover = rr | prio
    Failover strategy for multiple "connect" targets.
        rr (round robin) - fair load distribution
        prio (priority) - use the order specified in config file
    default: rr

Our config file contains a list of connect targets, like this:

    [...]
    connect = h1.example.com
    connect = h2.example.com
    connect = h3.example.com
    connect = h4.example.com
    [...]

Based on the description, we expect the connection to try h1 first,
and if h1 fails, failover through the remainder of the list, in-order.
However, what we observe is that the connections start at a seemingly
random place in the list. It does appear to proceed in-order...it's
just the starting place that seems incorrect. For example, we might
get a sequence that proceeds "h3->h4->h1->h2".

Please let me know if I am misunderstanding the intended behavior of
the 'prio' failover strategy.

Here is a trial patch that hard-codes the starting place for the
failover to the beginning of the list.  I am not sure if this change
has any unintended consequences for other configurations.  But as a
proof-of-concept, it does seem to fix the behavior to be consistent
with our reading of the man page.  This patch is against 5.20 but it
appears that 5.21b2 acts the same way.

Thanks!

Michael

diff -ru stunnel-5.20.orig/src/client.c stunnel-5.20-prio-fix/src/client.c
--- stunnel-5.20.orig/src/client.c 2015-07-16 15:55:52.213064746 -0700
+++ stunnel-5.20-prio-fix/src/client.c 2015-07-17 16:13:12.129184507 -0700
@@ -1285,7 +1285,8 @@
         *c->connect_addr.rr_ptr=(i+1)%c->connect_addr.num;
         s_log(LOG_INFO, "failover: round-robin");
     } else {
-        s_log(LOG_INFO, "failover: priority");
+        s_log(LOG_INFO, "failover: priority, starting at first entry");
+        i=0;
     }
     return i;
 }