For a while I've been turning off iptables to let my secondary machine connect as synergy client to let me use one keyboard and mouse across both machines.  Finally today I decided to set up the rules properly to do it, so here's how:

Create a new rule file in the iptables chain directory for incoming rules:

sudo vi /etc/iptables.d/filter/INPUT/53-custom-synergys.rule

with the following lines in it:

# Allows incoming flows for Synergy.
-A INPUT -p tcp -m tcp --dport 24800 -j ACCEPT
-A INPUT -p udp -m udp --dport 24800 -j ACCEPT

Restart iptables for the rules to take effect

sudo /etc/init.d/iptables restart