The interest of the firesendid utility is limited, unless you hook it to a service already providing authentication for users on your network. The Samba server does exactly that. Once a user is logged to your Samba server, you can retrieve his IP number and you can trust the authentication (the user provided a password).
To hook firesendid, you modify one share configuration (homes for one) and place a "root-preexec" directive. Using the %U and %I macro, you can update your firewall in real time from your Samba server:
[homes]
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root-preexec = /usr/lib/linuxconf/lib/firesendid --fhost host \
--port 999 --id your_server login %U %I
root-postexec = /usr/lib/linuxconf/lib/firesendid --fhost host \
--port 999 --id your_server logout %U
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