tor server Address

[gabrix]

My lan is so configured:
[router netgear] -------
[linuxbox2ethernets-firewall]------[switch]-------[wwwserver][mailserver]
the tor server is on the linuxbox listening on the internet eth0 and it
can't update  his address  because of the router netgear in front of him
, so i have to update manually his pubblic ip anytime it changes ,the
netgear reboots or whatever, how could i fix this ???
Thanks !!!