Source: http://keystoneit.wordpress.com/2007/11/25/tftp-through-ipcop-or-other-iptables-firewalls/

TFTP will not work well behind most routers unless special settings have been implemented. Here is a way to implement the settings on IPCop and possibly SmoothWall. Maybe even a couple of other Linux based firewalls as well. I have only tested this on an IPCop box.

  1. On the firewall system: nano /etc/rc.d/rc.network
  2. Add these two lines:
    modprobe ip_conntrack_tftp
    modprobe ip_nat_tftp
  3. For immediate effect, repeat the two lines at the command prompt, or reboot the firewall.