I have many Cisco IP phones that operate in the following manner (oversimplified):
- Negotiate with DHCP for IP, DNS, TFTP, etc.
- Look for SEPXXXXXXXXXXXX.cnf.xml configuration file on TFTP server where X is the MAC address of the phone
- Parse the configuration file to load its configuration and update firmware (also stored on TFTP server) if necessary
The issue here is that I have some phones that need to be put in small offices or peoples' homes. I need to be able to update the configuration files at all times so I can't just preconfigure the phone and send it out. How can I make the TFTP access secure over the internet and prevent someone unauthorized from getting to the configuration files? I know I could do an IP based ACL but this doesn't stop the possibility of someone spoofing the IP.