Technical Tip: How to configure DHCP option 43 with sub-option 66 to TFTP server on VoIP phones

[DiegoBernardelli]

Description	This article describes how to configure the DHCP server on FortiGate to proper send the TFTP server to provision IP phones.
Scope	FortiGate, FortiOS v6.4, v7.0, v7.2.
Solution	Some IP phones need to receive a TFTP server IP on the DHCP OFFER. Most of the IP phones take this parameter as DHCP option 43 with sub-option 66. This can be configured in FortiOS as follows: config system dhcp server edit 3 set lease time <number of seconds> set default-gateway <DG IP> set next-server <next server IP> set netmask < y.y.y.y > set interface <interface name> config ip-range edit 1 set start-ip <x.x.x.x> set end-ip <x.x.y.y> next end config options edit 1 set code 43 set value "4204ac102001" next end set dns-server1 <dns server IP> next end end The above configuration sends the DHCP OFFER the TFTP IP 172.16.32.1 as option 43 suboption 66. The HEX string '4204ac102001' should be interpreted as follows: 42 = 66 in decimal <- Sub-option 66. 04 = 04 in decimal <- Next 4 bytes. AC = 172 in decimal 10 = 16 in decimal 20 = 32 in decimal 01 = 01 in decimal The generated DHCP offer seen from Wireshark should look like the following: The capture shows the string from the HEX dump with the preamble '2B 06' which is option 43, 6 bytes length. The TFTP IP is not displayed in Wireshark as option 43 is a generic vendor specific option, so it could contain anything: Wireshark is not able to understand how to parse the HEX string.