How to get the TFTP server address from the DHCP configuration

alexanderzhirov · 08-15-2022, 12:24 PM

I need to find out the address of the TFTP server that is specified in the DHCP configuration.

When the PC is started on, the computer receives the IP address via DHCP, then downloads the image from the PXE server. During the download of the distribution, I need to run a utility that accesses the Database to the server from which this image was downloaded (where the TFTP server is running).

In theory, it would be possible to register the necessary address of the TFTP server in the downloadable image of the distribution. But the bottom line is that such a scheme exists in various subnets. And specifying its TFTP server for each subnet is an irrational approach. It would be more convenient to get the address of the TFTP server from the DHCP server, which is listed there as next-server.

I found something similar at Busybox.

Is it possible to implement something like this in C/C++ and how can it be done? I don't have any ideas.

jefro · 08-15-2022, 04:01 PM

Most of the time the IP addresses scroll on screen. If not then monitor with wireshark or go to server for it's config??

TB0ne · 08-15-2022, 04:59 PM

Quote:

Originally Posted by alexanderzhirov

I need to find out the address of the TFTP server that is specified in the DHCP configuration. When the PC is started on, the computer receives the IP address via DHCP, then downloads the image from the PXE server. During the download of the distribution, I need to run a utility that accesses the Database to the server from which this image was downloaded (where the TFTP server is running).

In theory, it would be possible to register the necessary address of the TFTP server in the downloadable image of the distribution. But the bottom line is that such a scheme exists in various subnets. And specifying its TFTP server for each subnet is an irrational approach. It would be more convenient to get the address of the TFTP server from the DHCP server, which is listed there as next-server.

I found something similar at Busybox. Is it possible to implement something like this in C/C++ and how can it be done? I don't have any ideas.

Not sure how C/C++ comes into play here...the system isn't booted to a point where it could run a program, if it's only doing a PXE/network boot. But I do know if you're using a Cisco device, you can set up a TFTP helper address on the router. Older article, but still valid I think:
https://community.cisco.com/t5/routi...r/td-p/1552749

Not sure that DHCP can do this by itself; IANA docs list DHCP option #66 as a TFTP server name field.