This was perhaps a bit of an obscure case, but just in case this helps anyone…
I had an old thin-client terminal (not sure how old, but probably circa 2005). I was getting so frustrated because I couldn’t get it to successfully PXE network boot. It kept giving the error File not found. I tried many different tftpboot files (ipxe.pxe, undionly.pxe, etc.). It wasn’t until I started live monitoring tftp traffic that I found the problem.
By running this command in a terminal on the Fog Server: tcpdump port 69, I was able to detect that for some reason the client was requesting a filename that was missing the last character. For example, if my DHCP config was set to request filename ipxe.pxe, the problem client would actually end up requestiong ipxe.px (missing the e on the end). It’s so odd because only one client was doing this and another client was working fine. To get this client to work I had to add an extra character on the end in the DHCP config (for example, ipxe.pxez).