Diving straight in here, this article is not supposed to give you any background or explanation on PXE boot. It is meant for trouble shooting and verifying that your PXE boot infrastructure is set up properly. So follow (or skip) these steps.
All commands to be run as root from a linux client.
##1 DHCP Server
# First test if the DHCP server redirects the PXE client to the right TFTP Server.
# THIS SECTION IS NOT CREATED YET.
nmap --script broadcast-dhcp-discover -e eth0
##2 TFTP Server
# Check if the TFTP Server is reachable (correct IP and ports are open). tftp must be installed.
# Make sure the client has the right UDP opened (or disable the fireall). Otherwise you get a timeout
TFTP_SERVER=192.168.0.90
tftp $TFTP_SERVER
verbose
trace
status
get /pxelinux.0
quit
ls -l
# If you get timeouts, open the client's firewall port (not sure which ones). On server you will see messages like in /var/log/messages: in.tftpd[1186181]: tftpd: read(ack): No route to host
# Compare the size of the file
# In the ftp server logs you should see these messages:
# tail -f /var/log/messages
#Jan 20 11:31:15 hostname xinetd[1064]: START: tftp pid=90266 from=192.168.0.152
#Jan 20 11:31:15 hostname in.tftpd[90267]: Client 192.168.0.152 finished pxelinux.0
# Other trouble shooting: http://www.ing.iac.es/~docs/external/realport/ps-xxxx-tsht-tftp.html