You get this error when trying to use sanboot (with http and maybe other sources to) and autoboot is undefined when compiling ipxe.

You may also get this error if you try to use sanboot against an Apache 2.2 server with KeepAlive set to “off”. (Which is the default setting for Apache in RHEL 6 and derivatives) Try setting KeepAlive to “on” to see if that resolves the issue.