How to configure dnsmasq for gPXE

A network card

Why DNSMasq

  • It is small and lightweight
  • Easy to setup (includes its own tftp server)
  • It may already be running on your linux-based router :-)

Defining the gPXE-specific options

Add the following settings to your /etc/dnsmasq.conf file:

  dhcp-boot=pxelinux.0
  enable-tftp
  tftp-root=/var/lib/tftpboot

for pxe chaining use this:

  dhcp-match=gpxe,175
  dhcp-boot=net:#gpxe,gpxe.pxe
  dhcp-boot=pxelinux.0
  

the '#' above is a 'not', not a comment

See: http://www.thekelleys.org.uk/dnsmasq/docs/dnsmasq.conf.example for more information

Chain-booting gPXE into SAN-boot / installation

These settings will cause your existing PXE to load gPXE, and then gPXE to boot off an iSCSI target. If the target can not be booted, it is left registered so that you can install an operating system to it.

  enable-tftp
  tftp-root=/var/lib/tftpboot    # place your gpxe.pxe (either built yourself or from http://rom-o-matic.net here)
  dhcp-match=gpxe,175            # tags the request with net:gpxe if the gPXE option was supplied in DHCP request
  dhcp-option=175,8:1:1          # turn on the keep-san option to allow installation
  dhcp-boot=net:#gpxe,gpxe.pxe   # Here #gpxe means 'not gpxe': that is the tag is not set
  dhcp-option=net:gpxe,17,"iscsi:ta.rg.et.ip::::iqn.yyyy-mm.reversed-domain-name:identifier"

A potential problem and a fix

According to this page: http://blog.nella.org/?p=335

gPXE wants to find the boot filename in the dedicated slot in the reply packet. 
dnsmasq does something clever/stupid (I’m not sure which) and puts it 
someplace else. You have to use the –dhcp-no-override option to 
dnsmasq to make gPXE see the filename.

Here is an online version of the man page for dnsmasq. it contains this switch explanation:

--dhcp-no-override
Disable re-use of the DHCP servername and filename fields as extra 
option space. If it can, dnsmasq moves the boot server and filename 
information (from dhcp-boot) out of their dedicated fields into DHCP 
options. This make extra space available in the DHCP packet for 
options but can, rarely, confuse old or broken clients. This flag 
forces "simple and safe" behaviour to avoid problems in such a case.

Under OpenWRT

OpenWRT is a replacement for factory firmware commonly found in wireless routers. dnsmasq is configured a bit differently when you use OpenWRT. Use these commands to set the dhcp-supplied filename. This eventually gets passed as the –dhcp-boot command-line option to dnsmasq. But, this way, it is saved in the router's flash and will get saved/restored with the rest of the system's configuration.

  uci set dhcp.@dnsmasq[0].dhcp_boot=http://4.3.2.1/gpxe-dir/script-name,
  uci commit dhcp