How does the variable "pxe_default_server" get set in Red Hat 7.9?

phydeauxman

6/21/24, 9:45 PM

We have a RHEL 7.9 server that is acting as a DHCP/TFTP (PXE) server servicing multiple subnets. Below is a copy of our dhcpd.conf file (the DHCP/PXE server is X.Y.145.98):

allow bootp;
allow booting;
max-lease-time 1200;
default-lease-time 900;

subnet X.Y.145.96 netmask 255.255.255.224 {
  option routers X.Y.145.126;
  range X.Y.145.100 X.Y.145.105;
  next-server X.Y.145.98;
  filename "pxelinux.0"
}

subnet X.Y.145.144 netmask 255.255.255.248 {
  option routers X.Y.145.145;
  range X.Y.145.146 X.Y.145.150;
  next-server X.Y.145.98;
  filename "pxelinux.0"
}

The problem we are seeing is that the grub.cfg is grabbing the IP of the options router parameter for the variable pxe_default_server instead of the next-server parameter. This (pxe_default_server) is then used to define the variable net_default_server parameter and our PXE fails because it is point to the gateway IP instead of the DHCP/PXE server IP. If we edit the grub.cfg file to hard code the pxe_default_server parameter to be equal to the IP of the DHCP/PXE server...we can boot fine and pull down an image. Is there a different parameter I should be using?

2 + 0

dhcp

redhat

pxe-boot

rhel7

Score:0

Server

VSYS Host

7/6/24, 8:56 AM

allow bootp;
allow booting;
max-lease-time 1200;
default-lease-time 900;

subnet X.Y.145.96 netmask 255.255.255.224 {
  option routers X.Y.145.126;
  option pxe-server-name X.Y.145.98;
  range X.Y.145.100 X.Y.145.105;
  next-server X.Y.145.98;
  filename "pxelinux.0";
}

subnet X.Y.145.144 netmask 255.255.255.248 {
  option routers X.Y.145.145;
  option pxe-server-name X.Y.145.98;
  range X.Y.145.146 X.Y.145.150;
  next-server X.Y.145.98;
  filename "pxelinux.0";
}

+ 0

Score:0

Server

Pat

6/29/24, 9:37 PM

Grub is surely getting confused, but your dhcpd.confg has issues.

You have 2 subnets but you are providing in both the same "next-server" IP that means that for one subnet the TFTP server might not be visible depending on your layout.
You are not defining the "broadcast-address" per subnet
You are providing PXE data even to not PXE booting clients.

See here an example: https://www.redhat.com/sysadmin/pxe-boot-uefi

# DHCP Server Configuration File
#
#. see /usr/share/doc/dhcp*/dhcpd.conf.example
#
#. see dhcpd.conf(5) man page


option rfc3442-classless-static-routes code 121 = array of integer 8;
option ms-classless-static-routes code 249 = array of integer 8;
option space pxelinux;
option pxelinux.magic code 208 = string;
option pxelinux.configfile code 209 = text;
option pxelinux.pathprefix code 210 = text;
option pxelinux.reboottime code 211 = unsigned integer 32;
option architecture-type code 93 = unsigned integer 16;
subnet 192.168.1.0 netmask 255.255.255.0 {
   option routers 192.168.1.0;
   option subnet-mask 255.255.255.0;
   option broadcast-address 192.168.1.255;
}

subnet 192.168.2.0 netmask 255.255.255.0 {
   option routers 192.168.2.1;
   option subnet-mask 255.255.255.0;
   option broadcast-address 192.168.2.255;
   range 192.168.2.2 192.168.2.254;

   class "pxeclients" {
     match if substring (option vendor-class-identifier, 0, 9) = "PXEClient";
     next-server 192.168.1.10;
     if option architecture-type = 00:07 {
        filename "shim.efi";
        } else {
        filename "pxelinux/pxelinux.0";
     }
   }
}

subnet 192.168.3.0 netmask 255.255.255.0 {
   option routers 192.168.3.1;
   option subnet-mask 255.255.255.0;
   option broadcast-address 192.168.3.255;
   range 192.168.3.2 192.168.3.254;

   class "pxeclients" {
     match if substring (option vendor-class-identifier, 0, 9) = "PXEClient";
     next-server 192.168.1.10;
     if option architecture-type = 00:07 {
        filename "shim.efi";
        } else {
        filename "pxelinux/pxelinux.0";
     }
   }
}

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