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PXE Boot or DHCP Failure on Guest - Fix it now

This article covers how to fix PXE Boot (or DHCP) Failure on Guest.

Nature of this error:

A guest virtual machine starts successfully, but is then either unable to acquire an IP address from DHCP or boot using the PXE protocol, or both. There are two common causes of this error: having a long forward delay time set for the bridge, and when the iptables package and kernel do not support checksum mangling rules.


Long forward delay time on bridge.

This is the most common cause of this error. If the guest network interface is connecting to a bridge device that has STP (Spanning Tree Protocol) enabled, as well as a long forward delay set, the bridge will not forward network packets from the guest virtual machine onto the bridge until at least that number of forward delay seconds have elapsed since the guest connected to the bridge. This delay allows the bridge time to watch traffic from the interface and determine the MAC addresses behind it, and prevent forwarding loops in the network topology. If the forward delay is longer than the timeout of the guest's PXE or DHCP client, then the client's operation will fail, and the guest will either fail to boot (in the case of PXE) or fail to acquire an IP address (in the case of DHCP).


If this is the case, change the forward delay on the bridge to 0, or disable STP on the bridge.

This solution applies only if the bridge is not used to connect multiple networks, but just to connect multiple endpoints to a single network (the most common use case for bridges used by libvirt).

If the guest has interfaces connecting to a libvirt-managed virtual network, edit the definition for the network, and restart it. 

For example, edit the default network with the following command:

# virsh net-edit default

Add the following attributes to the <bridge> element:

<name_of_bridge='virbr0' delay='0' stp='on'/>


If this problem is still not resolved, the issue may be due to a conflict between firewalld and the default libvirt network.

To fix this, stop firewalld with the service firewalld stop command, then restart libvirt with the service libvirtd restart command.

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