DHCP clients in virtual networks fail with "bad UDP checksum" error

This document (7016810) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server 12 Xen or KVM host
SUSE Linux Enterprise Server 11 Service Pack 4 (SLES 11 SP4) para-virtualized guest

Situation

The DHCP client on a para-virtulized (PV) guest system prints the error message "err, eth0: bad UDP checksum, ignoring" and subsequently does not configure the interface.

Resolution

Transmit checksum offloading on the host bridge needs to be turned off. For KVM hosts, this is documented at http://libvirt.org/formatdomain.html#elementsDriverBackendOptions

For Xen hosts, libvirt doesn't allow to configure Ethernet offloading options for dynamically created bridges. A recommended way is to set up the bridge statically on the host system, add the line ETHTOOL_OPTIONS='-K iface tx off to its /etc/sysconfig/network/ifcfg-iface file (replace iface with the name of the bridge interface) and refer to the documentation at http://libvirt.org/formatnetwork.html#examplesBridge on how to configure libvirt to use the existing bridge.

If a static bridge isn't an option, a custom udev rule can be used as well. It can be created by running

cat >/etc/udev/rules.d/81-virbr.rules <<EOF
ACTION=="add", SUBSYSTEM=="net", NAME=="virbr*", RUN+="/sbin/ethtool -K $env{INTERFACE} tx off"
EOF

as the root user in a shell. This will turn off tx checksum offloading for any network interface that gets created with a name starting with "virbr".

Cause

Additional Information

Disclaimer

This Support Knowledgebase provides a valuable tool for SUSE customers and parties interested in our products and solutions to acquire information, ideas and learn from one another. Materials are provided for informational, personal or non-commercial use within your organization and are presented "AS IS" WITHOUT WARRANTY OF ANY KIND.