Communications fail between Azure Load Balancer and SLE instance

This document (7024034) is provided subject to the disclaimer at the end of this document.

Environment

SUSE Linux Enterprise Server for Azure

Situation

Communications fail to Public Cloud instance through the Azure Load Balancer.

Resolution

Warning: This solution comes from a 3rd party and recommends a solution which SUSE TCP support and development strongly discourage using, as it will disable the use of the best TCP packet recovery algorithms and may lead to significant network delays or failures.

It may be better to avoid the Azure Load Balancer than to follow this advice. Use this at your own risk:

Microsoft recommendation:

Disable net.ipv4.tcp_timestamps setting.

In-memory change:

hostname:~ # echo 0 > /proc/sys/net/ipv4/tcp_timestamps

Persistent change:

Create a tcp_timestamps.conf drop-in configuration file.

hostname:~ # echo "net.ipv4.tcp_timestamps = 0" > /etc/sysctl.d/tcp_timestamps.conf
Load new sysctl parameter from configuration file.

hostname:~ # sysctl -p /etc/sysctl.d/tcp_timestamps.conf

CAUTION: The "best practice" modern packet recovery algorithms, as well as certain application workloads and HPC features require tcp_timestamps. When in doubt, consult the manual or other technical resources to be sure disabling this setting will not negatively impact the running application(s). If the application requires this parameter, use of the Azure Load Balancer is not recommended. Test thoroughly before disabling this setting in production. The Linux TCP development community generally does not do routine testing with tcp_timestamps disabled, so there is greater risk of problems and bugs arising (with current or future code) if you disable tcp_timestamps.

Cause

With TCP Timestamps enabled, health probe packets sent through the Azure Load Balancer may be dropped by the guest OS, causing the load balancer to report the endpoint as down.

Full explanation can be found at the bottom of the "Design guidance" section here:

https://docs.microsoft.com/en-us/azure/load-balancer/load-balancer-custom-probe-overview#design

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.