Faulty proxy prevents initial Agent setup but allows upgrades

[AndersonQ]

Consider there is a policy with a proxy associated to fleet-server. This proxy does not work for the elastic-agent in question.

The agent can be successfully installed if not proxy is provided on the installation cli flags. The agent will be stuck on Updating on fleet UI and it's state. For:

• 8.9.4, the agent state will be Waiting for initial configuration and composable variables,
• 8.15.0, validating Fleet client config: validating fleet client config: fail to communicate with Fleet Server API client hosts: all hosts failed[...]

Either version will allow the agent to upgrade, the upgrade will fail for both versions as the new agent cannot connect to fleet. For 8.9.4 it's harder to detect why the upgrade fails as it does not properly logs why the agent is failing, at least for 8.15+ the logs explain well why the upgrade is rolled back.

Version: 8.9.4, 8.15.0

Operating System: all

Discuss Forum URL: N/A

Steps to Reproduce:

• create a policy
• add a faulty proxy on Fleet > Settings
• add the proxy to the default fleet server
• install the elastic-agent without passing --proxy-url=http://10.151.86.11:8082
• upgrade the agent

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)