[Fleet] Add support for proxy in Elastic Agent

[mostlyjason]

We've gotten a few requests from users of Elastic Agent to add support for proxy URLs in the output. Apparently, a user tried adding the proxy_url setting in the YAML output settings in Fleet settings but it did not work.

This could have the same behavior as proxy_url in Filebeat. This could initially be done in the YAML configuration because this is an advanced setting. We could add UI support later.

Elastic Agent should also be able to communicate with Fleet Server through a proxy. This allows Fleet Server to run in a central location, while Elastic Agent is running on the edge behind a proxy.

As an Elastic Agent user, I want the ability to:

• Set a proxy url in the ES output of my standalone agent policy
• Set a proxy url in my Kibana Fleet settings for the ES output
• Set a proxy url for Elastic Agent to communicate with Fleet Server
• Set a proxy url in my Kibana Fleet settings for the Fleet Server connection

Non-goals (deferred to later):

• Set a proxy for Fleet Server to communicate with Elasticsearch

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[henurah]

The user mentioned they tried to use the workaround in this Discuss forum topic (https://discuss.elastic.co/t/elastic-agent-fleet-deployment-behind-a-proxy/254183/5) as well as the proxy_url setting, but it did not work for them. They tried to set up Elastic Agent on a Windows host if that helps. Thank you very much for your help!

[henurah]

@mostlyjason The user also mentioned that they tried the workaround from the discuss forums on a Linux host and it still did not appear to work (the agent would start but no data was being sent to ES). Any update from engineering? Thanks!

[mostlyjason]

Thanks I think the next step is confirm the current behavior on our end, and then we can assess if we need to make any changes to support it.

[mostlyjason]

@henurah can you check with the user whether agent standalone mode would provide the ability for them to test this feature out? Do they also need support for Fleet central management with the proxy, and could that potentially be a phase 2?

[henurah]

@mostlyjason How would they go about configuring integrations in standalone mode? Is it just manually set up the config on the agent side and then the agent would pull from Elasticsearch via the proxy configuration? Have we verified this would work on Windows by any chance? I think the customer just wants to have the integrations working through a proxy for now using Elastic Agent (with the assurance that full proxy support will be coming soon). Thanks!

[jen-huang]

@ph I'm moving this to the agent board since it seems that it is work on agent side. Let me know otherwise.

[elasticmachine]

Pinging @elastic/agent (Team:Agent)

[ph]

Closing as duplicate of #25021