Change default APM server_url to 'http://127.0.0.1:8200' to avoid IPv6 ambiguity

[trentm]

The current default APM server URL -- 'http://localhost:8200' -- is ambiguous. "localhost" can resolve to a '127.0.0.1' (IPv4) or '[::1]' (IPv6).

At least in Node v17 the default dns.lookup() ordering of results was changed to no longer explicitly sort IPv4 addresses first. That means that on systems where the default resolver returns '[::1]' first for "localhost" will result in a broken attempt to talk to a default-configured local APM server:
elastic/apm-agent-nodejs#3045
This is because APM server only binds to the IPv4 port:
elastic/apm-server#1405

Checklist

• May the instrumentation collect sensitive information, such as secrets or PII (ex. in headers)?
  • Yes
    • Add a section to the spec how agents should apply sanitization (such as sanitize_field_names)
  • No
    • Why?
  • n/a
• Create PR as draft
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• Mark as Ready for Review (automatically requests reviews from all agents and PM via CODEOWNERS)
  • Remove PM from reviewers if impact on product is negligible
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• Approved by at least 2 agents + PM (if relevant)
• Merge after 7 days passed without objections
  To auto-merge the PR, add /schedule YYYY-MM-DD to the PR description.
• Create implementation issues through the meta issue template (this will automate issue creation for individual agents) -> Change default APM server_url to 'http://127.0.0.1:8200' to avoid IPv6 ambiguity #735
• If this spec adds a new dynamic config option, add it to central config.

/schedule 2022-12-13

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[SergeyKleyman]

Is it possible for the system to be IPv6 only or less drastic case when list of addresses to which localhost resolves does not include 127.0.0.1?

[trentm]

(Sorry for not having opened this as draft to start.)

[trentm]

Is it possible for the system to be IPv6 only or less drastic case when list of addresses to which localhost resolves does not include 127.0.0.1?

IPv6-only is likely possible yes. I’ve no encountered it. I think it would be rare, but am not sure.

I can try to look at the Go lang docs to see if we think APM server would even listen on the IPv6 localhost by default in that case.

Re: ‘localhost’ not resolving to 127.0.0.1. Oof, I would think that would be very unlikely but possible. I am not sure if some RFC has a “MUST” statement about “localhost” resolving so. If some user had this case, I am not sure we would feel the need to have the default agent and server configs successfully reach each other. I may be missing a legitimate use case though?

[basepi]

Seems reasonable to me. I agree with Trent that while there may be edge cases, I think a user with a host file that redirects localhost or an ipv6-only system will be used to defaults not working.

[z1c0]

The change makes sense 👍 .

(On a side note, I'm just wondering though:

• how many applications will be impacted by this change of behavior in Node v17?
• why APM Server binds only to IPv4?)

[trentm]

• how many applications will be impacted by this change of behavior in Node v17?

@z1c0 Possibly a lot. Here is the core node issue, which shows a number of incoming links from impacted projects. Also the links in that issue's description point to a number of back issues related to this.

• why APM Server binds only to IPv4?

There is some discussion in elastic/apm-server#1405
I suspect it is mainly just because that's what Go lang's native net library works and just binding to IPv4 has always mostly been fine.

Interestingly https://pkg.go.dev/net#Listen says:

The address can use a host name, but this is not recommended, because it will create a listener for at most one of the host's IP addresses.

So it might be worth asking APM Server devs. I'll ask separately.

[trentm]

APM server will be making the same change in elastic/apm-server#9749