feat: update minimal supported version of Node.js to v20

[bliuchak]

This PR upgrades minimal supported version of Node.js to v20.

Why Node.js 20?

I think for us it makes sense to support last 3 LTS versions of node. As of today they are: 24, 22 and 20.

Also I'll keep tests at Github Actions for Node.js 16, 18 and 20 (will be added) to make sure we're not breaking too much things. At least right now. However this might change in next major release.

Migration Flow

Migration from Node.js 18 to 20 wasn't complicated. Here are the main issues which we've faced:

• Node.js 20 enables keep-alive by default. This should be turned off in tests explicitly.
• Some tests should be skipped since Node.js became more strict 👍
• For anonymized proxy script Node.js 20 throw socket hang up instead of ECONNREFUSED for some special cases

Skipped Tests On Node.js 20

Overview

There was 2242 tests cases for Node.js v18. Currently there are 2191 (minus 51 test case) for Node.js v20. See details below:

1. "ignores non-standard server HTTP headers" - Skipped when !useUpstreamProxy:
   • Main proxy configurations: 2 (http/https) × 2 (ssl) × 4 (auth) = 16 configs
   • Custom response suites: 2 x 2 x 4 = 16 configs
   • Direct tests (HTTP/HTTPS -> Target): 2 configs
   • Total: 34 tests
2. "gracefully fails on invalid HTTP status code" - Skipped when !useUpstreamProxy && !useSsl:
   • Main proxy configurations: 2 x 1 x 4 = 8 configs
   • Custom response suites: 2 x 1 x 4 = 8 configs
   • Direct tests: 1 config (HTTP -> Target only)
   • Total: 17 tests

Grand total: 34 + 17 = 51 skipped tests

Why these tests are skipped

Both tests verify the proxy's behavior when receiving malformed HTTP responses from upstream servers:

1. "ignores non-standard server HTTP headers" - tests handling of headers with spaces in names (e.g., Invalid Header With Space: value) which violate HTTP RFC
2. "gracefully fails on invalid HTTP status code" - tests handling of invalid status codes (like HTTP/1.1 55 OK)

In Node.js 20, the --insecure-http-parser flag no longer relaxes HTTP client parsing for these edge cases. The HTTP client throws parse errors before the proxy can even process them. This is a Node.js security hardening change, not a proxy bug.

Close #601

[Copilot]

Pull request overview

This PR upgrades the minimal supported Node.js version from v14 to v20 to align with active LTS versions (20, 22, 24) and address the Node.js 18 end-of-life. The upgrade addresses Node.js 20's stricter HTTP parsing and default keep-alive behavior changes.

Changes:

• Updated minimum Node.js version requirement from >=14 to >=20
• Disabled HTTP keep-alive across test servers to prevent connection tracking issues
• Added logic to skip 51 tests on Node.js 20+ that rely on lenient HTTP parsing
• Updated error message handling for connection failures to account for Node.js 20 behavior changes

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
package.json	Updated engines.node requirement from >=14 to >=20
.github/workflows/check.yaml	Updated test matrix to Node.js 16, 18, 20 and changed lint condition to run on Node 20
test/Dockerfile	Updated base Docker image from Node 18.20.8 to Node 20.19.6
test/utils/target_server.js	Disabled keep-alive on HTTP test server by setting keepAliveTimeout to 0
test/server.js	Added Connection: 'close' header to test requests and disabled keep-alive on proxy servers; added Node.js version check to skip tests for stricter HTTP parsing
test/https-stress-test.js	Disabled keep-alive on global HTTP agent and proxy server
test/anonymize_proxy.js	Updated error assertion to accept both 'ECONNREFUSED' and 'socket hang up' messages
test/anonymize_proxy_no_password.js	Updated error assertion to accept both 'ECONNREFUSED' and 'socket hang up' messages

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[danpoletaev]

Looks good!

Left few suggestions that needs to be reviewed before merging

[danpoletaev]

Nice ❤️