Summary

Fixes #10805.

When sending a request through a proxy with the http adapter, axios was unconditionally rewriting options.headers.host to the request URL's hostname:port, overwriting any Host header the caller had set in config.headers. That made it impossible to direct a proxy to a virtual host that differs from the request URL.

Before

axios.get("http://127.0.0.1:4000", {
  headers: { host: "example.com" },
  proxy: { protocol: "http", host: "127.0.0.1", port: 8888 },
});

The proxy received:

GET / HTTP/1.1
host: 127.0.0.1:4000

After

The proxy receives:

GET / HTTP/1.1
host: example.com

Change

In lib/adapters/http.js#setProxy, only assign the default Host header when the caller has not already provided one. The check is case-insensitive so users can pass host, Host, or HOST.

Test plan

• New unit tests in tests/unit/adapters/http.test.js covering the default behavior, lowercase user header, and mixed-case user header (Host header preservation when forwarding through a proxy (#10805) describe block).
• All existing proxy-related unit tests still pass (vitest run --project unit -t "proxy" — 46 passed).
• End-to-end smoke check with a real HTTP proxy + target server confirms the proxy now sees the user-supplied Host value while the target still sees its own hostname.

Summary by cubic

Preserves a user-supplied Host header when requests go through a proxy in the Node http adapter for axios. This lets callers route to virtual hosts via proxies without the header being overwritten.

Description

• Summary of changes: Skip setting options.headers.host if the user already set any Host/host header (case-insensitive). Otherwise default to hostname:port.
• Reasoning: Needed to target virtual hosts through proxies and avoid clobbering caller intent.
• Additional context: Behavior is limited to proxy forwarding logic; non-proxy requests are unchanged.

Docs

Please update /docs/ to note that axios respects a user-supplied Host header when using a proxy, with a short example showing virtual host routing via proxies.

Testing

• Added unit tests for:
  • Defaulting when no Host is provided.
  • Preservation with lowercase host and mixed-case Host.
• All existing proxy tests still pass.
• Manual smoke check with a real HTTP proxy confirms the proxy sees the user-supplied Host.

Semantic version impact

Patch (bug fix, no API changes).

^{Written for commit 584f284. Summary will update on new commits. Review in cubic}