X-Forwarded-Host

The HTTP X-Forwarded-Host request header is an unofficial HTTP header indicating the original Host header value sent by the client before a proxy or load balancer modified the request.

Usage

Reverse proxies and load balancers often rewrite the Host header when forwarding a request to a backend server. The X-Forwarded-Host header preserves the hostname the client originally targeted, allowing the origin server to generate correct URLs, Redirects, and cookie domains.

A typical scenario involves a CDN or reverse proxy accepting requests for shop.example.re and forwarding them to an internal backend at backend-pool-3.internal. Without the X-Forwarded-Host header, the backend only sees the rewritten host value and produces incorrect absolute URLs in responses.

The standardized replacement is the host parameter of the Forwarded header.

Values

The value is a single hostname, optionally followed by a port number separated by a colon.

X-Forwarded-Host: <host>
X-Forwarded-Host: <host>:<port>

Example

A reverse proxy receives a request targeting shop.example.re and forwards the request to the backend. The proxy sets the X-Forwarded-Host header so the backend knows the original hostname.

X-Forwarded-Host: shop.example.re

When the original request targeted a non-standard port, the port number is included.

X-Forwarded-Host: api.example.re:8443

A backend application generating an absolute redirect URL reads the X-Forwarded-Host value instead of the rewritten Host header to construct the correct Location response.

X-Forwarded-Host: www.example.re
Host: backend-7.internal

Takeaway

The HTTP X-Forwarded-Host header preserves the original hostname from the client request when reverse proxies rewrite the Host header. The host parameter of the Forwarded header is the standardized replacement.

See also

• Forwarded
• Host
• X-Forwarded-For
• X-Forwarded-Proto
• HTTP headers