ProxyPreserveHost

ProxyPreserveHost is a configuration option in Nginx's HTTP proxy module that governs how the Host header from a client request is treated when the request is proxied to an upstream server. When the directive is enabled, Nginx forwards the original Host header received from the client to the upstream; when it is disabled, Nginx may overwrite the Host header with the hostname of the upstream server or rely on explicit header configuration.

Why it matters: many applications rely on the Host header to perform virtual hosting, tenant routing, or host-based features. Preserving the client’s Host header enables upstream services to see the external domain the user requested, which is important in multi-tenant or domain-based deployments.

Context and usage: proxy_preserve_host can be set in http, server, or location blocks and accepts on or off, with the default being off in many builds. In practice, enabling it is often paired with careful header management—such as using proxy_set_header to specify Host explicitly—to ensure the upstream receives a meaningful value and to avoid exposing internal hostnames.

Relation to other headers: aside from the Host header, reverse proxy configurations may add X-Forwarded-For and X-Forwarded-Proto headers to pass client information. Understanding how proxy_preserve_host interacts with proxy_set_header Host is important when configuring proxies that front multi-tenant or virtual-hosted services.