Expert Nginx SME for Reverse Proxy Setup
I need expert level nginx SME for setting up a reverse proxy.
Need to add a nginx as a reverse proxy for accessing a site in a restricted network.
I am currently getting cookies error when trying to login to the site.
Your cookie error is most likely caused by the reverse proxy server changing the interaction context between the client and the backend application.
Cookies set by the backend for the internal context (domain, path, protocol) become invalid for the client's browser.
I consider 5 hypotheses to be the most likely in your situation.
Domain attribute mismatchThe backend application sets a Cookie with a Domain attribute that matches its internal address (e.g., internal.local).
The client's browser, which connects via the external address (e.g., proxy.com), rejects this Cookie due to the domain mismatch.
In the project description, you specified access to the website in a «restricted network».
This strongly implies the use of internal domain names that differ from external ones.
By default, Nginx does not modify the Domain attribute in the Set-Cookie header returned by the backend.
The proxy_cookie_domain directive is required to rewrite this attribute correctly.
Secure flagNginx terminates SSL/TLS (accepts HTTPS from the client), but interacts with the backend over insecure HTTP.
The backend is not aware of the original secure connection and perceives the request as HTTP.
This leads to authentication errors because the backend generates incorrect absolute URLs for redirection (using http:// instead of https://) or refuses to establish a session, considering the transport insecure.
Your phrase «Ensure secure access» implies the use of HTTPS.
SSL termination on the reverse proxy is a standard practice.
If Nginx does not forward information about the original protocol (e.g., via the X-Forwarded-Proto: https header), the backend considers the connection insecure, leading to the issues described in 2.1.
Furthermore, if the backend incorrectly perceives the connection as insecure (HTTP), it will likely not set the Secure flag on cookies.
This is problematic because modern browsers require the Secure flag for any cookie marked with SameSite=None (which is often necessary in reverse proxy scenarios).
Host header forwardingNginx forwards an incorrect Host header to the backend (e.g., the upstream server's name or its IP address instead of the external domain name used by the client).
The backend uses this value to generate absolute redirect URLs after login.
Additionally, this value is used as the target origin for Cross-Site Request Forgery (CSRF) protection.
Many web frameworks compare the value of the Host header (target origin) with the values of the Origin and/or Referer headers (source origin) for CSRF protection.
A mismatch, caused by an incorrect proxy_set_header Host configuration, causes this check to fail (e.g., a 403 Forbidden error), resulting in authentication failure.
If the application uses the Host header to set the Domain attribute in the Cookie, this scenario is the root cause of Hypothesis #1.
The backend application sets a cookie with a Path attribute matching its internal path (e.g., /internal-app/).
The client's browser accesses the application via the proxy using a different path (e.g., /).
The browser does not send the cookie in subsequent requests because the current request path does not match the cookie's Path attribute.
Changing the URL structure is a common practice when publishing an internal application via a reverse proxy.
By default, Nginx does not modify the Path attribute in the Set-Cookie header.
The proxy_cookie_path directive is required to rewrite this attribute correctly.
SameSite attributeWhen the reverse proxy configuration causes a change of domain (site), the interaction context, from the browser's perspective, shifts from same-site to cross-site.
Consequently, the browser blocks authentication cookies, as they do not meet the requirements for the cross-site context.
Modern browsers apply the SameSite=Lax policy by default if the SameSite attribute is not explicitly set.
Both Lax and Strict policies prevent cookies from being sent in cross-site requests (e.g., POST requests after a redirect).
Using cookies in a cross-site context requires explicitly setting SameSite=None.
This setting must be accompanied by the Secure flag.
If the backend does not set these attributes correctly, or if they are not modified by Nginx (e.g., using the proxy_cookie_flags directive), authentication will fail.