How to Configure Symfony to Work behind a Load Balancer or a Reverse Proxy

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When you deploy your application, you may be behind a load balancer (e.g. an AWS Elastic Load Balancing) or a reverse proxy (e.g. Varnish for caching).

For the most part, this doesn't cause any problems with Symfony. But, when a request passes through a proxy, certain request information is sent using either the standard Forwarded header or X-Forwarded-* headers. For example, instead of reading the REMOTE_ADDR header (which will now be the IP address of your reverse proxy), the user's true IP will be stored in a standard Forwarded: for="..." header or a X-Forwarded-For header.

If you don't configure Symfony to look for these headers, you'll get incorrect information about the client's IP address, whether or not the client is connecting via HTTPS, the client's port and the hostname being requested.

Solution: setTrustedProxies()

To fix this, you need to tell Symfony which reverse proxy IP addresses to trust and what headers your reverse proxy uses to send information:

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# config/packages/framework.yaml
framework:
    # ...
    # the IP address (or range) of your proxy
    trusted_proxies: '192.0.0.1,10.0.0.0/8'
    # trust *all* "X-Forwarded-*" headers
    trusted_headers: ['x-forwarded-for', 'x-forwarded-host', 'x-forwarded-proto', 'x-forwarded-port', 'x-forwarded-prefix']
    # or, if your proxy instead uses the "Forwarded" header
    trusted_headers: ['forwarded']

5.2

In previous Symfony versions, the above example used HEADER_X_FORWARDED_ALL to trust all "X-Forwarded-" headers, but that constant is deprecated since Symfony 5.2 in favor of the individual HEADER_X_FORWARDED_* constants.

Tip

You can set a TRUSTED_PROXIES env var to configure proxies on a per-environment basis:

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# .env
TRUSTED_PROXIES=127.0.0.1,10.0.0.0/8
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# config/packages/framework.yaml
framework:
    # ...
    trusted_proxies: '%env(TRUSTED_PROXIES)%'

Danger

Enabling the Request::HEADER_X_FORWARDED_HOST option exposes the application to HTTP Host header attacks. Make sure the proxy really sends an x-forwarded-host header.

The Request object has several Request::HEADER_* constants that control exactly which headers from your reverse proxy are trusted. The argument is a bit field, so you can also pass your own value (e.g. 0b00110).

5.2

The feature to configure trusted proxies and headers with trusted_proxies and trusted_headers options was introduced in Symfony 5.2. In earlier Symfony versions you needed to use the Request::setTrustedProxies() method in the public/index.php file.

Caution

The "trusted proxies" feature does not work as expected when using the nginx realip module. Disable that module when serving Symfony applications.

But what if the IP of my Reverse Proxy Changes Constantly!

Some reverse proxies (like AWS Elastic Load Balancing) don't have a static IP address or even a range that you can target with the CIDR notation. In this case, you'll need to - very carefully - trust all proxies.

  1. Configure your web server(s) to not respond to traffic from any clients other than your load balancers. For AWS, this can be done with security groups.

  2. Once you've guaranteed that traffic will only come from your trusted reverse proxies, configure Symfony to always trust incoming request:

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    # config/packages/framework.yaml
framework:
    # ...
    # trust *all* requests (the 'REMOTE_ADDR' string is replaced at
    # run time by $_SERVER['REMOTE_ADDR'])
    trusted_proxies: '127.0.0.1,REMOTE_ADDR'

That's it! It's critical that you prevent traffic from all non-trusted sources. If you allow outside traffic, they could "spoof" their true IP address and other information.

If you are also using a reverse proxy on top of your load balancer (e.g. CloudFront), calling $request->server->get('REMOTE_ADDR') won't be enough, as it will only trust the node sitting directly above your application (in this case your load balancer). You also need to append the IP addresses or ranges of any additional proxy (e.g. CloudFront IP ranges) to the array of trusted proxies.

Reverse proxy in a subpath / subfolder

If your Symfony application runs behind a reverse proxy and it's served in a subpath/subfolder, Symfony might generate incorrect URLs that ignore the subpath/subfolder of the reverse proxy.

To fix this, you need to pass the subpath/subfolder route prefix of the reverse proxy to Symfony by setting the X-Forwarded-Prefix header. The header can normally be configured in your reverse proxy configuration. Configure X-Forwarded-Prefix as trusted header to be able to use this feature.

The X-Forwarded-Prefix is used by Symfony to prefix the base URL of request objects, which is used to generate absolute paths and URLs in Symfony applications. Without the header, the base URL would be only determined based on the configuration of the web server running Symfony, which leads to incorrect paths/URLs, when the application is served under a subpath/subfolder by a reverse proxy.

For example if your Symfony application is directly served under a URL like https://symfony.tld/ and you would like to use a reverse proxy to serve the application under https://public.tld/app/, you would need to set the X-Forwarded-Prefix header to /app/ in your reverse proxy configuration. Without the header, Symfony would generate URLs based on its server base URL (e.g. /my/route) instead of the correct /app/my/route, which is required to access the route via the reverse proxy.

The header can be different for each reverse proxy, so that access via different reverse proxies served under different subpaths/subfolders can be handled correctly.

Custom Headers When Using a Reverse Proxy

Some reverse proxies (like CloudFront with CloudFront-Forwarded-Proto) may force you to use a custom header. For instance you have Custom-Forwarded-Proto instead of X-Forwarded-Proto.

In this case, you'll need to set the header X-Forwarded-Proto with the value of Custom-Forwarded-Proto early enough in your application, i.e. before handling the request:

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// public/index.php

// ...
$_SERVER['HTTP_X_FORWARDED_PROTO'] = $_SERVER['HTTP_CUSTOM_FORWARDED_PROTO'];
// ...
$response = $kernel->handle($request);
This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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