---
layout: page
title: Express detrás de proxies
description: Learn how to configure Express.js applications to work correctly behind reverse proxies, including using the trust proxy setting to handle client IP addresses.
menu: guide
redirect_from: "  "
---

# Express detrás de proxies

When running an Express app behind a reverse proxy, some of the Express APIs may return different values than expected. In order to adjust for this, the `trust proxy` application setting may be used to expose information provided by the reverse proxy in the Express APIs. The most common issue is express APIs that expose the client's IP address may instead show an internal IP address of the reverse proxy.

<div class="doc-box doc-info" markdown="1">
When configuring the `trust proxy` setting, it is important to understand the exact setup of the reverse proxy. Since this setting will trust values provided in the request, it is important that the combination of the setting in Express matches how the reverse proxy operates.
</div>

The application setting `trust proxy` may be set to one of the values listed in the following table.

<table class="doctable" border="1" markdown="1">
  <thead><tr><th>Type</th><th>Value</th></tr></thead>
  <tbody>
    <tr>
      <td>Booleano</td>
<td markdown="1">
Si es `true`, la dirección IP del cliente se entiende como la entrada más a la izquierda en la cabecera `X-Forwarded-*`.

Si es `false`, la aplicación se entiende como orientada directamente a Internet, y la dirección IP del cliente se obtiene de `req.connection.remoteAddress`. Este es el valor predeterminado.

<div class="doc-box doc-warn" markdown="1">
When setting to `true`, it is important to ensure that the last reverse proxy trusted is removing/overwriting all of the following HTTP headers: `X-Forwarded-For`, `X-Forwarded-Host`, and `X-Forwarded-Proto`, otherwise it may be possible for the client to provide any value.
</div>
</td>
    </tr>
    <tr>
      <td>IP addresses</td>
<td markdown="1">
An IP address, subnet, or an array of IP addresses and subnets to trust as being a reverse proxy. The following list shows the pre-configured subnet names:

- loopback - `127.0.0.1/8`, `::1/128`
- linklocal - `169.254.0.0/16`, `fe80::/10`
- uniquelocal - `10.0.0.0/8`, `172.16.0.0/12`, `192.168.0.0/16`, `fc00::/7`

Puede establecer direcciones IP de varias formas:

```js
app.set('trust proxy', 'loopback') // specify a single subnet
app.set('trust proxy', 'loopback, 123.123.123.123') // specify a subnet and an address
app.set('trust proxy', 'loopback, linklocal, uniquelocal') // specify multiple subnets as CSV
app.set('trust proxy', ['loopback', 'linklocal', 'uniquelocal']) // specify multiple subnets as an array
```

Cuando se especifican, las direcciones IP o las subredes se excluyen del proceso de determinación de direcciones, y la dirección IP no de confianza más próxima al servidor de aplicaciones se establece como la dirección IP del cliente. This works by checking if `req.socket.remoteAddress` is trusted. If so, then each address in `X-Forwarded-For` is checked from right to left until the first non-trusted address.

</td>
    </tr>
    <tr>
      <td>Number</td>
<td markdown="1">
Use the address that is at most `n` number of hops away from the Express application. `req.socket.remoteAddress` is the first hop, and the rest are looked for in the `X-Forwarded-For` header from right to left. A value of `0` means that the first untrusted address would be `req.socket.remoteAddress`, i.e. there is no reverse proxy.

<div class="doc-box doc-warn" markdown="1">
When using this setting, it is important to ensure there are not multiple, different-length paths to the Express application such that the client can be less than the configured number of hops away, otherwise it may be possible for the client to provide any value.
</div>
</td>
    </tr>
    <tr>
      <td>Function</td>
<td markdown="1">
Custom trust implementation.

```js
app.set('trust proxy', (ip) => {
  if (ip === '127.0.0.1' || ip === '123.123.123.123') return true // trusted IPs
  else return false
})
```

</td>
    </tr>
  </tbody>
</table>

Enabling `trust proxy` will have the following impact:

<ul>
  <li markdown="1">El valor de [req.hostname](/{{ page.lang }}/api.html#req.hostname) se obtiene del valor definido en la cabecera `X-Forwarded-Host`, que puede estar establecido por el cliente o el proxy.
  </li>
  <li markdown="1">El proxy inverso puede establecer `X-Forwarded-Proto` para indicar a la aplicación si es `https`, `http` o incluso un nombre no válido. [req.protocol](/{{ page.lang }}/api.html#req.protocol) refleja este valor.
  </li>
  <li markdown="1">Los valores [req.ip](/{{ page.lang }}/api.html#req.ip) y [req.ips](/{{ page.lang }}/api.html#req.ips) se rellenan con la lista de direcciones de `X-Forwarded-For`.
  </li>
</ul>

El valor `trust proxy` se implementa utilizando el paquete [proxy-addr](https://www.npmjs.com/package/proxy-addr). For more information, see its documentation.
