Question

When running an Express app behind a proxy, set (by using app.set()) the application variable trust proxy to one of the values listed in the following table.

Although the app will not fail to run if the application variable trust proxy is not set, it will incorrectly register the proxy’s IP address as the client IP address unless trust proxy is configured.

Type	Value
Boolean	If true, the client’s IP address is understood as the left-most entry in the X-Forwarded-* header. If false, the app is understood as directly facing the Internet and the client’s IP address is derived from req.connection.remoteAddress. This is the default setting.
IP addresses	An IP address, subnet, or an array of IP addresses and subnets to trust. 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 You can set IP addresses in any of the following ways: 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 When specified, the IP addresses or the subnets are excluded from the address determination process, and the untrusted IP address nearest to the application server is determined as the client’s IP address.
Number	Trust the nth hop from the front-facing proxy server as the client.
Function	Custom trust implementation. Use this only if you know what you are doing. app.set('trust proxy', function (ip) { if (ip === '127.0.0.1' || ip === '123.123.123.123') return true // trusted IPs else return false })

Enabling trust proxy will have the following impact:

• The value of req.hostname is derived from the value set in the X-Forwarded-Host header, which can be set by the client or by the proxy.

• X-Forwarded-Proto can be set by the reverse proxy to tell the app whether it is https or http or even an invalid name. This value is reflected by req.protocol.

• The req.ip and req.ips values are populated with the list of addresses from X-Forwarded-For.

The trust proxy setting is implemented using the proxy-addr package. For more information, see its documentation.