HTTP headers and Classic Load Balancers

HTTP requests and HTTP responses use header fields to send information about the HTTP messages. Header fields are colon-separated name-value pairs that are separated by a carriage return (CR) and a line feed (LF). A standard set of HTTP header fields is defined in RFC 2616, Message Headers. There are also non-standard HTTP headers available (and automatically added) that are widely used by the applications. Some of the non-standard HTTP headers have an X-Forwarded prefix. Classic Load Balancers support the following X-Forwarded headers.

For more information about HTTP connections, see Request routing in the Elastic Load Balancing User Guide.

Prerequisites

• Confirm that your listener settings support the X-Forwarded headers. For more information, see Listener configurations for Classic Load Balancers.

• Configure your web server to log client IP addresses.

X-Forwarded-For

The X-Forwarded-For request header is automatically added and helps you identify the IP address of a client when you use an HTTP or HTTPS load balancer. Because load balancers intercept traffic between clients and servers, your server access logs contain only the IP address of the load balancer. To see the IP address of the client, use the X-Forwarded-For request header. Elastic Load Balancing stores the IP address of the client in the X-Forwarded-For request header and passes the header to your server. If the X-Forwarded-For request header is not included in the request, the load balancer creates one with the client IP address as the request value. Otherwise, the load balancer appends the client IP address to the existing header and passes the header to your server. The X-Forwarded-For request header may contain multiple IP addresses that are comma separated. The left-most address is the client IP where the request was first made. This is followed by any subsequent proxy identifiers, in a chain.

The X-Forwarded-For request header takes the following form:

X-Forwarded-For: client-ip-address

The following is an example X-Forwarded-For request header for a client with an IP address of 203.0.113.7.

X-Forwarded-For: 203.0.113.7

The following is an example X-Forwarded-For request header for a client with an IPv6 address of 2001:DB8::21f:5bff:febf:ce22:8a2e.

X-Forwarded-For: 2001:DB8::21f:5bff:febf:ce22:8a2e

X-Forwarded-Proto

The X-Forwarded-Proto request header helps you identify the protocol (HTTP or HTTPS) that a client used to connect to your load balancer. Your server access logs contain only the protocol used between the server and the load balancer; they contain no information about the protocol used between the client and the load balancer. To determine the protocol used between the client and the load balancer, use the X-Forwarded-Proto request header. Elastic Load Balancing stores the protocol used between the client and the load balancer in the X-Forwarded-Proto request header and passes the header along to your server.

Your application or website can use the protocol stored in the X-Forwarded-Proto request header to render a response that redirects to the appropriate URL.

The X-Forwarded-Proto request header takes the following form:

X-Forwarded-Proto: originatingProtocol

The following example contains an X-Forwarded-Proto request header for a request that originated from the client as an HTTPS request:

X-Forwarded-Proto: https

X-Forwarded-Port

The X-Forwarded-Port request header helps you identify the destination port that the client used to connect to the load balancer.