Which system’s IP will be a part of this header?

An application hosted at the EC2 instance receives an HTTP request from ELB. The same request has an X-Forwarded-For header, which has three IP addresses.

Which system’s IP will be a part of this header?
A .  Previous Request IP address.
B .  Client IP address.
C .  All of the answers listed here.
D .  Load Balancer IP address.

View Answer

Answer: C

Explanation:

When a user sends a request to ELB over HTTP/HTTPS, the request header log at the instance will only receive the IP of ELB. This is because ELB is the interceptor between the EC2 instance and the client request. To get the client IP, use the header X-Forwarded-For in header. The client IP address in the X-Forwarded-For request header is followed by the IP addresses of each successive proxy that passes along the request. The last IP address is the IP address that connects to the back-end application instance. e.g. if the HTTP request already has a header when it reaches the Load Balancer, the IP address from which the request came is appended at the end of the header followed by the IP address of the Load Balancer. In such cases, the X-Forwarded-For request header takes the following form: X-Forwarded-For: clientIPAddress, previousRequestIPAddress, LoadBalancerIPAddress.

Reference: http://docs.aws.amazon.com/ElasticLoadBalancing/latest/DeveloperGuide/TerminologyandKeyConcepts.html