What does the X-Forwarded-For note in the docs mean?

Question

The docs:  
https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-mapping-template-reference.html  
  
say that:   
  
**$context.identity.sourceIp** — The source IP address of the TCP connection making the request to API Gateway. Warning. You should not trust this value if there is any chance that the **X-Forwarded-For** header could be forged.  
  
What does that even mean?  
Anyone could "forge" the **X-Forwarded-For** header easily.  
So what? Does it mean, that **$context.identity.sourceIp** shouldn't be trusted at all? Why are the docs so elusive about it? The whole internet is using **$context.identity.sourceIp**, and you're saying that it doesn't actually guarantee anything? Why not remove this property entirely then, in order to stop confusing people.

Answer

As stated in documentation, $context.identity.sourceIp is the source IP address of the TCP connection making the request to API Gateway.  
  
Whenever X-Forwarded-For header is available, the source IP would come from it, but we don't trust all entries in the header. We only trust the entry who made a call directly to endpoint.  
  
If the call chain is like this,  
```  
ClientIp, \[proxyServer1, ..., proxyServerN], CloudFront/Regional  
```  
  
proxyServerN would be used for the sourceIp address, when X-Forwarded-For header is available.  
  
CloudFront adds itself to the X-Forwarded-For header but Regional endpoint API/Regional custom domain name does not add itself to the X-Forwarded-For header. Therefore, In case of CloudFront, IP address appended by CloudFront would not be used as sourceIp, but the IP of the last proxy prior to CloudFront IP in X-Forwarded-For header. In case of Regional,  the last IP address in X-Forwarded-For header would be used as sourceIp.  
  
Please ignore the warning in the documentation. The team is aware of it and the documentation would be updated.

Answer

I think the note is from this thread https://forums.aws.amazon.com/message.jspa?messageID=669697.  
At that time, the value of $context.identity.sourceIp is came from X-Forwarded-For header but it is fixed now.  
AWS team should remove that note.

What does the X-Forwarded-For note in the docs mean?

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