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amazon web services - How to properly set AWS inbound rules to accept response from external REST API call

My use case
I have an AWS lambda hosted function that calls an external API. In my case it is Trello's terrific and well-defined API.

My problem in a nutshell - TL;DR Option: Feel Free to Jump to Statement Below
I had my external API call to Trello working properly. Now it is not working. I suspect I changed networking permissions within AWS that now block the returned response from the service provider. Details to follow.

My testing
I have tested my call to the API using Postman, so I know I have a well-formed request and a useful returned response from the service provider. The business logic is OK. For reference, here is the API call I am using. I have obfuscated my key and token for obvious reasons:

https://api.trello.com/1/cards?key=<myKey>&token=<myToken&idList=<a_real_list_here>&name=New+cards+are+cool

This should put a new card on my Trello board, and in POSTMAN (running on my local machine) it does so successfully. In fact, I had this working in an AWS lambda function I recently deployed. Here is the call. (Note that I'm using the recommended urllib3 library recommended by AWS:

    http.request("POST", "https://api.trello.com/1/cards?key=<myKey>&token=<myToken>&idList=<a_real_list_here>&name="+card_name+"&desc="+card_description)

Furthermore, I have tested the same capability a CURL version of that same request. It is formed like this:

    curl --location --request POST 'https://api.trello.com/1/cards?key=338d5b193d43e95712005fd2bcb4cd12&token=d0e3c4cd6281f43e4ec257ae5f05cd902230cbbca7e26b99664cd620f6479f7a&idList=600213811e171376755c7ed5&name=New+cards+are+cool'

I can summarize the behavior like this

+------------+---------------+----------------------+---------------+
|            | Local Machine | Previously on Lambda | Now on Lambda |
+------------+---------------+----------------------+---------------+
| cURL       |     GOOD      |         GOOD         |      N/A      |
+------------+---------------+----------------------+---------------+
| HTTP POST  |     GOOD      |         GOOD         |    443 Error  |
+------------+---------------+----------------------+---------------+

Code and Errors
I am not getting a debuggable response. I get a 443, which I presume is the error code, but even that is not clear. Here is the code snippet:

#send to trello board 
try: 
    http.request("<post string from above>") 
except: 
    logger.debug("<post string from above>")

The code never seems to get to the logger.debug() call. I get this in the AWS log:

[DEBUG] 2021-01-19T21:56:24.757Z 729be341-d2f7-4dc3-9491-42bc3c5d6ebf 
Starting new HTTPS connection (1): api.trello.com:443 

I presume the "Starting New HTTPS connection..." log entry is coming fromurllib3 libraries

PROBLEM SUMMARY
I know from testing that my actual API call to the external service is properly formed. At one point it was working well, but now it is not. Previously, in order to get it to work well, I had to fiddle with AWS permissions to allow the response to come back from the service provider. I did it, but I didn't fully understand what I did and I think I was just lucky. Now it's broken and I want to do it in a thoughtful way.

What I'm looking for is an understanding of how to set up the AWS permission structure to enable that return message from the service provider. AWS provides a comprehensive guide to how to use the API Gateway to give others access to services hosted on AWS, but it's much more sketchy about how to open permissions for responses from other service providers.

Thanks to the folks at Hava, I have this terrific diagram of the permissions in place for my AWS infrastructure: Security Structure The two nets marked in red are unrelated to this project. The first green check points to one of my EC2 machines and the second points to a related security group.

I'm hoping the community can help me to understand what the key permission elements (IAM roles, security groups, etc) are in play and what I need to look for in the related AWS permissions/networking/security structure.


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1 Answer

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by (71.8m points)

As the lambda is in your VPC you need to make extra configurations to allow it to communicate beyond the VPC as the lambda runner does not have a public IP. Thus you'll need an internet or NAT gateway as described here: https://aws.amazon.com/premiumsupport/knowledge-center/internet-access-lambda-function/

You'll need either additional managed services or infrastructure running a NAT gateway.


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