UPDATE: Solved 
See my reply to this post, below.
Original post
Hello, I’ve been experiencing strange behavior from my Netlify function from my deploy-preview. I’m totally stumped and would appreciate any help.
tl;dr
When a Netlify function receives a POST request from the Slack Event API with the following header, Netlify interprets the request as a GET request. Is there any way to correct or circumvent this problem?
'User-Agent: Slackbot 1.0 (+https://api.slack.com/robots)'
The full story:
My function below handles the Slack challenge without fail when I run it locally using an ngrok tunnel.
exports.handler = async (event, context) => {
console.log('EVENT', { event })
const { body } = event
const data = body ? JSON.parse(body) : { challenge: 'none' }
console.log('DATA', { data })
return {
statusCode: 200,
headers: {
'Content-type': 'text/plain',
},
body: data.challenge,
}
}
However my function fails when run from the deploy, in preview or production. Slack returns the following message:
Our Request:
POST
"body": {
"type": "url_verification",
"token": "eiX__obscured_by_me__916",
"challenge": "MMoUApArQ6VWQO7TElAhp2nEGXxztQrGpcUeVdfEW9neGEbbLpX1"
}
Your Response:
"code": 200
"error": "challenge_failed"
"body": {
<html><head></head><body><pre style="word-wrap: break-word; white-space: pre-wrap;">none</pre></body></html>
}
Looking at the body from my function’s response, it appears as though it received as a GET. I confirmed this by looking at the function’s log from the deploy:
8:07:05 PM: 2021-03-05T02:07:05.717Z ba94b36d-54b4-41ca-83d8-a59c69d6195f INFO EVENT {
event: {
path: '/.netlify/functions/tester-text',
httpMethod: 'GET',
headers: {
accept: '*/*',
'accept-encoding': 'gzip',
'client-ip': '2001:470:12b:3:0:2ce:fc90:6e5',
cookie: '__cfduid=d210a36bef68471de0c744646845d3cbd1614191895; experimentation_subject_id=eyJfcmFpbHMiOnsibWVzc2FnZSI6IklqRTFNREkyTnpZMUxUUTRNalF0TkRFMFl5MWhPVEUzTFRnM04yWTRNemxtTlRnd01TST0iLCJleHAiOm51bGwsInB1ciI6ImNvb2tpZS5leHBlcmltZW50YXRpb25fc3ViamVjdF9pZCJ9fQ%3D%3D--3db436929a6121bc7f6f9213cc34aa1a98b2aad9',
host: 'deploy-preview-758--seeds.netlify.app',
'user-agent': 'Discourse Forum Onebox v2.7.0.beta4',
via: 'https/1.1 Netlify[14218673-7aa6-4b45-9c99-daa848251ad4] (Netlify Edge Server)',
'x-bb-ab': '0.976536',
'x-bb-client-request-uuid': '14218673-7aa6-4b45-9c99-daa848251ad4-53456025',
'x-bb-ip': '2001:470:12b:3:0:2ce:fc90:6e5',
'x-bb-loop': '1',
'x-country': 'US',
'x-forwarded-for': '2001:470:12b:3:0:2ce:fc90:6e5',
'x-forwarded-proto': 'https',
'x-nf-client-connection-ip': '2001:470:12b:3:0:2ce:fc90:6e5',
'x-nf-request-id': '14218673-7aa6-4b45-9c99-daa848251ad4-53456025'
},
multiValueHeaders: {
Accept: [Array],
'Accept-Encoding': [Array],
'Client-Ip': [Array],
Cookie: [Array],
'User-Agent': [Array],
Via: [Array],
'X-Bb-Ab': [Array],
'X-Bb-Client-Request-Uuid': [Array],
'X-Bb-Ip': [Array],
'X-Bb-Loop': [Array],
'X-Country': [Array],
'X-Forwarded-For': [Array],
'X-Forwarded-Proto': [Array],
'X-Nf-Client-Connection-Ip': [Array],
'X-Nf-Request-Id': [Array],
host: [Array]
},
queryStringParameters: {},
multiValueQueryStringParameters: {},
body: '',
isBase64Encoded: true
}
}
8:07:05 PM: 2021-03-05T02:07:05.717Z ba94b36d-54b4-41ca-83d8-a59c69d6195f INFO DATA { data: { challenge: 'none' } }
At Slack support’s suggestion, I used a listener hook at https://dev.oscato.com/ to generate an HTTP inspector to look at the request composition from Slack’s challenge request:
Method
[POST]
Headers
Accept: */*
Accept-Encoding: gzip,deflate
Connection: close
Content-Length: 129
Content-Type: application/json
User-Agent: Slackbot 1.0 (+https://api.slack.com/robots)
X-Slack-Request-Timestamp: 1614870550
X-Slack-Signature: v0=045d5e7aad57b99767ae25377627ed2264b353a658a2212f44e92964f4afe3a7
Body
{
"token": "eiX__obscured_by_me__916",
"challenge": "6O5sHgTpfsMYP3lEYzFkIeTvR0Gmzv2CyhnUKDQgVHu8qjjf9JJt",
"type": "url_verification"
}
Experimenting with curl, I discovered that the User-Agent header is causing the false GET interpretation by Netlify.
curl -X POST \
'https://deploy-preview-758--seeds.netlify.app/.netlify/functions/tester-text' \
-H 'User-Agent: Slackbot 1.0 (+https://api.slack.com/robots)' \
-d '{"token":"eiX__obscured_by_me__916","challenge":"Mbg6GreKIIMXL0Y5Yr64gCpvs2VKj9Ly88265qkSBy9U6jdZqJKE","type":"url_verification"}'
When this header is removed or set to another value, the request is properly interpreted as a POST.
I would greatly appreciate any help in resolving this problem.
Thanks!
Maurice
