Nodejs lambda request function gets 411 Length Required - node.js

Question

I use a node.js lambda that makes post requests to remote api, which installed on several remote locations.
I have no access to the remote api code or logs.
The lambda gets called with HTTP gateway by external application which I do not control as well.
It works perfectly for all the location but one. For one location I get this error:
411 Length Required.
I have tried to neutralize the HTTP gateway, and run lambda posts with test events.
I get the same result.
I have sent the same exact request to other locations, and got a response.
I can't find the problem as I do send a ContentLength header.
This is the lambda code:
const https = require("https");
const iconv = require("iconv-lite");
const charset = require("charset");
const qs = require("qs");
const url = require("url");
exports.handler = (event, context, callback) => {
event = JSON.parse(JSON.stringify(event));
let enc ="";
let multiValueHeaders = event["multiValueHeaders"]["Content-Type"];
let PostParams = null;
let domain = event["queryStringParameters"]["domain"] ;
let buf = Buffer.from(JSON.stringify(event["body"]), "base64");
let tstring = buf.toString("utf8");
PostParams = qs.parse(tstring);
var postData = PostParams ? qs.stringify(PostParams) : {};
let ContentLength = new Buffer.from(postData).length;
let headers = "" ;
headers += (multiValueHeaders) ? (' { "Content-Type": "'+ multiValueHeaders + '",') : '{';
headers += ('"Content-Length":'+ ContentLength + '}');
headers = JSON.parse(headers);
var q = url.parse(domain, true);
let options = {
'method': 'POST',
'hostname': q.hostname,
'path': q.pathname,
'headers': {headers}
};
var req = http.request(options, function (res) {
let chunks = [];
res.on("data", function (chunk) {
chunks.push(chunk);
enc = charset(res.headers, chunk);
});
res.on("end", function (chunk) {
var decodedBody = iconv.decode(Buffer.concat(chunks), enc);
const response = {
statusCode: 200,
body: decodedBody
};
callback(null ,response );
});
res.on("error", function (error) {
console.error(error);
});
});
if (PostParams != null) req.write(postData);
req.end();
}
When a request sent to the endpoint straight form postman there is no error. Only from lambda.

Answer 1

Apart from why this event = JSON.parse(JSON.stringify(event));?
Apart from this is a very ugly way to build the headers object:
let headers = "";
headers += (multiValueHeaders) ? (' { "Content-Type": "'+ multiValueHeaders + '",') : '{';
headers += ('"Content-Length":'+ ContentLength + '}');
headers = JSON.parse(headers);
and I would have written as:
const headers = { "Content-Length": ContentLength };
if(multiValueHeaders) headers["Content-Type"] = multiValueHeaders;
The root cause of your problem is in this line:
'headers': {headers}
it needs to be changed in:
'headers': headers
Hope this helps

Answer 2

411 is returned when the server demands a valid Content-Length
event argument passed through the HTTP gateway is the entire client request object. You don't have to parse it.
event.body is an escaped string. Double-escaping it gives the wrong content-length. For example,
JSON.stringify({'double': 2}) !== JSON.stringify(JSON.stringify({'double': 2))
// false
With this in mind, you can perform your request like this:
exports.handler = (event, context, callback) => {
let enc = "";
let multiValueHeaders = event["multiValueHeaders"];
let domain = event["queryStringParameters"]["domain"] ;
const postHeaders = {...multiValueHeaders};
let postData = null;
if (event.body !== null) {
postData = qs.stringify(
qs.parse(
Buffer.from(event.body, "base64").toString("utf-8")
)
);
postHeaders['Content-Length'] = [ Buffer.byteLength(postData) ];
}
var q = url.parse(domain, true);
let options = {
'method': 'POST',
'hostname': q.hostname,
'path': q.pathname,
'headers': postHeaders
};
var req = http.request(options, function (res) {
let chunks = [];
res.on("data", function (chunk) {
chunks.push(chunk);
enc = charset(res.headers, chunk);
});
res.on("end", function (chunk) {
var decodedBody = iconv.decode(Buffer.concat(chunks), enc);
const response = {
statusCode: 200,
body: decodedBody
};
callback(null, response);
});
res.on("error", function (error) {
console.error(error);
});
});
if (postData !== null) req.write(postData);
req.end();
}