ASM monitor error Unrecognized content encoding type

Products

• CA App Synthetic Monitor

Issue/Introduction

ASM monitoring is showing following error.

Description:     9261: Unrecognized content encoding type. libcurl understands deflate, gzip, br, zstd content encodings. (Unrecognized or bad HTTP Content or Transfer-Encoding)

This error occurs when we try to monitor a site with OPMS version 24.7.1.  This site is an application. Have tried several different header combination from others having similar issues with Curl.

Headers when running it from browser (currently running with these headers):
accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,/;q=0.8,application/signed-exchange;v=b3;q=0.7
accept-encoding: gzip, deflate, br, zstd
accept-language: en-US,en;q=0.9

Cause

It looks like improperly configured web/app server.

The Detailed analysis page shows.

Server response
Page content

HTTP/1.1 200 OK
Content-Length: 4596
Content-Type: text/html
Content-Encoding: UTF-8
Server: Microsoft-IIS/7.0
X-Powered-By: ASP.NET

Server is sending an invalid value for Content-Encoding (based on above response from the server).  

Content-Encoding: UTF-8

The valid values are listed here: 

https://www.iana.org/assignments/http-parameters/http-parameters.xhtml#content-coding

Resolution

As a workaround, we suggest trying a JMeter monitor for this monitor. Try different versions of JMeter (in ASM Downloads section) and different HTTP client versions. Using JMeter monitor helped resolve this issue.