Requests to third party API from .NET Core app timeout in IIS - iis

Question

I have a .NET Core Web API app running in IIS that can receive and respond to requests, but it needs to make requests to a third party API (Twilio). Those requests succeed when hosted locally but they time out when hosted in IIS.
The .NET Core Web API app is targeting v2.1 and running on Windows Server and IIS 7. I setup a dedicated app pool for the site following guidelines in this article.
App pool settings:
I'm using Postman to make GET and POST requests to the app.
GET and POST requests to a test controller work fine. All I'm doing with these test endpoints is returning the parameters that are received.
[Route("api/[controller]")]
[ApiController]
public class TestController : ControllerBase
{
[HttpGet]
public string Get(int id)
{
return $"response to GET; id:{id}";
}
[HttpPost]
public string Post([FromBody] int id)
{
return $"response to POST; foo:{id}";
}
}
The above endpoints work locally running debug mode in VS2019 using IISExpress, and they also work when deployed to the web server and hosted with IIS7.
The trouble starts when I try to call out to a third party API (Twilio). Again these calls succeed when the app runs locally. There's almost no delay. I post to the SMSController's post endpoint, the request is triggered, and I get a text message within seconds. However, GET and POST requests to these endpoints fails from the web server and IIS.
[Route("api/[controller]")]
[ApiController]
public class SMSController : ControllerBase
{
private string twilioSID = "xxxxxx";
private string twilioAuthToken = "xxxxxx";
private string twilioPhoneNumber = "[phone]";
[HttpGet]
public string Get(string phoneNumber)
{
TwilioClient.Init(twilioSID, twilioAuthToken);
var message = MessageResource.Create(
body: $"TEST message via Twilio",
from: new Twilio.Types.PhoneNumber(twilioPhoneNumber),
to: new Twilio.Types.PhoneNumber(phoneNumber)
);
return message.Status.ToString();
}
[HttpPost]
public string Post([FromBody] GeoEvent geoEvent)
{
TwilioClient.Init(twilioSID, twilioAuthToken);
var message = MessageResource.Create(
body: $"TEST message via Twilio",
from: new Twilio.Types.PhoneNumber(twilioPhoneNumber),
to: new Twilio.Types.PhoneNumber(geoEvent.PhoneNumber)
);
return message.Status.ToString();
}
}
GET and POST requests to the above endpoints get a 500 response when hosted in IIS. The following exception is thrown:
System.Net.Http.HttpRequestException: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond ---> System.Net.Sockets.SocketException: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond
Lastly, here is the web.config that the SDK generated for me when I published the app in VS2019.
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<location path="." inheritInChildApplications="false">
<system.webServer>
<handlers>
<add name="aspNetCore" path="*" verb="*" modules="AspNetCoreModule" resourceType="Unspecified" />
</handlers>
<aspNetCore processPath=".\TwilioService.exe" stdoutLogEnabled="false" stdoutLogFile=".\logs\stdout" />
</system.webServer>
</location>
</configuration>
Neither my local machine nor the web server running IIS are behind a corporate proxy.
Am I missing an IIS or web.config setting that will allow .NET Core to make requests through IIS to a third party?
UPDATE:
I have now reproduced the same issue on a different web server running Windows Server 2016. This machine is setup with .NET 5, IIS 10, and the hosting bundle for .NET 5. I also installed VS2019 and recreated the web api project to run locally on the server. Here are the results:
Running locally in IIS Express, the api controller can successfully
make a request to the Twilio api and gets a response. The SMS
message is sent successfully. No errors.
Running locally in IIS 10, the api controller calls Twilio but
fails to get a response. The following error occurs:
Twilio.Exceptions.ApiConnectionException: Connection Error: POSThttps://api.twilio.com/2010-04-01/Accounts//Messages.json
---> System.Net.Http.HttpRequestException: A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond. (api.twilio.com:443)
To reiterate, both of the above scenarios are running on the same machine. The only difference is IIS Express vs IIS 10. Is this an SSL cert issue? Because IIS 10 works as a reverse proxy for .NET core apps, could there be a problem with a cert getting recognized or forwarded from the .net core app through IIS?

Answer 1

Try to ping the third-party service URL from your machine command prompt.
Make sure the firewall port is set to allow. check the antivirus is now blocking the connection.
try to add the defaultProxy section to my web.config in order to get my website to connect to a web service.
https://learn.microsoft.com/en-US/troubleshoot/dotnet/framework/cannot-consume-web-service-via-http-proxy

Answer 2

Solved. It turned out to be a corporate proxy after all. I had bad information that the web server was not behind a proxy but it was. Twilio has a solution for this exact scenario in their documentation involving the use of a custom proxied rest client class. It's a wrapper around System.Net.HttpClient. The service can now make calls to Twilio through the proxy using this class with the proxy server url set.