I’m trying to call an API in my .NET Core web app project but it returns these errors as a response:

1. 504 Gateway Timeout
2. 502 Bad Gateway

Although, this API responds correctly using Postman and CURL.

I investigated all the headers of Postman and CURL in Fiddler and async them in my code, tried different HTTP Client Libraries like ‘RestSharp’ and ‘HttpClient’, even tested a simplified code in a raw .NET Core console application, ran the code on two different local machines.

public static async Task GetQuote()
{
    try
    {
        var content = "<?xml version="1.0" encoding="ISO-8859-1"?><job><job_action>QUOTE</job_action></job>";

        using (var handler = new HttpClientHandler())
        {
            handler.Proxy = null; // Explicitly set proxy to null
            handler.UseProxy = false; // Ensure proxy is disabled
            handler.SslProtocols = System.Security.Authentication.SslProtocols.Tls12; // Ensure correct SSL/TLS protocol

            using (var client = new HttpClient(handler))
            {
                client.Timeout = TimeSpan.FromMinutes(5); // Increase timeout duration
                // Additional Headers to match POSTMAN request
                client.DefaultRequestHeaders.Accept.Clear();
                client.DefaultRequestHeaders.Accept.Add(new MediaTypeWithQualityHeaderValue("*/*"));
                client.DefaultRequestHeaders.AcceptEncoding.Add(new StringWithQualityHeaderValue("gzip"));
                client.DefaultRequestHeaders.AcceptEncoding.Add(new StringWithQualityHeaderValue("deflate"));
                client.DefaultRequestHeaders.AcceptEncoding.Add(new StringWithQualityHeaderValue("br"));
                client.DefaultRequestHeaders.Connection.Add("keep-alive");
                client.DefaultRequestHeaders.CacheControl = new CacheControlHeaderValue
                {
                    NoCache = true
                };

                var form = new MultipartFormDataContent();
                var byteArray = Encoding.GetEncoding("ISO-8859-1").GetBytes(content);
                var byteContent = new ByteArrayContent(byteArray);
                byteContent.Headers.ContentType = new MediaTypeHeaderValue("application/xml");

                form.Add(byteContent, "jobxml", "job_quote.xml");

                var response = await client.PostAsync("https://www.apiServer.com/online/getxmljob.cl", form);
                Console.WriteLine("Response Status Code: " + response.StatusCode);
                response.EnsureSuccessStatusCode();

                var responseBody = await response.Content.ReadAsStringAsync();
                Console.WriteLine("Response Body: " + responseBody);
            }
        }
    }
    catch (HttpRequestException ex)
    {
        Console.WriteLine($"HttpRequestException: {ex.Message}");
        Console.WriteLine($"Status Code: {ex.StatusCode}");
    }
    catch (TaskCanceledException ex)
    {
        Console.WriteLine("TaskCanceledException: Request timed out.");
        Console.WriteLine($"Exception: {ex.Message}");
    }
    catch (Exception ex)
    {
        Console.WriteLine($"Exception: {ex.Message}");
    }
}

I did everything I knew but I’m still encountering a ‘504 Gateway Timeout’ error.

Unfortunately, the API server isn’t under my control and I can’t check the server-side logs for more detailed error information.

Is there anything that I can check and I haven’t done it yet?

Any Clues and thoughts can save me.