Error handling in asp.net core (.net8), how do I get URL, form data and client IP

LeoMuller
May 15, 2024
191 views
1 vote
2 Answers

I am creating a new API website in .NET8, which is the latest version of ASP.NET Core

In my program.cs I added this:

builder.Services.AddExceptionHandler<ErrorHandler>();
app.UseExceptionHandler("/Home/Error");

In case of an exception this goes indeed to my custom error handling class:

public class ErrorHandler : IExceptionHandler

I access the extra data I need like this:

private readonly ILogger<ErrorHandler> logger;
private readonly IHttpContextAccessor _httpContextAccessor;

public ErrorHandler(ILogger<ErrorHandler> logger, IHttpContextAccessor httpContextAccessor)
{
    this.logger = logger;
    this._httpContextAccessor = httpContextAccessor;
}

So far so good, but now I try to get some data for my log:

Request URL
Form data (RAW)
Client IP Address
Referrer

    private string GetErrorReport(Exception e)
    {
        StringBuilder sbErrorReport = new StringBuilder(1024);
        try
        {
    
            HttpResponse response = _httpContextAccessor.HttpContext?.Response; 
            HttpRequest request = _httpContextAccessor.HttpContext?.Request; 
            string CurUrl = "";
            string CurReferrer = "";
            string formData = "";
            string clientIP = "";
    
            if(request != null)
            {
                CurUrl = request.GetEncodedPathAndQuery();
                CurReferrer = request.Headers["Referer"].ToString();
                if(request.Body != null)            //is this the same as request.Form.ToString()?
                {
                    using (StreamReader sr = new StreamReader(request.Body))
                    {
                        formData = sr.ReadToEnd();
                    }
                }
    
                if(request.HttpContext != null && request.HttpContext.Connection != null && request.HttpContext.Connection.RemoteIpAddress != null)
                {
                    clientIP = request.HttpContext.Connection.RemoteIpAddress.ToString();
                }
            }
    
        
        }
        catch (Exception ex)
        {
            //nothing?
        }
    
        return sbErrorReport.ToString();
    }

None of them seem to work…

CurUrl gives "/Home/Error", not the original URL that had the error.
CurReferrer gives an error about headers not being available.
formData crashes completely.
clientIP actually seems to work, if ::1 is correct for localhost.

Any ideas how to best write errors with all these details to a log file?

Answers

Chosen as BEST ANSWER

After much help of Emre Bener, thank you for that, I figured it out. My mistake was to try to use the IHttpContextAccessor , whilst everything I needed was already in the TryHandleAsync from the IExceptionHandler.

Here is my code that works: In program.cs:

builder.Services.AddExceptionHandler<ErrorHandler>();   //custom error log, still needs UseExceptionHandler
var app = builder.Build();
app.UseExceptionHandler("/Home/Error");

And in my general exception handling class ErrorHandler.cs:

public class ErrorHandler : IExceptionHandler
{

    public ErrorHandler()
    {

    }

    public ValueTask<bool> TryHandleAsync(
        HttpContext httpContext,
        Exception exception,
        CancellationToken cancellationToken)
    {

        string errorReport = GetErrorReport(exception, httpContext);
        SaveError(errorReport);

        return ValueTask.FromResult(false);
    }



    private string GetErrorReport(Exception e, HttpContext httpContext)
    {
        StringBuilder sbErrorReport = new StringBuilder(1024);
        try
        {

            string errorMessage;
            string CurUrl = "";
            string CurReferrer = "";
            string reqBodyRaw = "";
            string clientIP = "";
            string extra = "";

            var exceptionHandlerFeatures = httpContext.Features.Get<IExceptionHandlerFeature>();

            //error message:
            errorMessage = e.Message;   //also check e.InnerException

            //client IP
            if (httpContext.Connection.RemoteIpAddress != null)
            {
                clientIP = httpContext.Connection.RemoteIpAddress.ToString();
            }

            //Referer / headers:
            CurReferrer = httpContext.Request.Headers["Referer"].ToString();


            //request posted body 
            using (var reader = new StreamReader(httpContext.Request.BodyReader.AsStream(false), Encoding.UTF8))
            {
                reqBodyRaw = reader.ReadToEnd();
            }

            //get the requested URL
            if (exceptionHandlerFeatures != null)
            {
                //CurUrl = httpContext.Request.Path;  //this does not work, gives the error page path.
                CurUrl = exceptionHandlerFeatures.Path; //this does work.
            }

        // write details to sbErrorReport....
            
        }
        catch (Exception ex)
        {
            //nothing?
        }

        return sbErrorReport.ToString();
    }
}

(Edit)

In the error handler action, you can retrieve error details like this:

var exceptionHandlerPathFeature = HttpContext.Features.Get<IExceptionHandlerFeature>();

This will retrieve the error information from the current context. then, you can use exceptionHandlerFeature.Error to retrieve the actual error. for example:

public string? ExceptionMessage { get; set; }

var exceptionHandlerFeature = HttpContext.Features.Get<IExceptionHandlerFeature>();

var error = exceptionHandlerFeature.Error;

You can also read where the error occurred (request URL) from the Path property of exceptionHandlerFeature like this:

if (exceptionHandlerFeature.Path == "/")
{
    ExceptionMessage ??= string.Empty;
    ExceptionMessage += " Page: Home.";
}

HttpContext.Connection.RemoteIpAddress will give you the client IP. In fact, you can get the current user like this:

if (context.User.Identity is { IsAuthenticated: true })
    userName = context.User.Identity.Name;

Note that you will have problems retrieving IP if your server is located behind a load balancer or if there is a proxy in place.

HttpContext.Request.Form will give you the form data.
Request.Headers["Referer"].ToString() will give you the referrer header value.

By the way, you can opt for using an inline handler for error handling, something like this:

if (!app.Environment.IsDevelopment())
{
    app.UseExceptionHandler(exceptionHandlerApp =>
    {
        exceptionHandlerApp.Run(async context =>
        {
            context.Response.StatusCode = StatusCodes.Status500InternalServerError;
            var errorCode = Guid.NewGuid().ToString();
            var exceptionHandlerFeature =
                context.Features.Get<IExceptionHandlerFeature>();

            // do stuff

            context.Response.Redirect("/error?customErrorCode=" + errorCode); // perform 302 redirection with error code in query string
        });
    });

    app.UseHsts(); // you can configure any other non-development features in this if block too, like hsts or https redirection (note that you can do can set up these two in IIS too, although irrelevant)
}

see https://learn.microsoft.com/en-us/aspnet/core/fundamentals/error-handling?view=aspnetcore-8.0 for more info on error handling in asp.net core with the built-in error handler middleware.

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