IIS, Visual Studio, unable to start debugging on the web server.


So this is another logging this issue after some frustrating problem sovling process. This is to help future me when I forget the same windows configuration step in another 8-15 months, maybe it’ll help you too.

On Windows 8 you have go in and turn on many individual items to just get an ASP.NET IIS hosted web app running, I’ve a variation of this problem in the past and blogged about it.

This time round it’s basically the same problem, but with Visual Studio upon trying to spin up and attach to an IIS hosted web project kicks up 1 of 3 errors.


The most common one was:

Unable to start debugging on the web server. The debugger cannot connect to the remote computer. The debugger was unable to resolve the specified computer name.

Which is not helpful, and the posts I found were people trying to remote debug machines or some thing else not helpful.

Unable to start debugging on the web server

Other variations included:

… error occured on a send

Unable to start debugging on the web server.

If you dig further into the windows event store you’ll be sent down an even further wrong path with errors such as:

The program can’t start because SecRuntime.dll is missing from your computer. Try reinstalling the program to fix this problem.


Ensure that you turn on ‘Internet Information Services Hostable Web Core’ along with all the other .NET / IIS feautes you need that I mentioned in the previous post.

Windows Features

RavenDB invalid HTTP Header characters bug

So you’ve done a search and you’ve arrived here, and you’re using RavenDB (at least version 2). In my case 2.5.2907, if this is fixed in 3.0 that’s good but not in our case we’re not ready to move to v3 yet based on our challenges in production with v2 (which is stable enough now).

I’ll try and help you out first (or a future me if I make this mistake again) then I’ll explain more. This may help you also in another place where you’re using HTTP headers and have used invalid characters.

The Error

Specified value has invalid HTTP Header characters

“Error”: “System.ArgumentException: Specified value has invalid HTTP Header characters.
Parameter name: name
at System.Net.WebHeaderCollection.CheckBadChars(String name, Boolean isHeaderValue)
at System.Net.WebHeaderCollection.SetInternal(String name, String value)
at Raven.Database.Extensions.HttpExtensions.WriteHeaders(IHttpContext context, RavenJObject headers, Etag etag)
at Raven.Database.Extensions.HttpExtensions.WriteData(IHttpContext context, Byte[] data, RavenJObject headers, Etag etag)
at Raven.Database.Server.Responders.Document.GetDocumentDirectly(IHttpContext context, String docId)
at Raven.Database.Server.Responders.Document.Respond(IHttpContext context)
at Raven.Database.Server.HttpServer.DispatchRequest(IHttpContext ctx)
at Raven.Database.Server.HttpServer.HandleActualRequest(IHttpContext ctx)”

The Fix

Check for invalid characters as per the Http spec (RFC 2616) Thanks to this StackOverflow answer in finding it in the spec faster.

Good luck, in my case it was an email, the ‘@’ is invalid.

So make sure you’re only storing US-ASCII and not using any of the control characters or separators:

“(” | “)” | “” | “@”
| “,” | “;” | “:” | “\” |
| “/” | “[” | “]” | “?” | “=”
| “{” | “}” | SP | HT

The Details

Using metadata on RavenDB documents is quite helpful and the data stored there so far has been pretty simple, to support a special case the storage of an email was being worked in to the metadata. The nature of this bug in the RavenDB IDE was that when you list all the documents of that collection they show up, and you see their selected fields, but when you click to load the document you get the “Specified value has invalid HTTP Header characters” error, and you’re scratching your head about how the document is in the database but you can’t load it.


I encountered what really feels like a bug, as a developer using the metadata mechanism of Raven documents, it shouldn’t be the responsibility of that developer to ensure they are meeting the specification HTTP headers (see page 16 of RFC 2616).

RavenDB Invalid metadata

This is invalid metadata on a raven document (see the ‘@’)


You do want the tools you use to really help you, and it’s frustrating when something obscure like this happens, it may be more complex or difficult but what I would like to see is a check on the data on its way in, so you can clearly see when this became a problem, instead of hunting it down after the fact.

I raised this on the RavenDB support group, and it’s been since raised as a bug, so if it’ll eventually (hopefully) come in v3.