asp.net - Override host of webapi odata links

Question

I'm using WebAPI 2.2 and Microsoft.AspNet.OData 5.7.0 to create an OData service that supports paging.

When hosted in the production environment, the WebAPI lives on a server that is not exposed externally, hence the various links returned in the OData response such as the @odata.context and @odata.nextLink point to the internal IP address e.g. http://192.168.X.X/<AccountName>/api/... etc.

I've been able to modify the Request.ODataProperties().NextLink by implementing some logic in each and every ODataController method to replace the internal URL with an external URL like https://account-name.domain.com/api/..., but this is very inconvenient and it only fixes the NextLinks.

Is there some way to set an external host name at configuration time of the OData service? I've seen a property Request.ODataProperties().Path and wonder if it's possible to set a base path at the config.MapODataServiceRoute("odata", "odata", GetModel()); call, or in the GetModel() implementation using for instance the ODataConventionModelBuilder?

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UPDATE: The best solution I've come up with so far, is to create a BaseODataController that overrides the Initialize method and checks whether the Request.RequestUri.Host.StartsWith("beginning-of-known-internal-IP-address") and then do a RequestUri rewrite like so:

var externalAddress = ConfigClient.Get().ExternalAddress;  // e.g. https://account-name.domain.com
var account = ConfigClient.Get().Id;  // e.g. AccountName
var uriToReplace = new Uri(new Uri("http://" + Request.RequestUri.Host), account);
string originalUri = Request.RequestUri.AbsoluteUri;
Request.RequestUri = new Uri(Request.RequestUri.AbsoluteUri.Replace(uriToReplace.AbsoluteUri, externalAddress));
string newUri = Request.RequestUri.AbsoluteUri;
this.GetLogger().Info($"Request URI was rewritten from {originalUri} to {newUri}");

This perfectly fixes the @odata.nextLink URLs for all controllers, but for some reason the @odata.context URLs still get the AccountName part (e.g. https://account-name.domain.com/AccountName/api/odata/$metadata#ControllerName) so they still don't work.

Answer 1

Rewriting the RequestUri is sufficient to affect @odata.nextLink values because the code that computes the next link depends on the RequestUri directly. The other @odata.xxx links are computed via a UrlHelper, which is somehow referencing the path from the original request URI. (Hence the AccountName you see in your @odata.context link. I've seen this behavior in my code, but I haven't been able to track down the source of the cached URI path.)

Rather than rewrite the RequestUri, we can solve the problem by creating a CustomUrlHelper class to rewrite OData links on the fly. The new GetNextPageLink method will handle @odata.nextLink rewrites, and the Link method override will handle all other rewrites.

public class CustomUrlHelper : System.Web.Http.Routing.UrlHelper
{
    public CustomUrlHelper(HttpRequestMessage request) : base(request)
    { }

    // Change these strings to suit your specific needs.
    private static readonly string ODataRouteName = "ODataRoute"; // Must be the same as used in api config
    private static readonly string TargetPrefix = "http://localhost:8080/somePathPrefix"; 
    private static readonly int TargetPrefixLength = TargetPrefix.Length;
    private static readonly string ReplacementPrefix = "http://www.contoso.com"; // Do not end with slash

    // Helper method.
    protected string ReplaceTargetPrefix(string link)
    {
        if (link.StartsWith(TargetPrefix))
        {
            if (link.Length == TargetPrefixLength)
            {
                link = ReplacementPrefix;
            }
            else if (link[TargetPrefixLength] == '/')
            {
                link = ReplacementPrefix + link.Substring(TargetPrefixLength);
            }
        }

        return link;
    }

    public override string Link(string routeName, IDictionary<string, object> routeValues)
    {
        var link = base.Link(routeName, routeValues);

        if (routeName == ODataRouteName)
        {
            link = this.ReplaceTargetPrefix(link);
        }

        return link;
    }

    public Uri GetNextPageLink(int pageSize)
    {
        return new Uri(this.ReplaceTargetPrefix(this.Request.GetNextPageLink(pageSize).ToString()));
    }
}

Wire-up the CustomUrlHelper in the Initialize method of a base controller class.

public abstract class BaseODataController : ODataController
{
    protected abstract int DefaultPageSize { get; }

    protected override void Initialize(System.Web.Http.Controllers.HttpControllerContext controllerContext)
    {
        base.Initialize(controllerContext);

        var helper = new CustomUrlHelper(controllerContext.Request);
        controllerContext.RequestContext.Url = helper;
        controllerContext.Request.ODataProperties().NextLink = helper.GetNextPageLink(this.DefaultPageSize);
    }

Note in the above that the page size will be the same for all actions in a given controller class. You can work around this limitation by moving the assignment of ODataProperties().NextLink to the body of a specific action method as follows:

var helper = this.RequestContext.Url as CustomUrlHelper;
this.Request.ODataProperties().NextLink = helper.GetNextPageLink(otherPageSize);