Change the Urls of OData V4 when running behind reverse proxy

[kvskranthikumar]

With DotNet Core WebAPI I would like to change the OData URLs in the output. API is running behind a reverse proxy and I want all the URLs in the OData output to be relative to the gateway.

Current Output from API -

{
    "@odata.type": "#Company.Users.Models.User",
    "@odata.id": "http://localhost:7004/Users/api/v1/odata/Users(1)",
    "@odata.editLink": "http://localhost:7004/Users/api/v1/odata/Users(1)",
    "userId": 1,
    "userName": "Test",
    "email": null,
    "isWindowsUser": false,
    "isLocked": false,
    "createDate@odata.type": "#DateTimeOffset",
    "createDate": "2019-05-20T07:46:59.187+05:30",
    "lastLoginDate@odata.type": "#DateTimeOffset",
    "lastLoginDate": "2019-05-22T15:52:11.437+05:30",
    "lastPwdChangeDate": null,
    "failedPwdAttemptCount": null,
    "failedPwdAttemptWinStart": null,
    "failedPwdAnswerAttemptCount": null,
    "failedPwdAnswerAttemptWinStart": null,
    "lastLockedOutDate": null,
    "ipAddress": null,
    "userComment": "Test User",
    "userRoles@odata.associationLink": "http://localhost:7004/Users/api/v1/odata/Users(1)/userRoles/$ref",
    "userRoles@odata.navigationLink": "http://localhost:7004/Users/api/v1/odata/Users(1)/userRoles"
}

Change the url - "http://localhost:7004/Users/api/v1/odata" to "http://localhost:7000/Users"
I have already gone through this link - https://issues.oasis-open.org/browse/ODATA-527 but this doesn't provide details on how to make it work.

[MJeorrett]

+1 I see that this was added to vNext (all be it a year ago) but would be good to know if there is any workaround in the mean time?

Thanks

Matthew

[Mhirji]

If you are asking to change the host and port, I believe you can do this using the BaseAddressFactory on the ODataOutputFormatter and ODataInputFormatter. Something like this:

services.AddMvcCore(
options =>
{
foreach ( var outputFormatter in options.OutputFormatters.OfType() )
{
outputFormatter.BaseAddressFactory = ( m ) =>
{
var std = ODataInputFormatter.GetDefaultBaseAddress( m );
if ( HostingEnvironment.IsDevelopment() )
{
return new Uri( std.ToString().Replace( m.Scheme + "://" + m.Host.Host + ":" + m.Host.Port, Configuration["Host"], StringComparison.InvariantCultureIgnoreCase ) );
}

                        return new Uri( std.ToString().Replace( m.Scheme + "://" + m.Host.Host, Configuration["Host"], StringComparison.InvariantCultureIgnoreCase ) );
                    };
                }

                foreach ( var inputFormatter in options.InputFormatters.OfType<ODataInputFormatter>() )
                {
                    inputFormatter.BaseAddressFactory = ( m ) =>
                    {
                        var std = ODataInputFormatter.GetDefaultBaseAddress( m );
                        if ( HostingEnvironment.IsDevelopment() )
                        {
                            return new Uri( std.ToString().Replace( m.Scheme + "://" + m.Host.Host + ":" + m.Host.Port, Configuration["Host"], StringComparison.InvariantCultureIgnoreCase ) );
                        }

                        return new Uri( std.ToString().Replace( m.Scheme + "://" + m.Host.Host, Configuration["Host"], StringComparison.InvariantCultureIgnoreCase ) );
                    };
                }

});

[MJeorrett]

Thanks @Mhirji. I ended up doing similar.

Would be interested to know why the ODataInputFormatter needs to be affected, as my intuition says that it is only the response written by the ODataOutputFormatter that should be affected?

For anyone that is interested, my solution is slightly different as I did not want to lose the ability to call the Api Gateway in development environment so, instead of always settings the base url to the downstream service (i.e. always using GetDefaultBaseAddress) I have:

• Added a header in the reverse proxy containing the external host name
• If running in dev I do this:

private static Uri DevODataCustomBaseAddressFactory(string baseUrlHeaderKey, HttpRequest request)
{
    var apiGatewayBaseUrlHeader = request.Headers.FirstOrDefault(h => h.Key == baseUrlHeaderKey);

    if (apiGatewayBaseUrlHeader.Equals(default(KeyValuePair<string, StringValues>)))
    {
        return ODataOutputFormatter.GetDefaultBaseAddress(request);
    }

    return new Uri(apiGatewayBaseUrlHeader.Value);
}

• If not running in dev I do this:

private static Uri ODataCustomBaseAddressFactory(string baseUrlHeaderKey, HttpRequest request)
{
    var apiGatewayBaseUrlHeader = request.Headers.FirstOrDefault(h => h.Key == baseUrlHeaderKey);

    if (apiGatewayBaseUrlHeader.Equals(default(KeyValuePair<string, StringValues>)))
    {
        throw new ArgumentException($"Failed to retrieve header containing external base url ('{baseUrlHeaderKey}').");
    }

    return new Uri(apiGatewayBaseUrlHeader.Value);
}

Thanks

Matthew

P.S. Thanks for the tip of grabbing the default base address before applying the custom base address factory, didn't think of that!