DiscoveryEndpoint.cs

    // Courtesy of https://github.com/IdentityServer/IdentityServer4/blob/18897890ce2cb020a71b836db030f3ed1ae57882/src/IdentityServer4/src/Endpoints/DiscoveryEndpoint.cs
    // This slightly adjusts the original implementation to play nicely behind an application gateway so it checks if we have an X-original-host header and uses that as our baseUrl
    internal class DiscoveryEndpointHandler : IEndpointHandler
    {
        private readonly ILogger _logger;

        private readonly IdentityServerOptions _options;

        private readonly IDiscoveryResponseGenerator _responseGenerator;

        public DiscoveryEndpointHandler(
            IdentityServerOptions options,
            IDiscoveryResponseGenerator responseGenerator,
            ILogger<DiscoveryEndpointHandler> logger)
        {
            _logger = logger;
            _options = options;
            _responseGenerator = responseGenerator;
        }

        public async Task<IEndpointResult> ProcessAsync(HttpContext context)
        {
            _logger.LogTrace("Processing discovery request.");

            // validate HTTP
            if (!HttpMethods.IsGet(context.Request.Method))
            {
                _logger.LogWarning("Discovery endpoint only supports GET requests");
                return new StatusCodeResult(HttpStatusCode.MethodNotAllowed);
            }

            _logger.LogDebug("Start discovery request");

            if (!_options.Endpoints.EnableDiscoveryEndpoint)
            {
                _logger.LogInformation("Discovery endpoint disabled. 404.");
                return new StatusCodeResult(HttpStatusCode.NotFound);
            }

            var baseUrl = context.GetIdentityServerBaseUrl();
            if (context.Request.Headers.TryGetValue("X-original-host", out var xOriginalHostValue))
            {
                // NOTE The value of X-original-host should only contain the host portion
                var xOriginalHost = xOriginalHostValue.ToString();
                var baseUrlBuilder = new UriBuilder(new Uri(baseUrl));

                if (xOriginalHost.Length > 0 && xOriginalHost != baseUrlBuilder.Host)
                {
                    baseUrlBuilder.Host = xOriginalHost;
                    baseUrl = baseUrlBuilder.Uri.AbsoluteUri;
                }
            }

            var issuerUri = context.GetIdentityServerIssuerUri();

            // generate response
            _logger.LogTrace("Calling into discovery response generator: {type}", _responseGenerator.GetType().FullName);
            var response = await _responseGenerator.CreateDiscoveryDocumentAsync(EnsureTrailingSlash(baseUrl), issuerUri);

            return new DiscoveryDocumentResult(response, _options.Discovery.ResponseCacheInterval);

            static string EnsureTrailingSlash(string url)
            {
                if (url != null && !url.EndsWith("/"))
                {
                    return url + "/";
                }

                return url;
            }
        }
    }