Identity server 4 throws redirect uri not define error - webforms

Question

I am trying connect the asp.net webform client to identity server 4 for authentication and athorization. When user is redirected to identity server for login I am getting an error and Identity server log says "redirect_uri is missing or too long" but I have defined the redirect uri on client config. Not sure why it is throwing an error on identity server side?
Client Configuration:
new Client {
ClientId = "testclient1",
ClientSecrets = { new Secret("client_secret_webform".ToSha256()) },
AllowedGrantTypes = GrantTypes.Implicit,
RequirePkce = true,
RedirectUris = { "http://localhost:54602/signin-oidc" },
PostLogoutRedirectUris = { "http://localhost:54602/signin-oidc" },
AllowedScopes = {
IdentityServerConstants.StandardScopes.OpenId,
IdentityServerConstants.StandardScopes.Profile,
},
AllowAccessTokensViaBrowser = true,
RequireConsent = false,
}
Webform Client setting
app.UseOpenIdConnectAuthentication(
new OpenIdConnectAuthenticationOptions
{
ClientId = "testclient1",
Authority = "https://localhost:44314/",
ClientSecret = "client_secret_webform",
ResponseType = "id_token token",
SaveTokens = true
});
}
IdentityServer4.Validation.AuthorizeRequestValidator: Error: redirect_uri is missing or too long

Answer 1

The easiest way to figure out the correct redirectUrl is to locate the request to the Authenticate endpoint in Fiddler, and see what RedirectUrl the OpenIDConnect handler is sending to IdentityServer.
In Fiddler you can then locate the redirect_uri that should match exactly the url defined in IdentityServer

Answer 2

Try to change OpenID Connect settings on client to add RedirectUri and PostLogoutRedirectUri, like this:
app.UseOpenIdConnectAuthentication(
new OpenIdConnectAuthenticationOptions
{
ClientId = "testclient1",
Authority = "https://localhost:44314/",
ClientSecret = "client_secret_webform",
ResponseType = "id_token token",
SaveTokens = true,
RedirectUri = "http://localhost:54602/signin-oidc",
PostLogoutRedirectUri = "http://localhost:54602/signin-oidc",
});
}
Also as other answer mentioned use Fiddler or activate log to check the values sent to IDS4

Answer 3

The redirect uri you configure in IdentityServer is not the same as the one you are actually using in the client request. Capture your request to see this redirect uri.
The exception can be caused by two resons:
The redirect Uri is empty, null or white space
The redirect Uri is longer than the maximum allowed. The default value is 400.
You can modify this value in the IdentityServerOptions:
services
.AddIdentityServer(options =>
{
options.InputLengthRestrictions.RedirectUri = 1000;
...
}
...
...