Postman调用Keycloak API遇400错误，提示grant_type参数缺失求助

Hey there, let's get this 400 error sorted out for you. The issue here is a common gotcha with OAuth2 password grant flows and how Keycloak expects parameters to be sent.

What's Going Wrong #

Even though you included all the required parameters in the URL query string and set the right Content-Type header, Keycloak's token endpoint doesn't accept grant-type parameters via URL query parameters for the password flow. It expects these values to be sent as form data in the request body instead.

Step-by-Step Fix #

Here's exactly what to adjust in Postman:

1. Use the correct request method: Make sure your request is set to POST (not GET) — this is required for the password grant flow.
2. Remove parameters from the URL: Trim your request URL down to just the base endpoint:

   http://{hostname}:8080/auth/realms/master/protocol/openid-connect/token
   

3. Add parameters to the request body:
   • Switch to the "Body" tab in Postman
   • Select the x-www-form-urlencoded option (Postman will automatically set the correct Content-Type header for you, so you don't need to manually add it)
   • Add these key-value pairs one by one:
     • username: admin
     • password: admin
     • client_id: admin-cli
     • grant_type: password
4. Send the request: You should now get a valid token response instead of the 400 error.

Extra Checks (If It Still Fails) #

• If you're using Keycloak 17 or newer, note that the /auth prefix was removed from the API path. Your URL should be:

  http://{hostname}:8080/realms/master/protocol/openid-connect/token
  

• Verify that the admin user in the master realm has the correct password, and that the admin-cli client is enabled in Keycloak's admin console.
• Double-check that Postman isn't sending any conflicting custom headers that might interfere with the request.

内容的提问来源于stack exchange，提问作者Programmer