How to request for the crumb issuer for Jenkins

Or you can use Python and requests instead

req = requests.get('http://JENKINS_URL/crumbIssuer/api/xml?xpath=concat(//crumbRequestField,":",//crumb)', auth=(username, password))
print(req.text)

will give you the name and the crumb:

Jenkins-Crumb:e2e41f670dc128f378b2a010b4fcb493

This Python function gets the crumb, and additionally uses the crumb to post to a Jenkins endpoint. This is tested with Jenkins 2.46.3 with CSRF protection turned on:

import urllib.parse
import requests

def build_jenkins_job(url, username, password):
    """Post to the specified Jenkins URL.

    `username` is a valid user, and `password` is the user's password or
    (preferably) hex API token.
    """
    # Build the Jenkins crumb issuer URL
    parsed_url = urllib.parse.urlparse(url)
    crumb_issuer_url = urllib.parse.urlunparse((parsed_url.scheme,
                                                parsed_url.netloc,
                                                'crumbIssuer/api/json',
                                                '', '', ''))
    # Use the same session for all requests
    session = requests.session()

    # GET the Jenkins crumb
    auth = requests.auth.HTTPBasicAuth(username, password)
    r = session.get(crumb_issuer_url, auth=auth)
    json = r.json()
    crumb = {json['crumbRequestField']: json['crumb']}

    # POST to the specified URL
    headers = {'Content-Type': 'application/x-www-form-urlencoded'}
    headers.update(crumb)
    r = session.post(url, headers=headers, auth=auth)

username = 'jenkins'
password = '3905697dd052ad99661d9e9f01d4c045'
url = 'http://jenkins.example.com/job/sample/build'
build_jenkins_job(url, username, password)

Meanwhile you can generate an API token in order to prevent having to include your password in the source code provided by the solutions above:

https://wiki.jenkins.io/display/JENKINS/Authenticating+scripted+clients

I haven't found this in the documentation either. This code is tested against an older Jenkins (1.466), but should still work.

To issue the crumb use the crumbIssuer

// left out: you need to authenticate with user & password -> sample below
HttpGet httpGet = new HttpGet(jenkinsUrl + "crumbIssuer/api/json");
String crumbResponse = toString(httpclient, httpGet);
CrumbJson crumbJson = new Gson().fromJson(crumbResponse, CrumbJson.class);

This will get you a response like this

{"crumb":"fb171d526b9cc9e25afe80b356e12cb7","crumbRequestField":".crumb"}

This contains two pieces of information you need

1. the field name with which you need to pass the crumb
2. the crumb itself

If you now want to fetch something from Jenkins, add the crumb as header. In the sample below I fetch the latest build results.

HttpPost httpost = new HttpPost(jenkinsUrl + "rssLatest");
httpost.addHeader(crumbJson.crumbRequestField, crumbJson.crumb);

Here is the sample code as a whole. I am using gson 2.2.4 to parse the response and Apache's httpclient 4.2.3 for the rest.

import org.apache.http.auth.*;
import org.apache.http.client.*;
import org.apache.http.client.methods.*;
import org.apache.http.impl.client.*;

import com.google.gson.Gson;

public class JenkinsMonitor {

    public static void main(String[] args) throws Exception {

        String protocol = "http";
        String host = "your-jenkins-host.com";
        int port = 8080;
        String usernName = "username";
        String password = "passwort";

        DefaultHttpClient httpclient = new DefaultHttpClient();
        httpclient.getCredentialsProvider().setCredentials(
                new AuthScope(host, port), 
                new UsernamePasswordCredentials(usernName, password));

        String jenkinsUrl = protocol + "://" + host + ":" + port + "/jenkins/";

        try {
            // get the crumb from Jenkins
            // do this only once per HTTP session
            // keep the crumb for every coming request
            System.out.println("... issue crumb");
            HttpGet httpGet = new HttpGet(jenkinsUrl + "crumbIssuer/api/json");
            String crumbResponse= toString(httpclient, httpGet);
            CrumbJson crumbJson = new Gson()
                .fromJson(crumbResponse, CrumbJson.class);

            // add the issued crumb to each request header
            // the header field name is also contained in the json response
            System.out.println("... issue rss of latest builds");
            HttpPost httpost = new HttpPost(jenkinsUrl + "rssLatest");
            httpost.addHeader(crumbJson.crumbRequestField, crumbJson.crumb);
            toString(httpclient, httpost);

        } finally {
            httpclient.getConnectionManager().shutdown();
        }

    }

    // helper construct to deserialize crumb json into 
    public static class CrumbJson {
        public String crumb;
        public String crumbRequestField;
    }

    private static String toString(DefaultHttpClient client, 
        HttpRequestBase request) throws Exception {
        ResponseHandler<String> responseHandler = new BasicResponseHandler();
        String responseBody = client.execute(request, responseHandler);
        System.out.println(responseBody + "\n");
        return responseBody;
    }

}