From Nexus log:

2018-01-02 12:52:07,859+0000 WARN  [proxy-3-thread-1335] jnord org.sonatype.nexus.proxy.maven.maven2.M2Repository - Remote peer of proxy repository "Jenkins CI Releases" [id=jenkins-ci.org] threw a org.sonatype.nexus.proxy.RemoteStorageException exception. Connection/transport problems occured while connecting to remote peer of the repository. - Cause(s): Unexpected response code while executing GET method [repositoryId="jenkins-ci.org", requestPath="/", remoteUrl="https://repo.jenkins-ci.org/public/"]. Expected: "SUCCESS (200)". Received: 405 : Not Allowed

From Chrome:

{
  "log": {
    "version": "1.2",
    "creator": {
      "name": "WebInspector",
      "version": "537.36"
    },
    "pages": [
      {
        "startedDateTime": "2018-01-02T12:52:16.037Z",
        "id": "page_3",
        "title": "https://repo.jenkins-ci.org/public/",
        "pageTimings": {
          "onContentLoad": 357.73799999969924,
          "onLoad": 357.2529999996732
        }
      }
    ],
    "entries": [
      {
        "startedDateTime": "2018-01-02T12:52:16.386Z",
        "time": 554.0869999997303,
        "request": {
          "method": "GET",
          "url": "https://repo.jenkins-ci.org/public/",
          "httpVersion": "http/2.0",
          "headers": [
            {
              "name": ":path",
              "value": "/public/"
            },
            {
              "name": "dnt",
              "value": "1"
            },
            {
              "name": "accept-encoding",
              "value": "gzip, deflate, br"
            },
            {
              "name": "accept-language",
              "value": "en-GB,en;q=0.9"
            },
            {
              "name": "upgrade-insecure-requests",
              "value": "1"
            },
            {
              "name": "user-agent",
              "value": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.108 Safari/537.36"
            },
            {
              "name": "accept",
              "value": "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8"
            },
            {
              "name": "cache-control",
              "value": "max-age=0"
            },
            {
              "name": ":authority",
              "value": "repo.jenkins-ci.org"
            },
            {
              "name": "cookie",
              "value": "_ga=GA1.2.1488753192.1514896812; _gid=GA1.2.795396387.1514896812"
            },
            {
              "name": ":scheme",
              "value": "https"
            },
            {
              "name": ":method",
              "value": "GET"
            }
          ],
          "queryString": [],
          "cookies": [
            {
              "name": "_ga",
              "value": "GA1.2.1488753192.1514896812",
              "expires": null,
              "httpOnly": false,
              "secure": false
            },
            {
              "name": "_gid",
              "value": "GA1.2.795396387.1514896812",
              "expires": null,
              "httpOnly": false,
              "secure": false
            }
          ],
          "headersSize": -1,
          "bodySize": 0
        },
        "response": {
          "status": 405,
          "statusText": "",
          "httpVersion": "http/2.0",
          "headers": [
            {
              "name": "date",
              "value": "Tue, 02 Jan 2018 12:52:16 GMT"
            },
            {
              "name": "via",
              "value": "1.1 google"
            },
            {
              "name": "server",
              "value": "nginx"
            },
            {
              "name": "x-node",
              "value": "nginx-shared3c.gcoss-use1"
            },
            {
              "name": "content-type",
              "value": "text/html"
            },
            {
              "name": "status",
              "value": "405"
            },
            {
              "name": "alt-svc",
              "value": "clear"
            },
            {
              "name": "content-length",
              "value": "568"
            }
          ],
          "cookies": [],
          "content": {
            "size": 568,
            "mimeType": "text/html"
          },
          "redirectURL": "",
          "headersSize": -1,
          "bodySize": -1,
          "_transferSize": 682
        },
        "cache": {},
        "timings": {
          "blocked": 1.2519999997712201,
          "dns": 208.56099999991778,
          "ssl": 15.880999999809006,
          "connect": 239.75199999995277,
          "send": 0.5320000000210143,
          "wait": 103.99000000006748,
          "receive": 0,
          "_blocked_queueing": -1
        },
        "serverIPAddress": "130.211.20.35",
        "connection": "11765",
        "pageref": "page_3"
      }
    ]
  }
}

I hit it as well.

We run an nginx proxy only for Azure-based agents for ci.jenkins.io, which is located at repo.azure.jenkins.io.

This appears to be yet-another-issue with Artifactory Online.

indeed.  using https://jenkinsci.jfrog.io/jenkinsci/public  also gives a 405

it's like they are actively trying to break Nexus users ;-p

OT: - who does the reverse from https://repo.jenkins.io to artifactory online then?

I doubt they will repsond.  they didn't last time they broke Nexus caches infront of them, but https://twitter.com/JamesTeilo/status/948241747917922306

not sure we have any support with them...

Well, Artifactory support definitely tried to help us with INFRA-1447.
And they sponsor us, so probably we should not grumble in the current state

FFS  https://issues.jenkins-ci.org/browse/INFRA-1447?focusedCommentId=323757&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-323757

Thank you for contacting JFrog Support.
We have experienced performance difficulties with some servers, including jenkins-ci. Please be advised that in the meantime, to mitigate this, we have blocked the directory browsing for https://repo.jenkins-ci.org/public. You may still use this repository for uploading and downloading files. In addition, we improved performance related to Access service.
repo.jenkins-ci.org is currently up and running.

Flag added

Good catch James Nord, I emailed JFrog support about lifting the restriction

Is there an update to this issue?
We are currently unable to build projects because of the 405 error, preventing the download of jar files.
Works for most team members as long as they have the jar locally in their maven repository, but people needing to download it simply cannot.

Arne De Herdt most likely you are using a Nexus in front of the Jenkins repos, as Maven itself doesn't care.

Ask your Nexus admin to temporarily disable "Auto Blocking Enabled" for it.

I'll forward the information and see if it helps.

However, when I try to access the Jenkins Repo, I still get the 405 however.
So if our Nexus is missing the jar, we will still run into the same problem however.

Arne De Herdt you only get the 405 on the root page.

downloading artifacts and browsing anywhere not the root is unaffected.

e.g. https://repo.jenkins-ci.org/public/org/jenkins-ci/main/

From JFrog:

Following up on my last email, we wanted to share with you that we are planning
to deploy the fixed version to your instance on next Sunday, the 14th, (during
Israel daytime hours). When the deployment is done, we will perform our
validations, and remove the /public directory browsing restriction.

James Nord Is this still an issue?

This is not resolved or occurs again. 

Getting:

405 Not Allowed

nginx

Sascha Retter Please explain why you care.

Daniel Beck we're operating a nexus proxy-repository for jenkins-plugin development. 

Sascha Retter As I've told James Nord out of band I'm telling you now:

Please don't murder our Artifactory. You're not the only one using it, and proxying the virtual repo is really expensive.

https://repo.jenkins-ci.org/api/repositories/public is a JSON response that lists the actual repos included in this virtual one.

The only ones we deployed anything to in the last five years are snapshots and releases. so those should be mirrored/proxied.

Apparent lack of legitimate use case.

Daniel Beck thanks for elucidation! It was not transparent to us that this is a virtual repo.

Sascha Retter Arne De Herdt if you have a commercial nexus please file a support ticket with Sonatype and reference (ZD-23504 which is from a different customer so you will not be able to see but they will).

If you don't have a commercial contract (and possibly even if you do) please vote for https://issues.sonatype.org/browse/NEXUS-9508