Nexus proxy repo cannot fetch from remote in CI

A Nexus proxy repository fronts an upstream registry. When Nexus cannot reach the remote (network, auth, or an auto-blocked outbound), it serves a 404 or transfer failure for packages that do exist upstream.

What this error means

Downloads through the Nexus proxy fail with 404 or "Could not transfer artifact" for a package present on the upstream registry, and the Nexus UI shows the proxy remote status as blocked or unhealthy.

Nexus

[ERROR] Failed to read artifact descriptor for org.example:lib:jar:2.0.0:
Could not transfer artifact org.example:lib:pom:2.0.0 from/to nexus
(https://nexus.example.com/repository/maven-central/): status code: 404

Common causes

The proxy remote is unreachable or auto-blocked

Nexus auto-blocks a proxy after repeated remote failures; until it recovers, requests return failures for uncached artifacts.

Missing outbound credentials or a firewall block

The upstream needs authentication Nexus does not have, or egress is blocked, so the proxy cannot fetch new packages.

How to fix it

Restore and unblock the proxy remote

Open the proxy repo in Nexus and check the remote status.
Fix outbound connectivity or add the upstream credentials under HTTP authentication.
Clear the auto-block so Nexus retries the remote.

Verify the remote from the Nexus host

Curl the upstream URL from the Nexus server to confirm connectivity independent of the proxy.

Terminal

curl -I https://repo1.maven.org/maven2/org/example/lib/2.0.0/lib-2.0.0.pom

How to prevent it

Monitor proxy remote health and alert on auto-block.
Store upstream credentials on the proxy for authenticated remotes.
Allow Nexus egress to the registries it proxies.