Jenkins CI/CD Pipeline Fails with Docker-in-Docker Missing ca.pem

by

Summary

An error surfaced during a Jenkins CI/CD pipeline that uses Docker-in-Docker (DIND) and a custom Jenkins agent image.
During the Building stage the agent tried to access /certs/client/ca.pem, but the file was missing, leading to:

ERROR: open /certs/client/ca.pem: no such file or directory

The pipeline failed immediately, preventing image build and any downstream stages.

Root Cause

  • Volume mounting issue: The jenkins-docker-certs volume was mounted correctly on the jenkins-docker (DIND) container but omitted the ca.pem file from the bind mount for the agent container.
  • Incorrect Dockerfile: The agent image was based on jenkins/agent:bookworm-jdk21 and did not install the Docker client or copy the CA certificates during build.
  • Environment variable mismatch: The agent expected /certs/client to contain ca.pem, cert.pem, and key.pem, but only key.pem existed.

Hence, the agent could not negotiate a secure TLS connection to the DIND daemon.

Why This Happens in Real Systems

  • Subtle volume sharing: In Compose setups, each service can mount the same named volume but with different permissions. A typo (:ro vs :rw) can silently drop a file.
  • Image layering: Custom agent images often reuse base images that do not include the Docker CLI or CA bundles unless explicitly added.
  • TLS misconfiguration: Production builds often rely on Docker’s TLS for security; missing certificates quickly break the entire pipeline.

Real-World Impact

  • Build pipeline stalls: A single missing certificate crashes the entire pipeline, wasting compute resources.
  • Developer frustration: Junior contributors may spend hours debugging “no such file” errors without realizing it’s a volume mount problem.
  • Security exposure: Attempting to connect without proper certs can fall back to insecure transport, exposing data.

Example or Code (if necessary and relevant)

services:
  jenkins-docker:
    ...
    volumes:
      - jenkins-docker-certs:/certs/client   # contains ca.pem, cert.pem, key.pem
  jenkins-blueocean:
    ...
    volumes:
      - jenkins-docker-certs:/certs/client:ro   # read-only, but still should include ca.pem

(No additional code is required; the issue is with the missing file, not the Dockerfile.)

How Senior Engineers Fix It

  • Verify volume contents: 
    docker exec jenkins-docker ls -l /certs/client
docker exec jenkins-blueocean ls -l /certs/client
  • Ensure ca.pem is present in the jenkins-docker-certs volume. If it isn’t, regenerate the certificates or copy them from the host into the volume.
  • Update Dockerfile for the agent to install the Docker CLI and copy the certificates: 
    FROM jenkins/agent:bookworm-jdk21
USER root
RUN apt-get update && apt-get install -y docker-ce-cli
COPY --from=jenkins-docker-certs /certs /certs
USER jenkins
  • Recreate the volume so that all files are synchro‑nized: 
    docker volume rm jenkins-docker-certs
docker volume create jenkins-docker-certs
# copy certs into the volume before starting containers
  • Check Compose file: Remove any overriding :ro that might prevent read access, and ensure all certificates are listed explicitly.
  • Add health checks: Configure Jenkins to verify Docker connectivity before running jobs.

Why Juniors Miss It

  • Assumption that volumes are automatically synced: They expect that mounting a named volume shares everything without seeing missing files.
  • Lack of understanding of Docker TLS flow: Junior engineers often overlook the necessity of ca.pem, cert.pem, and key.pem for secure communication.
  • Overreliance on documentation: Following generic examples without tailoring paths or permissions to their environment leads to subtle misconfigurations.
  • Neglecting file existence checks: They seldom run ls inside containers to confirm required files are present before pipeline execution.

By addressing the volume contents, ensuring proper image layers, and validating environment variables, senior developers prevent costly pipeline failures and enforce secure, reliable CI/CD workflows.

Leave a Comment