A project inside a docker container cannot run curl to access or monitor external web URLs

AI Summary

This article describes an issue where a Docker container cannot access external URLs and provides a solution. First, enter the container to troubleshoot the issue. If Curl is not installed, install it. Add Curl to the package list in the Dockerfile to ensure it is installed when the container is built and prevent similar issues. After installation, test again; external URLs should now be accessible.

Troubleshoot first

Enter the docker container and access the URL directly from the command line.

sudo docker exec -it [container_id] /bin/bash #Enter your container ID or name to enter the container

If this step fails with an error saying that /bin/bash is missing, such as the following:

OCI runtime exec failed: exec failed: unable to start container process: exec: "/bin/bash": stat /bin/bash: no such file or directory: unknown

This suggests that the container does not have /bin/bash. It may be based on a minimal image containing only essential components, such as an alpine image.

Try using /bin/sh as the shell inside the container:

sudo docker exec -it [container_id] /bin/sh

You should now have a working shell inside the container. Run the following command to test access to the external network:

curl -s -o /dev/null -w '%{http_code}' https://www.czl.net

If this works and returns 200, you are not facing the issue I found. You can skip the rest and ask chatgpt.

But if it fails with an error like this:

/bin/sh: curl: not found

Then you have found the same issue I did: the container image does not include curl at all, so it cannot access external URLs using curl. Follow the steps below to install it.

Install curl

It appears that curl is not installed in the container. This may be why the script cannot run. To fix this, install curl inside the container.

Since the container uses Alpine Linux, run the following commands to install curl:

apk update
apk add curl

Once installation is complete, test the curl command again:

curl -s -o /dev/null -w '%{http_code}' https://www.czl.net

If curl now runs correctly inside the container, try restarting the service and checking its status.

To prevent similar issues in the future, add curl to the package list in the Dockerfile so it is installed when the container is built. Here is an example Dockerfile snippet that installs curl:

FROM alpine
RUN apk update && apk add curl
...

Add this snippet to the Dockerfile and rebuild the container. This ensures that curl is always available to your scripts.

The installation output should look like this:

/usr/src/vigil # apk update
fetch https://dl-cdn.alpinelinux.org/alpine/v3.17/main/x86_64/APKINDEX.tar.gz
fetch https://dl-cdn.alpinelinux.org/alpine/v3.17/community/x86_64/APKINDEX.tar.gz
v3.17.3-250-g30850c73a4c [https://dl-cdn.alpinelinux.org/alpine/v3.17/main]
v3.17.3-250-g30850c73a4c [https://dl-cdn.alpinelinux.org/alpine/v3.17/community]
OK: 17823 distinct packages available
/usr/src/vigil # apk add curl
(1/5) Installing ca-certificates (20230506-r0)
(2/5) Installing brotli-libs (1.0.9-r9)
(3/5) Installing nghttp2-libs (1.51.0-r0)
(4/5) Installing libcurl (8.1.2-r0)
(5/5) Installing curl (8.1.2-r0)
Executing busybox-1.35.0-r29.trigger
Executing ca-certificates-20230506-r0.trigger
OK: 9 MiB in 20 packages

Test again after installation—it should now work

curl -s -o /dev/null -w '%{http_code}' https://www.czl.net
200

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