Protocol "telnet" not supported or disabled in libcurl

[norve]

Prior to the last release, it used to be possible to run, for example:
curl -v telnet://towel.blinkenlights.nl:23 to open a telnet/socket connection. Right now it errors the following message:

* Protocol "telnet" not supported or disabled in libcurl
* Closing connection -1
curl: (1) Protocol "telnet" not supported or disabled in libcurl

This was a nice to have feature as there aren't any other tools on the system for quickly troubleshooting TCP connectivity issues.

Please re-add telnet support in libcurl.

[margamanterola]

We use curl directly from Gentoo without changes. When we updated curl to the latest version, the curl ebuild file changed from always enabling telnet, to conditionally enabling telnet (and other protocols) depending on USE flags: flatcar-archive/portage-stable@d42ea09#diff-87c1bc20b512568d3faf0582bc9289e7bf81d8febdb4da8741ac8c78fe6e9a5fL178

Whether a protocol is enabled or not depends on the protocol. e.g. ftp is enabled by default, while telnet is not. We would need to enable the telnet USE flag for curl if we want telnet support.

[oliwer]

There is a better tool than curl for this: ncat.

ncat towel.blinkenlights.nl 23

[tormath1]

since it's a good first issue, let's bring some elements to work on this:

• building a custom Flatcar image will be required to validate the changes: https://kinvolk.io/docs/flatcar-container-linux/latest/reference/developer-guides/sdk-modifying-flatcar/
• once the SDK installed, we should assert that curl is installed without telnet USE flag.

equery-amd64-usr u net-misc/curl
...
 - - telnet           : Enable Telnet protocol support

• In the making changes section, we should not have to modify curl ebuild - the telnet useflag needs to be set directly into ::coreos-overlay in the package.use file: https://github.com/kinvolk/coreos-overlay/blob/main/profiles/coreos/base/package.use
• once the file edited, we can assert that curl will be reinstalled with telnet USE flag

emerge-amd64-usr -pv net-misc/curl

• Once emerged, we can build the image by following the step of the developer-guides
• we can boot the new instance and SSH into it to validate that the issue is actually solved
• changes can be committed and pushed to a feature branch in order to be reviewed by the flatcar-maintainers

useful links:

• https://wiki.gentoo.org/wiki/Handbook:AMD64/Working/USE
• example: Set oniguruma USE flag for the jq package flatcar-archive/coreos-overlay#320
• CNC meetup: introduction to Flatcar contribution

[peanutduck]

@tormath1, I would like to work on this issue

[sayanchowdhury]

@peanutduck Feel free to start working on the issue. The steps mentioned above should help you understand the issue and get started. For any more queries, join our Matrix channel.

[dongsupark]

PR flatcar-archive/coreos-overlay#1099

[sayanchowdhury]

The fix is merged now and will be available in the next Alpha release flatcar-archive/coreos-overlay#1099