close curl connection after telnet

Question

What I Want:

After a successful connection, I want curl to exit successfully. I am running this command inside a container, so I want the curl command to exit successfully so that the container will too.

Here is my example:

$ curl -v telnet://google.com:443/
*   Trying 172.217.197.113...
* TCP_NODELAY set
* Connected to google.com (172.217.197.113) port 443 (#0)

Options I have tried:

No Keep Alive:

$ curl -v --no-keepalive telnet://google.com:443/
*   Trying 172.217.197.102...
* TCP_NODELAY set
* Connected to google.com (172.217.197.102) port 443 (#0)

Connection Timeout:

$ curl -v --connect-timeout 5 telnet://google.com:443/
*   Trying 172.217.197.139...
* TCP_NODELAY set
* Connected to google.com (172.217.197.139) port 443 (#0)

Keep Alive Time:

$ curl -v --keepalive-time 5 telnet://google.com:443/
*   Trying 172.217.197.139...
* TCP_NODELAY set
* Connected to google.com (172.217.197.139) port 443 (#0)

Flag Definitions

--no-keepalive (Disable keepalive use on the connection)

--connect-timeout (SECONDS Maximum time allowed for connection)

--keepalive-time (SECONDS Wait SECONDS between keepalive probes)

Answer 1

What worked for me was sending "exit" to telnet and since you are not intressted in the output, redirect it

$ echo "exit" | curl --connect-timeout 1 -s telnet://google.com:443 > /dev/null
$ echo $?
0
$ echo "exit" | curl --connect-timeout 1 -s telnet://google.com:4433 > /dev/null
$ echo $?
7

Answer 2

To make curl exit immediately upon successful telnet connection, purposely pass an unknown telnet option and test if the exit code is 48:

curl -t 'DUMMY=1' --connect-timeout 2 -s telnet://google.com:443 </dev/null
[ "$?" -eq 48 ]
echo "$?"
# 0

48 is curl's error code for "An option passed to libcurl is not recognized/known."

^{— https://curl.se/libcurl/c/libcurl-errors.html}

This works because the telnet options are processed only after a successful connection.

Edit: May 27, 2022: Watch this todo item:
https://curl.se/docs/todo.html#exit_immediately_upon_connection
Via: https://curl.se/mail/archive-2022-04/0027.html