Note: these are my self-help notes. Please let me know if they're wrong :)
The only thing you have is pure bash - no net tools. How to check the port visibility on a remote machine?
1. Opening stream for the TCP connection
exec [stream number]<>/dev/tcp/[host]/[port]
e.g.:
exec 5<>/dev/tcp/10.21.24.41/22
2. Sending something to that port
echo "hello" >&5
3. Reading the answer
cat <&5
In case the port 22 is open, we'll get something like:
SSH-2.0-OpenSSH_7.4
Protocol mismatch.
4. Closing the stream descriptor
exec 5<&-
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