test port no telnet

gistfile1.txt

# test port when no telnet 

#/dev/tcp/host/port
#    If host is a valid hostname or Internet address, and port is an integer port number
#    or service name, bash attempts to open a TCP connection to the corresponding socket.
#/dev/udp/host/port
#    If host is a valid hostname or Internet address, and port is an integer port number
#    or service name, bash attempts to open a UDP connection to the corresponding socket.

cat < /dev/tcp/127.0.0.1/22

Python (si devuelve 1 hay conexión.)

Python 2.7.5 (default, Aug  4 2017, 00:39:18)
[GCC 4.8.5 20150623 (Red Hat 4.8.5-16)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import socket
>>> clientsocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>>> clientsocket.connect(('smtp.office365.com', 587))
>>> clientsocket.send('\n')
1


Netcat:

nc -zv -w 5 127.0.0.1 22
Connection to 127.0.0.1 22 port [tcp/ssh] succeeded!

$ nc -zv -w 5 127.0.0.1 23
nc: connect to 127.0.0.1 port 23 (tcp) failed: Connection refused


curl:

$ curl -v telnet://127.0.0.1:22
* About to connect() to 127.0.0.1 port 22 (#0)
*   Trying 127.0.0.1... connected
* Connected to 127.0.0.1 (127.0.0.1) port 22 (#0)
SSH-2.0-OpenSSH_5.3
^C

$ curl -v telnet://127.0.0.1:23
* About to connect() to 127.0.0.1 port 23 (#0)
*   Trying 127.0.0.1... Connection refused
* couldn't connect to host
* Closing connection #0
curl: (7) couldn't connect to host