Linux command: telnet

telnet stands for "Teletype Network".

RHEL install telnet: yum install -y telnet

telnet-ssl on Ubuntu

Before installing telnet-ssl, there is no -z options

tuyendq@2:~$ telnet -h
telnet: invalid option -- 'h'
Usage: telnet [-4] [-6] [-8] [-E] [-L] [-a] [-d] [-e char] [-l user]
        [-n tracefile] [ -b addr ] [-r] [host-name [port]]
tuyendq@2:~$

tuyendq@2:~$ apt show telnet-ssl
Package: telnet-ssl
Version: 0.17.41+0.2-3build1
Priority: extra
Section: universe/net
Source: netkit-telnet-ssl
Origin: Ubuntu
Maintainer: Ubuntu Developers <ubuntu-devel-discuss@lists.ubuntu.com>
Original-Maintainer: Mats Erik Andersson <mats.andersson@gisladisker.se>
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Installed-Size: 217 kB
Provides: telnet-client
Depends: libc6 (>= 2.15), libssl1.1 (>= 1.1.0), libstdc++6 (>= 5)
Download-Size: 85.2 kB
APT-Sources: http://us.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
Description: telnet client with SSL encryption support
 The telnet command is used for interactive communication with another host
 using the TELNET protocol.
 .
 SSL telnet replaces normal telnet using SSL authentication and
 encryption. It interoperates with normal telnetd in both directions.
 It checks if the other side is also talking SSL, if not it falls back
 to normal telnet protocol.
 .
 Advantages over normal telnet: Your passwords and the data you send
 will not go in cleartext over the line. Nobody can get it with
 tcpdump or similar tools. With SSLtelnet you can also connect to
 https-server like https://www.mozilla.org. Just do
 'telnet -z ssl www.mozilla.org 443'

tuyendq@2:~$ sudo apt install telnet-ssl

After install telnet-ssl

tuyendq@2:~$ telnet -h
telnet: invalid option -- 'h'
Usage: telnet [-4] [-6] [-8] [-E] [-K] [-L] [-X atype] [-a] [-d] [-e char]
        [-l user] [-n tracefile] [ -b addr ] [-r]
        [-z ssl] [-z secure] [-z debug] [-z verify=int]
        [-z cacert=file] [-z cert=file] [-z key=file]
        [host-name [port]]
tuyendq@2:~$

telnet -z ssl smtp.gmail.com 465

tuyendq@2:~$ telnet -z ssl smtp.gmail.com 465
Trying 142.250.101.108...
Connected to smtp.gmail.com.
Escape character is '^]'.
220 smtp.gmail.com ESMTP t21sm4101108pfe.82 - gsmtp
EHLO tuyen
250-smtp.gmail.com at your service, [173.82.240.78]
250-SIZE 35882577
250-8BITMIME
250-AUTH LOGIN PLAIN XOAUTH2 PLAIN-CLIENTTOKEN OAUTHBEARER XOAUTH
250-ENHANCEDSTATUSCODES
250-PIPELINING
250-CHUNKING
250 SMTPUTF8