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We can use the following in order to test telnet VIA port; in the following example we test port 6667:

[root@kafka03 ~]# telnet kafka02 6667

Trying 103.64.35.86...

Connected to kafka02.

Escape character is '^]'.

^CConnection closed by foreign host

Since on some machines we can't use telnet (for internal reasons) what are the alternatives to check ports, as telnet?

If using Bash Shell, then you can use its feature to check if a port is open or closed:

(timeout 1 bash -c '</dev/tcp/127.0.0.1/17500 && echo PORT OPEN || echo PORT CLOSED') 2>/dev/null

PORT OPEN



(timeout 1 bash -c '</dev/tcp/127.0.0.1/7500 && echo PORT OPEN || echo PORT CLOSED') 2>/dev/null

PORT CLOSED

Note that if the server does not respond after 1 second the timeout is reached, the commands between ' interrupted, and thus nothing is printed.

netcat is one option.

nc -zv kafka02 6667

• 
  -z = sets nc to simply scan for listening daemons, without actually sending any data to them


• 
  -v = enables verbose mode

The gold standard is undoubtedly nmap (nmap.org), but it typically requires root for “best results”. However, standalone binaries are available and it is possible to run it as an unprivileged user, just with degraded capabilities. For example, instead of a stealth syn scan (-sS), it falls back to a standard TCP connect scan (-sT). This is functionally equivalent to netcat, but with the nice multi-host, sped-up capabilities that it has.

An example:

not-root$ nmap -sT google.com

Starting Nmap 7.70 ( https://nmap.org ) at 2018-11-04 21:01 GMT

Nmap scan report for google.com (172.217.23.14)

Host is up (0.12s latency).

rDNS record for 172.217.23.14: lhr35s01-in-f14.1e100.net

Not shown: 998 filtered ports

PORT    STATE SERVICE

80/tcp  open  http

443/tcp open  https

If Perl is an option, you can use its IO::Socket module to test a connection to a particular host and port; the script below hard-codes TCP as the protocol (which is what telnet would use):

#!/usr/bin/perl -w



# tries to connect to the given IP and port (tcp)



use strict;

use IO::Socket;



my $desthost = shift or die "Usage: $0 host portn";

my $destport = shift or die "Usage: $0 host portn";



gethostbyname($desthost) || die "Invalid host givenn";



my $handle = IO::Socket::INET->new(

        PeerAddr => $desthost,

        PeerPort => $destport,

        Proto    => 'tcp')

    or die "can't connect to $desthost:$destport: $!n";

close $handle;

print "Success!n"

Sample output from a closed port:

$ ./above-script kafka02 6667

can't connect to kafka02:6667: Connection refused

Sample output from an open port:

$ ./above-script kafka02 4200

Success!