What are the alternatives for checking open ports, besides telnet?

Question

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?

Answer 1

Netcat (nc) 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

Answer 2

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.

Answer 3

'curl' can make life easier. No root require ; curl is readily available on all the linux systems

1) If port is not open will show below output

[niti@SourceServerName ~]$ curl -vv telnet://DestinationServerName:80

* About to connect() to DestinationServerName port 80 (#0)
*   Trying 192.168.0.100...

2) If port is open will show below output

[niti@SourceServerName ~]$ curl -vv telnet://DestinationServerName:443

* About to connect() to DestinationServerName port 443 (#0)
*   Trying 192.168.0.100...
* Connected to DestinationServerName (192.168.0.100) port 443 (#0)

---

CtrlC to exit

Answer 4

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

Answer 5

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 port\n";
my $destport = shift or die "Usage: $0 host port\n";

gethostbyname($desthost) || die "Invalid host given\n";

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!

Answer 6

openssl s_client -connect host:port

Answer 7

Device file /dev/tcp and /dev/udp can be used instead of telnet. Example: echo 0 > /dev/tcp/103.64.35.86/6667 . Then check the exit status using echo $? . If exit status is 0 then the port is open. If exit status is non-zero then the port is closed. For checking udp packets, use echo 0 > /dev/udp/103.64.35.86/6667 .

Answer 8

I found fast and simple this way, throgh Python interpreter (from https://serverfault.com/a/500062/168647):

[gryphius@ut ~]$ python
Python 2.6.6 (r266:84292, Feb 22 2013, 00:00:18) 
[GCC 4.4.7 20120313 (Red Hat 4.4.7-3)] on linux2                                                                                                                                                                                                                               
Type "help", "copyright", "credits" or "license" for more information.                                                                                                                                                                                                         
>>> import socket                                                                                                                                                                                                                                                              
>>> conn=socket.create_connection(('gmail-smtp-in.l.google.com',25))                                                                                                                                                                                                           

If that didn't throw an error so far the connection is ok.

Answer 9

ss -lt 

this is another command you can use.