18

What do people use when Telnet is not installed to check a port is open and reachable? E.g. I used to use the technique of telnet <destination> and know it was there, even if telnet could not interact with the system on the other end.

With Windows 2008 telnet is not installed so I've been a bit lost. So what can I use instead. And something if its not there in Linux or Solaris, too please.

I am a consultant who works on different sites. For a number of reasons (access rights, change control times, if I install it someone uses it next year we have some liability, etc) I cannot install on someone else's server. But a USB or other self contained, non-installed tool would be wonderful ...

25

Use Powershell like a boss


Basic code

$ipaddress = "4.2.2.1"
$port = 53
$connection = New-Object System.Net.Sockets.TcpClient($ipaddress, $port)

if ($connection.Connected) {
    Write-Host "Success"
}
else {
    Write-Host "Failed"
}

Turn it into a cmdlet

[CmdletBinding()]
Param(
  [Parameter(Mandatory=$True,Position=1)]
   [string]$ip,

   [Parameter(Mandatory=$True,Position=2)]
   [int]$port
)

$connection = New-Object System.Net.Sockets.TcpClient($ip, $port)
if ($connection.Connected) {
    Return "Connection Success"
}
else {
    Return "Connection Failed"
}

Save as a script and use all the time

Then you use the command in your powershell or cmd prompt like so:

PS C:\> telnet.ps1 -ip 8.8.8.8 -port 53

or

PS C:\> telnet.ps1 8.8.8.8 53

| improve this answer | |
  • 4
    You can also just execute: New-Object System.Net.Sockets.TcpClient("IP or DomainName", 80) And you'll get an error if it can't connect, or information on the created object if the connection succeeded. – Tsuushin Dec 13 '13 at 22:14
  • 1
    How does that work in terms of closing the socket? For example I know if I open a webrequest to IIS, but don't close the connection, it will reach a limit and I won't be able to test anymore. – Vasili Syrakis Dec 15 '13 at 22:10
  • Good question. I imagine it does the same thing as in your answer (since you/we don't dispose of it or close the socket manually); The TcpClient should get garbage collected at some point, or the socket will hit a read timeout before that. Just a guess, haven't tested it! – Tsuushin Dec 16 '13 at 1:32
  • 1
    If you don't want to worry about leaving the the socket open for a while you can do this: (New-Object System.Net.Sockets.TcpClient($ip, $port)).Close Same deal as before; Error if you can't connect, but it'll print this out if it connected then closed the port successfully: void Close() – Tsuushin Dec 16 '13 at 1:36
42

Here are several different ways to test a TCP port without telnet.

BASH (man page)

# cat < /dev/tcp/127.0.0.1/22
SSH-2.0-OpenSSH_5.3
^C

# cat < /dev/tcp/127.0.0.1/23
bash: connect: Connection refused
bash: /dev/tcp/127.0.0.1/23: 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


Python

# python
Python 2.6.6 (r266:84292, Oct 12 2012, 14:23:48)
[GCC 4.4.6 20120305 (Red Hat 4.4.6-4)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import socket
>>> clientsocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>>> clientsocket.connect(('127.0.0.1', 22))
>>> clientsocket.send('\n')
1
>>> clientsocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>>> clientsocket.connect(('127.0.0.1', 23))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<string>", line 1, in connect
socket.error: [Errno 111] Connection refused


Perl

# perl
use IO::Socket::INET;
$| = 1;
my $socket = new IO::Socket::INET(
  PeerHost => '127.0.0.1',
  PeerPort => '22',
  Proto => 'tcp',
);
die "cannot connect to the server $!\n" unless $socket;
print "connected to the server\n";
^D
connected to the server
| improve this answer | |
  • +1 Was not aware of the curl method, very cool! :) – blong Nov 3 '16 at 18:10

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