Basic networking TCP test using telnet

When telneting to an IP at a given port, there are various telnet responses. Knowing the difference in telnet responses could easily point you in the right direction when a telnet to a host on a particular port in unsuccessful.

There are a distinct differences in getting ‘refused’ or ‘timeout’ responses.

You will get a connection refused message for one of the following reasons:

• The application you are trying to test hasn’t been started/installed on the remote server.
• There is a firewall rejecting the connection attempt by terminating the connection setup.

Example output from a Linux box:

$ telnet server2 7063
Trying 172.1.1.1...
telnet: connect to address 172.1.1.1: Connection refused
telnet: Unable to connect to remote host: Connection refused

The similar Connection refused message from a Solaris box :

$ telnet server3 7055
Trying 172.2.1.1...
telnet: Unable to connect to remote host: Connection refused

The Connect failed message is the equivalent but from a Windows box :

Microsoft Windows XP [Version 5.1.2600]
(C) Copyright 1985-2001 Microsoft Corp.

C:\Documents and Settings\vickwan>telnet 172.2.1.1 7062
Connecting To 172.2.1.1...Could not open connection to the host, on port 7062: Connect failed

The telnet command will abort the attempted connection after waiting a predetermined time for a response. This is called a timeout response.

In some cases, telnet won’t abort, but will just wait indefinitely. This is also known as hanging. These symptoms can be caused by the one of the following reasons:

• The remote server doesn’t exist on the destination network. It could be turned off.
• The could be a routing issue, either the request or the response never gets to the destination.
• A firewall could be blocking the connection attempt, causing it to timeout instead of being quickly refused.

Here is an example of the output:

$ telnet server3 7055
Trying 172.2.1.1...
telnet: connect to address 172.2.1.1: Connection timed out
telnet: Unable to connect to remote host: Connection timed out

The script, command file and input file.

Reference Adapted from : http://blog.ru.co.za/2009/09/29/telnet/

This little script is written to helps cut down time needed to test if ACL allows connection from server A to server B at a given port. It will attempt to suggest the remedy actions.

Script tested on AIX 6.1, AIX 7.1, RHEL 4.6 and Solaris 9.

#!/bin/ksh
# Written By   : Victor Kwan
# Written On   : 25 Oct 2009
# EMAIL   : victorkk [AT] gmail [DOT] com
# Description  : Utility to test TCP ACL via telnet
# Updated On   : 27 Oct 2009 : Attempt to interpret telnet response.
#              : 09 Mar 2011 : Test if telnet command is executable
#              : 07 Apr 2011 : Support AIX, Improve code to be not chatty and terminate telnet session properly.

FILE=${1}
OUTPUTFILE="$0.output"
LOGFILE="$0.log"
TELNETCMD="$0.telnetcmd"
TELNET=`which telnet`
CAT=`which cat`
ECHO=`which echo`
OS="`uname -s`"

#UNIX Normal "Connection to 10.106.50.10 closed."
#UNIX No route "No route to host"
#UNIX Conn refused "telnet: Unable to connect to remote host: Connection refused"
#UNIX timed out "telnet: Unable to connect to remote host: Connection timed out"

RESPONSE_NORMAL="gn host."
RESPONSE_NO_ROUTE="to host"
RESPONSE_CONN_REFUSED="refused"
RESPONSE_TIMED_OUT="med out"

#AIX Normal "Connection closed."
#AIX No route "No route to host"
#AIX Conn refised "telnet: connect: A remote host refused an attempted connect operation."
#AIX timed out "telnet: connect: A remote host did not respond within the timeout period."

AIXRESPONSE_NORMAL="Connection closed."
AIXRESPONSE_NO_ROUTE="No route to host"
AIXRESPONSE_CONN_REFUSED="connect operation."
AIXRESPONSE_TIMED_OUT="he timeout period."

THISRESPONSE_NORMAL="$RESPONSE_NORMAL"
THISRESPONSE_NO_ROUTE="$RESPONSE_NO_ROUTE"
THISRESPONSE_CONN_REFUSED="$RESPONSE_CONN_REFUSED"
THISRESPONSE_TIMED_OUT="$RESPONSE_TIMED_OUT"

COLOR_BLUE="\033[0;34m"
COLOR_GREEN="\033[32m"
COLOR_RED="\033[31m"
COLOR_BRIGHTRED="\033[1;31m"
COLOR_WHITE="\033[0m"
COLOR_BRIGHTWHITE="\033[1;37m"

if [ ! -x $TELNET ]
then
        echo "${COLOR_BRIGHTRED}Telnet command is not executable!!${COLOR_WHITE}"
        echo "${COLOR_WHITE}Script will now terminate.${COLOR_WHITE}"
        exit
fi

echo "Commence telnet test based on [$FILE] file."
echo

cat ${FILE} | grep -v "#" | while read LINE do {
        IP=`echo $LINE | awk -F: '{print $1}'`
        PORT=`echo $LINE | awk -F: '{print $2}'`

        ($CAT $TELNETCMD) | $TELNET $IP $PORT >> $OUTPUTFILE 2>&1

        RESPONSE=`tail -1 $OUTPUTFILE | tr -d "\r" | tr -d "\n"`
	if [ "$OS" = "SunOS" ]
	then
	{
		STR_TO_CMP=`echo "$RESPONSE" | awk '{print substr($0,length-7)}'`
	}
	elif [ "$OS" = "AIX" ]
	then
	{
		STR_TO_CMP=`echo "$RESPONSE" | awk '{print substr($0,length-18)}'`
		THISRESPONSE_NORMAL="$AIXRESPONSE_NORMAL"
		THISRESPONSE_NO_ROUTE="$AIXRESPONSE_NO_ROUTE"
		THISRESPONSE_CONN_REFUSED="$AIXRESPONSE_CONN_REFUSED"
		THISRESPONSE_TIMED_OUT="$AIXRESPONSE_TIMED_OUT"
	}
	fi

        if [ ! "$STR_TO_CMP" = "$THISRESPONSE_NORMAL" ]
        then
        {
                echo "Telnet ${COLOR_BRIGHTRED}FAILED${COLOR_WHITE} for ${COLOR_BRIGHTWHITE}$IP:$PORT${COLOR_WHITE}."
                echo "${COLOR_BRIGHTRED}Error Message${COLOR_WHITE} : [$RESPONSE]!"

                if [ "$STR_TO_CMP" = "$THISRESPONSE_NO_ROUTE" ]
                then
                {
                        echo "${COLOR_GREEN}Suggestion${COLOR_WHITE}: Check routing at both source and destination"
                }
                fi

                if [ "$STR_TO_CMP" = "$THISRESPONSE_CONN_REFUSED" ]
                then
                {
                        echo "${COLOR_GREEN}Suggestion${COLOR_WHITE}: Destination may not be listening, routable or firewall is blocking the connection."
                }
                fi

                if [ "$STR_TO_CMP" = "$THISRESPONSE_TIMED_OUT" ]
                then
                {
                        echo "${COLOR_GREEN}Suggestion${COLOR_WHITE}: Destination may not be listening, routable or firewall is blocking the connection."
                }
                fi
        }
        fi
     	echo "Done for $IP:$PORT."
        echo " "
}
done
echo "Telnet test ends."

For the input file, e.g. IP_PORT It is okay to have commented lines as the script will ignore them.

~$more IP_PORT
#WLS
server2:7003
server2:7004
server2:7022
server2:7023
...
...

For the command file, the 2 telnet control commands must be used.

~$ more testACL.telnetcmd
^]
quit

Final outcome. Output may look similar to the following. No output for telnet success.

> ./testACL_telnet.ksh IP_PORT
Commence telnet test based on [IP_PORT] file.
Telnet FAILED for server4:7053.
Error Message : [telnet: Unable to connect to remote host: Connection refused]!
Suggestion: Destination may not be listening on this IP and Port, routable or firewall is blocking the connection.
...
...
...
telnet test ends.