Router Connectivity Tools
TELNET
Telnet is a virtual terminal protocol that is part of the TCP/IP protocol
suite. Telnet allows you to make connections to remote devices, gather
information, and run programs. After your routers and switches are
configured, you can use Telnet to configure and check your routers and switches
remotely. You can run telnet by typing telnet along with the IP
address from the command line or just the IP address (an attempt will be made to
open a telnet connection to the IP address). Before you can login via
telnet you will have to enable logins and
set the VTY password. Telnet allows a user at one site to establish a
TCP connection to a login server at another site, and then passes the keystrokes
from one system to the other. Telnet can accept either an IP address or a fully
qualified domain name as the remote system address. Telnet tests
connectivity up to the application layer of the OSI model.
In short, Telnet offers three main services:
- Network virtual terminal connection
- Option negotiation
- Symmetric connection
The Cisco Systems implementation of Telnet supports the
following Telnet options:
- Remote echo
- Binary transmission
- Suppress go ahead
- Timing mark
- Terminal type
- Send location
- Terminal speed
- Remote flow control
- X display location
Telnet to devices
Below is an example telnet session, I telneted from the router to a Linux
server. {Never telnet and login as the root user, telnet sends passwords in
plain text. If you must login remotely to a server as root, use Secure
Shell SSH instead}.
Router_2>192.168.1.70
Trying 192.168.1.70 ... Open
Welcome to SuSE Linux (i386) - Kernel 2.4.5 (pts/0).
suse login: lxuser
Password:
lxuser@suse~ >
Below is an example telnet session, I telneted into another router.
Router_2>192.168.1.1
Trying 192.168.1.1 ... Open
User Access Verification
Password:
Router3>
You can telnet into multiple devices simultaneously, telnet into the first
device then switch back to the original device by pressing the following keys:
[Ctrl]+[Shift]+6, let go and then press X.
Checking Telnet Users
You can list all active consoles and VTY ports in use with the show users
command. The asterisk denotes who entered the command. Host(s)
shows outgoing connections. Idle is the time in minutes since a
user has typed something. Location is either the hardwired
location for the line or, if there is an incoming connection, the host the
incoming connection is from.
Router3#show users
Line User Host(s) Idle Location
0 con 0 idle
* 2 vty 0 mb idle 0 SUSE.BELL.NET
Checking Telnet Sessions
To see the connections made from your router to a remote host, use the
show sessions command. The asterisk denotes the console was the last
session used. Host shows the remote connection to which the
router is connected through a Telnet session. Address is the
address of the remote host. Byte is the number of unread bytes
that are waiting for the user to see on the connection. Idle is
the interval (in minutes) since data was last sent on the line. Conn
Name is the Assigned name of the connection.
Router3#show sessions
Conn Host Address Byte Idle Conn Name
1 192.168.1.1 192.168.1.1 0 0 192.168.1.1
* 2 192.168.1.2 192.168.1.2 0 0 192.168.1.2
Ending Telnet Sessions
There are a couple of different ways to end telnet sessions. They are
the exit or
disconnect [session number, session name] commands.
You can disconnect users with the clear line [number]
command, you can see the users with the show users command.
TRACE
Trace can be used to show the path a packet takes through the internetwork to
a remote host. Trace is similar to the windoze tracert command or
the UNIX traceroute command.
PING
You can use ping to test network connectivity and test name resolution.
Router_2#ping ?
WORD Ping destination address or hostname
apollo Apollo echo
appletalk Appletalk echo
clns CLNS echo
decnet DECnet echo
ip IP echo
ipx Novell/IPX echo
vines Vines echo
xns XNS echo
<cr>
An example ping session.
Router_2>en
Router_2#ping 192.168.1.70
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 192.168.1.70, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 4/4/4 ms
Name Resolution
In order to use hostnames instead of IP address to connect to remote devices,
you need a method to resolve names. One method is to use a host table and
another is to use a DNS (Domain Name System) Server.
Host Table
The syntax for building a host table is: ip host [name] [tcp
port #] [IP_address]. The command is used in global configuration
mode (conf t) and you can assign up to eight IP address to a hostname.
To see the host table use the command show hosts.
DNS Server
If you have many devices and don't want to create a host table at each
device, you can use a central DNS server to resolve hostnames. DNS lookups
are enabled by default and can be disabled with the following command in global
configuration mode no ip domain-lookup. To specify the DNS server
to use, use the command
ip name-server [ip_address] in
global configuration mode. The last command you should use to enable DNS
resolution of hostnames is to specify the domain you are in with the command
ip domain-name [domain] since DNS server resolve FQDN (Fully
Qualified Domain Names).
Here is how to set up a DNS server in Linux (BIND 8)
Here is how to set it up in NT 4.0.
Router_2>en
Router_2#conf t
Enter configuration commands, one per line. End with CNTL/Z.
Router_2(config)#ip domain-lookup
Router_2(config)#ip name-server 192.168.1.70
Router_2(config)#ip domain-name bell.net
Router_2(config)#^Z
After entering the domain information, you can telnet and ping using the
hostnames. Every hostname will be cached for faster access next time.
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