Telnet
Lesson Overview
Introduction
Telnet, like NSLookup, is a key tool for troubleshooting inbound and
outbound Internet (SMTP) mail issues.
As a Domino Server Administrator, you might consider using Telnet if you
are having trouble sending or receiving Internet mail for your an entire
domain or for a particular Domino server.
Objectives
After completing this lesson, you will be able to:
•
Explain two reasons to use Telnet in troubleshooting Internet routing
issues
• Use Telnet to connect to a server to confirm port 25 is open to receive
Inbound Internet messages
• Use Telnet to determine what application is listening for inbound
Internet messages on port 25
• Use Telnet to send a test message to a particular user on a Domino
server
In this Lesson
This lesson is divided into the following topics:
Topic
Introduction to Telnet
Combining Multiple Tools
The Telnet Dialog
Using Windows 2000/XP Version of Telnet
Exercise: Using Telnet to Troubleshoot Routing Issues
Appendix: How to Use the Windows GUI version of Telnet
Timing
See Page
2
3
4
8
12
18
The timing of this lesson’s components is as follows:
Component
Lessons/Lesson Overview
Exercise
Total:
Last Edited: 11/4/2005
Approximate Time
30 minutes
30 minutes
1 hour
1
©IBM 2005
Introduction to Telnet
Overview
Telnet is a multifaceted tool that can perform many functions. Telnet is
available on a variety of platforms, however, this lesson will use the Windows
version. Our focus on Telnet is specific to troubleshooting SMTP Internet
mail issues. In particular, you would use Telnet for the following three
reasons:
Reason to Use Telnet
1
2
3
Last Edited: 11/4/2005
Details
To determine whether
port 25 is open to receive
inbound Internet mail.
For a domain to receive inbound Internet
mail, an SMTP server (such as a Domino
server configured for SMTP) must be
listening on port 25 and must be accessible
from external hosts.
Note: Port 25 is the wellknown TCP port for
If you can establish a connection to that
SMTP.
server over port 25 using Telnet, you can
confirm that the port is open to receive
inbound mail.
To identify what
Although multiple applications can be
application is currently
running on a server, only one can be bound to
bound to port 25 (for
port 25. In particular, a Domino server
sending and receiving
configured for SMTP cannot send or receive
SMTP messages).
Internet mail if another application has bound
to port 25.
When you establish a connection to a server
over Telnet, you can identify the application
that is currently bound to the port.
Sending a message via Telnet is a means to
test whether it’s possible to send and receive
a message using the commands designated
for SMTP.
To build and send a test
Internet message to a
particular SMTP server
listening on port 25.
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©IBM 2005
Combining Multiple Tools for Testing
NSLookup and
Telnet – Often
Used in
Combination
When troubleshooting SMTP Internet mail issues, Administrators frequently
use NSLookup and Telnet in combination. The following scenario describes
one way these tools can be used together to troubleshoot an issue.
Typical Scenario:
An Administrator determines that her company (978388 Co.) is not receiving
any Internet mail that is sent to its domain (978388.com) by external senders.
A Domino 6.5.4 server to setup and configured to receive all inbound SMTP
mail for the domain. It has already been confirmed that Domino SMTP
Listener task is enabled on the Server document.
Troubleshooting:
•
The first step would be to verify the MX records for xyz.com are present
in DNS using NSLookup. Typically, most will have 2 MX records listed
for a given domain. The host with the lowest MX preference would be the
first choice to connect to.
•
The next step would be to use Telnet to attempt a connection to the host
with the lowest preference on port 25. If the Connection is established but
the banner message displayed does not indicate a Domino 6.5.4 Server is
running, another SMTP application could have this port.
Results:
If Domino does not have use of port 25, its ability to receive inbound Internet
mail is unlikely. Changes would be needed on the local machine, so that the
other SMTP application no longer binds to port 25. Once the port is free, the
Domino SMTP server would able to bind to port 25 and receive inbound
Internet mail as intended.
Why use Telnet
too?
In the previous example, Telnet could be used to determine if the SMTP
servers listed in the MX record is capable of receiving mail for a given
domain or recipient. Telnet allows you, as the Administrator; to emulate the
exact behavior for sending messages manually, it would be done as if the
message was sent by an SMTP server.
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©IBM 2005
The Telnet Dialog
A Telnet
Session is a
Dialog
With Telnet, you are in a dialog with a server (the host). When you enter a
Telnet command, the host responds. The host’s response is either a positive
response or an error.
•
A positive response indicates that the host recognizes and accepts the
command. Positive responses begin with a numeric code in the 200’s.
•
An error indicates that the host either does not recognize or cannot
accept the command. There are two types of errors.
1. Transient errors (temporary errors), which begin with a numeric code in
the 400’s.
2. Permanent errors, which begin with a numeric code in the 500’s.
RFC821
A Telnet dialog over port 25 corresponds to the RFC (Request for Comments)
standards for SMTP: RFC821. For more information, refer to:
http://www.ietf.org/rfc/rfc821.txt?number=821
Telnet Dialog
Begins with the
Connection
The Telnet dialog begins when you successfully connect to the host. In most
cases, the host’s initial response includes two particularly important pieces of
information:
•
The number 220, indicating connection was established. (This
information confirms for you that port 25 is open.)
•
The host’s application type, such as Domino, Sendmail (a UNIX-based
mail program), or Microsoft Exchange. (This information tells you
whether the intended application is bound to port 25.)
It is important to note, however, that Administrators can customize the
response a server generates. You may not always see the host’s application
type. “Proper” implementations return at least the 220-response code and the
date.
Continued on next page
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©IBM 2005
The Telnet Dialog, Continued
Examples of
Successful
Connections
Below are two examples of responses from hosts after successful connections
were established via Telnet.
Example 1: Response from a Domino server:
220 mailserver1.lotus.com ESMTP Service (Lotus
Domino Release 7.0) ready at Wed, 19 Oct 2005
09:29:42 -0400
Example 2: Response from Sendmail:
220 mailserver100.boulder.ibm.com ESMTP Sendmail
8.12.11/8.12.11; Wed, 19 Oct 2005 07:31:25 -0600
If Connection
Attempt Fails
If a connection to a host cannot be established over Telnet, you will see a
message such as the following:
Microsoft Telnet> open wtfmail0500.lotus.com 25
Connecting To wtfmail0500.lotus.com...
Could not open connection to the host, on port 25: Connect failed
Possible reasons for a failed connection include the following:
•
The port is blocked by a firewall.
•
The server is down.
•
The server is not running the SMTP service. (For a Domino server, this
means the SMTP listener task is not enabled.)
•
An incorrect TCP/IP port is listening on a multi-homed server.
Continued on next page
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©IBM 2005
The Telnet Dialog, Continued
Dialog
Continues with
the Message
Once you establish a connection to the server, the dialog continues as you
build and then send the test Internet message.
As you enter each command, the host responds with either a positive (250)
response or an error. Again, the error may be a transient error (in the 400’s)
or a permanent error (in the 500’s).
A sample dialog is below:
Helo mailserver100.boulder.ibm.com
250 mailserver100.boulder.ibm.com Hello
[9.17.195.170], pleased to meet you.
mail from:John_Sample@lotus.com
250 John_Sample@lotus.com…Sender ok
recpt to:Jane_Doe@us.ibm.com
500 Command not recognized “recpt to:
Jane_Doe@us.ibm.com”
rcpt to:Jane_Doe@us.ibm.com
250 Jane_Doe@us.ibm.com…Recipient ok
data
354 Enter mail, end with “.” on a line by itself
This is a test message sent by John to Jane via
Telnet.
.
250 Message accepted for delivery
In this dialog, most of the responses were positive, but one 500 error was
returned when a command was entered incorrectly.
Note: For more information on the response codes that you can see in a
Telnet conversation with a host over port 25 (SMTP), refer to the following
Lotus Knowledge Base document:
Document 1153776, What Are the Available SMTP Reply Codes?
Continued on next page
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©IBM 2005
The Telnet Dialog, Continued
Dialog
Termination
The Telnet connection remains active after the message is sent. To terminate
the connection either exit Telnet using the UI or type quit (and press
ENTER).
For example:
quit
221 mailserver100.boulder.ibm.com closing connection
Note: If you do not manually terminate the connection, the server will drop
the connection after the designated amount of time (usually, after 2 minutes
of inactivity).
Telnet
Guidelines
On the next page, we’ll actually connect to a host via Telnet and send a test
message. First, however, consider the following guidelines for using Telnet.
Do…
• Turn on Local Echo so that you can see the commands you are typing.
• Use the proper syntax for each command; otherwise, the host will
generate a 500 error.
Don’t…
• Use either the Backspace or Delete key. Telnet does not recognize
either key. If you mistype a command, you must retype the entire
command.
Using the BACKSPACE or DELETE key in an attempt to correct a
mistyped Telnet command causes a 501-error code (a syntax error). In the
data fields, the BACKSPACE and DELETE keys cause garbage
characters, such as pipe symbols and squares.
Don’t need to…
Bother with the case of the Telnet commands. Any combination of upper and
lower case characters is accepted. (On UNIX platforms, you may need to pay
attention to the case of certain elements, such as file names.)
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©IBM 2005
Using Windows 2000/XP version of Telnet
Introduction
Unlike previous versions of Windows, the Windows 2000/XP does not
include a GUI-based version of Telnet. As a result, you must issue manual
commands to perform the following steps:
•
•
•
Open the connection with the server
Set local echo
Disconnect from the server
The procedure below lists the steps to Telnet a message to an SMTP server
using a Windows 2000/XP client.
Before You
Begin
To use the procedure below, you must know either the fully qualified domain
name or the IP address for the server to which you wish to connect.
Procedure –
For Win2k/XP
To Telnet to a Domino server over the SMTP protocol, follow these steps:
Step
1
2
Action
From the Windows 2000/XP task bar, select Start and then Run.
In the Run dialog box, enter Telnet and click OK.
Result: The following window displays:
3
Turn on Local Echo by typing the following command:
set local_echo
Press Enter
Continued on next page
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©IBM 2005
Using Windows 2000/XP version of Telnet, Continued
Procedure – For Win2k/XP (continued)
Step
4
5
Action
Connect to the Domino server via the appropriate command.
To connect using the
server’s…
hostname
Type…
IP Address
Open <IPAddress> 25
Open <hostname> 25
Result: If the server is listening on port 25, the Telnet window
displays a 220 connection message.
Create the message using the steps below:
a
b
Command
Type helo <hostname> and
press ENTER.
Note: ehlo can be used to
conduct the conversation in
ESMTP mode.
Type mail from: <your valid
email address> and press
ENTER.
c
Type rcpt to: <recipient’s
Internet address> and press
ENTER.
d
Type data and press ENTER.
Description
In the helo command, the
host sending the
command identifies
itself. The command can
be interpreted as saying
“Hello, I am <Domain>.”
This step fills in the from
field of the message. The
name should be a valid
address so that you can
receive, if necessary, a
delivery failure.
This step fills in the
SendTo field. The name
entered must be a valid
Internet address.
This step signals the
beginning of the data
fields.
Continued on next page
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©IBM 2005
Using Windows 2000/XP version of Telnet, Continued
Procedure – For Win2k/XP (continued)
Step
5
con’t
Action
e
Command
Optional:
Details
This step fills in the cc:
field.
f
Type cc: <recipient> and press
ENTER.
Optional:
g
Type date: <date> and press
ENTER.
Optional:
Type from: <> and press
ENTER.
h
i
j
Type subject: <your subject>
and press ENTER.
Type the body of the message
and press ENTER.
Type a period: .
and press ENTER.
This step fills in the date
field.
This step fills in the from
field. If included, it
overrides the address
used in the mail from
specified in Step b.
This step fills in the
subject field.
This step creates the
body text of the message.
This step signals the end
of the body field, and the
message is sent.
Result: The following message displays at the bottom of the
Telnet window:
250 Message accepted for delivery
6
7
To disconnect from the current host, type quit and press Enter.
To close the window, click the or type exit and press Enter.
Note: Other optional entries can be included within the data command. For
more information, refer to RFC821:
http://www.ietf.org/rfc/rfc821.txt?number=821
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©IBM 2005
Using the GUI version of Telnet on Windows 2000/XP
Obtaining a
GUI-Based
Telnet for
Win2K/XP
According to Microsoft, you can copy the executable file for the GUI-based
Telnet program from Windows NT 4.0 to Windows 2000. The steps to do
this are listed in the table below.
Step
1
2
3
4
On-Line
Instructions
Action
Obtain a copy of the TELNET.EXE file from a Windows NT 4.0
system.
Note: The file should be located in the c:\winnt\system32 folder.
Make a copy of the file and rename the copy to TELNET32.EXE.
Copy TELNET.EXE to the Windows 2000 system.
Optional: Copy the TELNET.HLP and TELNET.CNT files from
Windows NT to the folder where you copied TELNET32.EXE
into Windows 2000.
The steps to use the Windows NT 4.0 GUI-based version of Telnet in
Windows 2000 are available online in the following Lotus Knowledge Base
document:
Document 1095047, The GUI Version of Telnet in Windows 95, 98, and NT
Can be Used in Windows 2000
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©IBM 2005
Exercise: Using Telnet to Troubleshoot Routing Issues
Overview
This exercise is designed to reinforce your ability to use Telnet in
troubleshooting routing issues.
Timing
The timing for this exercise is as follows:
Component
Overview
Exercise
Recap
Total:
Intended
Outcome
Approximate Time
1 minute
20 minutes
9 minutes
30 minutes
In completing this exercise, you will demonstrate your ability to…
•
Explain two reasons to use Telnet in troubleshooting Internet routing
issues
• Use Telnet to connect to a server to confirm port 25 is open to receive
Inbound Internet messages
• Use Telnet to determine what application is listening for inbound
Internet messages on port 25
• Use Telnet to send a test message to a particular user on a Domino
server
Requirements
To complete this exercise, you need the following:
•
•
•
•
A pen
Telnet
The procedure: How to Telnet Using Windows 2000/XP.
The FQDN of your inbound SMTP server: _____________________
The name of a local test user to send messages to: __________
Continued on next page
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©IBM 2005
Exercise: Using Telnet to Troubleshoot Routing Issues,
Continued
Instructions
There are two parts to this exercise.
• In Part 1, you will use Telnet to establish a connection to an SMTP host (a
Domino server) and send a message. Along the way, you will describe
why you are performing certain steps and track the responses from the
server.
In Part 2, you will provide short answers to a set of scenarios on
troubleshooting with Telnet.
Part 1 – Telnet
Follow the steps in the table below and answer the corresponding questions.
Step
Action
1
Load Telnet and set local echo.
2
3
Answers
What is the reason local echo
should be enabled?
Connect to your SMTP server.
a. What port did you specify
for SMTP?
a.
b. What is the numeric
response returned by the
server?
b.
c. What type of application is
the server?
c.
Enter the Helo command.
What is the response returned
by the server?
Continued on next page
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©IBM 2005
Exercise: Using Telnet to Troubleshoot Routing Issues,
Continued
Part 1 – Telnet (continued)
Step
Action
4
Using the following
commands, send a test
message to one of your mail
users.
Answers
mail from:
rcpt to:
data
subject:
5
What is the response when the
message is successfully sent?
Send a second message in
which you deliberately
mistype one of the commands.
For example, type rpt to:
instead of rcpt to:
a. What is the response code
indicating an error?
a.
b. How do you correct the
mistyped command?
b.
Continued on next page
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©IBM 2005
Exercise: Using Telnet to Troubleshoot Routing Issues,
Continued
Part 1 – Telnet (continued)
6
Disconnect from the server.
Disable the SMTP listener task
on your server (tell it to quit).
Now attempt to reconnect to
the server via Telnet on port
25.
What response do you get from
the server?
Part 2 – Short
Answer
Provide short answers to each of the following scenarios.
Scenario 1
As the Administrator, you’ve attempted to configure a server, running
Domino 7.0, to receive all inbound mail sent to your company’s domain
(AFakeCompany.com). However, no messages sent to your domain by
external senders are getting to the intended recipients.
You have used NSLookup to identify that there is a single MX record in DNS
for a server within that domain: smtp.AFakeCompany.com. What two things
would you attempt to determine about the smtp.AFakeCompany.com server
using Telnet?
Your Response
Continued on next page
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©IBM 2005
Exercise: Using Telnet to Troubleshoot Routing Issues,
Continued
Scenario 2
You’ve received a report from one of your fellow Administrators, who is
currently working on an SMTP mail routing issue. You attempted to assist
and have used Telnet to connect to an SMTP server and sent a test message to
this person. She reports that the test message arrived, but it has garbage
characters (such as squares) in the message body.
Do these odd characters indicate yet another problem with her SMTP
configuration? What is the most likely reason for the garbage characters?
Your Response
Scenario 3
Your help desk reports that all Internet messages sent from your company to a
key business partner (somebuspartner.com) are resulting in non-delivery
notifications after a 24-hour period. The outbound SMTP server is running
Domino 6.5.4.
You’ve determined that all other outbound Internet messages are routing
correctly. Using NSLookup, you determined that the business partner’s
Internet domain has no MX records, but it does have an A record. You
attempt to connect to the server listed in the A record via Telnet, but an error
occurs: “Could not open a connection to inbound.somebuspartner.com.”
At this point, what can you say “with certainty” is needed to fix this? What
are some possible causes for this issue?
Your Response
Continued on next page
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©IBM 2005
Exercise: Using Telnet to Troubleshoot Routing Issues,
Continued
Scenario 4 –
(BONUS)
You’ve recently switched ISP’s and now are unable to send any outbound
SMTP mail. Your outbound SMTP servers are using a relay host so from the
Domino prospective, your configuration has not changed.
How might you verify the connection to the relay? Where is the relay defined
in the Domino Directory? Is there anything you needed to do on the Domino
side to accommodate the change to the new internet service provider?
Your Response
Results
In completing this exercise, you have demonstrated your ability to…
• Explain two reasons to use Telnet in troubleshooting Internet routing
issues
• Use Telnet to connect to a server to confirm port 25 is open to receive
inbound Internet messages
• Use Telnet to determine what application is listening for inbound Internet
messages on port 25
• Use Telnet to send a test message to a particular user on a Domino server
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©IBM 2005
Appendix: How to Use the Windows GUI version of Telnet
Overview
Telnet (TELNET.EXE) is a TCP/IP terminal application that is available on
most Operating Systems. The procedure below describes how to use the
version of Telnet that is included with Windows 95, Windows 98, and
Windows NT 4.0.
Before You
Begin
Before using the procedure below, you must know either the fully qualified
domain name or the IP Address for the server to which you wish to connect
via Telnet.
Procedure How to Telnet
To connect to an SMTP host and send a message via Telnet, follow the steps
below.
Step
1
2
Action
From the Windows 95, 98 or NT 4.0 task bar, select Start and then
Run.
In the Run dialog box, enter Telnet and click OK.
Result: The following Telnet window displays:
Note: The Telnet executable can be launched a number of ways.
If you prefer, you can launch Telnet from a DOS prompt or
configure a shortcut for the Windows desktop.
Continued on next page
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©IBM 2005
Appendix: How to Use the Windows GUI version of Telnet,
Continued
Procedure - How to Telnet (continued)
Step
3
Action
Turn on Local Echo. To do this:
a. From the menu, select Terminal, Preferences.
b. Under Terminal Options, select Local Echo.
c. Click OK.
4
From the menu, select Connect, Remote System.
Continued on next page
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©IBM 2005
Appendix: How to Use the Windows GUI version of Telnet,
Continued
Procedure - How to Telnet (continued)
Step
5
Action
In the Connect dialog box:
a. Enter the appropriate Host Name or IP Address.
b. Change the port from Telnet to the appropriate number
(such as 25 for SMTP).
c. Leave TermType at the default (vt100).
Result: The Connect dialog box resembles the following:
6
Click Connect.
Result: If the server is listening on port 25, the Telnet window
displays the 220 response and additional information, such as the
following:
Continued on next page
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©IBM 2005
Appendix: How to Use the Windows GUI version of Telnet,
Continued
Procedure - How to Telnet (continued)
Step
7
Action
Create the test message using the steps below:
a
b
Command
Type helo <hostname> and
press ENTER.
Note: ehlo can be used to
conduct the conversation in
ESMTP mode.
Type mail from: <your valid
email address> and press
ENTER.
c
Type rcpt to: <recipient’s
Internet address> and press
ENTER.
d
Type data and press ENTER.
e
Optional:
f
Type cc: <recipient> and
press ENTER.
Optional:
g
Type date: <date> and press
ENTER.
Optional:
Details
In the helo command, the
host sending the command
identifies itself. The
command can be
interpreted as saying
“Hello, I am <Domain>.”
This step fills in the from
field of the message. The
name should be a valid
address so that you can
receive, if necessary, a
delivery failure.
This step fills in the
SendTo field. The name
entered must be a valid
Internet address.
This step signals the
beginning of the data
fields.
This step fills in the cc:
field.
This step fills in the date
field.
This step fills in the from
field. If included, it
Type from: <name> and press overrides the address used
ENTER.
in the mail from specified
in Step b.
Continued on next page
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©IBM 2005
Appendix: How to Use the Windows GUI version of Telnet,
Continued
Procedure - How to Telnet (continued)
Step
7
con’t
Action
Command
h Type subject: <your subject>
and press ENTER.
i Type the body of the message
and press ENTER.
j Type a period: .
and press ENTER.
8
9
10
Details
This step fills in the
subject field.
This step creates the body
text of the message.
This step signals the end
of the body field, and the
message is sent.
Result: The following message displays at the bottom of the
Telnet window:
250 Message accepted for delivery
To disconnect from the current host, type quit and press ENTER.
Alternately, you can select Connect and then Disconnect from the
Telnet menu.
At this point, you can connect to another host if you wish; or, to
exit Telnet, proceed to Step 10.
To exit from Telnet, either select Connect, Exit or click the x to
close the window.
Note: If, at any point during the conversation you would like to start sending
a new message, enter the rset command. (Type rset and then press ENTER.)
The rset command brings the session back to the mail from command, at
which point you can start again. You do not need to repeat the helo command
because the session is already established with the host.
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©IBM 2005