Question:
- What is TELNET?
- How can I test email connectivity to a server?
- Are there Telnet commands specific for testing Trustwave SEG/MailMarshal?
Information:
The first step in troubleshooting protocol problems is to verify that you can establish a successful TCP connection to the host. The easiest way to test this is by using the telnet command to connect to the required port. Pinging a host is not a reliable test. The Ping utility does not tell you that the host is listening on a specific port. It is only an indication that a lower-level packet was able to find its way to the correct IP address. In addition, many companies block ping or ICMP packets from entering their networks. Telnet allows you to test exactly the same type of connection that is used to send email.
Note: With recent versions of Windows, the Telnet client is not installed by default. If necessary, use Server Manager to add the feature "Telnet Client".
You can use a Telnet utility to test the basic operation of the SEG Receiver. To get a feel for how Telnet works, try to use Telnet to connect to your SEG computer on port 25. The syntax is:
For example:
Both of the examples above can be used (with the correct values); however, the first example is the most reliable. When addressing systems by their IP address, you eliminate the possibility of name resolution problems. The example above should bring up a Telnet window and a successful connect message indicating that you are ready to send to the host. This indicates that the host is listening on port 25, and that you can probably send an SMTP message to the host. The example above is similar to what the SEG Sender does when it connects to send email. At this point, the Sender starts sending SMTP commands to the host to initiate message transfer.
Many other protocols including POP3, IMAP4, and NNTP (Net News) work in a similar way. They include commands that are sent to the host and the replies that are expected. After you have concluded that a successful connection can be made to the correct port, you can start reviewing the actual responses given to a host when commands are issued.
The best testing method is to actually issue the commands to the host by using a Telnet command. This method is effective if you know exactly what you need to send and how to do it, or if you know what your client/server program is trying to send.
Tip: If you find you are making mistakes when typing commands in a command window, you might want to use a script file to enter the set of commands. A simple Windows Scripting File example is attached to this article. Unzip the archive, modify the WSF file as required, and then simply double-click it to run the script. The progress will display in a command window.
Basic Mail and News Protocol Command and Response Overview
All commands are terminated with a carriage return/line feed (CR/LF).
(.tld stands for top level domain, for instance "com".)
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SMTP Commands
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Expected Response
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Description
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HELO
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250 OK
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Initiates the conversation between hosts.
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MAIL FROM: <user@domain.tld>
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250 OK
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Identifies who is sending the message. Include the email address between angle brackets (the brackets are required).
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RCPT TO: <user@domain.tld>
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250 OK
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Identifies whom the message is to. Include the email address between angle brackets (the brackets are required).
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DATA
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354 Send Data
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Identifies the start point for message data.
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. (a single period)
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250 OK
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Identifies the end point for message data.
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QUIT
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221
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Terminates the session.
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- The HELO command is issued and the server responds, indicating that it understands and accepts the command.
- MAIL FROM tells the host who is sending the message. Again, the server responds successfully.
- RCPT TO tells the host whom to deliver this message to.
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POP3 Commands
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Expected Response
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Description
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USER ntdomain/ntaccount/alias
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+OK
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Initiates the authentication process.
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PASS password
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+OK
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Specifies the password for the user's account.
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LIST
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+OK
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Lists available messages to download.
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RETR Message#
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The Message Text
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Retrieves the message text of Message#.
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DELE Message#
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+OK
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Deletes Message#.
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QUIT
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+OK
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Terminates the session.
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- The USER command, followed by the appropriate logon credentials, is used to log on to the POP3 mailbox.
- The PASS command issues the password to gain access.
- Exchange Server returns "+OK User successfully logged on."
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IMAP4 Commands
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Expected Response
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Description
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LOGIN ntdomain/ntaccount/alias password
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OK LOGIN
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Logs on to the mailbox.
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SELECT folder
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Folder mode & OK SELECT
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Selects a folder to view.
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FETCH Message#
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Message text & OK FETCH
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Retrieves the message by Message#.
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STORE Message Flags/flag
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OK STORE
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Marks a message for deletion or read/unread.
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EXPUNGE
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OK
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Deletes all marked messages.
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LOGOUT
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OK
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Terminates the session.
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- All commands issued to an IMAP4 server must be prefixed with a command identifier. The client can interpret that to keep track of command and response pairs.
- For example, when your IMAP client issues local SELECT inbox, the server responds with local OK.
- A NO response is given when a command is denied.
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NNTP Commands
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Expected Response
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Description
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AUTHINFO USER ntdomain/ntaccount/username
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381 More Info Needed
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Supplies authentication information.
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AUTHINFO PASS password
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281 Accepted.
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Supplies password for authentication.
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LIST
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List Of Groups
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Lists all groups available.
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GROUP group
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Group Specifications
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Sets the current group.
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ARTICLE #
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Article Text
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Retrieves an article by #.
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QUIT
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205
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Terminates the session.
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There are two modes of NNTP access: authenticated and anonymous. Authentication requires the first two commands listed (AUTHINFO USER and AUTHINFO PASS).
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- This article was previously published as:
- NETIQKB2699
NETIQKB35161
