free barcode generator dll for vb.net The Postfix MTA does not implement the HELP command. in Font

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The Postfix MTA does not implement the HELP command.
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CHAPTER 7 s SECURING YOUR MAIL SERVER
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Disabling Dangerous and Legacy SMTP Commands
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One of the first things you need to do is to look at some SMTP commands. SMTP was designed with some useful commands, such as VRFY, that used to make sending e-mail easier. Those commands now represent more of a liability for your SMTP server than a benefit. I will go through all of these legacy commands and examine how to deal with them. You can see a list of all the potentially unsafe SMTP commands like this in Table 7-1. Some of these commands may be disabled, may be turned off, or are simply not available in future versions of Sendmail and Postfix, but it is better to be sure you have addressed these commands. Table 7-1. SMTP Commands That Are Potentially Insecure
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VRFY EXPN
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Verifies the presence of an e-mail address Expands an e-mail address and shows a list of all the mailboxes or users who will receive messages when e-mail is sent to this address Allows dial-up hosts to retrieve only the mail destined for their domain
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ETRN
Disable if not used
Disabling VRFY
VRFY, for example, is a way for a remote SMTP server to verify that a user or e-mail addresses exists and is valid at another SMTP server. For example, if you Telnet to a Sendmail server with VRFY enabled, you should see something like Listing 7-14. Listing 7-14. Using the VRFY Command [john@kitten]$ telnet puppy.yourdomain.com 25 Trying 192.168.0.1... Connected to puppy.yourdomain.com. Escape character is '^]'. 220 puppy.yourdomain.com ESMTP VRFY root 250 2.1.5 root@puppy.yourdomain.com VRFY jim 550 5.1.1 jim... User unknown This is a Sendmail server, and I have Telneted into it and asked it to check for the presence of some local users. First, I try to VRFY root. Sendmail gives SMTP response code 250 and provides root s e-mail address. In the second attempt I try to VRFY jim. Sendmail reports that jim is an unknown user and returns response code 550. With the VRFY option enabled only attackers and the harvesters of e-mail addresses for spam purposes are able to do two things confirm the presences of a username on your system or confirm that an e-mail address will receive mail and is thus of some value as a target for spam. You can control most of the SMTP command options in Sendmail using this option: define(`confPRIVACY_FLAGS', `flags').
CHAPTER 7 s SECURING YOUR MAIL SERVER
In Sendmail to disable VRFY and other SMTP commands, you need to add flags to the confPRIVACY_FLAGS option in sendmail.mc and then rebuild your sendmail.cf. So to disable VRFY in Sendmail, do this: define(`confPRIVACY_FLAGS', `novrfy') Restart Sendmail, and the VRFY command will be disabled. Listing 7-15 shows the results if you try a VRFY with it disabled in Sendmail Listing 7-15. Results from Sendmail with VRFY Disabled VRFY jim 252 2.5.2 Cannot VRFY user; try RCPT to attempt delivery (or try finger) In Postfix you need to add the option in Listing 7-16 to the main.cf file and then reload or restart Postfix. Listing 7-16. Disabling VRFY in Postfix disable_vrfy_command = yes With this option, Postfix should respond similarly to Listing 7-17. Listing 7-17. Results from Postfix with VRFY Disabled VRFY jim 502 VRFY command is disabled
Disabling EXPN
EXPN stands for expand and allows someone to Telnet to your MTA and query a name. If that name is an alias for multiple recipients, that EXPN command expands that alias into a list of those users. On a Sendmail server using a .forward file, the EXPN command will also show the real forwarding destination of mail. Or you can issue EXPN for the root user and see who receives mail addressed to the system administrator. As you can imagine, this is dangerous both from a security point of view, as attackers can identify a variety of potential user accounts on your system, and from a spam point of view, as spammers can gather considerable numbers of addresses by expanding aliases. As with disabling VRFY, you use the same confPRIVACY_FLAGS option for EXPN. In Listing 7-18 you can see the argument for disabling EXPN added to the confPRIVACY_FLAGS option. Listing 7-18. Disabling EXPN in Sendmail define(`confPRIVACY_FLAGS', `novrfy,noexpn') Rebuild sendmail.cf, and restart Sendmail. When you issue an EXPN, as shown in Listing 7-19, you should see results. Listing 7-19. Results from Sendmail with EXPN Disabled EXPN 502 5.7.0 Sorry, we do not allow this operation
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