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Listing 9-23. Using the imtest -s Command puppy# imtest -s puppy TLS connection established: TLSv1 with cipher AES256-SHA (256/256 bits) S: * OK puppy Cyrus IMAP4 v2.2.3 server ready C: C01 CAPABILITY S: * CAPABILITY IMAP4 IMAP4rev1 ACL QUOTA LITERAL+ MAILBOX-REFERRALS NAMESPACE UIDPLUS ID NO_ATOMIC_RENAME UNSELECT CHILDREN MULTIAPPEND BINARY SORT THREAD=ORDEREDSUBJECT THREAD=REFERENCES ANNOTATEMORE IDLE AUTH=PLAIN AUTH=LOGIN AUTH=DIGEST-MD5 AUTH=CRAM-MD5 SASL-IR S: C01 OK Completed C: A01 AUTHENTICATE DIGEST-MD5 S: + bm9uY2U9Ikl3MHZGNHhGY05pbzVXa0N4VU8vSUI0RjhwZS9uTldJbTNqMXR0dTFrQ1k9 IixyZWFsbT0icHVwcHkiLHFvcD0iYXV0aCIsbWF4YnVmPTQwOTYsY2hhcnNldD11dGYtOCxhb Gdvcml0aG09bWQ1LXNlc3M= Please enter your password: You can see that the IMAP AUTH command has displayed the LOGIN and PLAIN mechanisms. If I run this against the non-SSL-enabled IMAP on port 143, you would not see these mechanisms, because the plain-text mechanisms are enabled only with TLS. Table 9-7 lists a variety of other options you can input as command-line options to imtest. Table 9-7. imtest/pop3test Command-Line Options
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Specifies the location of a TLS keyfile that contains the TLS public and private keys. Specify "" to negotiate a TLS encryption layer but not use TLS authentication. Allows you to specify the port to connect to. Specifies the particular authentication mechanism to use (for example, PLAIN). Specifies the username to log in as, which defaults to the current user. Enables verbose mode.
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-p port -m mechanism -u user -v
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Listing 9-24 shows the process of logging into my Cyrus IMAP server, puppy, using the PLAIN authentication method on the SSL-enabled port 993 as the user cyrus, with the verbose mode enabled, and using a specific TLS keyfile. Listing 9-24. Further imtest Options puppy# imtest -s -a plain -p 993 -u cyrus -t /var/imap/certs/puppy.pem -v puppy
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Fetchmail is a popular tool authored by Eric S. Raymond (of open-source fame8) to provide remote e-mail retrieval and forwarding functionality using a variety of protocols, including POP3 (and related protocols such as APOP RPOP and KPOP) and most flavors of IMAP ETRN, , , , and ODMR.
s Note I briefly discuss ETRN in 7. ODMR is On-Demand Mail Relay, also known as ATRN, which is best defined as an authenticated version of the SMTP TURN command. It allows SMTP functionality using dynamic IP addresses instead of static IP addresses. See RFC 26459 for more details.
As previously mentioned, Fetchmail is a remote mail retrieval and forwarding tool that uses a variety of protocols to retrieve e-mail via TCP/IP links. Fetchmail retrieves mail from remote MTAs and forwards it via SMTP or local delivery such as LMTP to your mailbox or message store. This allows you to gather e-mail from a variety of remote mailboxes and servers and centralize that e-mail at your local server. This also allows the spam filtering, antivirus, or aliasing functionality capabilities of your MTA to be used on these remote messages as they come in. Figure 9-3 shows how this process works. So why is Fetchmail potentially insecure Some of the insecurities are related to its use of particular protocols, such as IMAP and POP to retrieve e-mail. For example, Fetchmail by , default runs POP and IMAP unencrypted and transmits your password to the remote system in the clear. This exposes you to the potential risk, which also exists with regular unencrypted IMAP or POP that with relatively simple network-sniffing tools an attacker could sniff out the , username and password you are using to connect. Additionally, if you are using .fetchmailrc, your password is often hard-coded into that file to allow Fetchmail to act as a daemon without requiring you to input your password for each session. Finally, as with any other similar tool, I see the possibility of Fetchmail being used as a conduit for Denial of Service (DoS) attacks. I will cover installing Fetchmail and then cover some examples of common uses, such as securely retrieving e-mail from a POP and IMAP server using SSL authentication with Fetchmail, encapsulating Fetchmail transactions using ssh, and demonstrating the most secure ways to configure and use Fetchmail. I will also address how to secure your .fetchmailrc file and limit the potential of Fetchmail being used for a DoS attack. In doing this, I will focus on Fetchmail s use as a mail retrieval tool for POP and IMAP accounts and not cover its use with other protocols such as ETRN or look at using more specialized authenticated protocols such as APOP and KPOP I am taking this approach because . I am attempting to address the practical issues facing most users rather than the more specialized approaches.10 I will not cover any complex Fetchmail configurations or any of the methods of integrating Fetchmail with your MTA. (I will assume you are injecting the mail into an SMTP server located on your local host.)
8. 9.
http://www.catb.org/~esr/who-is-ESR.html and http://www.opensource.org/ http://www.faqs.org/rfcs/rfc2645.html
10. I recommend if you want to investigate these other protocol and configuration options that you start with the Fetchmail home page, which is referenced in the Resources section, and specifically its FAQ section.
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