PC World 1999 August/PCWorld_1999-08_cd.bin/doc/HOWTO/mini/Secure-POP+SSH

Text File | 1998-10-24 | 12KB | 397 lines

  Secure POP via SSH mini-HOWTO
  Manish Singh, <[e-mail]>
  v1.0, 30 September 1998

  This document explains how to set up secure POP connections using ssh.
  ______________________________________________________________________

  Table of Contents


  1. Introduction

  2. The Basic Technique

     2.1 Setting up Port Forwarding
     2.2 Testing it out

  3. Using it With Your Mail Software

     3.1 Setting up fetchmail
     3.2 Automating it all
     3.3 Not using fetchmail

  4. Miscellany

     4.1 Disclaimer
     4.2 Copyright
     4.3 Acknowledgements


  ______________________________________________________________________

  1.  Introduction


  Normal POP mail sessions, by their very nature, are insecure. The
  password goes across the network in cleartext for everyone to see.
  Now, this may be perfectly acceptable in a trusted or firewalled
  environment. But on a public network, such as a university or your
  run-of-the-mill ISP, anyone armed with a simple network sniffer can
  grab your password right off the wire. This is compounded by the fact
  that many people set their computers to check for mail at regular
  intervals, so the password is sent out quite frequently, which makes
  it easy to sniff.

  With this password, an attacker can now access your email account,
  which may have sensitive or private information. It is also quite
  common that this password is the same as the user's shell account, so
  there is the possibility for more damage.

  By doing all POP traffic using an encrypted channel, nothing goes in
  cleartext over the network. We can use ssh's diverse methods of
  authentication, instead of a simple plaintext password. That is the
  real point of using this method: not because we get encrypted content
  (which is futile at this point, since it's probably gone unencrypted
  over several networks already before reaching your mailbox; securing
  those communications is the job of GNU Privacy Guard or PGP, not ssh),
  but the secure authentication.

  There are other methods of achieving secure authentication already,
  such as APOP, KPOP, and IMAP. However, using ssh has the advantage
  that it works with normal POP configurations, without requiring
  special client (not all mail clients support advanced protocols) or
  server support (except for sshd running on the server). You mail
  provider may be unable or unwilling to use a more secure protocol.
  Besides, by using ssh you can compress the traffic too, which is a
  nice little extra for people with slow connections.


  2.  The Basic Technique


  This technique relies on a fundamental feature of ssh: port forwarding

  There are many variations on this theme, which depend on your desired
  mail setup. They all require ssh, which is available from
  http://www.ssh.fi/ and mirrors.  RPMs are available at
  ftp://ftp.replay.com/pub/crypto/ and Debian packages are available at
  ftp://non-us.debian.org/debian-non-US/ (and their respective mirrors).


  2.1.  Setting up Port Forwarding


  To start port forwarding, run the following command:



       ssh -C -f popserver -L 11110:popserver:110 sleep 5




  Let's take a closer look at that command:



     ssh
        The ssh binary itself, the magic program that does it all.


     -C This enables compression of the datastream. It's optional, but
        usually useful, especially for dialup users.


     -f Once ssh has done authentication and established port
        forwarding, fork to background so other programs can be run.
        Since we're just using the port forwarding features of ssh, we
        don't need a tty attached to it.


     popserver
        The POP server we're connecting to.


     -L 11110:popserver:110
        Forward local port 11110 to port 110 on the remote server
        popserver. We use a high local port (11110) so any user can
        create forwardings.


     sleep 5
        After ssh has forked itself into the background, it runs a
        command. We use sleep so that the connection is maintained for
        enough time for our mail client to setup a connection to the
        server. 5 seconds is usually sufficient time for this to happen.


  You can use most other options to ssh when appropriate. A common
  setting may be a username, since it might be different on the POP
  server.

  This requires sshd running on the remote server popserver. However,
  you do not need to have an active shell account there. The time it
  takes to print a message ``You cannot telnet here'' is enough to setup
  a connection.


  2.2.  Testing it out


  Once you've figured out the details command to run to establish port
  forwarding, you can try it. For example:



       $ ssh -C -f msingh@popserver -L 11110:popserver:110 sleep 1000




  popserver is the ol' POP server. My username on my local machine is
  manish so I need to explicitly specify the username msingh. (If your
  local and remote usernames are the same the msingh@ part is
  unnecessary.

  Then it prints:



       msingh@popserver's password:




  And I type in my POP password (you may have different shell and POP
  passwords though, so use your shell one). Now we're done! So we can
  try:



       $ telnet localhost 11110




  which should print something like:



       QUALCOMM POP v3.33 ready.




  Woohoo! It works! The data is sent out over the network encrypted, so
  the only cleartext is over the loopback interfaces of my local box and
  the POP server.


  3.  Using it With Your Mail Software


  This section describes setting up your POP client software to use the
  ssh forwarded connection. It's primary focus is fetchmail (ESR's
  excellent mail-retrieval and forwarding utility), since that is the
  most flexible software I have found for dealing with POP. fetchmail
  can be found at http://www.tuxedo.org/~esr/fetchmail/.  It will do you
  a great service to read the excellent documentation that comes with
  fetchmail.


  3.1.  Setting up fetchmail


  The following is my .fetchmailrc



       ______________________________________________________________________
       defaults
               user msingh is manish
               no rewrite

       poll localhost with protocol pop3 and port 11110:
               preconnect "ssh -C -f msingh@popserver -L 11110:popserver:110 sleep 5"
               password foobar;
       ______________________________________________________________________




  Pretty simple, huh? fetchmail has a wealth of commands, but the key
  ones are the preconnect line and the poll option.

  We're not connecting directly to the POP server, but instead localhost
  and port 11110. The preconnect does the forwarding each time fetchmail
  is run, leaving open the connection for 5 seconds, so fetchmail can
  make it's own connect. The rest fetchmail does itself.

  So each time you run fetchmail, you're prompted for your ssh password
  for authentication. If you run fetchmail in the background (like I
  do), it's inconvenient to have to do that. Which brings us to the next
  section.


  3.2.  Automating it all


  ssh can authenticate using many methods. One of these is an RSA
  public/private key pair. You can generate an authentication key for
  your account using ssh-keygen. An authetication key can have a
  passphrase associated with it, or the passphase can be blank. Whether
  you want a passphrase depends on how secure you think the account you
  are using locally is.

  If you think your machine is secure, go ahead and have a blank
  passpharase.  Then the above .fetchmailrc works just by running
  fetchmail. You can then run fetchmail in daemon mode when you dial up
  and mail is fetched automatically. You're done.

  However, if you think you need a passphrase, things get more complex.
  ssh can run under control of an agent, which can register keys and
  authenticate whatever ssh connections are made under it. So I have
  this script getmail.sh:



       ______________________________________________________________________
       #!/bin/sh
       ssh-add
       while true; do fetchmail --syslog --invisible; sleep 5m; done
       ______________________________________________________________________

  When I dialup, I run:



       $ ssh-agent getmail.sh




  This prompts me for my passphrase once, then checks mail every 5
  minutes. When the dialup connection is closed, I terminate ssh-agent.
  (This is automated in my ip-up and ip-down scripts)


  3.3.  Not using fetchmail


  What if I can't/don't want to use fetchmail? Pine, Netscape, and some
  other clients have their own POP mechanisms. First, consider using
  fetchmail! It's far more flexible, and mail clients shouldn't be doing
  that kind of stuff anyway. Both Pine and Netscape can be configured to
  use local mail systems.

  But if you must, unless your client has a preconnect feature like
  fetchmail, you're going to have to keep the ssh port forward active
  for the entire time you're connected. Which means using sleep
  100000000 to keep the connection alive. This might not go over well
  with your network admins.

  Secondly, some clients (like Netscape) have the port number hardcoded
  to 110.  So you need to be root to do port forwarding from privledged
  ports. This is also annoying. But it should work.


  4.  Miscellany

  4.1.  Disclaimer


  There is no guarantee that this document lives up to its intended
  purpose. This is simply provided as a free resource. As such, the
  author of the information provided within cannot make any guarentee
  that the information is even accurate. Use at your own risk.

  Cryptographic software such as ssh may be subject to certain
  restrictions, depending on where you live. In some countries, you must
  have a license to use such software. If you are unsure of your local
  laws, please consult someone who is familiar with your situation for
  more information.

  The use of the information provided in this document is most likely
  not anticipated by your mail service provider. The author does not
  encourage the abuse and misuse of network services, and provides this
  document for informational purposes only. If you are in doubt about
  whether the use of these techniques falls within the service agreement
  of your mail provider, please clear that up beforehand.


  4.2.  Copyright


  This document is copyright ⌐ 1998 Manish Singh <[e-mail]>

  Permission is granted to make and distribute verbatim copies of this
  manual provided the copyright notice and this permission notice are
  preserved on all copies.
  Permission is granted to copy and distribute modified versions of this
  document under the conditions for verbatim copying, provided that this
  copyright notice is included exactly as in the original, and that the
  entire resulting derived work is distributed under the terms of a
  permission notice identical to this one.

  Permission is granted to copy and distribute translations of this
  document into another language, under the above conditions for
  modified versions.

  Commercial redistribution is allowed and encouraged; however, the
  author would like to be notified of any such distributions.

  All trademarks used in this document are acknowledged as being owned
  by their respective owners.


  4.3.  Acknowledgements


  Special thanks goes to Seth David Schoen
  <[e-mail]>, who enlightened me in the ways of ssh
  port forwarding.