How do I prevent unauthorized ssh login attempts?

Bill Campbell freebsd at
Thu Apr 26 18:39:50 UTC 2007

On Thu, Apr 26, 2007, Kevin Hunter wrote:
>At 11:22a -0400 on 26 Apr 2007, Hal wrote:
>>On Apr 26, 2007, at 8:34 AM, Kevin Hunter wrote:
>>>In general, utilizing public/private keys for remote  
>>>authentication is /much/ more secure than passwords.
>>There is some debate about which is more secure
>>public/private keys or username/password.
>Yep, thank you for that reminder.  :-)  I suppose we now know what  
>I'm arguing!
>>        With public/private keys anyone who has
>>        access to your machine has access to any machine
>>        your machine has a key on.
>Without a passphrase, I'd agree.  The key word that I made sure to  
>put in was 'remote'.  With passphrases, it becomes a two-step  
>authentication, one locally to unlock the private key, and one  
>remotely to at least confirm that you have the other half of the key.

As a rule, we restrict systems to public/private key access with
secure shell.  On a few systems where people claim they need to
use username/password authentication, we restrict access using
tcp wrappers to specific systems.

One of the biggest problems I see is how to handle things like
cron jobs and automatic updates securely without opening up the
can of worms of identities without pass phrases.

Using rsync modules, restricted to specific hosts is very useful
as it permits fine grained control by directory and IP address,
and can be done in many cases over insecure channels as updates
of existing files with rsync only transmits pieces of the files,
not the whole thing.  Many of the things we use this for aren't
very sensitive information, djbdns data files, postfix
configuration, and such.

We also use XMLRPC with https to control some things such as
running make in the /etc/postfix directory where we have the
Makefile set up to rebuild anything necessary as changes are
made.  We have an XMLRPC server (written in python not php thank
you very much :-), which is accessible under the normal Apache
restricted security.  This server has a limited set of procedures
than can do things like restrict access based on the IP address
of the client.  This server can then make an XMLRPC call to
another XMLRPC server running with root priviledges on localhost
to do the processing.

INTERNET:   bill at Celestial.COM  Bill Campbell; Celestial Software LLC
URL:  PO Box 820; 6641 E. Mercer Way
FAX:            (206) 232-9186  Mercer Island, WA 98040-0820; (206) 236-1676

``People from East Germany have found the West so confusing. It's so much
easier when you have only one party.'' -- Linus Torvalde, Linux Expo Canada
when asked about confusion over many Linux distributions.

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