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RE: ssh login protection
From: Tony Kava <securityfocus () pottcounty com>
Date: Thu, 4 Dec 2003 14:04:24 -0600

-----Original Message-----
From: Burak Bilen [mailto:bilen () metu edu tr] 
Sent: Wednesday, 03 December, 2003 05:35
To: Edmund
Subject: Re: ssh login protection

 You could try a two-tier approach. Put an external server(a Pentium-133 
is enough) between your mail servers and the world.
Then allow ssh access(disabling root access) to this external server 
from all of the world. And configure your mail servers that only the
external server is able to ssh your mail servers.

Edmund wrote:


I was wondering if someone could clarify something for me.
I often ssh into two mail servers from dialup(thus dynamic
ip) at home.

Right now, I specify which IPs that can ssh into the two machines but 
for dynamic IPs, I can't do that unless I go crazy and allow 
xx.xx.xx.xx/16, which is not very secure.  But due to the importance 
of me needing to ssh to the servers, I've been 'slacking' off the 
security and allowing a certain range of IPs (those that I'm
certain are from my ISP at home).
Can someone tell me if this is the appropriate way?
Or do I allow any IPs from sshing?

The reason why I'm asking is that I'll be taking
a holiday and believe I'll also need to ssh to the
mail servers.   I don't know the IPs ahead of
time since where I'll be staying, it'll also be
dynamically assigned.

Is there a solution to this problem?  I don't
want to open the servers to attacks from any
SSH-related issues that crackers would take
advantage of.

There is another way to work around these issues.  If you setup that single
SSH machine that still means you'll have to have a machine left open, and
since it can access the other machines it may serve a potential intruder
just as easily as it would serve you.  I personally would not recommend
exposing SSH on any public IP address.  You would be better to setup some
form of encrypted tunnel (even CIPE or OpenVPN would work) from your home to
work, and then you can SSH through the tunnel to these machines across your

Of course you will still have to resolve the dynamic IP issue.  Both CIPE
and OpenVPN (for example) can work with a dynamic client, and your key would
be the method of authentication.  I would not personally rely only on a key
for authentication so I would generally lock down the tunnel to a specific
IP address, but this brings you to your other issue of having a dynamic IP.

You may find that using a dynamic DNS host would be a good way to work
around your changing IP address.  You could use, for example, cjb.net.  They
have a free dynamic DNS service, and there are good clients for both Windows
and Linux.  If you have a Linux machine at home you can setup a cronjob to
update your dynamic DNS hostname at cjb.net every fifteen minutes or so.  If
you're creative you could easily script this to update only when your
address changes.

The next step would be to allow connections from your dynamic DNS hostname
to either a tunnel (better option) or to your SSH daemon.  I'm not sure if
TCP wrappers will let you ALLOW a hostname in place of an IP address.  There
may be an issue if your IP has to reverse resolve to the allowed hostname.
If you can't ALLOW a hostname then you could setup another simple script
(I'm a Perl fan, I guess that's becoming evident) to resolve your dynamic
hostname and put the IP it resolves to into your hosts.allow file.


Use dynamic DNS to work around your dynamic IP address
Use a tunnel to get secure access to your network from remote

If you can do this then you can limit your SSH daemon to only accept
internal connection (i.e. internal or through the tunnel).  The dynamic DNS
would work while you are away on holiday as well.

This is just one idea.  I like this sort of approach better than leaving
anything wide open.

Tony Kava
Network Administrator
Pottawattamie County, Iowa


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