Security Basics mailing list archives
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:Hi, 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 LAN. 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. Basically: 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 --------------------------------------------------------------------------- ----------------------------------------------------------------------------
Current thread:
- ssh login protection Edmund (Dec 02)
- Re: ssh login protection Andreas Schubert (Dec 02)
- Re: ssh login protection Burak Bilen (Dec 03)
- <Possible follow-ups>
- RE: ssh login protection Shawn Jackson (Dec 02)
- RE: ssh login protection LordInfidel (Dec 03)
- RE: ssh login protection Tony Kava (Dec 04)