[CentOS] Security advice, please

Anne Wilson cannewilson at googlemail.com
Mon Mar 23 17:26:34 UTC 2009


On Monday 23 March 2009 15:29:53 JohnS wrote:
> On Mon, 2009-03-23 at 14:31 +0000, Anne Wilson wrote:
> > On Tuesday 23 December 2008 15:38:17 Warren Young wrote:
> > > Michael Simpson wrote:
> > > >> GRC reports that ports are stealthed
> > > >
> > > > Try www.auditmypc.com or nmap-online.com rather than grc to look for
> > > > open ports
> > >
> > > What advantages do they have, in your opinion?
> > >
> > > >> there a better way than opening port 143?
> > > >
> > > > ssh tunnelling?
> > >
> > > I agree, though the default CentOS sshd configuration requires some
> > > tightening down to trust it on Internet-facing servers, IMHO:
> > >
> > > 1. In /etc/ssh/sshd_config, set "PasswordAuthentication no".  No matter
> > > how good your password, it isn't as good as using keys.  Remember,
> > > forwarding ssh opens it to pounding 24x7 from any of the millions on
> > > zombie boxes on the Internet.
> > >
> > > 2. On the machine(s) that you want to allow logins from, run
> > > "ssh-keygen -t rsa" to generate a key pair, if you haven't already. 
> > > Then copy the contents of ~/.ssh/id-rsa.pub into ~/.ssh/authorized_keys
> > > on your home server.  These keys are used to authenticate the remote
> > > system, in lieu of a password or physical token.  You could put these
> > > keys on a USB stick instead, if you didn't want to keep them
> > > permanently on the remote hosts.
> > >
> > > 3. Disable SSHv1 protocol support in /etc/ssh/sshd_config: "Protocol
> > > 2", not "Protocol 2,1".  SSHv1 has known weaknesses.  Boggles my mind
> > > that it's still enabled by default....
> > >
> > > 4. Same file, set "PermitRootLogin no" if it isn't already.
> > >
> > > (Aside: I also like to set up sudo with one account allowed to do
> > > anything, then lock the root account, so the only way to get root
> > > access is to log in as a regular user then sudo up, reducing the risk
> > > of passwordless keys.)
> > >
> > > Having done all this, you're ready to allow remote access:
> > >
> > > 5. In your router, forward a high-numbered port to 22 on the server. 
> > > If it's not smart enough to use different port numbers on either side,
> > > you can change the sshd configuration so it listens on a different port
> > > instead.  I like to use 22022 for this.
> > >
> > > This is *not* security through obscurity.  It's simply a way to reduce
> > > the amount of log spam you have to dig through when monitoring your
> > > system's behavior.  Everything that appears in your logs should be
> > > *interesting*.  Constant port knocking from worms and script kiddies is
> > > not interesting.
> > >
> > > In case you've not done ssh tunelling, Anne, the command that does what
> > > you want, having done all the above is:
> > >
> > > 	$ ssh -p22022 -L10143:my.server.com:143 anne at my.server.com
> > >
> > > This sets up port 10143 on the local system to be redirected through
> > > the ssh session to the IMAP port on your home server.  You don't want
> > > to redirect 143 to 143 because that would require you to run ssh as
> > > root. It also prevents you from using this on a system that itself has
> > > an IMAP server.
> > >
> > > With the tunnel up, you can set up your mail client to connect to port
> > > 10143 on localhost, and you'll be looking at your remote mail server.
> >
> > Hello again.  You were kind enough to give me this advice last December. 
> > I've another holiday approaching and thought it was time that I got this
> > sorted. Unfortunately, I'm not sure that I can do this, so I'm asking
> > your opinion.
> >
> > My router is a Netgear DG834G.  I can create a service, tell it which
> > ports to open, and say which local IP I want it sent to.  However, I
> > can't see any way to set the port to which it should be forwarded as
> > anything other than the incoming port.  IOW, I can enable the new service
> > Ext-ssh, which accepts incoming traffic on port 22022, and direct it to
> > my server on 192.168.0.40, but I can't see how to make it send that
> > traffic to port 22 on the server.
> >
> > Am I totally misunderstanding this?  Really all I want is to be able to
> > log in to the server if I get an email alert that there is a problem or
> > security updates pending.  If I can get this sorted, I'll look again at
> > how to route the IMAP mail through the tunnel too.
>
> ---
> http://kbserver.netgear.com/kb_web_files/n101145.asp
> http://kbserver.netgear.com/kb_web_files/n101145.asp#FR114PAnchor
>
Sure, but those pages are very much like the router's doc pages.  I don't see 
any info about forwarding to ports different from the incoming one.

Anne
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