[CentOS] Security advice, please

JohnS jses27 at gmail.com
Mon Mar 23 16:29:53 UTC 2009


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

JohnStanley



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