[CentOS] Security advice, please

Anne Wilson cannewilson at googlemail.com
Mon Mar 23 19:37:18 UTC 2009


On Monday 23 March 2009 16:57:45 JohnS wrote:
> On Mon, 2009-03-23 at 16:26 +0000, Anne Wilson wrote:
> > 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.
>
> ---
> Her's another example it will do what you want, your just
> misunderstanding it. I have 2 customers that use Netgear routers. I
> think your not setting up the Nat - Add Page.
> http://portforward.com/english/routers/port_forwarding/Netgear/DG834G/eMule
>.htm One thing are you using it for the DSL or another modem/router for dsl?
> If your using two only one can be Natted and the other Main router in
> Bridged Mode.

The router is also the DSL modem.

OK - I'm thick.  I've looked at that page and seen only what I'm already 
familiar with.  Please, in plain English, how do I set ssh to come in on port 
22022 (service called ext-ssh already set up for that) to be forwarded to 
192.168.0.xx port 22?

Anne
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