Re: understanding netstat -ap

[Eric Bambach <eric@cisu.net>]

On Sunday 18 September 2005 09:59 am, Ray Olszewski wrote:

Hello,

 These are SSH brute force attempts to guess username password combonations. 
There are alot of such attacks going on as my boxes at work are getting 
hammered by thousands of authentication attempts. The best solution I found 
that doesnt involve a lot of firewall tricks was the pam_abl module. Turn on 
PAM authentication in the SSHD config file usually /etc/ssh/sshd_config and 
add "required auth pam_abl.so" (or something like that read the docs) 
to /etc/pam.d/ssh. 

 Although it wont stop the connections, what pam_abl does is auto-blacklist 
the host after so many failed attempts. They can still try to log in and it 
looks like they're authenticating but even if they have a correct 
username/password pair they will be denied! Its quite a nifty module. Combine 
that with "RootLogin no" in the sshd config file and you can feel safe that 
they will never break in by brute force.

 I use this at work on production boxes and I feel safe against this kind of 
attack.

> Karthik Vishwanath wrote:
> > Hello,
> >
> > As reported previously (Friday 12 August 2005, thread:
> > programs/daemons/PIDs using the network), I happened to notice a lot of
> > activity on the ethernet applet on my desktop. Here are lines that I
> > thought looked most strange from the output of netstat -ap. What do they
> > mean? For instance, does the line (from output below)
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:51222
> > ESTABLISHED
> >
> > mean that someone (?) had an ssh session into this machine?
>
> Yes. Probably some one from IP address 61.218.77.13 ... but to be sure
> of that, use netatat with the -n switch.
>
> > last -adi does not show any untoward activity, however /var/log/auth.log
> > has a whole horde of entries like:
> >
> > Sep 16 21:16:56 mithrandir sshd[16946]: Illegal user a from 64.91.253.157
> > Sep 16 21:16:57 mithrandir sshd[16946]: error: Could not get shadow
> > information for NOUSER
> > Sep 16 21:16:57 mithrandir sshd[16946]: Failed password for illegal user
> > a from 64.91.253.157
> >  port 60348 ssh2
> > Sep 16 21:16:57 mithrandir sshd[16948]: Illegal user b from 64.91.253.157
> > Sep 16 21:16:57 mithrandir sshd[16948]: error: Could not get shadow
> > information for NOUSER
> > Sep 16 21:16:57 mithrandir sshd[16948]: Failed password for illegal user
> > b from 64.91.253.157
> >  port 60369 ssh2
>
> This records a failed login attempt. Actually, two different ones (or
> maybe the same one trying to authenticate twice; depends on how you have
> sshd set up) from the same IP address.
>
> > Must I reinstall the , to feel "safe"?
>
> Huh? Reinstall "the" what?
>
> In general, anyone trying to tell you what to do to "feel" safe is
> saying more than is possible for relative strangers like us.
>
> But to *be* safe, I'd suggest you do the following:
>
> 1. Figure out how connections from external IP addresses are getting to
> a private-interface address at all. Decide if there is a good reason for
> having this access. If not, eliminate it (probably at your router, but I
> don't know enough about your setup to be sure).
>
> 2. If you do need this access, make sure it is secure by
>  A. limiting it to reasonably safe services and their ports. ssh
> qualifies as reasonably safe, for example, while telnet does not.
>  B. seeing to it that all accounts on the system have strong password.
>
> 3. Make sure you are applying security updates regularly and promptly.
> (I forget what distro you use, but most have decent support for security
> udating of their own packages these days.)
>
> 4. If this system does have direct access to the Internet somehow
> (despite its using a private address, I mean), use iptables (or its
> 2.6.x equivalent) to create a good firewall on the system.
>
> It seems that you are the victim of **attempted** breakins. I don't see
> any indication in what you posted (with one possible exception; see
> below) of a **successful** breakin. A successsful breakin would, of
> course, call for an OS-plus-applications reinstall.
>
> > Thanks, regards and sorry for the long post.
> >
> > -K
> >
> > --------------------------------------
> > # netstat -ap
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:50481
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:49720
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         adsl-220-228-117-:50266
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:49175
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:51222
> > ESTABLISHED
> > tcp        0      0 192.168.0.3:ssh         adsl-220-228-117-:49928
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:50040
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:50811
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:49506
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         adsl-220-228-117-:50706
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:51029
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         adsl-220-228-117-:48933
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         adsl-220-228-117-:50373
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:51135
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:49824
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:50584
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:49281
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         adsl-220-228-117-:49394
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:35283       galaxian.gpcc.itd.u:ssh
> > ESTABLISHED
> > tcp        0      0 192.168.0.3:ssh         adsl-220-228-117-:49053
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:50150
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:50921
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:48832
> > TIME_WAIT
> > tcp        0      0 192.168.0.3:ssh         61-218-77-13.HINE:49615
> > TIME_WAIT
> > udp        0      0 192.168.0.3:netbios-ns  *:*
> > udp        0      0 *:netbios-ns            *:*
> > udp        0      0 *:discard               *:*
> > udp        0      0 192.168.0.3:netbios-dgm *:*
> > udp        0      0 *:netbios-dgm           *:*
> > udp        0      0 192.168.0.3:32841       ns.cmc.co.denver:domain
> > ESTABLISHED
> > udp        0      0 *:sunrpc                *:*
>
> This looks to me like someone (or maybe 2 someones, since there are 2
> source addresses) is making a bunch of ssh connections and trying to
> find a userid/password combo that will work. Note that all but 1 of the
> ssh entries is status TIME_WAIT, which in practice means they are
> terminated connections that have not timed out on your system yet. But
> compare these addresses/ports to your logs to be sure of what happened.
>
> The other ESTABLISHED connection is an *outgoing* ssh connection. If you
> don't know what that one it, then I suggest you do need to worry about a
> successful penetration having occurred.
>
> BTW, the 61-218-77-13 address is a dialup IP address in Taiwan. The
> other one is incomplete (try using the -n option) so I cannot check it
> for sure, but 220-228-117-0 also is from Taiwan (probably a DSL block,
> judging from the "adsl" in the name).
>
> -
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-- 
----------------------------------------
--EB

> All is fine except that I can reliably "oops" it simply by trying to read
> from /proc/apm (e.g. cat /proc/apm).
> oops output and ksymoops-2.3.4 output is attached.
> Is there anything else I can contribute?

The latitude and longtitude of the bios writers current position, and
a ballistic missile.

                --Alan Cox LKML-December 08,2000 

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