My linux box is rejecting remote logins

My linux box is rejecting remote logins

Post by Michael McGarr » Wed, 24 Apr 2002 01:01:40



Hi,

All of the sudden my linux box is rejecting remote logins and ftps. As soon
as I log in I get a remote disconnect.

Does anyone know why this is happening?

Thanks,

Michael

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My linux box is rejecting remote logins

Post by Juergen Heinz » Wed, 24 Apr 2002 06:55:49



> Hi,

> All of the sudden my linux box is rejecting remote logins and ftps. As soon
> as I log in I get a remote disconnect.

> Does anyone know why this is happening?

[-]
An a wee bit more precise problem description might have possibly
proven helpful, aehm, still ...

* inetd not running anymore
* expired passwords
* f****** up inted configuration
* f****** up tcpd configuration
* f****** up xinetd configuration
* kernel crash
* ...

The files in /var/log/* are generally a good start,
Juergen

--
\ Real name     : Juergen Heinzl       \       no flames      /


 
 
 

My linux box is rejecting remote logins

Post by Bit Twist » Wed, 24 Apr 2002 08:14:32



> Hi,

> All of the sudden my linux box is rejecting remote logins and ftps. As soon
> as I log in I get a remote disconnect.

> Does anyone know why this is happening?

Did you change firewall or msec settings.

I did see a post where a cracker, cracked a box and installed security
updates and hardened the box so other crackers could not crack his new
box. Hope that did not happen to you.

IF your box is cracked, unplug your system from the internet, Your
machine is a menace to society and you until it's cleaned it up.

Here is why you need a FORMAT and clean install when your box IS cracked.
   http://www.linuxdoc.org/LDP/LG/issue36/kuethe.html
4'th paragraph.

Think about that paragraph.
You cannot use ANY of your pc's utilities to see if your box is cracked
and find what addtional files are installed.

http://www.chkrootkit.org   has a program for checking for rootkit installs
on the cracked box. That will tell you about known root kits if you have one.
The cracker may not have installed a rootkit.

What you can do is have a dual boot system. You install a second copy of
your OS and label it Auditor. You never, EVER mount it from the internet OS.

Anytime you THINK you've been cracked, you can boot into Auditor, mount the
internet os partition and start checking the internet OS partitions for new
files and whatnot.

Any time you KNOW your're box is cracked, you should:
o       Pull the box off the network. You do not want the police taking
        you and your equipment to jail because a cracker used it
        to crack a bank or military site. If the cracker removes their
        backtracks to their box, you get to do the jail time.

o       Put the hardrive(s) into a standalone machine,
        mount the disk(s) readonly,
        save any data, user files, ...,

o       Save a full copy of the disk(s) for your forensic attempt,
        save the disk(s) for FBI forensics if it's a Big, BIG dollar loss.

o       Re-FORMAT disk drives and do a fresh install from known clean
        source to remove any possible back doors and/or password sniffers
        the cracker installed.

o       Restore your saved files, verify that the restored files
        do not have the suid bit set  "find / -perm +6000 -ls".

o       Have everyone on the box's network change passwords and
        tell them that the cracker may have been running a
        password sniffer so they will not use the passwords ever again.
        Any other boxes logged into from the cracked box should
        have their passwords changed.

Install a modern firewall. Example: iptables is better than ipchains.
If you have a spare linux computer, you can use it to port scan
your box with nmap from http://www.insecure.org/nmap/

Get all the vendor updates to your distro.

You might want to read Armoring Linux
        http://www.linuxdoc.org/HOWTO/Security-Quickstart-HOWTO/index.html
        http://www.enteract.com/~lspitz/linux.html
http://www.ibiblio.org/pub/Linux/docs/HOWTO/Security-Quickstart-Redha...
http://www.ibiblio.org/pub/Linux/docs/linux-doc-project/solrhe/Secur
ing-Optimizing-Linux-RH-Edition-v1.3.txt

        http://www.linuxsecurity.com/docs/colsfaq.html
        http://www.securityportal.com/lskb/articles/
        http://www.securityportal.com/lasg/
keep an eye on
        http://www.cert.org/advisories/

For cheap install cd's
http://cart.cheapbytes.com/cgi-bin/cart
top left under Products.
For people accross the pond,
        http://www.linuxemporium.co.uk
        http://www.linux123.co.uk/

Never login as root unless you have to.
Always login from the console, no su, telnet, ssh,..
That way a keystroke logger in your user account cannot
catch your root login password.

You can audit your system if you are using the  rpm   package manager with
  rpm -Va | grep '..5' > /tmp/verify.log
Runs for a while; more than 5 minutes.

/tmp/verify.log  will contain changes which you have made using
configuration tools

Hope crackers do not put in a rootkit which makes the rpm check obsolete.
I think this has happened, though not sure. On one of my boxes
it cored after about 2 minutes, log looked like it ran but never completes
the audit.

rpm -Va | grep '..5' will give you a warm feeling about what changed.
That warm feeling might turn into the warm feeling you get when
you do not get to the bathroom in time.  :(

The cracker could install trojaned files some where else and modify
PATH to use them instead of the files you just checked.
You could look at the report and see
        S.5....T c /root/.bash_profile
        S.5....T c /root/.bashrc
You see that and say, "Ok, I did change those. No problem."
BZZZZzzit. WRONG answer, Cracker changed your PATH and you are
running his code.

It also does not show additional files. I have created a site file in
/etc/profile.d which puts my site/bin into PATH.

Cracker can add his own cracked.sh file to change/add to PATH and
create aliases to substitute a stock command for his code.