hosts.deny not working?

a2vr6 · 08-15-2006, 10:32 AM

After many years working in a Windows environment I have been moved into a different position at my job working on Unix/Linux systems. We had a c omplaint of hack attempts via ssh to some servers recently. I confirmed the IPS in the log files and compared to what was listed in hosts.deny.

Hosts.deny is setup to deny ssh sessions from all IPs except for a select few. Hosts.allow is empty. Can someone tell me why these rogue IP's are allowed connectivity if they are not on the allow list?? Any help would be much appreciated. Thanks.

unSpawn · 08-15-2006, 10:50 AM

I suggest not relying on one line of defense. Since this is really about SSH attempts best read the solutions listed in the Linux Security sticky thread "Failed SSH login attempts" and pick a solution appropriate for your situation.

Here's an excerpt from that may shed a light on what's happening:

Quote:

Originally Posted by ^H^H^H from man 5 hosts_access

. Access will be granted when a (daemon,client) pair matches an
entry in the /etc/hosts.allow file.

. Otherwise, access will be denied when a (daemon,client) pair
matches an entry in the /etc/hosts.deny file.

. Otherwise, access will be granted.

Basically you will want to drop everything from hosts.deny (ALL: ALL) and set up exceptions in hosts.allow.

HTH

a2vr6 · 08-15-2006, 01:43 PM

Just wondering how this is different? Will the person trying to ssh into the server get dropped immediately or will they get same message as if it was the other way around?

haertig · 08-15-2006, 02:30 PM

Quote:

Originally Posted by a2vr6

Can someone tell me why these rogue IP's are allowed connectivity if they are not on the allow list??

Is your ssh daemon compiled with tcp_wrappers support?

Look for "libwrap", like this:

Code:

$ whereis sshd
sshd: /usr/sbin/sshd /usr/share/man/man8/sshd.8.gz
$ ldd /usr/sbin/sshd
        linux-gate.so.1 =>  (0xffffe000)
        libwrap.so.0 => /lib/libwrap.so.0 (0xb7f87000)
        libpam.so.0 => /lib/libpam.so.0 (0xb7f7f000)
        libdl.so.2 => /lib/tls/libdl.so.2 (0xb7f7b000)
        libresolv.so.2 => /lib/tls/libresolv.so.2 (0xb7f68000)
        libcrypto.so.0.9.7 => /usr/lib/i686/cmov/libcrypto.so.0.9.7 (0xb7e69000)
        libutil.so.1 => /lib/tls/libutil.so.1 (0xb7e64000)
        libz.so.1 => /usr/lib/libz.so.1 (0xb7e52000)
        libnsl.so.1 => /lib/tls/libnsl.so.1 (0xb7e3c000)
        libcrypt.so.1 => /lib/tls/libcrypt.so.1 (0xb7e0e000)
        libpthread.so.0 => /lib/tls/libpthread.so.0 (0xb7dfc000)
        libc.so.6 => /lib/tls/libc.so.6 (0xb7cc4000)
        /lib/ld-linux.so.2 (0xb7fa0000)
$

You can see the 2nd line of my ldd output lists "libwrap.so.0", so mine is compiled with the required tcp_wrappers support. Most distros are compiled with this support in sshd, but it wouldn't hurt to check - just to make sure. Without such compiled in support, sshd will ignore your hosts.allow or hosts.deny files.

a2vr6 · 08-15-2006, 03:14 PM

thanks for the reply. I checked and confirmed it is in fact compiled with the required tcp_wrapper support.

a2vr6 · 08-16-2006, 01:14 PM

anyone else have some input?

haertig · 08-16-2006, 01:32 PM

Is there anything in your /var/log/auth.log file that might give you some clue? (file location may be different for your distro) I wouldn't expect much to be there, but check anyway.

Somebody might be able to spot something amiss if you cut/paste the contents of the following files:

/etc/ssh/sshd_config
/etc/hosts.allow
/etc/hosts.deny

druuna · 08-16-2006, 01:48 PM

Hi,

Something else you should check:

Quote:

When you installed TCP wrappers, you should have kept the tcpd.h and libwrap.a files. Usually tcpd.h goes in /usr/local/include and libwrap.a goes in /usr/local/lib. By default, installing TCP wrappers doesn't install these two files, so you may need to grab the TCP wrappers source and recompile it, then install those two files.

This info came from this site (chapter 5.9): ssh faq 5

It's not much, but you never know.

haertig · 08-16-2006, 02:15 PM

Since ldd reports that the OP's sshd is dynamically linked to the libwrap shared library (libwrap.so), he won't need the static version (libwrap.a)

druuna · 08-16-2006, 02:27 PM

Hi,

I see, I thought this had to be done after compiling sshd with wrapper support. Guess you must do it before you start the compile part.

Well, like I stated: It wasn't much and, as turns out it was nothing

Tinkster · 08-16-2006, 06:42 PM

Quote:

Originally Posted by a2vr6

anyone else have some input?

How about you show us the actual content of /etc/hosts.allow
and /etc/hosts.deny ?

If you're concerned about security you can always anonymise
the IP addresses if they're not in private IP ranges ....

Cheers,
Tink