Wed Mar 1 16:14:18 CET 2006
(as copied from :
http://rudd-o.com/archives/2006/02/27/hardening-a-linux-server-in-10-minutes/ )
Did you know that a freshly installed Linux server can be hardened in
less than 10 minutes? Heres how!
Print these instructions out, and keep them posted on a wall in your
office or home. Before plugging a freshly installed network server,
simply remember to follow these instructions. Make these instructions
second nature to you.
Youll need a bit of experience with the Linux command-line environment,
as the following commands are usually issued in a terminal. You will
need root access on your server as well. By the way, the following
instructions apply to any LSB-compliant Linux distribution, but Ill use
Fedora Core as an example.
Step 1: turn all unneeded services off
There are two kinds of network services:
those that get started as init.d services
those that get started by xinetd
This distinction is important, as xinetd can start services on demand,
while services started through init.d run all the time.
Okay, time to start securing your server. On a terminal, as root (and,
for the purposes of this tutorial, assume this from now on) run netstat
-ltunp. You should see a listing like this one:
[root@andrea rudd-o]# netstat -ltunp
Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address Foreign Address State PID/Program name
tcp 0 0 0.0.0.0:3493 0.0.0.0:* LISTEN 30562/upsd
tcp 0 0 0.0.0.0:3306 0.0.0.0:* LISTEN 12461/mysqld
tcp 0 0 0.0.0.0:6543 0.0.0.0:* LISTEN 12490/mythbackend
tcp 0 0 0.0.0.0:111 0.0.0.0:* LISTEN 1771/portmap
tcp 0 0 0.0.0.0:6544 0.0.0.0:* LISTEN 12490/mythbackend
tcp 0 0 127.0.0.1:631 0.0.0.0:* LISTEN 31537/cupsd
tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN 2143/sendmail: acce
tcp 0 0 :::80 :::* LISTEN 5024/httpd
tcp 0 0 :::22 :::* LISTEN 2009/sshd
tcp 0 0 0.0.0.0:19 0.0.0.0:* LISTEN 2019/xinetd
Those are all processes listening to specific ports. As you can see,
the PID (process ID) and the program name are displayed as well.
Make two lists: – one for the services you absolutely need (which you
should already know by heart), and – one for the services that are
expendable or you can start manually when theyre needed (tip: each
program name usually ships with a man page).
Shutdown each service on the second list (except for xinetd) Thats a
pretty straightforward task. Each one of those services are started by
init.d. To find out the name of the service control script, just hop to
/etc/rc.d/init.d and look for a file with a name similar to the program
name.
Example: suppose I dont need mythbackend. To stop it:
/etc/rc.d/init.d/mythbackend stop (some distributions provide the
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service mythbackend stop command, which is easier on your fingers).
Now, to disable it: chkconfig –del mythbackend. After doing this, you
should check to see if the offending service went away, with the same
netstat -ltunp command.
That pesky xinetd
Great. So you got rid of the unneeded services. But theres more. As we
saw earlier, xinetd has its own ways. In practice, this means that some
services will be started on demand thus, you wont see them under your
netstat -ltunp listing.
To find out which services xinetd manages, hop to /etc/xinetd.d and do
a directory listing. You should see some service configuration files.
Identify the ones you wont be using, and edit each one of them, adding
a line that says disable = yes between the curly braces.
Note that some services already ship with disable = yes, but some ship
with disable = no. If one of the configuration files says disable = no,
just change it to disable = yes. Now reload xinetd with the famous
/etc/rc.d/init.d/xinetd reload, and run netstat -ltunp again, just to
be sure.
Thats step 1. With a bit of practice, you should be doing this in five
minutes or less.
Step 2: limit access to running services using iptables
Great, our server now runs the absolutely required services, and no
more. But some of those services arent meant to be accessed from
everywhere, right? For example: I may have a MySQL database server
running, but that doesnt mean MySQL should be accessible from any
random IP address on the Internet, right?
So, well use the firewall to stop evil at the door. Again, make a list
of services. For each item on the list, identify which IP addresses
should be able to reach the service. For each service on your list,
write down the TCP/UDP port(s) they use.
In my example, MySQL uses TCP port 3306, and should only be accessible
by localhost (127.0.0.1).
Time to compose and activate the iptables rules. Doing a quick check
with iptables -L, I can see that my INPUT chain (the one Ill be working
with, since I want to disallow INPUTs to my server) is empty:
[root@andrea xinetd.d]# iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
Chain FORWARD (policy ACCEPT)
target prot opt source destination
Chain OUTPUT (policy ACCEPT)
target prot opt source destination
Your mileage may vary, because your distribution may already have set
up some basic iptables rules; to make these instructions foolproof, I
will be inserting rules at the beginning of the INPUT chain.
In this case, I want to allow access to 127.0.0.1:3306, and deny access
to everyone else on port 3306, in that order. So two rules are needed.
Ill add the allow rule into position 1 (the very first):
[root@andrea xinetd.d]# iptables -I INPUT 1 –protocol tcp –destination-port 3306 -s 127.0.0.1 -j ACCEPT
[root@andrea xinetd.d]# iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT tcp — localhost.localdomain anywhere tcp dpt:mysql
Great. Im telling the firewall to -j ACCEPT all –protocol tcp
connections to –destination-port 3306 from the address -s 127.0.0.1.
Now, Ill insert the deny rule into position 2:
[root@andrea xinetd.d]# iptables -I INPUT 2 –protocol tcp –destination-port 3306 -j REJECT
[root@andrea xinetd.d]# iptables -L
Chain INPUT (policy ACCEPT)
target prot opt source destination
ACCEPT tcp — localhost.localdomain anywhere tcp dpt:mysql
REJECT tcp — anywhere anywhere tcp dpt:mysql reject-with icmp-port-unreachable
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See how easy it is? Let me explain: rule 2 tells the firewall to -j
REJECT all –protocol tcp connections to –destination-port 3306 from
any address (since I omitted the address). Since rules are processed
top-down (from 1 to n), the first one that matches an incoming
connection is applied. If no rules match, then the default policy
(which is normally ACCEPT) kicks in.
Lather. Rinse. Repeat for every service that you want to secure.
Finally, save the rules. For this, youll need to use your distributions
tools. For Fedora Core, thats as easy as issuing the command service
iptables save and ensuring that the iptables service runs at boot time:
chkconfig –add iptables.
Its worth noting that some people prefer to -j DROP instead of DENYing.
DROP means that your server will ignore connection attempts (neither
denying connections nor accepting them). I prefer DENY, because its
easier to pinpoint a problem with iptables rules that way, and (most
importantly) DROP rules make those ports appear as filtered to a
hostile port scanner (which hints to the attacker that a service is
running).
So thats it, from insecure to secure in 10 minutes! If you have any
suggestions or questions, please leave them as comments below. Happy
hacking!
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This entry was posted on Monday, February 27th, 2006 at 18:55 and is
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