Archive for August, 2007

Aug 04 2007

Securing Linux with Netfilter, IPTABLES and Tcp Wrappers

Published by under linux,Security

First, Netfilter, IPTables are huge subjects that requires lots of time and practice to master it and i don’t claim to be a master of this art. Hence, this post is targeted towards those people who subscribe to a Virtual Private Server (VPS) plan and need to secure their instance against unwanted intruders.

Even though Netfilter and IPTABLES are pretty involved subjects, it turns out that for filtering out unwanted packets it is pretty straight forward, at least in my case.

Before venturing into IPTABLE configuration, i would like to provide few links that i referred to setup my firewall rules.

IPTABLE Tutorial

A Neat write up on securing Cent OS

Example Filter Script

Here is the filter rule that I use. The filter rule must be in /etc/sysconfig/iptables for CentOS. i have tested it on CentOS only.


# By Default Drop all incoming and forwarded packets


#By default allow all packets that originates from your machine to the outside world


#now allow the incoming packets from an established outgoing connection from your machine

# Allow required Incoming ports. Here  i am allowing packets whose destination port is for

# http, https, and SSH
-A INPUT -p tcp –dport 80 -j ACCEPT
-A INPUT -p tcp –dport 443 -j ACCEPT
-A INPUT -p tcp –dport 22 -j ACCEPT

# accept from local host.
-A INPUT -i lo -j ACCEPT

# accept echo requests. This will be good to test whether your server is alive
-A INPUT -p icmp –icmp-type ping -j ACCEPT



Save the file

To be safe make the file readable only by root

$chmod 600 /etc/sysconfig/iptables


Now restart iptables service as follows in CentOS. This should be similar in other flavours as well.

$service iptables restart  /* if you are a root*/
$sudo service iptables restart  /*if you have privilege to gain root permission*/


Test it out

Enabling TCP WRAPPERS TO Add Another Layer of Security

 Many network applications consults two files named hosts.deny and hosts.allow before granting access to the users who want to use those network applications. In securing linux, it is recommended to add several layers of security. So even if one is compromised, others could hold gaurd. TCP Wrappers is another layer of security against the network intruders.

Any application that consults these two files has the following flow:

  1. hosts.allow is checked for “service name:connection address” pair match
  2. If it matches then access is granted.
  3. if it does not match then hosts.deny is checked. If “service name:connection address” pari matches, then access is denied.
  4. If it does not match in hosts.deny as well, then access is granted.


Here are  the basic rules for hosts.allow

# hosts.allow   This file describes the names of the hosts which are
#               allowed to use the local INET services, as decided
#               by the ‘/usr/sbin/tcpd’ server.

#allow connection from (localhost) to all INET services

#Allow connection from all internet address to sshd service
sshd: ALL


Here are the basic rules for hosts.deny

# hosts.deny    This file describes the names of the hosts which are
#               *not* allowed to use the local INET services, as decided
#               by the ‘/usr/sbin/tcpd’ server.

#Simple Deny access from any address to any service. This is like “Deny first, allow required” #policy


Saves the files. Now you are set.

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Aug 03 2007

Doing Away with the Need for Root Account in Linux

Published by under linux,Security

After your installation of Linux, it is a better idea to do away with root account. This is part of making your system as secure as possible. Almost all Linux installations comes with a program named sudo that provides root privileges to normal users without knowing the root password. This way you can execute commands that are meant to be executed by root. Please follow the steps below to enable any user to obtain the root privileges:

  • Need to enable any user or group, who want to gain root privileges, in /etc/sudoers. The normal practice is to enable the users belonging to group wheel to run all root privileged commands.
  • /etc/sudoers is edited by visudo. You need to be root to edit this file.
  • $visudo
    =========== File snippet below =====================
    # sudoers file.
    # This file MUST be edited with the ‘visudo’ command as root.
    # See the sudoers man page for the details on how to write a sudoers file.
    # Host alias specification # User alias specification # Cmnd alias specification

    # Defaults specification

    # User privilege specification
    root ALL=(ALL) ALL

    # Uncomment to allow people in group wheel to run all commands. Uncomment the following line
    %wheel ALL=(ALL) ALL

    # Same thing without a password
    # %wheel ALL=(ALL) NOPASSWD: ALL

  • Now add the user you want to have root privilege into group wheel
    $usermod -a -G wheel “username here without the quotes”
    Now test whether that particular user can gain root privilege. First login to the system with username you want to test
    $wc /etc/sudoers
    wc: /etc/sudoers: Permission denied
    $sudo wc /etc/sudoers
    password: # enter your account password here
    28 94 579 /etc/sudoers
    if you can count the words in that file, that means you can any command that requires root privilege.

If you are using SSH to login to your machine remotely, then follow the procedure below to disallow “root” to login

  • Again it is a good practice to create a group whose users will be allowed SSH access.
  • create a group named sshusers.
  • $sudo groupadd sshusers
    $sudo usermod -a -G sshusers “username who needs to ssh access without quotes”
    Now open /etc/ssh/sshd_config to diable root login and enable the group sshusers
    $sudo vi /etc/sshd/sshd_config=========== File snippet. Only the required portion is shown=
    #LoginGraceTime 2m
    PermitRootLogin no
    #Add this line into the file AllowGroups sshusers

    Test it out just by logging in as root. You should get Access Denied message

    Now try to login using a user that belongs to sshusers group. You should be able to login

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