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Linux - RHEL - Ubuntu

Owned by Gabriel Hernandez
Last updated: Jan 09, 2018
6 min read

There are numerous distributions for Linux operating systems.  Below are a few items to review and implement when deploying a Linux system on the UT network.  Some examples reference configurations for a specific Linux distribution (e.g. RHEL/CentOS), but should translate to other distributions with some slight modifications.  For specific questions for certain distributions, please email help@ece.utexas.edu

 

Anti-Virus Considerations

You may choose any proven anti-virus product. One option is ClamAV.

For UT devices running RHEL or CentOS installations, Cisco AMP - (only for UT systems) is another option.  Please email help@ece.utexas.edu and provide the UT asset tag number (silver sticker The Property of The University of Texas at Austin) for further details.  Your system will need to be verified it is property of the University.

Applying Security Updates

CentOS (command line):

Install and use the yum-security plugin.
To install the plugin run:

 

sudo yum install yum-security

 

To list all updates that are security relevant, and get a reutrn code on whether there are security updates use:

 

sudo yum --security check-update

 

To apply updates that are security relevant use:

 

sudo yum --security update

 

Ubuntu (command line):

Update the Package Index:  The APT package index is essentially a database of available packages from the repositories defined in the /etc/apt/sources.list file.

sudo apt-get update

Upgrade Packages:  updated versions of packages (security updates).

sudo apt-get upgrade

Change SSH Default Port

Depending on your Linux distribution, the configuration file for SSH may reside in different locations.  Please perform a simple web search for instructions on how to change the port used by SSH on your specific Linux distribution.  Below is an example that may guide you through the process.

  • As root, use your favorite text editor to edit the sshd configuration file.

vi /etc/ssh/sshd_config

  • Edit the line which states Port 22. Choose a port that is not currently used on the system.

# What ports, IPs and protocols we listen for

Port 2002

It is recommended that a commonly known port number or a port number currently in use by another application is not selected.  This may cause technical issues with port allocation in the future.  A good secure range of ports you may want to use are ports from 49152 through 65535.

  • Switch over to the new port by restarting SSH.

/etc/init.d/ssh restart

  • Verify SSH is listening on the new port by connecting to it. Note how the port number now needs to be declared.

ssh username@hostname -p 2002

Configure NTP Server

The Information Security Office (ISO) has been proactively scanning systems on the UT wired network for outdated versions of the NTP (Network Time Protocol) service running.  Systems running a vulnerable version of NTP can be compromised, thus begin participation in NTP-based distributed denial of service (DDOS) attacks targeting various endpoints across the internet.  This ultimately leads to an inordinate amount of network usage from the system, and it will be identified from the ISO and the ITS-Networking group.

The Prevention
--------------
1. If you plan to continue running NTP, ensure it is upgraded to 4.2.6, or later.  ITS Networking operates two NTPv4 (NTP version 4) free of charge: 128.83.185.40 (ntp1.utexas.edu) or 128.83.185.41 (ntp2.utexas.edu)
2. Review NTP access restrictions and adjust as needed. Refer to the following
    resources: http://support.ntp.org/bin/view/Support/AccessRestrictions

Example 1:
edit /etc/inet/ntp.client -> ntp.conf
 added:
#added for DDoS prevention - don't allow any machine, except those w/o flags
restrict default notrust nomodify noquery    
restrict 127.0.0.1
restrict 128.83.185.40
restrict 128.83.185.41
 
Example 2 (with additional comments):
# Permit time synchronization with our time source, but do not
# permit the source to query or modify the service on this system.
restrict default ignore
restrict -6 default ignore
 
#restrict default kod nomodify notrap nopeer noquery
#restrict -6 default kod nomodify notrap nopeer noquery

 

# Permit all access over the loopback interface.  This could
# be tightened as well, but to do so would effect some of
# the administrative functions.
restrict 127.0.0.1
server 128.83.185.40
restrict 128.83.185.40 nomodify notrap nopeer noquery
server 128.83.185.41
restrict 128.83.185.41 nomodify notrap nopeer noquery
 
~~~~~~~~``
default servers are commented out
# Use public servers from the pool.ntp.org project.
# Please consider joining the pool (http://www.pool.ntp.org/join.html).
#server 0.centos.pool.ntp.org iburst
#server 1.centos.pool.ntp.org iburst
#server 2.centos.pool.ntp.org iburst
#server 3.centos.pool.ntp.org iburst

Configure Portmap (RPC) Service

A system allows its portmap service to be queried from the public Internet. Portmapper is an RPC service, which always listens on tcp and udp 111, and is used to map other RPC 
services (such as nfs, nlockmgr, quotad, mountd, etc.) to their corresponding port number on the server. When a remote host makes an RPC call to that server, it first consults with 
portmap to determine where the RPC server is listening.Querying portmapper is a small request (~82 bytes via UDP) which generates a large response (7x to 28x amplification), which 
makes it a good candidate for DDoS attacks--especially considering its prevalence among virtually all modern Unix systems.

Portmap must be restricted from the public internet with access controls or authentication.

The Prevention
--------------

Option 1: Uninstall NFS server, NFS client, and Portmapper (RPCbind)

Open a command-line terminal and then type the following command:

$ sudo apt-get --purge remove nfs-kernel-server nfs-common rpcbind

 

Option 2: Portmap lockdown via TCP Wrapper   

 **Note**

Solaris system TCP Wrappers not are enabled by default. Open a command-line terminal and enter the following commands to enable rpcbind TCP Wrappers:


# svccfg -s rpc/bind setprop config/enable_tcpwrappers=true
# svcadm refresh rpc/bind

      (Continue following the instructions below)

 

For all other Linux systems:

Open a command-line terminal and then type the following command:

$ sudo nano /etc/hosts.allowAdd the following lines:
rpcbind: 146.6.101.0/255.255.255.0
rpcbind: 128.83.190.0/255.255.255.0
rpcbind: 129.116.100.192/255.255.255.192
rpcbind: 129.116.238.128/255.255.255.192
rpcbind: 146.6.28.64/255.255.255.192
rpcbind: 146.6.53.0/255.255.255.0
rpcbind: 146.6.177.0/255.255.255.192
rpcbind: 129.116.140.0/255.255.255.0
rpcbind: 129.116.234.0/255.255.255.0
rpcbind: 172.25.1.0/255.255.255.224
rpcbind: 206.76.64.0/255.255.192.0
rpcbind: 198.213.192.0/255.255.192.0
rpcbind: 172.29.0.0/255.255.0.0
rpcbind: 10.0.0.0/255.0.0.0
rpcbind: 10.157.31.128/255.255.255.128
rpcbind: 10.157.33.0/255.255.255.0
rpcbind: 10.157.30.64/255.255.255.192
rpcbind: 10.157.34.0/255.255.255.0
rpcbind: 10.157.26.0/255.255.255.128
rpcbind: 10.157.27.0/255.255.255.0
rpcbind: 10.157.31.0/255.255.255.128
rpcbind: 10.157.29.0/255.255.255.128
rpcbind: 10.157.29.128/255.255.255.128
rpcbind: 10.157.30.0/255.255.255.192
   

Save the changes made to the file.

Type the following command:

$ sudo nano /etc/hosts.deny 

Add the following lines:

rpcbind: ALL

Save the changes made to the file.


Distribution Updates

Many distributions of Linux offer various distributions such as desktop, workstation, or server editions.  Please ensure you understand the differences in each edition and what additional services are installed/running by default in each addition.  Keep in mind unknown services running without proper configuration are vulnerable and lead system compromises.

Also, be sure the edition chosen offers a long-term support life cycle.  For example, Ubuntu publishes the current life cycle for their latest distributions.  If you run Ubuntu, or are considering Ubuntu, refer to the product support life cycle below.

Source:  https://wiki.ubuntu.com/LTS

Enable Firewall

Ubuntu:  https://help.ubuntu.com/community/UFW

RHEL/CentOS:  https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/security_guide/sec-using_firewalls

 

Encryption:  LUKS

Encryption is mandatory for all desktops and laptops with a Linux non-server operating system.

Refer to documentation at - https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/security_guide/sec-encryption

Note:  It is preferred that encryption is enabled during the installation of the operating system.  Initiating encryption after the system has been deployed will wipe all the user's data.

Implement Deny Hosts

DenyHosts is a script intended to be run by Linux system administrators to help prevent SSH server attacks (also known as dictionary based attacks and brute force attacks) - http://denyhosts.sourceforge.net/

Configuration example: set  /etc/hosts for specific restrictions. in this example, allowing *.utexas.edu domain hosts, and restricting everything else.

/etc/hosts./allow
ALL: .utexas.edu
and /etc/hosts.deny
ALL:PARANOID

/etc/hosts.allow is checked 1st, then /etc/hosts.deny.

Particular services can also be allowed to only particular machines,  e.g:

/etc/hosts.allow
sshd:hostname (allowed name of machine or IP address) 
and /etc/hosts.deny
sshd:ALL

Limit SSH Access to Campus Network

Example of IP Tables configuration that will only allow UT campus networks to access a system remotely via SSH.  The networks listed below include various wired, wireless, and VPN networks.

Note: To access these systems from off-campus, users will need to utilize the UT VPN client available at https://vpn.utexas.edu

 

:INPUT ACCEPT [0:0]
:FORWARD ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
-A INPUT -i lo -j ACCEPT
-A INPUT -s 128.62.0.0/16 -j ACCEPT
-A INPUT -s 128.83.0.0/16 -j ACCEPT
-A INPUT -s 129.116.0.0/16 -j ACCEPT
-A INPUT -s 146.6.0.0/16 -j ACCEPT
-A INPUT -s 172.29.0.0/16 -j ACCEPT
-A INPUT -s 198.213.192.0/18 -j ACCEPT
-A INPUT -s 206.76.64.0/18 -j ACCEPT
-A INPUT -s 10.144.0.0/12 -j ACCEPT
-A INPUT -s 128.62.0/16 -j ACCEPT
-A INPUT -s 146.6.248.0/21 -j ACCEPT
-A INPUT -S 198.213.192.0/18 -j ACCEPT
-A INPUT -s 206.76.64.0/18 -j ACCEPT 
-A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT
-A INPUT -j DROP
COMMIT

Password Complexity

On most Linux systems, you can use PAM to enforce password complexity. If you have a file in RHEL/CentOS named /etc/pam.d/system-auth-ac

Example:  Modify pam passwd requirements, length of 12 with special, upper, and lower cases plus a number:

/etc/pam.d/system-auth-ac

#password    requisite     pam_cracklib.so try_first_pass retry=3 type=
password    requisite     pam_cracklib.so try_first_pass retry=3 minlen=10 ucredit=-1 dcredit=-1 ocredit=-1 lcredit=-1

 

/etc/login.defs
PASS_MAX_DAYS   9999999
PASS_MIN_DAYS   0
PASS_MIN_LEN    12
PASS_WARN_AGE   7

To change some of the defaults at user creation time

/etc/default/useradd   

GROUP=1000   <-setting a default group doesn't seem to work. specify with useradd -g <groupname> <username>

#HOME=/home 
HOME=/group/users
INACTIVE=-1
EXPIRE=
SHELL=/bin/bash
SKEL=/etc/skel
#CREATE_MAIL_SPOOL=yes
CREATE_MAIL_SPOOL=no

 

System Accounts

Files/Directory Permissions/Access

  • Enable system accounting (install package sysstat).
  • Integrity checking of system accounts, group memberships, and their associated privileges should be enabled and tested.
    • Check in /etc/sudoers to see who has sudo rights
    • Check in /etc/groups to see what groups your users belong to
    • Check in /etc/passwd and/or /etc/shadow for blank passwords
  • All administrator or root access must be logged.

System Access, Authentication, and Authorization

  • Enable the terminal security file to restrict root logins to system console only.  Do not allow root logins via SSH.
  • Ensure the following are set in /etc/pam.d/other:
     auth  required pam_deny.so
     auth   required pam_warn.so
     account  required pam_deny.so
     account  required pam_warn.so
     password  required pam_deny.so
     password  required pam_warn.so
     session  required pam_deny.so
     session  required pam_warn.so
     session  required pam_deny.so 
      Warn will report alerts to syslog.

 

 

 

 

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