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Hardening TCP/IP stack in Linux

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TCP handshake Sequence numbers IP troubleshooting Humor Etc

Protecting TCP/IP stack is a very challenging task in a culture where easy access to information prevails over security concerns. The key problem here is that the need for an efficient enterprise to provide relatively unfettered access to data, combined with the highly decentralized nature of operations, is irrevocably connected with the potential for serious security breaches. Maintaining and, especially, improvement of large enterprises IT security is a huge challenge and introduction of new OSes like Linux is only one relatively minor problem among many others.

We sing out the following critical issues :

  1. That maturity of a OS platform from the security standpoint is highly dependent of the availability and quality of virtualization components

    Network infrastructure and server complexity in the large enterprises has increased so significantly that it has become a constraint on how flexible a business can be. Server consolidation based on virtual machine concept in a large enterprise environment is the necessity that no large enterprise can avoid. This movement already started in AIX space and Windows space (sometimes under VMware, which is this case can be reused for Linux virtualization purposes), but it will definitely accelerated in the future.

  2. "security via obscurity" is a viable strategy.  

    Linux's growing popularity is attracting unwanted attention from virus writers, script kiddies  and criminal elements. In response, Linux advocates are putting a new emphasis on security measures and working to reassure large enterprises that the OS is secure for important enterprise applications.

    Using different compiler to make application less susseptable to off-the-shelf exploits is one promising avenue. Recompiling such component as sendmail is easy.

    Many exploits are complier dependent and the necessity to cover both gcc and Sun Studio 10 compliers significantly complicates the creation of working exploit.  For example Intel compiler is definitely recommended for compiling bind and Sendmail.  Obscurity understood here as using less popular  software platforms with some additional security features is a viable method to secure any complex operating environment and being off the most popular (and the most vulnerable) applications is always a plus.  

  3. It's very important to distinguish between security of the Linux itself (OS platform) and security of major open source applications (like Apache, Bind, Perl, PHP, Postgress, Sendmail, etc).  Most vulnerabilities that are sited as Linux vulnerabilities actually are the vulnerabilities of the applications that are deployed on Linux.
  4. Open Source software are ideal for quick prototyping and can help to avoid costly deployment mistakes that often happen with proprietary products.  For this particular purpose Linux has an upper hand as most applications were tested on Linux and work "out of the box" in a Linux environment; the current Linux distributions can be installed on typical corporate PCs without problems.  The role of Linux as a antidote to red-tape should not be underestimated in a large corporate environment. Many prototypes on Linux can be created using regular workstations instead of servers with zero or minimal (the cost of additional memory) acquisition costs.   Often early prototyping can prove that open source solution are more economical than proprietary closed solutions  or can deliver at least 80% of functionality for, say, 20% of costs and thus can substantially lower software acquisition costs. In case the decision is make to go with the proprietary vendor experience gained with the open source prototype provides a much more realistic estimate of deployment costs than any other method as well as dramatically improves negotiating power in talks with the vendor and help to avoid costly mistakes.







route | grep link-local


* U 0


0 eth2


Edit /etc/sysconfig/network


Then remove the avahi package and its dependencies

Review Listening Daemons


netstat -tanp | grep LISTEN

Typical output:

[root ]# netstat -tanp | grep LISTEN

tcp 0 0* LISTEN 2256/nasd

tcp 0 0* LISTEN 2166/mysqld

tcp 0 0* LISTEN 2376/prelude-manage

tcp 0 0* LISTEN 2057/cupsd

tcp 0 0* LISTEN 2244/master

tcp 0 0 :::22 :::* LISTEN 2068/sshd

Disabling Listening Daemons

Locate the pid in the netstat command

cat /proc/<pid>/cmdline

If not full path, run which or locate to find utility

rpm -qf full-path-of-daemon

rpm -e package

If difficult to remove due to dependencies:

chkconfig <service> off

/etc/sysctl.conf settings

# Don't reply to broadcasts. Prevents joining a smurf attack

net.ipv4.icmp_echo_ignore_broadcasts = 1

# Enable protection for bad icmp error messages

net.ipv4.icmp_ignore_bogus_error_responses = 1

# Enable syncookies for SYN flood attack protection

net.ipv4.tcp_syncookies = 1

# Log spoofed, source routed, and redirect packets

net.ipv4.conf.all.log_martians = 1

net.ipv4.conf.default.log_martians = 1

# Don't allow source routed packets

net.ipv4.conf.all.accept_source_route = 0

net.ipv4.conf.default.accept_source_route = 0

# Turn on reverse path filtering

net.ipv4.conf.all.rp_filter = 1

net.ipv4.conf.default.rp_filter = 1

# Don't allow outsiders to alter the routing tables

net.ipv4.conf.all.accept_redirects = 0

net.ipv4.conf.default.accept_redirects = 0

net.ipv4.conf.all.secure_redirects = 0

net.ipv4.conf.default.secure_redirects = 0

# Don't pass traffic between networks or act as a router

net.ipv4.ip_forward = 0

net.ipv4.conf.all.send_redirects = 0

net.ipv4.conf.default.send_redirects = 0




smbd: 192.168.1.

Unused Daemon Removal

Remove all daemons (and packages) not being used

This reduces attack footprint and improves performance

Many daemons listen on the network and could be accessible


chkconfig –list


rpm -qf /etc/rc.d/init.d/name 
rpm -e package-name
chkconfig <service> off

Leave cpuspeed for speedshifting cpu and irqbalance for multicore


Disable readahead, mcstransd, firstboot, (and NetworkManager for machines without wireless networking) since they are not needed.

System Time

/usr/sbin/ntpdate ntp-server

At & cron

Only allow root and people with verified need to run cron jobs

Setup cron.allow and cron.deny

Setup equivalents if you have 'at' installed







TCP stack protection

Enable TCP SYN Cookie Protection

A "SYN Attack" is a denial of service attack that consumes all the resources on a machine. Any server that is connected to a network is potentially subject to this attack.

Disable IP Source Routing

Source Routing is used to specify a path or route through the network from source to destination. This feature can be used by network people for diagnosing problems. However, if an intruder was able to send a source routed packet into the network, then he could intercept the replies and your server might not know that it's not communicating with a trusted server.

To enable Source Route Verification, edit the /etc/sysctl.conf file and add the following line:

  net.ipv4.conf.all.accept_source_route = 0

To enable TCP SYN Cookie Protection, edit the /etc/sysctl.conf file and add the following line:

  net.ipv4.tcp_syncookies = 1
Disable ICMP Redirect Acceptance

ICMP redirects are used by routers to tell the server that there is a better path to other networks than the one chosen by the server. However, an intruder could potentially use ICMP redirect packets to alter the hosts's routing table by causing traffic to use a path you didn't intend.

To disable ICMP Redirect Acceptance, edit the /etc/sysctl.conf file and add the following line:

  net.ipv4.conf.all.accept_redirects = 0
Enable IP Spoofing Protection

IP spoofing is a technique where an intruder sends out packets which claim to be from another host by manipulating the source address. IP spoofing is very often used for denial of service attacks. For more information on IP Spoofing, I recommend the article IP Spoofing: Understanding the basics.

To enable IP Spoofing Protection, turn on Source Address Verification. Edit the /etc/sysctl.conf file and add the following line:

  net.ipv4.conf.all.rp_filter = 1
Enable Ignoring to ICMP Requests

If you want or need Linux to ignore ping requests, edit the /etc/sysctl.conf file and add the following line:

  net.ipv4.icmp_echo_ignore_all = 1
This cannot be done in many environments.

Enable Ignoring Broadcasts Request

If you want or need Linux to ignore broadcast requests, edit the /etc/sysctl.conf file and add the following line:

  net.ipv4.icmp_echo_ignore_broadcasts = 1
Enable Bad Error Message Protection

To alert you about bad error messages in the network, edit the /etc/sysctl.conf file and add the following line:

  net.ipv4.icmp_ignore_bogus_error_responses = 1
Enable Logging of Spoofed Packets, Source Routed Packets, Redirect Packets

To turn on logging for Spoofed Packets, Source Routed Packets, and Redirect Packets, edit the /etc/sysctl.conf file and add the following line:

  net.ipv4.conf.all.log_martians = 1

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Last updated: March 12, 2019