For each tunable kernel parameters you need to be add it to the /etc/sysctl.conf configuration file to make the change permanent after reboots. To activate the configured kernel parameters immediately at runtime, use:
# sysctl -p
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.
To enable TCP SYN Cookie Protection, edit the /etc/sysctl.conf file and add the following line:
net.ipv4.tcp_syncookies = 1
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
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
References: Network Security with /proc/sys/net/ipv4
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