Sysctl is an interface that allows you to make changes to a running Linux kernel. With /etc/sysctl.conf you can configure various Linux networking and system settings such as:

1. Limit network-transmitted configuration for IPv4
2. Limit network-transmitted configuration for IPv6
3. Turn on execshield protection
4. Prevent against the common 'syn flood attack'
5. Turn on source IP address verification
6. Prevents a cracker from using a spoofing attack against the IP address of the server.
7. Logs several types of suspicious packets, such as spoofed packets, source-routed packets, and redirects.


sysctl command:

The sysctl command is used to modify kernel parameters at runtime. /etc/sysctl.conf is a text file containing sysctl values to be read in and set by sysct at boot time. To view current values, enter:

Code:
# sysctl -a
# sysctl -A
# sysctl mib
# sysctl net.ipv4.conf.all.rp_filter
To load settings, enter:

Code:
# sysctl -p

Sample /etc/sysctl.conf

Edit /etc/sysctl.conf and update it as follows. I recommend reading the official Linux kernel sysctl tuning help file (see below):


Code:
# The following is suitable for dedicated web server, mail, ftp server etc.
# ---------------------------------------
# BOOLEAN Values:
# a) 0 (zero) - disabled / no / false
# b) Non zero - enabled / yes / true
# --------------------------------------
# Controls IP packet forwarding
net.ipv4.ip_forward = 0
 
# Controls source route verification
net.ipv4.conf.default.rp_filter = 1
 
# Do not accept source routing
net.ipv4.conf.default.accept_source_route = 0
 
# Controls the System Request debugging functionality of the kernel
kernel.sysrq = 0
 
# Controls whether core dumps will append the PID to the core filename
# Useful for debugging multi-threaded applications
kernel.core_uses_pid = 1
 
# Controls the use of TCP syncookies
#net.ipv4.tcp_syncookies = 1
net.ipv4.tcp_synack_retries = 2
 
########## IPv4 networking start ##############
# Send redirects, if router, but this is just server
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0
 
# Accept packets with SRR option? No
net.ipv4.conf.all.accept_source_route = 0
 
# Accept Redirects? No, this is not router
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.all.secure_redirects = 0
 
# Log packets with impossible addresses to kernel log? yes
net.ipv4.conf.all.log_martians = 1
net.ipv4.conf.default.accept_source_route = 0
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.conf.default.secure_redirects = 0
 
# Ignore all ICMP ECHO and TIMESTAMP requests sent to it via broadcast/multicast
net.ipv4.icmp_echo_ignore_broadcasts = 1
 
# Prevent against the common 'syn flood attack'
net.ipv4.tcp_syncookies = 1
 
# Enable source validation by reversed path, as specified in RFC1812
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1
 
########## IPv6 networking start ##############
# Number of Router Solicitations to send until assuming no routers are present.
# This is host and not router
net.ipv6.conf.default.router_solicitations = 0
 
# Accept Router Preference in RA?
net.ipv6.conf.default.accept_ra_rtr_pref = 0
 
# Learn Prefix Information in Router Advertisement
net.ipv6.conf.default.accept_ra_pinfo = 0
 
# Setting controls whether the system will accept Hop Limit settings from a router advertisement
net.ipv6.conf.default.accept_ra_defrtr = 0
 
#router advertisements can cause the system to assign a global unicast address to an interface
net.ipv6.conf.default.autoconf = 0
 
#how many neighbor solicitations to send out per address?
net.ipv6.conf.default.dad_transmits = 0
 
# How many global unicast IPv6 addresses can be assigned to each interface?
net.ipv6.conf.default.max_addresses = 1
 
########## IPv6 networking ends ##############
 
#Enable ExecShield protection
kernel.exec-shield = 1
kernel.randomize_va_space = 1
 
# TCP and memory optimization
# increase TCP max buffer size setable using setsockopt()
#net.ipv4.tcp_rmem = 4096 87380 8388608
#net.ipv4.tcp_wmem = 4096 87380 8388608
 
# increase Linux auto tuning TCP buffer limits
#net.core.rmem_max = 8388608
#net.core.wmem_max = 8388608
#net.core.netdev_max_backlog = 5000
#net.ipv4.tcp_window_scaling = 1
 
# increase system file descriptor limit
fs.file-max = 65535
 
#Allow for more PIDs
kernel.pid_max = 65536
 
#Increase system IP port limits
net.ipv4.ip_local_port_range = 2000 65000
Reference :: http://www.cyberciti.biz/faq/linux-k...ity-hardening/