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In this tutorial I'll show you how you can optimise server load via kernel sysctl .          
Sysct is an interface that allows you to make changes to a running Linux/Unix system. This includes many advanced options of the TCP/IP stack and virtual memory system that can dramatically improve performance for an experienced system administrator. Over five hundred system variables can be read and set using sysctl.conf.You can find every variable explained here:
/usr/src/linux-`uname -r`/Documentation/networking/ip-sysctl.txt

Next configuration brought my load down from the teens to 99% of the time under 2.

Now, I have a dedicated Linux box running dual xeons and 2gb ram. If you don't have SSh access, you can pretty much forget about trying this.
Make backups of anything and everything before trying this. I can't be held responsible for anything u mess up. proceed with caution.
Open /etc/sysctl.conf and replace what is in there with this

# Kernel sysctl configuration file for Red Hat Linux

# Controls IP packet forwarding
net.ipv4.ip_forward = 0

# Controls source route verification
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.lo.rp_filter = 1
net.ipv4.conf.eth0.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1

# Disables IP source routing
net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.lo.accept_source_route = 0
net.ipv4.conf.eth0.accept_source_route = 0
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

# Increase maximum amount of memory allocated to shm

# kernel.shmmax = 1073741824

 


# Disable ICMP Redirect Acceptance

net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.lo.accept_redirects = 0
net.ipv4.conf.eth0.accept_redirects = 0
net.ipv4.conf.default.accept_redirects = 0

# Enable Log Spoofed Packets, Source Routed Packets, Redirect Packets

net.ipv4.conf.all.log_martians = 1

net.ipv4.conf.lo.log_martians = 1
net.ipv4.conf.eth0.log_martians = 1

# Decrease the time default value for tcp_fin_timeout connection
net.ipv4.tcp_fin_timeout = 30

# Decrease the time default value for tcp_keepalive_time connection
net.ipv4.tcp_keepalive_time = 1800

# Turn on the tcp_window_scaling
net.ipv4.tcp_window_scaling = 1

# Turn on the tcp_sack
net.ipv4.tcp_sack = 1
# tcp_fack should be on because of sack
net.ipv4.tcp_fack = 1

# Turn on the tcp_timestamps
net.ipv4.tcp_timestamps = 1

# Enable TCP SYN Cookie Protection
net.ipv4.tcp_syncookies = 1

# Enable ignoring broadcasts request
net.ipv4.icmp_echo_ignore_broadcasts = 1

# Enable bad error message Protection
net.ipv4.icmp_ignore_bogus_error_responses = 1

# Make more local ports available
# net.ipv4.ip_local_port_range = 1024 65000

# Set TCP Re-Ordering value in kernel to '5'
net.ipv4.tcp_reordering = 5

# Set SYN ACK retry attempts to '3'
net.ipv4.tcp_synack_retries = 3

# Various Settings
net.core.netdev_max_backlog = 1024

# Increase the maximum number of skb-heads to be cached
net.core.hot_list_length = 256

# Increase the tcp-time-wait buckets pool size
net.ipv4.tcp_max_tw_buckets = 360000

# This will increase the amount of memory available for socket input/output queues
net.core.rmem_default = 65535

net.core.rmem_max = 8388608
net.ipv4.tcp_rmem = 4096 87380 8388608
net.core.wmem_default = 65535
net.core.wmem_max = 8388608
net.ipv4.tcp_wmem = 4096 65535 8388608
net.ipv4.tcp_mem = 8388608 8388608 8388608
net.core.optmem_max = 40960

After you make the changes to make them effective without rebooting, simply run the following commands:

/sbin/sysctl -p
/sbin/sysctl -w net.ipv4.route.flush=1
Another example is of a host wich was under a 5mbit syn flood attack for the past few days. Enabling syncookies instantly dropped the load averages. Despite still being under attack, the site is as responsive as ever:

 

net.ipv4.ip_forward=0

kernel.sysrq=0
kernel.core_uses_pid=1
kernel.shmmax = 134217728
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.lo.rp_filter = 1
net.ipv4.conf.eth0.rp_filter = 1
net.ipv4.conf.default.rp_filter = 1
net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.lo.accept_source_route = 0
net.ipv4.conf.eth0.accept_source_route = 0
net.ipv4.conf.default.accept_source_route = 0
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.lo.accept_redirects = 0
net.ipv4.conf.eth0.accept_redirects = 0
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.tcp_fin_timeout = 20
net.ipv4.tcp_keepalive_time = 1800
net.ipv4.tcp_sack = 1
net.ipv4.tcp_fack = 1
net.ipv4.tcp_syncookies = 1
net.ipv4.icmp_echo_ignore_broadcasts = 1
net.ipv4.icmp_ignore_bogus_error_responses = 1
net.ipv4.ip_local_port_range = 16384 65536
net.core.netdev_max_backlog = 512
net.core.rmem_default = 65535
net.core.rmem_max = 8388608
net.ipv4.tcp_rmem = 4096 87380 8388608
net.core.wmem_default = 65535
net.core.wmem_max = 8388608
net.ipv4.tcp_wmem = 4096 65535 8388608
net.ipv4.tcp_mem = 8388608 8388608 8388608
net.core.optmem_max = 40960

fs.file-max=16384

kernel.threads-max=2048

A tcp_max_syn_backlog variable defines how many half-open connections can be kept by the backlog queue. For instance 256 is a total number of half-open connections handled in memory by Linux RedHat 7.3. The TCP/IP stack variables can be configured by sysctl or standard Unix commands. The following example shows how to change the default size of the backlog queue by the sysctl command:

# sysctl -w net.ipv4.tcp_max_syn_backlog="2048"


You can check also those values on:
http://www.netadmintools.com/html/7tcp.man.html
http://ipsysctl-tutorial.frozentux.net/ipsysctl-tutorial.html

Note:

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