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Friday, January 7, 2011

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Some Linux tweaks...

Change the values above as desired, depending on your internet connection and maximum bandwidth/latency.There are other parameters you can change from the default if you'reconfident in what you're doing - just find the correct syntax of thevalues in /proc/sys/net/... and add a line in the above code analogousto the others. To revert to the default parameters, you can justcomment or delete the above code from /etc/rc.local and restart. The TCP/IP parameters for tweaking a Linux-based machine for fast internet connectionsare located in /proc/sys/net/... (assuming 2.1+ kernel). This locationis volatile, and changes are reset at reboot. There are a couple ofmethods for reapplying the changes at boot time, ilustrated below.

Locating the TCP/IP related parameters
All TCP/IPtunning parameters are located under /proc/sys/net/...  

For example,here is a list of the most important tunning parameters, along withshort description of their meaning:

/proc/sys/net/core/rmem_max - Maximum TCP Receive Window
/proc/sys/net/core/wmem_max - Maximum TCP Send Window
/proc/sys/net/ipv4/tcp_timestamps - timestamps (RFC 1323) add 12 bytes to the TCP header...
/proc/sys/net/ipv4/tcp_sack - tcp selective acknowledgements.
/proc/sys/net/ipv4/tcp_window_scaling - support for large TCP Windows (RFC 1323). 

Needs to be set to 1 if the Max TCP Window is over 65535.
Keep in mind everything under /proc is volatile, so any changes you make are lost after reboot.
There are some additional internal memory buffers for the TCP Window, allocated for each connection:
/proc/sys/net/ipv4/tcp_rmem - memory reserved for TCP rcv buffers (reserved memory per connection default)
/proc/sys/net/ipv4/tcp_wmem  - memory reserved for TCP snd buffers (reserved memory per connection default)

Thetcp_rmem and tcp_wmem contain arrays of three parameter values: the 3numbers represent minimum, default and maximum memory values. Those 3values are used to bound autotunning and balance memory usage whileunder global memory stress.

Applying TCP/IP Parameters at System Boot
You can edit /etc/rc.local, or /etc/boot.local depending on your distribution so the parameters get automatically reapplied at boot time. 
The TCP/IP parameters should be self-explanatory: we're basically setting the TCP Window to 256960, disabling timestamps (to avoid 12 byte header  overhead), enabling tcp window scaling, and selective acknowledgements:

echo 256960 > /proc/sys/net/core/rmem_default
echo 256960 > /proc/sys/net/core/rmem_max
echo 256960 > /proc/sys/net/core/wmem_default
echo 256960 > /proc/sys/net/core/wmem_max
echo 0 > /proc/sys/net/ipv4/tcp_timestamps echo 1 > /proc/sys/net/ipv4/tcp_sack echo 1 > /proc/sys/net/ipv4/tcp_window_scaling
Another method to reapply the values upon boot is to include the following in your /etc/sysctl.conf (adjust RWIN values as needed):
net.core.rmem_default = 256960
net.core.rmem_max = 256960
net.core.wmem_default = 256960
net.core.wmem_max = 256960
net.ipv4.tcp_timestamps = 0 
net.ipv4.tcp_sack = 1
net.ipv4.tcp_window_scaling = 1
Execute sysctl -p to make these new settings take effect.
To manually set the MTU value under Linux, use the command: 

ifconfig eth0 mtu 1500   (where 1500 is the desired MTU size)

Changing Current Values without rebooting  
The current TCP/IP parameters can be edited without the need for reboot in the following locations:

/proc/sys/net/core/rmem_default = Default Receive Window
rmem_max = Maximum Receive Window
wmem_default = Default Send Window
wmem_max = Maximum Send Window
You'll find timestamps, window scalling, selective acknowledgements, etc.
Keepin mind the values in /proc will be reset upon reboot. You still needto add the code in /etc/rc.local or /etc/boot.local in order to havethe changes applied at boot time as described above.

Other TCP Parameters to consider

This setting determines the time that must elapse before TCP/IPcan release a closed connection and reuse its resources. During thisTIME_WAIT state, reopening the connection to the client costs less thanestablishing a new connection.
By reducing the value of this entry, TCP/IPcan release closed connections faster, making more resources availablefor new connections. Addjust this in the presense of many connectionssitting in the TIME_WAIT state:
# echo 30 > /proc/sys/net/ipv4/tcp_fin_timeout(default: 60 seconds, recommended 15-30 seconds)
You can use any of the earlier described methods to reapply these settings at boot time.
Here is a quick way to view the number of connections and their states:
netstat -tan | grep ':80 ' | awk '{print $6}' | sort | uniq -c

This determines the wait time between isAlive interval probes. To set:
echo 30 > /proc/sys/net/ipv4/tcp_keepalive_intvl
(default: 75 seconds, recommended: 15-30 seconds)

This determines the number of probes before timing out. To set:
echo 5 > /proc/sys/net/ipv4/tcp_keepalive_probes
(default: 9, recommended 5)

TCP_TW_RECYCLEIt enablesfast recycling of TIME_WAIT sockets. The default value is 0 (disabled).
The sysctl documentation incorrectly states the default as enabled. Itcan be changed to 1 (enabled) in many cases. Known to cause some issueswith hoststated (load balancing and fail over) if enabled, should be used with caution.
echo 1 > /proc/sys/net/ipv4/tcp_tw_recycle
(boolean, default: 0)  
This allows reusing sockets in TIME_WAIT state for new connections when it is safe from protocol viewpoint. Default value is 0 (disabled). It is generally a safer alternative to tcp_tw_recycle
echo 1 > /proc/sys/net/ipv4/tcp_tw_reuse(boolean, default: 0)
Note:The tcp_tw_reuse setting is particularly useful in environments wherenumerous short connections are open and left in TIME_WAIT state, suchas web servers. Reusing the sockets can be very effective in reducing server load. 

Kernel Recompile Option
There is another method one can use to set TCP/IP parameters, involving kernel recompile... If you're brave enough. Look for the parameters in the following files:

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About 'Anu': My name is 'Anu' also Known as 'ANU 007 TIGER' .I'm administrator of 'PC Tips by Anu' blog .This blog was opened for sharing contents about hacking n cracking.


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