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Disable uptime detection from external mashines

With the right tools you can "sniff" the uptime of a linux mashine only by sending and analysing tcp packets. The tool nmap does such a test.

Example output from nmap:

Interesting ports on 192.168.70.121: 
PORT STATE SERVICE 
80/tcp open http 
81/tcp closed hosts2-ns 
Device type: general purpose 
Running: Linux 2.4.X|2.5.X 
OS details: Linux Kernel 2.4.18 - 2.5.70 (X86) 
Uptime 0.667 days (since Thu Oct 21 03:21:03 2004)

This uptime statistics can be disabled by running the following command:

echo 0>/proc/sys/net/ipv4/tcp_timestamp

running nmap after that:

... 
Running: Linux 2.4.X|2.5.X 
OS details: Linux Kernel 2.4.0 - 2.5.20 w/o tcp_timestamps 
...

You can add that line above to you init script or modify /etc/sysctl.conf and add that line:

net.ipv4.tcp_timestamp=0

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