Linux Networking HOWTO
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7.1. Kernel Level Options

This section includes information on setting IP options within the kernel at boot time. An example of these options are ip_forward or ip_bootp_agent. These options are used by setting the value to a file in the
/proc/sys/net/ipv4/
directory. The name of the file is the name of the command.

For example, to set ip_forward enabled, you would type
echo 1 > /proc/sys/net/ipv4/ip_forward

7.1.1. General IP option listing

ip_forward
If ip_forward is set to 0 it is disabled. If it is sforwardany other number it is enabled. This option is used in conjunction with technologies such as routing between interfaces with IP Masquerading. .
ip_default_ttl
This is the time to live for an IP Packet. The default is 64 milliseconds.
ip_addrmask_agent - BOOLEAN
Reply to ICMP ADDRESS MASK requests. default TRUE (router) FALSE (host)
ip_bootp_agent
- BOOLEAN Accept packets with source address of sort 0.b.c.d and destined to this host, broadcast or multicast. Such packets are silently ignored otherwise. default FALSE
ip_no_pmtu_disc
- BOOLEAN Disable Path MTU Discovery. default FALSE
ip_fib_model - INTEGER
0 - (DEFAULT) Standard model. All routes are in class MAIN. 1 - default routes go to class DEFAULT. This mode should be very convenient for small ISPs making policy routing. 2 - RFC1812 compliant model. Interface routes are in class MAIN. Gateway routes are in class DEFAULT.

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