/proc/sys/net/ipv4/* variables:

ip_forwarding - "SNMP" BOOLEAN
	2 - disabled (default)
	1 - enabled

	This variable is special, its change resets all configuration
	parameters to their default state (RFC1122 for hosts, RFC1812
	for routers)

ip_default_ttl - INTEGER
	default 64

ip_log_martians - BOOLEAN
	log packets with strange or impossible addresses.
	default TRUE (router)
		FALSE (host)

ip_accept_redirects - BOOLEAN
	Accept ICMP redirect messages.
	default TRUE (host)
		FALSE (router)

ip_secure_redirects - BOOLEAN
	Accept ICMP redirect messages only for gateways,
	listed in default gateway list.
	default TRUE

ip_addrmask_agent - BOOLEAN
	Reply to ICMP ADDRESS MASK requests.
	default TRUE (router)
		FALSE (host)

ip_rfc1620_redirects - BOOLEAN
	Send(router) or accept(host) RFC1620 shared media redirects.
	Overrides ip_secure_redirects.
	default TRUE (should be FALSE for distributed version,
		      but I use it...)

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_bootp_relay - BOOLEAN
	Accept packets with source address 0.b.c.d destined
	not to this host as local ones. It is supposed, that
	BOOTP relay deamon will catch and forward such packets.

	default FASLE
	Not Implemented Yet.


ip_source_route - BOOLEAN
	Accept packets with SRR option.
	default TRUE (router)
		FALSE (host)


ip_no_pmtu_disc - BOOLEAN
	Disable Path MTU Discovery.
	default FALSE

ip_rfc1812_filter - INTEGER
	2 - do source validation by reversed path, as specified in RFC1812
	    Recommended option for single homed hosts and stub network
	    routers. Could cause troubles for complicated (not loop free)
	    networks running a slow unreliable protocol (sort of RIP),
	    or using static routes.

	1 - (DEFAULT) Weaker form of RP filtering: drop all the packets
	    that look as sourced at a directly connected interface, but
	    were input from another interface.
	    
	0 - No source validation. 

	NOTE: do not disable this option! All BSD derived routing software
	(sort of gated, routed etc. etc.) is confused by such packets,
	even if they are valid.

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.



Alexey Kuznetsov.
kuznet@ms2.inr.ac.ru