The sysctl variables in IPVS

All the sysctl variables in IPVS are under the directory /proc/sys/net/ipv4/vs/. The following description is for the sysctl variables in IPVS 1.0.x for the Linux kernel 2.4.xx.

The /proc/sys/net/ipv4/vs/* Variables:

debug_level - INTEGER
	It is only available when IPVS is compiled with the IPVS
	debugging option.

	The default value is 0. The higher value is set to
	debug_level, the more debugging information is output in the
	kernel messages. See the doc/debug-levels.txt (shipped in ipvs
	source code) for more information about the debugging levels.

expire_nodest_conn - BOOLEAN
	0 - disabled (default)
	not 0 - enabled

	The default value is 0, the load balancer will silently drop
	packets when its destination server is not available. It may
	be useful, when user-space monitoring program deletes the
	destination server (because of server overload or wrong
	detection) and add back the server later, and the connections
	to the server can continue.

	If this feature is enabled, the load balancer will expire the
	connection immediately when a packet arrives and its
	destination server is not available, then the client program
	will be notified that the connection is closed. This is
	equivalent to the feature some people requires to flush
	connections when its destination is not available.

nat_icmp_send - BOOLEAN
	0 - disabled (default)
	not 0 - enabled

	It controlls sending icmp error messages (ICMP_DEST_UNREACH)
	for VS/NAT when the load balancer receives packets from real
	servers but the connection entries don't exist.

cache_bypass - BOOLEAN
	0 - disabled (default)
	not 0 - enabled

	If it is enabled, forward packets to the original destination
	directly when no cache server is available and destination
	address is not local (iph->daddr is RTN_UNICAST). It is mostly
	used in transparent web cache cluster.

sync_threshold - INTEGER
	default 3

	It sets synchronization threshold, which is the minimum number
	of incoming packets that a connection needs to receive before
	the connection will be synchronized. A connection will be
	synchronized, every time the number of its incoming packets
	modulus 50 equals the threshold. The range of the threshold is
	from 0 to 49.

drop_entry - INTEGER
	0  - disabled (default)

	The drop_entry defense is to randomly drop entries in the
	connection hash table, just in order to collect back some
	memory for new connections. In the current code, the
	drop_entry procedure can be actived every second, then it
	randomly scans 1/32 of the whole and drops entries that are in
	the SYN-RECV/SYNACK state, which should be effective against
	syn-flooding attack.

	The valid values of drop_entry are from 0 to 3, where 0 means
	that this strategy is always disabled, 1 and 2 mean automatic
	modes (when there is no enough available memory, the strategy
	is enabled and the variable is automatically set to 2,
	otherwise the strategy is disabled and the variable is set to
	1), and 3 means that that the strategy is always enabled.

drop_packet - INTEGER
	0  - disabled (default)

	The drop_packet defense is designed to drop 1/rate packets
	before forwarding them to real servers. If the rate is 1, then
	drop all the incoming packets.

	The value definition is the same as that of the drop_entry. In
	the automatic mode, the rate is determined by the follow
	formula: rate = amemthresh / (amemthresh - available_memory)
	when available memory is less than the available memory
	threshold. When the mode 3 is set, the always mode drop rate
	is controlled by the /proc/sys/net/ipv4/vs/am_droprate.

secure_tcp - INTEGER
	0  - disabled (default)

	The secure_tcp defense is to use a more complicated state
	transtition table and some possible short timeouts of each
	state. In the VS/NAT, it delays the entering the ESTABLISHED
	until the real server starts to send data and ACK packet
	(after 3-way handshake).

	The value definition is the same as that of drop_entry or
	drop_packet.
	
amemthresh - INTEGER
	default 1024

	It sets the available memory threshold (in pages), which is
        used in the automatic modes of defense. When there is no
        enough available memory, the respective strategy will be
        enabled and the variable is automatically set to 2, otherwise
        the strategy is disabled and the variable is  set  to 1.

am_droprate - INTEGER
	default 10

	It sets the always mode drop rate, which is used in the mode 3
	of the drop_rate defense.

timeout_* - INTEGER
	Secure TCP timeouts (in seconds).