[IPVS]: Add sysctl documentation

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

am_droprate - INTEGER

        default 10

        It sets the always mode drop rate, which is used in the mode 3

        of the drop_rate defense.

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.

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.

debug_level - INTEGER

	0          - transmission error messages (default)

	1          - non-fatal error messages

	2          - configuration

	3          - destination trash

	4          - drop entry

	5          - service lookup

	6          - scheduling

	7          - connection new/expire, lookup and synchronization

	8          - state transition

	9          - binding destination, template checks and applications

	10         - IPVS packet transmission

	11         - IPVS packet handling (ip_vs_in/ip_vs_out)

	12 or more - packet traversal

	Only available when IPVS is compiled with the CONFIG_IPVS_DEBUG

	Higher debugging levels include the messages for lower debugging

	levels, so setting debug level 2, includes level 0, 1 and 2

	messages. Thus, logging becomes more and more verbose the higher

	the level.

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 activated 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.

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.

expire_quiescent_template - BOOLEAN

	0 - disabled (default)

	not 0 - enabled

	When set to a non-zero value, the load balancer will expire

	persistent templates when the destination server is quiescent.

	This may be useful, when a user makes a destination server

	quiescent by setting its weight to 0 and it is desired that

	subsequent otherwise persistent connections are sent to a

	different destination server.  By default new persistent

	connections are allowed to quiescent destination servers.

	If this feature is enabled, the load balancer will expire the

	persistence template if it is to be used to schedule a new

	connection and the destination server is quiescent.

nat_icmp_send - BOOLEAN

        0 - disabled (default)

        not 0 - enabled

        It controls 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.

secure_tcp - INTEGER

        0  - disabled (default)

        The secure_tcp defense is to use a more complicated state

        transition 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.

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.