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).