Persistence And Firewall Marks; Persistence - Red Hat ENTERPRISE LINUX 5 - VIRTUAL SERVER ADMINISTRATION Manual

Persistence and Firewall Marks

1.4.2.1. Direct Routing and the ARP Limitation
While there are many advantages to using direct routing in LVS, there are limitations as well. The most
common issue with LVS via direct routing is with Address Resolution Protocol (ARP).
In typical situations, a client on the Internet sends a request to an IP address. Network routers typically
send requests to their destination by relating IP addresses to a machine's MAC address with ARP.
ARP requests are broadcast to all connected machines on a network, and the machine with the
correct IP/MAC address combination receives the packet. The IP/MAC associations are stored in
an ARP cache, which is cleared periodically (usually every 15 minutes) and refilled with IP/MAC
associations.
The issue with ARP requests in a direct routing LVS setup is that because a client request to an IP
address must be associated with a MAC address for the request to be handled, the virtual IP address
of the LVS system must also be associated to a MAC as well. However, since both the LVS router
and the real servers all have the same VIP, the ARP request will be broadcast ed to all the machines
associated with the VIP. This can cause several problems, such as the VIP being associated directly
to one of the real servers and processing requests directly, bypassing the LVS router completely and
defeating the purpose of the LVS setup.
To solve this issue, ensure that the incoming requests are always sent to the LVS router rather than
one of the real servers. This can be done by using either the arptables_jf or the iptables packet
filtering tool for the following reasons:
• The arptables_jf prevents ARP from associating VIPs with real servers.
• The iptables method completely sidesteps the ARP problem by not configuring VIPs on real
servers in the first place.
For more information on using arptables or iptables in a direct routing LVS environment, refer to
Section 3.2.1, "Direct Routing and arptables_jf" Section 3.2.2, "Direct Routing and
or
iptables".
1.5. Persistence and Firewall Marks
In certain situations, it may be desirable for a client to reconnect repeatedly to the same real server,
rather than have an LVS load balancing algorithm send that request to the best available server.
Examples of such situations include multi-screen web forms, cookies, SSL, and FTP connections. In
these cases, a client may not work properly unless the transactions are being handled by the same
server to retain context. LVS provides two different features to handle this: persistence and firewall
marks.

1.5.1. Persistence

When enabled, persistence acts like a timer. When a client connects to a service, LVS remembers
the last connection for a specified period of time. If that same client IP address connects again within
that period, it is sent to the same server it connected to previously — bypassing the load-balancing
mechanisms. When a connection occurs outside the time window, it is handled according to the
scheduling rules in place.
Persistence also allows the administrator to specify a subnet mask to apply to the client IP address
test as a tool for controlling what addresses have a higher level of persistence, thereby grouping
connections to that subnet.
Grouping connections destined for different ports can be important for protocols which use more than
one port to communicate, such as FTP. However, persistence is not the most efficient way to deal with
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