Relayd and Httpd Mastery (IT Mastery Book 11) by Michael W Lucas

Author:Michael W Lucas [Lucas, Michael W]
Language: eng
Format: azw3
Tags: General Fiction
Publisher: Tilted Windmill Press
Published: 2017-05-17T04:00:00+00:00

* * *

1 If you want to cause yourself pain, though, who am I to say no?

2 My guess would be “because the firewall administrator is a jerk,” but it’s possibly another reason. Possibly.

Chapter 8: Common Address Redundancy Protocol

Every host needs patching and hardware maintenance. No matter how hard you work to keep a system up, at some time it’s going to need downtime. Can your organization handle having its web site down after hours for maintenance? Or is the web site a key part of its business, making downtime unacceptable?

One solution for keeping a service like HTTP available at all times is the Common Address Redundancy Protocol, or CARP. Like the similar protocols, CARP is intended for a situation where you have multiple hosts on the same network segment offering the same service, such as a web site.

The trick with CARP is that the service is tied to one or more IP addresses that are not permanently attached to any single host. It’s often called a floating address. Each host has an entirely different IP address for management. CARP temporarily attaches the service address to one of the hosts. When that host dies, the CARP protocol on the other hosts notice. One of the other hosts picks up the address and offers the service. The host holding the floating address is called the CARP master, while the others is a CARP backup.

CARP cannot transparently switch active TCP session termination from one host to another. (It can pass state on firewalls via pfsync(8), but that’s a separate thing.) If you’re serving large files off on your web site, the client will notice the interruption. Most clients can create a new connection and pick up where they left off, though.

If you’re familiar with either Virtual Router Redundancy Protocol (VRRP) or Hot Standby Router Protocol (HSRP), CARP will feel very familiar. CARP has stronger authentication and integrity checks than either of those protocols, however.

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