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ucarp - Common Address Redundancy Protocol (CARP) for Unix… more info»
.:. UCARP .:. Documentation for version 1.5 ------------------------ BLURB ------------------------ UCARP allows a couple of hosts to share common virtual IP addresses in order to provide automatic failover. It is a portable userland implementation of the secure and patent-free Common Address Redundancy Protocol (CARP, OpenBSD's alternative to the patents-bloated VRRP). Strong points of the CARP protocol are: very low overhead, cryptographically signed messages, interoperability between different operating systems and no need for any dedicated extra network link between redundant hosts. Home page is http://www.ucarp.org/ ------------------------ COMPILATION ------------------------ libpcap (http://www.tcpdump.org/) must be installed on your system, with development files (headers). Then, follow the boring traditional procedure: ./configure make install-strip For details, have a look at the INSTALL file. The software has been successfully tested on Linux 2.4, Linux 2.6, MacOS X, OpenBSD, MirBSD and NetBSD. ------------------------ REQUIREMENTS ------------------------ A couple of virtual hosts must be given: - A shared virtual IP, which will be dynamically answered by one alive host. Services that need high availability need to be assigned to that virtual IP. - A real IP address for each host. - A shared identifier for the virtual IP address, which is a number between 1 and 255. - For each host : an advertisement interval, comprised of a base and skew value, which is the frequency the host will tell the other one that it's still alive. By default, base is 1 and skew is 0, which basically means one advertisement a second. The protocol is very light, a tiny packet every second won't have any noticeable impact on your network. - A shared password (that will never go plaintext to the network). - A script to bring the virtual address up when a host becomes the master. - Another script to bring the virtual address down when a host is no more the master. ------------------------ USAGE ------------------------ The server will usually be installed as : /usr/local/sbin/ucarp Everything is driven through command-line options. In order to see the list of available options, try : /usr/local/sbin/ucarp -h Better than a long technical discussion, here's a real-life setup example. Your company has an internal mail relay whose IP address is 10.1.1.252. Every user has configured his mail client with that host or IP address and the service must always be up and running without having to reconfigure every user's mail client in case of a failure. Instead of assigning 10.1.1.252 to a particular mail server, you decide to use ucarp to allow two hosts to share this IP address. Of course, only one server can answer for this address at a time, while the other sits idle. However the other server will automatically become active in case the first one fails. Thus you're providing a simple but powerful IP failover solution. So you set up two mail servers hosts with an identical configuration. Their real IP addresses are 10.1.1.1 and 10.1.1.2. First, we will create a script that brings the virtual IP address up. Let's save that file as /etc/vip-up.sh : #! /bin/sh /sbin/ip addr add 10.1.1.252/24 dev eth0 Now another script to bring it down, /etc/vip-down.sh : #! /bin/sh /sbin/ip addr del 10.1.1.252/24 dev eth0 Of course, anything can go in these scripts. For instance, you may want to add routes, to add something to log files or to send mail. And last, but not least, you can use a script that will connect to your switches and flush their ARP cache. Some users reported that transitions were way faster when also switching MAC addresses. The called scripts are passed arguments, in this order: <interface name> <virtual address> <optional extra parameter> For instance, as the is passed as the first argument to the called scripts, feel free to replace "eth0" with "$1" and 10.1.1.252 by "$2" in the previous examples. Don't forget to make those files executable : chmod +x /etc/vip-up.sh /etc/vip-down.sh Right. What we need now is an identifier for the virtual IP. Let's take "42". And we also need a password. Let's take "love". Now, on the first host (whoose real IP is 10.1.1.1), run : /usr/local/sbin/ucarp -v 42 -p love -a 10.1.1.252 -s 10.1.1.1 & On the second host, whose real IP is 10.1.1.2, run : /usr/local/sbin/ucarp -v 42 -p love -a 10.1.1.252 -s 10.1.1.2 & You should see that one of those hosts quickly becomes the master, and the other one the backup. Related scripts are spawned on change. Now unplug the master. After a few seconds, the other host becomes the new master. ------------------------ MASTER ELECTION PROCESS ------------------------ When ucarp first runs, it starts as a backup and listens to the network to determine if it should become the master. If at any time more than three times the node's advertising interval (defined as the advertising base (seconds) plus a fudge factor, the advertising skew) passes without hearing a peer's CARP advertisement, the node will transition itself to being a master. Transitioning from backup to master means: 1. running the specified up script to assign the vip to the local system 2. sending a gratuitous arp to the network to claim the vip 3. continuously sending CARP advertisements to the network every interval. Transitioning from master to backup means: 1. running the specified down script to remove the vip from the local system To understand how ucarp works, it's important to note that the advertisement interval is not only used as the time in between which each CARP advertisement is sent by the master, but also as a priority mechanism where shorter (i.e. more frequent) is better. The interval base and skew values are stored in the CARP advertisement and are used by other nodes to make certain decisions. By default, once a node becomes the master, it will continue on indefinitely as the master. If you like/want/need this behavior, or don't have a preferred master, then choose the same interval on all hosts. If for whatever reason you were to choose different intervals on the hosts, then over time the one with the shortest interval would tend to become the master as machines are rebooted, after failures, etc. Also of note is a conflict resolution algorithm that in case a master hears another, equal (in terms of its advertised interval) master, the one with the lower IP address will remain master and the other will immediately demote itself. This is simply to eliminate flapping and quickly determine who should remain master. This situation should not happen very often but it can. If you want a "preferred" master to always be the master (even if another host is already the master), add the preempt switch (--preempt or -P) and assign a shorter interval via the advertisement base (--advbase or -b) and skew (--advskew or -k). This will cause the preferred node to ignore a master who is advertising a longer interval and promote itself to master. The old master will quickly hear the preferred node advertising a shorter interval and immediately demote itself. In summary, a backup will become master if: - no one else advertises for 3 times its own advertisement interval - you specified --preempt and it hears a master with a longer interval and a master will become backup if: - another master advertises a shorter interval - another master advertises the same interval, and has a lower IP address ------------------------ OTHER NOTES ------------------------ Specify the --neutral (-n) switch for ucarp to not run the downscript at startup. --shutdown (-z) will run the downscript at exit, unless ucarp is already in the backup state. The "dead ratio" (--deadratio=...) knob basically changes how long a backup server will wait for an unresponsive master before considering it as dead, and becoming the new master. In the original protocol, the ratio is 3. This is also the default when this command-line switch is missing. Notices are sent both to stderr/stdout and to the syslog daemon (with the "daemon" facility) by default. stderr/stdout are bypassed if the daemon is started in background (--daemonize). Facilities can be changed with the --syslog switch. Use --syslog=none to disable syslog logging, for instance if prefer using something like multilog. You can send the ucarp process a SIGUSR1 to have it log a status line to syslog, like: Jan 7 17:38:22 localhost ucarp: [INFO] BACKUP on eth0 id 198 You can send the ucarp process a SIGUSR2 to have it demote itself from master to backup, pause 3 seconds, then proceed as usual to listen for other masters and promote itself if necessary. This could be useful if you wish another node to take over master. --ignoreifstate (-S) option tells ucarp to ignore unplugged network cable. It is useful when you connect ucarp nodes with a crossover patch cord (not via a hub or a switch). Without this option the node in MASTER state will switch to BACKUP state when the other node is powered down, because network interface shows that cable is unplugged (NO-CARRIER). Some network interface drivers don't support NO-CARRIER feature, and this option is not needed for these network cards. The card that definitely supports this feature is Realtek 8139. ------------------------ TRANSLATIONS ------------------------ UCARP can speak your native language through gettext / libintl. If you want to translate the software, have a look at the po/ directory. Copy the ucarp.pot file to <your locale name>.po and use software like Kbabel or Emacs to update the file. Better use use your local charset than UTF-8. ------------------------ DOWNLOADING UCARP ------------------------ UCARP home page is: http://www.ucarp.org/ . UCARP mailing-list: http://www.ucarp.org/ml/ . Be sure to scroll all the way down to see the UCARP mailing list info! Thank you, -Frank DENIS "Jedi/Sector One" <email@example.com> .