Hitchhiker's Guide to Openbsd

6.11.2 - Configuration
CARP's controls are located in two places: 
sysctl(8)
 and 
ifconfig(8)
. Let's look at the sysctls first. 
The first sysctl, 
net.inet.carp.allow
, defines whether the host handles CARP packets at all. 
Clearly, this is necessary to use CARP. This sysctl is enabled by default. 
The second, 
net.inet.carp.log
, logs CARP state changes, bad packets and other errors. Set to log 
state changes by default. 
Third, 
net.inet.carp.preempt
enables natural selection among CARP hosts. The most fit for the 
job (that is to say, able to advertise most frequently) will become master. Disabled by default, meaning a 
http://www.openbsd.org/faq/faq6.html (26 of 33)9/4/2011 10:02:06 AM

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6 - Networking
system that is not a master will not attempt to (re)gain master status. 
All these sysctl variables are documented in 
sysctl(3)
. 
For the remainder of CARP's configuration, we rely on 
ifconfig(8)
. The CARP-specific commands 
advbase
and 
advskew
deal with the interval between CARP advertisements. The formula (in 
seconds) is 
advskew
divided by 256, then added to 
advbase
. 
advbase
can be used to decrease 
network traffic or allow longer latency before a backup host takes over; 
advskew
lets you control 
which host will be master without much delaying failover (should that be required). 
Next, 
pass
sets a password, and 
vhid
sets the virtual host identifier number of the CARP group. You 
need to assign a unique number for each CARP group, even if (for load balancing purposes) they share 
the same IP address. CARP is limited to 255 groups. 
Finally, 
carpdev
specifies which physical interface belongs to this particular CARP group. By default, 
whichever interface has an IP address in the same subnet assigned to the CARP interface will be used. 
Let's put all these settings together in a basic configuration. Let's say you're deploying two identically 
configured Web servers, rachael (192.168.0.5) and pris (192.168.0.6), to replace an older system that 
was at 192.168.0.7. The commands: 
rachael# ifconfig carp0 create
rachael# ifconfig carp0 vhid 1 pass tyrell carpdev fxp0 \