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CARP(4) Kernel Interfaces Manual CARP(4) NAME carp -- Common Address Redundancy Protocol SYNOPSIS device carp DESCRIPTION The CARP allows multiple hosts on the same local network to share a set of IPv4 and/or IPv6 addresses. Its primary purpose is to ensure that these addresses are always available. To use carp, the administrator needs to configure at a minimum a common virtual host ID (vhid), and attach at least one IP address to this vhid on each machine which is to take part in the virtual group. Additional para- meters can also be set on a per-vhid basis: advbase and advskew, which are used to control how frequently the host sends advertisements when it is the master for a virtual host, and pass which is used to authenticate carp ad- vertisements. The advbase parameter stands for "advertisement base". It is measured in seconds and specifies the base of the advertisement inter- val. The advskew parameter stands for "advertisement skew". It is mea- sured in 1/256 of seconds. It is added to the base advertisement interval to make one host advertise a bit slower that the other does. Both advbase and advskew are put inside CARP advertisements. These values can be con- figured using ifconfig(8), or through the SIOCSVH ioctl(2). CARP defaults to using multicast messages, but can be configured to unicast announcements to peers using the peer and peer6 parameters. Default ad- dresses can be restored using mcast and mcast6. Note that TTL verification is disabled if the peer address is not a multicast address. These values can be configured using ifconfig(8), or through the SIOCSPEER ioctl(2). carp(4) can be configured to use either the non-standard CARP protocol, or VRRPv3 (RFC 5798). Use the carpver parameter to select either 2 (CARP) or 3 (VRRPv3). VRRPv3 specific parameters can be configured using the vrrpprio and vrrpinterval parameters. CARP virtual hosts can be configured on multicast-capable interfaces: Eth- ernet, layer 2 VLAN, FDDI and Token Ring. An arbitrary number of virtual host IDs can be configured on an interface. An arbitrary number of IPv4 or IPv6 addresses can be attached to a particular vhid. It is important that all hosts participating in a vhid have the same list of prefixes configured on the vhid, since all the prefixes are included in the cryptographic checksum supplied in each advertisement. Multiple vhids running on one in- terface participate in master/backup elections independently. Additionally, there are a number of global parameters which can be set us- ing sysctl(8): net.inet.carp.allow Allow carp operation. When disabled, virtual hosts remain in initial state, neither sending nor receiving announcements or traffic. Enabled by default. net.inet.carp.preempt Allow virtual hosts to preempt each other. When enabled, a vhid in a backup state would preempt a master that is announcing itself with a lower advskew. Disabled by default. net.inet.carp.dscp DSCP value in carp packet. Valid Values are 0 to 63. A value of 4 is equivalent to the old standard of TOS LOW_DELAY. TOS values were depre- cated and replaced by DSCP in 1998. The default value is 56 (CS7/Network Control). net.inet.carp.log Determines what events relating to carp vhids are logged. A value of 0 disables any logging. A value of 1 enables logging state changes of carp vhids. Values above 1 enable logging of bad carp packets. The default value is 1. net.inet.carp.demotion This value shows the current level of CARP demotion. The value is added to the actual advskew sent in announce- ments for all vhids. During normal system operation the demotion factor is zero. However, problematic condi- tions raise its level: when carp ex- periences problem with sending an- nouncements, when an interface run- ning a vhid goes down, or while the pfsync(4) interface is not synchro- nized. The demotion factor can be adjusted writing to the sysctl oid. The signed value supplied to the sysctl(8) command is added to current demotion factor. This allows to con- trol carp behaviour depending on some external conditions, for example on the status of some daemon utility. net.inet.carp.ifdown_demotion_factor This value is added to net.inet.carp.demotion when an inter- face running a vhid goes down. The default value is 240 (the maximum ad- vskew value). net.inet.carp.senderr_demotion_factor This value is added to net.inet.carp.demotion when carp ex- periences errors sending its an- nouncements. The default value is 240 (the maximum advskew value). STATE CHANGE NOTIFICATIONS Sometimes it is useful to get notified about carp status change events. This can be accomplished by using devd(8) hooks. Master/slave events are signalled under system CARP. The subsystem specifies the vhid and name of the interface where the master/slave event occurred. The type of the mes- sage displays the new state of the vhid. Please see devd.conf(5) and the "EXAMPLES" section for more information. EXAMPLES For firewalls and routers with multiple interfaces, it is desirable to failover all of the addresses running carp together, when one of the physi- cal interfaces goes down. This is achieved by the use of the preempt op- tion. Enable it on both hosts A and B: sysctl net.inet.carp.preempt=1 Assume that host A is the preferred master and we are running the 192.168.1.0/24 prefix on em0 and 192.168.2.0/24 on em1. This is the setup for host A (advskew is above 0 so it could be overwritten in the emergency situation from the other host): ifconfig em0 vhid 1 advskew 100 pass mekmitasdigoat 192.168.1.1/24 ifconfig em1 vhid 2 advskew 100 pass mekmitasdigoat 192.168.2.1/24 The setup for host B is identical, but it has a higher advskew: ifconfig em0 vhid 1 advskew 200 pass mekmitasdigoat 192.168.1.1/24 ifconfig em1 vhid 2 advskew 200 pass mekmitasdigoat 192.168.2.1/24 When one of the physical interfaces of host A fails, advskew is demoted to a configured value on all its carp vhids. Due to the preempt option, host B would start announcing itself, and thus preempt host A on both interfaces instead of just the failed one. Processing of carp status change events can be set up by using the follow- ing devd.conf rule: notify 0 { match "system" "CARP"; match "subsystem" "[0-9]+@[0-9a-z.]+"; match "type" "(MASTER|BACKUP)"; action "/root/carpcontrol.sh $subsystem $type"; }; To see carp packets decoded in tcpdump(1) output, one needs to specify the -T carp option, otherwise tcpdump(1) will interpret them as VRRP packets: tcpdump -npi vlan0 -T carp SEE ALSO tcpdump(1), inet(4), pfsync(4), devd.conf(5), rc.conf(5), ifconfig(8), sysctl(8) HISTORY The carp device first appeared in OpenBSD 3.5. The carp device was im- ported into FreeBSD 5.4. In FreeBSD 10.0, carp was significantly rewrit- ten, and is no longer a pseudo-interface. FreeBSD ports 15.quarterly April 12, 2024 CARP(4)
NAME | SYNOPSIS | DESCRIPTION | STATE CHANGE NOTIFICATIONS | EXAMPLES | SEE ALSO | HISTORY
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