[From OpenBSD Man Page]CARP

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简介:

NAME

carp – Common Address Redundancy Protocol

SYNOPSIS

pseudo-device carp

DESCRIPTION

The carp interface is a pseudo-device which implements and controls the

CARP protocol.  carp allows multiple hosts on the same local network to

share a set of IP addresses.  Its primary purpose is to ensure that these

addresses are always available, but in some configurations carp can also

provide load balancing functionality.

A carp interface can be created at runtime using the ifconfig carpN

create command or by setting up a hostname.if(5) configuration file for

netstart(8).

To use carp, the administrator needs to configure at minimum a common

virtual host ID (VHID) and virtual host IP address on each machine which

is to take part in the virtual group.  Additional parameters can also be

set on a per-interface basis: advbase and advskew, which are used to con-

trol how frequently the host sends advertisements when it is the master

for a virtual host, and pass which is used to authenticate carp adver-

tisements.  Finally carpdev is used to specify which interface the carp

device attaches to.  If unspecified, the kernel attempts to set it by

looking for another interface with the same subnet.  These configurations

can be done using ifconfig(8), or through the SIOCSVH ioctl.

carp can also be used in conjunction with ifstated(8) to respond to

changes in CARP state; however, for most uses this will not be necessary.

See the manual page for ifstated(8) for more information.

Additionally, there are a number of global parameters which can be set

using sysctl(8):

net.inet.carp.allow         Accept incoming carp packets.  Enabled by de-

fault.

net.inet.carp.preempt       Allow virtual hosts to preempt each other.

It is also used to failover carp interfaces

as a group.  When the option is enabled and

one of the carp enabled physical interfaces

goes down, advskew is changed to 240 on all

carp interfaces.  See also the first example.

Disabled by default.

net.inet.carp.log           Make carp log state changes, bad packets, and

other errors.  May be a value between 0 and 7

corresponding with syslog(3) priorities.  The

default value is 2, which limits logging to

changes in CARP state.

LOAD BALANCING

carp provides two mechanisms to load balance incoming traffic over a

group of carp hosts: ARP balancing and IP balancing.

Which one to use mainly depends on the network environment carp is being

used in.  ARP balancing has limited abilities for load balancing the in-

coming connections between hosts in an Ethernet network.  It only works

for clients in the local network, because ARP balancing spreads the load

by varying ARP replies based on the source MAC address of the host send-

ing the query.  Therefore it cannot balance traffic that crosses a

router, because the router itself will always be balanced to the same

virtual host.

IP balancing is not dependent on ARP and therefore also works for traffic

that comes over a router.  This method should work in all environments

and can also provide more fine grained load balancing than ARP balancing.

The downside of IP balancing is that it requires the traffic that is des-

tined towards the load balanced IP addresses to be received by all carp

hosts.  While this is always the case when connected to a hub, it has to

play some tricks in switched networks, which will result in a higher net-

work load.

A rule of thumb might be to use ARP balancing if there are many hosts on

the same network segment and to use IP balancing for all other cases.

To configure load balancing one has to specify multiple carp nodes using

the carpnodes option.  Each node in a load balancing cluster is repre-

sented by at least one “vhid:advskew” pair in a comma separated list.

carp tries to distribute the incoming network load over all configured

carpnodes.  The following example creates a load balancing group consist-

ing of three nodes, using vhids 3, 4 and 6:

# ifconfig carp0 carpnodes 3:0,4:0,6:100

The advskew value of the last node is set to 100, so that this node is

designated to the BACKUP state.  It will only become MASTER if all nodes

with a lower advskew value have failed.  By varying this value throughout

the machines in the cluster it is possible to decide which share of the

network load each node receives.  Therefore, all carp interfaces in the

cluster are configured identically, except for a different advskew value

within the carpnodes specification.

See the EXAMPLES section for a practical example of load balancing.

ARP BALANCING

For ARP balancing, one has to configure multiple carpnodes and choose the

balancing mode arp.

Once an ARP request is received, the CARP protocol will use a hashing

function against the source MAC address in the ARP request to determine

which carpnode the request belongs to.  If the corresponding carpnode is

in master state, the ARP request will be answered, otherwise it will be

ignored.

The ARP load balancing has some limitations.  Firstly, ARP balancing only

works on the local network segment.  It cannot balance traffic that

crosses a router, because the router itself will always be balanced to

the same carpnode.  Secondly, ARP load balancing can lead to asymmetric

routing of incoming and outgoing traffic, thus combining it with

pfsync(4) requires special care, because this can create a race condition

between balanced routers and the host they are serving.  ARP balancing

can be safely used with pfsync if the pf(4) ruleset translates the source

address to an unshared address on the outgoing interface using a NAT

rule.  This requires multiple CARP groups with different IP addresses on

the outgoing interface, configured so that each host is the master of one

group.

ARP balancing also works for IPv6, but instead of ARP the Neighbor Dis-

covery Protocol (NDP) is used.

IP BALANCING

IP load balancing works by utilizing the network itself to distribute in-

coming traffic to all carp nodes in the cluster.  Each packet is filtered

on the incoming carp interface so that only one node in the cluster ac-

cepts the packet.  All the other nodes will just silently drop it.  The

filtering function uses a hash over the source and destination address of

the IPv4 or IPv6 packet and compares the result against the state of the

carpnode.

IP balancing is activated by setting the balancing mode to ip.  This is

the recommended default setting.  In this mode, carp uses a multicast MAC

address, so that a switch sends incoming traffic towards all nodes.

However, there are a few OS and routers that do not accept a multicast

MAC address being mapped to a unicast IP.  This can be resolved by using

one of the following unicast options.  For scenarios where a hub is used

it is not necessary to use a multicast MAC and it is safe to use the ip-

unicast mode.  Manageable switches can usually be tricked into forwarding

unicast traffic to all cluster nodes ports by configuring them into some

sort of monitoring mode.  If this is not possible, using the ip-stealth

mode is another option, which should work on most switches.  In this mode

carp never sends packets with its virtual MAC address as source.  Stealth

mode prevents a switch from learning the virtual MAC address, so that it

has to flood the traffic to all its ports.  Please note that activating

stealth mode on a carp interface that has already been running might not

work instantly.  As a workaround the VHID of the first carpnode can be

changed to a previously unused one, or just wait until the MAC table en-

try in the switch times out.  Some Layer-3 switches do port learning

based on ARP packets.  Therefore the stealth mode cannot hide the virtual

MAC address from these kind of devices.

If IP balancing is being used on a firewall, it is recommended to config-

ure the carpnodes in a symmetrical manner.  This is achieved by simply

using the same carpnodes list on all sides of the firewall.  This ensures

that packets of one connection will pass in and out on the same host and

are not routed asymmetrically.

EXAMPLES

For firewalls and routers with multiple interfaces, it is desirable to

failover all of the carp interfaces together, when one of the physical

interfaces goes down.  This is achieved by the preempt option.  Enable it

on both host A and B:

# sysctl net.inet.carp.preempt=1

Assume that host A is the preferred master and 192.168.1.x/24 is config-

ured on one physical interface and 192.168.2.y/24 on another.  This is

the setup for host A:

# ifconfig carp0 192.168.1.1 vhid 1

# ifconfig carp1 192.168.2.1 vhid 2

The setup for host B is identical, but it has a higher advskew:

# ifconfig carp0 192.168.1.1 vhid 1 advskew 100

# ifconfig carp1 192.168.2.1 vhid 2 advskew 100

Because of the preempt option, when one of the physical interfaces of

host A fails, advskew is adjusted to 240 on all its carp interfaces.

This will cause host B to preempt on both interfaces instead of just the

failed one.

LOAD BALANCING

In order to set up a load balanced virtual host, it is necessary to con-

figure one carpnodes entry for each physical host.  In the following ex-

ample, two physical hosts are configured to provide balancing and

failover for the IP address 192.168.1.10.

First the carp interface on Host A is configured.  The advskew of 100 on

the second carpnode entry means that its advertisements will be sent out

slightly less frequently and will therefore become the designated backup.

# ifconfig carp0 192.168.1.10 carpnodes 1:0,2:100 balancing ip

The configuration for host B is identical, except the skew is on the

carpnode entry with virtual host 1 rather than virtual host 2.

# ifconfig carp0 192.168.1.10 carpnodes 1:100,2:0 balancing ip

If ARP balancing or a different mode of IP balancing is desired the

balancing mode can be adjusted accordingly.

SEE ALSO

sysctl(3), inet(4), pfsync(4), hostname.if(5), ifconfig(8), ifstated(8),

netstart(8), sysctl(8)

NAME     carp – Common Address Redundancy Protocol

SYNOPSIS     pseudo-device carp

DESCRIPTION     The carp interface is a pseudo-device which implements and controls the     CARP protocol.  carp allows multiple hosts on the same local network to     share a set of IP addresses.  Its primary purpose is to ensure that these     addresses are always available, but in some configurations carp can also     provide load balancing functionality.

A carp interface can be created at runtime using the ifconfig carpN     create command or by setting up a hostname.if(5) configuration file for     netstart(8).

To use carp, the administrator needs to configure at minimum a common     virtual host ID (VHID) and virtual host IP address on each machine which     is to take part in the virtual group.  Additional parameters can also be     set on a per-interface basis: advbase and advskew, which are used to con-     trol how frequently the host sends advertisements when it is the master     for a virtual host, and pass which is used to authenticate carp adver-     tisements.  Finally carpdev is used to specify which interface the carp     device attaches to.  If unspecified, the kernel attempts to set it by     looking for another interface with the same subnet.  These configurations     can be done using ifconfig(8), or through the SIOCSVH ioctl.

carp can also be used in conjunction with ifstated(8) to respond to     changes in CARP state; however, for most uses this will not be necessary.     See the manual page for ifstated(8) for more information.

Additionally, there are a number of global parameters which can be set     using sysctl(8):

net.inet.carp.allow         Accept incoming carp packets.  Enabled by de-                                 fault.

net.inet.carp.preempt       Allow virtual hosts to preempt each other.                                 It is also used to failover carp interfaces                                 as a group.  When the option is enabled and                                 one of the carp enabled physical interfaces                                 goes down, advskew is changed to 240 on all                                 carp interfaces.  See also the first example.                                 Disabled by default.

net.inet.carp.log           Make carp log state changes, bad packets, and                                 other errors.  May be a value between 0 and 7                                 corresponding with syslog(3) priorities.  The                                 default value is 2, which limits logging to                                 changes in CARP state.

LOAD BALANCING     carp provides two mechanisms to load balance incoming traffic over a     group of carp hosts: ARP balancing and IP balancing.

Which one to use mainly depends on the network environment carp is being     used in.  ARP balancing has limited abilities for load balancing the in-     coming connections between hosts in an Ethernet network.  It only works     for clients in the local network, because ARP balancing spreads the load     by varying ARP replies based on the source MAC address of the host send-     ing the query.  Therefore it cannot balance traffic that crosses a     router, because the router itself will always be balanced to the same     virtual host.

IP balancing is not dependent on ARP and therefore also works for traffic     that comes over a router.  This method should work in all environments     and can also provide more fine grained load balancing than ARP balancing.     The downside of IP balancing is that it requires the traffic that is des-     tined towards the load balanced IP addresses to be received by all carp     hosts.  While this is always the case when connected to a hub, it has to     play some tricks in switched networks, which will result in a higher net-     work load.

A rule of thumb might be to use ARP balancing if there are many hosts on     the same network segment and to use IP balancing for all other cases.

To configure load balancing one has to specify multiple carp nodes using     the carpnodes option.  Each node in a load balancing cluster is repre-     sented by at least one “vhid:advskew” pair in a comma separated list.     carp tries to distribute the incoming network load over all configured     carpnodes.  The following example creates a load balancing group consist-     ing of three nodes, using vhids 3, 4 and 6:

# ifconfig carp0 carpnodes 3:0,4:0,6:100

The advskew value of the last node is set to 100, so that this node is     designated to the BACKUP state.  It will only become MASTER if all nodes     with a lower advskew value have failed.  By varying this value throughout     the machines in the cluster it is possible to decide which share of the     network load each node receives.  Therefore, all carp interfaces in the     cluster are configured identically, except for a different advskew value     within the carpnodes specification.

See the EXAMPLES section for a practical example of load balancing.

ARP BALANCING     For ARP balancing, one has to configure multiple carpnodes and choose the     balancing mode arp.

Once an ARP request is received, the CARP protocol will use a hashing     function against the source MAC address in the ARP request to determine     which carpnode the request belongs to.  If the corresponding carpnode is     in master state, the ARP request will be answered, otherwise it will be     ignored.

The ARP load balancing has some limitations.  Firstly, ARP balancing only     works on the local network segment.  It cannot balance traffic that     crosses a router, because the router itself will always be balanced to     the same carpnode.  Secondly, ARP load balancing can lead to asymmetric     routing of incoming and outgoing traffic, thus combining it with     pfsync(4) requires special care, because this can create a race condition     between balanced routers and the host they are serving.  ARP balancing     can be safely used with pfsync if the pf(4) ruleset translates the source     address to an unshared address on the outgoing interface using a NAT     rule.  This requires multiple CARP groups with different IP addresses on     the outgoing interface, configured so that each host is the master of one     group.

ARP balancing also works for IPv6, but instead of ARP the Neighbor Dis-     covery Protocol (NDP) is used.

IP BALANCING     IP load balancing works by utilizing the network itself to distribute in-     coming traffic to all carp nodes in the cluster.  Each packet is filtered     on the incoming carp interface so that only one node in the cluster ac-     cepts the packet.  All the other nodes will just silently drop it.  The     filtering function uses a hash over the source and destination address of     the IPv4 or IPv6 packet and compares the result against the state of the     carpnode.

IP balancing is activated by setting the balancing mode to ip.  This is     the recommended default setting.  In this mode, carp uses a multicast MAC     address, so that a switch sends incoming traffic towards all nodes.

However, there are a few OS and routers that do not accept a multicast     MAC address being mapped to a unicast IP.  This can be resolved by using     one of the following unicast options.  For scenarios where a hub is used     it is not necessary to use a multicast MAC and it is safe to use the ip-     unicast mode.  Manageable switches can usually be tricked into forwarding     unicast traffic to all cluster nodes ports by configuring them into some     sort of monitoring mode.  If this is not possible, using the ip-stealth     mode is another option, which should work on most switches.  In this mode     carp never sends packets with its virtual MAC address as source.  Stealth     mode prevents a switch from learning the virtual MAC address, so that it     has to flood the traffic to all its ports.  Please note that activating     stealth mode on a carp interface that has already been running might not     work instantly.  As a workaround the VHID of the first carpnode can be     changed to a previously unused one, or just wait until the MAC table en-     try in the switch times out.  Some Layer-3 switches do port learning     based on ARP packets.  Therefore the stealth mode cannot hide the virtual     MAC address from these kind of devices.

If IP balancing is being used on a firewall, it is recommended to config-     ure the carpnodes in a symmetrical manner.  This is achieved by simply     using the same carpnodes list on all sides of the firewall.  This ensures     that packets of one connection will pass in and out on the same host and     are not routed asymmetrically.

EXAMPLES     For firewalls and routers with multiple interfaces, it is desirable to     failover all of the carp interfaces together, when one of the physical     interfaces goes down.  This is achieved by the preempt option.  Enable it     on both host A and B:

# sysctl net.inet.carp.preempt=1

Assume that host A is the preferred master and 192.168.1.x/24 is config-     ured on one physical interface and 192.168.2.y/24 on another.  This is     the setup for host A:

# ifconfig carp0 192.168.1.1 vhid 1           # ifconfig carp1 192.168.2.1 vhid 2

The setup for host B is identical, but it has a higher advskew:

# ifconfig carp0 192.168.1.1 vhid 1 advskew 100           # ifconfig carp1 192.168.2.1 vhid 2 advskew 100

Because of the preempt option, when one of the physical interfaces of     host A fails, advskew is adjusted to 240 on all its carp interfaces.     This will cause host B to preempt on both interfaces instead of just the     failed one.

LOAD BALANCING     In order to set up a load balanced virtual host, it is necessary to con-     figure one carpnodes entry for each physical host.  In the following ex-     ample, two physical hosts are configured to provide balancing and     failover for the IP address 192.168.1.10.

First the carp interface on Host A is configured.  The advskew of 100 on     the second carpnode entry means that its advertisements will be sent out     slightly less frequently and will therefore become the designated backup.

# ifconfig carp0 192.168.1.10 carpnodes 1:0,2:100 balancing ip

The configuration for host B is identical, except the skew is on the     carpnode entry with virtual host 1 rather than virtual host 2.

# ifconfig carp0 192.168.1.10 carpnodes 1:100,2:0 balancing ip

If ARP balancing or a different mode of IP balancing is desired the     balancing mode can be adjusted accordingly.

SEE ALSO     sysctl(3), inet(4), pfsync(4), hostname.if(5), ifconfig(8), ifstated(8),     netstart(8), sysctl(8)

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