Difference between revisions of "Using arp announce/arp ignore to disable ARP"

Latest revision as of 21:29, 22 March 2007

arp_announce/arp_ignore sysctl

The arp_announce/arp_ignore sysctl on interfaces is available at the Linux official kernel since 2.6.4 and 2.4.26. The description about arp_announce/arp_ignore taken from kernel documentation is as follows:

arp_announce - INTEGER
	Define different restriction levels for announcing the local
	source IP address from IP packets in ARP requests sent on
	interface:
	0 - (default) Use any local address, configured on any interface
	1 - Try to avoid local addresses that are not in the target's
	subnet for this interface. This mode is useful when target
	hosts reachable via this interface require the source IP
	address in ARP requests to be part of their logical network
	configured on the receiving interface. When we generate the
	request we will check all our subnets that include the
	target IP and will preserve the source address if it is from
	such subnet. If there is no such subnet we select source
	address according to the rules for level 2.
	2 - Always use the best local address for this target.
	In this mode we ignore the source address in the IP packet
	and try to select local address that we prefer for talks with
	the target host. Such local address is selected by looking
	for primary IP addresses on all our subnets on the outgoing
	interface that include the target IP address. If no suitable
	local address is found we select the first local address
	we have on the outgoing interface or on all other interfaces,
	with the hope we will receive reply for our request and
	even sometimes no matter the source IP address we announce.

	The max value from conf/{all,interface}/arp_announce is used.

	Increasing the restriction level gives more chance for
	receiving answer from the resolved target while decreasing
	the level announces more valid sender's information.

arp_ignore - INTEGER
	Define different modes for sending replies in response to
	received ARP requests that resolve local target IP addresses:
	0 - (default): reply for any local target IP address, configured
	on any interface
	1 - reply only if the target IP address is local address
	configured on the incoming interface
	2 - reply only if the target IP address is local address
	configured on the incoming interface and both with the
	sender's IP address are part from same subnet on this interface
	3 - do not reply for local addresses configured with scope host,
	only resolutions for global and link addresses are replied
	4-7 - reserved
	8 - do not reply for all local addresses

	The max value from conf/{all,interface}/arp_ignore is used
	when ARP request is received on the {interface}

Disable ARP for VIP

To disable ARP for VIP at real servers, we just need to set arp_announce/arp_ignore sysctls at the interface connected to the VIP network. For example, real servers have eth0 connected to the VIP network with the VIP at interface lo, we will have the following commands.

echo 1 > /proc/sys/net/ipv4/conf/eth0/arp_ignore
echo 2 > /proc/sys/net/ipv4/conf/eth0/arp_announce

Or, if /etc/sysctl.conf is used in the system, we have this config in /etc/sysctl.conf

net.ipv4.conf.eth0.arp_ignore = 1
net.ipv4.conf.eth0.arp_announce = 2

Note that the arp_announce/arp_ignore sysctls must be setup correctly, before the VIP address is brought up at a logical interface at real servers.

Linux Distributions

• RHEL 4 / CentOS 4
• SUSE 9 Enterprise
• Debian

References

• http://ssi.bg/~ja/#arp_announce