关于linux 双网卡arp响应问题分析

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:

Linux 官方内核自2.6.4和2.4.26开始，interface上的arp_announce/arp_ignore系统调用就可用了。下面是内核文档中关于arp_announce/arp_ignore的描述:

arp_announce - INTEGER
 Define different restriction levels for announcing the local
 source IP address from IP packets in ARP requests sent on
 interface:
 定义不同级别的限制，声明本地IP地址发送ARP请求的interface:

0 - (default) Use any local address, configured on any interface

0 - (缺省) 使用任何interface上的任何本地地址
 
 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.

1 - 避免这个interface的不在目标子网里的本地地址. 当发起ARP请求的
 源IP地址是被设置应该经由路由达到此网络接口的时候很有用.此时会
 检查来访IP是否为所有接口上的子网段内ip之一.如果该来访IP不属于各
 个网络接口上的子网段内,那么将采用级别2的方式来进行处理.

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:

为发送ARP的请求的回应，定义不同的模式， 

0 - (default): reply for any local target IP address, configured
 on any interface

0 - (缺省): 回应任何网络接口上对任何本地IP地址的arp查询请求

1 - reply only if the target IP address is local address
 configured on the incoming interface

1 - 仅当目标IP地址是当前收到ARP请求的interface上配置的本地IP时，才回应。

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

2 - 仅当目标IP地址是当前收到ARP请求的interface上配置的本地IP, 且发送者IP地址
 在这个interface的同子网内时，才回应。

 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

8 - 不回应所有本地地址。

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.