arp_ignore arp_announce

在搭建lvs_dr 或者lvs_tun模式时会用到两个重要的参数arp_ignore arp_announce：

下边分别说明

arp_announce :INTEGER 不同取值表示对网络接口上本地IP地址发出的ARP回应作出相应级别的限制：相关代码在 默认为0
确定不同程度的限制,宣布对来自本地源IP地址发出Arp请求的接口
0 - (默认) 在任意网络接口上的任何本地地址

1 -尽量避免不在该网络接口子网段的本地地址. 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 - 对查询目标使用最适当的本地地址.在此模式下将忽略这个IP数据包的源地址并尝试选择与能与该地址通信的本地地址.首要是选择所有的网络接口的子网中外出 访问子网中包含该目标IP地址的本地地址. 如果没有合适的地址被发现,将选择当前的发送网络接口或其他的有可能接受到该ARP回应的网络接口来进行发送；
当内网的机器要发送一个到外部的ip包，那么它就会请求路由器的Mac地址，发送一个arp请求，这个arp请求里面包括了自己的ip地址和Mac地址，而linux默认是使用ip的源ip地址作为arp里面的源ip地址，而不是使用发送设备上面的，如果设置 arp_announce 为2，则使用发送设备上的ip。


arp_ignore : INTEGER 定义对目标地址为本地IP的ARP询问不同的应答模式，相关代码在arp_announce 函数中 默认为0
0 - (默认值): 回应任何网络接口上对任何本地IP地址的arp查询请求（比如eth0=192.168.0.1/24,eth1=10.1.1.1/24,那么即使 eth0收到来自10.1.1.2这样地址发起的对10.1.1.1 的arp查询也会回应--而原本这个请求该是出现在eth1上，也该由eth1回应的）

1 - 只回答目标IP地址是来访网络接口本地地址的ARP查询请求（比如eth0=192.168.0.1/24,eth1=10.1.1.1/24,那么即使 eth0收到来自10.1.1.2这样地址发起的对192.168.0.1的查询会回答，而对10.1.1.1 的arp查询不会回应）

2 -只回答目标IP地址是来访网络接口本地地址的ARP查询请求,且来访IP必须在该网络接口的子网段内（比如 eth0=192.168.0.1/24,eth1=10.1.1.1/24,eth1收到来自10.1.1.2这样地址发起的对192.168.0.1 的查询不会回答，而对192.168.0.2发起的对192.168.0.1的arp查询会回应）

3 - 不回应该网络界面的arp请求，而只对设置的唯一和连接地址做出回应(do not reply for local addresses configured with scope host,only resolutions for global and link addresses are replied )

4-7 - 保留未使用
8 -不回应所有（本地地址）的arp查询

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.