Linux内核参数之arp_ignore和arp_announce
一、arp_ignore和arp_announce介绍
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}
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.
sysctl.conf中包含all和eth/lo（具体网卡）的arp_ignore参数，取其中较大的值生效。
二、arp_ignore和arp_announce参数示例
（1）当arp_ignore参数配置为0时，eth1网卡上收到目的IP为环回网卡IP的arp请求，但是eth1也会返回arp响应，把自己的mac地址告诉对端。
（2）当arp_ignore参数配置为1时，eth1网卡上收到目的IP为环回网卡IP的arp请求，发现请求的IP不是自己网卡上的IP，不会回arp响应。
（3）当arp_announce参数配置为0时，系统要发送的IP包源地址为eth1的地址，IP包目的地址根据路由表查询判断需要从eth2网卡发出，这时会先从eth2网卡发起一个arp请求，用于获取目的IP地址的MAC地址。该arp请求的源MAC自然是eth2网卡的MAC地址，但是源IP地址会选择eth1网卡的地址。
（4）当arp_announce参数配置为2时，eth2网卡发起arp请求时，源IP地址会选择eth2网卡自身的IP地址。
三、arp_ignore和arp_announce参数在DR模式下的作用
1. arp_ignore
因为DR模式下，每个真实服务器节点都要在环回网卡上绑定虚拟服务IP。这时候，如果客户端对于虚拟服务IP的arp请求广播到了各个真实服务器节点，如果arp_ignore参数配置为0，则各个真实服务器节点都会响应该arp请求，此时客户端就无法正确获取LVS节点上正确的虚拟服务IP所在网卡的MAC地址。假如某个真实服务器节点A的网卡eth1响应了该arp请求，客户端把A节点的eth1网卡的MAC地址误认为是LVS节点的虚拟服务IP所在网卡的MAC，从而将业务请求消息直接发到了A节点的eth1网卡。这时候虽然因为A节点在环回网卡上也绑定了虚拟服务IP，所以A节点也能正常处理请求，业务暂时不会受到影响。但时此时由于客户端请求没有发到LVS的虚拟服务IP上，所以LVS的负载均衡能力没有生效。造成的后果就是，A节点一直在单节点运行，业务量过大时可能会出现性能瓶颈。
所以DR模式下要求arp_ignore参数要求配置为1。
2. arp_announce
四、arp_ignore和arp_announce参数的配置方法
arp_ignore和arp_announce参数分别有all,default,lo,eth1,eth2...等对应不同网卡的具体参数。当all和具体网卡的参数值不一致时，取较大值生效。
一般只需修改all和某个具体网卡的参数即可（取决于你需要修改哪个网卡）。下面以修改lo网卡为例：
1. 修改/etc/sysctl.conf文件，然后sysctl -p刷新到内存。
2. 使用sysctl -w直接写入内存：
3. 修改/proc文件系统：
