linux双网卡服务器配置同一网段IP的ping问题

问题是这样的，我有一台服务器，两个网口分别配置了同一网段的IP，然后将一个网口的网线拔掉，发现该网卡的IP仍然能够ping通，但是arp -a却发现该IP地址返回的确是另一块网卡的mac。百思不得其解。最后发现居然是linux内核的问题。解释如下。

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:


	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 for at , we just need to set arp_announce/arp_ignore sysctls at the interface connected to the VIP network. For example, 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 address is brought up at a logical interface at .