Sep 24 2008

arp_announce 和 arp_ignore

Category: 技术ssmax @ 17:09:46

用来屏蔽arp请求,比较难理解,先看看linux核心2.6的定义: 

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}

 

arp_ignore 设置为1,这个比较好理解,当别人的arp请求过来的时候,如果接收的设备上面没有这个ip,就不响应,默认是0,只要这台机器上面任何一个设备上面有这个ip,就响应arp请求,并发送mac地址应答。

arp_announce 这个就比较难解释了,先看一段英文的:

Assume that a linux box X has three interfaces – eth0, eth1 and eth2. Each interface has an IP address IP0, IP1 and IP2. When a local application tries to send an IP packet with IP0 through the eth2.

Unfortunately, the target node’s mac address is not resolved. The linux box X will send the ARP request to know the mac address of the target(or the gateway). In this case what is the IP source address of the “ARP request message”? The IP0- the IP source address of the transmitting IP or IP2 – the outgoing interface?

Until now(actually just 3 hours before) ARP request uses the IP address assigned to the outgoing interface(IP2 in the above example) However the linux’s behavior is a little bit different. Actually the selection of source address in ARP request is totally configurable by the proc variable “arp_announce”

If we want to use the IP2 not the IP0 in the ARP request, we should change the value to 1 or 2. The default value is 0 – allow IP0 is used for ARP request.

其实就是路由器的问题,因为路由器一般是动态学习ARP包的(一般动态配置DHCP的话),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 ,这样在lvs这样的架构下,所有发送包都是同一个VIP地址,那么arp请求就会包括VIP地址和设备Mac,而路由器收到这个arp请求就会更新自己的arp缓存,这样就会造成ip欺骗了,VIP被抢夺,所以就会有问题。

arp缓存为什么会更新了,什么时候会更新呢,为了减少arp请求的次数,当主机接收到询问自己的arp请求的时候,就会把源ip和源Mac放入自己的arp表里面,方便接下来的通讯。如果收到不是询问自己的包(arp是广播的,所有人都收到),就会丢掉,这样不会造成arp表里面无用数据太多导致有用的记录被删除。
What happens when a host receives an ARP request packet?
The ARP request is received and processed by all the hosts in the network, since it is a broadcast packet. The following steps are carried out when a ARP request packet is received by a host:
If the IP address to be resolved is for this host, then the ARP module sends an ARP reply packet with its Ethernet MAC address.
If the IP address to be resolved is for this host, then the ARP module updates its ARP cache with the source Ethernet MAC address to source IP address mapping present in the ARP request packet. If the entry is already present in the cache, it is overwritten. If it is not present, it is added.
If the IP address to be resolved is not for this host, then the ARP module discards the ARP request packet.

Will a host update its ARP cache upon receiving any ARP request?
A host will update its ARP cache, only if the ARP request is for its IP address. Otherwise, it will discard the ARP request.

What is the disadvantage if a host updates its ARP cache upon receiving any ARP request?
The host will exhaust the ARP cache with a lot of unused ARP entries, if it updates the ARP cache for any ARP request.  

如果路由器使用静态ARP表,客户端也使用静态网关ARP的话,基本就不用管这两个值了。

也看了一下vpn,用了pppoe协议,也是不需要处理arp请求的。

查看某个设备上面绑定了多少个ip:>ip addr show dev eth0

绑定多个ip(临时,看操作系统不同加在不同的地方):>ip addr add x.x.x.x/32 dev eth0

临时修改arp_announce和arp_ignore:

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

永久修改:

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

One Response to “arp_announce 和 arp_ignore”

  1. Rex says:

    你这篇文档很有意思,受益良多~ 收录链接!

Leave a Reply