大型网站集群构建之六 (Real Server)

    在做完load balancing之后我们就要做提供真实服务的Real 服务器了。

    如果使用nat模式，这个没有什么问题，非常简单，只要每台机器的内容及服务正常就可以了，如果使用DR模式的话需要注意本地的网卡的广播问题，要不然就会出现只往一台服务器上发请求，而其他服务器无请求的情况，这里有几种解决方法。

 

lvs dr Disable ARP for VIP

1. Using arp_tables_jf to disable ARP
arptables -F
arptables -A IN -d $VIP -j DROP
arptables -A OUT -s $VIP -j mangle --mangle-ip-s $RIP


2. Using arp announce/arp ignore to disable ARP
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}

net.ipv4.conf.eth0.arp_ignore = 1
net.ipv4.conf.eth0.arp_announce = 2

3. Using the hidden interface to disable ARP

Linux kernel 2.4 and 2.6, you need to apply the hidden patch
# Start the hiding interface functionality
echo 1 > /proc/sys/net/ipv4/conf/all/hidden
# Hide all addresses for this interface
echo 1 > /proc/sys/net/ipv4/conf//hidden

# Insert the ipip module
insmod ipip
# Make the tunl0 device up
ifconfig tunl0 up
# Start the hiding interface functionality
echo 1 > /proc/sys/net/ipv4/conf/all/hidden
# Hide all addresses for this tunnel device
echo 1 > /proc/sys/net/ipv4/conf/tunl0/hidden
# Configure a VIP on an alias of tunnel device
ifconfig tunl0:0 up

4. Using redirect to disable ARP
iptables -t nat -A PREROUTING -p tcp -d --dport -j REDIRECT --to-port

5. Using policy routing to disable ARP
# Block access from the LAN to the real server's VIP. By
# this way we ignore the router's ARP probes. The drawback:
# we ignore the client's probes too. We have to do this
# because the client on the LAN can receive replies from all
# real servers
ip rule add prio 99 from 172.26.20/24 table 99
ip route add table 99 blackhole 172.26.20.118

# Now accept locally any other traffic, i.e. not from
# 172.26.20/24
ip rule add prio 100 table 100
ip route add table 100 local 172.26.20.118 dev lo

6. Using the noarp module to disable ARP
http://www.masarlabs.com/noarp/
noarpctl add VIP RIP

还有一点需要注意，尤其是在udp的服务器中更要注意：
    在DR方式下，服务器程序需要bind到VIP地址上，这样响应报文的源地址是VIP，对客户端程序来说这样的响应报文是有效的。注：如果监听 0.0.0.0 在udp服务的包中默认使用网卡的实际地址回报，而不是vip地址，这样客户端访问就会出现问题。