我在OpenStack中运行KVM实例，并没有从DHCP服务器获取IP地址。
使用tcpdump，我可以在计算主机的vnet0上看到请求和回复数据包：
# tcpdump -i vnet0 -n port 67 or port 68 tcpdump: WARNING: vnet0: no IPv4 address assigned tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on vnet0, link-type EN10MB (Ethernet), capture size 65535 bytes 19:44:56.176727 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from fa:16:3e:46:f6:11, length 300 19:44:56.176785 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from fa:16:3e:46:f6:11, length 300 19:44:56.177315 IP 10.40.0.1.67 > 10.40.0.3.68: BOOTP/DHCP, Reply, length 319 19:45:02.179834 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from fa:16:3e:46:f6:11, length 300 19:45:02.179904 IP 0.0.0.0.68 > 255.255.255.255.67: BOOTP/DHCP, Request from fa:16:3e:46:f6:11, length 300 19:45:02.180375 IP 10.40.0.1.67 > 10.40.0.3.68: BOOTP/DHCP, Reply, length 319 但是，如果我在KVM实例内的eth0上做同样的事情，我只能看到请求包，而不是回复包。 什么会阻止数据包从主机的vnet0到guest的eth0？
我的主机运行的是Ubuntu 12.04，我的客户正在运行CentOS 6.3。
请注意，我在我的iptables中添加了这个规则，但是它不能解决问题：
-A POSTROUTING -p udp -m udp --dport 68 -j CHECKSUM --checksum-fill
该实例对应于vnet0并通过br100连接：
# brctl show bridge name bridge id STP enabled interfaces br100 8000.54781a8605f2 no eth1 vnet0 vnet1 virbr0 8000.000000000000 yes
下面是完整的iptables-save：
# Generated by iptables-save v1.4.12 on Tue Apr 2 19:47:27 2013 *nat :PREROUTING ACCEPT [8323:2553683] :INPUT ACCEPT [7993:2494942] :OUTPUT ACCEPT [6158:461050] :POSTROUTING ACCEPT [6455:511595] :nova-compute-OUTPUT - [0:0] :nova-compute-POSTROUTING - [0:0] :nova-compute-PREROUTING - [0:0] :nova-compute-float-snat - [0:0] :nova-compute-snat - [0:0] :nova-postrouting-bottom - [0:0] -A PREROUTING -j nova-compute-PREROUTING -A OUTPUT -j nova-compute-OUTPUT -A POSTROUTING -j nova-compute-POSTROUTING -A POSTROUTING -j nova-postrouting-bottom -A POSTROUTING -s 192.168.122.0/24 ! -d 192.168.122.0/24 -p tcp -j MASQUERADE --to-ports 1024-65535 -A POSTROUTING -s 192.168.122.0/24 ! -d 192.168.122.0/24 -p udp -j MASQUERADE --to-ports 1024-65535 -A POSTROUTING -s 192.168.122.0/24 ! -d 192.168.122.0/24 -j MASQUERADE -A nova-compute-snat -j nova-compute-float-snat -A nova-postrouting-bottom -j nova-compute-snat COMMIT # Completed on Tue Apr 2 19:47:27 2013 # Generated by iptables-save v1.4.12 on Tue Apr 2 19:47:27 2013 *mangle :PREROUTING ACCEPT [7969:5385812] :INPUT ACCEPT [7905:5363718] :FORWARD ACCEPT [158:48190] :OUTPUT ACCEPT [6877:8647975] :POSTROUTING ACCEPT [7035:8696165] -A POSTROUTING -o virbr0 -p udp -m udp --dport 68 -j CHECKSUM --checksum-fill -A POSTROUTING -p udp -m udp --dport 68 -j CHECKSUM --checksum-fill COMMIT # Completed on Tue Apr 2 19:47:27 2013 # Generated by iptables-save v1.4.12 on Tue Apr 2 19:47:27 2013 *filter :INPUT ACCEPT [2196774:15856921923] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [2447201:1170227646] :nova-compute-FORWARD - [0:0] :nova-compute-INPUT - [0:0] :nova-compute-OUTPUT - [0:0] :nova-compute-inst-19 - [0:0] :nova-compute-inst-20 - [0:0] :nova-compute-local - [0:0] :nova-compute-provider - [0:0] :nova-compute-sg-fallback - [0:0] :nova-filter-top - [0:0] -A INPUT -j nova-compute-INPUT -A INPUT -i virbr0 -p udp -m udp --dport 53 -j ACCEPT -A INPUT -i virbr0 -p tcp -m tcp --dport 53 -j ACCEPT -A INPUT -i virbr0 -p udp -m udp --dport 67 -j ACCEPT -A INPUT -i virbr0 -p tcp -m tcp --dport 67 -j ACCEPT -A FORWARD -j nova-filter-top -A FORWARD -j nova-compute-FORWARD -A FORWARD -d 192.168.122.0/24 -o virbr0 -m state --state RELATED,ESTABLISHED -j ACCEPT -A FORWARD -s 192.168.122.0/24 -i virbr0 -j ACCEPT -A FORWARD -i virbr0 -o virbr0 -j ACCEPT -A FORWARD -o virbr0 -j REJECT --reject-with icmp-port-unreachable -A FORWARD -i virbr0 -j REJECT --reject-with icmp-port-unreachable -A OUTPUT -j nova-filter-top -A OUTPUT -j nova-compute-OUTPUT -A nova-compute-FORWARD -i br100 -j ACCEPT -A nova-compute-FORWARD -o br100 -j ACCEPT -A nova-compute-inst-19 -m state --state INVALID -j DROP -A nova-compute-inst-19 -m state --state RELATED,ESTABLISHED -j ACCEPT -A nova-compute-inst-19 -j nova-compute-provider -A nova-compute-inst-19 -s 10.40.0.1/32 -p udp -m udp --sport 67 --dport 68 -j ACCEPT -A nova-compute-inst-19 -s 10.40.0.0/16 -j ACCEPT -A nova-compute-inst-19 -p tcp -m tcp --dport 22 -j ACCEPT -A nova-compute-inst-19 -p icmp -j ACCEPT -A nova-compute-inst-19 -j nova-compute-sg-fallback -A nova-compute-inst-20 -m state --state INVALID -j DROP -A nova-compute-inst-20 -m state --state RELATED,ESTABLISHED -j ACCEPT -A nova-compute-inst-20 -j nova-compute-provider -A nova-compute-inst-20 -s 10.40.0.1/32 -p udp -m udp --sport 67 --dport 68 -j ACCEPT -A nova-compute-inst-20 -s 10.40.0.0/16 -j ACCEPT -A nova-compute-inst-20 -p tcp -m tcp --dport 22 -j ACCEPT -A nova-compute-inst-20 -p icmp -j ACCEPT -A nova-compute-inst-20 -j nova-compute-sg-fallback -A nova-compute-local -d 10.40.0.3/32 -j nova-compute-inst-19 -A nova-compute-local -d 10.40.0.4/32 -j nova-compute-inst-20 -A nova-compute-sg-fallback -j DROP -A nova-filter-top -j nova-compute-local COMMIT # Completed on Tue Apr 2 19:47:27 2013
有一个类似的问题，对我来说这个固定它：
echo 0 > /proc/sys/net/bridge/bridge-nf-call-iptables #To disable Iptables in the bridge.
详情请看这里： http : //www.linuxfoundation.org/collaborate/workgroups/networking/bridge#No_traffic_gets_trough_.28except_ARP_and_STP.29