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SDN环境中，每一个openvswitch的datapath实例中都会有大量的流表项，无论是使用各种关键字的grep手段或者是其他方法来确认是否由控制器下发了预期正确流表项，还是看关于特定数据包的匹配与最终action都是一件非常繁琐和头疼的事情。使用ovs-appctl工具结合linux自带的tcpdump抓包工具就可以很轻松直观的最终流表匹配情况，来完成自己繁琐的查找工作，还能避免自己的判断的错误。
主要步骤如下：
1、确认你需要跟踪的数据包的各项参数；
2、将其转化成openflow的match域的描述；
3、使用openvswitch提供的ofproto/trace功能跟踪流表匹配情况；
如何获取包特征参数？
可以找到自己需要验证的虚拟机，在其上发出需要验证的协议数据包，在物理计算节点上找到该虚拟机的后端虚拟网卡，在该虚拟网卡上使用tcpdump抓包，也可以从已有的抓包文件中获取，当然，也可以完全由自己指定openflow match域的内容。比如我读一下事先抓好的数据包。
[root@vdc ~]# tcpdump -ennvv -r /home/vnet31.0.pcap
reading from file /home/vnet31.0.pcap, link-type EN10MB (Ethernet)
10:25:17.693773 fa:16:3e:8c:eb:5b > fa:16:3e:a5:15:f3, ethertype IPv4 (0x0800), length 74: (tos 0x0, ttl 128, id 8060, offset 0, flags [none], proto ICMP (1), length 60)
20.20.20.104 > 20.20.20.101: ICMP echo request, id 1, seq 40197, length 40 转换成openflow的描述就是：
dl_src=fa:16:3e:8c:eb:5b,dl_dst=fa:16:3e:a5:15:f3,ip,nw_src=20.20.20.104,nw_dst=20.20.20.101,nw_proto=1
由于该虚拟网卡连接openvswitch的ofport 是37，所以要加上 in_port=37,完整的就如下所示：
in_port=37,dl_src=fa:16:3e:8c:eb:5b,dl_dst=fa:16:3e:a5:15:f3,ip,nw_src=20.20.20.104,nw_dst=20.20.20.101,nw_proto=1
确定了数据包的openflow的特征描述，就可以使用ovs-appctl提供的ofproto/trace功能来跟踪啦，命令如下：
[root@vdc ~]# ovs-appctl ofproto/trace dvs2_dp in_port=37,dl_src=fa:16:3e:8c:eb:5b,dl_dst=fa:16:3e:a5:15:f3,ip,nw_src=20.20.20.104,nw_dst=20.20.20.101,nw_proto=1 -generat
其中dvs2_dp是我实测环境中的bridge名称，-generate的意思是构造该数据报文，此时是确实有一个该报文通过ovs被处理了的。最终跟踪的效果下：
[root@vdc ~]# ovs-appctl ofproto/trace dvs2_dp in_port=37,dl_src=fa:16:3e:8c:eb:5b,dl_dst=fa:16:3e:a5:15:f3,ip,nw_src=20.20.20.104,nw_dst=20.20.20.101,nw_proto=1 -generate
Bridge: dvs2_dp
Flow: icmp,metadata=0,in_port=37,vlan_tci=0x0000,dl_src=fa:16:3e:8c:eb:5b,dl_dst=fa:16:3e:a5:15:f3,nw_src=20.20.20.104,nw_dst=20.20.20.101,nw_tos=0,nw_ecn=0,nw_ttl=0,icmp_type=0,icmp_code=0
Rule: table=0 cookie=0xd4 priority=0
OpenFlow actions=goto_table:1
Resubmitted flow: unchanged
Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0
Resubmitted odp: drop
Resubmitted megaflow: recirc_id=0,skb_priority=0,icmp,in_port=37,dl_src=fa:16:3e:8c:eb:5b,dl_dst=fa:16:3e:a5:15:f3,nw_src=20.20.20.104,nw_dst=20.20.20.101,nw_frag=no
Rule: table=1 cookie=0x616 priority=221,in_port=37
OpenFlow actions=write_metadata:0x3000009c4,goto_table:4
Resubmitted flow: icmp,metadata=0x3000009c4,in_port=37,vlan_tci=0x0000,dl_src=fa:16:3e:8c:eb:5b,dl_dst=fa:16:3e:a5:15:f3,nw_src=20.20.20.104,nw_dst=20.20.20.101,nw_tos=0,nw_ecn=0,nw_ttl=0,icmp_type=0,icmp_code=0
Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0
Resubmitted odp: drop
Resubmitted megaflow: recirc_id=0,skb_priority=0,icmp,in_port=37,dl_src=fa:16:3e:8c:eb:5b,dl_dst=fa:16:3e:a5:15:f3,nw_src=20.20.20.104,nw_dst=20.20.20.101,nw_frag=no
Rule: table=4 cookie=0x617 priority=161,dl_src=fa:16:3e:8c:eb:5b
OpenFlow actions=write_metadata:0x3000009c4,goto_table:5
Resubmitted flow: unchanged
Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0
Resubmitted odp: drop
Resubmitted megaflow:
recirc_id=0,skb_priority=0,icmp,in_port=37,dl_src=fa:16:3e:8c:eb:5b,dl_dst=fa:16:3e:a5:15:f3,nw_src=20.20.20.104,nw_dst=20.20.20.101,nw_frag=no
Rule: table=5 cookie=0xd9 priority=0
OpenFlow actions=goto_table:6
Resubmitted flow: unchanged
Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0
Resubmitted odp: drop
Resubmitted megaflow: recirc_id=0,skb_priority=0,icmp,metadata=0/0xffffff,in_port=37,dl_src=fa:16:3e:8c:eb:5b,dl_dst=fa:16:3e:a5:15:f3,nw_src=20.20.20.104,nw_dst=20.20.20.101,nw_frag=no
Rule: table=6 cookie=0x5e8 priority=102,metadata=0x9c4/0xffffff,dl_dst=fa:16:3e:a5:15:f3
OpenFlow actions=write_actions(set_field:0x9c4->tun_id,output:12)
Final flow: icmp,tun_id=0x9c4,metadata=0x3000009c4,in_port=37,vlan_tci=0x0000,dl_src=fa:16:3e:8c:eb:5b,dl_dst=fa:16:3e:a5:15:f3,nw_src=20.20.20.104,nw_dst=20.20.20.101,nw_tos=0,nw_ecn=0,nw_ttl=0,icmp_type=0,icmp_code=0
Megaflow: recirc_id=0,skb_priority=0,icmp,tun_id=0,metadata=0/0xffffff,in_port=37,dl_src=fa:16:3e:8c:eb:5b,dl_dst=fa:16:3e:a5:15:f3,nw_src=20.20.20.104,nw_dst=20.20.20.101,nw_ecn=0,nw_frag=no
Datapath actions: set(tunnel(tun_id=0x9c4,src=172.47.205.45,dst=172.47.205.46,tos=0x0,ttl=64,flags(df,key))),11上面的例子是最终数据包被打上了tun_id并从隧道端口被转发的跟踪，下面再举一个table miss被丢弃的例子：
[root@localhost ~]# ovs-appctl ofproto/trace sdn_dvs_dp in_port=127,dl_src=fa:16:3e:a5:85:78,dl_dst=00:d0:d0:1c:3d:2d,ip,nw_src=192.168.150.2,nw_dst=10.47.159.89,nw_proto=1 -generate
Bridge: sdn_dvs_dp
Flow: icmp,metadata=0,in_port=127,vlan_tci=0x0000,dl_src=fa:16:3e:a5:85:78,dl_dst=00:d0:d0:1c:3d:2d,nw_src=192.168.150.2,nw_dst=10.47.159.89,nw_tos=0,nw_ecn=0,nw_ttl=0,icmp_type=0,icmp_code=0
Rule: table=0 cookie=0x1ea priority=0
OpenFlow actions=goto_table:1
Resubmitted flow: unchanged
Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0
Resubmitted odp: drop
Resubmitted megaflow: recirc_id=0,skb_priority=0,icmp,in_port=127,dl_src=fa:16:3e:a5:85:78,dl_dst=00:d0:d0:1c:3d:2d,nw_src=192.168.150.2,nw_dst=10.47.159.89,nw_frag=no
Rule: table=1 cookie=0x294 priority=221,in_port=127
OpenFlow actions=write_metadata:0xa00000191,goto_table:4
Resubmitted flow: icmp,metadata=0xa00000191,in_port=127,vlan_tci=0x0000,dl_src=fa:16:3e:a5:85:78,dl_dst=00:d0:d0:1c:3d:2d,nw_src=192.168.150.2,nw_dst=10.47.159.89,nw_tos=0,nw_ecn=0,nw_ttl=0,icmp_type=0,icmp_code=0
Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0
Resubmitted odp: drop
Resubmitted megaflow: recirc_id=0,skb_priority=0,icmp,in_port=127,dl_src=fa:16:3e:a5:85:78,dl_dst=00:d0:d0:1c:3d:2d,nw_src=192.168.150.2,nw_dst=10.47.159.89,nw_frag=no
Rule: table=4 cookie=0x295 priority=161,dl_src=fa:16:3e:a5:85:78
OpenFlow actions=write_metadata:0xa00000191,goto_table:5
Resubmitted flow: unchanged
Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0
Resubmitted odp: drop
Resubmitted megaflow: recirc_id=0,skb_priority=0,icmp,in_port=127,dl_src=fa:16:3e:a5:85:78,dl_dst=00:d0:d0:1c:3d:2d,nw_src=192.168.150.2,nw_dst=10.47.159.89,nw_frag=no
Rule: table=5 cookie=0x1ef priority=0
OpenFlow actions=goto_table:6
Resubmitted flow: unchanged
Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0
Resubmitted odp: drop
Resubmitted megaflow: recirc_id=0,skb_priority=0,icmp,in_port=127,dl_src=fa:16:3e:a5:85:78,dl_dst=00:d0:d0:1c:3d:2d,nw_src=192.168.150.2,nw_dst=10.47.159.89,nw_frag=no
Rule: table=6 cookie=0x1f4 priority=111,dl_dst=00:d0:d0:1c:3d:2d
OpenFlow actions=goto_table:7
Resubmitted flow: unchanged
Resubmitted regs: reg0=0x0 reg1=0x0 reg2=0x0 reg3=0x0 reg4=0x0 reg5=0x0 reg6=0x0 reg7=0x0
Resubmitted odp: drop
Resubmitted megaflow: recirc_id=0,skb_priority=0,icmp,metadata=0/0xffffffff00000000,in_port=127,dl_src=fa:16:3e:a5:85:78,dl_dst=00:d0:d0:1c:3d:2d,nw_src=192.168.150.2,nw_dst=10.47.159.89,nw_frag=no
Rule: table=7 cookie=0x1f1 priority=0
OpenFlow actions=CONTROLLER:65535
Final flow: icmp,metadata=0xa00000191,in_port=127,vlan_tci=0x0000,dl_src=fa:16:3e:a5:85:78,dl_dst=00:d0:d0:1c:3d:2d,nw_src=192.168.150.2,nw_dst=10.47.159.89,nw_tos=0,nw_ecn=0,nw_ttl=0,icmp_type=0,icmp_code=0
Megaflow: recirc_id=0,skb_priority=0,icmp,metadata=0/0xffffffff00000000,in_port=127,dl_src=fa:16:3e:a5:85:78,dl_dst=00:d0:d0:1c:3d:2d,nw_src=192.168.150.2,nw_dst=10.47.159.89,nw_frag=no
Datapath actions: drop
This flow is handled by the userspace slow path because it:
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