追踪openvswitch对特定数据报文的流表匹配与处理结果的实例

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 -generateBridge: dvs2_dpFlow: 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=0Rule: table=0 cookie=0xd4 priority=0OpenFlow 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=0Megaflow: 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=noDatapath 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 -generateBridge: sdn_dvs_dpFlow: 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=0Rule: table=0 cookie=0x1ea priority=0OpenFlow 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:65535Final 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=0Megaflow: 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=noDatapath actions: dropThis flow is handled by the userspace slow path because it:        - Sends "packet-in" messages to the OpenFlow controller.