追蹤openvswitch對特定數據報文的流表匹配與處理結果的實例

SDN環境中，每一個openvswitch的datapath實例中都會有大量的流表項，無論是使用各種關鍵字的grep手段或者是其他方法來確認是否由控制器下發了預期正確流表項，還是看關于特定數據包的匹配與最終action都是一件非常繁瑣和頭疼的事情。使用ovs-appctl工具結合linux自帶的tcpdump抓包工具就可以很輕松直觀的最終流表匹配情況，來完成自己繁瑣的查找工作，還能避免自己的判斷的錯誤。

?? 主要步驟如下：
? ? 1、確認你需要跟蹤的數據包的各項參數；

? ? 2、將其轉化成openflow的match域的描述；

? ? 3、使用openvswitch提供的ofproto/trace功能跟蹤流表匹配情況；

如何獲取包特征參數？
? ? 可以找到自己需要驗證的虛擬機，在其上發出需要驗證的協議數據包，在物理計算節點上找到該虛擬機的后端虛擬網卡，在該虛擬網卡上使用tcpdump抓包，也可以從已有的抓包文件中獲取，當然，也可以完全由自己指定openflow match域的內容。比如我讀一下事先抓好的數據包。

[root@vdc ~]# tcpdump -ennvv -r /home/vnet31.0.pcap 

<code?class="language-plain">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</code>??

轉換成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

由于該虛擬網卡連接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

確定了數據包的openflow的特征描述，就可以使用ovs-appctl提供的ofproto/trace功能來跟蹤啦，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是我實測環境中的bridge名稱，-generate的意思是構造該數據報文，此時是確實有一個該報文通過ovs被處理了的。最終跟蹤的效果下：

[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

上面的例子是最終數據包被打上了tun_id并從隧道端口被轉發的跟蹤，下面再舉一個table miss被丟棄的例子：

[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:
        - Sends "packet-in" messages to the OpenFlow controller.