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Tracing packets inside Open vSwitch

Open vSwitch (OVS) is a programmable software switch that can execute actions at per packet level. This document explains how to use the tracing tool to know what is happening with packets as they go through the data plane processing.

The ovs-vswitchd(8) manpage describes basic usage of the ofproto/trace command used for tracing in Open vSwitch. For a tool with a goal similar to ofproto/trace for tracing packets through OVN logical switches, see ovn-trace(8).

Packet Tracing

In order to understand the tool, let's use the following flows as an example:

table=3,ip,tcp,tcp_dst=80,action=output:2
table=2,ip,tcp,tcp_dst=22,action=output:1
table=0,in_port=3,ip,nw_src=192.0.2.0/24,action=resubmit(,2)
table=0,in_port=3,ip,nw_src=198.51.100.0/24,action=resubmit(,3)

Note

If you can't use a "real" OVS setup you can use ovs-sandbox, as described in :doc:`/tutorials/ovs-advanced`, which also provides additional tracing examples.

The first line adds a rule in table 3 matching on TCP/IP packet with destination port 80 (HTTP). If a packet matches, the action is to output the packet on OpenFlow port 2.

The second line is similar but matches on destination port 22. If a packet matches, the action is to output the packet on OpenFlow port 1.

The next two lines matches on source IP addresses. If there is a match, the packet is submitted to table indicated as parameter to the resubmit() action.

Now let's see if a packet from IP address 192.0.2.1 and destination port 22 would really go to OpenFlow port 1:

$ ovs-appctl ofproto/trace br0 in_port=3,tcp,nw_src=192.0.2.2,tcp_dst=22
Flow: tcp,in_port=3,vlan_tci=0x0000,dl_src=00:00:00:00:00:00,dl_dst=00:00:00:00:00:00,nw_src=192.0.2.2,nw_dst=0.0.0.0,nw_tos=0,nw_ecn=0,nw_ttl=0,tp_src=0,tp_dst=22,tcp_flags=0

bridge("br0")
-------------
 0. ip,in_port=3,nw_src=192.0.2.0/24, priority 32768
    resubmit(,2)
 2. tcp,tp_dst=22, priority 32768
    output:1

Final flow: unchanged
Megaflow: recirc_id=0,tcp,in_port=3,nw_src=192.0.2.0/24,nw_frag=no,tp_dst=22
Datapath actions: 1

The first line is the trace command. The br0 is the bridge where the packet is going through. The next arguments describe the packet itself. For instance, the nw_src matches with the IP source address. All the packet fields are well documented in the ovs-fields(7) man-page.

The second line shows the flow extracted from the packet described in the command line. Unspecified packet fields are zeroed.

The second group of lines shows the packet's trip through bridge br0. We see, in table 0, the OpenFlow flow that the fields matched, along with its priority, followed by its actions, one per line. In this case, we see that this packet matches the flow that resubmit those packets to table 2. The "resubmit" causes a second lookup in OpenFlow table 2, described by the block of text that starts with "2.". In the second lookup we see that this packet matches the rule that outputs those packets to OpenFlow port #1.

In summary, it is possible to follow the flow entries and actions until the final decision is made. At the end, the trace tool shows the Megaflow which matches on all relevant fields followed by the data path actions.

Let's see what happens with the same packet but with another TCP destination port:

$ ovs-appctl ofproto/trace br0 in_port=3,tcp,nw_src=192.0.2.2,tcp_dst=80
Flow: tcp,in_port=3,vlan_tci=0x0000,dl_src=00:00:00:00:00:00,dl_dst=00:00:00:00:00:00,nw_src=192.0.2.2,nw_dst=0.0.0.0,nw_tos=0,nw_ecn=0,nw_ttl=0,tp_src=0,tp_dst=80,tcp_flags=0

bridge("br0")
-------------
 0. ip,in_port=3,nw_src=192.0.2.0/24, priority 32768
    resubmit(,2)
 2. No match.
    drop

Final flow: unchanged
Megaflow: recirc_id=0,tcp,in_port=3,nw_src=192.0.2.0/24,nw_frag=no,tp_dst=0x40/0xffc0
Datapath actions: drop

In the second group of lines, in table 0, you can see that the packet matches with the rule because of the source IP address, so it is resubmitted to the table 2 as before. However, it doesn't match any rule there. When the packet doesn't match any rule in the flow tables, it is called a table miss. The virtual switch table miss behavior can be configured and it depends on the OpenFlow version being used. In this example the default action was to drop the packet.

Credits

This document is heavily based on content from Flavio Bruno Leitner at Red Hat: