ovn-trace — Open Virtual Network logical network tracing utility


ovn-trace [options] datapath microflow

ovn-trace [options] --detach


This utility simulates packet forwarding within an OVN logical network. It can be used to run through ``what-if’’ scenarios: if a packet originates at a logical port, what will happen to it and where will it ultimately end up? Users already familiar with the Open vSwitch ofproto/trace command described in ovs-vswitch(8) will find ovn-trace to be a similar tool for logical networks.

ovn-trace works by reading the Logical_Flow and other tables from the OVN southbound database (see ovn-sb(5)). It simulates a packet’s path through logical networks by repeatedly looking it up in the logical flow table, following the entire tree of possibilities.

ovn-trace simulates only the OVN logical network. It does not simulate the physical elements on which the logical network is layered. This means that, for example, it is unimportant how VMs are distributed among hypervisors, or whether their hypervisors are functioning and reachable, so ovn-trace will yield the same results regardless. There is one important exception: ovn-northd, the daemon that generates the logical flows that ovn-trace simulates, treats logical ports differently based on whether they are up or down. Thus, if you see surprising results, ensure that the ports involved in a simulation are up.

The simplest way to use ovn-trace is to provide datapath and microflow arguments on the command line. In this case, it simulates the behavior of a single packet and exits. For an alternate usage model, see Daemon Mode below.

The datapath argument specifies the name of a logical datapath. Acceptable names are the name from the northbound Logical_Switch or Logical_Router table, the UUID of a record from one of those tables, or the UUID of a record from the southbound Datapath_Binding table.

The microflow argument describes the packet whose forwarding is to be simulated, in the syntax of an OVN logical expression, as described in ovn-sb(5), to express constraints. The parser understands prerequisites; for example, if the expression refers to ip4.src, there is no need to explicitly state ip4 or eth.type == 0x800.

For reasonable L2 behavior, the microflow should include at least inport and eth.dst, plus eth.src if port security is enabled. For example:

    inport == "lp11" && eth.src == 00:01:02:03:04:05 && eth.dst == ff:ff:ff:ff:ff:ff

For reasonable L3 behavior, microflow should also include ip4.src and ip4.dst (or ip6.src and ip6.dst) and ip.ttl. For example:

    inport == "lp111" && eth.src == f0:00:00:00:01:11 && eth.dst == 00:00:00:00:ff:11
    && ip4.src == && ip4.dst == && ip.ttl == 64

Here’s an ARP microflow example:

    inport == "lp123"
    && eth.dst == ff:ff:ff:ff:ff:ff && eth.src == f0:00:00:00:01:11
    && arp.op == 1 && arp.sha == f0:00:00:00:01:11 && arp.spa ==
    && arp.tha == ff:ff:ff:ff:ff:ff && arp.tpa ==

ovn-trace will reject erroneous microflow expressions, which beyond syntax errors fall into two categories. First, they can be ambiguous. For example, tcp.src == 80 is ambiguous because it does not state IPv4 or IPv6 as the Ethernet type. ip4 && tcp.src > 1024 is also ambiguous because it does not constrain bits of tcp.src to particular values. Second, they can be contradictory, e.g. ip4 && ip6.


ovn-trace supports the three different forms of output, each described in a separate section below. Regardless of the selected output format, ovn-trace starts the output with a line that shows the microflow being traced in OpenFlow syntax.

Detailed Output

The detailed form of output is also the default form. This form groups output into sections headed up by the ingress or egress pipeline being traversed. Each pipeline lists each table that was visited (by number and name), the ovn-northd source file and line number of the code that added the flow, the match expression and priority of the logical flow that was matched, and the actions that were executed.

The execution of OVN logical actions naturally forms a ``control stack’’ that resembles that of a program in conventional programming languages such as C or Java. Because the next action that calls into another logical flow table for a lookup is a recursive construct, OVN ``programs’’ in practice tend to form deep control stacks that, displayed in the obvious way using additional indentation for each level, quickly use up the horizontal space on all but the widest displays. To make detailed output more readable, without loss of generality, ovn-trace omits indentation for ``tail recursion,’’ that is, when next is the last action in a logical flow, it does not indent details of the next table lookup more deeply. Output still uses indentation when it is needed for clarity.

OVN ``programs’’ traces also tend to encounter long strings of logical flows with match expression 1 (which matches every packet) and the single action next;. These are uninteresting and merely clutter output, so ovn-trace omits them entirely even from detailed output.

The following excerpt from detailed ovn-trace output shows a section for a packet traversing the ingress pipeline of logical datapath ls1 with ingress logical port lp111. The packet matches a logical flow in table 0 (aka ls_in_port_sec_l2) with priority 50 and executes next(1); to pass to table 1. Tables 1 through 11 are trivial and omitted. In table 12 (aka ls_in_l2_lkup), the packet matches a flow with priority 50 based on its Ethernet destination address and the flow’s actions output the packet to the lrp11-attachement logical port.

    ingress(dp="ls1", inport="lp111")
    0. ls_in_port_sec_l2: inport == "lp111", priority 50
    12. ls_in_l2_lkup: eth.dst == 00:00:00:00:ff:11, priority 50
    outport = "lrp11-attachment";

Summary Output

Summary output includes the logical pipelines visited by a packet and the logical actions executed on it. Compared to the detailed output, however, it removes details of tables and logical flows traversed by a packet. It uses a format closer to that of a programming language and does not attempt to avoid indentation. The summary output equivalent to the above detailed output fragment is:

    ingress(dp="ls1", inport="lp111") {
    outport = "lrp11-attachment";

Minimal Output

Minimal output includes only actions that modify packet data (not including OVN registers or metadata such as outport) and output actions that actually deliver a packet to a logical port (excluding patch ports). The operands of actions that modify packet data are displayed reduced to constants, e.g. ip4.dst = reg0; might be show as ip4.dst =; if that was the value actually loaded. This yields output even simpler than the summary format. (Users familiar with Open vSwitch may recognize this as similar in spirit to the datapath actions listed at the bottom of ofproto/trace output.)

The minimal output format reflects the externally seen behavior of the logical networks more than it does the implementation. This makes this output format the most suitable for use in regression tests, because it is least likely to change when logical flow tables are rearranged without semantic change.

Stateful Actions

Some OVN logical actions use or update state that is not available in the southbound database. ovn-trace handles these actions as described below:


By default ovn-trace treats flows as ``tracked’’ and ``established.’’ See the description of the --ct option for a way to override this behavior.

ct_dnat (without an argument)

Forks the pipeline. In one fork, advances to the next table as if next; were executed. The packet is not changed, on the assumption that no NAT state was available. In the other fork, the pipeline continues without change after the ct_dnat action.

ct_snat (without an argument)

This action distinguishes between gateway routers and distributed routers. A gateway router is defined as a logical datapath that contains an l3gateway port; any other logical datapath is a distributed router. On a gateway router, ct_snat; is treated as a no-op. On a distributed router, it is treated the same way as ct_dnat;.


Forks the pipeline. In one fork, sets ip4.dst (or ip4.src) to ip and ct.dnat (or ct.snat) to 1 and advances to the next table as if next; were executed. In the other fork, the pipeline continues without change after the ct_dnat (or ct_snat) action.


Forks the pipeline. In one fork, sets ip4.dst (or ip6.dst) to one of the load-balancer addresses and the destination port to its associated port, if any, and sets ct.dnat to 1. With one or more arguments, gives preference to the address specified on --lb-dst, if any; without arguments, uses the address and port specified on --lb-dst. In the other fork, the pipeline continues without change after the ct_lb action.


These actions are treated as no-ops.

Daemon Mode

If ovn-trace is invoked with the --detach option (see Daemon Options, below), it runs in the background as a daemon and accepts commands from ovs-appctl (or another JSON-RPC client) indefinitely. The currently supported commands are described below.

trace [options] datapath microflow

Traces microflow through datapath and replies with the results of the trace. Accepts the options described under Trace Options below.


Causes ovn-trace to gracefully terminate.


Trace Options


These options control the form and level of detail in ovn-trace output. If more than one of these options is specified, all of the selected forms are output, in the order listed above, each headed by a banner line. If none of these options is given, --detailed is the default. See Output, above, for a description of each kind of output.


Selects all three forms of output.


Makes ovn-trace attempt to obtain and display the OpenFlow flows that correspond to each OVN logical flow. To do so, ovn-trace connects to remote (by default, unix:/br-int.mgmt) over OpenFlow and retrieves the flows. If remote is specified, it must be an active OpenFlow connection method described in ovsdb(7).

To make the best use of the output, it is important to understand the relationship between logical flows and OpenFlow flows. ovn-architecture(7), under Architectural Physical Life Cycle of a Packet, describes this relationship. Keep in mind the following points:

  • ovn-trace currently shows all the OpenFlow flows to which a logical flow corresponds, even though an actual packet ordinarily matches only one of these.
  • Some logical flows can map to the Open vSwitch ``conjunctive match’’ extension (see ovs-fields(7)). Currently ovn-trace cannot display the flows with conjunction actions that effectively produce the conj_id match.
  • Some logical flows may not be represented in the OpenFlow tables on a given hypervisor, if they could not be used on that hypervisor.
  • Some OpenFlow flows do not correspond to logical flows, such as OpenFlow flows that map between physical and logical ports. These flows will never show up in a trace.
  • When ovn-trace omits uninteresting logical flows from output, it does not look up the corresponding OpenFlow flows.

This option sets the ct_state flags that a ct_next logical action will report. The flags must be a comma- or space-separated list of the following connection tracking flags:

  • trk: Include to indicate connection tracking has taken place. (This bit is set automatically even if not listed in flags.
  • new: Include to indicate a new flow.
  • est: Include to indicate an established flow.
  • rel: Include to indicate a related flow.
  • rpl: Include to indicate a reply flow.
  • inv: Include to indicate a connection entry in a bad state.
  • dnat: Include to indicate a packet whose destination IP address has been changed.
  • snat: Include to indicate a packet whose source IP address has been changed.

The ct_next action is used to implement the OVN distributed firewall. For testing, useful flag combinations include:

  • trk,new: A packet in a flow in either direction through a firewall that has not yet been committed (with ct_commit).
  • trk,est: A packet in an established flow going out through a firewall.
  • trk,rpl: A packet coming in through a firewall in reply to an established flow.
  • trk,inv: An invalid packet in either direction.

A packet might pass through the connection tracker twice in one trip through OVN: once following egress from a VM as it passes outward through a firewall, and once preceding ingress to a second VM as it passes inward through a firewall. Use multiple --ct options to specify the flags for multiple ct_next actions.

When --ct is unspecified, or when there are fewer --ct options than ct_next actions, the flags default to trk,est.


Sets the IP from VIP pool to use as destination of the packet. --lb-dst is not available in daemon mode.


When cloud management systems such as OpenStack are layered on top of OVN, they often use long, human-unfriendly names for ports and datapaths, for example, ones that include entire UUIDs. They do usually include friendlier names, but the long, hard-to-read names are the ones that appear in matches and actions. By default, or with --friendly-names, ovn-trace substitutes these friendlier names for the long names in its output. Use --no-friendly-names to disable this behavior; this option might be useful, for example, if a program is going to parse ovn-trace output.

Daemon Options


Causes a file (by default, program.pid) to be created indicating the PID of the running process. If the pidfile argument is not specified, or if it does not begin with /, then it is created in .

If --pidfile is not specified, no pidfile is created.


By default, when --pidfile is specified and the specified pidfile already exists and is locked by a running process, the daemon refuses to start. Specify --overwrite-pidfile to cause it to instead overwrite the pidfile.

When --pidfile is not specified, this option has no effect.


Runs this program as a background process. The process forks, and in the child it starts a new session, closes the standard file descriptors (which has the side effect of disabling logging to the console), and changes its current directory to the root (unless --no-chdir is specified). After the child completes its initialization, the parent exits.


Creates an additional process to monitor this program. If it dies due to a signal that indicates a programming error (SIGABRT, SIGALRM, SIGBUS, SIGFPE, SIGILL, SIGPIPE, SIGSEGV, SIGXCPU, or SIGXFSZ) then the monitor process starts a new copy of it. If the daemon dies or exits for another reason, the monitor process exits.

This option is normally used with --detach, but it also functions without it.


By default, when --detach is specified, the daemon changes its current working directory to the root directory after it detaches. Otherwise, invoking the daemon from a carelessly chosen directory would prevent the administrator from unmounting the file system that holds that directory.

Specifying --no-chdir suppresses this behavior, preventing the daemon from changing its current working directory. This may be useful for collecting core files, since it is common behavior to write core dumps into the current working directory and the root directory is not a good directory to use.

This option has no effect when --detach is not specified.


By default this daemon will try to self-confine itself to work with files under well-known directories whitelisted at build time. It is better to stick with this default behavior and not to use this flag unless some other Access Control is used to confine daemon. Note that in contrast to other access control implementations that are typically enforced from kernel-space (e.g. DAC or MAC), self-confinement is imposed from the user-space daemon itself and hence should not be considered as a full confinement strategy, but instead should be viewed as an additional layer of security.


Causes this program to run as a different user specified in user:group, thus dropping most of the root privileges. Short forms user and :group are also allowed, with current user or group assumed, respectively. Only daemons started by the root user accepts this argument.

On Linux, daemons will be granted CAP_IPC_LOCK and CAP_NET_BIND_SERVICES before dropping root privileges. Daemons that interact with a datapath, such as ovs-vswitchd, will be granted three additional capabilities, namely CAP_NET_ADMIN, CAP_NET_BROADCAST and CAP_NET_RAW. The capability change will apply even if the new user is root.

On Windows, this option is not currently supported. For security reasons, specifying this option will cause the daemon process not to start.

Logging Options


Sets logging levels. Without any spec, sets the log level for every module and destination to dbg. Otherwise, spec is a list of words separated by spaces or commas or colons, up to one from each category below:

  • A valid module name, as displayed by the vlog/list command on ovs-appctl(8), limits the log level change to the specified module.
  • syslog, console, or file, to limit the log level change to only to the system log, to the console, or to a file, respectively. (If --detach is specified, the daemon closes its standard file descriptors, so logging to the console will have no effect.)

    On Windows platform, syslog is accepted as a word and is only useful along with the --syslog-target option (the word has no effect otherwise).

  • off, emer, err, warn, info, or dbg, to control the log level. Messages of the given severity or higher will be logged, and messages of lower severity will be filtered out. off filters out all messages. See ovs-appctl(8) for a definition of each log level.

Case is not significant within spec.

Regardless of the log levels set for file, logging to a file will not take place unless --log-file is also specified (see below).

For compatibility with older versions of OVS, any is accepted as a word but has no effect.


Sets the maximum logging verbosity level, equivalent to --verbose=dbg.


Sets the log pattern for destination to pattern. Refer to ovs-appctl(8) for a description of the valid syntax for pattern.


Sets the RFC5424 facility of the log message. facility can be one of kern, user, mail, daemon, auth, syslog, lpr, news, uucp, clock, ftp, ntp, audit, alert, clock2, local0, local1, local2, local3, local4, local5, local6 or local7. If this option is not specified, daemon is used as the default for the local system syslog and local0 is used while sending a message to the target provided via the --syslog-target option.


Enables logging to a file. If file is specified, then it is used as the exact name for the log file. The default log file name used if file is omitted is /var/log/ovn/program.log.


Send syslog messages to UDP port on host, in addition to the system syslog. The host must be a numerical IP address, not a hostname.


Specify method as how syslog messages should be sent to syslog daemon. The following forms are supported:

  • libc, to use the libc syslog() function. Downside of using this options is that libc adds fixed prefix to every message before it is actually sent to the syslog daemon over /dev/log UNIX domain socket.
  • unix:file, to use a UNIX domain socket directly. It is possible to specify arbitrary message format with this option. However, rsyslogd 8.9 and older versions use hard coded parser function anyway that limits UNIX domain socket use. If you want to use arbitrary message format with older rsyslogd versions, then use UDP socket to localhost IP address instead.
  • udp:ip:port, to use a UDP socket. With this method it is possible to use arbitrary message format also with older rsyslogd. When sending syslog messages over UDP socket extra precaution needs to be taken into account, for example, syslog daemon needs to be configured to listen on the specified UDP port, accidental iptables rules could be interfering with local syslog traffic and there are some security considerations that apply to UDP sockets, but do not apply to UNIX domain sockets.
  • null, to discard all messages logged to syslog.

The default is taken from the OVS_SYSLOG_METHOD environment variable; if it is unset, the default is libc.

PKI Options

PKI configuration is required to use SSL for the connection to the database (and the switch, if --ovs is specified).

-p privkey.pem

Specifies a PEM file containing the private key used as identity for outgoing SSL connections.

-c cert.pem

Specifies a PEM file containing a certificate that certifies the private key specified on -p or --private-key to be trustworthy. The certificate must be signed by the certificate authority (CA) that the peer in SSL connections will use to verify it.

-C cacert.pem

Specifies a PEM file containing the CA certificate for verifying certificates presented to this program by SSL peers. (This may be the same certificate that SSL peers use to verify the certificate specified on -c or --certificate, or it may be a different one, depending on the PKI design in use.)

-C none

Disables verification of certificates presented by SSL peers. This introduces a security risk, because it means that certificates cannot be verified to be those of known trusted hosts.

Other Options

--db database

The OVSDB database remote to contact. If the OVN_SB_DB environment variable is set, its value is used as the default. Otherwise, the default is unix:/db.sock, but this default is unlikely to be useful outside of single-machine OVN test environments.


Prints a brief help message to the console.


Prints version information to the console.

Referenced By

ovn-detrace(1), ovn-sbctl(8).

Open vSwitch 2.12.0 Open vSwitch Manual