ovn-trace(8) — Linux manual page


ovn-trace(8)                   OVN Manual                   ovn-trace(8)

NAME         top

       ovn-trace - Open Virtual Network logical network tracing utility

SYNOPSIS         top

       ovn-trace [options] [datapath] microflow

       ovn-trace [options] --detach

DESCRIPTION         top

       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

       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 the microflow
       (and optional datapath) 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 optional 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 datapath is optional because
       ovn-trace can figure it out from the inport that the microflow

       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

           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.

OUTPUT         top

       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 19 (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
           19. 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.


       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

                   These actions are treated as no-ops.

DAEMON MODE         top

       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

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

              exit   Causes ovn-trace to gracefully terminate.

OPTIONS         top

   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

            •      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.

            Specify the id to be selected by the select action. id must
            be one of the values listed in the select action. Otherwise,
            a random id is selected from the list, as if --select-id
            were not specified. --select-id is not available in daemon

            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

              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
              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 determined 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

            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

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

            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

            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

            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.

COLOPHON         top

       This page is part of the Open Virtual Network (Daemons for Open
       vSwitch that translate virtual network configurations into
       OpenFlow) project.  Information about the project can be found at
       ⟨https://www.ovn.org/⟩.  If you have a bug report for this manual
       page, send it to bugs@openvswitch.org.  This page was obtained
       from the project's upstream Git repository
       ⟨https://github.com/ovn-org/ovn⟩ on 2023-12-22.  (At that time,
       the date of the most recent commit that was found in the
       repository was 2023-12-18.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
       is a better or more up-to-date source for the page, or you have
       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

OVN 23.06.90                    ovn-trace                   ovn-trace(8)

Pages that refer to this page: ovn-detrace(1)ovn-sbctl(8)