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NAME | SYNOPSIS | DESCRIPTION | OUTPUT | STATEFUL ACTIONS | DAEMON MODE | OPTIONS | COLOPHON |
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ovn-trace(8) OVN Manual ovn-trace(8)
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 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
matches.)
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 == 192.168.11.1 && ip4.dst == 192.168.22.2 && 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 == 192.168.1.11
&& arp.tha == ff:ff:ff:ff:ff:ff && arp.tpa == 192.168.2.22
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 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
next(1);
19. ls_in_l2_lkup: eth.dst == 00:00:00:00:ff:11, priority 50
outport = "lrp11-attachment";
output;
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";
output;
...
};
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 = 192.168.0.1; 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:
ct_next
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;.
ct_dnat(ip)
ct_snat(ip)
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.
ct_lb;
ct_lb(ip[:port]...);
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.
ct_commit
put_arp
put_nd
These actions are treated as no-ops.
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 ovn-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.
exit Causes ovn-trace to gracefully terminate.
Trace Options
--detailed
--summary
--minimal
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.
--all
Selects all three forms of output.
--ovs[=remote]
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.
--ct=flags
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.
--lb-dst=ip[:port]
Sets the IP from VIP pool to use as destination of the
packet. --lb-dst is not available in daemon mode.
--select-id=id
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 mode.
--friendly-names
--no-friendly-names
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
--pidfile[=pidfile]
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.
--overwrite-pidfile
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.
--detach
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.
--monitor
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.
--no-chdir
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.
--no-self-confinement
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.
--user=user:group
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
-v[spec]
--verbose=[spec]
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.
-v
--verbose
Sets the maximum logging verbosity level, equivalent to
--verbose=dbg.
-vPATTERN:destination:pattern
--verbose=PATTERN:destination:pattern
Sets the log pattern for destination to pattern. Refer to
ovs-appctl(8) for a description of the valid syntax for
pattern.
-vFACILITY:facility
--verbose=FACILITY:facility
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.
--log-file[=file]
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
/usr/local/var/log/ovn/program.log.
--syslog-target=host:port
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.
--syslog-method=method
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/TLS for the connection to
the database (and the switch, if --ovs is specified).
-p privkey.pem
--private-key=privkey.pem
Specifies a PEM file containing the private key used
as identity for outgoing SSL connections.
-c cert.pem
--certificate=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
--ca-cert=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
--ca-cert=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.
-h
--help
Prints a brief help message to the console.
-V
--version
Prints version information to the console.
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 2025-08-11. (At that time,
the date of the most recent commit that was found in the
repository was 2025-08-08.) 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
man-pages@man7.org
OVN 25.03.90 ovn-trace ovn-trace(8)
Pages that refer to this page: ovn-detrace(1), ovn-sbctl(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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