Provided by: ovn-common_25.03.0-1_amd64 
NAME
ovn-trace - Open Virtual Network logical network tracing utility
SYNOPSIS
ovn-trace [options] [datapath] microflow
ovn-trace [options] --detach
DESCRIPTION
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.
OUTPUT
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.
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:
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.
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.
exit Causes ovn-trace to gracefully terminate.
OPTIONS
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 /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/TLS
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/TLS 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/TLS peers. (This may be the same certificate that SSL/TLS 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/TLS 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.
OVN 25.03.0 ovn-trace ovn-trace(8)