Invoking iperf3

iperf3 includes a manual page listing all of the command-line options. The manual page is the most up-to-date reference to the various flags and parameters.

For sample command line usage, see:

https://fasterdata.es.net/performance-testing/network-troubleshooting-tools/iperf/

Using the default options, iperf3 is meant to show typical well designed application performance. “Typical well designed application” means avoiding artificial enhancements that work only for testing (such as splice()-ing the data to /dev/null). iperf3 does also have flags for “extreme best case” optimizations but they must be explicitly activated. These flags include the -Z (--zerocopy) and -A (--affinity) options.

iperf3 Manual Page

This section contains a plaintext rendering of the iperf3 manual page. It is presented here only for convenience; the text here might not correspond to the current version of iperf3. The authoritative iperf3 manual page is included in the source tree and installed along with the executable.

IPERF3(1)                        User Manuals                        IPERF3(1)

NAME
       iperf3 - perform network throughput tests

SYNOPSIS
       iperf3 -s [ options ]
       iperf3 -c server [ options ]


DESCRIPTION
       iperf3  is  a  tool for performing network throughput measurements.  It
       can test TCP, UDP, or SCTP throughput.  To perform an iperf3  test  the
       user must establish both a server and a client.

       The  iperf3  executable  contains both client and server functionality.
       An iperf3 server can be started using either of the -s or --server com-
       mand-line parameters, for example:

              iperf3 -s

              iperf3 --server

       Note that  many  iperf3  parameters  have  both  short  (-s)  and  long
       (--server) forms.  In this section we will generally use the short form
       of  command-line  flags,  unless only the long form of a flag is avail-
       able.

       By default, the iperf3 server listens on TCP port 5201 for  connections
       from  an iperf3 client.  A custom port can be specified by using the -p
       flag, for example:

              iperf3 -s -p 5002

       After the server is started, it will listen for connections from iperf3
       clients (in other words, the iperf3 program run in client  mode).   The
       client mode can be started using the -c command-line option, which also
       requires a host to which iperf3 should connect.  The host can by speci-
       fied by hostname, IPv4 literal, or IPv6 literal:

              iperf3 -c iperf3.example.com

              iperf3 -c 192.0.2.1

              iperf3 -c 2001:db8::1

       If  the  iperf3  server is running on a non-default TCP port, that port
       number needs to be specified on the client as well:

              iperf3 -c iperf3.example.com -p 5002

       The initial TCP connection is used to exchange test parameters, control
       the start and end of the test, and to exchange test results.   This  is
       sometimes  referred  to  as  the "control connection".  The actual test
       data is sent over a separate TCP connection, as a separate flow of  UDP
       packets, or as an independent SCTP connection, depending on what proto-
       col was specified by the client.

       Normally, the test data is sent from the client to the server, and mea-
       sures  the  upload  speed  of the client.  Measuring the download speed
       from the server can be done by specifying the -R flag  on  the  client.
       This causes data to be sent from the server to the client.

              iperf3 -c iperf3.example.com -p 5202 -R

       Results  are displayed on both the client and server.  There will be at
       least one line of output per measurement interval (by  default  a  mea-
       surement  interval lasts for one second, but this can be changed by the
       -i option).  Each line of output includes (at least) the time since the
       start of the test, amount of data transferred during the interval,  and
       the  average bitrate over that interval.  Note that the values for each
       measurement interval are taken from the point of view of  the  endpoint
       process  emitting that output (in other words, the output on the client
       shows the measurement interval data for the client.

       At the end of the test is a set of statistics that shows (at  least  as
       much  as possible) a summary of the test as seen by both the sender and
       the receiver, with lines tagged accordingly.  Recall  that  by  default
       the  client  is  the sender and the server is the receiver, although as
       indicated above, use of the -R flag will reverse these roles.

       The client can be made to retrieve the server-side output for  a  given
       test by specifying the --get-server-output flag.

       Either the client or the server can produce its output in a JSON struc-
       ture,  useful for integration with other programs, by passing it the -J
       flag.  Normally the contents of the JSON structure are only  completely
       known after the test has finished, no JSON output will be emitted until
       the  end of the test.  By enabling line-delimited JSON multiple objects
       will be emitted to provide a real-time parsable JSON output.

       iperf3 has a (overly) large set of command-line  options  that  can  be
       used  to  set the parameters of a test.  They are given in the "GENERAL
       OPTIONS" section of the manual page below, as  well  as  summarized  in
       iperf3's help output, which can be viewed by running iperf3 with the -h
       flag.

GENERAL OPTIONS
       -p, --port n
              set server port to listen on/connect to to n (default 5201)

       -f, --format
              [kmgtKMGT]   format to report: Kbits/Mbits/Gbits/Tbits

       -i, --interval n
              pause  n seconds between periodic throughput reports; default is
              1, use 0 to disable

       -I, --pidfile file
              write a file with the process ID, most useful when running as  a
              daemon.

       -F, --file name
              Use  a  file  as  the source (on the sender) or sink (on the re-
              ceiver) of data, rather than  just  generating  random  data  or
              throwing  it  away.  This feature is used for finding whether or
              not the storage subsystem is the bottleneck for file  transfers.
              It  does not turn iperf3 into a file transfer tool.  The length,
              attributes, and in some cases contents of the received file  may
              not match those of the original file.

       -A, --affinity n/n,m
              Set  the  CPU affinity, if possible (Linux, FreeBSD, and Windows
              only).  On both the client and server  you  can  set  the  local
              affinity  by using the n form of this argument (where n is a CPU
              number).  In addition, on the client side you can  override  the
              server's  affinity for just that one test, using the n,m form of
              argument.  Note that when using this  feature,  a  process  will
              only  be  bound  to a single CPU (as opposed to a set containing
              potentially multiple CPUs).

       -B, --bind host[%dev]
              bind to the specific interface associated with address host.  If
              an optional interface is specified, it is treated as a  shortcut
              for  --bind-dev dev.  Note that a percent sign and interface de-
              vice name are required for IPv6 link-local address literals.

       --bind-dev dev
              bind to the  specified  network  interface.   This  option  uses
              SO_BINDTODEVICE,  and  may require root permissions.  (Available
              on Linux and possibly other systems.)

       -V, --verbose
              give more detailed output

       -J, --json
              output in JSON format

       --json-stream
              output in line-delimited JSON format

       --logfile file
              send output to a log file.

       --forceflush
              force flushing output at every interval.  Used to avoid  buffer-
              ing when sending output to pipe.

       --timestamps[=format]
              prepend  a  timestamp  at the start of each output line.  By de-
              fault, timestamps have the format emitted by ctime(1).   Option-
              ally, = followed by a format specification can be passed to cus-
              tomize the timestamps, see strftime(3).  If this optional format
              is  given, the = must immediately follow the --timestamps option
              with no whitespace intervening.

       --rcv-timeout #
              set idle timeout for receiving data during active tests. The re-
              ceiver will halt a test if no data is received from  the  sender
              for this number of ms (default to 120000 ms, or 2 minutes).

       --snd-timeout #
              set  timeout  for unacknowledged TCP data (on both test and con-
              trol connections) This option can be used to force a faster test
              timeout in case of a network partition during a  test.  The  re-
              quired  parameter is specified in ms, and defaults to the system
              settings.  This functionality depends  on  the  TCP_USER_TIMEOUT
              socket  option, and will not work on systems that do not support
              it.

       --use-pkcs1-padding
              This option is only meaningful when using  iperf3's  authentica-
              tion  features.  Versions  of  iperf3  prior  to 3.17 used PCKS1
              padding in the RSA-encrypted credentials, which  was  vulnerable
              to  a  side-channel  attack that could reveal a server's private
              key.  Beginning with iperf-3.17, OAEP padding is  used,  however
              this  is  a  breaking  change  that is not compatible with older
              iperf3 versions.  Use this option to preserve the  less  secure,
              but more compatible, behavior.

       -m, --mptcp
              use mptcp variant for the current protocol. This only applies to
              TCP and enables MPTCP usage.

       -d, --debug
              emit  debugging  output.  Primarily (perhaps exclusively) of use
              to developers.

       -v, --version
              show version information and quit

       -h, --help
              show a help synopsis


SERVER SPECIFIC OPTIONS
       -s, --server
              run in server mode

       -D, --daemon
              run the server in background as a daemon

       -1, --one-off
              handle one client connection, then exit.  If  an  idle  time  is
              set, the server will exit after that amount of time with no con-
              nection.

       --idle-timeout n
              restart  the  server  after n seconds in case it gets stuck.  In
              one-off mode, this is the number of seconds the server will wait
              before exiting.

       --server-bitrate-limit n[KMGT]
              set a limit on the server side, which will cause a test to abort
              if the client specifies a test of more than n bits  per  second,
              or if the average data sent or received by the client (including
              all  data  streams)  is greater than n bits per second.  The de-
              fault limit is zero, which implies no limit.  The interval  over
              which  to average the data rate is 5 seconds by default, but can
              be specified by adding a '/' and a number to the bitrate  speci-
              fier.

       --rsa-private-key-path file
              path to the RSA private key (not password-protected) used to de-
              crypt  authentication credentials from the client (if built with
              OpenSSL support).

       --authorized-users-path file
              path to the configuration file containing authorized users  cre-
              dentials  to  run  iperf  tests (if built with OpenSSL support).
              The file is a comma separated list  of  usernames  and  password
              hashes;  more  information  on  the structure of the file can be
              found in the EXAMPLES section.

       --time-skew-thresholdsecond seconds
              time skew threshold (in seconds) between the server  and  client
              during the authentication process.

CLIENT SPECIFIC OPTIONS
       -c, --client host[%dev]
              run  in client mode, connecting to the specified server.  By de-
              fault, a test consists of sending data from the  client  to  the
              server,  unless the -R flag is specified.  If an optional inter-
              face is specified, it is treated as a  shortcut  for  --bind-dev
              dev.  Note that a percent sign and interface device name are re-
              quired for IPv6 link-local address literals.

       --sctp use SCTP rather than TCP (FreeBSD and Linux)

       -u, --udp
              use UDP rather than TCP

       --connect-timeout n
              set  timeout  for establishing the initial control connection to
              the server, in milliseconds.  The default behavior is the  oper-
              ating  system's  timeout for TCP connection establishment.  Pro-
              viding a shorter value may speed up detection of a  down  iperf3
              server.

       -b, --bitrate n[KMGT]
              set  target  bitrate  to n bits/sec (default 1 Mbit/sec for UDP,
              unlimited for TCP/SCTP).  If  there  are  multiple  streams  (-P
              flag),  the  throughput  limit  is  applied  separately  to each
              stream.  You can also add a '/' and  a  number  to  the  bitrate
              specifier.  This is called "burst mode".  It will send the given
              number  of packets without pausing, even if that temporarily ex-
              ceeds the specified throughput limit.  Setting  the  target  bi-
              trate  to 0 will disable bitrate limits (particularly useful for
              UDP tests).  This throughput limit is implemented internally in-
              side iperf3, and is available on all  platforms.   Compare  with
              the  --fq-rate flag.  This option replaces the --bandwidth flag,
              which is now deprecated but (at least for now) still accepted.

       --pacing-timer n[KMGT]
              set pacing timer interval  in  microseconds  (default  1000  mi-
              croseconds,  or  1  ms).  This controls iperf3's internal pacing
              timer for the -b/--bitrate option.  The timer fires at  the  in-
              terval  set  by  this  parameter.   Smaller values of the pacing
              timer parameter smooth out the traffic emitted  by  iperf3,  but
              potentially  at  the  cost  of  performance due to more frequent
              timer processing.

       --fq-rate n[KMGT]
              Set a rate to be used with fair-queueing based socket-level pac-
              ing, in bits per second.  This pacing (if specified) will be  in
              addition  to any pacing due to iperf3's internal throughput pac-
              ing (-b/--bitrate flag), and both can be specified for the  same
              test.   Only  available  on platforms supporting the SO_MAX_PAC-
              ING_RATE socket option (currently only Linux).  The  default  is
              no fair-queueing based pacing.

       --no-fq-socket-pacing
              This option is deprecated and will be removed.  It is equivalent
              to specifying --fq-rate=0.

       -t, --time n
              time in seconds to transmit for (default 10 secs)

       -n, --bytes n[KMGT]
              number of bytes to transmit (instead of -t)

       -k, --blockcount n[KMGT]
              number of blocks (packets) to transmit (instead of -t or -n)

       -l, --length n[KMGT]
              length  of  buffer to read or write.  For TCP tests, the default
              value is 128KB.  In the case of UDP, iperf3 tries to dynamically
              determine a reasonable sending size based on the  path  MTU;  if
              that  cannot be determined it uses 1460 bytes as a sending size.
              For SCTP tests, the default size is 64KB.

       --cport port
              bind data streams to a specific client port  (for  TCP  and  UDP
              only, default is to use an ephemeral port)

       -P, --parallel n
              number  of parallel client streams to run. iperf3 will spawn off
              a separate thread for each test stream. Using  multiple  streams
              may result in higher throughput than a single stream.

       -R, --reverse
              reverse  the  direction of a test, so that the server sends data
              to the client

       --bidir
              test in both directions (normal  and  reverse),  with  both  the
              client and server sending and receiving data simultaneously

       -w, --window n[KMGT]
              set  socket  buffer size / window size.  This value gets sent to
              the server and used on that side too; on both sides this  option
              sets  both  the sending and receiving socket buffer sizes.  This
              option can be used to set (indirectly) the  maximum  TCP  window
              size.   Note that on Linux systems, the effective maximum window
              size is approximately double what is specified  by  this  option
              (this  behavior  is  not  a bug in iperf3 but a "feature" of the
              Linux kernel, as documented by tcp(7) and socket(7)).

       -M, --set-mss n
              set TCP/SCTP maximum segment size (MTU - 40 bytes)

       -N, --no-delay
              set TCP/SCTP no delay, disabling Nagle's Algorithm

       -4, --version4
              only use IPv4

       -6, --version6
              only use IPv6

       -S, --tos n
              set the IP type of service. The usual prefixes for octal and hex
              can be used, i.e. 52, 064 and 0x34 all specify the same value.

       --dscp dscp
              set the IP DSCP bits.  Both numeric and symbolic values are  ac-
              cepted.  Numeric  values  can be specified in decimal, octal and
              hex (see --tos above).

       -L, --flowlabel n
              set the IPv6 flow label (currently only supported on Linux)

       -X, --xbind name
              Bind SCTP associations to  a  specific  subset  of  links  using
              sctp_bindx(3).   The  --B  flag  will be ignored if this flag is
              specified.  Normally SCTP will include the protocol addresses of
              all active links on the local host when setting up  an  associa-
              tion.  Specifying at least one --X name will disable this behav-
              iour.  This flag must be specified for each link to be  included
              in  the association, and is supported for both iperf servers and
              clients (the latter are supported by passing the first --X argu-
              ment to bind(2)).  Hostnames are accepted as arguments  and  are
              resolved  using  getaddrinfo(3).   If  the  --4 or --6 flags are
              specified, names which do not resolve to  addresses  within  the
              specified protocol family will be ignored.

       --nstreams n
              Set number of SCTP streams.

       -Z, --zerocopy
              Use  a  "zero copy" method of sending data, such as sendfile(2),
              instead of the usual write(2).

       --skip-rx-copy
              Ignored received packet data, using the MSG_TRUNC  flag  to  the
              recv(2) system call.

       -O, --omit n
              Perform pre-test for N seconds and omit the pre-test statistics,
              to skip past the TCP slow-start period.

       -T, --title str
              Prefix every output line with this string.

       --extra-data str
              Specify  an  extra data string field to be included in JSON out-
              put.

       -C, --congestion algo
              Set the congestion control algorithm (Linux and  FreeBSD  only).
              An  older  --linux-congestion  synonym for this flag is accepted
              but is deprecated.

       --get-server-output
              Get the output from the server.  The output format is determined
              by the server (in particular, if the server was invoked with the
              --json flag, the output will be in  JSON  format,  otherwise  it
              will  be  in  human-readable format).  If the client is run with
              --json, the server output is included in a JSON  object;  other-
              wise it is appended at the bottom of the human-readable output.

       --udp-counters-64bit
              Use 64-bit counters in UDP test packets.  The use of this option
              can  help  prevent counter overflows during long or high-bitrate
              UDP tests.  Both client and server need to be running  at  least
              version  3.1 for this option to work.  It may become the default
              behavior at some point in the future.

       --repeating-payload
              Use repeating pattern in payload, instead of random bytes.   The
              same  payload  is  used  in iperf2 (ASCII '0..9' repeating).  It
              might help to test and reveal problems in networking  gear  with
              hardware  compression (including some WiFi access points), where
              iperf2 and iperf3 perform differently, just based on payload en-
              tropy.

       --dont-fragment
              Set the IPv4 Don't Fragment (DF) bit on outgoing packets.   Only
              applicable to tests doing UDP over IPv4.

       --username username
              username to use for authentication to the iperf server (if built
              with OpenSSL support).  The password will be prompted for inter-
              actively  when  the  test is run.  Note, the password to use can
              also be specified via the IPERF3_PASSWORD environment  variable.
              If  this  variable  is  present,  the  password  prompt  will be
              skipped.

       --rsa-public-key-path file
              path to the RSA public key used to encrypt  authentication  cre-
              dentials (if built with OpenSSL support)


EXAMPLES
   Authentication - RSA Keypair
       The  authentication  feature  of iperf3 requires an RSA public keypair.
       The public key is used to encrypt the authentication  token  containing
       the  user credentials, while the private key is used to decrypt the au-
       thentication token.  The private key must be in PEM  format  and  addi-
       tionally  must  not have a password set.  The public key must be in PEM
       format and use SubjectPrefixKeyInfo encoding.  An example of a  set  of
       UNIX/Linux  commands  using OpenSSL to generate a correctly-formed key-
       pair follows:

            > openssl genrsa -des3 -out private.pem 2048
            > openssl rsa -in private.pem -outform PEM -pubout -out public.pem
            > openssl rsa -in private.pem -out private_not_protected.pem -out-
            form PEM

       After these commands, the public key will be contained in the file pub-
       lic.pem and the  private  key  will  be  contained  in  the  file  pri-
       vate_not_protected.pem.

   Authentication - Authorized users configuration file
       A  simple plaintext file must be provided to the iperf3 server in order
       to specify the authorized user credentials.  The file is a simple  list
       of  comma-separated  pairs  of  a username and a corresponding password
       hash.  The password hash is a SHA256 hash of the string  "{$user}$pass-
       word".   The file can also contain commented lines (starting with the #
       character).  An example of commands to generate the password hash on  a
       UNIX/Linux system is given below:

            > S_USER=mario S_PASSWD=rossi
            > echo -n "{$S_USER}$S_PASSWD" | sha256sum | awk '{ print $1 }'

       An example of a password file (with an entry corresponding to the above
       username and password) is given below:
            > cat credentials.csv
            # file format: username,sha256
            mario,bf7a49a846d44b454a5d11e7ac-
            faf13d138bbe0b7483aa3e050879700572709b



AUTHORS
       A list of the contributors to iperf3 can be found within the documenta-
       tion located at https://software.es.net/iperf/dev.html#authors.


SEE ALSO
       libiperf(3), https://software.es.net/iperf

ESnet                              May 2025                          IPERF3(1)

The iperf3 manual page will typically be installed in manual section 1.