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:

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)

       iperf3 - perform network throughput tests

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

       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-

       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 -c

              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 -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 -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  competely
       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

       -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

       -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

              output in line-delimited JSON format

       --logfile file
              send output to a log file.

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

              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

              This  option  is only meaningful when using iperf3's authentica-
              tion features. Versions of iperf3 prior to 3.17 used PCKS1  pad-
              ding 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  ver-
              sions.   Use  this  option to preserve the less secure, but more
              compatible, behavior.

       -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

       -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-

       --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-

       --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.

       -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

       -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.

              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

              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).

       -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-

       -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 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.

              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.

              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-

              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

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

   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-

   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

       A list of the contributors to iperf3 can be found within the documenta-
       tion located at


ESnet                              May 2024                          IPERF3(1)

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