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.  Because 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.

       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
              receiver)  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
              device 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

       --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
              default,   timestamps  have  the  format  emitted  by  ctime(1).
              Optionally, = followed by a format specification can  be  passed
              to  customize 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
              receiver will halt a test if no data is received from the sender
              for this number of ms (default to 12000 ms, or 2 minutes).

       -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
              default 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

       --rsa-private-key-path file
              path  to  the  RSA  private key (not password-protected) used to
              decrypt 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
              default,  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
              required 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
              exceeds  the  specified  throughput  limit.   Setting the target
              bitrate to 0 will disable bitrate  limits  (particularly  useful
              for UDP tests).  This throughput limit is implemented internally
              inside 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
              microseconds,  or 1 ms).  This controls iperf3's internal pacing
              timer for the -b/--bitrate  option.   The  timer  fires  at  the
              interval  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. Note  that  iperf3  is
              single  threaded,  so  if you are CPU bound, this will not yield
              higher throughput.

       -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
              accepted.  Numeric values can be specified in decimal, octal and
              hex (see --tos above). To set both the DSCP  bits  and  the  ECN
              bits, use --tos.

       -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
              Omit the first n seconds of the test, 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

              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
       authentication 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                            January 2022                        IPERF3(1)

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