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.
				 Note that the server output is available only if the test
				 completes, not if it is interrupted.
   
          --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.
