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.
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 be 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]
Set 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. This option is 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. This option is unavailable
when doing --udp tests.
-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, in
order to set the link-local scope.
--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
Produce more detailed output.
-J, --json
Output in JSON format instead of the default human-readable out-
put.
--json-stream
Output in line-delimited JSON format instead of the default hu-
man-readable output. This option overrides the --json option, if
that option was also specified.
--json-stream-full-output
Output in JSON format with JSON streams enabled. This flag only
takes effect if the --json-stream option was also specified.
--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 the MPTCP variant for the current protocol. This only ap-
plies 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 (at most) one client connection, then exit. If an idle
time is set, the server will exit after that amount of time with
no connection.
--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-max-duration
The maximum time, in seconds, that an iperf client can run
against the server. When the sum of the client's time and omit
values exceeds the max duration set by the server or the
client's time value is 0, the measurement is rejected.
--server-bitrate-limit n[KMGT][/n]
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 0, 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 / character and a number to the bitrate
specifier.
--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-threshold seconds
Specify the allowable 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 for tests rather than TCP (FreeBSD and Linux). Note
that TCP communication is still used for the control connection
between client and server.
-u, --udp
Use UDP for tests rather than TCP. Note that TCP communication
is still used for the control connection between client and
server.
--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 perform the
given number of sends without pausing, even if that temporarily
exceeds 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
Set the test duration in seconds (default 10 secs). The -t , -n
", and" -k options are mutually exclusive.
-n, --bytes n[KMGT]
Set the number of bytes to transmit. The -t , -n ", and" -k op-
tions are mutually exclusive.
-k, --blockcount n[KMGT]
Set the number of blocks (packets) to transmit. The -t , -n ",
and" -k options are mutually exclusive.
-l, --length n[KMGT]
Set the length of the 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 TCP or UDP client port (for TCP
and UDP only, default is to use an ephemeral port).
-P, --parallel n
Set the number of parallel client streams to run. Beginning with
iperf-3.16, iperf3 will spawn off a separate thread for each
test stream. Using multiple streams may result in higher
throughput than a single stream, in cases where network through-
put is CPU-limited.
-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 t he socket buffer size / window size. This value gets sent
to the server and used on that side too; on both sides this op-
tion sets both the sending and receiving socket buffer sizes.
This option can be used to set (indirectly) the maximum TCP win-
dow 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 the TCP/SCTP maximum segment size (MTU - 40 bytes).
-N, --no-delay
Set the TCP/SCTP no delay option, disabling Nagle's Algorithm.
-4, --version4
Force the use of IPv4.
-6, --version6
Force the use of IPv6.
-S, --tos n
Set the IP type of service bits. 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 also
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 the string str.
--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 com-
pletes, 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
Specify username to use for authentication to the iperf server
(if built with OpenSSL support). The password will be prompted
for interactively when the test is run. Note the password 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
Set path to the RSA public key used to encrypt authentication
credentials (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 pub-
lic.pem
> openssl rsa -in private.pem -out private_not_protected.pem \
-outform 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 November 2025 IPERF3(1)
The iperf3 manual page will typically be installed in manual section 1.