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 command- 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 available. 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 re- quires a host to which iperf3 should connect. The host can be specified 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 in- dependent SCTP connection, depending on what protocol was specified by the client. Normally, the test data is sent from the client to the server, and measures 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 measurement 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 bi- trate over that interval. Note that the values for each measurement inter- val 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 in- terval 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 re- ceiver, 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 structure, 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" sec- tion of the manual page below, as well as summarized in iperf3's help out- put, 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 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. 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 device 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_BIND- TODEVICE or IP_BOUND_IF, and may require root permissions. (Avail- able on Linux and possibly other systems.) -V, --verbose Produce more detailed output. -J, --json Output in JSON format instead of the default human-readable output. --json-stream Output in line-delimited JSON format instead of the default human- 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 buffering when sending output to pipe. --timestamps[=format] Prepend a timestamp at the start of each output line. By default, timestamps have the format emitted by ctime(1). Optionally, = fol- lowed by a format specification can be passed to customize the time- stamps, 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 control connections) This option can be used to force a faster test timeout in case of a network partition during a test. The required parameter is specified in ms, and defaults to the system settings. This func- tionality 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 authentication 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 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 (at most) 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-max-duration n 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 ex- ceeds 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 default 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 decrypt authentication credentials from the client (if built with OpenSSL support). --authorized-users-path file Path to the configuration file containing authorized users creden- tials to run iperf tests (if built with OpenSSL support). The file is a comma separated list of usernames and password hashes; more in- formation 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 default, a test consists of sending data from the client to the server, un- less the -R flag is specified. If an optional interface is speci- fied, 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 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 operating sys- tem's timeout for TCP connection establishment. Providing 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, unlim- ited 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 bitrate to 0 will disable bitrate limits (particularly useful for UDP tests). This throughput limit is im- plemented internally inside iperf3, and is available on all plat- forms. 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 microsec- onds, 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 pacing, in bits per second. This pacing (if specified) will be in addition to any pacing due to iperf3's internal throughput pacing (-b/--bi- trate flag), and both can be specified for the same test. Only available on platforms supporting the SO_MAX_PACING_RATE socket op- tion (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 options 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 dynami- cally 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 throughput 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 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 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 speci- fied. Normally SCTP will include the protocol addresses of all ac- tive links on the local host when setting up an association. Speci- fying at least one --X name will disable this behaviour. 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 argument 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), in- stead 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 output. -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 hu- man-readable format). If the client is run with --json, the server output is included in a JSON object; otherwise 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. --gsro Enable UDP Generic Segmentation Offload (GSO) on the sender and Generic Receive Offload (GRO) on the receiver, where supported by the operating system and network hardware (currently Linux only). GSO allows the network stack to aggregate multiple UDP datagrams into larger packets, improving throughput and reducing CPU overhead. This is a client-only option; the client communicates the GSO/GRO settings to the server automatically. This feature is disabled by default and must be explicitly enabled with this flag. --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 com- pression (including some WiFi access points), where iperf2 and iperf3 perform differently, just based on payload entropy. --dont-fragment Set the IPv4 Don't Fragment (DF) bit on outgoing packets. Only ap- plicable 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 in- teractively 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 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 authentication token. The private key must be in PEM format and additionally must not have a password set. The public key must be in PEM format and use Subject- PrefixKeyInfo encoding. An example of a set of UNIX/Linux commands using OpenSSL to generate a correctly-formed keypair 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 \ -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 private_not_pro- tected.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 "{$user}$password". The file can also contain commented lines (starting with the # character). An exam- ple 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,bf7a49a846d44b454a5d11e7acfaf13d138bbe0b7483aa3e050879700572709b AUTHORS A list of the contributors to iperf3 can be found within the documentation located at https://software.es.net/iperf/dev.html#authors. SEE ALSO libiperf(3), https://software.es.net/iperf ESnet April 2026 IPERF3(1)
NAME | SYNOPSIS | DESCRIPTION | GENERAL OPTIONS | SERVER SPECIFIC OPTIONS | CLIENT SPECIFIC OPTIONS | EXAMPLES | AUTHORS | SEE ALSO
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