iPerf3 Cheat Sheet

Server

-s	Start server mode
-p <port>	Listen on custom port (default: 5201)
-B <addr>	Bind to specific interface/IP
-D	Run as daemon (background)
-1	Handle one client then exit
--forceflush	Force output flush after each interval

iperf3 -s
iperf3 -s -p 9999
iperf3 -s -B 192.168.2.11
iperf3 -s -D   # daemonize
      

Client

-c <host>	Connect to server at host/IP
-p <port>	Connect to custom port
-t <sec>	Test duration in seconds (default: 10)
-n <bytes>	Transfer exactly N bytes instead of timed
-i <sec>	Interval between reports (default: 1)
-R	Reverse mode (server→client)
--bidir	Bidirectional simultaneous test

iperf3 -c 192.168.2.11
iperf3 -c 192.168.2.11 -t 30 -i 5
iperf3 -c 192.168.2.11 -R
iperf3 -c 192.168.2.11 --bidir
      

Bandwidth Control

-b <rate>	Target bitrate (K/M/G suffix). Default: unlimited TCP, 1M UDP
-b 0	Unlimited (TCP only)

iperf3 -c host -b 100M   # 100 Mbps TCP
iperf3 -c host -b 500K   # 500 Kbps
iperf3 -c host -b 1G     # 1 Gbps
iperf3 -c host -u -b 50M  # 50 Mbps UDP
      

⚠ TCP bitrate is a target, not a hard cap. UDP bitrate is precise.

UDP Mode

-u	Use UDP instead of TCP
-b <rate>	Bandwidth (required for UDP, default 1M)
-l <bytes>	UDP datagram / packet size

# UDP at 100 Mbps, 1400 byte datagrams
iperf3 -c host -u -b 100M -l 1400

# Check jitter and packet loss in output
iperf3 -c host -u -b 10M -t 30
      

TCP Tuning

-w <size>	Socket/window buffer size (K/M)
-M <mss>	Set TCP max segment size
-N	Disable Nagle's algorithm
-4 / -6	Force IPv4 or IPv6
-P <n>	Parallel streams

# 4 parallel streams, 256K buffer
iperf3 -c host -P 4 -w 256K

# IPv6 test
iperf3 -c host -6
      

Output & Logging

-J	Output results as JSON
--logfile	Save output to file
-f <fmt>	Format: k/m/g/K/M/G (bits or bytes)
-V	Verbose output
-d	Debug output

iperf3 -c host -J   # JSON output
iperf3 -c host -f m  # Mbps display
iperf3 -c host --logfile test.log

# Parse JSON for bandwidth
iperf3 -c host -J | jq '.end.sum_received.bits_per_second'
      

Common Test Scenarios

LAN / Throughput

# Basic TCP throughput
iperf3 -c host -t 30

# Max throughput (multi-stream)
iperf3 -c host -P 8 -t 30

# Full duplex
iperf3 -c host --bidir

# Upload + download (separate runs)
iperf3 -c host
iperf3 -c host -R
          

VPN / WAN

# Bandwidth-limited VPN test
iperf3 -c host -b 50M -t 30

# UDP jitter (VoIP sim)
iperf3 -c host -u -b 1M
       -l 160 -t 30

# Long test, 5s intervals
iperf3 -c host -t 120 -i 5
       --logfile vpn_test.log
          

Reading Output

[ ID] Interval Transfer Bitrate [ 5] 0.00-1.00 sec 112 MBytes 940 Mbits/sec [ 5] 1.00-2.00 sec 111 MBytes 935 Mbits/sec ... - - - - - - - Summary - - - - - - - [ ID] 0-10s Transfer 1.09 GBytes Bitrate 940 Mbits/sec Retr: 2

Bitrate	Throughput per interval
Retr	TCP retransmits (lower = better)
Jitter	UDP only — variance in delay
Lost/Total	UDP only — packet loss count/%
Cwnd	TCP congestion window size

Advanced Flags

-O <sec>	Omit first N seconds (skip slow-start)
--connect-timeout	Connection timeout in ms
--rcv-timeout	Idle read timeout (ms)
-T <title>	Add prefix string to output
-Z	Use zero-copy send (sendfile)
--pacing-timer	Pacing timer interval (us)
--get-server-output	Retrieve output from server side

TLS / Auth (v3.9+)

--authorized-users-file	Path to user credentials file
--rsa-private-key-path	RSA key for auth
--username	Client username for auth

# Authenticated server
iperf3 -s
  --authorized-users-file users.csv
  --rsa-private-key-path server.key

# Authenticated client
iperf3 -c host
  --username chris
  --rsa-public-key-path server.pub
      

Tips & Gotchas

Firewall	Open TCP/UDP 5201 on server (or use `-p`)
Skip slow-start	Use `-O 2` for cleaner TCP results
CPU bound?	Use `-Z` (zero-copy) to reduce CPU overhead
VPN testing	Run server on both ends; test each direction with `-R`
Parallel streams	`-P 4` often reveals true max throughput on high-latency paths
Only 1 client	iPerf3 accepts one client at a time; use `-s` per port for multi

Flag Summary

-s	Server mode
-c <h>	Client → host
-p <n>	Port
-t <s>	Duration (sec)
-i <s>	Report interval
-n <b>	Transfer N bytes

-u	UDP mode
-b <r>	Bandwidth/bitrate
-P <n>	Parallel streams
-R	Reverse direction
--bidir	Bidirectional
-w <b>	Window size

-4 / -6	IPv4 / IPv6
-J	JSON output
-f <k/m/g>	Output format
-O <s>	Omit N seconds
-Z	Zero-copy mode
-D	Daemon (server)