iperf & iperf3 Network Testing

calendarDec 12, 2024 · 6 min read · net-knowhow iperf bandwidth performance  ·
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Network bandwidth testing with iperf and iperf3. Understand differences and common usage patterns.

iperf vs iperf3

Key Differences

Featureiperf2iperf3
CompatibilityNot compatible with iperf3Not compatible with iperf2
Parallel StreamsMultiple streams in both directionsOne direction at a time
Simultaneous Bi-directionalYesNo (sequential only)
JSON OutputNoYes
UDP BandwidthMust specifyAuto-detects
Active DevelopmentLess activeActively maintained
Default Port50015201

Recommendation: Use iperf3 for new projects (better maintained, JSON output, simpler).

Installation

 1# Linux
 2sudo apt install iperf3
 3sudo apt install iperf  # For iperf2
 4
 5# macOS
 6brew install iperf3
 7brew install iperf2
 8
 9# Windows (Scoop)
10scoop install iperf3

iperf3 Commands

Basic TCP Test

1# Server mode (run on server)
2iperf3 -s
3
4# Client mode (run on client)
5iperf3 -c server_ip
6
7# Output:
8# [ ID] Interval           Transfer     Bitrate
9# [  5]   0.00-10.00  sec  1.09 GBytes   941 Mbits/sec

Test Duration

1# Test for 30 seconds (default is 10)
2iperf3 -c server_ip -t 30
3
4# Test for 60 seconds
5iperf3 -c server_ip -t 60

Parallel Streams

1# Use 4 parallel streams
2iperf3 -c server_ip -P 4
3
4# Use 10 parallel streams
5iperf3 -c server_ip -P 10

Reverse Mode

1# Server sends data to client (reverse direction)
2iperf3 -c server_ip -R
3
4# Useful for testing upload vs download speeds

UDP Testing

1# UDP test with 100 Mbps bandwidth
2iperf3 -c server_ip -u -b 100M
3
4# UDP test with 1 Gbps bandwidth
5iperf3 -c server_ip -u -b 1G
6
7# Show packet loss
8iperf3 -c server_ip -u -b 100M
9# Output includes: Jitter, Lost/Total Datagrams

Specific Port

1# Server on custom port
2iperf3 -s -p 5555
3
4# Client to custom port
5iperf3 -c server_ip -p 5555

JSON Output

1# Output results in JSON format
2iperf3 -c server_ip -J > results.json
3
4# Parse with jq
5iperf3 -c server_ip -J | jq '.end.sum_received.bits_per_second'

Interval Reporting

1# Report every 1 second (default)
2iperf3 -c server_ip -i 1
3
4# Report every 5 seconds
5iperf3 -c server_ip -i 5
6
7# No interval reports (only final)
8iperf3 -c server_ip -i 0

Bandwidth Limit

1# Limit to 10 Mbps
2iperf3 -c server_ip -b 10M
3
4# Limit to 100 Mbps
5iperf3 -c server_ip -b 100M

Bi-directional Test

1# Test both directions (sequential in iperf3)
2iperf3 -c server_ip --bidir
3
4# First tests client->server, then server->client

IPv6

1# Server with IPv6
2iperf3 -s -6
3
4# Client with IPv6
5iperf3 -c server_ipv6 -6

iperf2 Commands

Basic TCP Test

1# Server
2iperf -s
3
4# Client
5iperf -c server_ip

Simultaneous Bi-directional

1# Test both directions simultaneously (iperf2 only)
2iperf -c server_ip -d
3
4# Full duplex test
5iperf -c server_ip -r  # Sequential bi-directional

Window Size

1# Set TCP window size
2iperf -c server_ip -w 256K

Common Use Cases

Test LAN Speed

1# Server
2iperf3 -s
3
4# Client (expect ~940 Mbps on gigabit ethernet)
5iperf3 -c 192.168.1.100 -t 30

Test WiFi Speed

1# Server (wired connection)
2iperf3 -s
3
4# Client (WiFi device)
5iperf3 -c server_ip -t 60 -i 1
6
7# Monitor for fluctuations in WiFi performance

Test VPN Throughput

1# Server (outside VPN)
2iperf3 -s
3
4# Client (inside VPN)
5iperf3 -c vpn_server_ip -t 30
6
7# Compare with direct connection

Test Multiple Connections

1# Simulate multiple users
2iperf3 -c server_ip -P 10 -t 60
3
4# Useful for testing router/switch capacity

Test Packet Loss (UDP)

1# Server
2iperf3 -s
3
4# Client - UDP test
5iperf3 -c server_ip -u -b 100M -t 30
6
7# Check output for packet loss percentage
8# Jitter: 0.015 ms
9# Lost/Total Datagrams: 0/75000 (0%)

Continuous Monitoring

 1# Run server in background
 2iperf3 -s -D
 3
 4# Or with systemd
 5sudo systemctl start iperf3
 6
 7# Run periodic tests
 8while true; do
 9    iperf3 -c server_ip -t 10 -J >> results.log
10    sleep 300  # Every 5 minutes
11done

Docker Setup

 1version: '3.8'
 2
 3services:
 4  iperf3-server:
 5    image: networkstatic/iperf3
 6    container_name: iperf3-server
 7    command: -s
 8    ports:
 9      - "5201:5201"
10    restart: unless-stopped
11
12  # Client (for testing)
13  iperf3-client:
14    image: networkstatic/iperf3
15    container_name: iperf3-client
16    command: -c iperf3-server -t 10
17    depends_on:
18      - iperf3-server

1# Run server
2docker run -d --name iperf3-server -p 5201:5201 networkstatic/iperf3 -s
3
4# Run client
5docker run --rm networkstatic/iperf3 -c host.docker.internal

Interpreting Results

TCP Results

1[ ID] Interval           Transfer     Bitrate         Retr
2[  5]   0.00-10.00  sec  1.09 GBytes   941 Mbits/sec    0    sender
3[  5]   0.00-10.04  sec  1.09 GBytes   937 Mbits/sec         receiver
  • Transfer: Amount of data transferred
  • Bitrate: Speed in Mbits/sec or Gbits/sec
  • Retr: Retransmissions (should be 0 or very low)

Good Results:

  • Gigabit Ethernet: ~940 Mbps (TCP overhead)
  • 10 Gigabit: ~9.4 Gbps
  • WiFi 5 (802.11ac): 200-600 Mbps
  • WiFi 6 (802.11ax): 600-1200 Mbps

UDP Results

1[ ID] Interval           Transfer     Bitrate         Jitter    Lost/Total Datagrams
2[  5]   0.00-10.00  sec   119 MBytes  99.8 Mbits/sec  0.015 ms  0/85000 (0%)
  • Jitter: Variation in packet delay (lower is better)
  • Lost/Total: Packet loss (should be 0% or very low)

Good Results:

  • Jitter: < 1 ms (excellent), < 10 ms (good)
  • Packet Loss: 0% (excellent), < 1% (acceptable), > 5% (poor)

Troubleshooting

Low Throughput

 1# Check for retransmissions
 2iperf3 -c server_ip -t 30
 3
 4# If high retransmissions:
 5# - Check network cables
 6# - Check switch/router
 7# - Check for interference (WiFi)
 8# - Check CPU usage on both ends
 9
10# Try increasing TCP window
11iperf3 -c server_ip -w 256K
12
13# Try parallel streams
14iperf3 -c server_ip -P 4

Firewall Issues

1# Allow iperf3 port
2sudo ufw allow 5201/tcp
3sudo ufw allow 5201/udp
4
5# Or iptables
6sudo iptables -A INPUT -p tcp --dport 5201 -j ACCEPT
7sudo iptables -A INPUT -p udp --dport 5201 -j ACCEPT

Connection Refused

 1# Check if server is running
 2ps aux | grep iperf
 3
 4# Check if port is listening
 5netstat -tuln | grep 5201
 6ss -tuln | grep 5201
 7
 8# Test connectivity
 9ping server_ip
10telnet server_ip 5201

Automation Script

 1#!/bin/bash
 2# iperf3-test.sh - Automated network testing
 3
 4SERVER="192.168.1.100"
 5DURATION=30
 6OUTPUT_DIR="./iperf_results"
 7
 8mkdir -p $OUTPUT_DIR
 9
10# TCP Test
11echo "Running TCP test..."
12iperf3 -c $SERVER -t $DURATION -J > $OUTPUT_DIR/tcp_$(date +%Y%m%d_%H%M%S).json
13
14# UDP Test
15echo "Running UDP test..."
16iperf3 -c $SERVER -u -b 100M -t $DURATION -J > $OUTPUT_DIR/udp_$(date +%Y%m%d_%H%M%S).json
17
18# Parallel streams
19echo "Running parallel streams test..."
20iperf3 -c $SERVER -P 4 -t $DURATION -J > $OUTPUT_DIR/parallel_$(date +%Y%m%d_%H%M%S).json
21
22# Reverse test
23echo "Running reverse test..."
24iperf3 -c $SERVER -R -t $DURATION -J > $OUTPUT_DIR/reverse_$(date +%Y%m%d_%H%M%S).json
25
26echo "Tests complete. Results in $OUTPUT_DIR"

Quick Reference

 1# Server
 2iperf3 -s                          # Start server
 3
 4# Basic tests
 5iperf3 -c server_ip                # TCP test
 6iperf3 -c server_ip -u -b 100M     # UDP test
 7iperf3 -c server_ip -R             # Reverse (server to client)
 8iperf3 -c server_ip -P 4           # 4 parallel streams
 9
10# Advanced
11iperf3 -c server_ip -t 60          # 60 second test
12iperf3 -c server_ip -J             # JSON output
13iperf3 -c server_ip -i 5           # Report every 5 seconds
14iperf3 -c server_ip -p 5555        # Custom port

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