public-iperf3-servers/docs.md

iperf3

iperf3 is a widely used command-line utility designed for actively measuring the maximum achievable network throughput between two endpoints. In short, it's a powerful tool for testing and analyzing network performance. It facilitates testing using both TCP and UDP protocols, offering valuable insights into bandwidth, latency, jitter, and packet loss. While commonly employed by network and system administrators for diagnosing bottlenecks and verifying link performance, iperf3 is also an effective tool for general network speed testing.

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How iperf3 Works

iperf3 operates using a simple client-server model:

1. Server: One machine runs iperf3 in server mode using the -s flag. It listens for incoming connections from an iperf3 client on a specific network port (defaulting to 5201).

   # Start a server on the default port 5201
   iperf3 -s
   

2. Client: Another machine runs iperf3 in client mode using the -c flag, followed by the hostname or IP address of the server it needs to connect to.

   # Connect to a server running on 'server.example.com'
   iperf3 -c server.example.com
   

Once connected, the client initiates the test by sending data to the server (default direction). iperf3 then measures performance metrics like throughput, and depending on the protocol (TCP or UDP), it can also report retransmissions (TCP), jitter, and packet loss (UDP).

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Installation

iperf3 is available for most common operating systems.

Debian / Ubuntu:

sudo apt update && sudo apt install -y iperf3

CentOS / RHEL / Rocky Linux: (Requires the EPEL repository)

sudo yum install epel-release -y
sudo yum install iperf3 -y

Fedora:

sudo dnf install iperf3 -y

Windows: Download the latest pre-compiled Windows binaries from a reliable source, such as builds linked from the official iperf.fr or directly from: ar51an/iperf3-win-builds recommended.

Extract the archive. You need to run iperf3.exe from the Command Prompt (cmd) or PowerShell. Either navigate (cd) to the directory containing the .exe file or add the directory to your system's PATH environment variable for easier access.

winget install -e --id ar51an.iPerf3

macOS (using Homebrew): If you have Homebrew installed:

brew install iperf3

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Basic Usage

Connecting a Client

To run a default 10-second TCP test connecting to a server:

iperf3 -c iperf-ams-nl.eranium.net

To connect to a server running on a non-default port:

iperf3 -c iperf-ams-nl.eranium.net -p 5205

To connect to a server on a random port between 5201 and 5210:

iperf3 -c iperf-ams-nl.eranium.net -p $((5201+(RANDOM%10)))

Common Command-Line Options

Here are some frequently used options for both client (-c) and server (-s) modes, although some are specific to one mode:

• -s, --server: Run in server mode.
• -c, --client <host>: Run in client mode, connecting to <host> (IP or hostname).
• -p, --port <port>: Set the server port to listen on (-s) or connect to (-c). Default is 5201.
• -i, --interval <sec>: Set the interval time in seconds between periodic bandwidth, jitter, and loss reports. Default is 1 second. Set to 0 to disable periodic reports.
• -t, --time <sec>: Set the duration of the test in seconds. Default is 10 seconds.
• -P, --parallel <num>: Client only. Number of parallel client streams to run. Default is 1.
• -R, --reverse: Client only. Reverse the direction of the test (server sends, client receives). Useful for testing download speed from the client's perspective.
• -u, --udp: Use UDP protocol instead of the default TCP.
• -b, --bitrate <bits/s>: Client only. Set the target bitrate in bits per second. Supports K, M, G suffixes (e.g., 100M, 1G). Crucial for UDP tests. Default is 1 Mbps for UDP, unlimited for TCP.
• --bidir: Client only. Run the test in both directions simultaneously.
• -4, --version4: Use IPv4 only.
• -6, --version6: Use IPv6 only.
• -J, --json: Output results in JSON format.
• -V, --verbose: Provide more detailed output.

Example: 30-second TCP test with 5 parallel streams

iperf3 -c ams.speedtest.clouvider.net -t 30 -P 5

Example: 20-second UDP test targeting 500 Mbps bitrate

iperf3 -c ams.speedtest.clouvider.net -t 20 -u -b 500M

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Test Scenarios & Protocol Differences

Choosing between TCP and UDP depends on what aspect of network performance you want to measure.

TCP Testing (Default)

TCP provides reliable, ordered, and error-checked delivery. It's the foundation for most common internet traffic like web browsing (HTTP/S), file transfers (FTP, SMB), and email.

• When to Use: To measure the realistic, usable throughput for applications that rely on reliable data transfer. It reflects how actual applications might perform under current network conditions (latency, loss, congestion).
• Pros: Simulates common application traffic, measures effective throughput considering TCP's behavior (congestion control, retransmissions).
• Cons: Throughput can be significantly impacted by network latency and packet loss due to TCP's congestion control mechanisms. It might not reveal the absolute maximum raw bandwidth of the link.

UDP Testing (-u)

UDP is a connectionless protocol that prioritizes speed over reliability. It doesn't guarantee delivery or order. Used for real-time applications like VoIP, online gaming, and video streaming.

• When to Use: To estimate the maximum raw link capacity, or to measure network jitter and packet loss, which are critical for real-time applications.
• Pros: Minimal protocol overhead, allows direct measurement of jitter and packet loss, can potentially saturate the link to find its maximum capacity (if hardware allows).
• Cons: Unreliable by nature. Requires setting a target bitrate (-b); without it, iperf3 defaults to ~1 Mbps, making results useless for capacity testing. Setting -b too high can overwhelm the network or devices, leading to artificial packet loss.

👍 ​Rule of Thumb:

• For a realistic view of application performance → Use TCP.
• To test maximum raw capacity or analyze real-time traffic characteristics (jitter, loss) → Use UDP (with an appropriate -b value).

Reverse Mode (-R)

By default, the client sends data to the server. The -R flag reverses this.

• When to Use: To specifically test the download speed from the client's perspective (server sends, client receives).

  # Test download speed from server for 15 seconds
  iperf3 -c ams.speedtest.clouvider.net -t 15 -R
  

Bidirectional Mode (--bidir)

Runs tests simultaneously in both directions (client sends to server AND server sends to client).

• When to Use: To simulate full-duplex traffic and see how the network performs under load in both directions, like during a video conference or P2P file sharing.

  # Run a 10-second bidirectional TCP test
  iperf3 -c ams.speedtest.clouvider.net --bidir
  

  Note: Output will show separate results for transmit (client->server) and receive (server->client) legs.

TCP vs. UDP Summary

Protocol	Reliability	Best for	Pros	Cons
TCP	Yes	Web, file transfer	Realistic, error correction	Lower peaks, affected by TCP behavior
UDP	No	VoIP, games, video	Measures jitter/packet loss, fast	Unreliable, must set bandwidth manually

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Advanced Usage & Tips

Find & Test (findtest.sh)

Find & Test is a script that automatically locates the geographically closest server to the client's location from a list and runs a test against it.

curl -s https://raw.githubusercontent.com/R0GGER/public-iperf3-servers/main/findtest.sh | bash

Note: Requires curl, bash, jq, and iperf3 to be installed.

Using Docker

Running iperf3 via Docker avoids local installation dependencies.

Run as Client:

# Example: Connect to a known public server using TCP
docker run -it --rm --name iperf3-client ghcr.io/r0gger/public-iperf3-servers:latest -c iperf-ams-nl.eranium.net -p 5208

# Example: UDP test, mapping local UDP port if needed
docker run -it --rm --name iperf3-udp-client ghcr.io/r0gger/public-iperf3-servers:latest -c iperf-ams-nl.eranium.net -p 5209 -u -b 1G

Run as Server:

# Run a server listening on default TCP/UDP port 5201
docker run -it --rm --name iperf3-server -p 5201:5201 -p 5201:5201/udp ghcr.io/r0gger/public-iperf3-servers:latest -s

Understanding Output (Brief)

• [ ID] Interval: Time window for the measurement line.
• Transfer: Amount of data moved during the interval.
• Bitrate: Throughput achieved during the interval (Mbps or Gbps).
• Retr (TCP): Number of TCP segments retransmitted; a high number indicates packet loss or congestion.
• Jitter (UDP): Variation in packet arrival times; crucial for real-time applications (lower is better).
• Lost/Total Datagrams (UDP): Number and percentage of lost UDP packets.
• Sender/Receiver: Summary lines showing total results for the entire test duration.

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More Information

For the most comprehensive details, consult the official iperf3 resources:

• Official iperf3 Documentation: https://iperf.fr
• Public Server List Project: https://iperf3serverlist.net/

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Thank you

This project is sponsored and supported by HostHatch and Censys.