How to simulate slow internet connections on the mac

Apple’s official tool to slow down the network connections on you Mac for testing purposes is Network Link Conditioner

• Additional Tools for Xcode [version].

Additionally, iOS has similar function accessible from within Xcode and iOS 6 or later.

Older versions of Xcode before version 4.3.2 embedded a copy of this tool. This SO thread documents some history of the tool in a similar manner to the iOS simulators and developer documentation.

There are 11 built in profiles from a Lossy Edge network with 400ms delay to a cable modem. If you need other limits, you can create custom profiles with your own settings or you can also use ipfw yourself as described in Craig Hockenberry's article slow ride, make it easy It also mentions the Speed Limit panel by Mike Schrag that is a smaller download than Xcode, but has fewer options than Apple's tool.

It slows down the entire network stack, so you can't throttle on a per app basis without doing things like install lion in a virtual machine and set that VM with a throttled stack.

OS X 10.9 and earlier provide ipfw and it allows you to define custom firewall rules. Create a pipe with limited bandwidth using ipfw and you can run your tests and simulations.

1. Create a pipe "1" limited to 500KBytes/s via

   sudo ipfw pipe 1 config bw 500KByte/s
   

2. Guide all network traffic of port 80 through pipe "1" using

   sudo ipfw add 1 pipe 1 src-port 80
   

3. When you don't need the pipe anymore, remove it from the port using

   sudo ipfw delete 1
   

Other

• If you want to set higher traffic barriers, you can use MByte/s
• Port 80: standard port for unencrypted http traffic. This port is used for most browsing and downloading. You should be fine with this in most cases.
• Port 443: standard port for SSL encrypted https traffic.

Speed Limit is a System Preferences pane for intentionally and selectively slowing down specific ports and domains.

To add to the accepted answer: it looks like you shouldn't need XCode, just an account at the Apple Developer website (simpler than first downloading the 2GB XCode package if you don't have it already).

Go to https://developer.apple.com/downloads and search for "Network Link Conditioner" or "Additional Tools for XCode", the latter being the name of the package it's found in.

To download it via Xcode 8.x:

• Click on the Xcode menu
• Go to Open Developer Tool > More Developer Tools...
• This will open a page at developer.apple.com (Note: you may need to log in first)
• Click on the '+' sign next to Additional Tools for Xcode 8.x
• Click on the download URL

Mac OS X 10.10+ users need to use dnctl and pfctl but documented usage examples aren't easy to find.

# Configure `pfctl` to use `customRule`. 
(cat /etc/pf.conf && echo "dummynet-anchor \"customRule\"" && echo "anchor \"customRule\"") | sudo pfctl -f -

# Define `customRule` to pipe traffic to `pipe 1`.
# Note this is the actual port definition, not a textual comment
echo "dummynet in quick proto tcp from any to any port 443 pipe 1" | sudo pfctl -a customRule -f -

# Define what `pipe 1` should do to traffic
sudo dnctl pipe 1 config delay 10000
sudo dnctl pipe 1 config bw 10Kbit/s

# DO NOT FORGET to undo these when you're done
sudo dnctl -q flush
sudo pfctl -f /etc/pf.conf

If you want to go all out and shape everything you can use:

echo "dummynet in quick proto tcp from any to any pipe 1" | sudo pfctl -a customRule -f -

I believe this also affects localhost pipes which slowed down my vs-code debugging, so be mindful of that.