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low lag mouse/keyboard sharing with Synergy and an ethernet cable

23 Mar 2019

Synergy lets you share a desktop and keyboard over the network, which is quite convenient if you’re like me and have two desktop machines (one Mac, one Linux in my case) and don’t want to waste desk space with duplicate mice/keyboards.

I’ve tried to use Synergy in the past with a wireless connection, but I’ve always been discouraged by the input lag on the client machine. Recently, I realized that since the boxes are physically located close to each other, I could probably connect them directly to each other over ethernet, while still allowing them to use wireless for the regular internet connection.

So I did that.

On Mac, the configuration simply consisted of going to the Network preferences, and configuring the ethernet with a manual IP address (say that didn’t conflict with any of the wireless network prefixes (e.g. if your wireless uses, configure your wired “network” to something like I didn’t specify a router, since it’s just a direct connection between the two machines.

On Linux, the configuration is:

# enable ethernet
ip link set eth0 up

# add the address to the route table
ip route add dev eth0

# pick an IP address that's different than the other computer

# statically set the IP addres for the eth0 device
ip addr add dev eth0 $IP

Then, you can just configure Synergy to use the IP address (of the eth0 device) when connecting the client to the server. With this new configuraiton, I don’t notice any input latency on the client machine anymore.

I realize that none of these ideas are particularly novel, but I thought it was neat that a very usable configuration could be set up over ethernet. It definitely beats a KVM switch.

macOS ethernet packet drops

After performing the above configuration, the mouse was still stuttering every few seconds. A ping between the two devices over the ethernet device revealed packet drops. To fix this, I had to disable “energy efficient ethernet” on the Mac. The setting is under Network > Ethernet > Advanced > Hardware, set the Configure setting to Manually, then change Duplex from full-duplex, energy-efficient-ethernet to full-duplex.