IPv6 should be disabled when ssh of Raspberry Pi 4 is very slow

environment

For details on the setup environment, refer to Set up raspbian buster on Raspberry Pi 4.

• Raspberry Pi 4 Model B (RAM 4GB)
• raspbian buster (2019-09-26-raspbian-buster.zip)

phenomenon

When I ssh connect to Raspberry Pi, it takes time to connect.

Cause

The default is IPv6 connection → IPv4 connection, and it took time for the IPv6 connection to time out. (Maybe I made a mistake during setup)

I wondered if it was a problem on the Raspberry Pi side because it is slow to connect from Windows Powershell or other Linux.

Debug information can be displayed by executing it with the -v option when sshing. Here, you can see that you are connecting to Raspberry Pi from Windows, and after failing to connect like an IPv6 address due to a timeout, you are connecting with IPv4.

$ ssh -v Username@Machine name
OpenSSH_for_Windows_7.7p1, LibreSSL 2.6.5
debug1:Connecting to machine name[xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx] port 22.
debug1: connect to address xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx:xxxx port 22: Connection timed out
debug1:Connecting to machine name[yyy.yyy.yyy.yyy] port 22.
debug1: Connection established.

How to improve

Change the Raspberry Pi settings.

$ sudo vi /etc/sysctl.conf

Add the following to sysctl.conf.

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1

Reflect changes in sysctl.conf.

$ sudo sysctl -p

Confirm that there is no item called inet6 with the following command.

$ ip a

Close the ssh connection once.

$ exit

Connect with ssh again and check.

$ ssh -v Username@Machine name
OpenSSH_for_Windows_7.7p1, LibreSSL 2.6.5
debug1:Connecting to machine name[yyy.yyy.yyy.yyy] port 22.
debug1: Connection established.

(..) φ memo

The above disables IPv6 on all interfaces. If you want to disable only a specific interface, add the following items to /etc/sysctl.conf. Below is an example of disabling only eth0.

net.ipv6.conf.eth0.disable_ipv6 = 1