Ttyjfyk

(Virtualbox OS: Linux Mint 19.1 Tessa)

I am trying to disable the network-connectivity-check on Linux Mint.
This website shows how to disable it in the GUI for Ubuntu 17.10.
Unfortunately in Mint the path
Settings > Privacy > Turn off "network connectivity checking"
does not exist.
According to this post the GUI just changes a line entry in the file /var/lib/NetworkManager/NetworkManager-intern.conf to false:

[connectivity] 
.set.enabled=false

By using cat I saw that these two lines didn't even exist, therefore I inserted them and tried the following procedure again:

1. network-tray-icon > Wired_connection_1 > Disconnect
2. wireshark start packet capturing for interface "enp0s3"
3. network-tray-icon > Wired_connection_1 > Connect
4. wireshark stop packet capturing

By applying the display-filter (dns.qry.name == "connectivity-check.ubuntu.com"), inside wireshark, there are exactly 100 DNS-queries and their corresponding responses.
So the insertion of those two lines didn't work.

I also did overwrite the following file as described here:

sudo touch /etc/NetworkManager/conf.d/20-connectivity-ubuntu.conf

Still exact 100 DNS-messages.

How do I disable the network-connectivity-check on Linux Mint,
by using the terminal?

Based on the connectivity section in gnome docs:

Note that your distribution might set /proc/sys/net/ipv4/conf/*/rp_filter to strict filtering. That works badly with per-device connectivity checking, which uses SO_BINDDEVICE to send requests on all devices. A strict rp_filter setting will reject any response and the connectivity check on all but the best route will fail.

command lines (as root):

sysctl -w net.ipv4.conf.all.rp_filter=0
sysctl -w net.ipv4.conf.default.rp_filter=0
sysctl -w net.ipv4.conf.<NET_INTERFACE_HERE>.rp_filter=0
sysctl -p

You can also remove the package network-manager-config-connectivity-ubuntu:

apt purge network-manager-config-connectivity-ubuntu