If you have ip_forwarding (NetworkManager) don't work:

/etc/rc.d/rc.ip_forward restart report fine, don't work !

May be resolve by updating user space configuration ?

i use inet from smartphone dhcpd (192.168.42.0/24)
now resolve via remove all connections and create bridge usb0+eth0, work fine !

i go to testing user PC...it triple show next config:

192.168.42.129 /24 is smartphone
192.168.42.55 is Slackware with name bridge0 (i'm typing Bridge1 in GUI) (Slackware64-current 5.14.6)
192.168.42.36 is Slackware Live eth0 (Slackware64-current 5.14.5)

Well continue testing !

this also help to you https://vk.com/wall-76526075_952 understanding !

So smartphone is connected to computer through usb. And computer get access to internet via smartphone, am I correct?

yes, smartphone and 2 computers work in one network, where smartphone i dhcp server and share internet include WIFI.
Also you can connect other: network printers, notebooks, ip-cam with internal vpn, wifi phones..
Example: Small or Home Office, where firewall is off or individual per PC.

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