Linux Cannot change tx-checksumming

I finally resolve problem. I use systemd (with networkd) and new feature about ip forwarding has been introduced in systemd-221: "IPForwarding=" - see https://github.com/systemd/systemd/blob/a2088fd025deb90839c909829e27eece40f7fce4/NEWS

From man systemd.network:

[NETWORK] SECTION OPTIONS

...

IPForward=

Configures IP forwarding for the network interface. If enabled incoming packets on the network interface will be forwarded to other interfaces according to the routing table. Takes either a boolean argument, or the values "ipv4" or "ipv6", which only enables IP forwarding for the specified address family, or "kernel", which preserves existing sysctl settings. This controls the net.ipv4.conf..forwarding and net.ipv6.conf..forwarding sysctl options of the network interface (see ip-sysctl.txt[1] for details about sysctl options). Defaults to "no".

Note: unless this option is turned on, or set to "kernel", no IP forwarding is done on this interface, even if this is globally turned on in the kernel, with the net.ipv4.ip_forward, net.ipv4.conf.all.forwarding, and net.ipv6.conf.all.forwarding sysctl options.

So now I use network file like following to enable ip forwarding (per interface):

# cat /etc/systemd/network/tun0.network
[Match]
Name=tun0

[Network]
IPForward=ipv4

The remote IP should be 100.1.1.1/100.1.1.2. This should work:

ip address add 100.1.1.1/24 dev enp0s8
ifconfig enp0s8 up
ip link add name vxlan42 type vxlan id 42 dev enp0s8 remote 100.1.1.2 local 100.1.1.1 dstport 4789
ip address add 50.1.1.1/24 dev vxlan42
ip link set up vxlan42

ip address add 100.1.1.2/24 dev enp0s8
ifconfig enp0s8 up
ip link add name vxlan42 type vxlan id 42 dev enp0s8 remote 100.1.1.1 local 100.1.1.2 dstport 4789
ip address add 50.1.1.2/24 dev vxlan42
ip link set up vxlan42