How to set MTU PERMANENTLY in CLI

I'm using Ubuntu 20.04 in WSL2, and cannot for the life of me permanently set the MTU. No matter what I do, after rebooting, it's 1500 again. I had the same problem running Ubuntu natively, and switched to windows because Ubuntu doesn't work over VPN because the MTU is wrong.

Is there really no way to permanently set the MTU?

I tried /etc/dhcp/dhclient.conf

  GNU nano 4.8                                                                                               /etc/dhcp/dhclient.conf                                                                                                         # Configuration file for /sbin/dhclient.                                                                                                                                                                                                     #                                                                                                                                                                                                                                            # This is a sample configuration file for dhclient. See dhclient.conf's                                                                                                                                                                      #       man page for more information about the syntax of this file                                                                                                                                                                          #       and a more comprehensive list of the parameters understood by                                                                                                                                                                        #       dhclient.                                                                                                                                                                                                                            #                                                                                                                                                                                                                                            # Normally, if the DHCP server provides reasonable information and does                                                                                                                                                                      #       not leave anything out (like the domain name, for example), then                                                                                                                                                                     #       few changes must be made to this file, if any.                                                                                                                                                                                       #                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         option rfc3442-classless-static-routes code 121 = array of unsigned integer 8;                                                                                                                                                                                                                                                                                                                                                                                                            send host-name = gethostname();                                                                                                                                                                                                              default interface-mtu 1340;                                                                                                                                                                                                                  supersede interface-mtu 1340;                                                                                                                                                                                                                request subnet-mask, broadcast-address, time-offset, routers,                                                                                                                                                                                        domain-name, domain-name-servers, domain-search, host-name,                                                                                                                                                                                  dhcp6.name-servers, dhcp6.domain-search, dhcp6.fqdn, dhcp6.sntp-servers,                                                                                                                                                                     netbios-name-servers, netbios-scope,                                                                                                                                                                                                         rfc3442-classless-static-routes, ntp-servers;                                                                                                                                                                                                                                                                                                                                                                                                                                     #send dhcp-client-identifier 1:0:a0:24:ab:fb:9c;                                                                                                                                                                                             #send dhcp-lease-time 3600;                                                                                                                                                                                                                  #supersede domain-name "fugue.com home.vix.com";                                                                                                                                                                                             #prepend domain-name-servers 127.0.0.1;                                                                                                                                                                                                      #require subnet-mask, domain-name-servers;                                                                                                                                                                                                   timeout 300;                                                                                                                                                                                                                                 #retry 60;                                                                                                                                                                                                                                   #reboot 10;                                                                                                                                                                                                                                  #select-timeout 5;                                                                                                                                                                                                                           #initial-interval 2;                                                                                                                                                                                                                         #script "/sbin/dhclient-script";                                                                                                                                                                                                             #media "-link0 -link1 -link2", "link0 link1";                                                                                                                                                                                                #reject 192.33.137.209;                                                                                                                                                                                                                                                                                                                                                                                                                                                                   #alias {                                                                                                                                                                                                                                     #  interface "eth0";                                                                                                                                                                                                                         #  fixed-address 192.5.5.213;                                                                                                                                                                                                                #  option subnet-mask 255.255.255.255;                                                                                                                                                                                                       #}                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        #lease {                                                                                                                                                                                                                                     #  interface "eth0";                                                                                                                                                                                                                         #  fixed-address 192.33.137.200;                                                                                                                                                                                                             #  medium "link0 link1";                                                                                                                                                                                                                     #  option host-name "andare.swiftmedia.com";                                                                                                                                                                                                 #  option subnet-mask 255.255.255.0;                                                                                                                                                                                                         #  option broadcast-address 192.33.137.255;                                                                                                                                                                                                  #  option routers 192.33.137.250;                                                                                                                                                                                                            #  option domain-name-servers 127.0.0.1;                                                                                                                                                                                                     #  renew 2 2000/1/12 00:00:01;                                                                                                                                                                                                               #  rebind 2 2000/1/12 00:00:01;                                                                                                                                                                                                              #  expire 2 2000/1/12 00:00:01;                                                                                                                                                                                                              #}       

I also tried messing with netplan, but I don't think my system uses that.

Also tried /etc/network/interfaces

mtu 1340

How did you verify your real MTU?

I suppose your Ubuntu uses Netplan manager. You have asked CLI, not GUI. Here is my suggestion.

MTU verifying

Ping your default GW or another live closest node in your LAN by packets with "do not fragment" option and with specified packet size. Change packet size and find the limit size which is responded by the peer node:

ping 192.168.1.1 -c 2 -M do -s 2000
ping 192.168.1.1 -c 2 -M do -s 1500
ping 192.168.1.1 -c 2 -M do -s 1400
ping 192.168.1.1 -c 2 -M do -s 1472
ping 192.168.1.1 -c 2 -M do -s 1473

If packet size is bigger than MTU then ping response is e.g.:

ping: local error: message too long, mtu=1500

Please note that packet size you use in the ping command (-s option) must be MTU minus 18 bytes, i.e. for instance 1500-18=1472.

The response if the MTU is not exceeded is e.g.

2 packets transmitted, 2 received, 0% packet loss, time 1014ms

Setting

I will suppose your interface name is eth1.

cd /etc/netplan
ls -l
sudo cp 01-network-manager-all.yaml  02-eth1-mtu.yaml
sudo vi 02-eth1-mtu.yaml

The content example of new file 02-eth1-mtu.yaml:

network:
  version: 2
  renderer: NetworkManager
  ethernets:
    eth1:
      dhcp4: true
      mtu: 1000

New setting will be applied by command netplan try. If you press Enter while executing a command, the wait will be less than 120 s.

sudo netplan try

Now it is time to verify new MTU size by method described above.

Disable new setting without deleting

sudo mv /etc/netplan/02-eth1-mtu.yaml /etc/netplan/02-eth1-mtu.yaml.bak
sudo netplan try