Jtdcftul

I go to this file /etc/dhcp/dhclient.confand add these two line before request line

default interface-mtu 1200;



supercede interface-mtu 1200;

and after I restart and system boot up and run ifconfig command , it says my MTU size is 1280 , Why??
How can I change it to exact 1200 ??

Update:

cat /etc/resolv.conf

lrwxrwxrwx 1 root root 29 Mar 19 13:05 /etc/resolv.conf -> ../run/resolvconf/resolv.conf

and the resolv.conf file

nameserver 1.1.1.1

nameserver 8.8.8.8

nameserver 2606:4700:4700::1111

nameserver 127.0.0.53

options edns0

the resolvectl output is :

Current DNS Server: 1.1.1.1

     DNS Servers: 1.1.1.1

                  8.8.8.8

                  2606:4700:4700::1111

      DNSSEC NTA: 10.in-addr.arpa

and the dpkg -l *dnsmasq* output is :

Desired=Unknown/Install/Remove/Purge/Hold

| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig- 

aWait/Trig-pend

|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)

||/ Name                   Version          Architecture     

Description

+++-======================-================-================- 

==================================================

un  dnsmasq                <none>           <none>           (no 

description available)

ii  dnsmasq-base           2.79-1           amd64            Small 

caching DNS proxy and DHCP/TFTP server

un  dnsmasq-base-lua       <none>           <none>           (no 

description available)

You can automate the setting of the mtu for the VPN port when the VPN goes up as suggested in script
/etc/NetworkManager/dispatcher.d/01-ifupdown. Scripts in the dispatcher.d folder are called with the port and event as argument.
So, create a script in folder dispatcher.d, with following contents (if you want mtu 1200)

    #!/bin/sh  



    #info : pre-up and pre-down are not implemented in network-manager



    if [ "$2" = "vpn-up" ]; then  

        /sbin/ifconfig "$1" mtu 1200  

    fi  

The name of the script does not matter, make it executable with chmod +x.

To determine the correct MTU setting, start with all MTU settings = 1500 and VPN = off. (VPN requires different testing).

Note: first remove your mods to /etc/dhcp/dhclient.conf and reboot.

In terminal:

ping [-c count] [-M do] [-s packet_size] [host]

The options used are:

• 
  c count: number of times to ping


• 
  M hint: Select Path MTU Discovery strategy. may be either do (prohibit fragmentation, even local one), want (do PMTU discovery, fragment locally when packet size is large), or dont (do not set DF flag).


• 
  s packet_size: Specifies the number of data bytes to be sent.

You should always start at 1472 and work your way down by 10 each time. Once you get a reply, go up by 1 until you get a fragmented packet. Take that value (last good value) and add 28 to the value to account for the various TCP/IP headers. Eg. let's say that 1452 was the proper packet size (where you first got an ICMP reply to your ping). The actual MTU size would be 1480, which is the optimum for the network we're working with.

ping -c 4 -M do -s 1472 8.8.8.8 # this will probably show fragmentation



ping -c 4 -M do -s 1462 8.8.8.8 # may show fragmentation



ping -c 4 -M do -s 1452 8.8.8.8 # no fragmentation?



ping -c 4 -M do -s 1453 8.8.8.8 # still no fragmentation?

reference: How to determine the proper MTU size with ICMP pings

Update #1:

I would recommend that you remove the 3 extra nameservers that you added to /etc/resolvconf/resolv.conf.d/head and let systemd-resolved properly manage /etc/resolv.conf. That should work better with VPN.

Update #2:

You can also setup a DIFFERENT wired/wireless connection profile, just for VPN, that hardwires the MTU=1200. In terminal, type nm-connection-editor and there you can customize a profile just for VPN with autoconnect and MTU.

You could use tracepath ( from the iputils-tracepath package) to determine the Path MTU:

walt@bat:~(0)$ dpkg -S $(type -p tracepath)

iputils-tracepath: /usr/bin/tracepath

walt@bat:~(0)$ tracepath primus.ip4.torontointernetxchange.net

 1?: [LOCALHOST]                                         pmtu 1500

 1:  spark3y                                               0.525ms 

 1:  spark3y                                               0.497ms 

 2:  spark3y                                               0.465ms pmtu 1492

 2:  dsl-xxx-xxx-xxx-xxx.tor.primus.ca                    32.701ms 

 3:  10.201.117.2                                         34.726ms asymm  5 

 4:  bb1.tor.primus.ca                                    39.238ms 

 5:  primus.ip4.torontointernetxchange.net                33.295ms reached

     Resume: pmtu 1492 hops 5 back 4