Set MTU on Guest

you can try to force mtu 1400 on vmbr, but I think you'll have fragmentation if your guest use 1500 at install.

 

seem that qemu 2.8 support a new option for virtio-net : host_mtu=xxx

https://git.qemu.org/?p=qemu.git;a=commit;h=a93e599d4a04c3cf7edcf5a24f3397e27431c027

but it's not yet implemented in proxmox

 

you can try to hack

/usr/share/perl5/PVE/QemuServer.pm

sub print_netdevice_full {
...
$tmpstr .= ",bootindex=$net->{bootindex}" if $net->{bootindex} ;
$tmpstr .= ",host_mtu=1400" if $net->{model} eq 'virtio';

then

systemctl restart pvedaemon


and start your vm.

 

does it work for you ? (I don't have tested it)

 

I would also prefer to implement this option in Proxmox. It worked great with your modification and the Vlans from Hetzner.

 

Hello there, can anyone help to add this patch to PVE to configure MTU by Network Edit dialog
There is only to patches

First adds ability to specify in network config parameter ,mtu=MTU_NUM /etc/pve/qemu-server/VMID.conf

Code:

--- /usr/share/perl5/PVE/QemuServer.pm.orig     2018-08-30 03:07:40.885517605 +0300
+++ /usr/share/perl5/PVE/QemuServer.pm  2018-08-30 02:56:40.191065007 +0300
@@ -715,6 +715,12 @@
        description => "Rate limit in mbps (megabytes per second) as floating point number.",
        optional => 1,
     },
+    mtu => {
+       type => 'number',
+       minimum => 1200, maximum=> 9000,
+       description => "MTU",
+       optional => 1,
+    },
     tag => {
        type => 'integer',
        minimum => 1, maximum => 4094,
@@ -1839,6 +1845,7 @@
        $tmpstr .= ",vectors=$vectors,mq=on";
     }
     $tmpstr .= ",bootindex=$net->{bootindex}" if $net->{bootindex} ;
+    $tmpstr .= ",host_mtu=$net->{mtu}" if $net->{mtu} ;

     if ($use_old_bios_files) {
        my $romfile;
@@ -4634,7 +4641,7 @@
                &$safe_string_ne($oldnet->{trunks}, $newnet->{trunks}) ||
                &$safe_num_ne($oldnet->{firewall}, $newnet->{firewall})) {
                PVE::Network::tap_unplug($iface);
-               PVE::Network::tap_plug($iface, $newnet->{bridge}, $newnet->{tag}, $newnet->{firewall}, $newnet->{trunks}, $newnet->{rate});
+               PVE::Network::tap_plug($iface, $newnet->{bridge}, $newnet->{tag}, $newnet->{firewall}, $newnet->{trunks}, $newnet->{rate}, $newnet->{mtu});
            } elsif (&$safe_num_ne($oldnet->{rate}, $newnet->{rate})) {
                # Rate can be applied on its own but any change above needs to
                # include the rate in tap_plug since OVS resets everything.

Second patch is for GUI to edit MTU settings

Code:

--- /usr/share/pve-manager/js/pvemanagerlib.js.orig     2018-08-30 03:08:07.430582086 +0300
+++ /usr/share/pve-manager/js/pvemanagerlib.js  2018-08-30 02:37:28.504841119 +0300
@@ -1517,6 +1517,8 @@
                res.bridge = match_res[1];
            } else if ((match_res = p.match(/^rate=(\d+(\.\d+)?)$/)) !== null) {
                res.rate = match_res[1];
+           } else if ((match_res = p.match(/^mtu=(\d+)$/)) !== null) {
+               res.mtu = match_res[1];
            } else if ((match_res = p.match(/^tag=(\d+(\.\d+)?)$/)) !== null) {
                res.tag = match_res[1];
            } else if ((match_res = p.match(/^firewall=(\d+)$/)) !== null) {
@@ -1558,6 +1560,9 @@
        if (net.rate) {
            netstr += ",rate=" + net.rate;
        }
+       if (net.mtu) {
+           netstr += ",mtu=" + net.mtu;
+       }
        if (net.queues) {
            netstr += ",queues=" + net.queues;
        }
@@ -1785,7 +1790,7 @@
            if (!p || p.match(/^\s*$/)) {
                return; // continue
            }
-           var match_res = p.match(/^(bridge|hwaddr|mtu|name|ip|ip6|gw|gw6|firewall|tag|rate)=(\S+)$/);
+           var match_res = p.match(/^(bridge|hwaddr|mtu|name|ip|ip6|gw|gw6|firewall|tag|mtu|rate)=(\S+)$/);
            if (!match_res) {
                // todo: simply ignore errors ?
                return; // continue
@@ -18401,6 +18406,12 @@
            delete me.network.rate;
        }

+       if (values.mtu) {
+           me.network.mtu = values.mtu;
+       } else {
+           delete me.network.mtu;
+       }
+
        var params = {};

        params[me.confid] = PVE.Parser.printQemuNetwork(me.network);
@@ -18466,6 +18477,16 @@
                xtype: 'proxmoxcheckbox',
                fieldLabel: gettext('Disconnect'),
                name: 'disconnect'
+           },
+           {
+               xtype: 'numberfield',
+               fieldLabel: gettext('MTU'),
+               minValue: 1200,
+               maxValue: 9000,
+               value: '',
+               emptyText: '1500',
+               name: 'mtu',
+                allowBlank: true
            }
        ];

@@ -18484,6 +18505,7 @@
                            'firewall',
                            'model',
                            'macaddr',
+                            'mtu',
                            'rate',
                            'queues'
                        ];

After that you should restart pvedaemon and pveproxy

 

@Tacid

I'll send patches based on this.
Do you known if the value can be changed online ? (hotplug)

 

@Tacid

I'll do some changes if it's ok for you:

min-max mtu : 576-65535
only do it for virtio-net (not supported for other model card)
hotplug not working, add a check for this
remove the mtu param in tap_plug (it's doing nothing and not related to tap)

 

patches sent:

https://pve.proxmox.com/pipermail/pve-devel/2018-August/033555.html
https://pve.proxmox.com/pipermail/pve-devel/2018-August/033556.html