Changing MTU via GUI does not show up in output from ifconfig

nassy · 5. September 2016

Using the GUI I changed MTU to 9000 geändert, see here Datei von filehorst.de laden

After a reboot is is seen in /etc/network/interfaces

Code

cat /etc/network/interfaces 
# The loopback network interface
auto lo
iface lo inet loopback


# eth0 network interface
auto eth0
allow-hotplug eth0
iface eth0 inet static
    address 192.168.2.34
    gateway 192.168.2.1
    netmask 255.255.255.0
    post-up /sbin/ifconfig $IFACE mtu 9000
    dns-nameservers 192.168.2.33 192.168.2.1
    dns-search hyper24.com
    pre-down ethtool -s $IFACE wol g
iface eth0 inet6 manual
    pre-down ip -6 addr flush dev $IFACE

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But this is the output of ifconfig -a - after reboot. What did I wrong?

Code

ifconfig -a
eth0      Link encap:Ethernet  Hardware Adresse a0:b3:cc:e3:f3:82  
          inet Adresse:192.168.2.34  Bcast:192.168.2.255  Maske:255.255.255.0
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metrik:1
          RX packets:105392 errors:0 dropped:0 overruns:0 frame:0
          TX packets:4271 errors:0 dropped:0 overruns:0 carrier:0
          Kollisionen:0 Sendewarteschlangenl\Uffffffffe:1000 
          RX bytes:13367609 (12.7 MiB)  TX bytes:5537860 (5.2 MiB)
          Interrupt:18 
lo        Link encap:Lokale Schleife  
          inet Adresse:127.0.0.1  Maske:255.0.0.0
          inet6-Adresse: ::1/128 G\Uffffffffeitsbereich:Maschine
          UP LOOPBACK RUNNING  MTU:65536  Metrik:1
          RX packets:156 errors:0 dropped:0 overruns:0 frame:0
          TX packets:156 errors:0 dropped:0 overruns:0 carrier:0
          Kollisionen:0 Sendewarteschlangenl\Uffffffffe:0 
          RX bytes:47669 (46.5 KiB)  TX bytes:47669 (46.5 KiB)
tun0      Link encap:UNSPEC  Hardware Adresse 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00  
          inet Adresse:10.8.0.1  P-z-P:10.8.0.2  Maske:255.255.255.255
          UP PUNKTZUPUNKT RUNNING NOARP MULTICAST  MTU:1500  Metrik:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          Kollisionen:0 Sendewarteschlangenl\Uffffffffe:100 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

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subzero79 · 5. September 2016

Is the nic gigabit?

If you run the command manually does it change mtu?

ifconfig eth0 mtu 9000

Check last lines of dmesg after this to see if any relevant message comes out.

nassy · 5. September 2016

Hi and thanks for your answer.

Well, neither

ifconfig eth0 mtu 1600

nor

ip link set dev eth0 mtu 1600

do work. I always get an "Invalid argument"-error.

dmesg tells me (and the bandwdth as well) that it is a gigabit-adapter.

[1015226.987903] device eth0 entered promiscuous mode
[1015229.279547] tg3 0000:02:00.0 eth0: Link is up at 1000 Mbps, full duplex
[1015229.279574] tg3 0000:02:00.0 eth0: Flow control is on for TX and on for RX
[1015229.279623] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready

and trying to restart the network with MTU 9000 in /etc/network/interfaces results in

/etc/init.d/networking restart
Running /etc/init.d/networking restart is deprecated because it may not re-enable some interfaces ... (warning).
Reconfiguring network interfaces...RTNETLINK answers: File exists
Failed to bring up eth0.
RTNETLINK answers: File exists
Failed to bring up eth0.
done.

subzero79 · 5. September 2016

Looks like the kernel driver doesn't support it. If you have omvextras installed you can try an upgrade to backport kernel one, maybe support was added later.
When you ran the command did dmesg showed something about that? check last 5-6 lines.
Google says some people report issues with mtu and the Broadcom tg3 driver.

nassy · 5. September 2016

Well, it seems to me, that I'm already using the backports kernel.

Code

Updating...
Install backports 3.16 kernel and headers
Reading package lists...
Building dependency tree...
Reading state information...
linux-base is already the newest version.
initramfs-tools is already the newest version.
The following packages were automatically installed and are no longer required:
  libmono-c5-1.1-cil libmonoboehm-2.0-dev
Use 'apt-get autoremove' to remove them.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Reading package lists...
Building dependency tree...
Reading state information...
firmware-linux-free is already the newest version.
linux-headers-3.16.0-0.bpo.4-amd64 is already the newest version.
linux-image-3.16.0-0.bpo.4-amd64 is already the newest version.
firmware-linux is already the newest version.
The following packages were automatically installed and are no longer required:
  libmono-c5-1.1-cil libmonoboehm-2.0-dev
Use 'apt-get autoremove' to remove them.
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
...
Please reboot to use new kernel

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dmesg does not tell me anything more than is seen in my previous post.

Well, I think it's the damn tg3. Thanks again

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