Change MTU not possible

  • Hi there,

    I try to change the MTU setting for our ipsec - connection because the tunnel get up, I can send packages (a lot of MB without errors) but If I try to donwload something, it stop working. So could be a MTU Problem… .

    On my PF-Sence Server using the shell and after a ifconfig get this output for the tunnel:
    enc0: flags=41 <up,running>mtu 1536

    The other side (not possible to change) uses MTU = 1456.
    Ok now enter:
    ifconfig enc0 mtu 1456
    and I get that back:
    ifconfig: ioctl (set mtu): Invalid argument

    Find a lot of persons have the same problem but no answers :(
    Hope somebody can help me?


  • Rebel Alliance Developer Netgate

    Try changing the MTU of your WAN interface instead. You can set that via the WebGUI.

  • I already try that but it's not working. Enter some other MTU size in the GUI and do a ifconfig after that, no change happend.
    Any other ideas?

  • Even though the field is marked MTU, it doesn't really set the MTU. Read the fine print:

    If you enter a value in this field, then MSS clamping for TCP connections to the value entered above minus 40 (TCP/IP header size) will be in effect. If you leave this field blank, an MTU of 1492 bytes for PPPoE and 1500 bytes for all other connection types will be assumed.

    If you really need to set the MTU you can do it from the shell.
    To survive a reboot, you would need to add a startup command to the xml.

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