dhcpcd-discuss

Hi list,

We have a system currently running dhcpcd 5.6.7.  While we'll look to update dhcpcd in the future, we are currently using this version, so this is the version relevant to this inquiry.

The dhcpcd-hooks/10-mtu file in question then is this version: http://roy.marples.name/projects/dhcpcd/artifact/b2bdd219284ceddd

I have found that when a DHCP server responds with an "Interface MTU" option such that the MTU specified is different than the currently configured MTU for the interface, the Ethernet interface appears to be reset, which causes dhcpcd to be notified that the carrier is lost.  Unfortunately, it has already updated the MTU, so after carrier is lost, it restores the original MTU that had been saved in $mtu_dir/$interface and then this process repeats indefinitely.

For example, if original MTU=1500 and new MTU=576, we end up with:
start:
        carrier acquired
        MTU set to 576, save 1500 to $mtu_dir/$interface
carrier lost
Restore MTU to $mtu_dir/$interface
Delete $mtu_dir/$interface
Goto start

I am apparently not the only one to notice this reset condition when changing the MTU[1].  I wouldn't know all the affected controllers, but for the record, this one is an Intel controller (e1000e) running on Linux 3.10.21.

I would like to honor the interface MTU, but also be able to restore the original MTU if needed, such as if the device is unplugged from the network where the MTU was changed into some other network where the DHCP server on that new network does not deliver the "Interface MTU" option.  To accomplish this, I have modified the 10-mtu file as follows:

      # Configure the MTU for the interface

      mtu_dir="$state_dir/mtu"

      set_mtu()
      {
        local mtu=$1

        if [ -w /sys/class/net/$interface/mtu ]; then
                echo "$mtu" >/sys/class/net/$interface/mtu
        else
                ifconfig "$interface" mtu "$mtu"
        fi
      }

      if [ -n "$new_interface_mtu" ] && $if_up; then
        # The smallest MTU dhcpcd can work with is 576
        if [[ "$new_interface_mtu" -ge 576 && "$new_interface_mtu" -ne "$ifmtu" ]]; then
                if set_mtu "$new_interface_mtu"; then
                        syslog info "$interface: MTU set to $new_interface_mtu"
                        # Save the original MTU so we can restore it later
                        if [ ! -e "$mtu_dir/$interface" ]; then
                                mkdir -p "$mtu_dir"
                                echo "$ifmtu" > "$mtu_dir/$interface"
                        fi
                fi
        fi
      elif [ -z "$new_interface_mtu" ] && $if_up; then
        # No MTU specified, so restore the original MTU
        if [ -e "$mtu_dir/$interface" ]; then
                mtu=$(cat "$mtu_dir/$interface")
                if [ $mtu -ne $ifmtu ]; then
                        set_mtu "$mtu"
                        syslog info "$interface: MTU restored to $mtu"
                fi
        fi
      fi

Basically, I don't care about what the MTU is set to if the interface is down, only when it comes up.  If it's up and a new MTU is specified, set it if it's valid and different from the current MTU, and then store the original MTU if we haven't already (storing it is only ever done once).  If it's up but no MTU is specified, then simply restore the original MTU if it had been saved and if it's different from the current one.  Never delete $mtu_dir/$interface, as that's the original MTU!

So, with this change, what ends up happening is:

start:
        carrier acquired
        MTU set to 576, save 1500 to $mtu_dir/$interface
carrier lost
carrier acquired
MTU already set to 576, so no need to change it.
Happy days!

The interface is still reset once but recovers quickly.  If I were to unplug from this network and plug into a different network with no "Interface MTU" option, then this would happen:

start:
        carrier acquired
        Change MTU from 576 to $mtu_dir/$interface (i.e., 1500)
carrier lost
carrier acquired
MTU already set to 1500, so no need to change it.
Happy days!

My apologies for the length of this message, but I wanted to provide as much information as possible so it would hopefully be clear what the problem.  Does this approach make sense?  Is there a better way to accomplish this perhaps?  Maybe there's something I missed or haven't thought of?  Any feedback is appreciated.  Thanks for reading.

- Chris
[1]: https://github.com/hartwork/image-bootstrap/issues/15

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