On Tue, Feb 18, 2014 at 04:19:58PM -0600, Serge Hallyn wrote: > Quoting Stéphane Graber ([email protected]): > > On Tue, Feb 18, 2014 at 03:12:52PM -0600, Serge Hallyn wrote: > > > If we are unprivileged and have asked for a veth device, then create > > > a pipe over which to pass the veth names. > > > > > > Network-related todos: > > > 1. set mtu on the container side of veth device > > > > > 2. set mtu in lxc-user-nic. Note that this probably requires an > > > update to the /etc/lxc/lxc-usernet file :( > > > > Hmm, that's an interesting problem and even without that change, we > > actually have a bug at the moment which may or may not qualify as a > > security issue. > > > > The bridge will set its own MTU to the lowest of all devices inside it > > (or so it looks like anyway), so say that a bridge has an MTU of 9000 > > (jumbo) and a user can join a container to it, that'll decrease the MTU > > to 1500 possibly breaking the other containers in the bridge. > > > > To fix that it looks like we indeed want an extra column in lxc-usernet > > which would specify the min and max MTU, a value of 0 (same as no value) > > would tell lxc-user-nic to copy that of the bridge, an value of > > 1500:4000 would mean that the mtu may not be set below 1500 or above > > 4000. > > > > Unfortunately as this would result in a rather user visible change as > > well as documentation changes, if we are going to do this, we really > > should do it before 1.0. > > > > > > Alternatively we could state that unprivileged containers may not use a > > custom MTU and that they will always default to the bridge's MTU value > > for both sides of the veth device. > > > > In which case we still need to change both lxc and lxc-user-nic to get > > the current MTU from the bridge and set it on both side of the veth > > device. > > Does lxc need to do it? We should just be able to have lxc-user-nic > copy the bridge's value right?
We probably would want to make sure that the behaviour is consistent and that if lxcbr0 has a mtu of say 1400 that both veth also get an mtu of 1400 in the system container case too. But yes, for the unprivileged case, we could simply have lxc-user-nic set the mtu to the bridge's value on both devices of the veth pair. -- Stéphane Graber Ubuntu developer http://www.ubuntu.com
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