bridge – Made by Mikal

Linux bridges have their MTU overwritten when you add an interface

Post author:mikal
Post published:September 7, 2021
Post category:Linux

I discovered last night that network bridges on linux have their Maximum Transmission Unit (MTU) overwritten by whatever is the MTU value of the most recent interface added to the bridge. This is bad. Very bad. Specifically this is bad because MTU matters for accurately describing the capabilities of the network path the packets will travel on, so it shouldn't be clobbered willy nilly. Here's an example of the behaviour: # ip link add egr-br-ens1f0 mtu 1500 type bridge # ip link show dev egr-br-ens1f0 3: egr-br-ens1f0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000 link/ether 7e:33:1b:30:d8:00 brd ff:ff:ff:ff:ff:ff # ip link add egr-eaa64a-o mtu 8950 type veth peer name egr-eaa64a-i # ip link show dev egr-br-ens1f0 3: egr-br-ens1f0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN mode DEFAULT group default qlen 1000 link/ether 7e:33:1b:30:d8:00 brd ff:ff:ff:ff:ff:ff # brctl addif egr-br-ens1f0 egr-eaa64a-o # ip link show dev egr-br-ens1f0 3: egr-br-ens1f0: <BROADCAST,MULTICAST> mtu 8950 qdisc noop state DOWN mode DEFAULT group default qlen 1000 link/ether da:82:cf:34:13:60 brd ff:ff:ff:ff:ff:ff So you can see here that the bridge had an MTU of 1,500 bytes. We create a veth pair with an MTU of 8,950 bytes and add it to…

Setting up VXLAN between nested virt VMs on Google Compute Engine

Post author:mikal
Post published:January 6, 2020
Post category:Google Google Compute Engine

I wanted to play with a VXLAN mesh between VMs on more than one hypervisor node, but the setup for VXLAN ended up being a separate post because it was a bit long. Read that post first if you want to follow the instructions here. Now that we have a working VXLAN mesh between our two nodes we can move on to installing libvirt (which is called libvirt-daemon-system on Debian, not libvirt-bin as on Ubuntu): sudo apt-get install -y qemu-kvm libvirt-daemon-system sudo virsh net-start default sudo virsh net-autostart --network default I'm going to use a little python helper to launch my VMs, so I need some other dependancies as well: sudo apt-get install -y python3-pip pkg-config libvirt-dev git git clone https://github.com/mikalstill/shakenfist cd shakenfist git checkout 6bfac153d249752b27d224ad9d079095b640498e sudo mkdir /srv/shakenfist sudo cp template.debian.xml /srv/shakenfist/template.xml sudo pip3 install -r requirements.txt Let's launch a quick test VM to make sure the helper works: sudo python3 daemon.py sudo virsh list You can destroy that VM for now, it was just testing the install. sudo virsh destroy ...name... Next we need to tweak the template that shakenfist is using to start instances so that it uses the bridge for networking (that template is the one…

Man tripping on LSD thinks he’s a bridge troll

Post author:mikal
Post published:July 15, 2006
Post category:Link

A most excellent hallucination if i do say so myself.