Extension

The extension backend provides an easy way for prototyping new backend types for flannel.

It is not recommended for production use, for example it doesn't have a built in retry mechanism.

This backend has the following configuration

Type (string): extension
PreStartupCommand (string): Command to run before allocating a network to this host
- The stdout of the process is captured and passed to the stdin of the SubnetAdd/Remove commands.
PostStartupCommand (string): Command to run after allocating a network to this host
- The following environment variable is set * SUBNET - The subnet of the remote host that was added.
SubnetAddCommand (string): Command to run when a subnet is added
- stdin - The output from PreStartupCommand is passed in.
- The following environment variables are set
  - SUBNET - The ipv4 subnet of the remote host that was added.
  - IPV6SUBNET - The ipv6 subnet of the remote host that was added.
  - PUBLIC_IP - The public IP of the remote host.
  - PUBLIC_IPV6 - The public IPv6 of the remote host.
SubnetRemoveCommand(string): Command to run when a subnet is removed
- stdin - The output from PreStartupCommand is passed in.
  - The following environment variables are set
    - SUBNET - The ipv4 subnet of the remote host that was removed.
    - IPV6SUBNET - The ipv6 subnet of the remote host that was removed.
    - PUBLIC_IP - The public IP of the remote host.
    - PUBLIC_IPV6 - The public IPv6 of the remote host.

All commands are run through the sh shell and are run with the same permissions as the flannel daemon.

Simple example (host-gw)

To replicate the functionality of the host-gw plugin, there's no need for a startup command.

The backend just needs to manage the route to subnets when they are added or removed.

An example

json

{
  "Network": "10.0.0.0/16",
  "Backend": {
    "Type": "extension",
    "SubnetAddCommand": "ip route add \"$SUBNET\" via \"$PUBLIC_IP\"",
    "SubnetRemoveCommand": "ip route del \"$SUBNET\" via \"$PUBLIC_IP\""
  }
}

Complex example (vxlan)

VXLAN is more complex. It needs to store the MAC address of the vxlan device when it's created and to make it available to the flannel daemon running on other hosts. The address of the vxlan device also needs to be set after the subnet has been allocated.

An example

json

{
  "Network": "10.50.0.0/16",
  "Backend": {
    "Type": "extension",
    "PreStartupCommand": "export VNI=1; export IF_NAME=flannel-vxlan; ip link del \"$IF_NAME\" 2>/dev/null; ip link add \"$IF_NAME\" type vxlan id \"$VNI\" dstport 8472 && cat \"/sys/class/net/${IF_NAME}/address\"",
    "PostStartupCommand": "export IF_NAME=flannel-vxlan; export SUBNET_IP=$(echo \"$SUBNET\" | cut -d'/' -f 1); ip addr add \"${SUBNET_IP}/32\" dev \"$IF_NAME\" && ip link set \"$IF_NAME\" up",
    "SubnetAddCommand": "export SUBNET_IP=$(echo \"$SUBNET\" | cut -d'/' -f 1); export IF_NAME=flannel-vxlan; read VTEP; ip route add \"$SUBNET\" nexthop via \"$SUBNET_IP\" dev \"$IF_NAME\" onlink && ip neigh replace \"$SUBNET_IP\" dev \"$IF_NAME\" lladdr \"$VTEP\" && bridge fdb add \"$VTEP\" dev \"$IF_NAME\" self dst \"$PUBLIC_IP\""
  }
}