Network Connectivity for Clones

This document presents a strategy to ensure continued network connectivity for multiple clones created from a single Firecracker microVM snapshot.

[!CAUTION]

This should be considered as just an example to get you started, and we don't claim this is a performant or secure setup.

Setup

There are two things which prevent network connectivity from resuming out-of-the-box for clones created from the same snapshot: Firecracker currently saves and attempts to restore network devices using the initially configured TAP names, and each guest will be resumed with the same network configuration, most importantly with the same IP address(es). To work around the former, each clone should be started within a separate network namespace (we can have multiple TAP interfaces with the same name, as long as they reside in distinct network namespaces). The latter can be mitigated by leveraging iptables NAT support.

Let’s have a more detailed look at this approach. We assume each VM has a single network interface attached. If multiple interfaces with full connectivity are required, we simply repeat the relevant parts of this process for each additional interface. A typical setup right before taking a snapshot involves having a VM with a network interface backed by a TAP device (named vmtap0, for example) with an IP address (referred to as the TAP IP address, for example 192.168.241.1/29), and an IP address configured inside the guest for the corresponding virtio device (referred to as the guest IP address, for example 192.168.241.2/29).

Network namespaces

Attempting to restore multiple clones from the same snapshot faces the problem of every single one of them attempting to use a TAP device with the original name, which is not possible by default. Therefore, we need to start each clone in a separate network namespace. This is already possible using the --netns jailer parameter, described in the documentation. The specified namespace must already exist, so we have to create it first using

sudo ip netns add fc0

(where fc0 is the name of the network namespace we plan to use for this specific clone - clone0). A new network namespace is initially empty, so we also have to create a new tap interface within using

sudo ip netns exec fc0 ip tuntap add name vmtap0 mode tap

The ip netns exec <ns_name> <command> allows us to execute command in the context of the specified network namespace (in the previous case, the secondary command creates a new tap interface). Next we configure the new TAP interface to match the expectations of the snapshotted guest by running

sudo ip netns exec fc0 ip addr add 192.168.241.1/29 dev vmtap0
sudo ip netns exec fc0 ip link set vmtap0 up

At this point we can start multiple clones, each in its separate namespace, but they won’t have connectivity to the rest of the host, only the respective TAP interfaces. However, interaction over the network is still possible; for example we can connect over ssh to clone0 using

sudo ip netns exec fc0 ssh [email protected]

veth interfaces to connect the network namespaces

In order to obtain full connectivity we have to begin by connecting the network namespace to the rest of the host, and then solving the “same guest IP” problem. The former requires the use of veth pairs - virtual interfaces that are link-local to each other (any packet sent through one end of the pair is immediately received on the other, and the other way around). One end resides inside the network namespace, while the other is moved into the parent namespace (the host global namespace in this case), and packets flow in or out according to the network configuration. We have to pick IP addresses for both ends of the veth pair. For clone index idx, let’s use 10.<idx / 30>.<(idx % 30) * 8>.1/24 for the endpoint residing in the host namespace, and the same address ending with 2 for the other end which remains inside the clone's namespace. Thus, for clone 0 the former is 10.0.0.1 and the latter 10.0.0.2.

The first endpoint must have an unique name on the host, for example chosen as veth(idx + 1) (so veth1 for clone 0). To create and setup the veth pair, we use the following commands (for namespace fc0):

# create the veth pair inside the namespace
sudo ip link add name veth1 type veth peer name veth0 netns fc0

sudo ip netns exec fc0 ip addr add 10.0.0.2/24 dev veth0
sudo ip netns exec fc0 ip link set dev veth0 up

sudo ip addr add 10.0.0.1/24 dev veth1
sudo ip link set dev veth1 up

# designate the outer end as default gateway for packets leaving the namespace
sudo ip netns exec fc0 ip route add default via 10.0.0.1

iptables rules for VM egress connectivity

The last step involves adding the iptables rules which change the source/destination IP address of packets on the fly (thus allowing all clones to have the same internal IP).

# Find the host egress device
UPSTREAM=$(ip -j route list default |jq -r '.[0].dev')
# anything coming from the VMs, we NAT the address
ip netns exec fc0 iptables -t nat -A POSTROUTING -s 192.168.241.1/29 -o veth0 -j MASQUERADE
iptables -t nat -A POSTROUTING -s 10.0.0.0/30 -o $UPSTREAM -j MASQUERADE
# forward packets by default
iptables -P FORWARD ACCEPT
ip netns exec fc0 ip route add default via 10.0.0.1
ip netns exec fc0 iptables -P FORWARD ACCEPT

You may also want to configure the guest with a default route and a DNS nameserver:

ip route default via 10.0.0.1
echo nameserver 8.8.8.8 >/etc/resolv.conf

Connectivity from the clone should be present at this point.

To make sure the guest also adjusts to the new environment, you can explicitly clear the ARP/neighbour table in the guest:

ip -family inet neigh flush any
ip -family inet6 neigh flush any

Otherwise, packets originating from the guest might be using old Link Layer Address for up to arp cache timeout seconds. After said timeout period, connectivity will work both ways even without an explicit flush.

Renaming host device names

In some environments where the jailer is not being used, restoring a snapshot may be tricky because the tap device on the host will not be the same as the tap device that the original VM was mapped to when it was snapshotted, for example when the tap device comes from a pool of such devices.

In this case you can use the network_overrides parameter of the snapshot restore API to specify which guest network device maps to which host tap device.

For example, if we have a network interface named eth0 in the snapshotted microVM, we can override it to point to the host device vmtap01 during snapshot resume, like this:

curl --unix-socket /tmp/firecracker.socket -i \
    -X PUT 'http://localhost/snapshot/load' \
    -H  'Accept: application/json' \
    -H  'Content-Type: application/json' \
    -d '{
            "snapshot_path": "./snapshot_file",
            "mem_backend": {
                "backend_path": "./mem_file",
                "backend_type": "File"
            },
            "network_overrides": [
                 {
                     "iface_id": "eth0",
                     "host_dev_name": "vmtap01"
                 }
            ]
    }'

This may require reconfiguration of the networking inside the VM so that it is still routable externally. network setup documentation describes what the typical setup is. If you are not using network namespaces or the jailer, then the guest will have to be made aware (via vsock or other channel) that it needs to reconfigure its network to match the network configured on the tap device.

If the new TAP device, say vmtap3 has been configured to use a guest address of 172.16.3.2 then after snapshot restore you would run something like:

# In the guest

# Clear out the previous addr and route
ip addr flush dev eth0
ip route flush dev eth0

# Configure the new address
ip addr add 172.16.3.2/30 dev eth0
ip route add default via 172.16.3.1/30 dev eth0

Ingress connectivity

The above setup only provides egress connectivity. If in addition we also want to add ingress (in other words, make the guest VM routable outside the network namespace), then we need to choose a "clone address" that will represent this VM uniquely. For our example we can use IPs from 172.16.0.0/12, for example 172.16.0.1.

Then we can rewrite destination address heading towards the "clone address" to the guest IP.

ip netns exec fc0 iptables -t nat -A PREROUTING -i veth0 \
    -d 172.16.0.1 -j DNAT --to 192.168.241.2

And add a route on the host so we can access the guest VM from the host network namespace:

ip route add 172.16.0.1 via 10.0.0.2

To confirm that ingress connectivity works, try

ping 172.16.0.1
# or
ssh [email protected]

See also

For an improved setup with full ingress and egress connectivity to the individual VMs, see this discussion.