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Using systemd-nspawn containers with publicly routable ips (IPv6 and IPv4) via bridged mode for high density testing whilst balancing tenant isolation

Christopher Simpson

• 30 min read
Using systemd-nspawn containers with publicly routable ips (IPv6 and IPv4) via bridged mode for high density testing whilst balancing tenant isolation
If you're tight for time and want to build this right away see the "Tutorial" heading for how to setup systemd-nspawn with bridge mode and public ip addressing.

There's also an accompanying systemd-nspawn repo with a scripted process to deploy everything in this article automatically:

GitHub - KarmaComputing/nspawn-systemd-nspawn-containers
Contribute to KarmaComputing/nspawn-systemd-nspawn-containers development by creating an account on GitHub.
GitHubKarmaComputing

Disclaimer: Lennart Poettering specifically stated that systemd-nspawn was conceived  for only testing your software which has networking components, and what we're doing here might be considered misuse. However, Poettering said that 9 years ago, and things change. Today, systemd-nspawn is being used for more dependable workloads.

  • The first part of this article is focused on IPv4 because I want to get that clearer in my own mind first.
  • However, IPv6 is the far more natural fit because when you buy/rent a server you typically get an IPv6 /60 allocated to you for free (cost savings) which gives you 295,147,905,179,352,825,856 globally routable addresses. What's more, this entire public subnet is already routed to your server* making route configuration simpler compared to IPv4 where you have to battle with ARP, promiscuous mode, and port security much more.

*I'm not suggesting you try to run 295,147,905,179,352,825,856 services on a single host- please try though and get back to me!

I've previously written about IPv6 becoming more commonplace:

IPv6 Only Web Services Becoming Commonplace
IPv6 connectivity (native) continues to shoot up: This trend started as a slow “it won’t happen” almost joke within the IT community with efforts such as the 2012 “World IPv6 launch days” The economic incentive for IPv6 is here now What we’re seeing now is the economic incentive stepping in
Karma ComputingChristopher Simpson
See also IPv6 Only Web Services Becoming Commonplace similarly the raw costs of providing web services continue to hit the floor, and increasingly with more robust isolation promises leading to higher destiny and ultimately, more value for money.

Why now?

TLDR: Hermetic /usr/ is awesome; let's popularize image-based OSes with modernized security properties built around immutability, SecureBoot, TPM2, adaptability, auto-updating, factory reset, uniformity – built from traditional distribution packages, but deployed via images.
- Lennart Poettering, 'Fitting Everything Together' May 2022
- See also counter views on the topic at https://lwn.net/Articles/894396/

For an overview of systemd generally see: NYLUG Presents: Lennart Poettering -on- Systemd:

What we're building

  • Multi-tenancy: One sever, running n number of light-weight containers, each with their own publicly routable address- this is key, to the outside world each container looks like an ordinary host
  • Each light-weight container can run any application it chooses (we're using a Debian 11 install for each container)
  • Every container has some isolation promises**

From a network perspective to the outside world, each container will be globally reachable since each container gets its own public IP address assigned***.

This is decisively not IP Masquerading where you only have one src IP address translated to multiple internal IPs, we don't want that- we want publicly addressable endpoints. This is decisively not NAT.

No NAT? This sounds really insecure?

NAT != Firewall. The goals of this article are to explore systemd-nspawn. See 'The goals of this project'.

** Whilst remembering that promises may be broken
***Again, this is why IPv6 makes sense here but I digress- sticking with IPv4 for now).

Why? The goals of this project are:

  • Maximising frugality
  • Not sacrificing reachability  - every service must be directly contactable via its public ip address - yes really, no SNAT/DNAT masquarding here
  • Reproducibility - In the 'DevOps' world we're very familiar with infrastructure as code (buzzword for: I can audit, change, and  rebuild my application workloads easily if I have to). The same is useful for say, testing network topology generation

What's the use case?

I'm quite keen to spin up / tear down test k0s clusters ( k0s is a lightweight Kubenetes which is easy to install, which needs at least 3 nodes to be useful). With nspawn + ip addressing, frugal ephemeral clusters become possible.

How to install k0s kzeros Kubernetes
K0 (kzeros) in a new single binary distribution model for running and deployingKubernetes. The project is available on GitHub here:https://github.com/k0sproject/k0s The k0s project comes from engineers working on the original docker project andit’s some aims include making ruining and deploying…
Karma ComputingChristopher Simpson
  • But you could literally use nspawn containers for anything- Go for it. Anything which runs and binds to an address + port can be ran from nspawn containers. Want to run a database? Apache? an entire stack?
  • nspawn containers support the "Ephemeral" setting which "If enabled, the container is run with a temporary snapshot of its file system that is removed immediately when the container terminates." (docs)

Tutorial: How to setup systemd-nspawn bridge mode

The high-level steps are:

  1. Create a Debian 11 Virtual Private Server (VPS)

2. Install systemd-nspawn and create a machine (aka lightweight container)

3. Configure bridged networking from external -> host -> container(s) (the tricky bit)

We'll also be creating a floating IPv4 address and assigning that IP to our VPS, these instructions are for Hetzner but the concepts are the same regardless of your provider choices.

Creating the nspawn Debian container image

  1. First you must create a Debian 11 Hetzner Virtual Private server (note this is a referral link, you don't have to create a Hetzner vm, but all these instructions, assumes you have especially the floating ip part).
  2. Then follow instructions below to install systemd-nspawn and a few (optional) helper utilities for debugging (`tmux`, tracetoure, vim). Note qemu-guest-agent is absolutely not needed, it's helpful on Hetzner for resetting your password via console if you get locked-out, though.

SSH into your container and start setting up systemd-nspawn:

apt update
apt install -y systemd-container debootstrap bridge-utils tmux telnet traceroute vim  qemu-guest-agent
echo "set mouse=" > ~/.vimrc
echo 'kernel.unprivileged_userns_clone=1' >/etc/sysctl.d/nspawn.conf

# Permit ip forwarding

sed -i 's/#net.ipv4.ip_forward=1/net.ipv4.ip_forward=1/g' /etc/sysctl.conf  | grep 'forward'
echo 1 > /proc/sys/net/ipv4/ip_forward
sysctl -p

mkdir -p /etc/systemd/nspawn

Create a Debian nspawn container using debootstrap

debootstrap --include=systemd,dbus,traceroute,telnet,curl stable /var/lib/machines/debian
dbus is needed so that you can login to your guest from the host

Login to your guest container, set password and configure tty

Login to your container:

systemd-nspawn -D /var/lib/machines/debian -U --machine debian

Spawning container buster on /var/lib/machines/debian.
Press ^] three times within 1s to kill container.
Selected user namespace base 818610176 and range 65536.
Sidenote: the extremely high user id (uid) and user namespace. This is a systemd concept called "Dynamic User" for greater isolation which builds upon traditional *nix ownership and isolation models.

- If you're interested see
talk: "NYLUG Presents: Lennart Poettering -on- Systemd in 2018- Dynamic user chapter" or
text: Dynamic Users with systemd

The above will drop you inside your guest Debian container. Now still inside the guest shell configure a password:

# set root password
root@debian:~# passwd
New password:
Retype new password:
passwd: password updated successfully

# allow login via local tty # may not be needed any more
# 'pts/0' seems to be used when doing systemd-nspawn --boot, 'pts/1' with machinectl login
root@debian:~# printf 'pts/0\npts/1\n' >> /etc/securetty

# logout from container
root@debian:~# logout
Container debian exited successfully.
At the end of this you'l have configured a password for your guest container & logged out of the guest
It can be confusing to know if you're inside or outside of the guest (unless you change the container hostname).

A quick way to know if your inside a guest or at the host is to:
- Try and exit by pressing Ctrl + ]]] (Control key and right square bracket three time)
- ls /dev Inside a guest, since permissions are limited you won't see all /dev devices exposed in your container
- set a hostname for your container

On the host, create an /etc/systemd/nspawn/debian.nspawn config file for your container


Here we configure the guest container to connect to an existing bridge interface (we've not created it yet). We only need these two lines for a minimal nspawn container in bridged mode: 

vim /etc/systemd/nspawn/debian.nspawn

[Network]
Bridge=br0
For full configuration options see: https://www.freedesktop.org/software/systemd/man/systemd.nspawn.html 

Create host network configuration

Before we start here's an ip command primer of useful shortcuts I found helpful:

  • ip -c address (the -c will give you coloured output- easier to see 👀)
  • ip a is the same as ip address (save your fingers!)

Remove existing automatic cloud config:

Your VPS came with automatic address configuration which needs removing.

rm /etc/network/interfaces.d/50-cloud-init

Create a new network config with a bridge interface for eth0:

Assumptions:

Here we'll be using the following example IP addresses, yours of course will be mostly be different, and we highlight where not.
  • Your VPS public ip address is: 203.0.113.1/32 (yours will be different)
  • Your Hetzner gateway is: 172.31.1.1 (you don't want to change this, its Hetzners reserved IP address for the gateway. Hetzner FAQ
  • You have created and assigned a floating IP address to your VPS (docs)
    • e.g. 203.0.113.2/32 (yours will be different)
    • You have not assigned the floating ip to your interface eth0 (ignore Hetzners example, don't assign your floating ip address to eth0.

Edit your host: vim /etc/network/interfaces

# The loopback network interface
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet static
  address 203.0.113.1
  netmask 255.255.255.255
  pointtopoint 172.31.1.1
  gateway 172.31.1.1
  dns-nameservers 185.12.64.1 185.12.64.2

#Bridge setup
auto br0
iface br0 inet static
   bridge_ports none
   bridge_stp off
   bridge_fd 1
   pre-up brctl addbr br0
   up ip route add 203.0.113.2/32 dev br0
   down ip route del 203.0.113.2/32 dev br0
   address 203.0.113.1
   netmask 255.255.255.255
   dns-nameservers 185.12.64.1 185.12.64.2

You can manually bring up the interfaces by doing:

ip link set dev eth0 up
ifup br0

You can view interface statuses by the following (don't worry if br0 is down at the moment).

ip -c link
ip -c addr
(The -c gives you coloured output ! 🌈)
Help I'm locked out my server after configuring a bridge!

Playing with bridges with only 1 Ethernet interface is a great way to accidentally lock yourself our of your server.

Thankfully it's not hard to get back, use the console view (or equivalent on your provider) to directly login to your server and reset the networking if you need to (this is why we installed qemu-guest-agent earlier since you can 'rescue' and change the password from the console with the agent installed- not without.

Reboot and verify you're still able to ssh into your server

After a reboot, your interfaces (at least `eth0`) should come up and you'll still be able to ssh into your server.

Start your container and assign the public ip address to it

Starting your nspawn container is simple:

systemctl start systemd-nspawn@debian.service

# Enable container to automatically start upon reboot, if you like
systemctl enable  systemd-nspawn@debian.service
Since nspawn services are simply combining existing systemd features, they are systemd-services and you can journalctl them just like any other systemd service.
Since nspawn containers are just another systemd service, you can use systemctl status systemd-nspawn@debian.service to inspect, and even journalctl for logs.

See the container running with machinectl list --full:

machinectl list --full
MACHINE CLASS     SERVICE        OS     VERSION ADDRESSES
debian  container systemd-nspawn debian 11      203.0.113.2

1 machines listed.

You won't have any ip address listed yet- let's do that next! We're going to

  • login to your guest container (with machinectl login)
  • Assign the container your floating ip address (using ip)
  • Configure routing so that routes work from your VPS host -> container and back again

Login to your debian container, assign IP & configure routing


Logging into the guest container called debian:

machinectl login debian

Connected to machine debian. Press ^] three times within 1s to exit session.

Debian GNU/Linux 11 debian-guest pts/1

debian-guest login: root
Password: 

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
Last login: Fri Feb  3 22:26:43 UTC 2023 on pts/1
root@debian-guest:~#
Using machinectl to login to guest

Configuring an ip address for the nspawn guest container

Because we have an /etc/systemd/nspawn/debian.nspawn file which specifies "Bridge=br0", when you started your container, systemd performed some automatic bridge configuration for you.

Still in your guest container run: ip -c link: Note that systemd has created a host0 interface for you automatically.

# ip -c link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: host0@if6: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default qlen 1000


Now log out of your guest and run: ip -c link on your VPS host from outside of your guest:

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000

2: eth0: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000


3: br0: <BROADCAST,MULTICAST,PROMISC,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default qlen 1000

6: vb-debian@if2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br0 state UP mode DEFAULT group default qlen 1000
Notice that systemd has created a new interface called 'vb-debian'

You'll see a new interface has been created by systemd called vb-debian@if2 which binds one network namespace to your host, and the other to another interface created in your guest container. This is explained in detail in the systemd-nspawn network-bridge docs.

Login to your guest again  & configure it with the floating ip address

The floating ip address is to simulate having lots of available IP addresses (again, this is more suited to IPv6 but this still works in IPv4 with some effort).

# Login with your guest user root and password

machinectl login debian

# Set an ip address on the 
ip addr add 203.0.113.2/32 dev host0
ip link set host0 up

# Verify host0 interface is up
ip -c link

The above logs you back into the guest called debian, and adds an IP address (the floating ip address) to the host0 interface, which was created by systemd-nspawn and connects to our br0 bridge on the host.

Note that can't ping out host or reach the outside world from your container yet: 🥹

# Try to ping the host ip from the guest
# Remember that 203.0.113.1 is the example IP address of the VPS eth0
# yours will be different:

root@debian:~# ping 203.0.113.1
ping: connect: Network is unreachable

# Try to ping google from the guest
root@debian:~# ping 8.8.8.8
ping: connect: Network is unreachable
From the guest, we still can't contact the outside world (yet)

That's partly because the guests default routing table is empty:

ip route show default
# nothing 🥹
Empty routing table on the guest

Add default gateway from container to host

In order for the nspawn guest container to reach the host

ip route add default dev host0

Now we can ping the host ip from inside our guest! 🚀:

root@debian:~# ping 203.0.113.1
PING 203.0.113.1 (203.0.113.1) 56(84) bytes of data.
64 bytes from 203.0.113.1: icmp_seq=1 ttl=64 time=0.135 ms
64 bytes from 203.0.113.1: icmp_seq=2 ttl=64 time=0.057 ms
64 bytes from 203.0.113.1: icmp_seq=3 ttl=64 time=0.071 ms
^C
--- 203.0.113.1 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2030ms
rtt min/avg/max/mdev = 0.057/0.087/0.135/0.033 ms
This is pinging from inside the guest to our host eth0 / br0 interface

..and we can ping from our host -> to our floating ip which is assigned to our guest container:

# Logout of container
logout
# Back in main host:
root@host:~# ping 203.0.113.2
PING 203.0.113.2 (203.0.113.2) 56(84) bytes of data.
64 bytes from 203.0.113.2: icmp_seq=1 ttl=64 time=0.110 ms
64 bytes from 203.0.113.2: icmp_seq=2 ttl=64 time=0.061 ms
^C
--- 203.0.113.2 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 1020ms
rtt min/avg/max/mdev = 0.061/0.085/0.110/0.024 ms
203.0.113.2 is an example floating IP, yours will be different.

However, we sill can't reach external internet from our container. (log back into your container machinectl login debian and try to ping 8.8.8.8: 🥹 so close!

root@debian-test3:~# ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
From  icmp_seq=1 Destination Host Unreachable
From  icmp_seq=2 Destination Host Unreachable
From  icmp_seq=3 Destination Host Unreachable
^C
--- 8.8.8.8 ping statistics ---
4 packets transmitted, 0 received, +3 errors, 100% packet loss, time 3068ms
pipe 4
We do get there, honest! Read on

Configure proxy_arp to allow eth0 to reply to arp requests for ip address(es) on the guest interface host0

At this point, when you try to ping your floating IP externally (e.g. your own internet connection) you cannot ping,  and will see "Destination Host Unreachable". This is because proxy_arp is turned off.

If you're not interested in why this is the case, simply turn on proxy_arp on your host with:
echo 1 > /proc/sys/net/ipv4/conf/eth0/proxy_arp

Explanation of why proxy_arp is needed is below.

Since arp requests will be coming from your cloud providers switch to the 'physical' eth0, your server needs to reply to arp requests come the gateway, arriving at our host:

tcpdump -n -i eth0 -e arp

11:19:32.767498 d2:74:7f:6e:37:e3 > ff:ff:ff:ff:ff:ff, ethertype ARP (0x0806), length 42: Request who-has 203.0.113.2 tell 172.31.1.1, length 28
11:19:33.785587 d2:74:7f:6e:37:e3 > ff:ff:ff:ff:ff:ff, ethertype ARP (0x0806), length 42: Request who-has 203.0.113.2 tell 172.31.1.1, length 28
11:19:34.809684 d2:74:7f:6e:37:e3 > ff:ff:ff:ff:ff:ff, ethertype ARP (0x0806), length 42: Request who-has 203.0.113.2 tell 172.31.1.1, length 28
Our host, endlessly receiving ARP requests for our floating IP which are currently being unanswered. This is why we see Destination Host Unreachable when we try to ping our floating IP - they gateways ARP table for 203.0.113.2 has no entry because our host is not replying to the ARP request.

Above, we see Hetzner's gateway ip (172.31.1.1) endlessly sending ARP requests to our host which are currently not being answered at all. That is because our floating IP address is not assigned to eth0 (and shouldn't be).

  • We've associated (but not assigned) our floating IP to the VPS server, so the gateway is trying to populate its ARP table ✅
  • We've intentionally not assigned our floating ip address to our eth0, which is why arp requests from the gateway are going unanswered

And sure enough, our host's ARP table has no entry for either, only the gateway:

ip neigh 
172.31.1.1 dev eth0 lladdr d2:74:7f:6e:37:e3 REACHABLE
ip neigh is the more recent version of the arp command

So to resolve the ARP requests not being replied to issue, we need to turn on proxy_arp for the eth0 interface:

echo 1 > /proc/sys/net/ipv4/conf/eth0/proxy_arp

After which, finally ARP requests from the gateway are being replied to by our eth0 (even though our floating ip address is not directly assigned to that interface):

11:47:27.321712 d2:74:7f:6e:37:e3 > 96:00:01:e1:5c:65, ethertype ARP (0x0806), length 42: Request who-has 203.0.113.2 tell 172.31.1.1, length 28
11:47:27.321730 96:00:01:e1:5c:65 > d2:74:7f:6e:37:e3, ethertype ARP (0x0806), length 42: Reply 203.0.113.2 is-at 96:00:01:e1:5c:65, length 28
🚀 Our eth0 interface is replying to ARP requests from the gateway

And also since now we have enabled proxy_arp on our eth0, the arp table for that interface also contains a reference to our floating ip 203.0.113.2 reachable on our bridge:

ip neigh 
172.31.1.1 dev eth0 lladdr d2:74:7f:6e:37:e3 REACHABLE
203.0.113.2 dev br0 lladdr de:d4:27:20:44:78 REACHABLE
Don't let this ARP table confuse- the core issue for the Destination Host Unreachable is that our Gateway's ARP requests were going un-answered. However, in a rented 'cloud' environment we can't see the ARP table of the provider- but we do know now that we've managed to insert our appropriate entry into it for our floating IP. (very important we do that for ours alone).

Huzzah! Not so soon. Whilst we've resolved the Destination Host Unreachable issue, now when we ping our floating IP from an external connection, we get... silence*

ping 203.0.113.2
#.. nothing
🥹 

Thankfully we can get some visibility into whats happening by using tcmpdump at our host- are ICMP pings are reaching our host. But why are they not being answered?

tcpdump -n -i eth0 -e icmp
tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), snapshot length 262144 bytes
11:59:32.010084 d2:74:7f:6e:37:e3 > 96:00:01:e1:5c:65, ethertype IPv4 (0x0800), length 98: 37.157.51.68 > 203.0.113.2: ICMP echo request, id 123, seq 12071, length 64
11:59:33.034072 d2:74:7f:6e:37:e3 > 96:00:01:e1:5c:65, ethertype IPv4 (0x0800), length 98: 37.157.51.68 > 203.0.113.2: ICMP echo request, id 123, seq 12072, length 64
11:59:34.057892 d2:74:7f:6e:37:e3 > 96:00:01:e1:5c:65, ethertype IPv4 (0x0800), length 98: 37.157.51.68 > 203.0.113.2: ICMP echo request, id 123, seq 12073, length 64
^C
3 packets captured
3 packets received by filter
0 packets dropped by kernel
This is a tcpdump of me pinging the floating up from my internet connection. We can see that the pings are reaching the host (perhaps not the guest?) and are not being replied to. 🥹..we need to configure the guests default gateway correctly, let's go! 🛠️

Configure the guest default gateway so that the guest can speak to the world

tldr: If you want to see the steps to configure guests default gateway,
skip to:

"Configure the guest default gateway with the correct via route via hosts IP"

Explanation of why the guest default gateway is needed

At the moment,  ICMP packets are reaching our host, and guest virtual Ethernet bridge link which systemd-nspawn kindly created for us**** , however neither our host/nor guest is replying to those ICMP packets.

As a reminder, whilst pinging our floating IP from an external IP (e.g. your personal internet connection) , we also use tcpdump to confirm that our host is receiving the ICMP requests, but not replying to them:

tcpdump -n -i eth0 -e icmp
tcpdump: verbose output suppressed, use -v[v]... for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), snapshot length 262144 bytes
11:59:32.010084 d2:74:7f:6e:37:e3 > 96:00:01:e1:5c:65, ethertype IPv4 (0x0800), length 98: 37.157.51.68 > 203.0.113.2: ICMP echo request, id 123, seq 12071, length 64

So right now our guest can only ping its own IP (203.0.113.2), and the IP address of the host on the directly connected eth0 interface ( 203.0.113.1), and we can't ping out from our guest to the wider internet (e.g. google 8.8.8.8), and when we ping from our personal internet connection to our guest, neither can our guest reply back- this is a big hint that there may be a gateway issue: 

# Currently our guest routing table is empty:
ip route show

# .. nothing
# (or contains invalid / bad defaults - delete them)

We can confirm that our guest has no route to 8.8.8.8 with:

# Make sure you're in your guest
machinectl login debian

ip route get 8.8.8.8
RTNETLINK answers: Network is unreachable
What about SNAT/DNAT etc?- Not here! Remember our goals here are to have each container appear as though it is publicly routable (which it will be)

What we need to do is configure the guests default gateway so that the routing table of the systemd container knows where to send packets not on its own network, and critically which src IP to give those packet- the src IP of the packers from our guests must be our floating IP, not the hosts IP. In other words, we don't want to masquerade our guests IP address at all.

Configure the guest default gateway with the correct via route via hosts IP
# Logged in to your guest container:

 machinectl login debian

# Delete any existing default route
ip route del default

# Add a static route to your host's IP
# This is the IP address assigned to your hosts eth0 interface
ip route add 203.0.113.1/32 dev host0

# Critically, now you can add a default route, which is mindful
# of your guest having to go via your host's IP for its default route
ip route add default via 203.0.113.1

# Validate you now have a route to 8.8.8.8
ip route get 8.8.8.8
8.8.8.8 via 203.0.113.1 dev host0 src 203.0.113.2 uid 0 
    cache

The order of adding a static route to your hosts IP before the default route is critical here, if you try to add ip route add default via 203.0.113.1 first, then you'll get "Error: Nexthop has invalid gateway." because, quite rightly, the kernel complains there's no route for 203.0.113.1 yet.

You'll now be able to ping from your container to the wider internet, and vice versa you can ping your containers public IP address 🚀! What just happened?*****

On your guest you:

  • Added a static root to your VPS's IP address 203.0.113.1 , which is reachable from your guest over its host0 interface (ip route add 203.0.113.1 dev host0)
  • Added a default gateway to your guest, specifying that not only must gateway destined packets go over host0, they have to go via the IP address of your host 203.0.113.1. But wait! When your packets do this, they are still sourced with your containers IP
ip route get 8.8.8.8
8.8.8.8 via 203.0.113.1 dev host0 src 203.0.113.2 uid 0 
    cache
(From our guest) this confirms that packets going out from our container via the gateway will be sourced with the containers IP address, and not masqueraded with the host's IP address. Great!

Lets ping to confirm we can:

  1. Ping from inside our container to an external network
  2. Ping from the internet to our container
Confirm we can ping from inside our container to an external network (1)
root@debian:~# ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_seq=1 ttl=57 time=9.08 ms
64 bytes from 8.8.8.8: icmp_seq=2 ttl=57 time=8.57 ms
64 bytes from 8.8.8.8: icmp_seq=3 ttl=57 time=8.49 ms
^C
--- 8.8.8.8 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2004ms
rtt min/avg/max/mdev = 8.492/8.713/9.082/0.262 ms
Woopie we can ping from inside our container,via our virtual Ethernet link -> to the bridge and get a reply back.
Confirm we can ping from the internet to our container (2)
$ ping 203.0.113.2
PING 203.0.113.2 (203.0.113.2) 56(84) bytes of data.
64 bytes from 203.0.113.2: icmp_seq=1 ttl=50 time=47.2 ms
64 bytes from 203.0.113.2: icmp_seq=2 ttl=50 time=49.1 ms
64 bytes from 203.0.113.2: icmp_seq=3 ttl=50 time=47.5 ms
^C
--- 203.0.113.2 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2003ms
rtt min/avg/max/mdev = 47.181/47.942/49.111/0.838 ms
Yes we can! 🚀🚀🚀


*****Jamie Prince sums up this gateway approach beautifully, we've simply extended to concept to the context of nspawn containers:

"Direct connection" can be part of a Layer 2 bridge, as I found, very useful for assigning spare public IPs to internal devices without having to resort to NAT nastiness and complexity.."
- Jamie Prince

Make network configuration static

At the moment we're having to manually perform the guest IP addresses using the IP command- but we don't need to. We can enable and start the systemd-networkd service to automatically configure the guest IP address for us.

machinectl login debian

vim /etc/systemd/network/80-container-host0.network

With the following:

[Match]
Virtualization=container
Name=host0

[Network]
# Yout floating IP
Address=203.0.113.2/32
DHCP=no
LinkLocalAddressing=yes
LLDP=yes
EmitLLDP=customer-bridge

[Route]
# Your servers main IP
Gateway=203.0.113.1/32
GatewayOnLink=yes


[DHCP]
UseTimezone=yes
Note: The LLDP and UseTimezone are not needed, they are preparations for IPV6

Now, when you reboot your server- the container is automatically started.
To apply addressing and gateway config without needing to use the ip command, reload the systemd daemon and restart systemd-networkd******

# Make sure you're still on the gues
machinectl login debian

# Apply the guest addressing:

systemctl daemon-reload
systemctl restart systemd-networkd

# Verify your gateway & route has been configured for you:

# Verify IP address 
ip -c a

# Verify gateway
ip route show

# Verify can ping externally
ping 8.8.8.8

******I've most certainly got the systemd drop-in configuration process wrong here, the goal is for systemd to automatically pick up these settings without having to reload/restart systemd-networkd when the container starts. See systemd.network.html config.

We did it! What did we do exactly?

We've basically configured a lightweight container to have its own publicly rotatable IP address. Big deal. But wow

Let's revisit the goals:

  • Maximising frugality.

    • ✅ CHECK this entire project raw costs were less than $5 (and I learnt a lot of valuable new insight into systemd-nspawn, networking and alike

  • Not sacrificed reachability

    • every service must be directly contactable via its public ip address
    • yes really, no SNAT/DNAT masquarding here
    • ✅ CHECK we can publically reach and there's no SNAT/DNAT going on which prepares us for IPv6 nicely too

  • Reproducibility - In the 'DevOps' world we're very familiar with infrastructure as code (buzzword for: I can audit, change, and rebuild my application workloads easily if I have to). The same is useful for say, testing network topology generation

    • Could do better, another person to follow this , and then codify coming soon!

Thoughts:

  • We didn't have to pull in and install any (I'm struggling for words here) 'external' packages
  • This is completely open source and learning purely on stable Linux kernel networking with minimal dependencies

Questions:

  • How useful is this for you?
  • How dependable is systemd-nspawn?
  • How maintained is systemd-nspawn?
  • How repeatable is this?
  • We're using proxy_arp , this likely means other containers will see, and may more easily answer other nodes arp requests, and that's a problem because it breaks down some isolation between the containers. To be tested to confirm.

What's next?

Doing the same, with IPv6!

Adding IPv6 support

At the beginning of this article we claimed that "IPV6 is the far more natural fit because when you buy/rent a server you typically get an IPv6 /60 allocated to you for free (cost savings) which gives you 295,147,905,179,352,825,856 globally routable addresses."

So let's see if that's true! Now we're going to to the same thing:

  • Get a publicly (globally) routable IPV6 address to your nspawn container(s)

What are the benefits of this?

  • Cost- we can trivially use our assigned /60 IPv6 assigned address to either carve out additional subnets (e.g. a subnet per customer) or, keep it simple and use the whole remaining /60 range to give each nspawn container 1 publicly routable IPv6 address (we'll start with that).

To achieve the above, much of the same concepts we used in IPv4 apply to IPv6 but with key differences:

  1. There's no Address Resolution Protocol (arp) in IPv6, instead there's Neighbour Discovery for IP version 6 (IPv6) (see RFC 4861) tldr: This is the new way that nodes find the hardware address (Mac address) of other nodes)
  2. Therefore there's no proxy_arp, instead we need to proxy the ICMPv6 neighbour solicitations otherwise our containers won't be able to 'find' the link layer address of our IPv6 gateway.
IPv6 by comparison to IPv4 is very chatty and really wants to auto configure, auto announce and even auto migrate connections between networks. It's very cool if you're a networking nerd, and you'll only keep hearing more about it as time goes on- finally...

See IPv6 Only Web Services Becoming Commonplace

1 - Create a new server with an IPv6 block assigned

Create a new server (which gives you a /60 IPv6 address).

git clone https://github.com/KarmaComputing/nspawn-systemd-nspawn-containers.git

cd nspawn-systemd-nspawn-containers


A thing of beauty? Layers. Lots of them.

No I do not understand all of this either, but stand back for a moment and appreciate the beauty of decades of network packet abstractions people have built for describing, controlling, shaping and manipulating datagrams through a network stack. Image credit: Packet flow paths in the Linux kernel

**** As you recall, the virtual Ethernet link vb-debian is created automatically for us at the host side by systemd-nspawn due to the Bridge=br0 setting we have in our /etc/systemd/nspawn/debian.nspawn config. systemd-nspawn also created the host0 interface for us on the guest side, connecting the two.

Helpful tools to debug routes

Get the default gateway using the ip command:
ip route show default
Show me all the routes in the routing table:
ip route show all
Show me which route a packet would take using ip
This command gets a single route to a destination and prints its contents exactly as the kernel sees it.
- man ip route

For example, this shows using ip route get to show the route to the host ip address from the guests perspective. 

# (Logged into the guest container)
ip route get 203.0.113.1
root@debian:~# ip route get 203.0.113.1
203.0.113.1 dev host0 src 203.0.113.2 uid 0 
    cache

Even though we can ping from the container to the host, we cannot currently ping from the container to 8.8.8.8 or anywhere else external.

Notes on tcpdump how to filter by ip address src / dst

How to flush arp table using the ip command

(How to clear the ARP table using the ip command)

ip -s -s neigh flush all
How to filter out ip addresses from tcpdump

Example, put tcpdump in verbose mode, but filter out ip address 10.0.0.1 and 10.0.0.2.

tcpdump -vv -n 'not (dst net 10.0.0.1/32) and not (src net 10.0.0.2/32)'

Further reading

Links & References

The general format of these links are "what I searched" -> Link to resource which was read to help deepen my understanding. Links visited during research & debugging.

Proxy ARP

Note that for IPv6 there is no arp, it's now called Neighbour Discovery Protocol which is part of ICMPv6. The IPv4 parts of this guide needed proxy_arp configured, and similarly for IPv6 we need to proxy NDP correctly.
Proxying IPv6 Neighbor Discovery

A thank you: Kevin P. Fleming's research and open source contributions were instrumental for understanding and debugging the nuances between networkd, nspawn and the proxying required for IPv6 neighbor discovery to work within the context of virtual ethernet links attached to a bridge.
Specifically highlighting that ip neigh show proxy may be used to verify an IPv6 NDP proxy is configured or not to your bridge interface, and raised a pr for network: Delay addition of IPv6 Proxy NDP addresses which appears to be included in Debian 11 now.

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