After setting up my mail server I already set up SPF, DKIM, DMARC and MTA-STS but was not yet sure about how to deal with DANE.

DANE uses a TLSA entry on the DNS server to publish a services certificate or key. This means that a client can verify that it is talking to the correct server without relying on any kind of CA. All this is based on DNSSEC which ensures the authenticity and data integrity of the DNS entries.

While this concept in general is pretty neat, it has the requirement of DNS entries to be updated when a certificate or key changes. Additionally just simply updating an entry will no be enough, since other DNS servers still might have the old key cached. Therefore a more sophisticated rollover mechanism would be required. Due to the short life of Let’s Encrypt certificates this is even more relevant.

Luckily Certbot provides the option --reuse-key to circumvent the requirement of updating the DNS entries by reusing the existing key. Therefore a TLSA entry can be generated from the current public key and will not require updating as well since the key does not change.

For generation of an according TLSA entry Shumon Huque has a very handy TLSA Record Generator on his website. The entry in this case must be for usage DANE-EE with selector 1 - SPKI. I use a SHA-256 hash and got following result:

_25._tcp.vs.senvang.org. IN TLSA 3 1 1 (
                  5634d5e1ce5f1e4a8ab25cd8335c97ab76e1215e09c157568e5c9a3dc39a
                  a491
)

Alternatively you can generate the hash with openssl:

openssl x509 -in cert.pem -pubkey -noout | openssl ec -pubin --outform der | sha256sum

Obviously this does not resolve the need for a clean rollover when a key change happens, but it massively reduces the frequency and therefore the risk and it gives me time to think about how to get this automated.

It’s a long time since I wrote something here so in this time things happened in the background. Here a quick update:

• I moved the server to a more powerful one (VPS 1000) - still from Netcup. The main reason for this upgrade was my decision to switch to Gentoo Linux on the server as well after my desktop computer is running flawlessly with it since years now and I wanted to make this switch before I started setting up a mail server.

• I got following services running on Gentoo now:

  1. Certbot to get certificates from Let’s Encrypt
  2. Nginx as web server / proxy
  3. Radicale as CalDAV / CardDAV server
  4. Git via ssh access (I keep it simple and do not run Gitea anymore)
  5. Postfix / Dovecot as mail server
  6. Wireguard as VPN server

• The mail server is setup with SPF, DKIM, DMARC and MTA-STS. Regarding DANE I am still investigating if I can/want to set this up with the frequently changing Let’s Encrypt certificates.

So what the heck is all that? Here in short:

• SPF: Specifies the mail servers allowed to send mail for the domain by publishing an entry in the DNS
• DKIM: Sign outgoing mail and publish the public key in the DNS, so the receipient can verify the signature
• DMARC: Publish a DMARC DNS entry with instructions on how to handle mail which doesn’t pass SPF/DKIM checks
• MTA-STS: A combination of DNS and HTTPS to instruct SMTP servers to encrypt communication and verify the used certificate
• DANE: A DNS entry containing the server certificate (usually just a hash of it) and therefore allowing the communication partner to compare the servers certificate against the one published in DNS

That’s what happened on here in the last months. More to come …

Since years I am running my own CalDAV / CardDAV server to synchronize my calendars and address books. I use Radicale for this and on my Uberspace it was installed via Python’s pip, but now it was time to move this to my own server too and I chose to install the package directly from the Devuan repository via apt-get install radicale.

The installation via apt-get just does the installation but some additional configuration was needed to set everything up.

At first I modified the configuration file /etc/radicale/config and did set
type = htpasswd and htpasswd_encryption = bcrypt (both in the [auth] section).

Then I created a new htpasswd file with my user:
htpasswd -c -B /etc/radicale/users <user>

Finally I started the service and enabled it to be started automatically:
/etc/init.d/radicale start
update-rc.d radicale enable

After this the radicale server is bound to localhost and listening on port 5232. For being accessible from the outside the following configuration must be added to the Nginx configuration (and Nginx being restarted once):

location /radicale/ { # The trailing / is important!
      proxy_pass        http://localhost:5232/; # The / is important!
      proxy_set_header  X-Script-Name /radicale;
      proxy_set_header  X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_set_header  Host $http_host;
      proxy_pass_header Authorization;
}

Now I only had to copy contact and calendar entries from my Uberspace instance. Done!

I started some investigation about how to run Linux Containers on Devuan Linux 5.0 (Daedalus), as I run it on my server. For this I first did set up a local VM which replicates my virtual server. I use qemu for this and created a script called vm‑netcup which is part of my dotfiles repository and sets qemu parameters accordingly.

Installation via apt-get install lxc works as expected. So far so good - but things should not go that smoothly for much longer.

cgroups

LXC relies on cgroups, a Linux kernel feature for isolation, limitation and accounting of resource usage of processes. On distributions using systemd this is set up by systemd, but with Devuan I chose to not use systemd.

You guessed it: lxc-checkconfig and ls /sys/fs/cgroup show that cgroups are not set up by default (at least not with my minimal installation). LXC can work with cgroups v1 but cgroups v2 provide a cleaner, unified hierarchy and therefore are my preferred way.

To mount the cgroup v2 filesystem at boot time, I simply created an additional entry in the /etc/fstab file:
none /sys/fs/cgroup cgroup2 defaults 0 0

In case you want to use cgroups v1 you can install the cgroupfs-mount package which installs a service to perform the required mounts at boot time.

That’s all that is required to set up cgroups for LXC.

unpriviledged vs. priviledged containers

In short: We want unpriviledged containers whenever possible. Those map user ids inside the container to a different range on the host and therefore are the safest option. For example user id 0 (root) in an unpriviledged container would be mapped to user id 100000 on the host. A user id 0 (root) on a priviledged container on the other hand also would be root on the host.

/etc/subuid and /etc/subgid contain the ranges for mapping uid and gid on the host, but those values also need to be reflected in the lxc configuration as described below.

configuration

For configuration LXC distinguishes between system and container configuration. See the according man pages lxc.container.conf and lxc.system.conf for further information.

By default LXC on my system used ~/.local/share/lxc/ for container storage. This resulted in LXC complaining about the users home directory not having x permission for the root user of the container and starting the container failed.

Therefore I decided to move the container storage to /var/lib/lxc/<user>. This directory needs to be created as root and owernship as well as permissions need to be set as follows:
chown <user>:<group> /var/lib/lxc/<user>
chmod 711 /var/lib/lxc/<user>

Once this is done we can continue as regular user. We create a file .config/lxc/lxc.conf and configure the container storage path in it:
lxc.lxcpath = /var/lib/lxc/<user>

/etc/lxc/default.conf contains the system wide container default configuration. We copy this file to ~/.config/lxc/default.conf and modify it to create our user specific defaults.

Be aware that this file only contains the default configuration used during creation of a new container. Once created every container has its own configuration file in its according folder within the container storage path.

We now add the following lines for configuration of the uid and gid mapping. The values used here must match the values defined in /etc/subuid and /etc/subgid for the according user:
lxc.idmap = u 0 100000 65536
lxc.idmap = g 0 100000 65536

Additionaly we change the AppArmor profile as follows:
lxc.apparmor.profile = unconfined

creating a new container

For creation of a new container we use the lxc-create command. lxc-create -t download -n <container-name> is a good start and provides an interactive way to create a new container. In my case I want to install the 3.18 release for the amd64 architecture of the alpine distribution.

This all also can be packed into the command directly:
lxc-create -t download -n <container-name> -- --dist alpine --release 3.18 --arch amd64

network

With all this set up the container still fails to start because it cannot set up its network. By default unprivileged containers cannot use any networking. We need to create an additional configuration file /etc/lxc/lxc-usernet as root. This file must have an entry to allow the user to add veth devices - up to 16 in in my case - to the lxcbr0 bridge:
<user> veth lxcbr0 16

using the container

To start an already existing container the command lxc-start -n <container-name> is used. With everything I described above done the container does start without issues. Previously appearing tmpfs: Bad value for 'uid' error message were related to the Devuan container image used. The errors disappeared after I switched to using the Alpine Linux image.

Once the container is running we can start a process in the container with lxc-attach. For example lxc-attach -n <container-name> bash will provide shell access to the container.

For stopping the container again we use lxc-stop -n <container-name>.

In case the container isn’t running we directly can run a command in it with lxc-execute. For example lxc-execute -n <container-name> bash will provide shell access as before.

conclusion

I am not done with this topic yet but for now this is a good foundation and a playground to further investigate if I want to run containers on the server. I might update this post at some point if I have any points worth mentioning.

Especially the network setup was barely touched here and could be a topic for a separate post at some point. For now that’s it …

After running my own server for some years in the past I did move on to using an Uberspace. Uberspace as a hoster is great and offers a lot of flexibility but it has its limitations anyway (for example it is not possible to run a VPN server). I am planning to run a VPN server and also like the admin work overall. So I ordered a small virtual server (VPS 200 G10s) from Netcup again.

Why did I choose Netcup?

The answer to that is easy. It’s cheap and I did have some good experience with it already. For 3.25 Euro/month I get a VPS with 2vCPUs, 2GB RAM and 40GB SSD. For the virtualization they use KVM and via the web interface I can mount any ISO file and install whatever OS I like. The drawback of their cheap virtual servers is the quite low minimum availability of just 99.6% but for my use case that’s not an issue.

Which OS did I install?

After initially even considering Gentoo Linux I quickly dropped that idea due to the limitations of the virtual hardware. I then - after some research - did a kind of test installation of Alpine Linux. It was the first time ever I used Alpine and I must say that I am pretty impressed with it - especially due to its low footprint. Anyway I decided to stick with something I know better and trust a bit more in the long term.

I installed Devuan Linux - Devuan Daedalus 5.0. For those who are not aware, Devuan is a fork of Debian Linux which does not use systemd and therfore I can count on the great selection of software available for Debian while still using the good old sysvinit.

As always I did a minimal installation and add what is required for my needs from there. My base installation with Nginx, Certbot and unattended upgrades uses about 60MB of RAM. A nice small footprint as well even if its not as small as Alpine Linux (but that’s expected).

Further plans?

With Nginx and Certbot up and running I already moved the website on to the new server. Additionally I want to install the following components:

• Gitea git server, bug tracker, etc.
• Radicale CalDAV and CardDAV server
• Mail server (probably Postfix and Dovecot)
• WireGuard VPN server
• … and eventually more

Regarding all this I am not in a hurry. For now I am using the Uberspace and my virtual server in parallel and before installing any of those components I want to do some research about Linux Containers. So far I do not have experience with containers of any kind and I am not a huge fan of Docker and its approach of isolating single applications. Linux Containers on the other hand are available on any Linux system, are OS-level virtualization and do not rely on infrastructure controlled by a single company. So my idea is to isolate some parts of my installation into Linux Containers (probably using Devuan images as well). Therefore I should gain some flexibility when updating the main server OS and be able to move and update some parts independently.

In any case containers are an interesting topic and digging deeper into it will be worth it …