Zchunk update

Putting it all together

Zchunk 1.0

Eight months ago, I started working on zchunk, and it’s now almost ready for its 1.0 release. Once zchunk 1.0 is released, we will offer a stability guarantee. Only additions to the API will be allowed, and the ABI will always be backwards-compatible. All files created by older versions of zchunk will be able to be opened by new versions of zchunk, and files created by newer versions of zchunk will be able to be opened by the old versions.

There is one important caveat to the last item: the zchunk format supports mandatory feature flags. It is possible that an older version of zchunk doesn’t support a certain feature flag, and, if so, that version of zchunk will be unable to open files that contain the new flag.

As of version 0.9.12, zchunk also supports optional feature flags that provide extra information about the zchunk file. If a newer version of zchunk sets an optional flag, and the file is read by an older version that doesn’t recognize that particular flag, it will ignore the optional flag data and continue reading the file. This feature was requested at Flock this year, and I’m glad it will be available when zchunk 1.0 is released.

Coverity coverage and CI

In September, we managed to get Coverity to scan zchunk as part of its open source project support, and managed to eliminate 15 potential bugs that Coverity identified. New releases will continue to be scanned by Coverity. Thanks, Stephen Gallagher, for the suggestion.

I’ve also setup a jenkins instance for continuous integration. Every commit is run through a series of tests to verify that we’re not breaking anything.

Fedora metadata integration

One of the features that we hoped to get into Fedora 29 was Zchunk metadata, creating zchunk-compressed metadata for DNF. It was a stretch, and we were unable to get the zchunk patches reviewed upstream in time for Fedora 29’s release. The goal now is to get the patches accepted in time for Fedora 30.

We have patches for libdnf, librepo, createrepo_c and libsolv. In a point for inter-distribution cooperation, Michael Schroeder from SUSE merged the libsolv patch first, but the other patches are still awaiting review. Now that Fedora 29 is out the door, I’m hoping we’ll be able to get this done quickly.

Unless we hit major problems, I anticipate that Zchunk metadata will be a Fedora 30 feature. A huge thank you to Neal Gompa and Igor Gnatenko for their help with this.

Future features

There are a couple of ideas that I have bouncing around in my head for a couple of other places where zchunk might be useful, and I figured I should commit them to (digital) paper here.

zchunk-compressed RPMs

Deltarpms offer amazing space savings, but are very limited. In order to take advantage of a deltarpm, you must not only want a specific new version of the RPM, but also have a specific old version of the RPM installed. Because zchunk doesn’t look at any old versions at compression time, the same zchunk-compressed RPM can be used whatever the old version you have installed (or even if you have no old version installed).

To make this work, we would need to create some new feature flags in zchunk, and it would require some changes in the RPM format itself (the third rail, I know), but it is possible and could provide us with significant download savings without having to generate deltarpms at all.

zchunk-compressed container images

This would require that container registries and container management systems both support zchunk, but would allow pull operations to be significantly smaller. I don’t know much about how podman or docker actually pull their images, but I’ve been pulling updated images over 3G and it’s not much fun.

Other ideas?

We would welcome ports of zchunk to other languages. At Flock, a few people were keen on doing a Rust implementation, and I think that’s a brilliant idea.

If you have any other ideas for new features (or find a bug), please create an issue. I won’t guarantee that we’ll implement your new ideas, but we’ll take a look at them and see if they’re feasible. Obviously, pull requests greatly increase the chances that your idea will become part of zchunk.

Using Wireguard, DigitalOcean and firewalld to give your roaming computer a static IP

"Have fun storming the castle"

While we’re waiting to figure out where in Ireland we’ll be living long-term, we’re currently using mobile broadband. I’d like to be able to ssh into my home network from outside, but when I looked into setting up DDNS, I found that I don’t even have a public IP address. My mobile dongle says that its IP address is in the range, so there’s no way to direct traffic to it from the internet.

After my first foray in using wireguard, I figured it should be a good fit to give myself ssh access to the home network. I could have just forwarded a specific port to my home network, but I prefer not to have to deal with non-standard ports. And, as this site is hosted on DigitalOcean, the natural next step was to see if I could pull it off with them.

It took a couple of hours (mainly dealing with firewalld as I’m much more comfortable with iptables), but I got there in the end, and I can now ssh into my home network (using key-based authentication, of course).

This guide will walk you through the process, step by step. It assumes you have a DigitalOcean droplet running, that you’re using Fedora on both the roaming computer and the DigitalOcean droplet, and that you have firewalld on both.

Step 1 - Give your droplet a Floating IP

  1. Log into DigitalOcean
  2. Go to Networking, Floating IPs
  3. Select the droplet you want to use as your gateway and click Assign Floating IP:
  4. Check to see what address you got. This will be your new static IP address

Do note that the floating IP will not appear when you run ip addr in your droplet, but there will be a local IP (most likely in the range) that all the floating IP’s traffic will be forwarded to.

Step 2 - Setup wireguard between DigitalOcean and your roaming computer

On both the DigitalOcean droplet and your roaming computer, install wireguard. I want to quickly note that it’s not available from the official Fedora repositories because the kernel module hasn’t been merged into the mainline kernel yet. It has just become available in the RPM Fusion Free testing repositories. This guide assumes you’ve already configured the RPM Fusion Free repository on your system.

$ sudo dnf --enablerepo=rpmfusion-free-updates-testing install wireguard

Now, it’s time to set wireguard up on the DigitalOcean droplet.

  1. We’ll use the wg-quick service to set everything up, so put the following in /etc/wireguard/wgnet0.conf:

    Address =
    SaveConfig = true
    ListenPort = 51820
    PrivateKey = 
    PublicKey = 
    PresharedKey = 
    AllowedIPs =

  2. Generate a private key and insert it into the line that says PrivateKey:

    $ wg genkey

  3. Generate a pre-shared key (which should protect your VPN from the omnipresent quantum computers that are undoubtedly listening in) and insert it into the line that says PresharedKey:

    $ wg genpsk

  4. Figure out the public key from the private key you generated in step 2:

    $ echo {private key} | wg pubkey

Now for the roaming computer. Because the roaming computer initiates the connection to the DigitalOcean droplet, we need to setup a keepalive so traffic that starts at the droplet (which is the whole point of this exercise) will get back to the roaming computer in 10 seconds max if the link is completely idle.

  1. Put the following in /etc/wireguard/wgnet0.conf, substituting your droplet’s hostname or public IP (but not the floating IP) for www.example.com:

    Address =
    PrivateKey = 
    PublicKey = 
    PresharedKey = 
    AllowedIPs =
    Endpoint = www.example.com:51820
    PersistentKeepalive = 10

  2. Insert the public key we extracted in step 4 of the droplet configuration into the line that says PublicKey

  3. Insert the pre-shared key generated in step 3 of the droplet configuration into the line that says PresharedKey

  4. Generate a private key and insert it into the line that says PrivateKey:

    $ wg genkey

  5. Now, extract the public key from the private key you just generated, go back to the droplet and insert it into the line that says PublicKey:

    $ echo {private key} | wg pubkey

At this point, you should have two configuration files with everything filled in.

Go back to the DigitalOcean droplet now and get the service running.

  1. First open the service port in the firewall:

    $ sudo firewall-cmd --add-port=51820/udp --permanent
    $ sudo firewall-cmd --reload

  2. Enable and start the service

    $ sudo systemctl enable [email protected]
    $ sudo systemctl start [email protected]

Assuming you haven’t hit any errors, you should now have wireguard running on your droplet

On your roaming computer, get the service running.

  1. Enable and start the service

    $ sudo systemctl enable [email protected]
    $ sudo systemctl start [email protected]

Test the service by pinging from your roaming computer and from your DigitalOcean droplet. Assuming both work, congratulations, you’ve now setup a VPN between your two systems!

Step 3 - Forward ports from the roaming IP to your roaming computer

So now you have a floating IP on your DigitalOcean droplet and a VPN between the droplet and your roaming computer, so the final step is to put them together.

This process would be easier if DigitalOcean used different interfaces for the public IP and the floating IP, but they don’t, so we have to use firewalld’s rich rules to make this work.

First, figure out which ports you want to open up. We’ll just open the ssh port in this example.

On the droplet, do the following steps:

  1. Verify that you can ssh into the roaming computer:

    $ ssh [email protected]

    If this step fails, you’ve made a mistake in your ssh configuration, and fixing it is beyond the scope of this guide.

  2. Figure out the private address that traffic to the floating IP is coming to:

    $ ip addr show eth0

    Look for the line that begins with inet 10.x.x.x. 10.x.x.x is the IP address you’re looking for. On my system it’s, so that’s what I’m going to use for the rest of the example.

  3. Turn on masquerading for any traffic going over the VPN:

    $ sudo firewall-cmd --add-rich-rule "rule family=ipv4 destination address= masquerade" --permanent

  4. Forward the ports from the floating IP to the VPN, making sure to substitute the IP address you found in step 2 for

    $ sudo firewall-cmd --add-rich-rule "rule family=ipv4 destination address= forward-port port=22 protocol=tcp to-port=22 to-addr=" --permanent

  5. Repeat step four for any other ports you want open.

  6. Reload the firewall rules:

    $ sudo firewall-cmd --reload

VoilĂ ! When you ssh into your floating IP, it should be forwarded to your roaming computer, while your droplet is still accessible on its public IP.

Security notice: you do not want to allow root to ssh into your system using a password. That’s just begging for someone to guess the password and get root access on your roaming computer. Setup SSH keys, and, at the minimum, make sure that root can only log in with an SSH key

One of the benefits of wireguard is that the client (in this case, the roaming computer) will automatically reconnect as it moves from network to network, so your roaming computer will automatically be available at the floating IP no matter where it is as long as it’s on the Internet.

Do note that DigitalOcean doesn’t seem to allow you to connect more than one floating IP to a droplet at a time, so you’ll be limited to one forwarded IP per droplet.

Enjoy your new static IP. If you run into any problems, please leave a message in the comments below.