Manage business documents with OpenAS2 on Fedora

Business documents often require special handling. Enter Electronic Document Interchange, or EDI. EDI is more than simply transferring files using email or http (or ftp), because these are documents like orders and invoices. When you send an invoice, you want to be sure that:

1. It goes to the right destination, and is not intercepted by competitors.
2. Your invoice cannot be forged by a 3rd party.
3. Your customer can’t claim in court that they never got the invoice.

The first two goals can be accomplished by HTTPS or email with S/MIME, and in some situations, a simple HTTPS POST to a web API is sufficient. What EDI adds is the last part.

This article does not cover the messy topic of formats for the files exchanged. Even when using a standardized format like ANSI or EDIFACT, it is ultimately up to the business partners. It is not uncommon for business partners to use an ad-hoc CSV file format. This article shows you how to configure Fedora to send and receive in an EDI setup.

Centralized EDI

The traditional solution is to use a Value Added Network, or VAN. The VAN is a central hub that transfers documents between their customers. Most importantly, it keeps a secure record of the documents exchanged that can be used as evidence in disputes. The VAN can use different transfer protocols for each of its customers

AS Protocols and MDN

The AS protocols are a specification for adding a digital signature with optional encryption to an electronic document. What it adds over HTTPS or S/MIME is the Message Disposition Notification, or MDN. The MDN is a signed and dated response that says, in essence, “We got your invoice.” It uses a secure hash to identify the specific document received. This addresses point #3 without involving a third party.

The AS2 protocol uses HTTP or HTTPS for transport. Other AS protocols target FTP and SMTP. AS2 is used by companies big and small to avoid depending on (and paying) a VAN.


OpenAS2 is an open source Java implemention of the AS2 protocol. It is available in Fedora since 28, and installed with:

$ sudo dnf install openas2
$ cd /etc/openas2

Configuration is done with a text editor, and the config files are in XML. The first order of business before starting OpenAS2 is to change the factory passwords.

Edit /etc/openas2/config.xml and search for ChangeMe. Change those passwords. The default password on the certificate store is testas2, but that doesn’t matter much as anyone who can read the certificate store can read config.xml and get the password.

What to share with AS2 partners

There are 3 things you will exchange with an AS2 peer.


Don’t bother looking up the official AS2 standard for legal AS2 IDs. While OpenAS2 implements the standard, your partners will likely be using a proprietary product which doesn’t. While AS2 allows much longer IDs, many implementations break with more than 16 characters. Using otherwise legal AS2 ID chars like ‘:’ that can appear as path separators on a proprietary OS is also a problem. Restrict your AS2 ID to upper and lower case alpha, digits, and ‘_’ with no more than 16 characters.

SSL certificate

For real use, you will want to generate a certificate with SHA256 and RSA. OpenAS2 ships with two factory certs to play with. Don’t use these for anything real, obviously. The certificate file is in PKCS12 format. Java ships with keytool which can maintain your PKCS12 “keystore,” as Java calls it. This article skips using openssl to generate keys and certificates. Simply note that sudo keytool -list -keystore as2_certs.p12 will list the two factory practice certs.


This is an HTTP URL that will access your OpenAS2 instance. HTTPS is also supported, but is redundant. To use it you have to uncomment the https module configuration in config.xml, and supply a certificate signed by a public CA. This requires another article and is entirely unnecessary here.

By default, OpenAS2 listens on 10080 for HTTP and 10443 for HTTPS. OpenAS2 can talk to itself, so it ships with two partnerships using http://localhost:10080 as the AS2 URL. If you don’t find this a convincing demo, and can install a second instance (on a VM, for instance), you can use private IPs for the AS2 URLs. Or install Cjdns to get IPv6 mesh addresses that can be used anywhere, resulting in AS2 URLs like http://[fcbf:fc54:e597:7354:8250:2b2e:95e6:d6ba]:10080.

Most businesses will also want a list of IPs to add to their firewall. This is actually bad practice. An AS2 server has the same security risk as a web server, meaning you should isolate it in a VM or container. Also, the difficulty of keeping mutual lists of IPs up to date grows with the list of partners. The AS2 server rejects requests not signed by a configured partner.

OpenAS2 Partners

With that in mind, open partnerships.xml in your editor. At the top is a list of “partners.” Each partner has a name (referenced by the partnerships below as “sender” or “receiver”), AS2 ID, certificate, and email. You need a partner definition for yourself and those you exchange documents with. You can define multiple partners for yourself. OpenAS2 ships with two partners, OpenAS2A and OpenAS2B, which you’ll use to send a test document.

OpenAS2 Partnerships

Next is a list of “partnerships,” one for each direction. Each partnership configuration includes the sender, receiver, and the AS2 URL used to send the documents. By default, partnerships use synchronous MDN. The MDN is returned on the same HTTP transaction. You could uncomment the as2_receipt_option for asynchronous MDN, which is sent some time later. Use synchronous MDN whenever possible, as tracking pending MDNs adds complexity to your application.

The other partnership options select encryption, signature hash, and other protocol options. A fully implemented AS2 receiver can handle any combination of options, but AS2 partners may have incomplete implementations or policy requirements. For example, DES3 is a comparatively weak encryption algorithm, and may not be acceptable. It is the default because it is almost universally implemented.

If you went to the trouble to set up a second physical or virtual machine for this test, designate one as OpenAS2A and the other as OpenAS2B. Modify the as2_url on the OpenAS2A-to-OpenAS2B partnership to use the IP (or hostname) of OpenAS2B, and vice versa for the OpenAS2B-to-OpenAS2A partnership. Unless they are using the FedoraWorkstation firewall profile, on both machines you’ll need:

# sudo firewall-cmd --zone=public --add-port=10080/tcp

Now start the openas2 service (on both machines if needed):

# sudo systemctl start openas2

Resetting the MDN password

This initializes the MDN log database with the factory password, not the one you changed it to. This is a packaging bug to be fixed in the next release. To avoid frustration, here’s how to change the h2 database password:

$ sudo systemctl stop openas2
$ cat >h2passwd <<'DONE'
java -cp "$AS2DIR"/lib/h2* \
-url jdbc:h2:"$AS2DIR"/db/openas2 \
-user sa -password "$1" <<EOF
alter user sa set password '$2';
$ sudo sh h2passwd ChangeMe yournewpasswordsetabove
$ sudo systemctl start openas2

Testing the setup

With that out of the way, let’s send a document. Assuming you are on OpenAS2A machine:

$ cat >testdoc <<'DONE'
This is not a real EDI format, but is nevertheless a document.
$ sudo chown openas2 testdoc
$ sudo mv testdoc /var/spool/openas2/toOpenAS2B
$ sudo journalctl -f -u openas2
... log output of sending file, Control-C to stop following log

OpenAS2 does not send a document until it is writable by the openas2 user or group. As a consequence, your actual business application will copy, or generate in place, the document. Then it changes the group or permissions to send it on its way, to avoid sending a partial document.

Now, on the OpenAS2B machine, /var/spool/openas2/OpenAS2A_OID-OpenAS2B_OID/inbox shows the message received. That should get you started!

Photo by Beatriz Pérez Moya on Unsplash.

Check storage performance with dd

This article includes some example commands to show you how to get a rough estimate of hard drive and RAID array performance using the dd command. Accurate measurements would have to take into account things like write amplification and system call overhead, which this guide does not. For a tool that might give more accurate results, you might want to consider using hdparm.

To factor out performance issues related to the file system, these examples show how to test the performance of your drives and arrays at the block level by reading and writing directly to/from their block devices. WARNING: The write tests will destroy any data on the block devices against which they are run. Do not run them against any device that contains data you want to keep!

Four tests

Below are four example dd commands that can be used to test the performance of a block device:

  1. One process reading from $MY_DISK:
    # dd if=$MY_DISK of=/dev/null bs=1MiB count=200 iflag=nocache
  2. One process writing to $MY_DISK:
    # dd if=/dev/zero of=$MY_DISK bs=1MiB count=200 oflag=direct
  3. Two processes reading concurrently from $MY_DISK:
    # (dd if=$MY_DISK of=/dev/null bs=1MiB count=200 iflag=nocache &); (dd if=$MY_DISK of=/dev/null bs=1MiB count=200 iflag=nocache skip=200 &)
  4. Two processes writing concurrently to $MY_DISK:
    # (dd if=/dev/zero of=$MY_DISK bs=1MiB count=200 oflag=direct &); (dd if=/dev/zero of=$MY_DISK bs=1MiB count=200 oflag=direct skip=200 &)

– The iflag=nocache and oflag=direct parameters are important when performing the read and write tests (respectively) because without them the dd command will sometimes show the resulting speed of transferring the data to/from RAM rather than the hard drive.

– The values for the bs and count parameters are somewhat arbitrary and what I have chosen should be large enough to provide a decent average in most cases for current hardware.

– The null and zero devices are used for the destination and source (respectively) in the read and write tests because they are fast enough that they will not be the limiting factor in the performance tests.

– The skip=200 parameter on the second dd command in the concurrent read and write tests is to ensure that the two copies of dd are operating on different areas of the hard drive.

16 examples

Below are demonstrations showing the results of running each of the above four tests against each of the following four block devices:

  1. MY_DISK=/dev/sda2 (used in examples 1-X)
  2. MY_DISK=/dev/sdb2 (used in examples 2-X)
  3. MY_DISK=/dev/md/stripped (used in examples 3-X)
  4. MY_DISK=/dev/md/mirrored (used in examples 4-X)

A video demonstration of the these tests being run on a PC is provided at the end of this guide.

Begin by putting your computer into rescue mode to reduce the chances that disk I/O from background services might randomly affect your test results. WARNING: This will shutdown all non-essential programs and services. Be sure to save your work before running these commands. You will need to know your root password to get into rescue mode. The passwd command, when run as the root user, will prompt you to (re)set your root account password.

$ sudo -i
# passwd
# setenforce 0
# systemctl rescue

You might also want to temporarily disable logging to disk:

# sed -r -i.bak 's/^#?Storage=.*/Storage=none/' /etc/systemd/journald.conf
# systemctl restart systemd-journald.service

If you have a swap device, it can be temporarily disabled and used to perform the following tests:

# swapoff -a
# MY_DEVS=$(mdadm --detail /dev/md/swap | grep active | grep -o "/dev/sd.*")
# mdadm --stop /dev/md/swap
# mdadm --zero-superblock $MY_DEVS

Example 1-1 (reading from sda)

# MY_DISK=$(echo $MY_DEVS | cut -d ' ' -f 1)
# dd if=$MY_DISK of=/dev/null bs=1MiB count=200 iflag=nocache
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 1.7003 s, 123 MB/s

Example 1-2 (writing to sda)

# MY_DISK=$(echo $MY_DEVS | cut -d ' ' -f 1)
# dd if=/dev/zero of=$MY_DISK bs=1MiB count=200 oflag=direct
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 1.67117 s, 125 MB/s

Example 1-3 (reading concurrently from sda)

# MY_DISK=$(echo $MY_DEVS | cut -d ' ' -f 1)
# (dd if=$MY_DISK of=/dev/null bs=1MiB count=200 iflag=nocache &); (dd if=$MY_DISK of=/dev/null bs=1MiB count=200 iflag=nocache skip=200 &)
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 3.42875 s, 61.2 MB/s
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 3.52614 s, 59.5 MB/s

Example 1-4 (writing concurrently to sda)

# MY_DISK=$(echo $MY_DEVS | cut -d ' ' -f 1)
# (dd if=/dev/zero of=$MY_DISK bs=1MiB count=200 oflag=direct &); (dd if=/dev/zero of=$MY_DISK bs=1MiB count=200 oflag=direct skip=200 &)
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 3.2435 s, 64.7 MB/s
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 3.60872 s, 58.1 MB/s

Example 2-1 (reading from sdb)

# MY_DISK=$(echo $MY_DEVS | cut -d ' ' -f 2)
# dd if=$MY_DISK of=/dev/null bs=1MiB count=200 iflag=nocache
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 1.67285 s, 125 MB/s

Example 2-2 (writing to sdb)

# MY_DISK=$(echo $MY_DEVS | cut -d ' ' -f 2)
# dd if=/dev/zero of=$MY_DISK bs=1MiB count=200 oflag=direct
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 1.67198 s, 125 MB/s

Example 2-3 (reading concurrently from sdb)

# MY_DISK=$(echo $MY_DEVS | cut -d ' ' -f 2)
# (dd if=$MY_DISK of=/dev/null bs=1MiB count=200 iflag=nocache &); (dd if=$MY_DISK of=/dev/null bs=1MiB count=200 iflag=nocache skip=200 &)
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 3.52808 s, 59.4 MB/s
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 3.57736 s, 58.6 MB/s

Example 2-4 (writing concurrently to sdb)

# MY_DISK=$(echo $MY_DEVS | cut -d ' ' -f 2)
# (dd if=/dev/zero of=$MY_DISK bs=1MiB count=200 oflag=direct &); (dd if=/dev/zero of=$MY_DISK bs=1MiB count=200 oflag=direct skip=200 &)
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 3.7841 s, 55.4 MB/s
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 3.81475 s, 55.0 MB/s

Example 3-1 (reading from RAID0)

# mdadm --create /dev/md/stripped --homehost=any --metadata=1.0 --level=0 --raid-devices=2 $MY_DEVS
# MY_DISK=/dev/md/stripped
# dd if=$MY_DISK of=/dev/null bs=1MiB count=200 iflag=nocache
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 0.837419 s, 250 MB/s

Example 3-2 (writing to RAID0)

# MY_DISK=/dev/md/stripped
# dd if=/dev/zero of=$MY_DISK bs=1MiB count=200 oflag=direct
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 0.823648 s, 255 MB/s

Example 3-3 (reading concurrently from RAID0)

# MY_DISK=/dev/md/stripped
# (dd if=$MY_DISK of=/dev/null bs=1MiB count=200 iflag=nocache &); (dd if=$MY_DISK of=/dev/null bs=1MiB count=200 iflag=nocache skip=200 &)
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 1.31025 s, 160 MB/s
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 1.80016 s, 116 MB/s

Example 3-4 (writing concurrently to RAID0)

# MY_DISK=/dev/md/stripped
# (dd if=/dev/zero of=$MY_DISK bs=1MiB count=200 oflag=direct &); (dd if=/dev/zero of=$MY_DISK bs=1MiB count=200 oflag=direct skip=200 &)
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 1.65026 s, 127 MB/s
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 1.81323 s, 116 MB/s

Example 4-1 (reading from RAID1)

# mdadm --stop /dev/md/stripped
# mdadm --create /dev/md/mirrored --homehost=any --metadata=1.0 --level=1 --raid-devices=2 --assume-clean $MY_DEVS
# MY_DISK=/dev/md/mirrored
# dd if=$MY_DISK of=/dev/null bs=1MiB count=200 iflag=nocache
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 1.74963 s, 120 MB/s

Example 4-2 (writing to RAID1)

# MY_DISK=/dev/md/mirrored
# dd if=/dev/zero of=$MY_DISK bs=1MiB count=200 oflag=direct
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 1.74625 s, 120 MB/s

Example 4-3 (reading concurrently from RAID1)

# MY_DISK=/dev/md/mirrored
# (dd if=$MY_DISK of=/dev/null bs=1MiB count=200 iflag=nocache &); (dd if=$MY_DISK of=/dev/null bs=1MiB count=200 iflag=nocache skip=200 &)
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 1.67171 s, 125 MB/s
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 1.67685 s, 125 MB/s

Example 4-4 (writing concurrently to RAID1)

# MY_DISK=/dev/md/mirrored
# (dd if=/dev/zero of=$MY_DISK bs=1MiB count=200 oflag=direct &); (dd if=/dev/zero of=$MY_DISK bs=1MiB count=200 oflag=direct skip=200 &)
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 4.09666 s, 51.2 MB/s
200+0 records in
200+0 records out
209715200 bytes (210 MB, 200 MiB) copied, 4.1067 s, 51.1 MB/s

Restore your swap device and journald configuration

# mdadm --stop /dev/md/stripped /dev/md/mirrored
# mdadm --create /dev/md/swap --homehost=any --metadata=1.0 --level=1 --raid-devices=2 $MY_DEVS
# mkswap /dev/md/swap
# swapon -a
# mv /etc/systemd/journald.conf.bak /etc/systemd/journald.conf
# systemctl restart systemd-journald.service
# reboot

Interpreting the results

Examples 1-1, 1-2, 2-1, and 2-2 show that each of my drives read and write at about 125 MB/s.

Examples 1-3, 1-4, 2-3, and 2-4 show that when two reads or two writes are done in parallel on the same drive, each process gets at about half the drive’s bandwidth (60 MB/s).

The 3-x examples show the performance benefit of putting the two drives together in a RAID0 (data stripping) array. The numbers, in all cases, show that the RAID0 array performs about twice as fast as either drive is able to perform on its own. The trade-off is that you are twice as likely to lose everything because each drive only contains half the data. A three-drive array would perform three times as fast as a single drive (all drives being equal) but it would be thrice as likely to suffer a catastrophic failure.

The 4-x examples show that the performance of the RAID1 (data mirroring) array is similar to that of a single disk except for the case where multiple processes are concurrently reading (example 4-3). In the case of multiple processes reading, the performance of the RAID1 array is similar to that of the RAID0 array. This means that you will see a performance benefit with RAID1, but only when processes are reading concurrently. For example, if a process tries to access a large number of files in the background while you are trying to use a web browser or email client in the foreground. The main benefit of RAID1 is that your data is unlikely to be lost if a drive fails.

Video demo

Testing storage throughput using dd


If the above tests aren’t performing as you expect, you might have a bad or failing drive. Most modern hard drives have built-in Self-Monitoring, Analysis and Reporting Technology (SMART). If your drive supports it, the smartctl command can be used to query your hard drive for its internal statistics:

# smartctl --health /dev/sda
# smartctl --log=error /dev/sda
# smartctl -x /dev/sda

Another way that you might be able to tune your PC for better performance is by changing your I/O scheduler. Linux systems support several I/O schedulers and the current default for Fedora systems is the multiqueue variant of the deadline scheduler. The default performs very well overall and scales extremely well for large servers with many processors and large disk arrays. There are, however, a few more specialized schedulers that might perform better under certain conditions.

To view which I/O scheduler your drives are using, issue the following command:

$ for i in /sys/block/sd?/queue/scheduler; do echo "$i: $(<$i)"; done

You can change the scheduler for a drive by writing the name of the desired scheduler to the /sys/block/<device name>/queue/scheduler file:

# echo bfq > /sys/block/sda/queue/scheduler

You can make your changes permanent by creating a udev rule for your drive. The following example shows how to create a udev rule that will set all rotational drives to use the BFQ I/O scheduler:

# cat << END > /etc/udev/rules.d/60-ioscheduler-rotational.rules
ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/rotational}=="1", ATTR{queue/scheduler}="bfq"

Here is another example that sets all solid-state drives to use the NOOP I/O scheduler:

# cat << END > /etc/udev/rules.d/60-ioscheduler-solid-state.rules
ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/rotational}=="0", ATTR{queue/scheduler}="none"

Changing your I/O scheduler won’t affect the raw throughput of your devices, but it might make your PC seem more responsive by prioritizing the bandwidth for the foreground tasks over the background tasks or by eliminating unnecessary block reordering.

Photo by James Donovan on Unsplash.

Use udica to build SELinux policy for containers

While modern IT environments move towards Linux containers, the need to secure these environments is as relevant as ever. Containers are a process isolation technology. While containers can be a defense mechanism, they only excel when combined with SELinux.

Fedora SELinux engineering built a new standalone tool, udica, to generate SELinux policy profiles for containers by automatically inspecting them. This article focuses on why udica is needed in the container world, and how it makes SELinux and containers work better together. You’ll find examples of SELinux separation for containers that let you avoid turning protection off because the generic SELinux type container_t is too tight. With udica you can easily customize the policy with limited SELinux policy writing skills.

SELinux technology

SELinux is a security technology that brings proactive security to Linux systems. It’s a labeling system that assigns a label to all subjects (processes and users) and objects (files, directories, sockets, etc.). These labels are then used in a security policy that controls access throughout the system. It’s important to mention that what’s not allowed in an SELinux security policy is denied by default. The policy rules are enforced by the kernel. This security technology has been in use on Fedora for several years. A real example of such a rule is:

allow httpd_t httpd_log_t: file { append create getattr ioctl lock open read setattr };

The rule allows any process labeled as httpd_t to create, append, read and lock files labeled as httpd_log_t. Using the ps command, you can list all processes with their labels:

$ ps -efZ | grep httpd
system_u:system_r:httpd_t:s0 root 13911 1 0 Apr14 ? 00:05:14 /usr/sbin/httpd -DFOREGROUND

To see which objects are labeled as httpd_log_t, use semanage:

# semanage fcontext -l | grep httpd_log_t
/var/log/httpd(/.)? all files system_u:object_r:httpd_log_t:s0
/var/log/nginx(/.)? all files system_u:object_r:httpd_log_t:s0

The SELinux security policy for Fedora is shipped in the selinux-policyRPM package.

SELinux vs. containers

In Fedora, the container-selinux RPM package provides a generic SELinux policy for all containers started by engines like podman or docker. Its main purposes are to protect the host system against a container process, and to separate containers from each other. For instance, containers confined by SELinux with the process type container_t can only read/execute files in /usr and write to container_file_t files type on host file system. To prevent attacks by containers on each other, Multi-Category Security (MCS) is used.

Using only one generic policy for containers is problematic, because of the huge variety of container usage. On one hand, the default container type (container_t) is often too strict. For example:

  • Fedora SilverBlue needs containers to read/write a user’s home directory
  • Fluentd project needs containers to be able to read logs in the /var/log directory

On the other hand, the default container type could be too loose for certain use cases:

  • It has no SELinux network controls — all container processes can bind to any network port
  • It has no SELinux control on Linux capabilities — all container processes can use all capabilities

There is one solution to handle both use cases: write a custom SELinux security policy for the container. This can be tricky, because SELinux expertise is required. For this purpose, the udica tool was created.

Introducing udica

Udica generates SELinux security profiles for containers. Its concept is based on the “block inheritance” feature inside the common intermediate language (CIL) supported by SELinux userspace. The tool creates a policy that combines:

  • Rules inherited from specified CIL blocks (templates), and
  • Rules discovered by inspection of container JSON file, which contains mountpoints and ports definitions

You can load the final policy immediately, or move it to another system to load into the kernel. Here’s an example, using a container that:

  • Mounts /home as read only
  • Mounts /var/spool as read/write
  • Exposes port tcp/21

The container starts with this command:

# podman run -v /home:/home:ro -v /var/spool:/var/spool:rw -p 21:21 -it fedora bash

The default container type (container_t) doesn’t allow any of these three actions. To prove it, you could use the sesearch tool to query that the allow rules are present on system:

# sesearch -A -s container_t -t home_root_t -c dir -p read 

There’s no allow rule present that lets a process labeled as container_t access a directory labeled home_root_t (like the /home directory). The same situation occurs with /var/spool, which is labeled var_spool_t:

# sesearch -A -s container_t -t var_spool_t -c dir -p read

On the other hand, the default policy completely allows network access.

# sesearch -A -s container_t -t port_type -c tcp_socket
allow container_net_domain port_type:tcp_socket { name_bind name_connect recv_msg send_msg };
allow sandbox_net_domain port_type:tcp_socket { name_bind name_connect recv_msg send_msg };

Securing the container

It would be great to restrict this access and allow the container to bind just on TCP port 21 or with the same label. Imagine you find an example container using podman ps whose ID is 37a3635afb8f:

# podman ps -q

You can now inspect the container and pass the inspection file to the udica tool. The name for the new policy is my_container.

# podman inspect 37a3635afb8f > container.json
# udica -j container.json my_container
Policy my_container with container id 37a3635afb8f created!

Please load these modules using:
# semodule -i my_container.cil /usr/share/udica/templates/{base_container.cil,net_container.cil,home_container.cil}

Restart the container with: "--security-opt label=type:my_container.process" parameter

That’s it! You just created a custom SELinux security policy for the example container. Now you can load this policy into the kernel and make it active. The udica output above even tells you the command to use:

# semodule -i my_container.cil /usr/share/udica/templates/{base_container.cil,net_container.cil,home_container.cil}

Now you must restart the container to allow the container engine to use the new custom policy:

# podman run --security-opt label=type:my_container.process -v /home:/home:ro -v /var/spool:/var/spool:rw -p 21:21 -it fedora bash

The example container is now running in the newly created my_container.process SELinux process type:

# ps -efZ | grep my_container.process
unconfined_u:system_r:container_runtime_t:s0-s0:c0.c1023 root 2275 434 1 13:49 pts/1 00:00:00 podman run --security-opt label=type:my_container.process -v /home:/home:ro -v /var/spool:/var/spool:rw -p 21:21 -it fedora bash
system_u:system_r:my_container.process:s0:c270,c963 root 2317 2305 0 13:49 pts/0 00:00:00 bash

Seeing the results

The command sesearch now shows allow rules for accessing /home and /var/spool:

# sesearch -A -s my_container.process -t home_root_t -c dir -p read
allow my_container.process home_root_t:dir { getattr ioctl lock open read search };
# sesearch -A -s my_container.process -t var_spool_t -c dir -p read
allow my_container.process var_spool_t:dir { add_name getattr ioctl lock open read remove_name search write }

The new custom SELinux policy also allows my_container.process to bind only to TCP/UDP ports labeled the same as TCP port 21:

# semanage port -l | grep 21 | grep ftp
ftp_port_t tcp 21, 989, 990
# sesearch -A -s my_container.process -c tcp_socket -p name_bind
allow my_container.process ftp_port_t:tcp_socket name_bind;


The udica tool helps you create SELinux policies for containers based on an inspection file without any SELinux expertise required. Now you can increase the security of containerized environments. Sources are available on GitHub, and an RPM package is available in Fedora repositories for Fedora 28 and later.

Photo by Samuel Zeller on Unsplash.


4 cool new projects to try in COPR for April 2019

COPR is a collection of personal repositories for software that isn’t carried in Fedora. Some software doesn’t conform to standards that allow easy packaging. Or it may not meet other Fedora standards, despite being free and open source. COPR can offer these projects outside the Fedora set of packages. Software in COPR isn’t supported by Fedora infrastructure or signed by the project. However, it can be a neat way to try new or experimental software.

Here’s a set of new and interesting projects in COPR.


Joplin is a note-taking and to-do app. Notes are written in the Markdown format, and organized by sorting them into various notebooks and using tags.
Joplin can import notes from any Markdown source or exported from Evernote. In addition to the desktop app, there’s an Android version with the ability to synchronize notes between them — using Nextcloud, Dropbox or other cloud services. Finally, there’s a browser extension for Chrome and Firefox to save web pages and screenshots.

Installation instructions

The repo currently provides Joplin for Fedora 29 and 30, and for EPEL 7. To install Joplin, use these commands with sudo:

sudo dnf copr enable taw/joplin
sudo dnf install joplin


Fzy is a command-line utility for fuzzy string searching. It reads from a standard input and sorts the lines based on what is most likely the sought after text, and then prints the selected line. In addition to command-line, fzy can be also used within vim. You can try fzy in this online demo.

Installation instructions

The repo currently provides fzy for Fedora 29, 30, and Rawhide, and other distributions. To install fzy, use these commands:

sudo dnf copr enable lehrenfried/fzy
sudo dnf install fzy


Fondo is a program for browsing many photographs from the website. It has a simple interface that allows you to look for pictures of one of several themes, or all of them at once. You can then set a found picture as a wallpaper with a single click, or share it.

Installation instructions

The repo currently provides Fondo for Fedora 29, 30, and Rawhide. To install Fondo, use these commands:

sudo dnf copr enable atim/fondo
sudo dnf install fondo


YACReader is a digital comic book reader that supports many comics and image formats, such as cbz, cbr, pdf and others. YACReader keeps track of reading progress, and can download comics’ information from Comic Vine. It also comes with a YACReader Library for organizing and browsing your comic book collection.

Installation instructions

The repo currently provides YACReader for Fedora 29, 30, and Rawhide. To install YACReader, use these commands:

sudo dnf copr enable atim/yacreader
sudo dnf install yacreader

Kubernetes on Fedora IoT with k3s

Fedora IoT is an upcoming Fedora edition targeted at the Internet of Things. It was introduced last year on Fedora Magazine in the article How to turn on an LED with Fedora IoT. Since then, it has continued to improve together with Fedora Silverblue to provide an immutable base operating system aimed at container-focused workflows.

Kubernetes is an immensely popular container orchestration system. It is perhaps most commonly used on powerful hardware handling huge workloads. However, it can also be used on lightweight devices such as the Raspberry Pi 3. Read on to find out how.

Why Kubernetes?

While Kubernetes is all the rage in the cloud, it may not be immediately obvious to run it on a small single board computer. But there are certainly reasons for doing it. First of all it is a great way to learn and get familiar with Kubernetes without the need for expensive hardware. Second, because of its popularity, there are tons of applications that comes pre-packaged for running in Kubernetes clusters. Not to mention the large community to provide help if you ever get stuck.

Last but not least, container orchestration may actually make things easier, even at the small scale in a home lab. This may not be apparent when tackling the the learning curve, but these skills will help when dealing with any cluster in the future. It doesn’t matter if it’s a single node Raspberry Pi cluster or a large scale machine learning farm.

K3s – a lightweight Kubernetes

A “normal” installation of Kubernetes (if such a thing can be said to exist) is a bit on the heavy side for IoT. The recommendation is a minimum of 2 GB RAM per machine! However, there are plenty of alternatives, and one of the newcomers is k3s – a lightweight Kubernetes distribution.

K3s is quite special in that it has replaced etcd with SQLite for its key-value storage needs. Another thing to note is that k3s ships as a single binary instead of one per component. This diminishes the memory footprint and simplifies the installation. Thanks to the above, k3s should be able to run k3s with just 512 MB of RAM, perfect for a small single board computer!

What you will need

  1. Fedora IoT in a virtual machine or on a physical device. See the excellent getting started guide here. One machine is enough but two will allow you to test adding more nodes to the cluster.
  2. Configure the firewall to allow traffic on ports 6443 and 8472. Or simply disable it for this experiment by running “systemctl stop firewalld”.

Install k3s

Installing k3s is very easy. Simply run the installation script:

curl -sfL | sh -

This will download, install and start up k3s. After installation, get a list of nodes from the server by running the following command:

kubectl get nodes

Note that there are several options that can be passed to the installation script through environment variables. These can be found in the documentation. And of course, there is nothing stopping you from installing k3s manually by downloading the binary directly.

While great for experimenting and learning, a single node cluster is not much of a cluster. Luckily, adding another node is no harder than setting up the first one. Just pass two environment variables to the installation script to make it find the first node and avoid running the server part of k3s

curl -sfL | K3S_URL=https://example-url:6443 \

The example-url above should be replaced by the IP address or fully qualified domain name of the first node. On that node the token (represented by XXX) is found in the file /var/lib/rancher/k3s/server/node-token.

Deploy some containers

Now that we have a Kubernetes cluster, what can we actually do with it? Let’s start by deploying a simple web server.

kubectl create deployment my-server --image nginx

This will create a Deployment named “my-server” from the container image “nginx” (defaulting to docker hub as registry and the latest tag). You can see the Pod created by running the following command.

kubectl get pods

In order to access the nginx server running in the pod, first expose the Deployment through a Service. The following command will create a Service with the same name as the deployment.

kubectl expose deployment my-server --port 80

The Service works as a kind of load balancer and DNS record for the Pods. For instance, when running a second Pod, we will be able to curl the nginx server just by specifying my-server (the name of the Service). See the example below for how to do this.

# Start a pod and run bash interactively in it
kubectl run debug --generator=run-pod/v1 --image=fedora -it -- bash
# Wait for the bash prompt to appear
curl my-server
# You should get the "Welcome to nginx!" page as output

Ingress controller and external IP

By default, a Service only get a ClusterIP (only accessible inside the cluster), but you can also request an external IP for the service by setting its type to LoadBalancer. However, not all applications require their own IP address. Instead, it is often possible to share one IP address among many services by routing requests based on the host header or path. You can accomplish this in Kubernetes with an Ingress, and this is what we will do. Ingresses also provide additional features such as TLS encryption of the traffic without having to modify your application.

Kubernetes needs an ingress controller to make the Ingress resources work and k3s includes Traefik for this purpose. It also includes a simple service load balancer that makes it possible to get an external IP for a Service in the cluster. The documentation describes the service like this:

k3s includes a basic service load balancer that uses available host ports. If you try to create a load balancer that listens on port 80, for example, it will try to find a free host in the cluster for port 80. If no port is available the load balancer will stay in Pending.


The ingress controller is already exposed with this load balancer service. You can find the IP address that it is using with the following command.

$ kubectl get svc --all-namespaces
default kubernetes ClusterIP 443/TCP 33d
default my-server ClusterIP 80/TCP 30m
kube-system kube-dns ClusterIP 53/UDP,53/TCP,9153/TCP 33d
kube-system traefik LoadBalancer 80:31596/TCP,443:31539/TCP 33d

Look for the Service named traefik. In the above example the IP we are interested in is

Route incoming requests

Let’s create an Ingress that routes requests to our web server based on the host header. This example uses to avoid having to set up DNS records. It works by including the IP adress as a subdomain, to use any subdomain of to reach the IP In other words, is used to reach the ingress controller in the cluster. You can try this right now (with your own IP instead of Without an ingress in place you should reach the “default backend” which is just a page showing “404 page not found”.

We can tell the ingress controller to route requests to our web server Service with the following Ingress.

apiVersion: extensions/v1beta1
kind: Ingress
name: my-server
- host:
- path: /
serviceName: my-server
servicePort: 80

Save the above snippet in a file named my-ingress.yaml and add it to the cluster by running this command:

kubectl apply -f my-ingress.yaml

You should now be able to reach the default nginx welcoming page on the fully qualified domain name you chose. In my example this would be The ingress controller is routing the requests based on the information in the Ingress. A request to will be routed to the Service and port defined as backend in the Ingress (my-server and 80 in this case).

What about IoT then?

Imagine the following scenario. You have dozens of devices spread out around your home or farm. It is a heterogeneous collection of IoT devices with various hardware capabilities, sensors and actuators. Maybe some of them have cameras, weather or light sensors. Others may be hooked up to control the ventilation, lights, blinds or blink LEDs.

In this scenario, you want to gather data from all the sensors, maybe process and analyze it before you finally use it to make decisions and control the actuators. In addition to this, you may want to visualize what’s going on by setting up a dashboard. So how can Kubernetes help us manage something like this? How can we make sure that Pods run on suitable devices?

The simple answer is labels. You can label the nodes according to capabilities, like this:

kubectl label nodes <node-name> <label-key>=<label-value>
# Example
kubectl label nodes node2 camera=available

Once they are labeled, it is easy to select suitable nodes for your workload with nodeSelectors. The final piece to the puzzle, if you want to run your Pods on all suitable nodes is to use DaemonSets instead of Deployments. In other words, create one DaemonSet for each data collecting application that uses some unique sensor and use nodeSelectors to make sure they only run on nodes with the proper hardware.

The service discovery feature that allows Pods to find each other simply by Service name makes it quite easy to handle these kinds of distributed systems. You don’t need to know or configure IP addresses or custom ports for the applications. Instead, they can easily find each other through named Services in the cluster.

Utilize spare resources

With the cluster up and running, collecting data and controlling your lights and climate control you may feel that you are finished. However, there are still plenty of compute resources in the cluster that could be used for other projects. This is where Kubernetes really shines.

You shouldn’t have to worry about where exactly those resources are or calculate if there is enough memory to fit an extra application here or there. This is exactly what orchestration solves! You can easily deploy more applications in the cluster and let Kubernetes figure out where (or if) they will fit.

Why not run your own NextCloud instance? Or maybe gitea? You could also set up a CI/CD pipeline for all those IoT containers. After all, why would you build and cross compile them on your main computer if you can do it natively in the cluster?

The point here is that Kubernetes makes it easier to make use of the “hidden” resources that you often end up with otherwise. Kubernetes handles scheduling of Pods in the cluster based on available resources and fault tolerance so that you don’t have to. However, in order to help Kubernetes make reasonable decisions you should definitely add resource requests to your workloads.


While Kubernetes, or container orchestration in general, may not usually be associated with IoT, it certainly makes a lot of sense to have an orchestrator when you are dealing with distributed systems. Not only does is allow you to handle a diverse and heterogeneous fleet of devices in a unified way, but it also simplifies communication between them. In addition, Kubernetes makes it easier to utilize spare resources.

Container technology made it possible to build applications that could “run anywhere”. Now Kubernetes makes it easier to manage the “anywhere” part. And as an immutable base to build it all on, we have Fedora IoT.

Command line quick tips: Cutting content out of files

The Fedora distribution is a full featured operating system with an excellent graphical desktop environment. A user can point and click their way through just about any typical task easily. All of this wonderful ease of use masks the details of a powerful command line under the hood. This article is part of a series that shows you some common command line utilities. So let’s drop into the shell, and have a look at cut.

Often when you work in the command line, you are working with text files. Sometimes these files may be quite long. Reading them in their entirety, while feasible, can be time consuming and prone to errors. In this installment you’ll learn how to extract content from text files, and get the information you want from them.

It’s important to recognize that there are many ways to accomplish similar command line tasks in Fedora. The Fedora repositories include entire language systems for parsing and working with text, as an example. Also, there are multiple command line utilities available for just about any purpose conceivable in the shell. This article will only focus on using a few of those utility choices, to extract some information from a file and present it in a readable format.

Making the cut

To illustrate this example use a standard sizable file on the system like /etc/passwd. As seen in a prior article in this series, you can execute the cat command to view an entire file:

$ cat /etc/passwd

This file contains information on all accounts present on the system. It has a specific format:


Imagine that you want to simply have a list of all the account names on the system. If you could only cut out the name value from each line. This is where the cut command comes in handy! This command treats any input one line at a time, and extracts a specific part of the line.

The cut command provides options for selecting parts of a line differently, and in this example two of them are needed, -d which is an option to specify a delimiter type to use, and -f which is an option to specify which field of the line to cut. The -d option lets you declare the delimiter that separates values in a line. In this case a colon (:) is used to separate values. The -f option lets you choose which field value or values to extract. So for this example the command entered would be:

$ cut -d: -f1 /etc/passwd

That’s great, it worked! But you get the printout to the standard output, which in a terminal session at least means the screen. What if you needed the information for another task to be done later? It would be really nice if there was a way to put the output of the cut command into a text file to save it. There is an easy builtin shell function for such a task, the redirect function (>).

$ cut -d: -f1 /etc/passwd > names.txt

This will place the output of cut into a file called names.txt and you can check the contents with cat:

$ cat names.txt

With two commands and one shell function, it was easy to identify using cat, extract using cut, and redirect the extracted information from one file, saving it to another file for later use.

Photo by Joel Mbugua on Unsplash.

3 cool text-based email clients

Writing and receiving email is a big part of everyone’s daily routine and choosing an email client is usually a major decision. The Fedora OS provides a large choice of email clients and among these are text-based email applications.


Mutt is probably one of the most popular text-based email clients. It supports all the common features that one would expect from an email client. Color coding, mail threading, POP3, and IMAP are all supported by Mutt. But one of its best features is it’s highly configurable. Indeed, the user can easily change the keybindings, and create macros to adapt the tool to a particular workflow.

To give Mutt a try, install it using sudo and dnf:

$ sudo dnf install mutt

To help newcomers get started, Mutt has a very comprehensive wiki full of macro examples and configuration tricks.


Alpine is also among the most popular text-based email clients. It’s more beginner friendly than Mutt, and you can configure most of Alpine via the application itself — no need to edit a configuration file. One powerful feature of Alpine is the ability to score emails. This is particularly interesting for users that are registered to a high volume mailing list like Fedora’s devel list. Using scores, Alpine can sort the email based on the user’s interests, showing emails with a high score first.

Alpine is also available to install from Fedora’s repository using dnf.

 $ sudo dnf install alpine

While using Alpine, you can easily access the documentation by pressing the Ctrl+G key combination.


nmh (new Mail Handling) follows the UNIX tools philosophy. It provides a collection of single purpose programs to send, receive, save, retrieve, and manipulate e-mail messages. This lets you swap the nmh command with other programs, or create scripts around nmh to create more customized tools. For example, you can use Mutt with nmh.

nmh can be easily installed using dnf.

$ sudo dnf install nmh

To learn more about nmh and mail handling in general you can read this GPL licenced book.


Let’s try dwm — dynamic window manger

If you like efficiency and minimalism, and are looking for a new window manager for your Linux desktop, you should try dwm — dynamic window manager. Written in under 2000 standard lines of code, dwm is extremely fast yet powerful and highly customizable window manager.

You can dynamically choose between tiling, monocle and floating layouts, organize your windows into multiple workspaces using tags, and quickly navigate through using keyboard shortcuts. This article helps you get started using dwm.


To install dwm on Fedora, run:

$ sudo dnf install dwm dwm-user

The dwm package installs the window manager itself, and the dwm-user package significantly simplifies configuration which will be explained later in this article.

Additionally, to be able to lock the screen when needed, we’ll also install slock — a simple X display locker.

$ sudo dnf install slock

However, you can use a different one based on your personal preference.

Quick start

To start dwm, choose the dwm-user option on the login screen.

After you log in, you’ll see a very simple desktop. In fact, the only thing there will be a bar at the top listing our nine tags that represent workspaces and a []= symbol that represents the layout of your windows.

Launching applications

Before looking into the layouts, first launch some applications so you can play with the layouts as you go. Apps can be started by pressing Alt+p and typing the name of the app followed by Enter. There’s also a shortcut Alt+Shift+Enter for opening a terminal.

Now that some apps are running, have a look at the layouts.


There are three layouts available by default: the tiling layout, the monocle layout, and the floating layout.

The tiling layout, represented by []= on the bar, organizes windows into two main areas: master on the left, and stack on the right. You can activate the tiling layout by pressing Alt+t.

The idea behind the tiling layout is that you have your primary window in the master area while still seeing the other ones in the stack. You can quickly switch between them as needed.

To swap windows between the two areas, hover your mouse over one in the stack area and press Alt+Enter to swap it with the one in the master area.

The monocle layout, represented by [N] on the top bar, makes your primary window take the whole screen. You can switch to it by pressing Alt+m.

Finally, the floating layout lets you move and resize your windows freely. The shortcut for it is Alt+f and the symbol on the top bar is ><>.

Workspaces and tags

Each window is assigned to a tag (1-9) listed at the top bar. To view a specific tag, either click on its number using your mouse or press Alt+1..9. You can even view multiple tags at once by clicking on their number using the secondary mouse button.

Windows can be moved between different tags by highlighting them using your mouse, and pressing Alt+Shift+1..9. 


To make dwm as minimalistic as possible, it doesn’t use typical configuration files. Instead, you modify a C header file representing the configuration, and recompile it. But don’t worry, in Fedora it’s as simple as just editing one file in your home directory and everything else happens in the background thanks to the dwm-user package provided by the maintainer in Fedora.

First, you need to copy the file into your home directory using a command similar to the following:

$ mkdir ~/.dwm
$ cp /usr/src/dwm-VERSION-RELEASE/config.def.h ~/.dwm/config.h

You can get the exact path by running man dwm-start.

Second, just edit the ~/.dwm/config.h file. As an example, let’s configure a new shortcut to lock the screen by pressing Alt+Shift+L.

Considering we’ve installed the slock package mentioned earlier in this post, we need to add the following two lines into the file to make it work:

Under the /* commands */ comment, add:

static const char *slockcmd[] = { "slock", NULL };

And the following line into static Key keys[]:

{ MODKEY|ShiftMask, XK_l, spawn, {.v = slockcmd } },

In the end, it should look like as follows: (added lines are highlighted)

/* commands */
static char dmenumon[2] = "0"; /* component of dmenucmd, manipulated in spawn() */
static const char *dmenucmd[] = { "dmenu_run", "-m", dmenumon, "-fn", dmenufont, "-nb", normbgcolor, "-nf", normfgcolor, "-sb", selbgcolor, "-sf", selfgcolor, NULL };
static const char *termcmd[]  = { "st", NULL };
static const char *slockcmd[] = { "slock", NULL };

static Key keys[] = {
/* modifier                     key        function        argument */
{ MODKEY|ShiftMask,             XK_l,      spawn,          {.v = slockcmd } },
{ MODKEY,                       XK_p,      spawn,          {.v = dmenucmd } },
{ MODKEY|ShiftMask,             XK_Return, spawn,          {.v = termcmd } },

Save the file.

Finally, just log out by pressing Alt+Shift+q and log in again. The scripts provided by the dwm-user package will recognize that you have changed the config.h file in your home directory and recompile dwm on login. And becuse dwm is so tiny, it’s fast enough you won’t even notice it.

You can try locking your screen now by pressing Alt+Shift+L, and then logging back in again by typing your password and pressing enter.


If you like minimalism and want a very fast yet powerful window manager, dwm might be just what you’ve been looking for. However, it probably isn’t for beginners. There might be a lot of additional configuration you’ll need to do in order to make it just as you like it.

To learn more about dwm, see the project’s homepage at


Building Flatpak apps in Gnome Builder on Fedora Silverblue

If you are developing software using Fedora Silverblue, and especially if what you are developing is a Gnome application, Gnome Builder 3.30.3 feels like an obvious choice of IDE.

In this article, I will show you how you can create a simple Gnome application, and how to build it and install it as a Flatpak app on your system.

Gnome and Flatpak applications

Builder has been a part of Gnome for a long time. It is a very mature IDE to me in terms of consistency and completeness.

The Gnome Builder project website offers extensive documentation regarding Gnome application development — I highly recommend spending some time there to anyone interested.

Editor’s note: Getting Builder

Because the initial Fedora Silverblue installation doesn’t include Builder, let’s walk through the installation process first.

Starting with a freshly installed system, the first thing you’ll need to do is to enable a repository providing Builder as a Flatpak — we’ll use Flathub which is a popular 3rd-party repository with many desktop apps.

To enable Flathub on your system, download the repository file from the Fedora Quick Setup page, and double-click it which opens Gnome Software asking you to enable this repository on your system.

After you’re done with that, you can search for Builder in Gnome Software and install it.

Creating a new project

So let’s walk through the creation of a new project for our Gnome app. When you start Gnome Builder, the first display is oriented towards project management.

To create a new project, I clicked on the New… button at the top-left corner which showed me the following view.

You’ll need to fill out the project name, choose your preferred language (I chose C, but other languages will work for this example as well), and the license. Leave the version control on, and select Gnome Application as your template.

I chose gbfprtfsb as the name of my project which means Hello from Gnome 3 on Fedora SilverBlue.

The IDE creates and opens the project once you press create.

Tweaking our new project

The newly created project is opened in the Builder IDE and on my system looks like the following.

This project could be run from within the IDE right now and would give you the ever popular “Hello World!” titled gnome windowed application with a label that says, yup “Hello World!”.

Let’s get a little disruptive and mess up the title and greeting a bit. Complacency leads to mediocrity which leads to entropy overcoming chaos to enforce order, stasis, then finally it all just comes to a halt. It’s therefore our duty to shake it up at every opportunity, if only to knock out any latent entropy that may have accumulated in our systems. Towards such lofty goals, we only need to change two lines of one file, and the file isn’t even a C language file, it’s an XML file used to describe the GUI named gbfprtfsb-window.ui. All we have to do is open it and edit the title and label text, save and then build our masterpiece!

Looking at the screenshot below, I have circled the text we are going to replace. The window is a GtkApplicationWindow, and uses a GtkHeaderBar and GtkLabel to display the text we are changing. In the GtkHeaderBar we will type GBFPRTFSB for the title property. In the GtkLabel we will type Hello from Gnome 3 on Fedora SilverBlue in the label property. Now save the file to record our changes.

Building the project

Well, we have made our changes, and expressed our individualism (cough) at the same time. All that is left is to build it and see what it looks like. The build panel is located near the top of the IDE, middle right, and is represented by the icon that appears to be a brick wall being built as shown on the following picture.

Press the button, and the build process completes. You can also preview your application by clicking on the “play” button next to it.

Building a Flatpak

When we’re happy with our creation, the next step will be building it as a Flatpak. To do that, click on the title in the middle of the top bar, and then on the Export Bundle button.

Once the export has successfully completed, Gnome Builder will open a Nautilus file browser window showing the export directory, with the Flatpak bundle already selected.

To install the app on your system, simply double-click the icon which opens Gnome Software allowing you to install the app. On my system I had to enter my user password twice, which I take to be due to the fact we had no configured GPG key for the project. After it was installed, the application was shown alongside all of the other applications on my system. It can be seen running below.

I think this has successfully shown how easy it is to deploy an application as a Flatpak bundle for Gnome using Builder, and then running it on Fedora Silverblue.