Linux: Empty swap space

Linux Comments

This is the translation of a post I wrote four years ago. I needed an english version of it for a friend of mine.

If you want to empty your swap space on Linux for whatever reason, so you can simply use these commands:

swapoff -a && swapon -a

This deactivates all swap storage first, leading to a drainage of the currently contained data. Afterwards the swap space is activated again.

If the swap space is heavily utilized, this can take a couple of minutes, so don’t worry if nothing happens. To check the progress, you can use this command in another window:

free -s 3 |grep Swap

This command shows the current used swap and refreshes every three seconds. Of course, the used swap space should continously decrease and finally be zero.

If you only want to empty one swap partition, you can substitute the -a switch in the first command with the paritition you want to drain, e.g. /dev/sda2.


Ansible: Extracting dictionary values into a list

Ansible Comments


In Ansible you have a list of dictionaries containing some values, e.g. a list of mount points:

  basedir: /srv/mymounts
      name: first_mount
      opts: defaults
      name: second_mount
      opts: noauto,x-systemd.automount,_netdev,reconnect
      name: third_mount
      opts: defaults

And you want to extract a list of mount pathes combined from basedir and name for further usage, so the result looks like this:

['/srv/mymounts/first_mount', '/srv/mymounts/second_mount', '/srv/mymounts/third_mount']

Ubuntu: How to fix the No Wi-Fi password dialogue error

Linux, Ubuntu Comments

I recently stumbled over a problem on my notebook running Ubuntu 16.04 “Xenial Xerus”. When trying to connect to a new, protected Wi-Fi network no password dialogue appeared, making a quick GUI connection impossible. Creating the connection manually, entering the Wi-Fi password in the “Wi-Fi-Security” setting in the process worked like a charm. Using nmtui from the terminal also worked flawlessly.

As I didn’t connect the notebook to new Wi-Fi networks in months, I didn’t encounter this error for a long time. Therefore I had a lot of problems debugging it as I installed bazillions of updates and new packages in the meantime.

I narrowed the problem down to NetworkManager not having a way of saving the Wi-Fi password, which is normally done via gnome-keyring, specifically via gnome-keyring-daemon. The daemon didn’t seem to start properly, but even starting it manually still didn’t make a difference. The logs still complained:

Okt 17 06:58:53 XPS13 NetworkManager[1064]: <info>  [1508216333.3058] device (wlan0): Activation: (wifi) access point 'MyTest' has security, but secrets are required.
Okt 17 06:58:53 XPS13 NetworkManager[1064]: <info>  [1508216333.3062] device (wlan0): state change: config -> need-auth (reason 'none') [50 60 0]
Okt 17 06:58:53 XPS13 NetworkManager[1064]: <warn>  [1508216333.3062] device (wlan0): No agents were available for this request.

Finally I found the solution in an askubuntu question. Nearly half a year ago I installed Flatpak so I could install FeedReader. One of the dependencies was dbus-user-session. This package seems to have problems with the gnome-keyring-daemon in 16.04 rendering it defective. Purging the dbus-user-session package and its dependencies solved my problem and the Wi-Fi password dialogue appeared once again.

apt purge dbus-user-session xdg-desktop-portal xdg-desktop-portal-gtk

Wi-Fi dialogue

I still had a problem with the NetworkManager applet in the tray, but this could be fixed with an apt install --reinstall network-manager-gnome and a reboot afterwards.

Java Keystore scribblings

Java Comments

Nearly two months ago I published my OpenSSL scribblings post. This one is the spiritual successor, addressing Java Keystore handling this time. There are already a lot of good web sites on how to handle the keytool, so I will limit myself to the issues I encounter from time to time which are more difficult to figure out. Similar to the last post I will use COMODO as CA and ssl.example.org as domain.

Adding a new certificate to a keystore


First concatenate the cert chain if not already in one file. A Comodo speciality is the occasional inclusion of Windows line breaks, so we use sed for output to substitute any occurence of these. Additionally we ensure that the certificate starts on a newline:

sed -e 's/\r$/\n/g' root-ca.pem intermediate-ca.pem > cabundle.pem

Running Docker swarm mode on Scaleway

Docker, Scaleway Comments

I’m currently running a couple of tests with Docker swarm mode on Scaleway. For those who don’t know Scaleway, it’s a PaaS provider which is part of online.net. Due to capped maximum prices per month and no Ingress/egress prices it’s good for smaller projects.

But small prices often also mean a couple of limitations as well. In my case I wasn’t able to run Docker swarm mode properly. I tried running a stack with a couple of web servers and Traefik as Reverse Proxy, but for some reason this didn’t work. When running a Traefik container manually in Docker, everything worked fine. But as soon as I tried using it in swarm mode context, there was no connectivity. Dockers logs showed some strange errors similar to these:

Apr 14 20:39:40 <hostname> docker[15787]: time="2017-04-14T20:39:40Z" level=error msg="Failed to write to /proc/sys/net/ipv4/vs/conntrack: open /proc/sys/net/ipv4/vs/conntrack: nno such file or directory"
Apr 14 20:39:40 <hostname> docker[15787]: time="2017-04-14T20:39:40.154260407Z" level=error msg="Failed to add firewall mark rule in sbox ingress (ingress): reexec failed: exit status 8"
Apr 14 20:41:17 <hostname> docker[15787]: time="2017-04-14T20:41:17.432619182Z" level=error msg="Failed to delete real server for vip fwmark 259 in sbox ingress (ingress): no such process"
Apr 14 20:41:17 <hostname> docker[15787]: time="2017-04-14T20:41:17.432762944Z" level=error msg="Failed to delete service for vip fwmark 259 in sbox ingress (ingress): no such process"