Linux has a sohpisticated firewall built right into the kernel: It’s called iptables!
I’m pretty sure you heard about it.
You can do realy crazy things with iptables.
But here I just want to log how to log+drop a packet in a single rule.
Usually, you would probably do something like that:
Works perfectly, but dramatically messes your rules table up..
Especially, if you want to log+drop packets that match a complicated filter.
You’ll end up with twice as many table entries as desired..
The trick is to instead create a new rule chain that will log+drop in sequence:
So here I created a new chain called LOG_DROP.
We can now append (-A) two new rules to that chain, which do the actual drop+log:
(similar like the first code above, just not for the INPUT chain but for the LOG_DROP chain)
That’s basically it!
If you now need to log+drop a packet you can append a new rule to e.g. the INPUT chain that routes the packet to the LOG_DROP chain:
You should consider to limit the number of redundant log entries per time to prevent flooding of your logs..
For more documentation you should consult the manual of iptables(8).
You can use OpenSSL to obtain a certificate, for example for binfalse.de:
Here, openssl will connect to the server behind binfalse.de at port 443 (default port for HTTPS) to request the SSL certificate and dump it to your terminal.
openssl can also print the details about a certificate. You just need to pipe the certificate into:
Thus, the whole command including the output may look like this:
As you can see in the X.509 extension this server’s SSL certificate does have a Subject Alternative Name:
To quick-check one of your websites you may want to use the following grep filter:
If that doesn’t print a proper Subject Alternative Name you should go and create a new SSL certificate for that server!
Hands up: who knows what an android device does when it sees a WiFi network coming up?
Exactly, since Lollipo (Android 5) your phone or tablet leaks a quick HTTP request to check if it has internet access.
This check is, for example, done with clients3.google.com/generate_204, a “webpage” that always returns an HTTP status code 204 No Content.
Thus, if the phone receives a 204 it is connected to the internet, otherwise it assumes that this network does not provide proper internet access or is just a captive portal.
However, that way Google of course always knows when you connect from where. And how often. And which device you’re using. etc… :(
How to prevent the leak
Even if people may like that feature,
that is of course a privacy issue – so how can we counter that?
However, blocking that “feature” also comes with some drawbacks…
The downside of blocking captive portal detection
The consequences of blocking all request of the captive portal detection are obvious:
your phone assumes that no network hat internet access.
And therefore, it wouldn’t connect automatically, saying
No Internet Access Detected, won’t automatically reconnect.
see image on top
That will probably increase your mobile data usage, as you always need (to remember) to do connect manually.
And even if you manually connect to a network “without internet” the WiFi icon will get an exclamation mark and the phone says
Connected, no Internet.
see second image
What can we do about it?
Disable captive portal detection
With a rooted phone you can simply disable captive portal detection.
Just get a root-shell through adb (or SSH etc) to run the following command:
One small drawback of that approach: you need to execute that again after flashing a new image…
However, I guess you’ll anyway have a small workflow for re-flashing your phone – just add that tiny bit to it ;-)
Another drawback is that you loose the captive portal detection…
Of course, that’s what you intended, but sometimes it may be useful to have that feature in hotels etc..
Change the server for captive portal detection with the Android API
You can also change the URL to the captive portal server to a server under your control.
Let’s say you have a site running at scratch.binfalse.de/generate_204 that simulates a captive portal detection server backend(!?) and always returns 204, no matter what request.
Then you can use that URL for captive portal detection!
Override the captive portal server on a root-shell (adb or SSH etc) by calling:
This way you retain the captive portal detection without leaking data to Google.
However, you will again loose the setting when flashing the phone again..
Change the server for captive portal detection using AdAway
Another option for changing the captive portal detection server is to change its IP address to one that’s under your control.
You can do that with AdAway, for example.
Let’s say your captive portal detection server has the IP address 22.214.171.124, then you may add the following to your AdAway configuration:
The webserver at 126.96.36.199 should then of course accept requests for the foreign domains.
This way, you also don’t leak the data to Google and you will also keep the settings after flashing the phone (as long as you leave AdAway installed).
However, there are also some things to keep in mind:
First, I could imagine that Google may be a bit upset if you redirect their domains to a different server?
And second, you don’t know if those are the only servers used for captive portal detection.
If Google at some point comes up with another domain for captive portal detection, such as captive.google.com, you’re screwed.
Even with Docker you need to care about backups.. ;-)
As you usually mount all the persistent data into the container the files will actually be on your host.
Thus, you can simply do the backup of these files.
However, for MySQL I prefer having an actual SQL-dump.
Therefore I just developed the Docker MySQL-Backup tool.
You will find the sources at the corresponding GitHub repository.
The script /etc/cron.daily/docker-mysql-backup parses the output of the docker ps command to find running containers of the MySQL image.
More precisely, it looks for containers of images that start with \smysql.
That of course only matches the original MySQL image names (if you have a good reason to derive an own version of that image please tell me!).
For every matching $container the script will exec the following command:
With the following variables:
$BACKUP_DIR is a concatenation of $BACKUP_BASE (configured in /etc/default/docker-mysql-backup) and the container name,
$NOW is the current time stamp as date +"%Y-%m-%d_%H-%M".
Thus, the backups are compressed, organised in subdirectories of $BACKUP_BASE, and the SQL-dumps have a time stamp in their names.
$BACKUP_BASE defaults to /srv/backup/mysql/, but can be configured in /etc/default/docker-mysql-backup.
Last but not least, the script also cleans the backups itself.
It will keep the backups of the last 30 days and all backups of days that end with a 2.
So you will keep the backups from the 2nd, the 12th, and the 22nd of every month.
As the script is stored in /etc/cron.daily/ the cron tool will execute the backup script on a daily basis.
Restore a dump
Restoring the dump is quite easy.
Let’s assume your container’s name is $container and the dump to restore carries the time stamp $date.
Then you just need to run:
This will mount the backup directory in /srv of the running container and then decompress and import the SQL-dump on the fly.
Docker is cool. Jails tools into containers. That of course sounds clean and safe and beautiful etc.
However, the tools are still buggy and subject to usual attacks, just as they were running on your main host!
Thus, you still need to make sure your containers are up to date.
But how would you do that?
Approaches so far
On the one hand, let’s assume you’re using Docker Compose, then you can go to the directory containing the docker-compose.yml and call
However, this will just update the images used in that Docker Compose setup – all the other images on your system wouldn’t be updated.
And you need to do that for all Docker Compose environments.
And if you’re running 30 containers of the same image it would check 30 times for an update of that image – quite a waste or power and time..
It is able to go through all your images and update them, one after the other.
That way, all the images on your system will be updated.
However, dupdate doesn’t know about running containers.
Thus, currently running tools and services won’t be restarted..
Better: Docker Auto-Update
Therefore, I just developed a tool called Docker Auto-Update that combines the benefits of both approaches.
It first calls dupdate -s to update all your images and then iterates over a pre-defined list of Docker Compose environments to call a docker-compose up -d --remove-orphans.
The tool consists of three files:
/etc/cron.daily/docker-updater reads the configuration in /etc/default/docker-updater and does the regular update
/etc/default/docker-updater stores the configuration. You need to set the ENABLED variable to 1, otherwise the update tool won’t run.
/etc/docker-compose-auto-update.conf carries a list of Docker Compose environments. Add the paths to the docker-compose.yml files on your system, one per line
As it’s installed in /etc/cron.daily/, cron will take care of the job and update your images and containers on a daily basis.
If your system is configured properly, cron will send an email to the systems administrator when it updates an image or restarts a container.
You see, no magic, but a very convenient workflow! :)
Let’s assume you rendered your FreeBSD system unbootable.. Yeah, happens to the best, but how can you still copy the data stored on a ZFS to another machine?
You probably just shouted RSYNC - but it’s not that easy.
You would need a FreeBSD live os (either on a USB pen drive or on a CD/DVD) and boot into that system.
However, by default you do not have network, the ZPool is not mounted, there is no rsync and SSH is not running, and the live os is not writable, which brings another few issues…
This is a step-by-step how-to through all the obstacles. Just boot into your live os (get it from freebsd.org) and go on with the following…
By default your live system does not have networking setup correctly.
Call ifconfig to see if the network interface is up. If it’s not you can bring it up using:
(assuming your inteface is called em0)
If it is up, you need to configure it.
When you’re using a DHCP server you can just ask for an IP address using:
Otherwise you need to configure the addresses manually:
Afterwards you should be able to ping other machines, such as
Mount the ZPool
Your ZPool won’t be mounted by default; you need to do it manually.
To list all pools available on that machine just call:
This searches through the devices in /dev to discover ZPools. You may specify a different directory with -d (see man page for zpool).
To actually import and mount your ZPool you need to provide its name, for example:
This will import the ZPool zroot. Moreover, the argument -o altroot=/mnt will mount it to /mnt instead of / and the -f will mount it even if it may be in use by another system (here we’re sure it isn’t, aren’t we?).
Create some Writeable Directories
The next problem is, that you do not have permissions to write to /etc, which you need to e.g. create SSH host keys etc.
However, that’s also not a big issue as we have the unionfs filesystem! :)
UnionFS will mount a directory as an overlay over another directory.
Let’s assume you have some space in $SPACE (maybe in the ZPool that you just mounted or on another USB drive), then you can just create a few directories:
and mount it as unionfs to the root’s equivalents:
Now we can write to /etc, while the actual changes will be written to $SPACE/etc! Isn’t that a great invention?
Start the SSH service
Now that /etc is writable we can start caring about the SSH daemon.
First, we need to configure it to allow root to login.
Add the follwing line to the /etc/ssh/sshd_config:
Then, we can start the ssh daemon using:
It will automatically create host keys and all the necessary things for a first start of SSH.
If that was successful, port 22 should now be open:
Set root Password
To be able to login you of course need to set a root password:
Aftwerwards, you should be able to login through SSH from any other machine. Go ahaed and give it a try!
Install and Run rsync
Almost there, but the freeBSD live image doesn’t come with rsync installed.
So we need to do it manually:
This will first tell us that not even pkg is installed, but answering the question with y it will automatically install itself.
And as everything is mounted as UnionFS, the stuff will actually be installed to $SPACE/... instead of /.
However, you should now be able to do the rsync job from where ever you want :)
Just tried to install Grub on a debootstrap‘ed hard drive, but Grub complained:
Never heard of that FlexNet thing, but according to Wikipedia it’s a software license manager.
And we all know how this whole DRM thing just bugs us..
So it bugged me because the new system wouldn’t boot properly..
Other people having similarproblems.
However, it seems impossible to force grub overriding this sector, but you may wipe it manually.
In my case sector 32 was infected by DRM, so I did the following:
If that’s done Grub installs like a charm,
the system booted again,
and the admin was happy that another DRM thing died :)
As I’m working with Docker quite intensively it was about time to develop some tools that help me managing different tasks.
Some of them have already been existing as functions in my environment or something, but now they are assembled in a git repository at GitHub.
The toolbox currently consists of the following tools:.
dclean cleans your setup
The Docker-Clean tool dclean helps getting rid of old, exited Docker containers.
Sometimes I forget the --rm flag during tests, and when I realise it there are already hundreds of orhpaned containers hanging around..
Running dclean without arguments removes all of them quickly.
Additionally, the dclean tool accepts a -i flag which will clean the images.
It will prune all dangling images.
Dangling images are orphaned and usually not needed anymore.
Thus, dclean -i will remove them.
denter gets you into a containers
The Docker-Enter tool denter beames you into a running Docker container.
Just provide the container’s name or CID as an argument to get a /bin/bash inside the container.
Internally, denter will just call
with $EXEC being /bin/bash by default.
So there is no magic, it’s just a shortcut..
You may overwrite the program to be executed by providing it as a second argument.
will execute ps -ef in the container with the id SOMEID.
dip shows IP addresses
The Docker-IP tool dip shows the IP addresses of running containers.
Without arguments it will print the IP addresses, names, and container ids of all running containers.
If your interested in the IP address of a specific container you may pass that container’s CID as an argument with -c, just like:
This will show the IP of the container with id SOMEID.
dkill stops all running containers
The Docker-Kill tool dkill is able to kill all running containers.
It doesn’t care what’s in the container, it will just iterate over the docker ps list to stop all running containers.
As this is quite dangerous, it requires a -f flag to actually kill the containers.
You may afterwards run the dclean tool from above to get rid of the cadavers..
dupdate updates images
The Docker-Update tool dupdate helps you staying up-to-date.
It will iterate over all your images and tries to pull new versions of that image from the Docker registry (or your own registry, if you have one).
By default, it will echo the images that have been updates and tells you which images cannot be found (anymore) on the registry.
You may pass the -v to dupdate to enable verbose mode and also get a report for images that do not have a newer version at the registry.
This way, you can make sure that all images are checked.
Similarly, you can pass -s to enable silent mode and suppress messages about images that cannot be found at the registry.
I’m using the i3 window manger.
As smart as possible, increases productivity, and feels clean.
Exactly how I like my desktop.
I’m still very happy that Uschy hinted me towards i3!
However, I’m experiencing a problem with highlighted text in Okular, my preferred PDF viewer.
When I highlight something in Okular the highlight-color (blue) is far too dark, the highlighted text isn’t readable anymore.
I used to live with that, but it was quite annoying.
Especially when you’re in a meeting/presentation and you want to highlight something at the projector.
I just saw that problem occurring in Okular.
Not sure why, but I honestly do not understand this whole desktop config thing – probably one of the reasons why I love i3 ;-)
Today, I eventually digged into the issue and found out what’s the problem how to solve the problem.
Apparently, Okular uses a Qt configuration, that can be modified using the qtconfig tool.
Just install it (here for Qt4 applications):
When you run qt4-qtconfig a window will pop up, as you can see in the figure on the right:
Select a GUI Style that is notDesktop Settings (Default), e.g. Cleanlooks.
Then you can click the Tune Palette… button in the Build Palette section.
A second window will pop up. Select Highlight in the Central color roles section.
Finally you’re good to select the hightlight color using the color chooser button! :)
Was a bit difficult to find, but the result is worth it!
The figure on the bottom shows the new highlight color – much better.
I will probably never understand all these KDE, QT, Gnome, GTK, blah settings.
Every environment does it differently and changes the configuration format and location like every few months.
At least for me that’s quite frustrating…