Linux has a sohpisticated firewall built right into the kernel: It’s called
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
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
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
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
You probably heard about the conflict between the fields Common Name (
CN) and Subject Alt Names (
subjectAltName) in SSL certificates.
It seems best practice for clients to compare the
CN value with the server’s name.
However, RFC 2818 already advised against using the Common Name and google now takes the gloves off.
Since Chrome version 58 they do not support the CN anymore, but throw an error:
Subject Alternative Name Missing
Good potential for some administrative work ;-)
Check for a Subject Alternative Names
You can use OpenSSL to obtain a certificate, for example for
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
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?
I already maintain a convenient configuration for AdAway at stuff.lesscomplex.org/adaway.txt, which blocks Google’s captive portal detection.
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
Changed as of Android 7, see update below!
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:
Changed as of Android 7, see update below!
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.
See also the CaptivePortal description at the android reference.
Create captive portal detection server with Nginx
Just add the following to your Nginx configuration:
Create captive portal detection server with Apache
Create captive portal detection server with PHP
A simple PHP script will also do the trick:
As of Android 7 the settings have changes.
To enable/disable captive portal detection you need to set
captive_portal_mode to either
0Don’t attempt to detect captive portals, see CAPTIVE_PORTAL_MODE_IGNORE.
1When detecting a captive portal, display a notification that prompts the user to sign in, see CAPTIVE_PORTAL_MODE_PROMPT.
2When detecting a captive portal, immediately disconnect from the network and do not reconnect to that network in the future, see CAPTIVE_PORTAL_MODE_AVOID.
To define the captive portal server you actually have three settings:
captive_portal_use_httpsshould the phone use HTTPS for captive portal detection? (
captive_portal_http_urlURL to the captive portal w/o HTTPS.
captive_portal_https_urlURL to the captive portal when using HTTPS.
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/MariaDB 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.
How does Docker MySQL-Backup work?
The tool basically consists of two scripts:
- a config file in
/etc/default/docker-mysql-backupto setup the path for the backup location and the path to gzip,
- the script
/etc/cron.daily/docker-mysql-backupwhich does the actual job.
/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 either
The actual filter command is
That of course only matches the original MySQL/MariaDB 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_DIRis a concatenation of
/etc/default/docker-mysql-backup) and the container name,
$NOWis the current time stamp as
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
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
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
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.
Manual installation through GitHub
Clone the Docker MySQL-Backup repository:
Copy the backup script to the
cron.daily (most likely
/etc/cron.daily/) directory on your system:
Copy the configuration to
Installation from my Apt repository
If you’re running a Debian-based system you may want to use my apt-repository to install the Docker MySQL-Backup tool. In that case you just need to run
Afterwards, look into
/etc/default/docker-mysql-backup for configuration options.
This way, you’ll always stay up-to-date with bug fixes and new features :)
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..
On the other hand, you may use the dupdate tool, introduced earlier:
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.
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-updaterreads the configuration in
/etc/default/docker-updaterand does the regular update
/etc/default/docker-updaterstores the configuration. You need to set the
1, otherwise the update tool won’t run.
/etc/docker-compose-auto-update.confcarries a list of Docker Compose environments. Add the paths to the
docker-compose.ymlfiles 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! :)
To install the Docker Auto-Update tool, you may clone the git repository at GitHub. Then,
- move the
- move the
./etc/default/docker-updaterconfig file to
- update the setup in
/etc/default/docker-updater– at least set
- create a list of Docker Compose config files in
/etc/docker-compose-auto-update.conf- one path to a
If you’re using a Debian based system you may install the Docker-Tools through my apt-repository:
/etc/default/docker-updater and at least set
This way, you’ll stay up-to-date with bug fixes etc.
The tool will update your images and containers automatically – very convenient but also dangerous! The new version of an image may break your tool or may require an updated configuration.