How to set custom time and date on virtual machine

If your virtual machine is managed by VirtualBox, there are 2 services which synchronize date and time. The first step is to stop them from restoring the time:

sudo service systemd-timesyncd stop
sudo service vboxadd-service stop

After that you can set time with date command, for example:

sudo date +%T -s "00:12:00"

or

date -s "27 OCT 2016 00:12:00"

or

date +%Y%m%d -s "20161027"

systemd: Create a service

Example of the service configuration file `selenium_hub.service`:

[Unit]
Description=Selenium hub

[Service]
Environment=DISPLAY=:99
ExecStart=/bin/bash -c 'java -jar /opt/selenium/selenium-server-standalone.jar -role hub'
User=root

[Install]
WantedBy=multi-user.target

Place this file to /etc/systemd/system/ (symlinking won't work) and refresh available services:

sudo systemctl daemon-reload

Now it is possible to start the service with command:

sudo service selenium_hub status

You can also make it start when a system starts by executing one of the following commands:

sudo chkconfig selenium_hub on
sudo systemctl enable selenium_hub.service

Overwrite response body with mitmproxy

It is possible to run python script with mitmproxy. For example:

mitmproxy -s "modify_response_body.py"

The modification process is very simple. Take a look at the modify_response_body.py code:

from mitmproxy.models import decoded


def response(context, flow):
    if "site.custom.min.js" in flow.request.path:
        with decoded(flow.response):  # automatically decode gzipped responses.
            flow.response.content = flow.response.content.replace(
                "surfly.com",
                "sitesupport.net")

The script will look for a request with path that contains site.custom.min.js and replace all occurences of surfly.com with sitesupport.net

mitmproxy and websocket connections

mitmproxy does not really supports websocket connections. However, there is a workaround: it is possible to just ignore them. For example all your websocket conections are encrypted and go to the special subdomain:

mitmproxy --ignore ws.sitesupport.net:443

Slow performance and no sound with pulseaudio-equalizer

I installed pulseaudio-equalizer on my laptop, but after 3 days my laptop became slow and there wes no sound at all. It happened because of the pulseaudio-equalizer package. Even if you remove it, the problem won't go because it adds some configuration to the pulseaudio itself and doesn't remove it.
Open pulseaudio configuration file, find pulseaudio-equalizer section and remove it completely:

sudo nano ~/.pulse/default.pa

The section should look like this:

### Generated from: pulseaudio-equalizer
load-module module-ladspa-sink sink_name=ladspa_output.mbeq_1197.mbeq master=alsa_output.pci-0000_05_00.0.analog-stereo plugin=mbeq_1197 label=mbeq #control=-0.2,-0.2,-0.2,-0.2,3.5,3.5,3.5,3.5,3.5,3.5,3.5,2.5,2.5,0.0,0.0
set-default-sink ladspa_output.mbeq_1197.mbeq
set-sink-volume alsa_output.pci-0000_05_00.0.analog-stereo 65536
set-sink-mute alsa_output.pci-0000_05_00.0.analog-stereo 0
### END: Equalized audio configuration

Reboot your computer and enjoy restored performance and sound!

fedora: Install latest VirtualBox

The latest virtualbox can be installed from the Oracle repository. First of all we need to install oracle repository:

cd /etc/yum.repos.d/
sudo wget http://download.virtualbox.org/virtualbox/rpm/fedora/virtualbox.repo

Install dependencies:

sudo dnf install binutils gcc make patch libgomp glibc-headers glibc-devel kernel-headers kernel-PAE-devel dkms

Install virtualbox. Note: pointing 5.1 is important as at the moment that is how it is called in the oracle's repository:

sudo dnf install VirtualBox-5.1

You might need to rebuild kernel modules with command:

sudo /usr/lib/virtualbox/vboxdrv.sh setup

And add current user to the xboxusers group:

sudo usermod -a -G vboxusers user_name

I had a problem reconfiguring kernel modules on my laptop. The error wasn't indicating anything specific. The problem was in the BIOS settings. Secure Boot should be disabled to allow module reconfiguration.

Speeding up Django tests: Disable migrations during tests

Running all migrations before the testrun can take a lot of time. For some reason, they are very slow. Running 50 migrations could easily take more than 1 minute! Fourtunately, disabling migrations is very easy. Just add the following code to the test's settings file:

# Disable migrations when running tests
MIGRATION_MODULES = {
    app[app.rfind('.') + 1:]: 'your_app.migrations_not_used_in_tests'
    for app in INSTALLED_APPS
}

Some of our migrations add data to the database and tests expect this data to be there. To fix that problem we will need to overwrite test runner. Add to the settings file:

TEST_RUNNER = 'your_app.test_runner.CustomDiscoverRunner'

Then create test_runner.py in the project root with the following content:

from django.test.runner import DiscoverRunner

from myapp.management.commands._setup_functions import create_groups


class CustomDiscoverRunner(DiscoverRunner):
    def setup_databases(self, **kwargs):
        result = super(CustomDiscoverRunner, self).setup_databases(**kwargs)
        create_groups()
        return result

After recreating test database directly from the models (what is really fast), test runner will execute create_groups() function which populates database with all necessary data.

Analyze http2 with Wireshark

Most of the time http2 requests are sent over TLS, which means that Wireshark needs to decrypt information before we can analyze it. Fortunately, Chrome allows to dump cryptographic keys in the file. To do that, before you start Chrome, export environmental variable SSLKEYLOGFILE:

export SSLKEYLOGFILE=/home/jsn/Downloads/sslkeys/sslkeys.log

Firefox also allows to do it, but as for now (48) it is disabled by default. Get more information Now we need to configure Wireshark to use exported keys. Go to Edit - Preferences - Protocols - SSL and point (Pre)-Master-Secret log filename to the file with keys (/home/jsn/Downloads/sslkeys/sslkeys.log) Now you can filter records by http2 and analyze them.

tar: Archives cheatsheet

Create archive:

tar -czvf compressed_file.tar.gz original_file.sql

Extract files:

tar -xzvf compressed_file.tar.gz

Number of connections to the specific port

To check how many connections are exists to the port 3306:

watch -n 1 "sudo netstat -anp | grep 3306 | wc -l"

fedora: Increase watches limit

Open configuration file:

sudo vim /etc/sysctl.d/90-override.conf

Add the following line to the end of the file to increase watches limit:

fs.inotify.max_user_watches = 524288

postgresql: How to fix ERROR: new encoding (UTF8) is incompatible

Sometimes I'm unable to create a database in UTF8 encoding because the template database is in the ASCII encoding. To fix it we need to recreate template database in UTF8. Start psql console:

psql -U postgres

Run the following commands:

UPDATE pg_database SET datistemplate = FALSE WHERE datname = 'template1';

DROP DATABASE template1;

CREATE DATABASE template1 WITH TEMPLATE = template0 ENCODING = 'UNICODE';

UPDATE pg_database SET datistemplate = TRUE WHERE datname = 'template1';

\c template1

VACUUM FREEZE;

Preview apiary.io docs

It is possible to preview apiary docs before pushing to the repository. To do that, install apiaryio CLI:

gem install apiaryio

The executable file of the installed application is located somewhere there:

/home/jsn/.gem/ruby/gems/apiaryio-0.3.3/bin/apiary

Generate html file to preview docs and open it in the browser:

/home/jsn/.gem/ruby/gems/apiaryio-0.3.3/bin/apiary preview --output="preview.html"

fedora: Install HipChat 4

UPDATED: Atlassian team has a rpm repository which they do not mention on the official download page. To install hipchat4 from it, create a repository file:

sudo gedit /etc/yum.repos.d/hipchat4.repo

With the following content:

[atlassian-hipchat4]
name=Atlassian Hipchat4
baseurl=https://atlassian.artifactoryonline.com/atlassian/hipchat-yum-client/
enabled=1
gpgcheck=0

Install hipchat:

sudo dnf install hipchat4 --refresh

-----
Atlassian team wasn't able to release rpm package for the latest hipchat for more than 2 months. Fortunately, it is possible to intsall hipchat from the debian package. Download the latest available debian package from here.

Install alien package which will convert deb package to the rpm package:

sudo dnf install alien

Convert hipchat debian package to rpm package:

sudo alien HipChat4-4.0.1635-Linux.deb -r

Install hipchat:

sudo dnf install hipchat4-4.0.1635-2.x86_64.rpm

fedora: X11 forwarding issue

Here I have a nice post how to open browser window on the remote machine. On fedora 23 that method stopped to work. It was possible to start simple application like xclock, but firefox or chrome.
The problem is in the xorg-x11-drv-intel package. The latest version which is available in the official fedora repo is xorg-x11-drv-intel-2.99.917-19.20151206.fc23 (as for 13 May 2016). Downgrading the package to the previous version fixes the issue.
The latest version of the package which is known to work is xorg-x11-drv-intel-2.99.917-19.20151206.fc23. Let's install it:

sudo dnf install xorg-x11-drv-intel-2.99.917-16.20150729.fc23.x86_64

It would be also nice to lock the package version, so the next time you start updating the system it won't be updated:

sudo dnf install 'dnf-command(versionlock)'
sudo dnf versionlock add xorg-x11-drv-intel

How to format USB stick as FAT32

First step is to find the name of the USB stick in the system:

sudo fdisk -l

In my case the name of the device is /dev/sdc If the USB stick is mounted, unmount it with command:

sudo umount /dev/sdc

Finally, run the command to format USB stick:

mkdosfs -F 32 -I /dev/sdc

apt-get: Install package with specific version and hold it on the next upgrade

If you need to install specific version of the package in your system, you can list all available package versions with the command:

apt-cache madison varnish

After you know the package version you can install it with:

sudo apt-get install varnish=4.0.2-1

To hold the package on the selected version during the next upgrade:

sudo apt-mark hold varnish

You can also "unhold it" with:

sudo apt-mark unhold varnish

fedora: Install accidentally removed sqlite package

If sqlite package (or, for example, sqlite-libs package) was accidentally removed from your system, you can't reinstall it using dnf or yum because they depend on it. To fix this problem, download missing rpm package from https://admin.fedoraproject.org/pkgdb/package/rpms/sqlite/. From the downloaded file location, run command:

rpm2cpio sqlite-libs-3.11.0-3.fc23.x86_64.rpm | cpio -idmv

It will create usr folder with extracted files. Run next command to copy them to the right location:

sudo cp -R usr/ /

Pay attention to the package architecture. You should be able to use dnf again. It would be good to reinstall missed package with dnf again:

sudo dnf --best --allowerasing install sqlite-libs

Set up logging with Flask and gunicorn

Let's say that we want to be able to log in the specified log file and syslog. Our application - is a simple app.py file. The first step, I suggest, is to set logger name:

import os

from flask import Flask


app = Flask(__name__)
app.logger_name = "flask.app"

# ...

if __name__ == "__main__":
    app.run()

You can log in your project like this:

app.logger.info("Hello")

A log configuration file for the gunicorn may look something like this:

[loggers]
keys=root, gunicorn.error, gunicorn.access, requests.packages.urllib3.connectionpool, __main__

[handlers]
keys=log_file, syslog

[formatters]
keys=generic

[logger_root]
level=INFO
handlers=log_file, syslog

[logger___main__]
level=DEBUG
handlers=log_file, syslog
propagate=0
qualname=__main__

[logger_gunicorn.error]
level=INFO
handlers=log_file, syslog
propagate=0
qualname=gunicorn.error

[logger_gunicorn.access]
level=INFO
handlers=log_file, syslog
propagate=0
qualname=gunicorn.access

[logger_requests.packages.urllib3.connectionpool]
level=WARN
handlers=log_file, syslog
propagate=0
qualname=requests.packages.urllib3.connectionpool

[handler_syslog]
class=logging.handlers.SysLogHandler
formatter=generic
args=()

[handler_log_file]
class=logging.FileHandler
formatter=generic
args=('/home/myuser/log/gunicorn.log',)

[formatter_generic]
format=%(asctime)s [%(process)d:%(name)s:%(lineno)s] [%(levelname)s] %(message)s
datefmt=%Y-%m-%d %H:%M:%S
class=logging.Formatter

The last step is starting gunicorn:

gunicorn app:app --log-config /home/myuser/gunicorn_logging.conf

Conditions in the Vagrantfile

Every developer could have different hardware configuration. Let's say one is ready to provision a virtual machine with 4 cpus and another - only with 2 cpus. It would be nice to handle this situation with environmental variables:

Vagrant.configure(2) do |config|
  config.vm.box = "box-cutter/debian81"

  config.vm.network "private_network", ip: "192.168.33.10"
  config.ssh.forward_agent = true

  config.vm.provider "virtualbox" do |vb|
    vb.memory = "4096"
    if ENV.has_key?('VAGRANT_CPUS')
      vb.cpus = ENV['VAGRANT_CPUS']
    else
      vb.cpus = "2"
    end
  end
end

With that config the number of the provisioned cpus can be controled via VAGRANT_CPUS environmental variable.

Gnome: How to record a screencast

Gnome 3 has in-built screencast functionality. To start record your screen simply press

Ctrl + Shift + Alt + R

The indicator (a red circle) will appear in the panel, informing you that the recording has started. Default video length is limited to 30s, what might be not enough in most cases. To adjust this parameter install dconf Editor and go to the:

org.gnome.settings-daemon.plugins.media-keys

And look for max-screencast-length to adjust.

komodo: Use custom version of the jshint

Komodo has an outdated version of the jshint. If you want to use, for example, moz:true option, you need to upgrade it. I tried it multiple times and never succeed. The problem was not in komodo (at least that was the developers say) but in jshint. All latest versions of the jshint does not work with komodo. The latest version of the jshint that is known to work with komodo is 2.6.0. To configure komodo to use jshint 2.6.0, open File -> Preferences -> Syntax Checking -> Language: Javascript and point Custom linter to the downloaded file:

The version 2.6.0 is somewhere from the early 2015 but still much better then the default one.