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Upgrading to Ubuntu 15.04 Vivid Vervet

Hello!

Yesterday you probably noticed some downtime. This is because I was upgradting this server's operating system from Ubuntu 14.10 to Ubuntu 15.04! Since I noticed a few things that you should watch out for when upgrading, I thought that I would make a post about it.

For the most part the upgrade went smoothly, but I did hit a few snags. Firstly, if you have got any custom ppas set up for apt-get, you will want to make a list of them (they are located in /etc/apt/sources.list.d) because the upgrade will annoyingly disable them all :( It's not too much trouble to fix but it is annoying.

Secondly, there are two new mime types that have been added /etc/mime.types. If you have made any customisations to this file (I have added text/x-markdown), then you will want to make a note of them and re-add them afterwards. Don't forget to restart your http servers after changing it!

There are some changes that require the ssh daemon to be stopped, so make sure you don't do the upgrade over ssh.

You will get asked which interfaces DCHPv6 should listen on / send requests to. If you use your linux box as a router and have it handing out IP addresses, then you will need to take note of which interfaces you have on your box and which one is which.

By far the biggest problem for me though was the switch from upstart to systemd. This server is hosted by OVH under one of their VPS hosting plans (which are great by the way!), which means that it is virtualised using OpenVZ. It also means that I can't choose my kernel :( I suspect that this is the reason that I can't use systemd, though if anyone has any other ideas, I would love to hears them - leave them in the comments below. When it has finished the upgrade, it couldn't reboot, instead telling me that it couldn't find an alternative telinit implementation to spawn. The solution to this is simple though (don't forget to run as root):

apt-get install upstart
apt-get remove systemd
apt-get install upstart-sysv

The last package in the above (upstart-sysv) should be install automatically, but you should make sure that it is installed - it is the package that prevents it from automatically trying to switch you back to systemd at the next available opportunity.

I hope this post is useful! If you do find it helpful, please leave a comment. If people seem to like it I might start posting full upgrade guides.

Securing a Linux Server Part 1: Firewall

Welcome to a new tutorial series, where I will show you what I have learnt so far about making sure that your linux server (and desktop too!) are secure so that nobody can get in (easily) and assume control.

Disclaimer: This tutorial series will not cover everything, and should not be taken to. There probably will be some mistakes in this post too. Check other guides online or consult a professional to make sure that your machine is secure. Please suggest improvements or point out mistakes in the comments.

To start this tutorial session off, I will talk about firewalls. Firewalls control how data is allowed to travel in and out of your computer. In Ubuntu, a firewall called ufw, the 'uncomplicated firewall' is already present. It acts as a nice frontend to iptables, which I find to be difficult to understand and use. We will be using that as our firewall.

I have done an asciinema recording on a virtual machine of this whole process:

Enabling the firewall

Ufw by default allows all outgoing connections and denys all incoming connections. This means that if you are using ssh to connect to your server, you will need to open the appropriate ports first before enabling ufw. Do that like this:

~$ sudo ufw allow 22/tcp

Ufw will automatically configure iptables to allow incoming connections on port 22 that use tcp. I will talk more about allowing and denying different connections later.

Just in case ufw blocks your ssh connection and you are unable to get back in, you can use another program called at to schedule the disabling of the ufw so that you can get back in again. If you don't have it installed, you can install it with sudo apt-get install at.

~$ sudo at -vM now +10 minutes
ufw disable
^D

Where ^D stands for CTRL + D. Now that you have it set such that ufw will disable itself in 10 minutes time, we go ahead and turn ufw on:

~$ sudo ufw enable

It will warn you that this may disrupt any existing ssh connections you have open. Reply yes to this. Once it have been enabled successfully, you should check that you can still ssh into your server (if that is the method that you are using to control it). If yes, great! If not, ufw will disable itself in 10 minutes and then you can try again.

Now that we have ufw enabled, we can cancel the at job we created to disable ufw. Type sudo atq to list the jobs you have schedules, and sudo atrm <number> to remove it, where <number> is the number of the jobs that you want to delete.

You may also want to cheeck the status of ufw to make sure that it is enabled, or to get a list of the rules that are currently in force. You can do that like this:

~$ sudo ufw status
Status: active

To                         Action      From
--                         ------      ----
80/tcp                     ALLOW       Anywhere
80/tcp (v6)                ALLOW       Anywhere

Allowing connections

Allowing connections through the firewall is easy. Simply type something like this:

~$ sudo ufw allow 80/tcp

Ufw will automatically configure iptables, in this example, to allow all connections on port 80 that use tcp. It will also configure it appropriately for both ipv4 and ipv6. Replace 80 with the port number you want to allow, and tcp with udp if needed. Ufw also understands several protocol names, and can configure itself accordingly:

~$ sudo ufw allow http
~$ sudo ufw allow imap

Denying connections

Denying all connections on a given port is very similar., Simply type something like this:

~$ sudo ufw deny 4722/tcp

The above would deny all tcp connections on port 4722.

You can also prevent a particular ip from gaining access to your server:

~$ sudo ufw deny from 123.123.123.123

The above would block all packets from the ip address 123.123.123.123. It works with IPv6 addresses too:

~$ sudo ufw deny from 2607:f8b0:4003:c05::65

The above would block all packets from the ip address 2607:f8b0:4003:c05::65, which just happens to belong to Google.

Port Ranges

You can open a range of ports with a colon:

~$ sudo ufw allow 60000:61000/udp

The above will allow udp connections on any port in the range 60,000 - 61,000 (the ports used for mosh).

Deleting Rules

Deleting rules can be done like this:

~$ sudo ufw delete allow 4724/tcp

The above would delete the rule(s) allowing tcp connections on port 4724.

Summary

In this post, I have shown you how to activate and configure a simple firewall that is bundled with Ubuntu. Next time, I will talk about securing you ssh daemon.

If you spotted a mistake in this post, have a suggestion, or are having trouble following along, please leave a comment below.

Other useful posts

These posts helped me to understand and use the uncomplicated firewall:

Recording animated gifs on Linux with Silentcast

Using Silentcast

A few months ago I was asked how I created animated gifs on Linux, and I said that I use silentcast. I also said that I'd write a blog post on it. I've been very busy since then, but now I have found some time when I remembered to post about it and am not exhausted.

Silentcast is a very versatile screen recording application that outputs either a set of png images, an animated gif, or 2 different types of video. It uses png files to store frames, so it isn't suitable for recording at a high fps or for very long, but it is still brilliant for recording short clips for your blog or to accopany a bug report.

Silentcast's dialogs stay in front of everything else that you have open, so you don't need to worry about loosing the window somewhere along the line. It integrates nicely with the Unity desktop (I haven't tried others yet), which makes it feel more intuitive and makes it easer to use. It also allows you to modify the intermediate png files before the final product is stitched together, too, allowing for precise edits to make the resulting gif loop perfectly.

It is written in bash, which makes it perfectly suited for usage on both Mac and Linux system , but I don't think that Windows is supported as of the time of posting. The other issue is that it took me a little while to work out how to record a custom area - this is done by the "Transparent Window Interior" option under "Area to be recorded". I also find it to be a little bit unpoliished around the edges (the icon especially needs some work), but overall it is an excellent piece of software that makes recording an animated gif on Linux a breeze - it's streets ahead of any other competing projects.

The animated gif above was taken and modified from Silentcast's GitHub project page.

Art by Mythdael