Upgrading Debian 8 ‘Jessie’ to Debian 9 ‘Stretch’


Debian 9 Stretch was released as the latest stable version of the Linux Distribution: Debian. While it’s always possible to install Debian 9 fresh from scratch, it’s also possible to perform an in-place upgrade from Debian 8. The following post describes the necessary steps to do so.

For a incredibly thorough documentation of the process, I suggest you also read through the official release notes.


  • Upgrading to Debian 9 Stretch is only supported from Debian 8 Jessie. If you are running a version older than 8, you must first upgrade to 8 before working through this process.
  • The upgrade involves a kernel update, so a reboot will be required toward the end of the process.
  • It is strongly recommended that you have a full system backup or backup of any important data before proceeding with the upgrade, ensure that you have a plan to roll back. In the case of a virtual machine, take a snapshot before starting.

Performing the upgrade to Debian 9 Stretch

Before proceeding with the upgrade, please read through the list of issues to be aware of when upgrading to Stretch.

  1. It is recommended that you have your Debian 8 Jessie installation completely up to date before starting, to do this run “apt-get update” followed by “apt-get upgrade” and install available updates.
root@debian8:~# apt-get update
root@debian8:~# apt-get upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

In this case all updates have been applied already, so it’s fine to proceed.

Edit the /etc/apt/sources.list file, my file is shown below. As you can see all of the lines are currently specifying “jessie”. Note that your mirror sources will likely be different which is fine.

deb http://ftp.ch.debian.org/debian/ jessie main
deb-src http://ftp.ch.debian.org/debian/ jessie main

deb http://security.debian.org/ jessie/updates main contrib
deb-src http://security.debian.org/ jessie/updates main contrib

# jessie-updates, previously known as 'volatile'
deb http://ftp.ch.debian.org/debian/ jessie-updates main contrib
deb-src http://ftp.ch.debian.org/debian/ jessie-updates main contrib

Change the instances of “jessie” to “stretch”, you can either do this manually, or automatically with the below sed command.

sed -i 's/jessie/stretch/g' /etc/apt/sources.list

You can either use “stretch” or “stable”, as Debian 9 Stretch is now the current stable version as of writing. However note that if you use stable instead of the specific release name, in future when Debian 10 is released that will be the stable version so you may upgrade to that unintentionally.

The recommended way to upgrade Debian is with the ‘apt-get’ command. First update the list of available packages with the below command, as we’ve just updated the sources.list file.

apt-get update

Use “apt list –upgradable” command to quickly see what will be installed, updated, and removed during the upgrade process without affecting the system.

apt list --upgradable

Sample Output:

unattended-upgrades/stable 0.93.1+nmu1 all [upgradable from:]
util-linux/stable 2.29.2-1 amd64 [upgradable from: 2.25.2-6]
uuid-runtime/stable 2.29.2-1 amd64 [upgradable from: 2.25.2-6]
vim/stable 2:8.0.0197-4 amd64 [upgradable from: 2:7.4.488-7+deb8u3]
vim-common/stable 2:8.0.0197-4 amd64 [upgradable from: 2:7.4.488-7+deb8u3]
vim-runtime/stable 2:8.0.0197-4 all [upgradable from: 2:7.4.488-7+deb8u3]
vim-tiny/stable 2:8.0.0197-4 amd64 [upgradable from: 2:7.4.488-7+deb8u3]

Now that the list of available packages has been updated from the mirror, run the below command to perform a minimal upgrade.

apt-get upgrade

This is known as a minimal system upgrade as it only upgrades packages that can be upgraded without needing any other packages to be removed or installed, so it’s a safe place to start. This upgraded 932 packages requiring 412MB on my system.

Now you’re ready to do the complete system upgrade, this will upgrade to the latest available version for all packages installed.

apt-get dist-upgrade

Ensure that you have enough free disk space to complete the operation, in my case it notes that afterwards 1,048MB of additional disk space will be used with 639 package upgrades and 479 newly installed packages.

During the distribution upgrade, services installed on your system needs to be restarted after up gradation of each service packages (ex. Apache, NTP) which may cause you the service interruptions. You can choose to restart automatically during upgrade or manually after the upgrade. Here, I opted to do “an automatic restart of services during the OS upgrade“.

Verify upgrade:

Reboot your machine after the distribution upgrade.


Verify the current version of Debian operating system.

lsb_release -a

Distributor ID: Debian
Description: Debian GNU/Linux 9.0 (stretch)
Release: 9.0
Codename: stretch

Users susceptible to man-in-the-middle attacks due to corporate https inspection

A large number of companies use “security” products to inspect HTTPS traffic for detecting malware and prevent other types of attacks. However, they might inadvertently make their user’s more susceptible to man-in-the-middle attacks by  decrypting and re-encrypting HTTPS connections.

The U.S. Computer Emergency Readiness Team (US-CERT) warns in an advisory that HTTPS inspection products don’t mirror the security attributes of the original HTTPS connections between the client and the server (Mirror: HTTPS Interception Weakens TLS Security | US-CERT).

HTTPS inspection is deployed in companies for checking the encrypted traffic coming from an HTTPS website to make sure it does not contain any malware or any other type of attacks. It basically performs a decryption and re-encryption of the client’s connection to an HTTPS server. The “security” products (proxy, web-gateway, firewall etc.) establish the connection on the client’s behalf by first decrypting the client’s HTTPS connection and re-encrypting the traffic sent to the HTTPS server. The client is served with a different, locally generated certificate by the security product which essentially perform a man-in-the-middle attack.

In some enterprise environments, an HTTPS connection may even be intercepted and re-encrypted multiple times. For example, at the network perimeter by a security gateway product and later, on the endpoint by a client’s antivirus program which needs to inspect the traffic for malware.

The problem revolves around the fact that the client’s browser no longer validates the real certificate issued by the server because its replaced with a locally generated certificate from the security product. In return, the task of validating the certificate now falls to the intercepting proxy.

According to the published advisory, those security products are evidently pretty bad at validating server certificates. An investigation conducted by researches from Google, Mozilla, Cloudfare, and multiple Universities states that the intercepted connections use weaker cryptographic algorithms (Source: interception-ndss17). The security products even advertise support for known-broken encryption ciphers that would allow an active man-in-the-middle attack by intercepting and downgrading a connection in order to decrypt it.

The analysis by the researches found that at least 32 percent of connections to e-comerce sites and 54 percent of Cloudflare HTTPS connections, which were intercepted, became less secure than they would have been if the user had connected directly to the server.

Browser makers had a long time to properly unterstand the quirks of TLS connections and certificate validation. Therefore, there is no better client-side implementation of TLS, the protocol used for encrypting HTTPS connection, than the one found in modern browsers.
In comparison, security product vendors use outdated, customised TLS libraries where they even back-port new protocol features. Re-implementing those features found in newer libraries makes them susceptible to serious vulnerabilities.

Furthermore, the US-CERT points out another widespread problem that many products intercepting HTTPS don’t properly validate the certificate chain presented by servers. Certificate-chain verification errors are infrequently forwarded to the client, leading the client to believe that operations were performed with the correct server.

The BadSSL website allows organisations and even employees to check if their HTTPS inspection products improperly validate certificates or allow for insecure ciphers. The client test from Qualys SSL Labs also provides checks for some known TLS vulnerabitiles and weakenesses.


Homebrew on macOS 10.12 Sierra

Apple introduced a few changes with the directory structure and permissions in macOS Sierra 10.12 which in the beginning broke Homebrew on macOS without some CLI magic to manually fix things. However, this has been fixed and Homebrew now full supports macOS 10.12. One of the issues was around a change in permissions on the directory /usr/local which on a new installation either doesn’t exist or wasn’t writeable.

Therefore Homebrew changed their installation routine and migrated the installation folder to /usr/local/homebrew in order to circumvent the whole permissions issues.

Install Homebrew

The following Terminal command will download and install Homebrew. Ruby comes pre-installed with macOS and therefore just copy and paste the following command in your Terminal:

ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"

Homebrew requires the command line tools which ship with Xcode. If you don’t have Xcode or the command line tools installed, macOS will automatically prompt you to install them. You can get away with the command line tools by clicking install.

However, you can also get Xcode by downloading it free of charge from the AppStore. Here, I will simply install the command line tools.

To check for any issues after the installation, run:

brew doctor

To search for an application:

brew search <application_name>

To install an application:

brew install <application_name>

To list all applications installed by Homebrew:

brew list

To remove an installation application:

brew remove <application_name>

To update Homebrew:

brew update

Fixing brew after upgrade to macOS 10.12

If you had brew installed before upgrading to macOS 10.12 you will notice that the upgrade broke brew. The following instructions will fix the broken install and will let you upgrade brew to the latest and greatest which supports 10.12

Accept the license agreement of the command line tools shipped with Xcode

sudo xcodebuild -license

Change ownership of /usr/local to yourself:

sudo chown -R $(whoami) /usr/local

Run brew doctor and brew update. The installer of the update will inform you that it will migrate the brew repository to a new directory:

brew doctor && brew update

=> Migrating HOMEBREW_REPOSITORY (please wait)...
==> Migrated HOMEBREW_REPOSITORY to /usr/local/Homebrew!
Homebrew no longer needs to have ownership of /usr/local. If you wish you can return /usr/local to its default ownership with:
sudo chown root:wheel /usr/local

Change the owner back to the defaults with:

sudo chown root:wheel /usr/local

Homebrew on OSX 10.11 El Capitan

Homebrew is a great package manager for OSX and one of the easiest ways to install Unix tools and open source software (Homebrew most certainly isn’t the only package manager out there: other popular choices: Macports and Fink but Homebrew is my personal favorite).

In order to install Homebrew you need XCode and the respective command lines tools. Xcode is available in the AppStore Screenshot 2016-01-01 13.53.40

Download and Install Xcode. You have to run it once and agree to the license. It will automatically start installing the components. Grab a cup of coffee/tea it will take a few minutes even on a fast machine with a SSD.

Screenshot 2016-01-01 13.54.45

Open up a Terminal and issue the following command to install the command line tools

xcode-select --install

Before you can download and run the script to install Homebrew you have to temporarily disable the System Integrity Protection of OSX in order to write to the /usr folder and create the installation root directory /usr/local.

If you encounter any permissions issues the following article provides instructions to fix the permissions problem. In case you upgraded an existing installation to El Capitan the /usr/local folder will most probably exist already and you will only have to issue the following command to fix the permission problem:

sudo chown -R $(whoami):admin /usr/local

If it’s a fresh installation (wiped the hard drive or simply bought a new Mac) /usr/local will not exist and you have to manually create the folder.

  • Reboot into recovery mode (Hold CMD + R on boot) → Utilities → Terminal
  • Temporarily disable SIP: csrutil disable
  • Reboot back into OSX
  • Open the Terminal application and issue the following command:

sudo mkdir /usr/local && sudo chflags norestricted /usr/local && sudo chown -R $(whoami):admin /usr/local

The command creates the installation directory for Homebrew and removes the directory flags that restrict access to it when SIP is re-enabled.

Reboot back into recovery mode and issue the following command to reenable SIP protection:

csrutil enable

Reboot back into OSX and now you will be able to write to /usr/local and install Homebrew by starting the following script in a Terminal:

ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"


After completion of the installation check for any outstanding issues as suggested at the end of the setup by issuing the following command:

brew doctor

A short list of commands for Homebrew, refer to the man page for more information:

To search for an application: brew search <search_keyword>

To install an application: brew install <application_name>

To list all applications installed by homebrew: brew list

To remove an installed application: brew remove <application_name>

To update Homebrew: brew update

Homebrew’s default installation directory: /usr/local/Cellar
This ensures that Homebrew doesn’t interfere with Unix utilities shipped by OSX (e.g. if you install python3 via Homebrew it will not interfere with the Version shipped with OSX)