Introduction

Aujourd'hui, j'aimerais parler à propos d'OpenVPN. Pour ceux qui ne connaissent pas, OpenVPN est un logiciel libre et avec le code est ouvert et qui implémente des différentes façons de créer des réseaux virtuels (VPN) sécurisés * Wikipedia. L'application est développée par OpenVPN Incorporation et il propose également différents services payants privatetunnel.

De nos jours, de nombreux tutoriaux ont été publiés qui décrivent comment l'installer et configurer. Contrairement à eux, je vais essayer de vous proposer un tutoriel complet d'une installation avec les paramètres de sécurités les plus élevés. Le tutoriel se basera sur OpenVPN 2.4.0 et Debian 9.5. Dans celui-ci, je décrirai autant que possible les différentes étapes afin qu'il soit le plus compréhensible et lisible possible, bonne lecture.

Préliminaire

Pour commencer, nous devons installer le paquet OpenVPN et quelques autres avec l'utilisateur root.

apt update
apt upgrade
apt install -y iptables-persistent openvpn vim sudo

La génération des certificats

Téléchargements et configuration de EasyRSA

Pour générer les certificats, nous allons utiliser EasyRSA. EasyRSA est une application en ligne de commande qui permet de générer et de gérer les clés et certificats nécessaires à l'authentification des clients et serveurs. Vous pouvez télécharger la dernière version disponible ici EasyRSA est un des outils les plus simples à utiliser pour générer les certificats. Cet article se concentre principalement sur la configuration de OpenVPN.

Malheureusement, bien que EasyRSA propose de générer des certificats et clés très sécurisés, ceux-ci ne sont pas les plus sécurisés possible. Pour les générer de façon la plus sécurisée possible, vous devrez utiliser directement OpenSSL. Un tutoriel est disponible ici.

mkdir /tmp/openvpn
cd /tmp/openvpn/
wget https://github.com/OpenVPN/easy-rsa/releases/download/v3.0.5/EasyRSA-nix-3.0.5.tgz
tar xf EasyRSA-nix-3.0.5.tgz
cd EasyRSA-3.0.5
cp vars.example vars
vim vars

Vous devez maintenant modifier le fichier var dans le but d'activer les courbes elliptiques et d'améliorer l'algorithme de hachage utilisé. En étudiant plus en détail les courbes elliptiques, il semblerait que celles proposaient par l'institut national des standards et de la technologie (NIST) soient délibérément affaiblies. C'est pourquoi je recommande d'utiliser la courbe Curve25519 parce qu'elle est aussi solide que son équivalente proposé par le NIST, plus rapide, plus sûr et elle n'a pas été crée par le NIST. Il y a d'autres courbes plus sécurisées disponibles, mais elles ralentissent beaucoup le trafic réseau.

# Activer les courbes elliptiques
set_var EASYRSA_ALGO ec

# Choix d'une courbe en particulier, les courbes supportées par OpenVPN peuvent être affichées avec - openvpn --show-curves
# Impossible de choisir la couber elliptic Curve25519 avec EasyRSA et openvpn ne la supporte pas. set_var EASYRSA_CURVE secp521r1
# Durée de validité du certificat racine avant expiration (en jours)
set_var EASYRSA_CA_EXPIRE 3650

# Durée de validité des certificats avant expiration (en jours)
set_var EASYRSA_CERT_EXPIRE 3650

# Nombre de jours avant vérification de la validité du certificat
set_var EASYRSA_CRL_DAYS 3650

# Choix de l'algorithme de hachage, malheureusement, seul MD5 et la famille SHA sont disponibles
set_var EASYRSA_DIGEST "sha512"

Génération des certificats

Merci de choisir le "Common name" soigneusement pour chaque certificat, en particulier pour le certificat du serveur. Celui-ci sera utilisé par la configuration cliente de OpenVPN pour vérifier le serveur et éviter les attaques de type Homme du milieu.

# Génération d'un répertoire pour conserver les fichiers
./easyrsa init-pki

# Génération du certificat racine de l'autorité
./easyrsa build-ca nopass

# Génération de la clé pour le serveur (server.key) et de la demande de signature du certificat (server.req)
./easyrsa gen-req server nopass

# Signature du certificat serveur par l'autorité de certification créée préalablement (server.ca)
./easyrsa sign-req server server nopass

# Génération de la clé pour le client (client.key) et de la demande de signature du certificat (client.req)
./easyrsa gen-req client nopass

# Signature du certificat client par l'autorité de certification créée préalablement (client.ca)
./easyrsa sign-req client client nopass

Structure des dossiers

# Nous allons maintenant créer une structure de fichiers facile à lire
mkdir /tmp/openvpn/server/
mkdir /tmp/openvpn/server/certificates
cp /tmp/openvpn/EasyRSA-3.0.5/pki/ca.crt /tmp/openvpn/server/certificates/ca.crt
cp /tmp/openvpn/EasyRSA-3.0.5/pki/issued/server.crt /tmp/openvpn/server/certificates/server.crt
cp /tmp/openvpn/EasyRSA-3.0.5/pki/private/server.key /tmp/openvpn/server/certificates/server.key

mkdir /tmp/openvpn/client/
mkdir /tmp/openvpn/client/certificates
cp /tmp/openvpn/EasyRSA-3.0.5/pki/ca.crt /tmp/openvpn/client/certificates/ca.crt
cp /tmp/openvpn/EasyRSA-3.0.5/pki/issued/client.crt /tmp/openvpn/client/certificates/client.crt
cp /tmp/openvpn/EasyRSA-3.0.5/pki/private/client.key /tmp/openvpn/client/certificates/client.key

Configuration réseau

Considérons que nous avons un serveur OpenSSH qui écoute le port 22 et que le port 443 est dédié au serveur OpenVPN.

Règles du pare-feu

*nat
:PREROUTING ACCEPT [0:0]
:INPUT ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
# Forward the VPN traffic to eth0
-A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE

COMMIT

*filter

# Autorise le trafic de l'interface loopback en provenance de l'interface loopback et rejette le reste
-A INPUT -i lo -j ACCEPT
-A INPUT ! -i lo -s 127.0.0.0/8 -j REJECT
-A OUTPUT -o lo -j ACCEPT

# Autorise le ping et les retours du protocole icmp
-A INPUT -p icmp -m state --state NEW --icmp-type 8 -j ACCEPT
-A INPUT -p icmp -m state --state ESTABLISHED,RELATED -j ACCEPT
-A OUTPUT -p icmp -j ACCEPT

# Authorise SSH.
-A INPUT -i eth0 -p tcp -m state --state NEW,ESTABLISHED --dport 22 -j ACCEPT
-A OUTPUT -o eth0 -p tcp -m state --state ESTABLISHED --sport 22 -j ACCEPT

# Authorise le traffic TCP.
-A INPUT -i eth0 -p tcp -m state --state NEW,ESTABLISHED --dport 443 -j ACCEPT
-A OUTPUT -o eth0 -p tcp -m state --state ESTABLISHED --sport 443 -j ACCEPT

# Autorise la résolution DNS et limite l'écoute du serveur OpenVPN à l'interface eth0
-A INPUT -i eth0 -p udp -m state --state ESTABLISHED --sport 53 -j ACCEPT
-A OUTPUT -o eth0 -p udp -m state --state NEW,ESTABLISHED --dport 53 -j ACCEPT

# Autorise le trafic pour l'interface tunnel créée par OpenVPN
-A INPUT -i tun0 -j ACCEPT
-A OUTPUT -o tun0 -j ACCEPT

# rejet du reste
-A INPUT -j REJECT
-A OUTPUT -j REJECT

COMMIT

Attention à bien adapter ces règles à vos besoins avant de les déployer.

sudo iptables-restore < /etc/iptables/rules.v4

Nous pouvons vérifier les règles appliquées avec:

sudo iptables -L

Autoriser le suivi IPV4 et désactivation de ipv6

Dans le fichier /etc/sysctl.d/99-sysctl.conf, ajouter les lignes suivantes pour activer le suivi IPV4 et désactiver le fonctionnement ipv6:

net.ipv4.ip_forward = 1

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1
net.ipv6.conf.eth0.disable_ipv6 = 1

puis activer la nouvelle configuration:

sudo sysctl -p

Suppression de la ligne ipv6 dans /etc/hosts:

#::1 localhost ip6-localhost ip6-loopback

*filter

-A INPUT -j REJECT
-A FORWARD -j REJECT
-A OUTPUT -j REJECT

COMMIT

puis appliquer:

sudo ip6tables-restore < /etc/iptables/rules.v6

Exemple de configuration très sécurisée

Afin de poursuivre, nous allons utiliser une configuration très sécurisée de OpenVPn que nous détaillerons ci-dessous.Attention, suivez bien le tutoriel jusqu'au bout pour vous assurez du bon fonctionnement de la configuration.

Exemple de la configuration serveur

Veuillez copier les lignes suivantes dans /tmp/openvpn/server/server.conf. Il s'agit de la configuration OpenVPN du serveur. N'oubliez pas de remplacer IP_OF_YOUR_OPENVPN_SERVER par l'adresse IP de votre serveur.

#/tmp/openvpn/server/server.conf
local IP_OF_YOUR_OPENVPN_SERVER
dev tun
topology subnet
proto tcp
port 443
server 10.8.0.0 255.255.255.0
tls-server

ca /etc/openvpn/server/certificates/ca.crt
# crl-verify /etc/openvpn/server/certificates/crl.pem
cert /etc/openvpn/server/certificates/server.crt
key /etc/openvpn/server/certificates/server.key
tls-crypt /etc/openvpn/server/certificates/tls_crypt.key

dh none
ecdh-curve ED25519
tls-cipher TLS-ECDHE-ECDSA-WITH-AES-256-GCM-SHA384
cipher AES-256-GCM
ncp-ciphers AES-256-GCM
tls-version-min 1.2
persist-tun
compress

persist-key
keepalive 10 120

user ovpn
group ovpn

status /var/log/openvpn-status.log
log /var/log/openvpn.log

push "redirect-gateway"
push "dhcp-option DNS 10.8.0.1"
push "dhcp-option WINS 10.8.0.1"
push "route-ipv6 2000::/3"

Exemple de la configuration client

Veuillez copier les lignes suivantes dans /tmp/openvpn/client/client.conf. Il s'agit de la configuration OpenVPN du serveur. N'oubliez pas de remplacer IP_OF_YOUR_OPENVPN_SERVER par l'adresse IP de votre serveur ainsi que COMMON_NAME_OF_THE_SERVER_CERTIFICATE par le Common name que vous avez choisi lors de la génération du certificat pour le serveur.

# /tmp/openvpn/client
client
dev tun
remote IP_OF_YOUR_OPENVPN_SERVER 443
proto tcp
resolv-retry infinite
compress
nobind
verify-x509-name "COMMON_NAME_OF_THE_SERVER_CERTIFICATE" name
remote-cert-tls server
tls-cipher TLS-ECDHE-ECDSA-WITH-AES-256-GCM-SHA384
cipher AES-256-GCM
tls-version-min 1.2
auth-nocache
persist-key
persist-tun

status /var/log/openvpn-status.log
log /var/log/openvpn.log
verb 3

ca /etc/openvpn/client/certificates/ca.crt
cert /etc/openvpn/client/certificates/client.crt
key /etc/openvpn/client/certificates/client.key
tls-crypt /etc/openvpn/client/certificates/tls_crypt.key

Durcissement de la configuration OpenVPN

Cette partie décrit la plupart des paramètres de configuration présents dans la configuration exemple.

TCP/IP protocole et port d'écoute

Dans le but d'éviter que le trafic du VPN ne soit bloqué par les pare-feu rencontrés, il est fortement recommandé de remplacer le protocole udp par le protocole tcp. OpenVPN configurait avec le protocole TCP est un petit plus lent, mais a plus de chance de passer outre les configurations réseaux restrictives.

Vous devez également configurer le serveur OpenVPN pour écouter sur le port 443. Le port 443 est le port standard pour un serveur web. Ce port est en général ouvert en sortie dans la plupart des configurations réseau. Pour information, le port standard d'OpenVPN est le 1194.

proto tcp4
port 443

Diffie-Hellman

L'échange de clé Diffie–Hellman key est une méthode sécurisée d'échange cryptographique de clé sur un canal publique. Dans Openvpn, ce protocole est utilisé dans les premières étapes de l'établissement d'une connexion TLS. Avec une clé de 1024 bits ou moins, le protocole est vulnérable à la vulnérabilité Logjam. Les auteurs de la vulnérabilité recommandent d'utiliser un nombre de bits égales ou supérieurs à 2048 bits pour Diffie-Hellman ou de basculer sur une courbe elliptique Diffie–Hellman. Dans ce tutoriel, nous allons utiliser la courbe Diffie–Hellman

HMAC signature pendant la poignée de main TLS

Depuis openVPN 2.4, il est recommandé de remplacer le paramètre tls-auth par tls-crypt. Principalement parce que tls-crypt va en plus de faire exactement la même chose que tls-auth, va chiffrer le canal de contrôle lors de l'établissement d'une connexion chiffrée avec TLS. Pour se faire, il faut ajouter les lignes suivantes:

tls-crypt /etc/openvpn/certificates/ta.key 0

et générer le fichier contenant la clé HMAC key dans le répertoire OpenVPN

openvpn --genkey --secret /tmp/openvpn/server/certificates/tls_crypt.key
cp /tmp/openvpn/server/certificates/tls_crypt.key /tmp/openvpn/client/certificates/tls_crypt.key

Persistent tun/tap device et clé

A chaque fois que votre connexion réseau est interrompue et qu'OpenVPN essaye de se reconnecter, il est possible que vous utilisiez votre réseau non sécurisé. Cela peut exposer des informations sensibles.

persist-key n'est pas un paramètre de sécurité, mais une option pour éviter un problème au redémarrage d'OpenVPN et à l'impossibilité que l'application peut avoir à relire les clés.

persist-tun
persist-key

Utilisateur limité

Probablement un des paramétrages les plus importants. Dans le but de réduire l'impact d'une vulnérabilité d'OpenVPN, il est hautement recommandé de l'exécuter avec un utilisateur possédant des droits limités. Pour ce faire, nous allons créer un utilisateur dédié à OpenVPN avec des droits limités.

adduser --system --shell /usr/sbin/nologin --no-create-home ovpn
groupadd ovpn
usermod -a -G ovpn ovpn

et spécifié à OpenVPN d'utiliser cet utilisateur:

user ovpn
group ovpn

Algorithme de chiffrement et fonction de hachage

tls-version-min 1.2
ncp-ciphers AES-256-GCM:AES-256-CBC
tls-cipher TLS-ECDHE-ECDSA-WITH-AES-256-GCM-SHA384
cipher AES-256-GCM
ecdh-curve ED25519
dh none

Nous allons désactiver la conservation en cache du mot de passe afin de réduire la surface d'attaque.

auth-nocache

Algorithmes de compression

Dans OpenVPN version 2.3, l'algorithme de compression LZ0 est celui par défaut. Depuis OpenVPN version 2.4, l'algorithme de compression LZ4 est disponible. Contrairement à ce que dit la documentation, la version 2 de LZ4 est est disponible et pas encore documenté.

Si la sécurité est votre seul critère, vous devez désactiver cette fonctionnalité. En effet, les vulnérabilités de type Beast, Crime et Voracle exploitent l'information relative à la taille différente de paquets suite à la compression. Plus précisément, si un attaquant connaît exactement les données qu'il est censé recevoir et qu'il réussit à capturer suffisamment de paquet, ces deux éléments combinés peuvent l'aider à réduire le temps nécessaire au cassage du chiffrement.

Pour moi, dans notre monde réel, cette attaque est très compliquée à mettre en oeuvre. La société OpenVPN recommande de désactiver les algorithmes de compression et à déjà procéder à cela sur leurs produits. Dans les fichiers de configuration fourni ici, axé sur la sécurité, la compression est désactivée. Si vous hésitez, vous pouvez faire des tests de performance pour mieux choisir.

Pousser les configurations réseau aux clients

OpenVPN est autorisé à pousser des règles réseau aux clients. La règle la plus importante concerne la directive push "redirect-gateway" qui modifie la route par défaut par celle spécifiée. Dans la plupart des cas, il s'agit de faire passer toutes les données par le tunnel ouvert par OpenVPN.

Une deuxième option intéressante est de pousser une configuration IPv6 erronée afin de s'assurer qu'aucun paquet ipv6 n'outrepasse le tunnel OpenVPN.

push "route-ipv6 2000::/3"

Revoquer et annuler la révocation du certificat d'un client

Revoquer le certificat d'un client

Il peut arriver que vous souhaitiez révoquer l'accès au VPN à un client. OpenVPN se base sur les Certification Revocation List (CRL) afin de déterminer si un certificat est valide. Pour révoquer le certificat du client que nous avons généré, vous devez exécuter les commandes suivantes:

./easyrsa revoke client
./easyrsa gen-crl

Ces commandes modifient également le fichier index.txt dans le répertoire /tmp/openvpn/EasyRSA-3.0.5/pki. Pour notre cas d'usage, le fichier devrait être ainsi:

V 893632988188Z CWGT67WFA9QLQ9YZ65VGB8FLJKUKV3JL unknown /CN=server R 943755258824Z Y9BFEB757EFV47Y9ENNBUGJLKGP9KQCD unknown /CN=client

Comme vous pouvez le voir, la première colonne est composée de la lettre V ou R. V signifie Validate (validé) et R signifie Revoke (révoqué). Un fichier crl.pem a été créé dans /tmp/openvpn/EasyRSA-3.0.5/pki. Ce fichier contient la liste des certificats révoqués. Nous devons le copier afin de le mettre dans un endroit accessible à OpenVPN et modifier server.conf pour lui indiquer.

cp /tmp/openvpn/EasyRSA-3.0.5/pki/crl.pem /etc/openvpn/server/certificates/crl.pem
chown ovpn:ovpn /etc/openvpn/server/certificates/crl.pem
# in server.conf
crl-verify /etc/openvpn/server/certificates/crl.pem
systemctl restart openvpn

Annuler la révocation d'un certificat client

La liste contenant les certificats révoqués est unique et contient les indications concernant tous les certificats révoqués. Si vous souhaitez annuler la révocation d'un certificat, il est nécessaire de générer une nouvelle liste. Pour se faire, vous devez modifier le fichier /tmp/OpenVPN/EasyRSA-3.0.5/index.txt et remplacer la lettre R pour le certificat en question par la lettre V puis générer une nouvelle liste de certificats révoqués et finalement remplacement le fichier crl.pem chargé par OpenVPN.

Configuration et amélioration de performance

OpenVPN peut être configuré de nombreuses façon afin d'améliorer les performances. Je n'aborderai pas ici cette question, car ce n'est pas l'objet de l'article. Pour information, vous pouvez par exemple modifier l'algorithme de chiffrement utilisé, le protocole de communication utilisé ou également modifier les paramètres ci-dessous:

• mssfix
• fragment
• tun-mtu
• compression algorithme

Si vous voulez allez plus loin, n'hésitez pas cette page .

Déploiment et nettoyage

Félicitations ! La partie la plus compliquée est terminée. Il ne reste plus qu'à déplacer les fichiers et répertoires de façon correcte et à nettoyer l'installation.

Structure des fichiers

A ce stade, vous devriez avoir cette structure dans le répertoire /tmp/openvpn/server/. Le server OpenVPN aura besoin des fichiers suivants pour fonctionner correctement:

• server.conf
• certificates/ca.crt
• certificates/crl.pem
• certificates/server.key
• certificates/server.crt
• certificates/tls_crypt.key

A ce stade, vous devriez avoir cette structure dans le répertoire /tmp/openvpn/client/. Le client OpenVPN aura besoin des fichiers suivants pour fonctionner correctement

• client.conf
• certificates/client.crt
• certificates/client.key
• tls_crypt.key

Un lecteur a fait un très judicieux commentaire à propos de la configuration du client. Pour plus de praticité, vous pouvez agréger en un seul fichier; client.conf, toutes les données cryptographiques nécessaires à l'établissement de la connexion avec le serveur OpenVPN. Cette méthode est plus simple à utiliser et à maintenir. Pour l'utiliser, vous devez modifier le fichier client.conf en supprimant les lignes ca, cert, key et tls-crypt et les remplacer avec les lignes suivantes:

<ca>
--STRIPPED INLINE CA CERT--
</ca>
<cert>
--STRIPPED INLINE CERT--
</cert>
<key>
--STRIPPED INLINE KEY--
</key>
<tls-crypt>
--STRIPPED INLINE KEY--
</tls-crypt>

Dernière configuration et déploiement

Avant de déplacer les répertoires, nous allons les rendre accessibles au seul utilisateur root en mode lecture uniquement.

chmod 400 -R /tmp/openvpn/

Nous allons également déplacer toutes les clés dans un endroit sur et déplacer le répertoire lié à la configuration du serveur à l'endroit prévu à cet effet:

cp -r /tmp/openvpn/server /etc/openvpn/
cp -r /tmp/openvpn/ /etc/ssl/openvpn-pki
ln -s /etc/openvpn/server/server.conf /etc/openvpn/server.conf

Concernant le client, le répertoire dédié à celui-ci doit être déplacé à un autre endroit, que ce soit sur un autre ordinateur ou autre. Vous devez copier le répertoire /tmp/openvpn/client vers /etc/openvpn/ et créer un lien symbolique depuis /etc/openvpn/client/client.conf vers /etc/openvpn/client.conf

Execution

Pour exécuter le programme OpenVPN avec Systemd, vous devez modifier le fichier /etc/default/openvpn et décommenter la ligne AUTOSTART="all" puis replacer "all" par le nom du fichier relatif à la configuration OpenVPN. Dans notre exemple, il faut remplacer "all" par "server".

systemctl daemon-reload

Vous pouvez maintenant démarrer le serveur OpenVPN avec la commande suivante:

systemctl start openvpn

Vous pouvez procéder de la même façon pour le client.

Cleaning

Attention, n'oubliez pas de conserver le répertoire /tmp/openvpn. Il contient tous les certificats, dont les certificats racines nécessaires à la génération de nouveaux clients.

Conclusion

Réseaux sociaux

Merci pour votre lecture, j'espère que cet article vous a été utile. N'hésitez pas à commenter et à le partager.

Sources

Cet article n'aura pu être possible sans les contributions d'autres personnes. Leur travail m'a beaucoup aidé, merci à vous !

• Durcissement de OpenVPN
• Durcissement de OpenVPN de linode
• Durcissement de OpenVPN par Ester
• Lien vers un tutoriel openVPN
• Lien vers un tutoriel avec la meilleure configuration pour les courbes Elliptic
• La différence entre l'algorithme de chiffrement GCM et CBC
• Pourquoi tun/tap est utile
• Page man officielle de OpenVPN 2.4
• Retours des lecteurs de lobste.rs

• 30/07/2019 : Ajoût de la mention concernant le non support de la courbe elliptic Curve25519 par EasyRSA et openvpn.
• 31/07/2019 : Remplacement de TLS-ECDHE-RSA-WITH-AES-256-GCM-SHA384 par TLS-ECDHE-ECDSA-WITH-AES-256-GCM-SHA384. Le certificat racine étant de type elliptic curve, il n'est pas possible d'utiliser l'algorithme de chiffrement RSA pour signer avec.

Introduction

Today I would like to talk about OpenVPN. For those who did not know, OpenVPN is a free and open-source software application that implements virtual private network (VPN) techniques to create secure point-to-point or site-to-site connections in routed or bridged configurations and remote access facilities * from Wikipedia. It is developed by OpenVPN Incorporation and they offered a service of VPN with the product privatetunnel.

Nowadays, there a plenty of OpenVPN tutorial which describe how to install and configure it. Contrary to them, I will try to choose the most secure configuration possible in 2018 of OpenVPN 2.4.0 with Debian 9.5. I will try to describe as much as possible each step of the tutorial in order to be clearly understood.

Preliminaries

First of all, we have to install the OpenVPN package and some extra tools with the user root

apt update
apt upgrade
apt install -y iptables-persistent openvpn vim sudo

Generate certificates

Downloads and configuration of EasyRSA

To generate certificates, we will use EasyRSA. EasyRSA is command line interface utility to build and manage keys and certificates. You can download the latest version for your desktop here EasyRSA is one the easiest tool to use to generate Certificate for OpenVPN. This article was focused on OpenVPN configuration and not the certificate part. That means I used the recommend certificate generation tool by OpenVPN, EASYRSA.

Unfortunately, it appears it does not allow to use the more secure cryptographic digest available today to generate a certificate. If you really want to use them, you need to use OpenSSL directly. An amazing tutorial is available here and could help you.

mkdir /tmp/openvpn
cd /tmp/openvpn/
wget https://github.com/OpenVPN/easy-rsa/releases/download/v3.0.5/EasyRSA-nix-3.0.5.tgz
tar xf EasyRSA-nix-3.0.5.tgz
cd EasyRSA-3.0.5
cp vars.example vars
vim vars

You must now modify the vars file in order to enable elliptic curve mode and improves the hash algorithm uses. Regarding the elliptic curve choosen, it appears that curves and cryptographic tools provided by the National Institute of Standards and Technology (NIST) were deliberately weaken. That's why, I recommend the curve Curve25519 because it is similarly strong than secp256, faster, safer and was not create by the NIST. They are curve which are more secure but they are also dramatically slow.

# Enable elliptic crypto mode
set_var EASYRSA_ALGO ec

# Define the named curve - choose what you like and what is supported - openvpn --show-curves
# Impossible to choose Curve25519 with EasyRSA
set_var EASYRSA_CURVE secp521r1
# In how many days should the root CA key expire?
set_var EASYRSA_CA_EXPIRE 3650

# In how many days should certificates expire?
set_var EASYRSA_CERT_EXPIRE 3650

# In how many days should the control happen?
set_var EASYRSA_CRL_DAYS 3650

# Define the Cryptographic digest use, unfortunately, only the md5 and sha family is currently available with EasyRSA
set_var EASYRSA_DIGEST "sha512"

Generate certificates

Please write carefully about the Common name you choose for each certificate, especially for the server certificate. This one will be used by the client to verify the server certificate in order to avoid Man in the middle attack.

# generate a directory to store all files
./easyrsa init-pki

# generate an autority certificate
./easyrsa build-ca nopass

# create server key (server.key) and certificate signing request (server.req)
./easyrsa gen-req server nopass

# sign server certificate signing request by autority certificate (server.ca)
./easyrsa sign-req server server nopass

# create client key (client.key) and certificate signing request (client.req)
./easyrsa gen-req client nopass

# sign client certificate signing request by autority certificate (client.ca)
./easyrsa sign-req client client nopass

Directory structure

# we will now create a more easy to read directory tree
mkdir /tmp/openvpn/server/
mkdir /tmp/openvpn/server/certificates
cp /tmp/openvpn/EasyRSA-3.0.5/pki/ca.crt /tmp/openvpn/server/certificates/ca.crt
cp /tmp/openvpn/EasyRSA-3.0.5/pki/issued/server.crt /tmp/openvpn/server/certificates/server.crt
cp /tmp/openvpn/EasyRSA-3.0.5/pki/private/server.key /tmp/openvpn/server/certificates/server.key

mkdir /tmp/openvpn/client/
mkdir /tmp/openvpn/client/certificates
cp /tmp/openvpn/EasyRSA-3.0.5/pki/ca.crt /tmp/openvpn/client/certificates/ca.crt
cp /tmp/openvpn/EasyRSA-3.0.5/pki/issued/client.crt /tmp/openvpn/client/certificates/client.crt
cp /tmp/openvpn/EasyRSA-3.0.5/pki/private/client.key /tmp/openvpn/client/certificates/client.key

Network configuration

Let's consider that the ssh server listens the 22 port and 443 for the VPN server.

Firewall rules

*nat
:PREROUTING ACCEPT [0:0]
:INPUT ACCEPT [0:0]
:OUTPUT ACCEPT [0:0]
:POSTROUTING ACCEPT [0:0]
# Forward the VPN traffic to eth0
-A POSTROUTING -s 10.8.0.0/24 -o eth0 -j MASQUERADE

COMMIT

*filter

# Allow all loopback (lo) traffic and reject anything
# to localhost that does not originate from lo.
-A INPUT -i lo -j ACCEPT
-A INPUT ! -i lo -s 127.0.0.0/8 -j REJECT
-A OUTPUT -o lo -j ACCEPT

# Allow ping and ICMP error returns.
-A INPUT -p icmp -m state --state NEW --icmp-type 8 -j ACCEPT
-A INPUT -p icmp -m state --state ESTABLISHED,RELATED -j ACCEPT
-A OUTPUT -p icmp -j ACCEPT

# Allow SSH.
-A INPUT -i eth0 -p tcp -m state --state NEW,ESTABLISHED --dport 22 -j ACCEPT
-A OUTPUT -o eth0 -p tcp -m state --state ESTABLISHED --sport 22 -j ACCEPT

# Allow TCP traffic.
-A INPUT -i eth0 -p tcp -m state --state NEW,ESTABLISHED --dport 443 -j ACCEPT
-A OUTPUT -o eth0 -p tcp -m state --state ESTABLISHED --sport 443 -j ACCEPT

# Allow DNS resolution and limited HTTP/S on eth0.
# Necessary for updating the server and keeping time.
-A INPUT -i eth0 -p udp -m state --state ESTABLISHED --sport 53 -j ACCEPT
-A OUTPUT -o eth0 -p udp -m state --state NEW,ESTABLISHED --dport 53 -j ACCEPT

# Allow traffic on the TUN interface.
-A INPUT -i tun0 -j ACCEPT
-A OUTPUT -o tun0 -j ACCEPT

# then reject them.
-A INPUT -j REJECT
-A OUTPUT -j REJECT

COMMIT

Be sure to adapt these rules to your needs before applying them.

sudo iptables-restore < /etc/iptables/rules.v4

We can check if the rules are correctly implied:

sudo iptables -L

Enable ipv4 forwarding and disable ipv6

In the file /etc/sysctl.d/99-sysctl.conf, add the following lines to enable ipv4 forwarding and disable ipv6:

net.ipv4.ip_forward = 1

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1
net.ipv6.conf.eth0.disable_ipv6 = 1

and then apply the new configuration

sudo sysctl -p

Remove ipv6 lines in /etc/hosts:

#::1 localhost ip6-localhost ip6-loopback

*filter

-A INPUT -j REJECT
-A FORWARD -j REJECT
-A OUTPUT -j REJECT

COMMIT

and apply:

sudo ip6tables-restore < /etc/iptables/rules.v6

Examples of very secure configuration

In order to continue, we will use the highly secure configuration below. Be careful, you need to follow the tutorial until the end to make it workable.

Example of server configuration

Please copy the following code in /tmp/openvpn/server/server.conf. It is the OpenVPN server configuration. Don't forget to replace local IP_OF_YOUR_OPENVPN_SERVER by your server ip.

#/tmp/openvpn/server/server.conf
local IP_OF_YOUR_OPENVPN_SERVER
dev tun
topology subnet
proto tcp
port 443
server 10.8.0.0 255.255.255.0
tls-server

ca /etc/openvpn/server/certificates/ca.crt
# crl-verify /etc/openvpn/server/certificates/crl.pem
cert /etc/openvpn/server/certificates/server.crt
key /etc/openvpn/server/certificates/server.key
tls-crypt /etc/openvpn/server/certificates/tls_crypt.key

dh none
ecdh-curve ED25519
tls-cipher TLS-ECDHE-ECDSA-WITH-AES-256-GCM-SHA384
cipher AES-256-GCM
ncp-ciphers AES-256-GCM
tls-version-min 1.2
persist-tun
compress

persist-key
keepalive 10 120

user ovpn
group ovpn

status /var/log/openvpn-status.log
log /var/log/openvpn.log

push "redirect-gateway"
push "dhcp-option DNS 10.8.0.1"
push "dhcp-option WINS 10.8.0.1"
push "route-ipv6 2000::/3"

Example of client configuration

Please copy the following code in /tmp/openvpn/client/client.conf. It is the OpenVPN client configuration. Don't forget to replace remote IP_OF_YOUR_OPENVPN_SERVER by your server ip and COMMON_NAME_OF_THE_SERVER_CERTIFICATE by the Common name you gave to the server certificate .cf Generating certificates.

# /tmp/openvpn/client
client
dev tun
remote IP_OF_YOUR_OPENVPN_SERVER 443
proto tcp
resolv-retry infinite
compress
nobind
verify-x509-name "COMMON_NAME_OF_THE_SERVER_CERTIFICATE" name
remote-cert-tls server
tls-cipher TLS-ECDHE-ECDSA-WITH-AES-256-GCM-SHA384
cipher AES-256-GCM
tls-version-min 1.2
auth-nocache
persist-key
persist-tun

status /var/log/openvpn-status.log
log /var/log/openvpn.log
verb 3

ca /etc/openvpn/client/certificates/ca.crt
cert /etc/openvpn/client/certificates/client.crt
key /etc/openvpn/client/certificates/client.key
tls-crypt /etc/openvpn/client/certificates/tls_crypt.key

Harden OpenVPN configuration

This part will describe most of the security parameters chosen in the final configuration.

TCP/IP protocol and port listening

In order to avoid most of firewall limit, it is highly recommended to switch from udp protocol to tcp. OpenVPN configured with TCP protocol is a little bit slower but it has more chances to be accessible from a network you do not control.

You also have to configure the OpenVPN server to listen the port 443. Port 443 is the usual port used by a web server configuration and it is most of the time open in firewall output. Moreover, it will also make your OpenVPN configuration harder to detect because it is not the standard OpenVPN port. The standard port is 1194.

proto tcp4
port 443

Diffie-Hellman

Diffie–Hellman key exchange is a method of securely exchanging cryptographic keys over a public channel. In OpenVPN, this protocol is used in the first steps of TLS establishment connection. With a key of 1024 bits and lower size, it is vulnerable to Logjam exploits. The authors of the vulnerability recommend using primes of 2048 bits or more as a defense or switching to elliptic-curve Diffie–Hellman. In this tutorial, we use elliptic-curve Diffie–Hellman

HMAC signature during TLS handshake

Since openVPN 2.4, it is recommended to replace tls-auth by tls-crypt. Mainly because tls-crypt will also encrypt the TLS control channel. That means, to add the following line in server.conf:

tls-crypt /etc/openvpn/certificates/ta.key 0

and generate the HMAC key file in the openvpn server directory and client directory:

openvpn --genkey --secret /tmp/openvpn/server/certificates/tls_crypt.key
cp /tmp/openvpn/server/certificates/tls_crypt.key /tmp/openvpn/client/certificates/tls_crypt.key

Persistent tun/tap device and key

While your connection might be interrupted and OpenVPN is trying to reconnect, you may be using the default network routes again, bypassing the tunnel. For accessing private networks this might not be a big issue as the network addresses may not be reachable from outside the tunnel, but it may expose information you'd rather keep private like an HTTP request containing cookies.

persist-key is not a security option but a problem solving in case OpenVPN restart and the OpenVPN user is not able to read the key file anymore. This parameter avoid this situation.

persist-tun
persist-key

Limited user

Probably one of the most important configuration setting. In order to limit the impact of an OpenVPN vulnerability, it is highly recommended to run it with a user with limited rights. To do that, we have to create a user for our OpenVPN applications. This user has limited privileges:

adduser --system --shell /usr/sbin/nologin --no-create-home ovpn
groupadd ovpn
usermod -a -G ovpn ovpn

and specify the user in the OpenVPN configuration:

user ovpn
group ovpn

Ciphers and digests

tls-version-min 1.2
ncp-ciphers AES-256-GCM:AES-256-CBC
tls-cipher TLS-ECDHE-ECDSA-WITH-AES-256-GCM-SHA384
cipher AES-256-GCM
ecdh-curve ED25519
dh none

We will force to use SHA512 to manage the authentication mechanism. unfortately the best digest available is still sha family. Enabling the auth-nocache parameter will prevent to cache passwords in memory.

auth-nocache
auth SHA512

Compression algorithms

In the OpenVPN version 2.3, it was the LZ0 compression algorithm which was by default. Since OpenVPN version 2.4, the compression algorithm LZ4 is available. Contrary to what the documentation says, it is not the best one available. There is also the version 2, lz4-v2 of it which is available but not documented yet.

If security if your only criteria, you should disable this feature. Indeed, a family of vulnerability like Beast, Crime and Voracle exist . Those vulnerabilities could allow an attacker to gain information about an encrypted communication in very specific circumstances. To explain differently, if the attacker knows exactly what he is supposed to receive and capture enough packet with little predictable changes, this could help him to reduce the time required to break the encryption.

For me, in the real world, it is really complicated to use this kind of vulnerability. The OpenVPN company recommends to disable compression algorithm and already did it on their products. In the configuration files provide here, it is, of course, disable because we are only focused on security. Feel free to make some performance test with speedtest if you hesitate.

Pushing configuration to the client

OpenVPN is allowed to push some network rules to the server. The most important is push "redirect-gateway" which forces to route all the client traffic throw the vpn. This parameter replaces the gateway route in the client configuration.

A second very useful possibility is to push a route for ipv6 which does not work. This will avoid in case your client is configured to work with ipv6 to make it unusable.

push "route-ipv6 2000::/3"

Revoke and unrevoke a client

Revoke a client

Sometime, you want to revoke an access to the vpn to a client. OpenVPN uses Certification Revocation List (CRL) to determine if a certificate is revoked or not. You need to execute the following command:

./easyrsa revoke client
./easyrsa gen-crl

Those commands also modify the file index.txt in the directory of /tmp/openvpn/EasyRSA-3.0.5/pki. For our setup and since we had revoked client, this file should be like that.

V 893632988188Z CWGT67WFA9QLQ9YZ65VGB8FLJKUKV3JL unknown /CN=server R 943755258824Z Y9BFEB757EFV47Y9ENNBUGJLKGP9KQCD unknown /CN=client

As you can see,the first column is V or R. V means valide and R means Revoke. We also have a new file crl.pem in /tmp/openvpn/EasyRSA-3.0.5/pki. It is this file which contains the list of revoked certificate. We have to copy it in a place accessible for the OpenVPN application and modify server.confwith a command.

cp /tmp/openvpn/EasyRSA-3.0.5/pki/crl.pem /etc/openvpn/server/certificates/crl.pem
chown ovpn:ovpn /etc/openvpn/server/certificates/crl.pem
# in server.conf
crl-verify /etc/openvpn/server/certificates/crl.pem
systemctl restart openvpn

Unrevoke a client

The certificate revocation list contains is a unique value even if you revoke multiple client certificate. If you want to unrevoke a client, you need to generate a new certificate revocation list. To do that, you need to modify /tmp/OpenVPN/EasyRSA-3.0.5/index.txt and replace the R for the certificate by V then generate a new certificate revocation list and finally replace the crl.pem loaded by OpenVPN.

Tuning and performance improvement

OpenVPN could be tuned in a lot of way to improve performance. I will not talk a lot about that because it requires an article by his own. You could change for example, the encryption algorithm, the TCP/IP protocol or modify those parameters to improve it.

• mssfix
• fragment
• tun-mtu
• compression algorithm

Feel free to read this page to know more about tuning OpenVPN.

Deployement and cleaning

Congratulation! The most complicated part is over. We just have now to deploy each directory in the proper place.

Directory structure

You should have this directory structure in /tmp/openvpn/server/. The server will need the following files to run properly:

• server.conf
• certificates/ca.crt
• certificates/crl.pem
• certificates/server.key
• certificates/server.crt
• certificates/tls_crypt.key

You should now have this directory structure in /tmp/openvpn/client/. The client will need the following files to run properly:

• client.conf
• certificates/client.crt
• certificates/client.key
• tls_crypt.key

A reader made a very good comment about the client configuration file. You can summarize in client.conf all the cryptography data required to establish the connection to the OpenVPN server. It makes it easier to transport and manage. To do that, you should remove the line about ca, cert, key and tls-crypt and replace them with the following lines.

<ca>
--STRIPPED INLINE CA CERT--
</ca>
<cert>
--STRIPPED INLINE CERT--
</cert>
<key>
--STRIPPED INLINE KEY--
</key>
<tls-crypt>
--STRIPPED INLINE KEY--
</tls-crypt>

Last configuration and deployement

Before moving directories, we will make them available only for the root user in reading mode.

chmod 400 -R /tmp/openvpn/

We will also move the all public and private key to a safe place and move the server directory to the right place:

cp -r /tmp/openvpn/server /etc/openvpn/
cp -r /tmp/openvpn/ /etc/ssl/openvpn-pki
ln -s /etc/openvpn/server/server.conf /etc/openvpn/server.conf

In the client which should be another computer. You just have to copy and paste the directory /tmp/openvpn/client to /etc/openvpn/ and create a symbolic link from /etc/openvpn/client/client.conf to /etc/openvpn/client.conf

Running

To run OpenVPN with Systemd, you need to modify /etc/default/openvpn by uncommenting AUTOSTART="all" and replacing "all" by the name of configuration file. For example with in the server side, you should replace "all" by "server".

systemctl daemon-reload

You are now able to run it with systemctl and the following command:

systemctl start openvpn

You need to do the same thing for the client.

Cleaning

Be careful, do not forget to remove the directory /tmp/openvpn. It contains all your certificates.

Conclusion

Social media

Thank you for your reading, I hope this article was helpful for you. Don't hesitate to comment and if you think I made a mistake, it will be a pleasure to discuss it. If you find this article interesting, feel free to subscribe to the RSS flux of the blog and to follow me on Mastodon.

Sources

This article would not have been possible if other people does not share information about it. Their work help me a lot, thank you!

• Hardening page from openVPN official website
• Hardening openVPN from linode
• Hardening openVPN from ester
• Link to a tutorial of openVPN
• Link to the best configuration of openvpn with Eliptic Curve
• Difference between GCM and CBC mode
• Why persistent tun/tap is usefull
• Official man page about OpenVPN 2.4
• Feedbacks from readers after the article was published

• 31/07/2019 : Replace TLS-ECDHE-RSA-WITH-AES-256-GCM-SHA384 by TLS-ECDHE-ECDSA-WITH-AES-256-GCM-SHA384. As the root certificate was built with elliptic curve secp521, it is not possible to use it to sign a certificate with the RSA algorithme, only ECDSA.

Firefox Account Server is not Firefox Sync Server.

In order to continue to be more and more independent and because I trust less and less Mozilla Foundation, I decided to manage by myself the Firefox authentication system (without Docker). For those who do not know, Firefox divided the whole authentication system and the storage management system. You can manage your data (bookmarks, history, tabs, profile) with Firefox Sync. I deployed it previously and a tutorial is available here. After hosting the most important part of my datas that Firefox manages, I wanted to host the all thing. I worked on it during 21 hours and was still not able to run it properly. I decided to share my experience.

Criticism

Firefox Authentication Server is built in following a microservices architecture. For those who do not know it, it divides an application into little smaller applications. Each of them should have a specific role and perimeter. For example, a microservice dedicated to send email or another dedicated to manage the user interface. However, this architecture, if not well built and documented could have some disadvantages. You can find below a list from Wikipedia:

• Services form information barriers
• Inter-service calls over a network have a higher cost in terms of network latency and message processing time than in-process calls within a monolithic service process
• Testing and deployment are more complicated
• Moving responsibilities between services is more difficult. It may involve communication between different teams, rewriting the functionality in another language or fitting it into a different infrastructure
• Viewing the size of services as the primary structuring mechanism can lead to too many services when the alternative of internal modularization may lead to a simpler design.

Unfortunately, I think the Firefox Accounts Server fall in most of them. They are improving it but there is so much work to do. Especially because it seems like Mozilla Foundation wants to maintain the compatibility with the past. You can find below the list of issues I found which made it really hard to deploy it and which demonstrates why it is obsolete.

• Each microservice has his own structure. In some of them, you have configuration in config/index.js, another one has it in /server/config/local.json, in another one you have two files to configure
• Each microservice has his own running process. For example, the running command could be different, in another case, you need to build the code to make it runnable.
• The documentation is clearly missing (no system-d unit, no reverse proxy configuration). Anybody who tries to run it in following the process in the documentation will most of the time failed because some part of it is not documented or is obsolete

Regarding the Firefox Authentication Server in general. I am sorry to say it but it is clearly out of date and has vulnerabilities inside. About obsolescence, I could talk about the need to use mysql 5.6 and about vulnerabilities, the node modules vulnerabilities. It is not ready to be deployed by anybody else than someone who works in this project or in the Mozilla Firefox platform. I do not imagine one second a system administrator without development skill being able to deploy it in less than 3 days.

Just another example about the mess, I made an issue here about the difficulties I got. Two people from Mozilla answered, the first answer was pertinent and helped me in the process. The second one was clearly out of subjects, I am not even sure he read it, he just repeats one thing I said, thing which does not work and he closed the issue without giving a fuck. Yes, it closed it, without waiting for my answer. I just took three days trying to make it works before asking for help and my issue was closed like "OK, thank you".

My responsability

My lack of knowledge was, of course, a reason of my impossibility to succeed in this task. Even if I deployed dozens of applications, I am not used to deploy microservices applications. The only comfort I have is I am not the only who did not succeed.

Installation process

I took three days deploying and configuring Firefox Accounts Server. For those who are interested, you can find below the process I follow to be able to run them. I was able to run 5 services, maybe it required more to make it runnable, but some of them still have issues and it. The list of microservices I deployed:

• fxa-auth-db-mysql
• fxa-auth-server
• fxa-content-server
• fxa-oauth-server
• fxa-profile-server

Global installation

In order to prepare the system, you need to to the following stuff:

In debian 9, you will need to install only mysql-server without mariadb

You have to choose mysql version 5.6, I tested with version 8 and mariadb and it doesn't work

Tips

In order to find the configuration file easily, I recommend you to use grep as much as possible and to read the packages.json file which could help you to find running command. You can find interesting stuff with:

Part of the installation process of Firefox Accounts database service

I still have issues with it. db.example.com

Firefox Accounts Server

I still have issues with it. auth.example.com

To change the listen address of the server, you have to modify the file config/index.js and replace it.

Firefox Accounts Content Server

All the configuration is in the file server/config/local.json-dist
Firefox Content Server loads his configuration from file we should create. It should be a copy of local.json-dist.

I recommend you to disable csp because they are completely obsolete. They still using x-content-security-policy even if it is obsolete since Firefox 23 !

Firefox Accounts OAuth Server

oauth.example.com

Firefox Accounts Profile Service

profile.example.com

Sources

• The list of all fxa Firefox repository.
• Github issues about Firefox Content Server.

Conclusion

I hope it will motivate you NOT to try to install it and save your time. I hope they will improve it and make it easier to configure and deploy. Maybe one day, we will be able to use only the Mozilla Firefox Browser and be able to manage everything behind, maybe.

Social media

If you find this article useful, feel free to follow my RSS flux and to follow me on Mastodon. Don't hesitate to share it if you think it could interested someone.

I decided to publish each month an article about the blog in general. Contrary to what I said, I decide to publish this article each two months. I think now it is not really relevant to publish it each two months. In this article, you will be able to know:

• What I achieved during this period.
• What I accomplished for the community.
• How is the blog and services popular.
• Balance sheet of the period.
• Some words about what I think for the next period.

This article is about the month of July-August 2018.

Period achievements

Articles

• Two articles about cybersecurity: article 1 article 2

Events

• Nothing special.

The blog

• Improve the accuracy of the statistic tool.
• I added a count of the RSS request by day group by IP address. It helps me to know if the blog interests people.
• I added a little text about personal data I store and how I manipulate them.

Give back to the community

• As I based my filter from a Github repository of bot, I can now extract automatically bot which request my website and which are not in the Github repository. I will extract this list of bot each month and add it to the Github repository.
• I published a little commit on Mastodon about the documentation.
• A little donation as each month for an association or service I find useful.

Balance sheet of the period

Statistics for this period

Some charts about the month of July:

Some charts about the month of August:

• Except the days around the publication in social media, visits are around 30 by days

My point of view

I got a little time off at the end of August. I did not really know what I can write about and I was questioning myself about the sense of it. Even if I know I primarily write for myself, I expected to bring something useful to the community. I begin to accept this blog will not change something and I begin to think about using my time in a better way.

For the next month

• I do not know.

Hello guys,

Sorry for the little delay but I was not sure about what I wanted to write for the month of September.

Introduction

Today, I would like to talk about operating system for mobile and especially those based on Android. For those who do not know, Android is an open-source operating system and each manufacturer may customise it with features or tweaks. A customise Android operating system is called a distribution. I do not know the IOS environment that is why I will not talk about it here.

A little lexicon below:

• IOS: Iphone operating system
• FAD: Factory Android Distributions
• CAD: Custom Android Distributions

Why and when install a custom Android distribution?

The issues with the Factory Android Distribution (FAD)

manufacturers make a lot of work to provide a good mobile phone. However, they are motivated by money contrary to the users who are motivated by good experience and good products.

Firstly, the most important issue is about updates. Android mobile phone tends to be in general updated for only two years. After this period, your smartphone will not be updated anymore. That means it will contain known vulnerabilities without any possibility to fix it.

As your phone has very sensitive features (GPS, microphone, camera, sensitive personal data). A mobile phone compromise could create a lot of issues. For example, the GPS could be used in an abusive way. An example with the recent vulnerability published the 29th of August.

You can find below the list of Android system deploy on smartphones.



You can see in February 2018, there are:

• Around 10% in Android 4.4 (published in October 31, 2013)
• Around 25% in Android 5.0-51 (published in November 12, 2014)
• Around 28% in Android 6.0 (published in October 5, 2015)
• Around 25% in Android 7.0-7.1 (published in August 22, 2016)

I do not know if you understand how bad it is. That just means around 90% of the FAD are not up to date and contain known vulnerabilities. Or, if we are less exigent, it is 65% which is obsolete. For me, that just means one thing. Never trust your Android smartphone or the Android smartphone of your friends. IOS (the operating system for Apple phone) is better but not perfect about security update. I do not find the chart but most of the devices are "up to date".

Secondly, as they are interested mainly by benefits or have to follow government rules. It appears that some device tracks phone calls, contacts, data and phone usage.

Pros

• Custom Android Distribution (CAD) generally tends to provide a more recent Android version. That means better security, better performance, better features and better autonomy
• CAD do not contain manufacturers features and improvements. You are also free not to install Google applications. That means no tracking features.
• CAD generally add features which are able to improve the management of your cellular phone. That means, for example, have a better tool to manage backup, update or security. They often have features to manage privacy more precisely. Some applications are made by the maintainers and are free to install.
• I do not know about the other distributions but LineageOS community provides a very good tutorial about how to install it on your smartphone. An example can be found here with the Galaxy S3.

Cons

• Replacing the Factory Android Distribution by one of your choices is not easy and required time. You need to understand the different steps of the process and how an Android operating system works in the main line. Contrary to what you could think, you will not develop at all. You also need to do a little analyse about what you will earn and lose and you need to make the required backup. It required me approximately 12 hours to do it and have a mobile phone which was fully operational whereas I had not a lot of knowledge about the process.
• CAD do not contain manufacturer features and improvement. It could be positive but it could also be negative. You could lose manufacturer tweaks and have worse performance. You will never know before making a try.
• Most of the time, unlocking the bootloader (which is a step required to replace your Android distribution) will stop the guaranty.
• Some features may not work properly (high consumption energy, cameras which do not work or even crash sometimes). However, it could be fixed in the next release which is published each week on LineageOS. For example, I was for one month without a front camera and GPS.
• Less stable than FAD, the mobile phone may crash and have a higher possibility to lose your data when update. Hopefully, you also have a better tool to get it back but it could not work all the time.

When to replace the factory Android distribution?



For casual users or users who do not want a lot of issues,
when your mobile phone is not updated anymore. When you are in this situation, that means your mobile phone is older than 2 years and the CAD should be quite stable. The tutorial should be quite complete. Issues should be known, fixed or with some work around available.

For expert users and experimental users,
some months after the manufacturer releases the new phone. It should let to the maintainers the time to develop enough stable version for your phone. In case of issues, you should be able to roll back to the previous release on your own.

Advice and warning about a mobile phone with CAD

• Choose a mobile phone quite popular. The most you have people who use it, the most it is probable than a custom Android distribution will support it well. Quite popular does not mean with a lot of hardware backdoors, you have some choices.

• Do as little as possible with your phone. First of all, because the mobile phone environment is far more dangerous than the desktop environment. Proprietary applications can literally siphon your data, track your location, use your camera, heard around you.

  Even if you are up to date with a recent phone, your mobile phone could be exploited to hear what it is around you, to locate you, to film around you. Secondly, because you use a CAD, it means less stability, you should be ready for it.

• Each custom Android distribution has his own purpose. Choose carefully the one you will install regarding stability, performance, security and maintainability.

Conclusion

You now have some arguments to make your decision.

Sources

• Wikipedia page about android version history.
• Wikipedia page about IOS version history.
• LineageOS changes for all devices
• Wikipedia page about LineageOS
• Wikipedia page about all custom Android distribution

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