mehr als nur ein Notizblock

VPN-Server sind ne tolle Geschichte, da man damit ver­schie­denste Dinge erle­di­gen kann:

• abge­si­cherte Kommunikation
• exklu­sive und ver­traute Verbindungen
• Umge­hung von pro­vi­de­r­ab­hän­gi­gen Gängeleien

Neben dem rei­nen Inter­esse an der Tech­nik möchte ich fol­gen­des davon erreichen:

• Über den VPN-Tunnel soll mein Heim-Exchange seine Mails an das Mail-Gateway abset­zen kön­nen — dabei kann ich die spe­zi­elle smtpd-Instanz mit weni­ger Restrik­tio­nen lau­fen las­sen (z.B. keine Über­prü­fung auf Viren und Spam)
• wenn ich das nächste Mail in China bin möchte ich über den VPN-Tunnel ohne Ein­schrän­kun­gen im Inter­net unter­wegs sein können

Aber nun zur Einrichtung:

Vor­be­rei­tung

Zuerst müs­sen wir die Pakete openvpn (klar, was sonst) und udev instal­lie­ren. udev benö­ti­gen wir, damit das Erstel­len des vir­tu­el­len Inter­face dyna­misch erfolgt.

aptitude install udev openvpn

Damit man über den Ser­ver eine Ver­bin­dung ins Inter­net auf­bauen kann muß man noch das IP-Forwarding ein­schal­ten. Dazu edi­tiert man die Datei /etc/sysctl.conf und akti­viert den Ein­trag net.ipv4.ip_forward = 1

Zer­ti­fi­kate erstellen

Wir benö­ti­gen für OpenVPN Zer­ti­fi­kate, d.h. eine beste­hende CA und müs­sen dann wei­tere Cli­ent­zer­ti­fi­kate erstel­len. Zuerst aber müs­sen wir einen soge­nann­ten Diffie-Hellman Para­me­ter erzeu­gen. Die­ser wird, laut dem OpenVPN.eu-Wiki, »vom OpenVPN Ser­ver ver­wen­det um bei vie­len gleich­zei­ti­gen Cli­ent­ver­bin­dun­gen den TLS-Handshake zu beschleunigen.«

/etc/ssl# openssl dhparam -out dh1024.pem 1024
Generating DH parameters, 1024 bit long safe prime, generator 2
This is going to take a long time
..................+......................+......+...............

Nun nutze ich mein Zer­ti­fi­kats­skript zur Erstel­lung des Server-Zertifikats

create_certs vpn.domain.de

Und damit habe ich fol­gende Dateien erstellt, die ich zukünf­tig brau­che /etc/ssl/vpn.domain.de-cert.pem /etc/ssl/private/vpn.domain.de-key.pem Natür­lich brau­che ich noch Zer­ti­fi­kate für die Cli­ents, diese erstell ich mit

create_certs client1

Wich­tig ist dabei, das als com­mon name der Cli­ent­name (in dem Bei­spiel client1) ange­ge­ben wird. Anschlie­ßend kön­nen die Dateien /etc/ssl/client1-cert.pem und /etc/ssl/private/client1-key.pem sowie /etc/ssl/cacert.pem auf den Cli­ent kopiert wer­den, damit die­ser sich mit einem gül­ti­gen Zer­ti­fi­kat spä­ter ver­bin­den kann.

Kon­fi­gu­ra­ti­ons­da­tei erstellen

Anschlie­ßend habe ich aus /usr/share/doc/openvpn/examples die Datei server.conf.gz nach /etc/openssl kopiert und dort aus­ge­packt. Beim Edi­tie­ren die­ser Datei habe ich die Pfade und die Namen der Zer­ti­fi­kats­da­teien sowie das Log­ging ange­paßt. Die Datei (meine Ände­run­gen sind rot mar­kiert) sieht anschlie­ßend so aus

#################################################
# Sample OpenVPN 2.0 config file for            #
# multi-client server.                          #
#                                               #
# This file is for the server side              #
# of a many-clients <-> one-server              #
# OpenVPN configuration.                        #
#                                               #
# OpenVPN also supports                         #
# single-machine <-> single-machine             #
# configurations (See the Examples page         #
# on the web site for more info).               #
#                                               #
# This config should work on Windows            #
# or Linux/BSD systems.  Remember on            #
# Windows to quote pathnames and use            #
# double backslashes, e.g.:                     #
# \"C:\\Program Files\\OpenVPN\\config\\foo.key\" #
#                                               #
# Comments are preceded with '#' or ';'         #
#################################################
	
# Which local IP address should OpenVPN
# listen on? (optional)
;local a.b.c.d
	
# Which TCP/UDP port should OpenVPN listen on?
# If you want to run multiple OpenVPN instances
# on the same machine, use a different port
# number for each one.  You will need to
# open up this port on your firewall.
port 1194
	
# TCP or UDP server?
;proto tcp
proto udp

# \"dev tun\" will create a routed IP tunnel,
# \"dev tap\" will create an ethernet tunnel.
# Use \"dev tap0\" if you are ethernet bridging
# and have precreated a tap0 virtual interface
# and bridged it with your ethernet interface.
# If you want to control access policies
# over the VPN, you must create firewall
# rules for the the TUN/TAP interface.
# On non-Windows systems, you can give
# an explicit unit number, such as tun0.
# On Windows, use \"dev-node\" for this.
# On most systems, the VPN will not function
# unless you partially or fully disable
# the firewall for the TUN/TAP interface.
;dev tap
dev tun
	
# Windows needs the TAP-Win32 adapter name
# from the Network Connections panel if you
# have more than one.  On XP SP2 or higher,
# you may need to selectively disable the
# Windows firewall for the TAP adapter.
# Non-Windows systems usually don't need this.
;dev-node MyTap
	
# SSL/TLS root certificate (ca), certificate
# (cert), and private key (key).  Each client
# and the server must have their own cert and
# key file.  The server and all clients will
# use the same ca file.
#
# See the \"easy-rsa\" directory for a series
# of scripts for generating RSA certificates
# and private keys.  Remember to use
# a unique Common Name for the server
# and each of the client certificates.
#
# Any X509 key management system can be used.
# OpenVPN can also use a PKCS #12 formatted key file
# (see \"pkcs12\" directive in man page).
ca /etc/ssl/cacert.pem
cert /etc/ssl/vpn.domain.de-cert.pem
key /etc/ssl/private/vpn.domain.de-key.pem  # This file should be kept secret
	
# Diffie hellman parameters.

# Generate your own with:
#   openssl dhparam -out dh1024.pem 1024
# Substitute 2048 for 1024 if you are using
# 2048 bit keys.
dh /etc/ssl/dh1024.pem
	
# Configure server mode and supply a VPN subnet
# for OpenVPN to draw client addresses from.
# The server will take 10.8.0.1 for itself,
# the rest will be made available to clients.
# Each client will be able to reach the server
# on 10.8.0.1. Comment this line out if you are
# ethernet bridging. See the man page for more info.
server 10.8.0.0 255.255.255.0
	
# Maintain a record of client <-> virtual IP address
# associations in this file.  If OpenVPN goes down or
# is restarted, reconnecting clients can be assigned
# the same virtual IP address from the pool that was
# previously assigned.
ifconfig-pool-persist ipp.txt
	
# Configure server mode for ethernet bridging.
# You must first use your OS's bridging capability
# to bridge the TAP interface with the ethernet
# NIC interface.  Then you must manually set the
# IP/netmask on the bridge interface, here we
# assume 10.8.0.4/255.255.255.0.  Finally we
# must set aside an IP range in this subnet
# (start=10.8.0.50 end=10.8.0.100) to allocate
# to connecting clients.  Leave this line commented
# out unless you are ethernet bridging.
;server-bridge 10.8.0.4 255.255.255.0 10.8.0.50 10.8.0.100
	
# Configure server mode for ethernet bridging
# using a DHCP-proxy, where clients talk
# to the OpenVPN server-side DHCP server
# to receive their IP address allocation
# and DNS server addresses.  You must first use
# your OS's bridging capability to bridge the TAP
# interface with the ethernet NIC interface.
# Note: this mode only works on clients (such as
# Windows), where the client-side TAP adapter is
# bound to a DHCP client.
;server-bridge

# Push routes to the client to allow it
# to reach other private subnets behind
# the server.  Remember that these
# private subnets will also need
# to know to route the OpenVPN client
# address pool (10.8.0.0/255.255.255.0)
# back to the OpenVPN server.
;push \"route 192.168.10.0 255.255.255.0\"
;push \"route 192.168.20.0 255.255.255.0\"
	
# To assign specific IP addresses to specific
# clients or if a connecting client has a private
# subnet behind it that should also have VPN access,
# use the subdirectory \"ccd\" for client-specific
# configuration files (see man page for more info).
	
# EXAMPLE: Suppose the client
# having the certificate common name \"Thelonious\"
# also has a small subnet behind his connecting
# machine, such as 192.168.40.128/255.255.255.248.
# First, uncomment out these lines:
;client-config-dir ccd
;route 192.168.40.128 255.255.255.248
	
# Then create a file ccd/Thelonious with this line:
#   iroute 192.168.40.128 255.255.255.248
# This will allow Thelonious' private subnet to
# access the VPN.  This example will only work
# if you are routing, not bridging, i.e. you are
# using \"dev tun\" and \"server\" directives.
	
# EXAMPLE: Suppose you want to give
# Thelonious a fixed VPN IP address of 10.9.0.1.
# First uncomment out these lines:
;client-config-dir ccd
;route 10.9.0.0 255.255.255.252
	
# Then add this line to ccd/Thelonious:
#   ifconfig-push 10.9.0.1 10.9.0.2
	
# Suppose that you want to enable different
# firewall access policies for different groups
# of clients.  There are two methods:
# (1) Run multiple OpenVPN daemons, one for each
#     group, and firewall the TUN/TAP interface
#     for each group/daemon appropriately.
# (2) (Advanced) Create a script to dynamically
#     modify the firewall in response to access
#     from different clients.  See man
#     page for more info on learn-address script.
;learn-address ./script
	
# If enabled, this directive will configure
# all clients to redirect their default
# network gateway through the VPN, causing
# all IP traffic such as web browsing and
# and DNS lookups to go through the VPN
# (The OpenVPN server machine may need to NAT
# or bridge the TUN/TAP interface to the internet
# in order for this to work properly).
;push \"redirect-gateway def1 bypass-dhcp\"
	
# Certain Windows-specific network settings
# can be pushed to clients, such as DNS
# or WINS server addresses.  CAVEAT:
# http://openvpn.net/faq.html#dhcpcaveats
# The addresses below refer to the public
# DNS servers provided by opendns.com.
;push \"dhcp-option DNS 208.67.222.222\"
;push \"dhcp-option DNS 208.67.220.220\"
	
# Uncomment this directive to allow different
# clients to be able to \"see\" each other.
# By default, clients will only see the server.
# To force clients to only see the server, you
# will also need to appropriately firewall the
# server's TUN/TAP interface.
;client-to-client
	
# Uncomment this directive if multiple clients
# might connect with the same certificate/key
# files or common names.  This is recommended
# only for testing purposes.  For production use,
# each client should have its own certificate/key
# pair.
#
# IF YOU HAVE NOT GENERATED INDIVIDUAL
# CERTIFICATE/KEY PAIRS FOR EACH CLIENT,
# EACH HAVING ITS OWN UNIQUE \"COMMON NAME\",
# UNCOMMENT THIS LINE OUT.
;duplicate-cn
	
# The keepalive directive causes ping-like
# messages to be sent back and forth over
# the link so that each side knows when
# the other side has gone down.
# Ping every 10 seconds, assume that remote
# peer is down if no ping received during
# a 120 second time period.
keepalive 10 120
	
# For extra security beyond that provided
# by SSL/TLS, create an \"HMAC firewall\"
# to help block DoS attacks and UDP port flooding.
#
# Generate with:
#   openvpn --genkey --secret ta.key
#
# The server and each client must have
# a copy of this key.
# The second parameter should be '0'
# on the server and '1' on the clients.
;tls-auth ta.key 0 # This file is secret
	
# Select a cryptographic cipher.
# This config item must be copied to
# the client config file as well.
;cipher BF-CBC        # Blowfish (default)
;cipher AES-128-CBC   # AES
;cipher DES-EDE3-CBC  # Triple-DES
	
# Enable compression on the VPN link.
# If you enable it here, you must also
# enable it in the client config file.
comp-lzo
	
# The maximum number of concurrently connected
# clients we want to allow.
;max-clients 100
	
# It's a good idea to reduce the OpenVPN
# daemon's privileges after initialization.
#
# You can uncomment this out on
# non-Windows systems.
;user nobody
;group nogroup
	
# The persist options will try to avoid
# accessing certain resources on restart
# that may no longer be accessible because
# of the privilege downgrade.
persist-key
persist-tun
	
# Output a short status file showing
# current connections, truncated
# and rewritten every minute.
status openvpn-status.log
	
# By default, log messages will go to the syslog (or
# on Windows, if running as a service, they will go to
# the \"\Program Files\OpenVPN\log\" directory).
# Use log or log-append to override this default.
# \"log\" will truncate the log file on OpenVPN startup,
# while \"log-append\" will append to it.  Use one
# or the other (but not both).
;log         openvpn.log
log-append  /var/log/openvpn.log
	
# Set the appropriate level of log
# file verbosity.
#
# 0 is silent, except for fatal errors
# 4 is reasonable for general usage
# 5 and 6 can help to debug connection problems
# 9 is extremely verbose
verb 3
	
# Silence repeating messages.  At most 20
# sequential messages of the same message
# category will be output to the log.
;mute 20

Natür­lich ist die Kon­fi­gu­ra­tion noch lange nicht opti­miert und ermög­licht noch nicht alles, was ich mir als Ziel gesetzt habe. Mit einem

/etc/init.d/openvpn start

star­ten wir den Ser­ver. Um zu schauen, ob alles funk­tio­niert schauen wir uns zuerst die Datei /var/log/openvpn.log an

Wed Jun  3 23:26:18 2009 OpenVPN 2.1_rc11 i486-pc-linux-gnu [SSL] [LZO2] [EPOLL] [PKCS11] built on Sep 18 2008
Wed Jun  3 23:26:18 2009 Diffie-Hellman initialized with 1024 bit key
Wed Jun  3 23:26:18 2009 WARNING: file '/etc/ssl/private/vpn.domain.de-key.pem' is group or others accessible
Wed Jun  3 23:26:18 2009 /usr/bin/openssl-vulnkey -q -b 1024 -m <modulus omitted>
Wed Jun  3 23:26:18 2009 TLS-Auth MTU parms [ L:1542 D:138 EF:38 EB:0 ET:0 EL:0 ]
Wed Jun  3 23:26:18 2009 ROUTE default_gateway=180.190.200.1
Wed Jun  3 23:26:18 2009 TUN/TAP device tun0 opened
Wed Jun  3 23:26:18 2009 TUN/TAP TX queue length set to 100
Wed Jun  3 23:26:18 2009 /sbin/ifconfig tun0 10.8.0.1 pointopoint 10.8.0.2 mtu 1500
Wed Jun  3 23:26:18 2009 /sbin/route add -net 10.8.0.0 netmask 255.255.255.0 gw 10.8.0.2
Wed Jun  3 23:26:18 2009 Data Channel MTU parms [ L:1542 D:1450 EF:42 EB:135 ET:0 EL:0 AF:3/1 ]
Wed Jun  3 23:26:18 2009 Socket Buffers: R=[129024->131072] S=[129024->131072]
Wed Jun  3 23:26:18 2009 UDPv4 link local (bound): [undef]:1194
Wed Jun  3 23:26:18 2009 UDPv4 link remote: [undef]
Wed Jun  3 23:26:18 2009 MULTI: multi_init called, r=256 v=256
Wed Jun  3 23:26:18 2009 IFCONFIG POOL: base=10.8.0.4 size=62
Wed Jun  3 23:26:18 2009 IFCONFIG POOL LIST
Wed Jun  3 23:26:18 2009 Initialization Sequence Completed

Damit steht unser Ser­ver schon mal und in einem wei­te­ren Bei­trag zeige ich, wie man sich mit einem Cli­ent verbindet.

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