2x HOW TO
OpenVPN 是一个功能齐全的 SSL VPN，它使用行业标准 SSL/TLS 协议实现 OSI 第 2 层或第 3 层安全网络扩展，支持基于证书、智能卡和/或用户名/密码凭证的灵活客户端身份验证方法，并允许用户或使用应用于 VPN 虚拟接口的防火墙规则的组特定访问控制策略。 OpenVPN 不是网络应用代理，不能通过网络浏览器运行。
本文档提供配置 OpenVPN 2.x 客户端/服务器 VPN 的分步说明，包括：
- OpenVPN Quickstart.
- Installing OpenVPN.
- Determining whether to use a routed or bridged VPN.
- Numbering private subnets.
- Setting up your own Certificate Authority (CA) and generating certificates and keys for an OpenVPN server and multiple clients.
- Creating configuration files for server and clients.
- Starting up the VPN and testing for initial connectivity.
- Configuring OpenVPN to run automatically on system startup.
- Controlling a running OpenVPN process.
- Expanding the scope of the VPN to include additional machines on either the client or server subnet.
- Pushing DHCP options to clients.
- Configuring client-specific rules and access policies.
- Using alternative authentication methods.
- How to add dual-factor authentication to an OpenVPN configuration using client-side smart cards.
- Routing all client traffic (including web-traffic) through the VPN.
- Running an OpenVPN server on a dynamic IP address.
- Connecting to an OpenVPN server via an HTTP proxy.
- Connecting to a Samba share over OpenVPN.
- Implementing a load-balancing/failover configuration.
- Hardening OpenVPN Security.
- Revoking Certificates.
- Additional Security Notes.
The impatient may wish to jump straight to the sample configuration files:
This HOWTO assumes that readers possess a prior understanding of basic networking concepts such as IP addresses, DNS names, netmasks, subnets, IP routing, routers, network interfaces, LANs, gateways, and firewall rules.
Please take a look at the OpenVPN books page.
OpenVPN 1.x HOWTO
The original OpenVPN 1.x HOWTO is still available, and remains relevant for point-to-point or static-key configurations.
While this HOWTO will guide you in setting up a scalable client/server VPN using an X509 PKI (public key infrastruction using certificates and private keys), this might be overkill if you are only looking for a simple VPN setup with a server that can handle a single client.
If you would like to get a VPN running quickly with minimal configuration, you might check out the Static Key Mini-HOWTO.
Static Key advantages
- Simple Setup
- No X509 PKI (Public Key Infrastructure) to maintain
Static Key disadvantages
- Limited scalability -- one client, one server
- Lack of perfect forward secrecy -- key compromise results in total disclosure of previous sessions
- Secret key must exist in plaintext form on each VPN peer
- Secret key must be exchanged using a pre-existing secure channel
For security, it's a good idea to check the file release signature after downloading.
The OpenVPN executable should be installed on both server and client machines, since the single executable provides both client and server functions.
Linux Notes (using RPM package)
If you are using a Linux distribution which supports RPM packages (SuSE, Fedora, Redhat, etc.), it's best to install using this mechanism. The easiest method is to find an existing binary RPM file for your distribution. You can also build your own binary RPM file:
rpmbuild -tb openvpn-[version].tar.gz
Once you have the .rpm file, you can install it with the usual
rpm -ivh openvpn-[details].rpm
or upgrade an existing installation with
rpm -Uvh openvpn-[details].rpm
Installing OpenVPN from a binary RPM package has these dependencies:
Furthermore, if you are building your own binary RPM package, there are several additional dependencies:
See the openvpn.spec file for additional notes on building an RPM package for Red Hat Linux 9 or building with reduced dependencies.
Linux Notes (without RPM)
If you are using Debian, Gentoo, or a non-RPM-based Linux distribution, use your distro-specific packaging mechanism such as apt-get on Debian or emerge on Gentoo.
It is also possible to install OpenVPN on Linux using the universal ./configure method. First expand the .tar.gz file:
tar xfz openvpn-[version].tar.gz
Then cd to the top-level directory and type:
./configure make make install
OpenVPN for Windows can be installed from the self-installing exe file on the OpenVPN download page. Remember that OpenVPN will only run on Windows XP or later. Also note that OpenVPN must be installed and run by a user who has administrative privileges (this restriction is imposed by Windows, not OpenVPN). The restriction can be sidestepped by running OpenVPN in the background as a service, in which case even non-admin users will be able to access the VPN, once it is installed. More discussion on OpenVPN + Windows privilege issues.
After you've run the Windows installer, OpenVPN is ready for use and will associate itself with files having the .ovpn extension. To run OpenVPN, you can:
- Right click on an OpenVPN configuration file (.ovpn) and select Start OpenVPN on this configuration file. Once running, you can use the F4key to exit.
- Run OpenVPN from a command prompt Window with a command such as:
Once running in a command prompt window, OpenVPN can be stopped by the F4 key.
- Run OpenVPN as a service by putting one or more .ovpn configuration files in \Program Files\OpenVPN\config and starting the OpenVPN Service, which can be controlled from Start Menu -> Control Panel -> Administrative Tools -> Services.
Mac OS X Notes
Angelo Laub and Dirk Theisen have developed an OpenVPN GUI for OS X.
Some notes are available in the INSTALL file for specific OSes. In general, the
./configure make make install
method can be used, or you can search for an OpenVPN port or package which is specific to your OS/distribution.
Determining whether to use a routed or bridged VPN
Overall, routing is probably a better choice for most people, as it is more efficient and easier to set up (as far as the OpenVPN configuration itself) than bridging. Routing also provides a greater ability to selectively control access rights on a client-specific basis.
I would recommend using routing unless you need a specific feature which requires bridging, such as:
- the VPN needs to be able to handle non-IP protocols such as IPX,
- you are running applications over the VPN which rely on network broadcasts (such as LAN games), or
- you would like to allow browsing of Windows file shares across the VPN without setting up a Samba or WINS server.
Numbering private subnets
Setting up a VPN often entails linking together private subnets from different locations.
The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private internets (codified in RFC 1918):
While addresses from these netblocks should normally be used in VPN configurations, it's important to select addresses that minimize the probability of IP address or subnet conflicts. The types of conflicts that need to be avoided are:
- conflicts from different sites on the VPN using the same LAN subnet numbering, or
- remote access connections from sites which are using private subnets which conflict with your VPN subnets.
For example, suppose you use the popular 192.168.0.0/24 subnet as your private LAN subnet. Now you are trying to connect to the VPN from an internet cafe which is using the same subnet for its WiFi LAN. You will have a routing conflict because your machine won't know if 192.168.0.1 refers to the local WiFi gateway or to the same address on the VPN.
As another example, suppose you want to link together multiple sites by VPN, but each site is using 192.168.0.0/24 as its LAN subnet. This won't work without adding a complexifying layer of NAT translation, because the VPN won't know how to route packets between multiple sites if those sites don't use a subnet which uniquely identifies them.
The best solution is to avoid using 10.0.0.0/24 or 192.168.0.0/24 as private LAN network addresses. Instead, use something that has a lower probability of being used in a WiFi cafe, airport, or hotel where you might expect to connect from remotely. The best candidates are subnets in the middle of the vast 10.0.0.0/8 netblock (for example 10.66.77.0/24).
And to avoid cross-site IP numbering conflicts, always use unique numbering for your LAN subnets.
Setting up your own Certificate Authority (CA) and generating certificates and keys for an OpenVPN server and multiple clients
The first step in building an OpenVPN 2.x configuration is to establish a PKI (public key infrastructure). The PKI consists of:
- a separate certificate (also known as a public key) and private key for the server and each client, and
- a master Certificate Authority (CA) certificate and key which is used to sign each of the server and client certificates.
OpenVPN supports bidirectional authentication based on certificates, meaning that the client must authenticate the server certificate and the server must authenticate the client certificate before mutual trust is established.
Both server and client will authenticate the other by first verifying that the presented certificate was signed by the master certificate authority (CA), and then by testing information in the now-authenticated certificate header, such as the certificate common name or certificate type (client or server).
This security model has a number of desirable features from the VPN perspective:
- The server only needs its own certificate/key -- it doesn't need to know the individual certificates of every client which might possibly connect to it.
- The server will only accept clients whose certificates were signed by the master CA certificate (which we will generate below). And because the server can perform this signature verification without needing access to the CA private key itself, it is possible for the CA key (the most sensitive key in the entire PKI) to reside on a completely different machine, even one without a network connection.
- If a private key is compromised, it can be disabled by adding its certificate to a CRL (certificate revocation list). The CRL allows compromised certificates to be selectively rejected without requiring that the entire PKI be rebuilt.
- The server can enforce client-specific access rights based on embedded certificate fields, such as the Common Name.
Note that the server and client clocks need to be roughly in sync or certificates might not work properly.
Generate the master Certificate Authority (CA) certificate & key
In this section we will generate a master CA certificate/key, a server certificate/key, and certificates/keys for 3 separate clients.
For PKI management, we will use easy-rsa 2, a set of scripts which is bundled with OpenVPN 2.2.x and earlier. If you're using OpenVPN 2.3.x, you need to download easy-rsa 2 separately from here.
For PKI management, we will use easy-rsa 2, a set of scripts which is bundled with OpenVPN 2.2.x and earlier. If you're using OpenVPN 2.3.x, you may need to download easy-rsa 2 separately from the easy-rsa-old project page. On *NIX platforms you should look into using easy-rsa 3 instead; refer to its own documentation for details.
If you are using Linux, BSD, or a unix-like OS, open a shell and cd to the easy-rsa subdirectory. If you installed OpenVPN from an RPM or DEB file, the easy-rsa directory can usually be found in /usr/share/doc/packages/openvpn or /usr/share/doc/openvpn(it's best to copy this directory to another location such as /etc/openvpn, before any edits, so that future OpenVPN package upgrades won't overwrite your modifications). If you installed from a .tar.gz file, the easy-rsa directory will be in the top level directory of the expanded source tree.
If you are using Windows, open up a Command Prompt window and cd to \Program Files\OpenVPN\easy-rsa. Run the following batch file to copy configuration files into place (this will overwrite any preexisting vars.bat and openssl.cnf files):
Now edit the vars file (called vars.bat on Windows) and set the KEY_COUNTRY, KEY_PROVINCE, KEY_CITY, KEY_ORG, and KEY_EMAIL parameters. Don't leave any of these parameters blank.
Next, initialize the PKI. On Linux/BSD/Unix:
. ./vars ./clean-all ./build-ca
vars clean-all build-ca
The final command (build-ca) will build the certificate authority (CA) certificate and key by invoking the interactive opensslcommand:
ai:easy-rsa # ./build-ca Generating a 1024 bit RSA private key ............++++++ ...........++++++ writing new private key to 'ca.key' ----- You are about to be asked to enter information that will be incorporated into your certificate request. What you are about to enter is what is called a Distinguished Name or a DN. There are quite a few fields but you can leave some blank For some fields there will be a default value, If you enter '.', the field will be left blank. ----- Country Name (2 letter code) [KG]: State or Province Name (full name) [NA]: Locality Name (eg, city) [BISHKEK]: Organization Name (eg, company) [OpenVPN-TEST]: Organizational Unit Name (eg, section) : Common Name (eg, your name or your server's hostname) :OpenVPN-CA Email Address [firstname.lastname@example.org]:
Note that in the above sequence, most queried parameters were defaulted to the values set in the varsor vars.bat files. The only parameter which must be explicitly entered is the Common Name. In the example above, I used "OpenVPN-CA".
Generate certificate & key for server
Next, we will generate a certificate and private key for the server. On Linux/BSD/Unix:
As in the previous step, most parameters can be defaulted. When the Common Name is queried, enter "server". Two other queries require positive responses, "Sign the certificate? [y/n]" and "1 out of 1 certificate requests certified, commit? [y/n]".
Generate certificates & keys for 3 clients
Generating client certificates is very similar to the previous step. On Linux/BSD/Unix:
./build-key client1 ./build-key client2 ./build-key client3
build-key client1 build-key client2 build-key client3
If you would like to password-protect your client keys, substitute the build-key-pass script.
Remember that for each client, make sure to type the appropriate Common Name when prompted, i.e. "client1", "client2", or "client3". Always use a unique common name for each client.
Generate Diffie Hellman parameters
Diffie Hellman parameters must be generated for the OpenVPN server. On Linux/BSD/Unix:
ai:easy-rsa # ./build-dh Generating DH parameters, 1024 bit long safe prime, generator 2 This is going to take a long time .................+........................................... ...................+.............+.................+......... ......................................