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openvpn_server

OpenVPN Server

The OpenVPN Server menu allows viewing and configuration of settings for the OpenVPN Server. Two distinct VPNs can be created, each using TUN/TAP virtual interfaces. Connections are TLS-encrypted.

Basic Tab

Start with WAN: Checking this makes OpenVPN Server run each time the WAN interface comes up. This is suitable if OpenVPN server needs to be available 24/7.

Start Now: If Start with WAN is not checked, clicking on this makes the OpenVPN server start immediately.

Interface Type

VPNs use virtual (software) network devices to simulate physical network adapters. OpenVPN has two main types of virtual interfaces:

TUN, (for “network TUNnel”), simulates a network layer device. TUN operates at OSI layer 3, and carries IP datagrams. TUN is used with routing. In general, TUN is used for VPN tunnels where only IP protocol is used.

Advantages

  • Has less overhead.
  • TUN only transports traffic destined for the VPN client.
  • TUN only handles layer 3 IP datagrams.

Disadvantages

  • Cannot be used to bridge networks.
  • Doesn’t normally transport broadcast traffic.
  • Limited to transporting IPv4 (OpenVPN 2.3 adds IPv6 support).

TAP, (“network TAP”), simulates a layer 2, link layer device to create a network bridge. TAP carries Ethernet frames. Since TAP allows full Ethernet frames to pass through the tunnel, it also supports non-routable protocols like IPX and AppleTalk.

Other important differences include:

  • TAP has slightly more overhead, since it encapsulates the full Ethernet frame.
  • All devices connected to a TAP-based network form a single broadcast domain,
    because TAP acts as a bridge.
  • Since TUN is used with routing, it can help networks to intercommunicate,
    while still remaining separate.

Common applications of TUN/TAP include:

  • VM networking.
  • Connections between real machines and network simulations
    (network behaviour modelled by software).
  • NAT (Network Address Translation).

Advantages

  • Can be used to bridge networks.
  • Only handles the transport of network protocols, such as IPv4 and IPv6.
  • Behaves like a real network adapter, even though it’s virtual.

Disadvantages

  • All packets TAP transports include the overhead from Ethernet headers.
  • TAP Tends to scale poorly.
  • TAP tends to cause more broadcast overhead.

Both clients and server must use the same Interface type(s). You cannot use TAP on clients and TUN on servers or vice versa.

Bridge TAP with ...

  • LAN (br0)
  • LAN1 (br1)
  • LAN2 (br2)
  • LAN3 (br3)

This option appears if you selected the TAP Interface Type. This lets you select the VLAN to which you want to bridge the clients that connect to your OpenVPN server. [Default: LAN (br0) ].

Protocol

OpenVPN can run over TCP or UDP transport protocols.

UDP OpenVPN Protocol

  • Faster — UDP is significantly faster than TCP.
  • Preferred connection for media streaming, VoIP and playing games online.
  • Lower reliability — Occasionally, UDP can drop packets.

TCP OpenVPN Protocol

  • Higher reliability — TCP offers more stable connections, since it
    guarantees delivery of packets.
  • Bypass Firewalls — TCP is rarely blocked, since it runs on common ports.
    TCP VPNs can bypass even strict corporate/government firewalls.
  • Lower speed — TCP's higher encryption tends to slow transfer rates
    when compared with UDP.

Port

This sets the port on which the OpenVPN server listens on the router's WAN interface. Make sure to configure firewall / IPTables rules to allow the traffic through this port. (Default: 443).

Firewall

  • Auto — FreshTomato creates all firewall / NAT rules required for the tunnel.
  • Custom — FreshTomato configures no firewall / NAT rules. The Administrator
    must create them manually.
  • External Only — This blocks LAN access (*To be Confirmed).

Authorization Mode

OpenVPN allows peers to authenticate each other using a Static (Pre‐Shared) Key or certificates. In client‐server configuration, OpenVPN allows the server to release an authentication certificate for each client, using signature and certificate authority. OpenVPN uses the OpenSSL encryption library, as well as the SSLv3/TLSv1 protocol. It contains many security and control features.

This option selects the authorization mode for the OpenVPN Server.

  • TLS — This setting uses Transport Layer Security authorization mode.
    This is the most secure authorization mode. Certificates can be generated and
    configured in the KEYS tab. An SSL session is established
    with bidirectional authentication. Each side must present its own certificate.
    OpenVPN uses a reliable transport layer on top of UDP, because SSL/TLS
    is designed to operate over a reliable transport. Once each peer has its set of keys,
    tunnel forwarding begins.
  • Static Key — This uses Pre-Shared key (pre‐shared or “PSK”) authorization mode.
    This mode offers the simplest setup, ideal for point-to-point VPNs or testing.
    However, there are security compromises. The static key size is fixed at 2048 bits.
    The key size doesn't correlate with the encryption level that protects the data
    between VPN peers. The –cipher and –keysize parameters set the encryption key size.
    Static Keys have security limitations. While easier to set up than an X.509 PKI,
    Static Keys lack Perfect Forward Secrecy. Since the key isn't automatically rotated,
    an attacker who gains access to the key may be able to decrypt any intercepted
    past or future communication using that key. Without automatic key rotation, there is a higher
    chance the key might be brute-force cracked for long-lived connections.
  • Custom — The administrator must configure all authentication parameters in the
    Advanced Tab /Custom Settings field.

Static Key

Advantages

  • Simple Setup.
  • No X.509 PKI (Public Key Infrastructure) to maintain.

Disadvantages

  • Limited scalability. Just one client, and one server.
  • Lacks Perfect Forward Secrecy. This compromise can result
    in total disclosure of previous sessions.
  • The secret key is stored in plaintext on each VPN peer.
  • The secret key must be exchanged on a pre-existing secure channel.

TLS Control channel security

This menu appears automatically if TLS Authorization mode is selected.

This option specifies how FreshTomato will generate the tls-auth configuration parameter where a direction constant needs to be given. This decides which set(s) of HMAC keys will be used (HMAC-send, cipher-encrypt, HMAC-receive, cipher-receive).

  • Disabled — No tls-auth will be used on the server. No direction is set.
  • Bi-directional Auth — [Direction] is set to 2.
    The HMAC-send and cipher-encrypt keys will be used.
  • Incoming Auth (0) — [Direction] is set to 0.
    HMAC keys won't be used on this server.
    However, they may be used on remote endpoints.
  • Outgoing Auth (1) — [Direction] is set to 1.
    The HMAC-send HMAC keys will be used.
  • Encrypt Channel — [Direction] is set to 3.
    The HMAC-send, cipher-encrypt, and HMAC-receive keys will be used.
  • Encrypt Channel v2 — [Direction] is set to 4.
    HMAC-send, cipher-encrypt, HMAC-receive and cipher-receive are used.

TLS requires a multi-packet exchange before it authenticates a peer. During this exchange, OpenVPN allocates memory and CPU resources to the potential peer. The potential peer exposes parts of OpenVPN and the OpenSSL library to the packets it is sending. Most successful network attacks today try to to either exploit bugs in programs (such as buffer overflow attacks) or force a program to consume so many resources that it becomes unusable. The first line of defense is always good programming. One of the main goals when coding OpenVPN was to prevent buffer overflow attacks. However, many widely-used network applications still occasionally fall to buffer overflow attacks.

OpenVPN's second line of defense is an authentication layer on top of the TLS Control channel. At this layer, every packet on the control channel is authenticated by an HMAC signature and a Unique ID. This authentication layer prevents replay attacks. The signature also helps protect against Denial of Service (DoS) attacks. When an unauthenticated client has limits on how much resources it can use, DoS attacks are less likely.

Enabling TLS Control Channel Security makes FreshTomato sign every control channel packet with an HMAC signature. This includes packets sent before the TLS layer has authenticated its peer. Packets without the correct signature will be immediately dropped on receipt. As a result, such packets don't have a chance to consume additional system resources.

However, the feature is optional. The key file used with –tls-auth gives a peer only the power to initiate a TLS handshake. It is not used to encrypt or authenticate any tunnel data. Encrypt Channel should be used instead if you want to use the key file to both authenticate and encrypt the TLS control channel.

(Default: Disabled).

Auth Digest

Auth Digest (Authentication Digest) is an authentication system which reduces the risks of the plaintext method used with Basic authentication. With Auth Digest, the client sends a hash of its data over the network. Thus, the client's user name and password are never sent in plaintext over the network. This reduces the risk that logon credentials could be snooped.

If Auth Digest is set to a value other than None, OpenVPN will authenticate data channel packets and tls-auth control channel packets with HMAC. To do this, it will use a message digest algorithm (SHA1, by default ). HMAC is a common Message Authentication Code algorithm (MAC) that uses a data string, a secure hash algorithm, and a key to produce a digital signature.

The OpenVPN data channel protocol uses Encrypt-then-Mac order. A packet is first encrypted, then the resulting ciphertext has HMAC applied against it. This helps prevent padding oracle attacks.

If an AEAD cipher mode (e.g. GCM) is chosen, the specified –auth algorithm is ignored for the data channel, and the authentication method of the AEAD cipher is used instead. Note that alg still specifies the digest used for tls-auth.

In Static Key encryption mode, the HMAC key is included in the key file generated by –genkey. In TLS mode, the HMAC key is dynamically generated and shared between peers via the TLS control channel. If OpenVPN receives a packet with a bad HMAC, it will drop the packet. HMAC usually adds 16 or 20 bytes per packet. To disable authentication, set alg=none.

For basic HMAC information, see:

https://www.tutorialspoint.com/cryptography/message_authentication.htm

VPN Subnet/Netmask

This option appears when the TUN interface type is selected. In this field, you enter the subnet and netmask used to assign addresses to OpenVPN clients.

Client Address Pool

This option appears when the TAP interface type is selected. This specifies which method will be used to assign addresses to OpenVPN clients.

DHCP — When checked, DHCP will assign addresses to your OpenVPN clients from your normal DHCP pool.
When unchecked, the Client Address Pool field appears. In this field, you can create a special pool of addresses for your VPN clients.

Advanced Tab

Poll Interval

If set greater than zero, a watchdog polls connectivity every n minutes, to verify that OpenVPN is activated. If it finds that OpenVPN is not activated, it will restart the OpenVPN service. If set to zero, the watchdog is disabled.

Direct clients to redirect Internet traffic

If checked, this instructs OpenVPN clients to redirect all their Internet traffic through this server. In other words, this server becomes their default gateway. If unchecked, routing will need to be configured on each client.

Respond to DNS

The OpenVPN server can push DHCP options like DNS and WINS server addresses to clients (with limitations). As is, Windows clients can accept pushed DHCP options. Non-Windows clients can accept options by using a client-side up script which parses the foreign_option_nenvironmental variable list. See the OpenVPN Linux man(ual) page for non-Windows foreign_option_n documentation and script examples.

openvpn_server_configuration-advanced_tab-3.jpg


Cipher Negotiation

  • Disable: * Disables encryption.
  • Enabled (with fallback): This setting uses the cipher set in the Legacy/fallback cipher field.
  • Enabled: This option removes the Legacy/fallback cipher field.

This selects the cipher algorithm used to encrypt data channel packets. The default is BF-CBC, (Blowfish in Cipher Block Chaining) mode. When cipher negotiation (NCP) is allowed, OpenVPN 2.4 and newer on both client and server will automatically upgrade to AES-256-GCM.
See –ncp-ciphers and –ncp-disable for more details on NCP.

Using BF-CBC is no longer recommended, because of its 64-bit block size. This small block size allows attacks based on collisions, such as the SWEET32 attacks. For details on preventing SWEET32 attacks, please see:
https://community.openvpn.net/openvpn/wiki/SWEET32

For this reason, support for BF-CBC, DES, CAST5, IDEA and RC2 ciphers will be removed starting in OpenVPN 2.6.

Negotiable ciphers

This field cannot be edited. It displays which ciphers can be negotiated during the handshake between OpenVPN Client and Server.

The ciphers available in existing firmware are:

  • AES-256-GCM
  • AES-128-GCM
  • AES-256-CBC
  • AES-128-CBC

Please note the ciphers recommended to avoid SWEET32 attacks:

Legacy/Fallback cipher

This field defines which cipher to use as a fallback, in case cipher negotiation between client and server fails using the available ciphers.

Compression

  • Disabled * (Default)
  • LZO
  • LZ4

This enables a compression algorithm. LZO and LZ4 are different compression algorithms. LZ4 generally offers the best balance between performance and low CPU usage. For backwards compatibility with OpenVPN versions before v2.4, use “lzo” (which is identical to the older option “–comp-lzo yes”).

If the algorithm is set to Disabled, compression will also be disabled. However, packet framing for compression will still be enabled, allowing a different setting to be pushed later.

Security Considerations

Combining compression and encryption is tricky. If an attacker knows or is able to control (parts of) the plaintext of packets that contain secrets, they might be able to extract the secret if compression is enabled. For example, the CRIME and BREACH attacks on TLS leverage compression to break encryption. If you can't be sure the above problems don't apply to your traffic, you are advised to disable compression.

TLS renegotiation time

This specifies how many seconds (n) will pass before OpenVPN renegotiates the data channel key (Default=3600). When using dual-factor authentication, the default value may cause the end user to be asked to reauthorize once per hour.

This option can be used on both client and server. Whichever host uses the lower value will trigger the renegotiation. It's a common mistake to set this parameter to a higher value on either the client or server, while the other end is still using the default value. In this case, renegotiation will still occur once every 3600 seconds. The solution is to increase –reneg-sec on both client and server, or set it to “0” (disabled) on one side of the connection, and to your preferred value on the other side.

Manage Client-Specific Options

The client-to-client option enables an OpenVPN client to access the other's, and/or the network behind each OpenVPN client. OpenVPN blocks the network access between the OpenVPN clients unless you enable the client-to-client option.

When you enable Client ↔ Client access on the server you can limit the networks accessible to your clients. Use the table “Allow Only These Clients” in order to specify and limit the access your client will have over your network.

Using the Client-specific options require each client to have a certificate with a unique common name, because it's used to identify which client rule to apply. However, duplicate CNs are now rejected by default, since this is the recommended configuration by OpenVPN, and it improves security.

If you still need to use client specific options and don`t want to generate unique client certificates for each client, then you will need to allow the use of the username as common name, or duplicate common names (which may not lead to the desired result however if you expect different client rules). Try either of these:

Setting:

duplicate-cn

Or:

username-as-common-name

in your Custom Settings.

The push checkbox on the clients table can be used in order to push the routes to the clients. If you decide to leave it unchecked, the routes should be set separately in each client configuration. This check box will turn to N/A if the Client ↔ Client feature is not enabled.

You can further customize your OpenVPN server by changing the server port from its default of 1194. You can also choose a different auth digest and encryption cipher. Choosing AES-128-CBC and an auth digest of SHA1 is sufficient encryption for maintaining proper security when connecting to your Server.

Custom Configuration

Here, you can specify a custom configuration to be used by the OpenVPN server.

For details about custom parameters that can be used, please see:
https://openvpn.net/community-resources/reference-manual-for-openvpn-2-4/
.

Keys Tab

If you chose TLS authorization mode, you MUST configure the certificates.

Instead of using a static key, you can let the machines use TLS to negotiate an encryption key and an encryption algorithm. During the TLS handshake process, one machine acts as “server” and the other as “client”. Once the handshake is completed, the OpenVPN interaction is on a peer-to-peer basis.

For a TLS handshake to occur, both “server” and “client” must have their respective digital certificates. Also, Diffie-Hellman parameters need to be generated for the “server” which will finally lead to creation of a shared secret number. You will generate your own certificates. Every digital certificate is issued by a Certificate Authority, (“CA”), so you will first create a CA. The tools to create a Certificate Authority, certificates, and Diffie-Hellman parameters are provided by OpenVPN and are all available within FreshTomato.

The first step in building an OpenVPN 2.x configuration is to establish a Public Key Infrastructure (or “PKI”). The PKI consists of:

  • A separate certificate (public key) and private key for the server
    and for each client
  • A master Certificate Authority (CA) certificate and key used to sign
    each of the server and client certificates

OpenVPN supports bidirectional authentication based on certificates. Both client and server must authenticate each other's certificate before they establish mutual trust.

Both server and client will authenticate the other, first, by verifying that the certificate presented was signed by the master certificate authority (CA). Then, they will test the information in the authenticated certificate header. That information includes the certificate common name (CN) or certificate type (client or server).

This security model has a number of desirable features for a VPN:

  • The server only needs its own certificate/key. It doesn’t need to know
    the certificate 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).
    Because the server can verify this signature without needing access
    to the CA private key itself, the CA key can be stored on another machine.
    It can even be stored on a device not connected to the network.
    This is crucial, since it's the most sensitive key in the entire PKI.
  • 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 fields
    embedded in the certificates, such as the Common Name.

Note that server and client clocks need to be roughly synchronized, or certificates might not function properly.

The user has the option to generate ALL the required keys directly from FreshTomato GUI using the button Generate Keys or simply using the easy-rsa tool provided by OpenVPN software and following these instructions:

Please be aware that using NVRAM space to store certificate values may be expensive, especially for routers with limited RAM. The certificates consume about 14 KB of NVRAM space. Therefore, if your router has limited NVRAM available, it might be a good idea to store the certificates in JFFS or other mounted storage in your router. Please consider the security risks before following this procedure, as certificates may be accessible if you enable Samba or NFS filesharing.

Here's an example of how to include certificates in your Custom Configuration field if you do not want to use NVRAM space:

#Path names and filenames below are used only as example
 
dh /jffs/certs/dh.pem      #Contains Diffie-Hellman parameters
cert /jffs/certs/srv.crt   #Contains Server Certificate   Server Key
ca /jffs/certs/ca.crt      #Contains CA Key

Please note that since FreshTomato does not use TLS with elliptic curve cryptography, you must configure the Diffie-Hellman parameters

Notes

Some warnings about certificates

A common mistake when setting up a new CA is to place all the CA files on the OpenVPN server. **DO NOT DO THAT**. A CA requires a private key used to sign the certificates your clients and servers use. If you lose control of your CA private key, you can no longer trust any certificates from that CA. Anyone with access to the CA private key can sign new certificates without your knowledge, and clients using those certificates then can connect to your OpenVPN server without needing to modify anything on the VPN server. Place your CA files on a storage medium that is offline as much as possible, only to be activated when you need to get a new certificate for a client or server.

There are just 3 files you need to copy from a CA to achieve this:

  1. A Private key (often a .key or .pem file)
  2. A Certificate (often a .crt or .pem file), and;
  3. A CA certificate (also a .crt or .pem file)

The server also needs a Diffie-Hellman parameters file.

You should avoid generating keys on devices which do not have a good entropy source for random data. This includes most common Wi-Fi routers and similar embedded devices. In many cases, virtual machines also don't have a good entropy source or can be manipulated by the hypervisor. Always try to generate keys and Diffie-Hellman parameters on bare metal equipment.

To better understand how PKI works, see this introduction:
https://github.com/OpenVPN/easy-rsa/blob/master/doc/Intro-To-PKI.md

Adding certificate revocation lists

Within the CA, you can also revoke certificates as needed. Using the CA management tool of your choice, you should be able to generate a Certificate Revocation List (CRL file). By adding this to the OpenVPN server, all client certificates will be checked against this revocation list. Clients which have their certificates listed in the CRL will not be able to connect. This is a common way to disable access to a VPN service on a per user level.

Add this line to the OpenVPN server configuration:

   crl /full/path/to/crl.pem

Routing notes

If you want to access particular network resources on other IP addresses via the VPN tunnel, you need to add network routes. A network route tells your operating system where it needs to send the network traffic when you want to access certain resources. An operating system can handle multiple routes via multiple gateways at the same time. So if you have a server on 192.168.1.10 behind your VPN server and you want to access this server via the VPN, you need to tell OpenVPN to configure a route for either a specific host or a network range to go via the tunnel.

So to configure this, you need to add one line in the server configuration and restart server and client.

   push "route 192.168.1.0 255.255.255.0"

When the client now connects, the server tells the VPN client that it should route all traffic for IP addresses in the 192.168.1.XXX scope via the VPN connection.

This is a very basic setup. And when we now start on the routing part, the VPN setup is mostly done. All you need now is to add the needed routes you need, just like you would do for normal TCP/IP routing.

BEWARE: Remember that you also need to consider what is called “return routes”. If your VPN client can access a host behind your VPN server, it does not mean that the host behind the VPN server will send the response via the same route. So you need to ensure that your hosts behind your VPN server also knows which gateway to use for your VPN. Nowadays this is most commonly fixed by adding a route on your existing default gateway. And if you run OpenVPN on an existing gateway, you have this return route already impllicitly configured.

For a more detailed example using routing, see the ​Using routing section in the 'Bridiging and routing' wiki page.

Routing everything over the VPN

It is possible to route absolutely all network traffic over the VPN. The configuration in OpenVPN is fairly simple. But you will need to investigate how to configure NAT on your VPN server for the virtual tun adapter.

You can either push such a “route everything over VPN” via the server, or you can add it explicitly in the client configuration. Do not use both at the same time.

Server push:

    push "redirect-gateway def1"

Client configuration alternative:

    redirect-gateway def1

What about IPv6?

OpenVPN v2.3 and later supports IPv6. To set up IPv6 in the tunnel is roughly similar to the IPv4 examples already discussed. You need to use the –server-ipv6 and –route-ipv6 options to configure IPv6.

For example, adding this will configure the IPv6 addresses for server and clients:

    server-ipv6 2001:db8:cada::/64

You can use the –route-ipv6 option, either by pushing it from the server, or using it directly in the client configuration. The same is true for the –route option. The syntax is similar too:

    route-ipv6 2001:db8:daca::/64

Other Considerations when configuring a VPN

There are a few more things which may need to be configured, but those are mostly outside of OpenVPN. The most common issues are related to adjusting your operating system to allow forwarding of packets and configuring the firewall properly.

Configuring firewall is so different between Linux and other Unix based OSes, in addition several Linux distributions have their own tools to manage iptables. So it is better to read the manuals for the firewall configuration on your operating system.

openvpn_server.txt · Last modified: 2021/08/28 19:08 by hogwild