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Data connections

Peers can transfer network data either over UDP or TCP. For both there are ways of securing the data (see below).

IPv6 support

IPv6 can be used for both server connections and data connections, alongside with IPv4. Additionally, both can be combined to allow gradual migration to IPv6.

Address selection

Because NATs and firewalls are widespread, it is harder for peer-to-peer services to operate. In general, for two computers to be able to communicate, one computer must bind to one of its addresses, and the other computer must connect to the computer that binded (both for TCP and UDP). In a network with point-to-point connectivity, the connecting computer can connect to the same address as the binding computer bound to, so it is sufficient for the binding computer to send its address to the connecting computer. However, NATs and firewalls break point-to-point connectivity. When a network is behind a NAT, it is, by default, impossible for computers outside of that network to connect to computers inside the network. This is because computers inside the network have no externally visible IP address, and only communicate with the outside world through the external IP address of the NAT router. It is however possible to manually configure the NAT router to forward a specific port number on its external IP address to a specific computer inside the network. This makes it possible for a computer outside of the network to connect to a computer inside a network, however, it must connect to the external address of the NAT router (rather than the address the computer inside bound to, which is its internal address). So there needs to be some way for the connecting peer to know what address to connect to.

BadVPN solves this problem with so-called address scopes. The peer that binds must have a list of external addresses for each address it can bind to, possibly ordered from best to worst. Each external address has its scope name. A scope name represents part of a network from which an external address can be reached. On the other hand, the peer that connects must have a list of scopes which it can reach. When a peer binds to an address, it sends the other peer a list of external addresses along with scope names. That peer than chooses the first external address whose scope it recognizes and attempts to connect to it (if there is one).

BadVPN also allows a peer to have multiple addresses for binding to. It is possible to specify both an IPv4 and an IPv6 address to work in a multi-protocol environment.

Relaying

BadVPN can be configured to allow pairs of peers that cannot communicate directly (i.e. because of NATs or firewalls) to relay network data through a third peer. Relaying is only attempted if none of the two peers recognize any of the other peer's external addresses (or there are none). For relaying to work, for each of the two peers (P1, other one P2) there must be at least one third peer (R) that P1 it is allowed to relay through and can communicate directly with, and all such peers R must be able to communicate directly with P2.

IGMP snooping

BadVPN nodes perform IGMP snooping in order to efficiently deliver multicast frames. For example, this makes it possible to use BadVPN as a tunnel into an IPTV network of an Internet Service Provider for you to watch TV from wherever you want (given sufficient link quality).

Code quality

BadVPN has great focus on code quality and reliability. BadVPN is written in the C programming language. It is a single-threaded event-driven program. This allows for low resource usage and fast response times. Even though C is a relatively low-level language, the programs are made of small, highly cohesive and loosely coupled modules that are combined into a complete program on a high level. Modules are accesed and communicate through small, simple and to-the-point interfaces. It utilizes a flow-based design which greatly simplifies processing of data and input and output of the programs.

BadVPN contains many security features, all of which are optional. The included security features are described here.

TLS for client-server connections

It is possible for the peers to communicate with the chat server securely with TLS. It is highly recommended that this feature is used if any security whatsoever is needed. Not using it renders all other security features useless, since clients exchange keys unencrypted via the server. When enabled, the chat server requires each client to identify itself with a certificate.

BadVPN uses Mozilla's NSS library for TLS support. This means that the required certificates and keys must be available in a NSS database. The database and certificates can be generated with the certutil command. See the examples section on how to generate and distribute the certificates.

TLS for peer messaging

If TLS is being used for client-server connections, it will also be used between each pair of peers communicating via the server, on top of the TLS connections to the server. This secures the messages from the server itself. It is important because the messages may include encryption keys and other private data.

TLS for TCP data connections

If TCP is used for data connections between the peers, the data connections can be secured with TLS. This requires using TLS for client-server connections. The clients need to trust each others' certificates to be able to connect. Additionally, each client must identify to its peers with the same certificates it used for connecting to the server.

Encryption for UDP data connections

If UDP is used for data connections, it is possible for each pair of peers to encrypt their UDP packets with a symmetric block cipher. Note that the encryption keys are transmitted through the server unencrypted, so for this to be useful, server connections must be secured with TLS. The encryption aims to prevent third parties from seeing the real contents of the network data being transfered.

Hashes for UDP data connections

If UDP is used for data connections, it is possible to include hashes in packets. Note that hashes are only useful together with encryption. If enabled, the hash is calculated on the packet with the hash field zeroed and then written to the hash field. Hashes are calculated and included before encryption (if enabled). Combined with encryption, hashes aim to prevent third parties from tampering with the packets and injecting them into the network.

One-time passwords for UDP data connections

If UDP is used for data connections, it is possible to include one-time passwords in packets. Note that for this to be useful, server connections must be secured with TLS. One-time passwords are generated from a seed value by encrypting zero data with a block cipher. The seed contains the encryption key for the block cipher and the initialization vector. Only a fixed number of passwords are used from a single seed. The peers exchange seeds through the server. One-time passwords aim to prevent replay attacks.

Control over peer communication

It is possible to instruct the chat server to only allow certain peers to communicate. This will break end-to-end connectivity in the virtual network. It is useful in certain cases to improve security, for example when the VPN is used only to allow clients to securely connect to a central service.

If you want to use TLS for server connections (recommended), the server and all the peers will need certificates. This section explains how to generate and distribute the certificates using NSS command line tools.

Setting up the Certificate Authority (CA)

On the system that will host the CA, create a NSS database for the CA and generate a CA certificate valid for 24 months:

vpnca $ certutil -d sql:/home/vpnca/nssdb -N
vpnca $ certutil -d sql:/home/vpnca/nssdb -S -n "vpnca" -s "CN=vpnca" -t "TC,," -x -2 -v 24
> Is this a CA certificate [y/N]? y
> Enter the path length constraint, enter to skip [<0 for unlimited path]: > -1
> Is this a critical extension [y/N]? n

Export the public CA certificate (this file is public):

vpnca $ certutil -d sql:/home/vpnca/nssdb -L -n vpnca -a > ca.pem

Setting up the server certificate

On the CA system, generate a certificate for the server valid for 24 months, with TLS server usage context:

vpnca $ certutil -d sql:/home/vpnca/nssdb -S -n "<insert_server_name>" -s "CN=<insert_server_name>" -c "vpnca" -t ",," -2 -6 -v 24
> 0
> -1
> Is this a critical extension [y/N]? n
> Is this a CA certificate [y/N]? n
> Enter the path length constraint, enter to skip [<0 for unlimited path]: >
> Is this a critical extension [y/N]? n

Export the server certificate to a PKCS#12 file (this file must be kept secret):

vpnca $ pk12util -d sql:/home/vpnca/nssdb -o server.p12 -n "<insert_server_name>"

On the system that will run the server, create a NSS database and import the CA certificate and the server cerificate:

vpnserver $ certutil -d sql:/home/vpnserver/nssdb -N
vpnserver $ certutil -d sql:/home/vpnserver/nssdb -A -t "CT,," -n "vpnca" -i /path/to/ca.pem
vpnserver $ pk12util -d sql:/home/vpnserver/nssdb -i /path/to/server.p12

Setting up peer certificates

On the CA system, generate a certificate for the peer valid for 24 months, with TLS client and TLS server usage contexts:

vpnca $ certutil -d sql:/home/vpnca/nssdb -S -n "peer-<insert_name>" -s "CN=peer-<insert_name>" -c "vpnca" -t ",," -2 -6 -v 24
> 0
> 1
> -1
> Is this a critical extension [y/N]? n
> Is this a CA certificate [y/N]? n
> Enter the path length constraint, enter to skip [<0 for unlimited path]: >
> Is this a critical extension [y/N]? n

Export the peer certificate to a PKCS#12 file (this file must be kept secret):

vpnca $ pk12util -d sql:/home/vpnca/nssdb -o peer-<insert_name>.p12 -n "peer-<insert_name>"

On the system that will run the VPN client, create a NSS database and import the CA certificate and the peer cerificate:

vpnclient $ certutil -d sql:/home/vpnclient/nssdb -N
vpnclient $ certutil -d sql:/home/vpnclient/nssdb -A -t "CT,," -n "vpnca" -i /path/to/ca.pem
vpnclient $ pk12util -d sql:/home/vpnclient/nssdb -i /path/to/peer-<insert_name>.p12

You need to create and configure TAP devices on all computers that will participate in the virtual network (i.e. run the client program). See badvpn-client(8), section `TAP DEVICE CONFIGURATION` for details.

Starting the server:

badvpn-server --listen-addr 0.0.0.0:7000

Starting the peers:

badvpn-client

Starting the server (other options as above):

badvpn-server ...

Starting the peers (other options as above):

badvpn-client ...

For each peer in the existing local network, configure the NAT router to forward its range of ports to it (assuming their port ranges do not overlap). The clients also need to know the external IP address of the NAT router. If you don't have a static one, you'll need to discover it before starting the clients. Also forward the server port to the server.

Starting the peers in the local network (other options as above):

badvpn-client

The --ext-addr option applies to the previously specified --bind-addr option, and must come after the first --ext-addr option which specifies a local address.

Now perform a similar setup in some other local network behind a NAT. However:
- Don't set up a new server, instead make the peers connect to the existing server in the first local network.
- You can't use {server_reported} for the local address --ext-addr options, because the server would report the NAT router's external address rather than the peer's internal address. Instead each peer has to know its internal IP address.
- Use a different scope name for it, e.g. "local2" instead of "local1".

If setup correctly, all peers will be able to communicate: those in the same local network will communicate directly through local addresses, and those in different local networks will communicate through the Internet.