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vpncloud(1) -- Peer-to-peer VPN

SYNOPSIS

vpncloud [options] [-t <type>] [-d <name>] [-l <addr>] [-c <addr>...]

OPTIONS

  • -t <type>, --type <type>:

    Set the type of network. There are two options: tap devices process Ethernet frames tun devices process IP packets. [default: tap]

  • -d <name>, --device <name>:

    Name of the virtual device. Any %d will be filled with a free number. [default: vpncloud%d]

  • -m <mode>, --mode <mode>:

    The mode of the VPN. The VPN can like a router, a switch or a hub. A hub will send all data always to all peers. A switch will learn addresses from incoming data and only send data to all peers when the address is unknown. A router will send data according to known subnets of the peers and ignore them otherwise. The normal mode is switch for tap devices and router for tun devices. [default: normal]

  • -l <addr>, --listen <addr>:

    The address to listen for data. [default: 0.0.0.0:3210]

  • -c <addr>, --connect <addr>:

    Address of a peer to connect to. The address should be in the form addr:port. If the node is not started, the connection will be retried periodically. This parameter can be repeated to connect to multiple peers.

  • --subnet <subnet>:

    The local subnets to use. This parameter should be in the form address/prefixlen where address is an IPv4 address, an IPv6 address, or a MAC address. The prefix length is the number of significant front bits that distinguish the subnet from other subnets. Example: 10.1.1.0/24.

  • --shared-key <key>:

    An optional shared key to encrypt the VPN data. If this option is not set, the traffic will be sent unencrypted.

  • --network-id <id>:

    An optional token that identifies the network and helps to distinguish it from other networks.

  • --peer-timeout <secs>:

    Peer timeout in seconds. The peers will exchange information periodically and drop peers that are silent for this period of time. [default: 1800]

  • --dst-timeout <secs>:

    Switch table entry timeout in seconds. This parameter is only used in switch mode. Addresses that have not been seen for the given period of time will be forgot. [default: 300]

  • --ifup <command>:

    A command to setup the network interface. The command will be run (as parameter to sh -c) when the device has been created to configure it. The name of the allocated device will be available via the environment variable IFNAME.

  • --ifdown <command>:

    A command to bring down the network interface. The command will be run (as parameter to sh -c) to remove any configuration from the device. The name of the allocated device will be available via the environment variable IFNAME.

  • -v, --verbose:

    Print debug information, including information for data being received and sent.

  • -q, --quiet:

    Only print errors and warnings.

  • -h, --help:

    Display the help.

DESCRIPTION

VpnCloud is a simple VPN over UDP. It creates a virtual network interface on the host and forwards all received data via UDP to the destination. It can work in 3 different modes:

  • Switch mode: In this mode, the VPN will dynamically learn addresses as they are used as source addresses and use them to forward data to its destination. Addresses that have not been seen for some time (option dst_timeout) will be forgot. Data for unknown addresses will be broadcast to all peers. This mode is the default mode for TAP devices that process Ethernet frames but it can also be used with TUN devices and IP packets.

  • Hub mode: In this mode, all data will always be broadcast to all peers. This mode uses lots of bandwidth and should only be used in special cases.

  • Router mode: In this mode, data will be forwarded based on preconfigured address ranges ("subnets"). Data for unknown nodes will be silently ignored. This mode is the default mode for TUN devices that work with IP packets but it can also be used with TAP devices and Ethernet frames.

All connected vpncloud nodes will form a peer-to-peer network and cross-connect automatically until the network is fully connected. The nodes will periodically exchange information with the other nodes to signal that they are still active and to allow the automatic cross-connect behavior. There are some important things to note:

  • To avoid that different networks that reuse each others addresses merge due to the cross-connect behavior, the network_id option can be used and set to any unique string to identify the network. The network_id must be the same on all nodes of the same VPN network.

  • The cross-connect behavior can be able to connect nodes that are behind firewalls or NATs as it can function as hole-punching.

  • The management traffic will increase with the peer number quadratically. It should still be reasonably small for high node numbers (below 10 KiB/s for 10.000 nodes). A longer peer_timeout can be used to reduce the traffic further. For high node numbers, router mode should be used as it never broadcasts data.

VpnCloud does not implement any loop-avoidance. Since data received on the UDP socket will only be sent to the local network interface and vice versa, VpnCloud cannot produce loops on its own. On the TAP device, however STP data can be transported to avoid loops caused by other network components.

For TAP devices, IEEE 802.1q frames (VLAN tagged) are detected and forwarded based on separate MAC tables. Any nested tags (Q-in-Q) will be ignored.

EXAMPLES

Switched TAP scenario

In the example scenario, a simple layer 2 network tunnel is established. Most likely those commands need to be run as root using sudo.

First, VpnCloud need to be started on both nodes (the address after -c is the address of the remote node and the the X in the interface address must be unique among all nodes, e.g. 0, 1, 2, ...):

vpncloud -c remote_node:3210 --ifup 'ifconfig $IFNAME 10.0.0.X/24 mtu 1400 up'

Afterwards, the interface can be used to communicate.

Routed TUN example

In this example, 4 nodes should communicate using IP. First, VpnCloud need to be started on both nodes:

vpncloud -t tun -c remote_node:3210 --subnet 10.0.0.X/32 --ifup 'ifconfig $IFNAME 10.0.0.0/24 mtu 1400 up'

Important notes

  • It is important to configure the interface in a way that all addresses on the VPN can be reached directly. E.g. if addresses 10.0.0.1 and 10.0.0.2 are used, the interface needs to be configured as /24. For TUN devices, this means that the prefix length of the subnets must be different than the prefix length that the interface is configured with.

  • VpnCloud can be used to connect two separate networks. TAP networks can be bridged using brctl and TUN networks must be routed. It is very important to be careful when setting up such a scenario in order to avoid network loops, security issues, DHCP issues and many more problems.

  • TAP devices will forward DHCP data. If done intentionally, this can be used to assign unique addresses to all participants. If this happens accidentally, it can conflict with DHCP servers of the local network and can have severe side effects.

  • VpnCloud is not designed for high security use cases. Although the used crypto primitives are expected to be very secure, their application has not been reviewed. The shared key is hashed using ScryptSalsa208Sha256 to derive a key, which is used to encrypt the payload of messages using ChaCha20. The authenticity of messages is verified using HmacSha512256 hashes. This method only protects the contents of the message (payload, peer list, etc.) but not the header of each message. Also, this method does only protect against attacks on single messages but not on attacks that manipulate the message series itself (i.e. suppress messages, reorder them, and duplicate them).

NETWORK PROTOCOL

The protocol of VpnCloud is kept as simple as possible to allow other implementations and to maximize the performance.

Every packet sent over UDP contains the following header (in order):

  • 3 bytes magic constant = [0x76, 0x70, 0x6e] ("vpn")

    This field is used to identify the packet and to sort out packets that do not belong.

  • 1 byte version number = 1 (currently)

    This field specifies the version and helps nodes to parse the rest of the header and the packet.

  • 2 reserved bytes that are currently unused

  • 1 byte for flags

    This byte contains flags that specify the presence of additional headers. The flags are enumerated from bit 1 (least significant bit) to bit 8 (most significant bit). The additional headers must be present in this same order. Currently the following additional headers are supported:

    • Bit 1: Network ID
    • Bit 2: Crypto information

  • 1 byte for the message type

    This byte specifies the type of message that follows after all additional headers. Currently the following message types are supported:

    • Type 0: Data packet
    • Type 1: Peer list
    • Type 2: Initial message
    • Type 3: Closing message

After this 8 byte header, the additional headers as specified in the flags field will follow in the order of their respective flag bits.

  • Network ID:

    The network id is encoded as 8 bytes.

  • Crypto information:

    If this header is present, the contents of the message are encrypted and must have to decrypted before decoding. This option contains 40 bytes. The first 8 bytes are the nonce for this message and the later 32 bytes are the authentication hash of the message.

After the additional headers, message as specified in the message type field will follow:

  • Data packet (message type 0): This packet contains payload. The format of the data depends on the device type. For TUN devices, this data contains an IP packet. For TAP devices it contains an Ethernet frame. The data starts right after all additional headers and ends at the end of the packet. If it is an Ethernet frame, it will start with the destination MAC and end with the payload. It does not contain the preamble, SFD, padding, and CRC fields.

  • Peer list (message type 1): This packet contains the peer list of the sender. The first byte after the switch byte contains the number of IPv4 addresses that follow. After that, the specified number of addresses follow, where each address is encoded in 6 bytes. The first 4 bytes are the IPv4 address and the later 2 bytes are port number (both in network byte order). After those addresses, the next byte contains the number of IPv6 addresses that follow. After that, the specified number of addresses follow, where each address is encoded in 18 bytes. The first 16 bytes are the IPv6 address and the later 2 bytes are port number (both in network byte order).

  • Initial message (message type 2): This packet contains all the local subnets claimed by the nodes. The subnet list is encoded in the following way: The first byte of data contains the number of encoded subnets that follow. After that, the given number of encoded subnets follow. For each subnet, the first byte is the length of bytes in the base address and is followed by the given number of base address bytes and one additional byte that is the prefix length of the subnet. The addresses for the subnet will be encoded like they are encoded in their native protocol (4 bytes for IPv4, 16 bytes for IPv6, and 6 bytes for a MAC address) with the exception of MAC addresses in a VLan which will be encoded in 8 bytes where the first 2 bytes are the VLan number in network byte order and the later 6 bytes are the MAC address.

  • Closing message (message type 3): This packet does not contain any further data.

Nodes are expected to send an initial message whenever they connect to a node they were not connected to before. As a reply to this message, another initial should be sent if the node was not known before. Also a peer list message should be sent as a reply.

When connected, nodes should periodically send their peer list to all of their peers to spread this information and to avoid peer timeouts. To avoid the cubic growth of management traffic, nodes should at a certain network size start sending partial peer lists instead of the full list. A reasonable number would be the square root of the number of peers. The subsets can be selected using round robin (making sure all peers eventually receive all information) or randomly.

Nodes should remove peers from their peer list after a certain period of inactivity or when receiving a closing message. Before shutting down, nodes should send the closing message to all of their peers in order to avoid receiving further data until the timeout is reached.

Nodes should only add nodes to their peer list after receiving an initial message from them instead of adding them right from the peer list of another peer. This is necessary to avoid the case of a large network keeping dead nodes alive.

COPYRIGHT

Copyright (C) 2015 Dennis Schwerdel