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wesher

wesher creates and manages an encrypted mesh overlay network across a group of nodes, using wireguard.

Its main use-case is adding low-maintenance security to public-cloud networks or connecting different cloud providers.

⚠ WARNING: since mesh membership is controlled by a mesh-wide pre-shared key, this effectively downgrades some of the security benefits from wireguard. See security considerations below for more details.

Quickstart

Before starting:
1. make sure the wireguard kernel module is installed on all nodes.
2. The following ports must be accessible between all nodes (see configuration options to change these):
  - 51820 UDP
  - 7946 UDP and TCP

Download the latest release for your architecture (assuming go is installed):

$ wget -O wesher https://github.com/costela/wesher/releases/latest/download/wesher-$(go env GOARCH)
$ chmod a+x wesher

On the first node:
```
# ./wesher
```
Running the command above on a terminal will currently output a generated cluster key as follows:
```
new cluster key generated: XXXXX
```
Note: the created key will only be shown if running on a terminal, to avoid keys leaking via logs.
Lastly, on any further node:
```
# wesher --cluster-key XXXXX --join x.x.x.x
```
Where XXXXX is the base64 encoded 256 bit key printed by the step above, and x.x.x.x is the hostname or IP of any of the nodes already joined to the mesh cluster.

Permissions

Note that wireguard - and therefore wesher - need root access to work properly.

It is also possible to give the wesher binary enough capabilities to manage the wireguard interface via:

# setcap cap_net_admin=eip wesher

This will enable running as an unprivileged user, but some functionality (like automatic adding peer entries to /etc/hosts; see configuration options below) will not work.

Installing from source

Alternatively, the latest wesher commit can be easily installed from sources:

Preferred:

$ git clone https://github.com/costela/wesher.git
$ make

Or:

$ GO111MODULE=on go get github.com/costela/wesher

Note: the latter will not provide a meaningful output for --version.

Features

The wesher tool builds a cluster and manages the configuration of wireguard on each node to create peer-to-peer connections between all nodes, thus forming a full mesh VPN. This approach may not scale for hundreds of nodes (benchmarks accepted 😉), but is sufficiently performant to join several nodes across multiple cloud providers, or simply to secure inter-node comunication in a single public-cloud.

Automatic Key management

The wireguard private keys are created on startup for each node and the respective public keys are then broadcast across the cluster.

The control-plane cluster communication is secured with a pre-shared AES-256 key. This key can be be automatically created during startup of the first node in a cluster, or it can be provided (see configuration). The cluster key must then be sent to other nodes via a out-of-band secure channel (e.g. ssh, cloud-init, etc). Once set, the cluster key is saved locally and reused on the next startup.

Automatic IP address management

The overlay IP address of each node is automatically selected out of a private network (10.0.0.0/8 by default; MUST be different from the underlying network used for cluster communication) and is consistently hashed based on the peer's hostname.

The use of consistent hashing means a given node will always receive the same overlay IP address (see limitations of this approach below).

Note: the node's hostname is also used by the underlying cluster management (using memberlist) to identify nodes and must therefore be unique in the cluster.

Automatic /etc/hosts management

To ease intra-node communication, wesher also adds entries to /etc/hosts for each peer in the mesh. This enables using the nodes' hostnames to ensure communication over the secured overlay network (assuming files is the first entry for hosts in /etc/nsswitch.conf).

See configuration below for how to disable this behavior.

Seamless restarts

If a node in the cluster is restarted, it will attempt to re-join the last-known nodes using the same cluster key. This means a restart requires no manual intervention.

Configuration options

All options can be passed either as command-line flags or environment variables:

Option	Env	Description	Default
`--cluster-key KEY`	WESHER_CLUSTER_KEY	shared key for cluster membership; must be 32 bytes base64 encoded; will be generated if not provided	autogenerated/loaded
`--join HOST,...`	WESHER_JOIN	comma separated list of hostnames or IP addresses to existing cluster members; if not provided, will attempt resuming any known state or otherwise wait for further members
`--init`	WESHER_INIT	whether to explicitly (re)initialize the cluster; any known state from previous runs will be forgotten	`false`
`--bind-addr ADDR`	WESHER_BIND_ADDR	IP address to bind to for cluster membership	autodetected
`--cluster-port PORT`	WESHER_CLUSTER_PORT	port used for membership gossip traffic (both TCP and UDP); must be the same across cluster	`7946`
`--wireguard-port PORT`	WESHER_WIREGUARD_PORT	port used for wireguard traffic (UDP); must be the same across cluster	`51820`
`--overlay-net ADDR/MASK`	WESHER_OVERLAY_NET	the network in which to allocate addresses for the overlay mesh network (CIDR format); smaller networks increase the chance of IP collision	`10.0.0.0/8`
`--interface DEV`	WESHER_INTERFACE	name of the wireguard interface to create and manage	`wgoverlay`
`--no-etc-hosts`	WESHER_NO_ETC_HOSTS	whether to skip writing hosts entries for each node in mesh	`false`
`--log-level LEVEL`	WESHER_LOG_LEVEL	set the verbosity (one of debug/info/warn/error)	`warn`

Security considerations

The decision of whom to allow in the mesh is made by memberlist and is secured by a cluster-wide pre-shared key. Compromise of this key will allow an attacker to:

access services exposed on the overlay network
impersonate and/or disrupt traffic to/from other nodes It will not, however, allow the attacker access to decrypt the traffic between other nodes.

This pre-shared key is currently static, set up during cluster bootstrapping, but will - in a future version - be rotated for improved security.

Current known limitations

Overlay IP collisions

Since the assignment of IPs on the overlay network is currently decided by the individual node and implemented as a naive hashing of the hostname, there can be no guarantee two hosts will not generate the same overlay IPs. This limitation may be worked around in a future version.

Split-brain

Once a cluster is joined, there is currently no way to distinguish a failed node from an intentionally removed one. This is partially by design: growing and shrinking your cluster dynamically (e.g. via autoscaling) should be as easy as possible.

However, this does mean longer connection loss between any two parts of the cluster (e.g. across a WAN link between different cloud providers) can lead to a split-brain scenario where each side thinks the other side is simply "gone".

There is currently no clean solution for this problem, but one could work around it by designating edge nodes which periodically restart wesher with the --join option pointing to the other side. Future versions might include the notion of a "static" node to more cleanly avoid this.

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