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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

  1. Before starting:

    1. make sure the wireguard kernel module is available on all nodes. It is bundled with linux newer than 5.6 and can otherwise be installed following the instructions here.

    2. The following ports must be accessible between all nodes (see configuration options to change these):

      • 51820 UDP
      • 7946 UDP and TCP
  2. Download the latest release for your architecture:

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

    # ./wesher
    

    This will start the wesher daemon in the foreground and - when running on a terminal - will currently output a generated cluster key as follows:

    new cluster key generated: XXXXX
    

    Note: to avoid accidentally leaking it in the logs, the created key will only be displayed if running on a terminal. When started via other means (e.g.: desktop session manager or init system), the key can be retreived with grep ClusterKey /var/lib/wesher/state.json.

  4. 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.

(optional) systemd integration

A minimal systemd unit file is provided under the dist folder and can be copied to /etc/systemd/system:

# wget -O /etc/systemd/system/wesher.service https://raw.githubusercontent.com/costela/wesher/master/dist/wesher.service
# systemctl daemon-reload
# systemctl enable wesher

The provided unit file assumes wesher is installed to /usr/local/sbin.

Note that, as mentioned above, the initial cluster key will not be displayed in the journal. It can either be initialized by running wesher manually once, or by pre-seeding via /etc/default/wesher as the WESHER_CLUSTER_KEY environment var (see configuration options below).

Installing from source

There are a couple of ways of installing wesher from sources:

Preferred:

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

This method can build a bit-by-bit identical binary to the released ones, assuming the same go version is used to build its respective git tag.

Alternatively:

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

Note: this method 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 (cannot be used with --bind-iface) autodetected
--bind-iface IFACE WESHER_BIND_IFACE Interface to bind to for cluster membership (cannot be used with --bind-addr)
--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

Running multiple clusters

To make a node be a member of multiple clusters, simply start multiple wesher instances.
Each instance must have different values for the following settings:

  • --interface
  • either --cluster-port, or --bind-addr or --bind-iface
  • --wireguard-port

The following settings are not required to be unique, but recommended:

  • --overlay-net (to reduce the chance of node address conflicts; see Overlay IP collisions)
  • --cluster-key (as a sensible security measure)

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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