weave/site/features.md

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Weave features	default

Weave features

Weave has a few more features beyond those illustrated by the basic example:

• Virtual ethernet switch
• Application isolation
• Dynamic network attachment
• Security
• Host network integration
• Service export
• Service import
• Service binding
• Service routing
• Multi-cloud networking
• Multi-hop routing
• Dynamic topologies
• Container mobility
• Fault tolerance
• DNS

Virtual Ethernet Switch

To application containers, the network established by weave looks like a giant Ethernet switch to which all the containers are connected.

Containers can easily access services from each other; e.g. in the container on $HOST1 we can start a netcat "service" with

root@28841bd02eff:/# nc -lk -p 4422

and then connect to it from the container on $HOST2 with

root@f76829496120:/# echo 'Hello, world.' | nc 10.2.1.1 4422

Note that any protocol is supported. Doesn't even have to be over TCP/IP, e.g. a netcat UDP service would be run with

root@28841bd02eff:/# nc -lu -p 5533

root@f76829496120:/# echo 'Hello, world.' | nc -u 10.2.1.1 5533

We can deploy the entire arsenal of standard network tools and applications, developed over decades, to configure, secure, monitor, and troubleshoot our container network. To put it another way, we can now re-use the same tools and techniques when deploying applications as containers as we would have done when deploying them 'on metal' in our data centre.

Application isolation

A single weave network can host multiple, isolated applications, with each application's containers being able to communicate with each other but not containers of other applications.

To accomplish that, we assign each application a different subnet. So, in the above example, if we wanted to add another application similar to, but isolated from, our first, we'd launch the containers with...

host1# D=$(weave run 10.2.2.1/24 -t -i ubuntu)
host2# D=$(weave run 10.2.2.2/24 -t -i ubuntu)

A quick 'ping' test in the containers confirms that they can talk to each other but not the containers of our first application...

host1# docker attach $D

root@da50502598d5:/# ping -c 1 -q 10.2.2.2
PING 10.2.2.2 (10.2.2.2): 48 data bytes
--- 10.2.2.2 ping statistics ---
1 packets transmitted, 1 packets received, 0% packet loss
round-trip min/avg/max/stddev = 0.562/0.562/0.562/0.000 ms

root@da50502598d5:/# ping -c 1 -q 10.2.1.1
PING 10.2.1.1 (10.2.1.1) 56(84) bytes of data.
--- 10.2.1.1 ping statistics ---
1 packets transmitted, 0 received, 100% packet loss, time 0ms

root@da50502598d5:/# ping -c 1 -q 10.2.1.2
PING 10.2.1.2 (10.2.1.2) 56(84) bytes of data.
--- 10.2.1.2 ping statistics ---
1 packets transmitted, 0 received, 100% packet loss, time 0ms

This isolation-through-subnets scheme is an example of carrying over a well-known technique from the 'on metal' days to containers.

NB: By default docker permits communication between containers on the same host, via their docker-assigned IP addresses. For complete isolation between application containers, that feature needs to be disabled by setting --icc=false in the docker daemon configuration. Furthermore, containers should be prevented from capturing and injecting raw network packets - this can be accomplished by starting them with the --cap-drop net_raw option.

Dynamic network attachment

In some scenarios containers are started independently, e.g. via some existing tool chain, or require more complex startup sequences than provided by weave run. And sometimes the decision which application network a container should be part of is made post-startup. For these situations, weave allows an existing, running container to be attached to the weave network. To illustrate, we can achieve the same effect as the first example with

host1# C=$(docker run -d -t -i ubuntu)
host1# weave attach 10.2.1.1/24 $C

There is a matching weave detach command:

host1# weave detach 10.2.1.1/24 $C

You can detach a container from one application network and attach it to another:

host1# weave detach 10.2.1.1/24 $C
host1# weave attach 10.2.2.1/24 $C

or attach a container to multiple application networks, effectively sharing it between applications:

host1# weave attach 10.2.1.1/24 $C
host1# weave attach 10.2.2.1/24 $C

Security

In order to connect containers across untrusted networks, weave peers can be told to encrypt traffic by supplying a -password option or WEAVE_PASSWORD environment variable when launching weave, e.g.

host1# weave launch -password wEaVe

or

host1# export WEAVE_PASSWORD=wEaVe
host1# weave launch

NOTE: The command line option takes precedence over the environment variable.

The same password must be specified for all weave peers; it is a component in the creation of ephemeral session keys for connections between peers. See the crypto documentation for more details.

Host network integration

Weave application networks can be integrated with a host's network, establishing connectivity between the host and application containers anywhere.

Let's say that in our example we want $HOST2 to have access to the application containers. On $HOST2 we run

host2# weave expose 10.2.1.102/24

choosing an unused IP address in the application subnet. (There is a corresponding 'hide' command to revert this step.)

Now

host2# ping 10.2.1.2

will work. And, more interestingly,

host2# ping 10.2.1.1

will work too, which is talking to a container that resides on $HOST1.

Service export

Services running in containers on a weave network can be made accessible to the outside world (and, more generally, other networks) from any weave host, irrespective of where the service containers are located.

Say we want to make our example netcat "service", which is running in a container on $HOST1, accessible to the outside world via $HOST2.

First we need to expose the application network to $HOST2, as explained above, i.e.

host2# weave expose 10.2.1.102/24

Then we add a NAT rule to route from the outside world to the destination container service.

host2# iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 2211 \
       -j DNAT --to-destination 10.2.1.1:4422

Here we are assuming that the "outside world" is connecting to $HOST2 via 'eth0'. We want TCP traffic to port 2211 on the external IPs to be routed to our 'nc' service, which is running on port 4422 in the container with IP 10.2.1.1.

With the above in place, we can connect to our 'nc' service from anywhere with

echo 'Hello, world.' | nc $HOST2 2211

(NB: due to the way routing is handled in the Linux kernel, this won't work when run on $HOST2.)

Similar NAT rules to the above can used to expose services not just to the outside world but also other, internal, networks.

Service import

Applications running in containers on a weave network can be given access to services which are only reachable from certain weave hosts, irrespective of where the application containers are located.

Say that, as an extension of our example, we have a netcat service running on $HOST3, port 2211, and that $HOST3 is not part of the weave network and is only reachable from $HOST1, not $HOST2. Nevertheless we want to make the service accessible to an application running in a container on $HOST2.

First we need to expose the application network to the host, as explained above, this time on $HOST1, i.e.

host1# weave expose 10.2.1.101/24

Then we add a NAT rule to route from the above IP to the destination service.

host1# iptables -t nat -A PREROUTING -p tcp -d 10.2.1.101 --dport 3322 \
       -j DNAT --to-destination $HOST3:2211

This allows any application container to reach the service by connecting to 10.2.1.101:3322. So if $HOST3 is indeed running a netcat service on port 2211, e.g.

host3# nc -lk -p 2211

then we can connect to it from our application container on $HOST2 with

root@f76829496120:/# echo 'Hello, world.' | nc 10.2.1.101 3322

The same command will work from any application container.

Service binding

Importing a service provides a degree of indirection that allows late and dynamic binding, similar to what can be achieved with a proxy. In our example, application containers are unaware that the service they are accessing at 10.2.1.101:3322 is in fact residing on $HOST3:2211. We can point application containers at another service location by changing the above NAT rule, without having to alter the applications themselves.

Service routing

The service export and service import features can be combined to establish connectivity between applications and services residing on disjoint networks, even when those networks are separated by firewalls and might have overlapping IP ranges. Each network imports its services into weave, and in turn exports from weave services required by its applications. There are no application containers in this scenario (though of course there could be); weave is acting purely as an address translation and routing facility, using the weave application network as an intermediary.

In our example above, the netcat service on $HOST3 is imported into weave via $HOST1. We can export it on $HOST2 by first exposing the application network with

host2# weave expose 10.2.1.102/24

and then adding a NAT rule which routes traffic from the $HOST2 network (i.e. anything which can connect to $HOST2) to the service endpoint in the weave network

host2# iptables -t nat -A PREROUTING -p tcp -i eth0 --dport 4433 \
       -j DNAT --to-destination 10.2.1.101:3322

Now any host on the same network as $HOST2 can access the service with

echo 'Hello, world.' | nc $HOST2 4433

Furthermore, as explained in service-binding, we can dynamically alter the service locations without having to touch the applications that access them, e.g. we could move the example netcat service to $HOST4:2211 while retaining its 10.2.1.101:3322 endpoint in the weave network.

Multi-cloud networking

Weave can network containers hosted in different cloud providers / data centres. So, for example, one could run an application consisting of containers on GCE, EC2 and in local data centres.

To enable this, the network must be configured to permit TCP and UDP connections to port 6783 of the docker hosts.

Multi-hop routing

A network of containers across more than two hosts can be established even when there is only partial connectivity between the hosts. Weave is able to route traffic between containers as long as there is at least one path of connected hosts between them.

So, for example, if a docker host in a local data centre can connect to hosts in GCE and EC2, but the latter two cannot connect to each other, containers in the latter two can still communicate; weave will route the traffic via the local data centre.

Dynamic topologies

To add a host to an existing weave network, one simply launches weave on the host, supplying the address of at least one existing host. Weave will automatically discover the other hosts in the network and establish connections to them if it can (in order to avoid unnecessary multi-hop routing).

In some situations all existing weave hosts may be unreachable from the new host due to firewalls, etc. However, it is still possible to add the new host, provided inverse connections, i.e. from existing hosts to the new hosts, are possible. To accomplish that, one launches weave on the new host without supplying any additional addresses, and then on one of the existing hosts runs

host# weave connect $NEW_HOST

Other hosts in the weave network will automatically attempt to establish connections to the new host too.

Container mobility

Containers can be moved between hosts without requiring any reconfiguration or, in many cases, restarts of other containers. All that is required is for the migrated container to be started with the same IP address as it was given originally.

Fault tolerance

Weave peers continually exchange topology information, and monitor and (re)establish network connections to other peers. So if hosts or networks fail, weave can "route around" the problem. This includes network partitions; containers on either side of a partition can continue to communicate, with full connectivity being restored when the partition heals.

The weave container is very light-weight - just over 8MB image size and a few 10s of MBs of runtime memory - and disposable. I.e. should weave ever run into difficulty, one can simply stop it (with weave stop) and restart it. Application containers do not have to be restarted in that event, and indeed may not even experience a temporary connectivity failure if the weave container is restarted quickly enough.

DNS

WeaveDNS is a distributed DNS service for weave networks, enabling containers to address each other by name rather than IP address. A preview release of weaveDNS is described on our blog.