---
title: Connecting to Tasks
---

When Calico is used to network Mesos tasks,
each task is assigned a uniquely addressable IP address.

Since these IP addresses are dynamically assigned and may change when a
task is re-deployed, it is recommended to use one of the available
service discovery solutions available for Mesos.

### Mesos-DNS

> **Important**: Modifying Mesos-DNS **in DC/OS** may break components 
> which rely on Mesos-DNS resolving to Agent IPs. It is instead 
> recommended to use Navstar DNS entries.
> Skip ahead for information on how to use this alternative DNS service.
{: .alert .alert-danger}

Mesos-DNS is an open source DNS service for Mesos which gathers task information
from the Mesos Master and uses it to serve DNS entries which resolve to a task's IP.
Mesos-DNS provides the following DNS entry for tasks launched by Marathon:

```
<task-name>.marathon.mesos
```

By default, Mesos-DNS is configured to return the IP address of the _Agent_ that is running the task.
When using Calico, it needs to be reconfigured to return the tasks' Container IP instead.
To do this, modify its configuration to preferentially resolve to a task's
`netinfo` for the `IPSources` field, as follows:

```
{
  "IPSources": ["netinfo", "host"]
}
```

For example, a Calico-networked task named `nginx` scaled to 3 instances will
respond to DNS lookups with the Calico IPs:

```
[vagrant@calico-01 ~]$ nslookup nginx.marathon.mesos
Server:         172.24.197.101
Address:        172.24.197.101#53

Name:   nginx.marathon.mesos
Address: 192.168.84.192
Name:   nginx.marathon.mesos
Address: 192.168.50.65
Name:   nginx.marathon.mesos
Address: 192.168.50.66
```

### Navstar / Spartan DNS

DC/OS 1.8+ includes Navstar & Spartan DNS services. These DNS services
provide _additional_ DNS entries which _always_ resolve to a task's Container IP.

The following address will resolve to a task's Calico IP:

```
<task-name>.marathon.containerip.dcos.thisdcos.directory
```

## Load Balancing / External Service Discovery

For accessing Calico tasks from _outside_ the cluster, Marathon-LB can be used to
forward requests on to Calico tasks. The following diagram demonstrates how this works:

![Marathon-LB]({{site.baseurl}}/images/marathon-lb.png)

As per the [official marathon-lb docs](https://dcos.io/docs/1.7/usage/service-discovery/marathon-lb/usage/#virtual-hosts), select a `HAPROXY_GROUP` and
optionally choose a `HAPROXY_VHOST` for your application in its Marathon application definition.

Additionally, for Calico, you'll need to explicitly tell it which port to load balance to.
Set `containerPort` in your application's `portMappings`:

```json
{
  "id": "nginx",
  "instances": 3,
  "labels": {
    "HAPROXY_GROUP":"external",
    "HAPROXY_0_VHOST":"mytask.acme.org"
  },
  "container": {
    "type": "DOCKER",
    "docker": {
      "image": "nginx",
      "network": "USER",
      "portMappings": [{"containerPort": 80}]
    }
  },
  "ipAddress": {
      "networkName": "calico"
  }
}
```

Requests made to the VHOST URL (from a host which resolves the address to the
public slave) will be fulfilled by the Calico task backing it. We can simulate this resolution with a simple curl:

```
curl -H "Host: mytask.acme.org" 172.24.197.101
```

> **Note**: Replace 172.24.197.101 with the IP of the public slave where
> Marathon-LB is running.
{: .alert .alert-info}

A look into the `haproxy.cfg` generated by Marathon-LB shows that
requests to this VIP will be round robin'd to the Calico IPs assigned
to our nginx instances:

```
docker exec marathon-lb cat haproxy.cfg

...

backend nginx_10000
  balance roundrobin
  mode http
  option forwardfor
  http-request set-header X-Forwarded-Port %[dst_port]
  http-request add-header X-Forwarded-Proto https if { ssl_fc }
  server 172_24_197_101_192_168_84_196_80 192.168.84.196:80
  server 172_24_197_101_192_168_84_197_80 192.168.84.197:80
  server 172_24_197_102_192_168_50_70_80 192.168.50.70:80
```

## Next

- [Configuring Policy: Docker Containerizer](policy/docker-containerizer)
- [Configuring Policy: Universal Containerizer](policy/universal-containerizer)
