# Networking

[TOC]

gVisor implements its own network stack called netstack. All aspects of the
network stack are handled inside the Sentry — including TCP connection state,
control messages, and packet assembly — keeping it isolated from the host
network stack. Data link layer packets are written directly to the virtual
device inside the network namespace setup by Docker or Kubernetes.

Configuring the network stack may provide performance benefits, but isn't the
only step to optimizing gVisor performance. See the [Production guide] for more.

The IP address and routes configured for the device are transferred inside the
sandbox. The loopback device runs exclusively inside the sandbox and does not
use the host. You can inspect them by running:

```bash
docker run --rm --runtime=runsc alpine ip addr
```

## Network passthrough

For high-performance networking applications, you may choose to disable the user
space network stack and instead use the host network stack, including the
loopback. Note that this mode decreases the isolation to the host.

Add the following `runtimeArgs` to your Docker configuration
(`/etc/docker/daemon.json`) and restart the Docker daemon:

```json
{
    "runtimes": {
        "runsc": {
            "path": "/usr/local/bin/runsc",
            "runtimeArgs": [
                "--network=host"
            ]
       }
    }
}
```

## Disabling external networking

To completely isolate the host and network from the sandbox, external networking
can be disabled. The sandbox will still contain a loopback provided by netstack.

Add the following `runtimeArgs` to your Docker configuration
(`/etc/docker/daemon.json`) and restart the Docker daemon:

```json
{
    "runtimes": {
        "runsc": {
            "path": "/usr/local/bin/runsc",
            "runtimeArgs": [
                "--network=none"
            ]
       }
    }
}
```

### Disable GSO {#gso}

If your Linux is older than 4.14.77, you can disable Generic Segmentation
Offload (GSO) to run with a kernel that is newer than 3.17. Add the
`--gso=false` flag to your Docker runtime configuration
(`/etc/docker/daemon.json`) and restart the Docker daemon:

> Note: Network performance, especially for large payloads, will be greatly
> reduced.

```json
{
    "runtimes": {
        "runsc": {
            "path": "/usr/local/bin/runsc",
            "runtimeArgs": [
                "--gso=false"
            ]
       }
    }
}
```
