add documentation for netavark plugins

Signed-off-by: Paul Holzinger <pholzing@redhat.com>

README.md

@@ -39,3 +39,7 @@ For general questions and discussion, please use Podman's
 For discussions around issues/bugs and features, you can use the GitHub
 [issues](https://github.com/containers/netavark/issues)
 and [PRs](https://github.com/containers/netavark/pulls) tracking system.
+
+## Plugins
+
+Netavark also supports executing external plugins, see [./plugin-API.md](./plugin-API.md).

plugin-API.md

@@ -0,0 +1,209 @@
+# Description of the netavark plugin API
+
+A netavark plugin is a external binary which must implement a specific set of subcommands that will be called by podman and netavark.
+ - `create`: creates a network config
+ - `setup`: setup the network configuration
+ - `teardown`: tear down the network configuration
+ - `info`: show info about this plugin
+
+## Create subcommand
+
+The create subcommand creates a new network config for podman.
+The subcommand will receive the JSON network config via STDIN. Podman will populate
+the network name and ID before calling the plugin. The name and ID cannot be changed
+by the plugin. The driver name must also not be changed. All other config
+fields can be changed in the plugin.
+
+Other fields such as subnet and options will also be populated by podman when
+these options are set on the podman network create command, i.e. `--subnet`
+and `--option`. The plugin validates the given values and errors out for
+invalid values.
+
+On success the plugin should print the generated config as JSON to STDOUT.
+
+Example JSON input and output format:
+```
+{
+    "name": "example1",
+    "id": "2f259bab93aaaaa2542ba43ef33eb990d0999ee1b9924b557b7be53c0b7a1bb9",
+    "driver": "mydriver",
+    "network_interface": "enp1",
+    "subnets": [
+        {
+            "subnet": "10.0.0.0/16",
+            "gateway": "10.0.0.1"
+        }
+    ],
+    "ipv6_enabled": false,
+    "internal": false,
+    "dns_enabled": false,
+    "ipam_options": {
+        "driver": "host-local"
+    },
+    "options": {
+        "custom": "opt"
+    }
+}
+```
+
+## Setup subcommand
+
+The setup subcommand sets-up the network configuration. This command is called when
+a container is started or network connect is used, assuming the container uses
+a network which was created by the plugin, see the create command above.
+
+On STDIN it receives a JSON config which contains the network config and container options.
+ON STDOUT the plugin must return a JSON status block. This contains information about the
+created interface, the assigned ip and mac address. This information will be visible in the
+podman inspect output.
+
+Also this command accepts one argument which is the path to the container network namespace.
+
+Example JSON input:
+```
+{
+    "container_id": "752947ff91f961eb3cb47ffe9315016979f3ffbec09e4d96a4fae3fb03391697",
+    "container_name": "testctr",
+    "port_mappings": [
+        {
+            "container_port": 80,
+            "host_ip": "127.0.0.1",
+            "host_port": 8080,
+            "protocol": "tcp",
+            "range": 1
+        }
+    ],
+    "network": {
+        "dns_enabled": false,
+        "driver": "bridge",
+        "id": "2f259bab93aaaaa2542ba43ef33eb990d0999ee1b9924b557b7be53c0b7a1bb9",
+        "internal": false,
+        "ipv6_enabled": false,
+        "name": "podman",
+        "network_interface": "podman0",
+        "options": null,
+        "ipam_options": {
+            "driver": "host-local"
+        },
+        "subnets": [
+            {
+                "gateway": "10.88.0.1",
+                "lease_range": null,
+                "subnet": "10.88.0.0/16"
+            }
+        ],
+        "network_dns_servers": null
+    },
+    "network_options": {
+        "aliases": [
+            "752947ff91f9"
+        ],
+        "interface_name": "eth0",
+        "static_ips": [
+            "10.88.0.50"
+        ],
+        "static_mac": "aa:bb:cc:dd:aa:00"
+    }
+}
+```
+
+Example JSON output:
+```
+{
+    "dns_search_domains": [],
+    "dns_server_ips": [],
+    "interfaces": {
+        "eth0": {
+            "mac_address": "aa:bb:cc:dd:aa:00",
+            "subnets": [
+                {
+                    "gateway": "10.88.0.1",
+                    "ipnet": "10.88.0.50/16"
+                }
+            ]
+        }
+    }
+}
+```
+
+
+## Teardown subcommand
+
+The teardown command is basically the reverse of the setup command. It should
+revert what the plugin did in setup.
+It accepts the same input as setup but it should not return anything on success.
+
+## Info subcommand
+
+Used to output information about this plugin. It must contain the version of your plugin and the API version.
+Extra fields can be added. The API version must be set to `1.0.0` at the moment, it is not used the moment
+but could be used in the future to allow for backwards compatibility in case the plugin types change.
+
+```
+{
+    "version": "0.1.0",
+    "api_version": "1.0.0"
+}
+```
+
+## Error handling
+
+If the plugin encounters an error it should return a special json message with the following format:
+```
+{"error": "message"}
+```
+where message should be replace with your actual error message. This message will be returned by
+netavark and will be visible to podman users.
+
+
+## Rust types
+
+Rust types can be found in [./src/network/types.rs](./src/network/types.rs), see the documentation
+[here](https://docs.rs/netavark/latest/netavark/network/types).
+Fields that are wrapped by an `Option<T>` can be omitted from the json, otherwise they must be set
+to allow proper deserialization.
+
+## Rust plugin interface
+
+There is a simple ready to use interface for writing your plugin in rust, see [./src/plugin.rs](./src/plugin.rs)
+```rust
+use netavark::{
+    network::types,
+    plugin::{Info, Plugin, PluginExec, API_VERSION},
+};
+
+fn main() {
+    // change the version to the version of your plugin
+    let info = Info::new("0.1.0".to_owned(), API_VERSION.to_owned(), None);
+
+    PluginExec::new(Exec {}, info).exec();
+}
+
+struct Exec {}
+
+impl Plugin for Exec {
+    fn create(
+        &self,
+        network: types::Network,
+    ) -> Result<types::Network, Box<dyn std::error::Error>> {
+        // your logic here
+    }
+
+    fn setup(
+        &self,
+        netns: String,
+        opts: types::NetworkPluginExec,
+    ) -> Result<types::StatusBlock, Box<dyn std::error::Error>> {
+        // your logic here
+    }
+
+    fn teardown(
+        &self,
+        netns: String,
+        opts: types::NetworkPluginExec,
+    ) -> Result<(), Box<dyn std::error::Error>> {
+        // your logic here
+    }
+}
+```
+Also see the examples in [./examples](./examples/).