Get dynamically mapped host port from within ECS container

ECSDynamicHostPort.go

package main

import (
	"encoding/json"
	"io/ioutil"
	"log"
	"net/http"
	"os"

	"bufio"
	"fmt"
	"github.com/aws/aws-sdk-go/aws"
	"github.com/aws/aws-sdk-go/aws/ec2metadata"
	"github.com/aws/aws-sdk-go/aws/session"
	"github.com/aws/aws-sdk-go/service/ecs"
	"path"
	"strings"
)

type EcsAgentMetadata struct {
	Cluster string `json:"Cluster"`
}

type EcsAgentTaskMetadata struct {
	Tasks []struct {
		Arn        string `json:"Arn"`
		Containers []struct {
			DockerId string `json:"DockerId"`
		} `json:"Containers"`
	} `json:"Tasks"`
}

func check(err error) {
	if err != nil {
		log.Panic(err)
	}
}

// See
// https://github.com/aws/amazon-ecs-agent/issues/258
// https://github.com/aws/amazon-ecs-agent/pull/709
func ecsAgentTaskMetadata() (EcsAgentTaskMetadata, error) {
	response, err := http.Get("http://172.17.0.1:51678/v1/tasks")
	if err != nil {
		return EcsAgentTaskMetadata{}, err
	}
	defer response.Body.Close()
	body, err := ioutil.ReadAll(response.Body)
	if err != nil {
		return EcsAgentTaskMetadata{}, err
	}

	metadata := EcsAgentTaskMetadata{}
	err = json.Unmarshal(body, &metadata)
	if err != nil {
		return EcsAgentTaskMetadata{}, err
	}

	return metadata, nil
}

func ecsAgentMetadata() (EcsAgentMetadata, error) {
	response, err := http.Get("http://172.17.0.1:51678/v1/metadata")
	if err != nil {
		return EcsAgentMetadata{}, err
	}
	defer response.Body.Close()
	body, err := ioutil.ReadAll(response.Body)
	if err != nil {
		return EcsAgentMetadata{}, err
	}

	metadata := EcsAgentMetadata{}
	err = json.Unmarshal(body, &metadata)
	if err != nil {
		return EcsAgentMetadata{}, err
	}

	return metadata, nil
}

func getDockerId() (string, error) {
	file, err := os.Open("/proc/self/cgroup")
	if err != nil {
		fmt.Println("Could not open /proc/self/cgroup, using hostname instead as container id")
		return os.Hostname()
	}
	defer file.Close()

	reader := bufio.NewReader(file)
	for {
		// Example: "2:cpu:/docker/93c562c426414f53582c9830a30bdb54d85642956e18115dd59bc9f435ae5644"
		line, err := reader.ReadString('\n')
		if err != nil {
			break
		}

		components := strings.Split(line, ":")
		if len(components) == 3 {
			return strings.TrimRight(path.Base(components[2]), "\n"), nil
		}
	}

	return "", fmt.Errorf("Failed to find Docker ID in /proc/self/group")
}

func main() {
	metasession := session.Must(session.NewSession())
	ec2metadataSvc := ec2metadata.New(metasession)

	region, err := ec2metadataSvc.Region()
	check(err)

	sess, err := session.NewSession(aws.NewConfig().WithRegion(region))
	check(err)

	// Create the AWS clients
	ecsSvc := ecs.New(sess)

	// find our task in the ECS agent metadata
	dockerId, err := getDockerId()
	check(err)

	agentMetadata, err := ecsAgentMetadata()
	check(err)

	if agentMetadata.Cluster == "" {
		log.Panic(fmt.Errorf("Could not find ECS cluster for docker container '%s'", dockerId))
	}

	agentTaskMetadata, err := ecsAgentTaskMetadata()
	check(err)

	taskArn := ""
	for _, task := range agentTaskMetadata.Tasks {
		for _, container := range task.Containers {
			if strings.HasPrefix(container.DockerId, dockerId) {
				taskArn = task.Arn
				break
			}
		}
	}
	if taskArn == "" {
		log.Panic(fmt.Errorf("Could not find ECS task for docker container '%s' on cluster '%s'", dockerId, agentMetadata.Cluster))
	}

	tasks, err := ecsSvc.DescribeTasks(&ecs.DescribeTasksInput{
		Cluster: aws.String(agentMetadata.Cluster),
		Tasks:   []*string{aws.String(taskArn)},
	})
	check(err)

	for _, task := range tasks.Tasks {
		for _, container := range task.Containers {
			for _, binding := range container.NetworkBindings {
				if binding.ContainerPort != nil && binding.HostPort != nil {
					log.Println("Found host port: ", *binding.HostPort)
					return
				}
			}
		}
	}
}