AWS System Manager start session: An error occurred (TargetNotConnected) when calling the StartSession operation: <instance_id> is not connected

Question

Problem:

When I try to locally connect to a running EC2 instance using the AWS System Session Manager CLI command: aws ssm start-session --target i-123456

I get the error:

An error occurred (TargetNotConnected) when calling the StartSession operation: i-123456 is not connected.

Background:

• Linux 2 instance hosted on a private subnet within a custom VPC
• VPC endpoints used to connect System Manager to managed instances without the need for a NAT GW or IGW.
• Endpoint Service Names:

com.amazonaws.us-west-2.s3
com.amazonaws.us-west-2.ec2
com.amazonaws.us-west-2.ec2messages
com.amazonaws.us-west-2.ssm
com.amazonaws.us-west-2.ssmmessages

• AWS CLI == 2.0.40
• Python == 3.7.4
• Custom Terraform module to launch airflow instance within one of the private subnets (see module "airflow_aws_resources" below)
• The only .tf file that would be relevant to this problem would be airflow.tf within the module "airflow_aws_resources". This file contains the security group and instance profile configuration for the EC2 instance that is being connected via SSM.

Reproduce with Terraform:

module "airflow_aws_resources" {
  source                      = "github.com/marshall7m/tf_modules/airflow-aws-resources"
  resource_prefix             = "test"
  vpc_id                      = module.vpc.vpc_id
  env                         = "testing"
  private_bucket              = "test-bucket"
  private_subnets_ids         = module.vpc.private_subnets
  private_subnets_cidr_blocks = module.vpc.private_subnets_cidr_blocks

  create_airflow_instance     = true
  create_airflow_instance_sg  = true
  create_airflow_db           = false
  create_airflow_db_sg        = false
  airflow_instance_ssm_access = true
  airflow_instance_ssm_region = "us-west-2"

  airflow_instance_ami  = "ami-0841edc20334f9287"
  airflow_instance_type = "t2.micro"

}

resource "aws_security_group" "vpc_endpoints" {
  name        = "test-vpc-endpoint-sg"
  description = "Default security group for vpc endpoints"
  vpc_id = module.vpc.vpc_id
  
  ingress {
    from_port   = 80
    to_port     = 80
    protocol    = "tcp"
    cidr_blocks = ["10.0.0.32/28", "10.0.0.64/28"]
  }

  ingress {
    from_port   = 443
    to_port     = 443
    protocol    = "tcp"
    #private subnet cidr blocks
    cidr_blocks = ["10.0.0.32/28", "10.0.0.64/28"]
  }

  egress {
    from_port   = 443
    to_port     = 443
    protocol    = "tcp"
    cidr_blocks = ["10.0.0.32/28", "10.0.0.64/28"]
  }
  egress {
    from_port   = 80
    to_port     = 80
    protocol    = "tcp"
    cidr_blocks = ["10.0.0.32/28", "10.0.0.64/28"]
  }
}

module "vpc" {
  source = "terraform-aws-modules/vpc/aws"
  version = "2.44.0"
  name = "test-vpc" 
  cidr = "10.0.0.0/24"

  azs = ["us-west-2a", "us-west-2b"]
  
  private_subnets = ["10.0.0.32/28", "10.0.0.64/28"]
  private_dedicated_network_acl = true
  private_subnet_suffix = "private"

  public_subnets = ["10.0.0.96/28", "10.0.0.128/28"]
  public_dedicated_network_acl = true
  public_subnet_suffix = "public"

  enable_s3_endpoint = true

  enable_ec2messages_endpoint = true
  ec2messages_endpoint_security_group_ids = [aws_security_group.vpc_endpoints.id]
  enable_ec2_endpoint = true
  ec2_endpoint_security_group_ids = [aws_security_group.vpc_endpoints.id]

  enable_ssm_endpoint = true
  ssm_endpoint_security_group_ids = [aws_security_group.vpc_endpoints.id]
  enable_ssmmessages_endpoint = true
  ssmmessages_endpoint_security_group_ids = [aws_security_group.vpc_endpoints.id]

  enable_nat_gateway = false
  single_nat_gateway = false
  enable_vpn_gateway = false

  create_database_subnet_route_table = false
  create_database_internet_gateway_route = false
  create_database_subnet_group = false
   
  manage_default_network_acl = false 
  enable_dns_hostnames = true
  enable_dns_support = true
  
  private_inbound_acl_rules = [
    {
      "description": "Allows inbound https traffic for aws s3 package requests"
      "cidr_block": "0.0.0.0/0",
      "from_port": 443,
      "to_port": 443,
      "protocol": "tcp",
      "rule_action": "allow",
      "rule_number": 101
    },
    { 
      "description": "Allows inbound http traffic for aws s3 package requests"
      "cidr_block": "0.0.0.0/0",
      "from_port": 80,
      "to_port": 80,
      "protocol": "tcp",
      "rule_action": "allow",
      "rule_number": 102
    }
  ]
  private_outbound_acl_rules = [
    {
      "description": "Allows outbound https traffic for aws s3 package requests"
      "cidr_block": "0.0.0.0/0",
      "from_port": 443,
      "to_port": 443,
      "protocol": "tcp",
      "rule_action": "allow",
      "rule_number": 101
    },
    { 
      "description": "Allows outbound http traffic for aws s3 package requests"
      "cidr_block": "0.0.0.0/0",
      "from_port": 80,
      "to_port": 80,
      "protocol": "tcp",
      "rule_action": "allow",
      "rule_number": 102
    }
  ]
  
  vpc_endpoint_tags = {
    type = "vpc-endpoint"
  }
}

Attempts:

#1

I tried the trouble shooting tips within the EC2 Console SSM (AWS Ec2 console >> instance-id >> Connect >> Session Manager):

1. SSM agent is already pre-installed on AWS Linux instance types. Although I doubled checked by accessing the instance via SSH and running sudo status amazon-ssm-agent which returned: amazon-ssm-agent start/running, process 1234

2. The EC2 instance profile displayed above includes the required AmazonSSMManagedInstanceCore policy

3. I completed the Session Manager Prerequisite.

#2

Attaching AmazonSSMFullAccess to the user using the command: aws ssm start-session --target i-123456

Same error while connecting the instance via SSM:

An error occurred (TargetNotConnected) when calling the StartSession operation: i-123456 is not connected.

#3

Adding HTTPS inbound/outbound traffic from the VPC endpoint's asscoiated private subnet to the EC2 instance security group (see airflow.tf)

Same error:

An error occurred (TargetNotConnected) when calling the StartSession operation: i-123456 is not connected.

#4

Within the System Manager console I used the Quick Setup option and configured the Quick Setup with the Instance profile specified in airflow.tf and the System Manager role with the default role. The ec2 instance successfully registered "Managed instances" within the quick setup page.

Same error:

An error occurred (TargetNotConnected) when calling the StartSession operation: i-123456 is not connected.

#5

Given this is a test VPC and EC2 instance, I tried allowing all types of traffic from all IPv4 sources (0.0.0.0/0) for the following resources:

• Private subnets NACL
• EC2 instance security group
• The security group associated with the following interface/gateway endpoints:

com.amazonaws.us-west-2.s3
com.amazonaws.us-west-2.ec2
com.amazonaws.us-west-2.ec2messages
com.amazonaws.us-west-2.ssm
com.amazonaws.us-west-2.ssmmessages

Same error while connecting the instance via SSM:

An error occurred (TargetNotConnected) when calling the StartSession operation: i-123456 is not connected.

Answer 1

I would refer here to make sure you have everything set up properly. I would first add the profile argument. If that still doesn't work, I ran into a similar issue when my profiles default region was not the same region I was looking to begin an active session. Thus, I needed to use the region argument as well. Sample .ssh/config below:

host ssh i-abc123
ProxyCommand sh -c "aws --region desired_region --profile my_profile ssm start-session --target %h --document-name AWS-StartSSHSession --parameters 'portNumber=%p'"

I would also encourage using AWS CLI v2. Once you configure your .ssh/config to look like that above, simply execute the following in a CLI:

ssh i-abc123