Filter out private subnets for default aws vpc

src/dstack/_internal/core/backends/aws/compute.py

@@ -1,5 +1,5 @@
 from concurrent.futures import ThreadPoolExecutor, as_completed
-from typing import Dict, List, Optional
+from typing import Dict, List, Optional, Tuple
 
 import boto3
 import botocore.client
@@ -113,25 +113,11 @@ def create_instance(
             {"Key": "dstack_user", "Value": instance_config.user},
         ]
         try:
-            subnet_id = None
-            vpc_id = None
-            if self.config.vpc_name is not None:
-                vpc_id = aws_resources.get_vpc_id_by_name(
-                    ec2_client=ec2_client,
-                    vpc_name=self.config.vpc_name,
-                )
-                if vpc_id is None:
-                    raise ComputeError(
-                        f"No VPC named {self.config.vpc_name} in region {instance_offer.region}"
-                    )
-                subnet_id = aws_resources.get_subnet_id_for_vpc(
-                    ec2_client=ec2_client,
-                    vpc_id=vpc_id,
-                )
-                if subnet_id is None:
-                    raise ComputeError(
-                        f"Failed to find public subnet for VPC {self.config.vpc_name} in region {instance_offer.region}"
-                    )
+            vpc_id, subnet_id = _get_vpc_id_subnet_id_or_error(
+                ec2_client=ec2_client,
+                vpc_name=self.config.vpc_name,
+                region=instance_offer.region,
+            )
             disk_size = round(instance_offer.instance.resources.disk.size_mib / 1024)
             response = ec2.create_instances(
                 **aws_resources.create_instances_struct(
@@ -159,7 +145,6 @@ def create_instance(
             instance = response[0]
             instance.wait_until_running()
             instance.reload()  # populate instance.public_ip_address
-
             if instance_offer.instance.resources.spot:  # it will not terminate the instance
                 ec2_client.cancel_spot_instance_requests(
                     SpotInstanceRequestIds=[instance.spot_instance_request_id]
@@ -224,15 +209,15 @@ def create_gateway(
                 user_data=get_gateway_user_data(ssh_key_pub),
                 tags=tags,
                 security_group_id=aws_resources.create_gateway_security_group(
-                    ec2_client, project_id
+                    ec2_client=ec2_client,
+                    project_id=project_id,
                 ),
                 spot=False,
             )
         )
         instance = response[0]
         instance.wait_until_running()
         instance.reload()  # populate instance.public_ip_address
-
         return LaunchedGatewayInfo(
             instance_id=instance.instance_id,
             region=region,
@@ -253,3 +238,30 @@ def _supported_instances(offer: InstanceOffer) -> bool:
         if offer.instance.name.startswith(family):
             return True
     return False
+
+
+def _get_vpc_id_subnet_id_or_error(
+    ec2_client: botocore.client.BaseClient,
+    vpc_name: Optional[str],
+    region: str,
+) -> Tuple[str, str]:
+    if vpc_name is not None:
+        vpc_id = aws_resources.get_vpc_id_by_name(
+            ec2_client=ec2_client,
+            vpc_name=vpc_name,
+        )
+        if vpc_id is None:
+            raise ComputeError(f"No VPC named {vpc_name} in region {region}")
+    else:
+        vpc_id = aws_resources.get_default_vpc_id(ec2_client=ec2_client)
+        if vpc_id is None:
+            raise ComputeError(f"No default VPC in region {region}")
+    subnet_id = aws_resources.get_subnet_id_for_vpc(
+        ec2_client=ec2_client,
+        vpc_id=vpc_id,
+    )
+    if subnet_id is not None:
+        return vpc_id, subnet_id
+    if vpc_name is not None:
+        raise ComputeError(f"Failed to find public subnet for VPC {vpc_name} in region {region}")
+    raise ComputeError(f"Failed to find public subnet for default VPC in region {region}")

src/dstack/_internal/core/backends/aws/resources.py

@@ -308,6 +308,13 @@ def get_vpc_id_by_name(
     return response["Vpcs"][0]["VpcId"]
 
 
+def get_default_vpc_id(ec2_client: botocore.client.BaseClient) -> Optional[str]:
+    response = ec2_client.describe_vpcs(Filters=[{"Name": "isDefault", "Values": ["true"]}])
+    if "Vpcs" in response and len(response["Vpcs"]) > 0:
+        return response["Vpcs"][0]["VpcId"]
+    return None
+
+
 def get_subnet_id_for_vpc(
     ec2_client: botocore.client.BaseClient,
     vpc_id: str,
@@ -318,7 +325,10 @@ def get_subnet_id_for_vpc(
     # Return first public subnet
     for subnet in subnets:
         subnet_id = subnet["SubnetId"]
-        if _is_public_subnet(ec2_client=ec2_client, subnet_id=subnet_id):
+        is_public_subnet = _is_public_subnet(
+            ec2_client=ec2_client, vpc_id=vpc_id, subnet_id=subnet_id
+        )
+        if is_public_subnet:
             return subnet_id
     return None
 
@@ -333,15 +343,36 @@ def _get_subnets_by_vpc_id(
 
 def _is_public_subnet(
     ec2_client: botocore.client.BaseClient,
+    vpc_id: str,
     subnet_id: str,
 ) -> bool:
     # Public subnet – The subnet has a direct route to an internet gateway.
     # Private subnet – The subnet does not have a direct route to an internet gateway.
+
+    # Check explicitly associated route tables
     response = ec2_client.describe_route_tables(
         Filters=[{"Name": "association.subnet-id", "Values": [subnet_id]}]
     )
     for route_table in response["RouteTables"]:
         for route in route_table["Routes"]:
             if "GatewayId" in route and route["GatewayId"].startswith("igw-"):
                 return True
+
+    # Main route table controls the routing of all subnetes
+    # that are not explicitly associated with any other route table.
+    if len(response["RouteTables"]) > 0:
+        return False
+
+    # Check implicitly associated main route table
+    response = ec2_client.describe_route_tables(
+        Filters=[
+            {"Name": "association.main", "Values": ["true"]},
+            {"Name": "vpc-id", "Values": [vpc_id]},
+        ]
+    )
+    for route_table in response["RouteTables"]:
+        for route in route_table["Routes"]:
+            if "GatewayId" in route and route["GatewayId"].startswith("igw-"):
+                return True
+
     return False