Default gateway in VPC with Public and Private Subnets

Question

Lets assume I have a VPC (172.16.0.0/16) with Public (172.16.0.0/24) and Private (172.16.1.0/24) subnets. Also I have one Internet Gateway (172.16.0.1/24) and one NAT Gateway (172.16.1.x/24). Note that the last bit is x, as IP address is assigned dynamically.

When I spawn an instances in the public subnet the default gateway is configured like so:

default via 172.16.0.1 dev eth0

When I spawn an instance in the private subnet I would like that instance had the default gateway configured like so:

default via 172.16.1.x dev eth0

Note: x in the last bit of the IP should match NAT-GW IP address.

At present I have to manually configure routing, thus change default gateway and add explicit route to other subnet:

default via 172.16.1.x dev eth0
172.16.0.0/24 via 172.16.1.1 dev eth0

Things like these should be possible to configure via DHCP, but aws DHCP seems to have a very limited set of settings.

EDIT - Route Tables:

Private subnet

172.16.0.0/16 local Active No
0.0.0.0/0 nat-id Active No

Public subnet

172.16.0.0/16 local Active No
0.0.0.0/0 igw-id Active No

Did you create your VPC manually or using the wizard in the dashboard? — helloV
I'm not sure how it was created, I'm just taking it over from someone else. What difference does it make anyway? — NarūnasK
It makes a lot a difference. You wouldn't be asking questions like this. — helloV

YFP YFP · Accepted Answer · 2016-08-25T13:34:30

I would guess that your routing table for the private subnets needs some attention and may need to be directed to a NAT instance to allow passage to the public internet. What do the route tables for your private subnets looks like? This should help you out as well - http://docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_Scenario2.html

Thanks, //P

Default gateway in VPC with Public and Private Subnets

1 Answers