My Dumb Guide to Amazon AWS EC2 - Security Groups, Bastion Hosts, and Firewalls

13 Nov 2009

Amazon's Security Groups provide the basic firewall and
networking security. You can of course suppliement it with your own
host-based firewall, but this provides the core.

By default, an instance has no connectivity, either internal or external. This is a like a firewall that starts with a "DENY ALL". By adding security groups, you are allowing different incoming connections. Either from your other instances, or from the outside world.

You assigned security groups before an instance starts. The ordering doesn't matter. Once the instance starts you can't change the membership.

Out of the box, every EC2 instance comes with a "Default group". It says any TCP/UDP/ICMP request from any other machine in your Default group is allowed. This allows internal connectivity. This security group creates your "personal" VLAN -- other instances that aren't yours can not connect to your instances.

If you don't think this is hot, let me give an example of another cloud provider. In the Rackspace Cloud, your instance can connect to every machine they have in the cloud (unless of course the instance is running it's own firewall). I'm sure Rackspace knows this is lame and is working on it. In the mean time, this is a problem you don't have with Amazon

In the AWS Management Console there is another group, "SSH/HTTP" which allows connections on ports 22, 80, and 443 (ssh, http, and https). If you have a one machine site, this is probably all you need.

If for some reason you needed to expose your database to outside world (say mysql on port 5506), you could create another group for that and assign that your instance.

Bastion Hosts

For more complicated systems, allowing everyone to log into any machine is problematic for a few reasons. You have manage updates to SSH (the binaries, keys) on every box. It makes running intrusion detection systems more difficult.

To simplify security management, create a bastion host. This is only machine that allows external ssh connectivity. From the bastion host you can ssh into your other instances.

In Amazon, this is a snap. Create a security group called "bastion" which allows TCP port 22. Create and instance with that group and the default. Create a plain "http" group that does not have SSH access. Then assign new machines to either the default group or the default group + the http group.

Ok here's the fun. Because amazon's firewall is based on group membership and not raw IP addresses. You can terminate your bastion host anytime. When you need to log in, fire up a new instance that has membership of default+bastion. Ta-da. This is actually better than a bastion host in a datacenter -- if it dies, you need to make a trip to the data center (or have a backup).

This allow means external SSH access is completely OFF when the bastion host is off. That may be handy if you suspect you are under attack.

Group Updates

While you can't change group membership when an instance is running, you can change the group's rules. You can delete or add rules, and the change is immediate. This allows you to make quick changes, although I have yet to find a real need for this. If your bastion host is dead and you needed to quickly log-in, you could add port 22 to the default group and you log in.

Host-Based Firewalls

This could go wrong, either in some horrible failure with Security Group, or more likely someone was tinkering with the security groups and opened it up too far. As a backup it's not a bad idea to run a host-based firewall as well.

In full form, you'd have a database of every machine in your network and be able to generate the iptables config and update every machine with it. THis is tricky.

A more lame version would to have the firewall reject any port that you don't use. Or allow from only the 10. network. For instance:

ALLOW 5606/TCP FROM 10.*.*.*

In other words, always allow SSH. Allow connections to MYSQL only from inside EC2. That's not great but will prevent people probing your MySQL from outside world.

Comment 2009-12-30 by None

Thanks for a very clear description of this topic! This helped me a lot.

Comment 2012-01-22 by None

Thanks for writing this!