Security Groups and Firewalling
Security Groups provide a modular way to define and compose firewall rules. The rules are managed at the hypervisor level in order to restrict incoming and outgoing network traffic.
Security Groups gives you the power of VLANs while keeping a single public IP. You get the best of both worlds and have never been safer.
Default firewalling rules
When you create an instance, you can attribute one or more Security Groups to it. Firewall rules defined in your Security Groups take precedence over the default rules, which are:
- All outgoing traffic is allowed
- All incoming traffic is forbidden
This means that a new instance with the default and unmodified security group attributed will be completely inaccessible from outside.
If you wish to Ping your instance or to access it via SSH you’ll have to define incoming rules for it.
About outgoing traffic
By default an unmodified security group without any rule specified allows any kind of outbound traffic.
However, as soon as you define an outbound rule, outbound traffic is only allowed for the defined outbound rules. Any outgoing traffic not allowed by a rule will be then blocked. See managing outbound security rules for more information.
Security group features
When adding a rule to a security group, you can set the following properties:
Traffic type: INGRESS (incoming) or EGRESS (outgoing).
Source type: this can be a CIDR or a security group. This allows you to define internal rules between Security Groups without the hassle of using IP addresses directly. Traffic between your machines will be routed internally and not exposed to the internet.
Protocol: TCP, UDP or ICMP. Special tunneling protocols AH, ESP and GRE are also available. (More on that here)
Start port and end port: this lets you define rules for a specific port (set the same port as start and end port) or for a whole range.
Additionally ICMP protocol let you specify Type and Code.
A simple example
You will probably want to access your instance via SSH. On a freshly created machine with no custom SSH configuration, you’ll need to set a rule in an attributed security group allowing TCP ingress on port 22. You can set it as follows:
- Type: INGRESS
- Protocol: TCP
- Source: 0.0.0.0/0
- Start Port: 22
- End Port: 22
For those most common cases the interface let’s you choose between some quick templates, PING, SSH and RDP.
Organizing Security Groups
You can add several Security Groups to an instance during its creation, and add or remove groups later on in the instance detail page.
It is important to carefully think about your Security Groups when setting up any infrastructure.
A common practice is to identify roles in your infrastructure. As an example, an application infrastructure could be composed of:
- application servers
- cache servers
- load balancers
- database servers
- etc, etc.
In this case you could have:
- A “common” security group that defines rules that are common to all machines, such as SSH access or internal communication
- One security group per role, defining rules specifically for database servers or load balancers.
When setting up a new database server, you would give it the “common” and the “database” Security Groups.
It is also a good practice to apply this technique even with small architectures where a single machine can play all different roles. This way your infrastructure is ready for growth and allows later separation of services across different machines.
Add a Security Group to your instance
Usually you attribute one or more Security Groups during the instance creation process. Note that an instance must belong to at least one Security Group.
During the creation process you’ll find your primary group already selected. You can change your primary group from the Security Groups list screen.
Would you need to change an instance’s groups, you may do so on the instance detail screen: you can add and remove groups as you like, although you can do so only when the instance is stopped.
Layer 2 filtering
Security Groups provide Layer 2 filtering to keep your instance safe from different types of spoofing and Man In The Middle attacks. This filtering is managed automatically so you don’t need to worry about it.
For example the following traffic will be dropped: ARP is allowed only when the source MAC matches the instance’s assigned MAC address, it is therefore not possible to spoof an instance mac address. An instance cannot send ARP responses for the IP address it does not own. An instance cannot spoof a DHCP server response. If you run Wireshark/tcpdump within your instance you won’t see your neighbors traffic even though your NIC is set to promiscuous mode.
With Security Groups on Exoscale, Layer 2 Ethernet isolation is enforced. This is commonly achieved using VLANs on a standard architecture.
Layer 3 (and 4) filtering
Security Groups provide Layer 3 filtering, which can be managed trough our console or API: Ingress and Egress IP traffic can filtered by Protocol / destination / destination port. By default all Ingress is denied and Egress is fully allowed until you create a first rule. As soon as you create an egress rule, only the matching traffic will be allowed * Egress filtering is preventing any broadcast / multicast traffic to leave your instance.
Layer 3 and 4 filtering typically take for source parameters: An IP address in the form of a single IP or network. For example: 188.8.131.52/32 or 0.0.0.0/0 are valid entries Or a Security Group; this can be a self declaration for allowing traffic from machines belonging to the same group or another Security Group
BUM is dropped by egress rule. This is an expected behavior as we don’t want anyone to receive this type of traffic, which may also leak sensitive information. Therefore any application relying on Broadcast, Unknown unicast and Multicast traffic type will not work.
More Security Groups resources
We just scratched the surface about Security Groups. You can find more tutorials and resources about them:
How to modify Security Groups attributed to an instance (from our blog)
Another simple example on how to set up PING, and understanding the ICMP protocol
A detailed explanation about outbound traffic rules
A great example on how to use Security Groups for a more complex setup, combining them to obtain network isolation (from our blog)