Pat Shuff's Blog

  • Iaas
    August 11, 2016

networking - practical

Today we are going to explore how to configure and setup the basics of networking for a Linux compute instance in the Oracle Cloud. If you would like to read more about network configurations you should refer to the Oracle Compute Cloud Services (IaaS) documentation. We are specifically interested in chapter 7, Configuring Network Settings. Terms like security list, security rules, and roles play a part of the configuration. By default security is locked down and no traffic can be received from outside the host. It is important to note that the demo accounts that you get when you click the Try Me button on http://cloud.oracle.com do not allow you to create a list of valid ip addresses but allow you to either share ports with the public internet or not. This is mainly for simplicity when running and configuring the demo accounts. When you get a commercial paid account you get full access to restrict access by ip address, ip range, or list of computers.

If we log into our compute console we can see a list of instances that exist for an account. In our example we have four servers defined. One is Oracle Linux, one is CentOS7, one is a database server, and the fourth is a WebLogic server. If we click on the Network tab we see the Security Rules, Security Lists, Security Applicaitons, and Security IP Lists.

It is important to realize that Oracle takes a different approach when provisioning servers. The server is first provisioned with only SSH or RDP as the default rule or a security list that you create. In this example we see four different lists. The bitnami-moodle definition on the list opens up port 80 and 443 for a web server. The database definition prs12cHP opens up port 1521. The prsJava definitions open up administration ports as well as ports 80 and 443. The default security list only opens up port 22 for ssh connections.

If we look at the default security list the default operation is to deny all inbound traffic for computers not in the security list and drop the packets with no reply. We could configure reject with reply but this might lead to a denial of service attack with someone constantly sending TCP/IP requests to our server just to overload the server and network with TCP ack packets. By default the configuration is to drop packets and this typically happens at the border gateway rather than at our compute server. The outbound definition gives you the option of allowing packets, rejecting packets with an ack, and dropping packets with no ack. It is important to communicate to your users how you configure your server. If you configure outbound for deny with no reply they might end up troubleshooting network connection issues when it is by design dropping packets and it is not a router or connection issue.

Note that the concept of security list is a little misleading. For all of our instances we have an inbound policy of deny and an outbound policy of permit. Why not go with one security list and map all instances to this configuration? The key is in the security rules definition. We create a definition of a rule that maps security applications to a source and destination. By application we really mean a port number for an application. The source is where the packet is coming from and the destination is where the packet is going to. Since we have a permit all outbound traffic we only need to define the exceptions to the rule for inbound traffic. If, for example, we defined a deny inbound and deny outbound we would need to define the exception for both directions. If you look at the security rule definitions we are defining the source as the public-internet and the destination as each of our servers.

Security rules are essentially firewall rules. This permits traffic from your compute instance and can be used in different security lists as well as specific definitions between instances and external hosts. Yesterday we talked about turning off public ssh for a database server and only allowing ssh into the database server from our Java server. We would do this by turning off public-internet access over port 22 into the database server and allowing port 22 from our Java server to our database server. To access the database we would have to have public access of port 22 into the Java server, require the user to log in to the command line then ssh across to the database server using port 22 from the Java server to the database server. With this we can hide our database instance from the public internet but still allow access to the console to manage it. We will need to define an outbound rule that allows the database server to reach out and pull down patches if we want or require staging patches from the Java server to the database server by turning off all outbound traffic and only allowing port 1521 to and from the Java server.

Note that we create a rule association by defining the security application and associating it with a source and destination. When we create a security rule we define if it is enable or disable as well as the port or port ranges that we want to open. We can identify the source either with a security list or specific ip lists. If we go with a Security IP List we can define a specific instance, a subnet (site), or the public internet. We can do the same for the destination and specify a security list or specific ip lists. This effectively creates a virtual software defined network that maps packet routing to and from your instance.

If we look at the moodle server that we have running, for example, we have three security applications open. The first is ssh which allows us to connect to a shell and configure services. The second is http which maps to port 80 if we look at the Security Applications. The third is https which maps to port 443. These three ports are the only ports that are open and they are open to the public-internet as the source. We have a permit outbound rule so that the moodle server can pull in images from our storage servers, get updates with command line tools from other web servers, and download updates to the moodle server as needed from bitnami. We could just as easily have set the outbound policy to deny and only allow http, https, and ssh connections to this server inbound and outbound.

Note that this process and procedure is very similar to the way that Amazon AWS and Microsoft Azure define network rules. With AWS you go through the VPC Dashboard and define a VPN Connection. You create Security Groups that defines the ports and access rights. For example the bch-shared-firewall-34877 opens up ports 22, 80, and 443 to the public internet. The source of is associated with the public internet. Note that we also have another rule that maps us to the server for management. Once we define the inbound rules we can associate it with a VPN connection or gateway and define the inbound and outbound rules as we do on the Oracle Compute Cloud.

Azure does something similar and allows you to define ports or sets of ports when you create the instance. Note that TCP and UDP are the protocols that are allowed. This tends to imply the ICMP and other protocols are restricted in the Microsoft network. This typically is not a big deal but does have implications on how and what you can deploy in the Microsoft network. Amazon appears to allow ICMP as a rule definition as well as Oracle.

In summary, it appears that all three cloud vendors provide basic inbound and outbound rules. Microsoft limits the protocols to TCP and UDP and does not allow ICMP rules. This might or might not matter when selecting a cloud vendor. Once you have the rules defined you effectively have a secure system and flexibility to define subnets, netmasks, router tables, and layers of security with software defined networks. All three vendors appear to address this basic networking issue the same with one small difference with Azure. Now that we know how to configure networks it might be important to talk about speed, blocking, and throttling of networks. More tomorrow.

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