wiki:GENIEducation/SampleAssignments/OpenFlowAssignment/ExerciseLayout/Execute

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<OpenFlow ASSIGNMENT>

Hello GENI index Hello GENI index Hello GENI index

STEPS FOR EXECUTING EXERCISE

Debugging an OpenFlow Controller

You will find it helpful to know what is going on inside your OpenFlow controller and its associated switch when implementing these exercises. This section contains a few tips that may help you out if you are using the Open vSwitch implementation provided with this tutorial. If you are using a hardware OpenFlow switch, your instructor can help you find equivalent commands.
The Open vSwitch installation provided by the RSpec included in this tutorial is located in /opt/openvswitch-1.6.1-F15. You will find Open vSwitch commands in /opt/openvswitch-1.6.1-F15/bin and /opt/openvswitch-1.6.1-F15/sbin. Some of these commands may be helpful to you. If you add these paths to your shell’s $PATH, you will be able to access their manual pages with man. Note that $PATH will not affect sudo, so you will still have to provide the absolute path to sudo; the absolute path is omitted from the following examples for clarity and formatting.

  • 2.1 ovs-vsctl
    Open vSwitch switches are primarily configured using the ovs-vsctl command. For exploring, you may find the ovs-vsctl show command useful, as it dumps the status of all virtual switches on the local Open vSwitch instance. Once you have some information on the local switch configurations, ovs-vsctl provides a broad range of capabilities that you will likely find useful for expanding your network setup to more complex configurations for testing and verification. In particular, the subcommands add-br, add-port, and set-controller may be of interest.
  • 2.2 ovs-ofctl
    The switch host configured by the given rspec listens for incoming OpenFlow connections on localhost port 6634. You can use this to query the switch state using the ovs-ofctl command. In particular, you may find the dump-tables and dump-flows subcommands useful. For example, sudo ovs-ofctl dump-flows tcp:127.0.0.1:6634 will output lines that look like this:
    cookie=0x4, duration=6112.717s, table=0, n packets=1, n bytes=74, idle age=78,priority=5,tcp,
    nw src=10.10.10.0/24 actions=CONTROLLER:65535
    
    This indicates that any TCP segment with source IP in the 10.10.10.0/24 subnet should be sent to the OpenFlow controller for processing, that it has been 78 seconds since such a segment was last seen, that one such segment has been seen so far, and the total number of bytes in packets matching this rule is 74. The other fields are perhaps interesting, but you will probably not need them for debugging. (Unless, of course, you choose to use multiple tables — an exercise in OpenFlow 1.1 functionality left to the reader.)
  • 2.3 Unix utilities
    You will want to use a variety of Unix utilities, in addition to the tools listed in ExerciseLayout, to test your controllers. The standard ping and /usr/sbin/arping tools are useful for debugging connectivity (but make sure your controller passes ICMP ECHO REQUEST and REPLY packets and ARP traffic, respectively!), and the command netstat -an will show all active network connections on a Unix host; the TCP connections of interest in this exercise will be at the top of the listing. The format of netstat output is out of the scope of this tutorial, but information is available online and in the manual pages.

Exercises

  • 3.1 Building a Firewall with OpenFlow
    A firewall observes the packets that pass through it, and uses a set of rules to determine whether any given packet should be allowed to pass. A stateless firewall does this using only the rules and the current packet. A stateful firewall keeps track of the packets it has seen in the past, and uses information about them, along with the rules, to make its determinations.
    In this exercise, you will build a stateful firewall controller for TCP connections in OpenFlow. The first packet of each connection will be handled by the controller, but all other connection packets will be handled by the OpenFlow-enabled router or switch without contacting your controller. This design will allow you to write powerful firewall rule sets without unduly impacting packet forwarding speeds. Your controller will parse a simple configuration file to load its rules. Complete stateful firewalls often handle multiple TCP/IP protocols (generally at least both TCP and UDP), track transport protocol operational states, and often understand some application protocols, particularly those utilizing multiple transport streams (such as FTP, SIP, and DHCP). The firewall you will implement for this exercise, however, needs handle only TCP, and will not directly process packet headers or data.
    Network Setup
    Follow instructions in the DesignSetup step to build a firewall experiment topology. The specific host names allocated for your experiment will be different, but the topology will be isomorphic. The host labeled left in the figure is “behind” the firewall, implemented by the Open vSwitch host labeled switch. The host labeled router handles IP routing for the firewalled network, and every host on the other side of this router (the host labeled right being the only example on this topology; you may wish to add others for your testing and experimentation) are “outside” of the firewall.
    The provided RSpec and the files it installs on the hosts it allocates will configure a complete, working network with an Open vSwitch running on the host labeled switch. The Open vSwitch switch is configured to connect to a controller on localhost (that is, the switch host), but no controller is started; until a controller is started on localhost, the Open vSwitch will act like a normal learning switch, forwarding all packets to the appropriate interface based on MAC address. Trema is installed in /opt/trema-trema-8e97343. Once you have implemented your switch, you can simply use this Trema install to run it and the Open vSwitch will obey its configuration.
    You can test that the network configured correctly by waiting a few moments after Flack or Omni (or whatever GENI tool you are using) suggests that the sliver is ready, and running ping right from the host allocated for left or vice-versa. Since the fallback switch configuration will act like a normal learning switch, the ping packets should go through.
    Firewall Configuration The firewall configuration language is very simple. All flows not specified in the configuration are assumed to be forbidden, and the default packet processing policy on the OpenFlow device you are managing should be to drop packets. The configuration language will specify, one flow to a line, the TCP flows that should be permitted to pass the firewall. The syntax is:
    <ip>[/<netmask>] <port> <ip>[/<netmask>] <port>
    
    Items in angle brackets (<>) represent variable values, items in square brackets ([]) represent optional syntax, and unquoted characters (e.g., the slash characters) represent themselves. The first subnet (IP address plus mask length) and port number are the subnet and port number of the host initiating the connection (that is, sending the first bare SYN), and the second subnet and port number are those of the host accepting the connection. IP addresses are specified as dotted quads (e.g., 192.168.1.0) and netmasks as bit lengths (e.g., 24). If a netmask is missing (the IP address for a given subnet is not followed by a slash and an integer), it is equivalent to /32. Port numbers are integers. Either or both of the IP address or port numbers may be replaced by the word any, equivalent to 0.0.0.0/0 in the case of IP address, or to any port number, in the case of port numbers.
    A sample configuration that implements a firewall permitting inbound connections to a web server at IP address 192.168.1.1 on port 80, and any outbound connections initiated by hosts inside the firewall (protecting 192.168.1.0/24) is as follows:
    any any 192.168.1.1 80
    192.168.1.0/24 any any any
    
    All whitespace will be a single ASCII space character, all newlines will be a single ASCII newline character (0x0a) Empty lines (two newlines back-to-back) are permitted.
    A connection is allowable if it matches any rule. A connection matches a rule if all four elements of the four-tuple match. Subnet matching uses standard rules, expressed in this pseudocode:
    boolean subnet_match(IP subnet, int bits, IP addr) {
        int32 bitmask =  ̃((1 << 32 - bits) - 1);
        IP addrnet = addr & bitmask;
        return addrnet ˆ subnet == 0;
    }
    
    Note that rules are not bidirectional; the presence of the first rule in this set does not imply the second:
    192.168.1.0/24 any any any
    any any 192.168.1.0/24 any
    
    This means that the first packet the controller sees that matches a flow causes the flow to be allowed. Packets that would trigger a reply that would be allowed are not necessarily allowed. The name of a firewall configuration file will be provided on the controller command line. To provide an argument to your controller application, it must be included with the controller file name. For example, to configure your firewall found in firewall.rb to load fw.conf, you would invoke:
    trema run ’firewall.rb fw.conf’
    
    You will then find [’firewall.rb’, ’fw.conf’] in ARGV when your controller’s start method is invoked.

Next: Teardown Experiment

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