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In this section we started to dig a little deeper into the innards of DRb We built a more complex application that had proprietary Ruby objects and made use of the DRbUndumped module We also learned how DRb decides which transmission mechanism it should use, pass by value or pass by reference As you build your own distributed applications using DRb, you will find that the DRbUndumped module is very powerful and is the real secret sauce in making writing distributed applications with Ruby incredibly easy
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The DRbUndumped module is actually only one method That one method contains only one line of code That one line of code does nothing but raise an error!
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module DRbUndumped def _dump(dummy) # :nodoc: raise TypeError, 'can\'t dump' end end
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When that error is raised, it tells DRb to reference the object instead of marshaling it Because of that one line, we can create complex applications using proprietary objects, and we don t have to worry about the end user not having access to objects
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Every developer needs to concern herself with the security of her application With distributed applications that security is even more of a concern and even more difficult to maintain Although the DRb package, and Ruby, provide some security, they fall short of a full, comprehensive solution This can make DRb less than desirable in a lot of real world situations However, in situations where security is a lesser concern, such as prototyping and intranet applications, the security that is provided with DRb and Ruby might just suffice Earlier I talked about how easy it is to execute code on a remote server from the client This enormous security hole should have jumped right off the page at you Let s look at an example from the DRb RDoc (an application that produces documentation for Ruby code):
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require 'drb' # !!! UNSAFE CODE DO NOT RUN !!! ro = DRbObject::new_with_uri("druby://yourservercom:8989") class << ro undef :instance_eval end roinstance_eval("`rm -rf *`")
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1 Distributed Ruby (DRb)
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What is this sample code doing, and why is it so dangerous Let s examine it First, we retrieve a new remote object from the server Then we undefine the instance_eval method on that remote object Because of how DRb works, as we learned earlier, when a method does not exist on the local copy of the remote object, the method is invoked on the server side So in the last line, when we call the instance_eval method and tell it to evaluate String "`rm -rf *`", we are telling the remote server to forcibly, and recursively, remove all files on the server Probably not the best idea How do we prevent this from happening It s best to run your server code with a safe mode of at least 1: $SAFE = 1 This disables eval() and related calls on strings passed across the wire Now, if you were to run our malicious client again, you would get the following error:
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SecurityError: Insecure operation - instance_eval
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Setting your safe level to 1 will not prevent you from calling the eval() family of methods locally, so it s a good safety measure to get in the habit of using
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With our servers now able to withstand some malicious attacks, the next step is to decide who can and cannot access the services we are providing Enter access control lists (ACLs) An ACL specifies which clients are allowed access and which are denied This is done with a simple Array defining these relationships Let s add an ACL to our Hello World application We know that we want only two machines to access the server the server itself and the client All other machines on the network should not be given access Our server has the internal IP address of 192168112, and our client has the IP address of 19216817 Here is what our HelloWorldServer code now looks like:
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require 'drb' require 'drb/acl' # Make sure no one can call eval() and related methods remotely! $SAFE = 1 class HelloWorldServer
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