SNMP GetRequest parsing

Author: blackhedd

lib/net/snmp.rb

@@ -51,5 +51,57 @@ class SNMP
 
     end
 
+    class SnmpPdu
+	class Error < Exception; end
+
+	attr_reader :version, :community, :pdu_type, :request_id, :variables
+
+	#--
+	# TODO, improve the error-trapping.
+	# We want to wrap up Ruby errors like array-ranges, which can appear if we get bad data.
+	# We should probably do the whole parse under a catch-all block.
+	def initialize ber_object
+	    begin
+		parse_ber_object ber_object
+	    rescue RuntimeError
+		# Wrap any basic parsing error so it becomes a PDU-format error
+		raise Error.new( "snmp-pdu format error" )
+	    end
+	end
+
+	def parse_ber_object ber_object
+	    @version = ber_object[0].to_i
+	    unless [0,2].include?(@version)
+		raise Error.new("unknown snmp-version: #{@version}")
+	    end
+
+	    @community = ber_object[1].to_s
+
+	    data = ber_object[2]
+	    app_tag = data.ber_identifier & 31
+	    case app_tag
+	    when 0
+		@pdu_type = :get_request
+		parse_get_request data
+	    else
+		raise Error.new( "unknown snmp-pdu type: #{app_tag}" )
+	    end
+	end
+
+	#--
+	# Defined in RFC1157, pgh 4.1.2.
+	def parse_get_request data
+	    @request_id = data[0].to_i
+	    # data[1] is error-status, always 0.
+	    # data[2] is error-index, always 0.
+	    @variables = data[3].map {|v|
+		# A variable-binding, of which there may be several,
+		# consists of an OID and a BER null.
+		# We're ignoring the null, we might want to verify it instead.
+		v[0]
+	    }
+	end
+
+    end
 end
 

tests/testsnmp.rb

@@ -11,7 +11,7 @@
 
 class TestSnmp < Test::Unit::TestCase
 
-  SnmpRequest = "0'\002\001\000\004\006public\240\032\002\002?*\002\001\000\002\001\0000\0160\f\006\b+\006\001\002\001\001\001\000\005\000"
+  SnmpGetRequest = "0'\002\001\000\004\006public\240\032\002\002?*\002\001\000\002\001\0000\0160\f\006\b+\006\001\002\001\001\001\000\005\000"
 
   def setup
   end
@@ -27,18 +27,40 @@ def test_invalid_packet
 
   end
 
+  # The method String#read_ber! added by Net::BER consumes a well-formed BER object
+  # from the head of a string. If it doesn't find a complete, well-formed BER object,
+  # it returns nil and leaves the string unchanged. If it finds an object, it returns
+  # the object and removes it from the head of the string. This is good for handling
+  # partially-received data streams, such as from network connections.
   def test_consume_string
       data = "xxx"
       assert_equal( nil, data.read_ber! )
       assert_equal( "xxx", data )
 
-      data = SnmpRequest + "!!!"
+      data = SnmpGetRequest + "!!!"
       ary = data.read_ber!( Net::SNMP::AsnSyntax )
       assert_equal( "!!!", data )
       assert ary.is_a?(Array)
       assert ary.is_a?(Net::BER::BerIdentifiedArray)
   end
 
+  def test_weird_packet
+      assert_raise( Net::SnmpPdu::Error ) {
+	Net::SnmpPdu.new("aaaaaaaaaaaaaa")
+      }
+  end
+
+  def test_packet
+      data = SnmpGetRequest.dup
+      pkt = data.read_ber(Net::SNMP::AsnSyntax)
+      assert pkt.is_a?(Net::BER::BerIdentifiedArray)
+      assert_equal( 48, pkt.ber_identifier) # Constructed [0], signifies GetRequest
+
+      pdu = Net::SnmpPdu.new(pkt)
+      assert_equal(:get_request, pdu.pdu_type )
+      assert_equal(16170, pdu.request_id ) # whatever was in the test data. 16170 is not magic.
+      assert_equal( [[1,3,6,1,2,1,1,1,0]], pdu.variables )
+  end
 end