Web site design and hosting - 3.3.3 A Simple SMB Connection Before we close
3.3.3 A Simple SMB Connection Before we close this chapter, let s take a look at a simple SMB connection. This is some pretty technical data - which isn t really necessary to administer Samba - so you can skip over it if you like. We present this information largely as a way to help you get familiar with how the SMB protocol negotiates connections with other computers on the network. There are four steps that the client and server must complete in order to establish a connection to a resource: 1. Establish a virtual connection. 2. Negotiate the protocol variant to speak. 3. Set session parameters. 4. Make a tree connection to a resource. We will examine each of these steps through the eyes of a useful tool that we mentioned earlier: the modified tcpdump that is available from the Samba web site. You can download this program at samba.org in the samba/ftp/tcpdump-smb directory; the latest version as of this writing is 3.4-5. Use this program as you would use the standard tcpdump application, but add the -s 1500switch to ensure that you get the whole packet and not just the first few bytes. 3.3.3.1 Establishing a virtual connection When a user first makes a request to access a network disk or send a print job to a remote printer, NetBIOS takes care of making a connection at the session layer. The result is a bidirectional virtual channel between the client and server. In reality, there are only two messages that the client and server need to establish this connection. This is shown in the following example session request and response, as captured by tcpdump : >>> NBT Packet NBT Session Request Flags=0×81000044 Destination=ESCRIME NameType=0×20 (Server) Source=WIZZIN NameType=0×00 (Workstation) >>> NBT Packet NBT Session Granted Flags=0×82000000 94
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