5.2 The NBDD and the Damage Done

To be blunt, Microsoft messed up the Datagram Service. Their NBNS implementation (WINS) does not report all of the IP addresses associated with a group name . Instead, group names are mapped to a single IP address the limited broadcast address: 255.255.255.255. This is contrary to the RFCs, and it causes a few problems.

Without a list of IPs for each group name, the NBDD cannot be implemented at all. With no NBDD and no IP list, there is no way to ensure that multicast and broadcast datagrams will be sent to all group members. This breaks the continuity of the NetBIOS virtual LAN. On a "real" NetBIOS LAN, a message sent to a group name would actually reach all members of that group. That is what should happen under NBT as well, but it doesn't. The best that can be done is to broadcast on the local subnet, in which case some members of the group may get the message.

Microsoft must have realized their mistake, because they later created what they call "Internet Group" names (also called "Special Group" names). For names in this category, WINS comes close to behaving like a proper NBNS; it will store up to 25 IP addresses per name, deleting the oldest entry to make room if necessary. For these names, a POSITIVE NAME QUERY RESPONSE from a WINS server will list up to 25 valid IP addresses.

Internet Group names are identified by their suffix. Originally only group names with the 0x1C suffix were given special treatment, but more recently (with W2K?) group names with a suffix value of 0x20 can be defined as having Internet Group status. Note that unique names may also have these suffixes but, since they are not group names, no special handling is required.

Sadly, most non-Microsoft implementations (including Samba) follow Microsoft's example. They map group names to the 255.255.255.255 IP address, store only 25 IPs for Special Group names, and fail to implement the NBDD. ^[1] This can cause trouble for some clients (OS/2, for example) which expect RFC behavior.

^[1] Network Telesystems, which has since been acquired by Efficient Networks, used to have an NBNS implementation that handled group names correctly and worked quite well with IBM's OS/2. Brian Landy has also written a set of patches for Samba's nmbd daemon which provide more complete NBDD support. See http://www.landy.cx/.

Sigh.