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/usr/lib/netsvc/yp/ypbind [-ypset | -ypsetme]
Both ypserv and ypbind are daemon processes typically activated at system startup time. ypserv runs only on NIS server machines with a complete NIS database. ypbind runs on all machines using NIS services, both NIS servers and clients.
The ypserv daemon's primary function is to look up information in its local database of NIS maps. Communication to and from ypserv is by means of RPC calls. Lookup functions are described in ypclnt(3rpc), and are supplied as C-callable functions in the NIS library. There are four lookup functions, all of which are performed on a specified map within some NIS domain: Match, Get_first, Get_next, and Get_all. The Match operation takes a key, and returns the associated value. The Get_first operation returns the first key-value pair from the map, and Get_next can be used to enumerate the remainder. Get_all ships the entire map to the requester as the response to a single RPC request.
Two other functions supply information about the map, rather than map entries: Get_order_number, and Get_master_name. In fact, both order number and master name exist in the map as key-value pairs, but the server will not return either through the normal lookup functions. If you examine the map with makedbm(1Mnis), however, they will be visible.
The function of ypbind is to remember information that lets client processes on a single node communicate with some ypserv process. ypbind must run on every machine which has NIS client processes; ypserv may or may not be running on the same node, but must be running somewhere on the network.
The information ypbind remembers is called a ``binding'' -- the association of a domain name with an NIS server.
The process of binding is driven by client requests. As a request for an unbound domain comes in, the ypbind process steps through the ypservers list (last entry first) trying to find a ypserv process that serves maps within that domain. There must be a ypserv process on at least one of the hosts in the ypservers file. Once a domain is bound by a particular ypbind, that same binding is given to every client process on the node. The ypbind process on the local node or a remote node may be queried for the binding of a particular domain by using the ypwhich(1nis) command.
If ypbind is unable to speak to the ypserv process it is bound to, it marks the domain as unbound, tells the client process that the domain is unbound, and tries to bind the domain once again. Requests received for an unbound domain will wait until the domain requested is bound. In general, a bound domain is marked as unbound when the node running ypserv crashes or gets overloaded. In such a case, ypbind will try to bind to another NIS server listed in /var/yp/binding/domainname/ypservers.
ypbind also accepts requests to set its binding for a particular domain. The request is usually generated by the NIS subsystem itself. ypset(1Mnis) is a command to access the Set_domain facility. Note that the Set Domain procedure only accepts requests from processes with appropriate privileges, and the -ypset or -ypsetme flags must have been set for ypbind.
Note that both ypbind and ypserv support multiple domains. The ypserv process determines the domains it serves by looking for directories of the same name in the directory /var/yp. Additionally, the ypbind process can maintain bindings to several domains and their servers.