YPSERV(8)

NAME
ypserv, ypbind - yellow pages server and binder processes

SYNOPSIS
/usr/etc/ypserv
/etc/ypbind

DESCRIPTION
The yellow pages (YP) provides a simple network lookup service consisting of databases and processes. The databases are dbm(3) files in a directory tree rooted at /etc/yp. These files are described in ypfiles(5). The processes are /usr/etc/ypserv, the YP database lookup server, and /etc/ypbind, the YP binder. The programmatic interface to YP is described in ypclnt(3N). Administrative tools are described in yppush(8), ypxfr(8), yppoll(8), ypwhich(1), and ypset(8). Tools to see the contents of YP maps are described in ypcat(1), and ypmatch(1). Database generation and maintenance tools are described in ypinit(8), ypmake(8), and makedbm(8).

Both ypserv and ypbind are daemon processes typically activated at system startup time from /etc/rc.local. Ypserv runs only on YP server machines with a complete YP database. Ypbind runs on all machines using YP services, both YP servers and clients.

The ypserv daemon's primary function is to look up information in its local database of YP maps. The operations performed by ypserv are defined for the implementor by the YP protocol specification, and for the programmer by the header file <rpcsvc/yp_prot.h>. Communication to and from ypserv is by means of RPC calls. Lookup functions are described in ypclnt(3N), and are supplied as C-callable functions in /lib/libc. There are four lookup functions, all of which are performed on a specified map within some YP 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 does not return either through the normal lookup functions. However, if you examine the map with makedbm(8), they are visible. Other functions are used within the YP subsystem itself, and are not of general interest to YP clients. They include: Do_you_serve_this_domain?, Transfer_map, and Reinitialize_internal_state (Refer to /usr/include/rpcsvc/yp_prot.h.)

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 YP 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 the internet address of the YP server, and the port on that host at which the ypserv process is listening for service requests. The process of binding is driven by client requests. As a request for an unbound domain comes in, the ypbind process broadcasts on the net trying to find a ypserv process that serves maps within that domain. Since the binding is established by broadcasting, there must be at least one ypserv process on every net. 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(1) command.

Bindings are verified before they are given out to a client process. If ypbind is unable to speak to the ypserv process it's 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 fail immediately. In general, a bound domain is marked as unbound when the node running ypserv crashes or gets overloaded. In such a case, ypbind tries to bind to any YP server available on the nettypically one that is less-heavily loaded.

Ypbind also accepts requests to set its binding for a particular domain. The request is usually generated by the YP subsystem itself. Ypset(8) is a command to access the Set_domain facility. It is for unsnarling messes, not for casual use.

FILES
If the file /usr/etc/yp/ypserv.log exists when ypserv starts up, log information is written to this file when error conditions arise.

SEE ALSO
ypcat(1), ypmatch(1), ypwhich(1), ypclnt(3N), ypfiles(5), yppush(8), ypset(8), ypxfr(8), YP protocol specification