4. NIS - Network Information System

4. NIS - Network Information System
	Chapter 6. Network Operating Systems

Network Information System
Definition:
NIS provides generic database access facilities that can be used to distribute information such as that contained in the passwd and groups files to all hosts on your network.
Objectives:
- To make the network appear just as a single system, with the same accounts on all hosts.
- To provide an environment to your users that makes the network transparent.
- To keep vital data such as user account information synchronized between all hosts.
NIS comprises:
- A RPC server.
- A client-side library.
- Several administrative tools.
Note:
Originally, NIS was called Yellow Pages, or YP, which is still widely used to informally refer this service. Yellow Pages is a trademark of British Telecom, which required Sun to drop that name.

Maps

NIS keeps database information is in so-called maps containing key-value pairs.

Maps are stored on a central host running the NIS server, from which clients may retrieve the information through various RPC calls.

Creation:

The maps are usually generated from master text files such as /etc/hosts or /etc/passwd.

For some files, several maps are created, one for each search key type:

Master File                                      Maps 
/etc/hosts                                       hosts.byname, hosts.byaddr 
/etc/networks                                    networks.byname, netwosks.byaddr 
/etc/passwd                                      passwd.byname, passwd.byuid 
/etc/group                                       group.byname, group.bygid 
/etc/services                                    services.byname, services.bynumber 
/etc/rpc                                         rpc.byname, rpc.bynumber 
/etc/protocols                                   protocols.byname, protocols.bynumber 
/usr/lib/aliases                                 mail.aliases

ypcat:

The command ypcat lists the maps in the same way the command cat would list the master files:

% ypcat passwd

NIS servers
The NIS server is traditionally called ypserv.
Types:
- For an average network: a single server usually suffices.
- For big networks: Many servers synchronized servers in a master-slave scheme:
  - Maps will be created only on the master server's host.
  - From there, they are distributed to all slaves.
NIS domains
Definition:
NIS domains are the collection of all hosts that share part of their system configuration data through NIS.
Characteristics:
- NIS domains have a purely administrative function only.
- They are mostly invisible to users, except for the sharing of passwords between all machines in the domain.
- The name given to a NIS domain is relevant only to the administrators.
- NIS domains determine which NIS server an application will query.
Finding a NIS server:
Configuration file:
A file that names the host on which to find the server.
Disadvantage: This approach is inflexible, because it doesn't allow clients to use different servers, depending on their availability.
ybbind:
ypbind probes for servers by broadcasting to the local IP-network; the first to respond is assumed to be the potentially fastest one and will be used in all subsequent NIS queries.
Disadvantage: ypbind blindly believes whoever answers, which could be a humble NIS server as well as a malicious intruder.
The Client Side of NIS
Without NIS:
Functions like:
- getpwnam(3), which return the account information associated with the given user name.
- getpwuid(3), which return the account information associated with the given user id.
Perform the requested lookup on the standard file, such as /etc/passwd.
With NIS:
There is a NIS-aware implementation of these functions in the C-library.
They place RPC calls to have the NIS server look up the user name or id.


3. NFS - Network File System		5. Electronic Mail