hosts Database

The hosts database contains the IPv4 addresses and host names of machines on your network. If you use the NIS, NIS+, or DNS name services (or LDAP as a name service), the hosts database is maintained in a database that is designated for host information. For example, on a network that runs NIS+, the hosts database is maintained in the host table.

If you use local files for the name service, the hosts database is maintained in the /etc/inet/hosts file. This file contains the host names and IPv4 addresses of the primary network interface, other network interfaces that are attached to the machine, and any other network addresses that the machine must check for.

For compatibility with BSD-based operating systems, the file /etc/hosts is a symbolic link to /etc/inet/hosts.

/etc/inet/hosts File Format

The /etc/inet/hosts file uses the basic syntax that follows. Refer to the hosts(4) man page for complete syntax information.

IPv4-address hostname [nicknames] [#comment]

IPv4-address contains the IPv4 address for each interface that the local host must recognize.

hostname contains the host name that is assigned to the machine at setup, plus the host names that are assigned to additional network interfaces that the local host must recognize.

[nickname] is an optional field that contains a nickname for the host.

[# comment] is an optional field for a comment.

Initial /etc/inet/hosts File

When you run the Solaris installation program on a machine, the program configures the initial /etc/inet/hosts file. This file contains the minimum entries that the local host requires. The entries include the loopback address, the host IPv4 address, and the host name.

For example, the Solaris installation program might create the following /etc/inet/hosts file for machine tenere shown in Figure 4–1:

Example 5–1 /etc/inet/hosts File for Machine ahaggar

127.0.0.1     localhost         loghost    #loopback address
192.9.200.3   tenere                      #host name

Loopback Address

In Example 5–1, the IPv4 address 127.0.0.1 is the loopback address. The loopback address is the reserved network interface that is used by the local machine to allow interprocess communication. This enables the host to send packets to itself. The ifconfig command uses the loopback address for configuration and testing, as explained in ifconfig Command. Every machine on a TCP/IP network must use the IP address 127.0.0.1 for the local host.

Host Name

The IPv4 address 192.9.200.1 and the name tenere are the address and host name of the local machine. They are assigned to the machine's primary network interface.

Multiple Network Interfaces

Some machines have more than one network interface, because they are either routers or multihomed hosts. Each additional network interface that is attached to the machine requires its own IPv4 address and associated name. When you configure a router or multihomed host, you must add this information manually to the router's /etc/inet/hosts file. See Configuring Routers for more information on configuring routers and multihomed hosts.

Example 5–2 is the /etc/inet/hosts file for machine timbuktu that is shown in Figure 4–1.

Example 5–2 /etc/inet/hosts File for Machine timbuktu

127.0.0.1      localhost     loghost
192.9.200.70   timbuktu      #This is the local host name
192.9.201.10   timbuktu-201  #Interface to network 192.9.201

With these two interfaces, timbuktu connects networks 192.9.200 and 192.9.201 as a router.

How Name Services Affect the hosts Database

The NIS, NIS+, and DNS name services (or LDAP as a name service) maintain host names and addresses on one or more servers. These servers maintain hosts databases that contain information for every host and router (if applicable) on the servers' network. Refer to System Administration Guide: Naming and Directory Services (DNS, NIS, and LDAP) and System Administration Guide: Naming and Directory Services (FNS and NIS+) for more information about these services.

When Local Files Provide Name Service

On a network that uses local files for name service, machines that run in local files mode consult their individual /etc/inet/hosts files for IPv4 addresses and host names of other machines on the network. Therefore, these machine's /etc/inet/hosts files must contain the following:

• Loopback address

• IPv4 address and host name of the local machine (primary network interface)

• IPv4 address and host name of additional network interfaces that are attached to this machine, if applicable

• IPv4 addresses and host names of all hosts on the local network

• IPv4 addresses and host names of any routers that this machine must know about, if applicable

• IPv4 address of any machine your machine wants to refer to by its host name

The figure below shows the /etc/inet/hosts file for machine tenere. This machine runs in local files mode. Notice that the file contains the IPv4 addresses and host names for every machine on the 192.9.200 network. The file also contains the IPv4 address and interface name timbuktu-201. This interface connects the 192.9.200 network to the 192.9.201 network.

A machine that is configured as a network client uses the local /etc/inet/hosts file for its loopback address and IPv4 address.

Figure 5–1 /etc/inet/hosts File for Machine Running in Local Files Mode