Managing NFS and NIS, 2nd Edition - Mike Eisler [175]
LOOPBACK
The interface is a loopback device: packets sent out on the device are immediately placed on a receive queue for other processes on the local host. Although the loopback device is implemented entirely in software, you must configure it as though it were a physical network interface.
MULTICAST
A valid multicast address has been assigned to this interface. Listening on a multicast address is analogous to listening to a particular band of the radio dial. The packet is not addressed to a particular interface, instead, it is addressed to all interfaces listening on that multicast address.
IPV4 / IPV6
Indicates the version of the Internet Protocol in use. The same interface can be configured to use both versions, although ifconfig prints the respective configuration on separate lines.
The mtu specifies the maximum transmission unit of the interface. IP uses path MTU discovery to determine the maximum transmission unit size across the link. On point-to-point links, the MTU is negotiated by the applications setting up the connection on both sides.
Every configured physical device is assigned a unique index number. The kernel associates the configuration values (IP address, MTU, etc.) with the index number for internal bookkeeping. It provides a useful means for network programming APIs to identify network interfaces.
The second line of ifconfig 's output shows the Internet (IP) address assigned to this interface, the broadcast (IPv4 only) address, and the network mask that is applied to the IPv4 address to derive the broadcast address. The previous example shows the ones form of the broadcast address. When invoked by root, ifconfig also displays the interface's Ethernet address where applicable.
The output of ifconfig resembles the first example for almost all Ethernet interfaces configured to use IPv4, and the third example for almost all Ethernet interfaces configured to use IPv6. ifconfig reports different state information if the interface is for a synchronous serial line, the underlying data link for point-to-point IP networks. Point-to-point links are one foundation of a wide-area network, since they allow IP packets to be run over long-haul serial lines. When configuring a point-to-point link, the broadcast address is replaced with a destination address for the other end of the point-to-point link, and the BROADCAST flag is replaced by the POINTTOPOINT flag:
this-side% ifconfig ipdptp0
ipdptp0: flags=10088d1 inet 131.40.46.1 --> 131.40.1.12 netmask ffffff00 This interface is a serial line that connects networks 131.40.46.0 and 131.40.1.0; the machine on the other end of the line has a similar point-to-point interface configuration with the local and destination IP addresses reversed: that-side% ifconfig ipdptp0 ipdptp0: flags=10088d1 inet 131.40.1.12 --> 131.40.46.1 netmask ffffff00 Marking the line PRIVATE means that the host-to-host connection will not be advertised to routers on the network. Note also that the Address Resolution Protocol (ARP) is not used over point-to-point links. Initializing an interface In addition to displaying the status of a network interface, ifconfig is used to configure the interface. During the boot process, Solaris identifies the network interfaces to be configured by searching for /etc/hostname.*[0-9] and /etc/hostname6.*[0-9] files. For example the presence of /etc/hostname.hme0 and /etc/hostname.hme1 indicate that the two network interfaces hme0 and hme1 need to be assigned an IPv4 address at boot time. Similarly, the presence of /etc/hostname6.hme0 indicates that hme0 needs to be configured to use IPv6. You can statically assign an IP address to the interface by specifying the corresponding hostname in