Concepts
TCP/IP
2-9
2.7.2.2 Rlogin
In addition to Telnet, ETS users can use the Rlogin application to connect to remote TCP/IP hosts. Rlogin
is similar to Telnet, but it can be configured to allow trusted users to log into a host without password
verification. For this reason, Rlogin can be a security problem, and is not enabled on all sites. TCP/IP Server
Parameters on page 4-5 explains how to set up Rlogin access and discusses the methods of user verification.
Like the UNIX version of the command, the ETS Rlogin command allows the user to specify a different
username for logging into remote devices. Both Telnet and Rlogin support the port’s Termtype field, which
allows hosts to discover the type of terminal device automatically.
Note:
Rlogin is explained in the Command Reference.
TCP/IP implementations generally have no knowledge of remote hosts until a connection is attempted.
There is no searching for a host until the actual connection request is made. The UNIX operating system
supports the rwho facility to allow hosts to see each other without an active connection, but it is not always
implemented, enabled, or supported. Hosts running rwho send out occasional broadcasts containing the host
name, address, and user information. The ETS will listen for rwho broadcasts and keep track of which UNIX
hosts are available. See Show/Monitor/List [Telnet] Hosts on page 12-113 or your local host
documentation for more information.
2.7.2.3 Subnetworks
TCP/IP Internets are usually broken down into networks, where a host is able to see only the hosts on its
own network or sub-unit. TCP/IP networks then rely on routers (or gateways) to transfer network traffic
to hosts on other networks.
Routers are typically connected to two or more networks, and will pass, or route, TCP/IP messages across
network boundaries. The ETS can be told explicitly which hosts are the gateways for the local network. If
no gateways are currently specified, the ETS will listen to routing protocol packets (for example, RIP) to
decide which hosts are acting as gateways. See Set/Define Server Gateway on page 12-66 for more
information.
The ETS decides at connection time whether the desired TCP/IP host is on the local network segment with
the help of the subnet mask on the server. This mask tells how much of the IP address is the network
address, and is applied to the IP addresses of both the ETS and the remote host. If the resulting addresses
are identical, the connection is deemed local and the host is contacted directly. If not, the connection attempt
and all subsequent messages to this host will be directed to the ETS’s gateway host for forwarding. The
subnet mask, if not set explicitly, will be automatically configured for the given IP address. See Set/Define
Server Subnet Mask on page 12-83 for more information.
2.7.3 Name Resolution
TCP/IP hosts generally have an alphanumeric host name, such as Phred, in addition to a numeric IP address,
such as 192.0.1.99. For this reason, the ETS supports the use of a local host table and domain name
servers— hosts that can translate text host names into the numeric addresses that are needed for a
connection.
