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Remote access is the hallmark of modern multiple-user operating systems such as Solaris and its antecedents, such as VAX/VMS. Solaris users can concurrently log into and interactively execute commands on Solaris server systems from any client that supports Transmission Control Protocol/Internet Protocol (TCP/IP), such as Solaris, Windows, and Macintosh OS. This section examines several historically popular methods of remote access, such as Telnet. It also outlines the much-publicized security holes and bugs that have led to the innovation of secure remote-access systems, such as SSH. These safer systems facilitate the encryption of the contents of user sessions and/or authentication sequences and provide an important level of protection for sensitive data. Although remote access is useful, the administrative overhead in securing a Solaris system can be significant, reflecting the increased functionality that remote-access services provide.
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Telnet
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Telnet is the standard remote-access tool for logging into a Solaris machine from a client using the original DARPA Telnet protocol. A client can be executed on most operating systems that support TCP/IP. Alternatively, a Java Telnet client is available (http://srp.stanford.edu/binaries.html), which is supported on any operating system that has a browser that runs Java natively or as a plug-in. Telnet is a terminal-like program that gives users interactive access to a login shell of their choice (for example, the C-shell, or csh). Most Telnet clients support VT100 or VT220 terminal emulations. The login shell can be used to execute scripts, develop applications, and read e-mail and news in short, everything a Solaris environment should provide to its users, with the exception of X11 graphics and Open Windows, and, more recently, the common desktop environment (CDE). A common arrangement in many organizations is for a Solaris server to be located in a secure area of a building with Telnet-only access allowed. This arrangement is shown in Figure 9-1. The sequence of events that occurs during a Telnet session begins with a request for a connection from the client to the server. The server either responds (or times out) with a connection being explicitly accepted or rejected. A rejection may occur because the port that normally accepts Telnet client connections on the server has been blocked by a packet filter or firewall. If the connection is accepted, the client is asked to enter a username followed by a password. If the username and password combination is valid, a shell is spawned, and the user is logged in. This sequence of events is shown in Figure 9-2.
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FIGURE 9-1
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Typical remote-access topology for client/server technology
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FIGURE 9-2
Identification and authentication of a Telnet session
The standard port for Telnet connections is 23. Thus, a command like this,
$ telnet server
is expanded to give the effective command:
$ telnet server 23
This means that Telnet can be used as a tool to access a service on virtually any port. Telnet is controlled by the super Internet daemon (inetd), which invokes the in.telnetd server. An entry is made in /etc/services that defines the port number for the Telnet service, which looks like this:
telnet 23/tcp
The configuration file /etc/inetd.conf also contains important details of the services provided by inetd. The telnet daemon s location and properties are identified here:
telnet stream tcp nowait root /pkgs/tcpwrapper/bin/tcpd in.telnetd
In this case, you can see that in.telnetd is protected by the use of TCP wrappers, which facilitate the logging of Telnet accesses through the Solaris syslog facility. In addition, inetd has some significant historical security holes and performance issues that, although
Part III:
Security
mostly fixed in recent years, have caused administrators to shy away from servers invoked by inetd. The Apache Web server (http://www.apache.org), for example, runs as a standalone daemon process and does not use inetd. inetd also controls many other standard remote-access clients, including the so-called r-commands, including the remote login (rlogin) and remote shell (rsh) applications. The rlogin application is similar to Telnet in that it establishes a remote connection through TCP/IP to a server, spawning an interactive login shell. For example, the command
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