TCP_IP协议源代码分析

NET User Reference Manual (NOS Version) Phil Karn, KA9Q 1. The NET.EXE Program The MS-DOS executable file net.exe provides Internet (TCP/IP), NET/ROM and AX.25 facilities. Because it has an internal multi- tasking operating system, net.exe can act simultaneously as a client, a server and a packet switch for all three sets of proto- cols. That is, while a local user accesses remote services, the system can also provide those same services to remote users while also switching IP, NET/ROM and AX.25 packets and frames between other client and server nodes. The keyboard and display is used by the local operator to control both host and gateway level functions, for which a number of com- mands are provided. 1.1. Installation Net.exe uses the following directory structure: /spool /spool/help /spool/mail /spool/mqueue /spool/rqueue /spool/news By default, the /spool directory is placed in the root directory of the current drive. However, a subdirectory may be specified with the -d command-line option described below. If a subdirec- tory is given, the alias, autoexec.net, dialer, domain.txt and ftpusers configuration files must also be located there. The "/spool" directory and its sub-directories are used by the bbs, SMTP and NNTP services. The areas, forward.bbs, history, mail.log, rewrite and signatur configuration files are located here. 1.2. net [-b] [-s <sockets>] [-d <directory>] [<startup file>] June 7, 1991 - 2 - 1.2.1. -b The -b option specifies the use of BIOS for console output; the default is to write directly to the video display buffer. Use this option if you are running under a windowing package and have trouble with output "bleeding through" on top of other windows. 1.2.2. -s The -s option specifies the size of the socket array to be allo- cated within net.exe. This limits the number of network connec- tions that may exist simultaneously. The default is 40. 1.2.3. -d The -d option allows the user to specify a directory for the con- figuration and spool files; it defaults to the root directory of the system. 1.2.4. Startup file After all command-line options, the name of a startup file may be specified. If no startup file is specified, net.exe attempts to open a file named autoexec.net in the configuration directory of the current drive. If the file exists, it is read and executed as though its contents were typed on the console as commands. (See the Commands chapter.) This feature is useful for attaching communication interfaces, configuring network addresses, and starting the various services. 2. Console modes The console may be in one of two modes: command mode and converse mode. In command mode, the prompt net> is displayed and any of the commands described in the Commands chapter may be entered. In converse mode, keyboard input is processed according to the current session. Sessions come in many types, including Telnet, FTP, AX25, NETROM, Ping, More, Hopcheck and Tip. In a Telnet, AX25, NETROM, or Tip session, keyboard input is sent to the remote system and any out- put from the remote system is displayed on the console. In a FTP session, keyboard input is first examined to see if it is a known local command; if so it is executed locally. If not, it is "passed through" to the remote FTP server. (See the FTP Subcom- mands chapter). In a Ping session the user may test the path to a remote site, and in a More session, the user may examine a local file. A Hopcheck session is used to trace the path taken by packets to reach a specified destination. A Tip session provides a "dumb terminal" service that bypasses all network protocols. The keyboard also has cooked and raw states. In cooked state, input is line-at-a-time; the user may use the line editing char- acters ^U, ^R and backspace to erase the line, redisplay the line June 7, 1991 - 3 - and erase the last character, respectively. Hitting either return or line feed passes the complete line up to the applica- tion. In raw state, each character is immediately passed to the application as it is typed. The keyboard is always in cooked state in command mode. It is also cooked in converse mode on an AX25, FTP or NET/ROM session. In a Telnet session it depends on whether the remote end has issued (and the local end has accepted) the Telnet WILL ECHO option (see the echo command). On the IBM-PC, the user may escape back to command mode by hit- ting the F10 key. On other systems, the user must enter the escape character, which is by default control-] (hex 1d, ASCII GS). (Note that this is distinct from the ASCII character of the same name). The escape character can be changed (see the escape command). In the IBM PC version, each session (including the command "ses- sion") has its own screen. When a new session is created, the command display is saved in memory and the screen is cleared. When the command escape key (usually F10) is hit, the current session screen is saved and the command screen is restored. When a session is resumed, its screen is restored exactly as it appeared when it was last current. 3. Commands This chapter describes the commands recognized in command mode, or within a startup file such as autoexec.net. These are given in the following notation: command command literal_parameter command subcommand <parameter> command [<optional_parameter>] command a | b Many commands take subcommands or parameters, which may be optional or required. In general, if a required subcommand or parameter is omitted, an error message will summarize the avail- able subcommands or required parameters. (Giving a '?' in place of the subcommand will also generate the message. This is useful when the command word alone is a valid command.) If a command takes an optional value parameter, issuing the command without the parameter generally displays the current value of the vari- able. (Exceptions to this rule are noted in the individual com- mand descriptions.) Two or more parameters separated by vertical bar(s) denote a choice between the specified values. Optional parameters are shown enclosed in [brackets], and a parameter enclosed in <angle brackets> should be replaced with an actual value or string. For June 7, 1991 - 4 - example, the notation <hostid> denotes an actual host or gateway, which may be specified in one of two ways: as a numeric IP address in dotted decimal notation (eg. 44.0.0.1), or as a sym- bolic name listed in the file domain.txt. All commands and many subcommands may be abbreviated. You only need type enough of a command's name to distinguish it from oth- ers that begin with the same series of letters. Parameters, how- ever, must be typed in full. Certain FTP subcommands (eg. put, get, dir, etc) are recognized only in converse mode with the appropriate FTP session; they are not recognized in command mode. (See the FTP Subcommands chapter.) Note that certain commands may have been configured out of a given copy of net.exe to save disk and memory. If a command has been configured out, it will not appear in the list produced by the "?" command, nor will it be recognized by the command inter- preter. 3.1. <CR> Entering a carriage return (empty line) while in command mode