ka9qnos.txt

嵌入式TCP/IP协议栈。 C 源码

             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>]





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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



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                           - 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



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                           - 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
puts  you  in converse mode with the current session. If there is
no current session, net.exe remains in command mode.

3.2.  !

An alias for the shell command.

3.3.  #

Commands starting with the hash mark (#)  are  ignored.  This  is
mainly useful for comments in the autoexec.net file.

3.4.  abort [<session #>]

Abort a FTP get, put or dir  operation  in  progress.  If  issued
without  an  argument, the current session is aborted. (This com-
mand works only on FTP sessions.) When receiving  a  file,  abort
simply  resets the data connection; the next incoming data packet
will generate a TCP RST (reset)  response  to  clear  the  remote
server.   When  sending  a  file, abort sends a premature end-of-
file. Note that in both cases abort will leave a partial copy  of
the  file on the destination machine, which must be removed manu-
ally if it is unwanted.

3.5.  arp

Display the  Address  Resolution  Protocol  table  that  maps  IP
addresses to their subnet (link) addresses on subnetworks capable
of broadcasting.  For each IP address entry the subnet type  (eg.
Ethernet, AX.25), subnet address and time to expiration is shown.



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                           - 5 -


If the link address  is  currently  unknown,  the  number  of  IP
datagrams awaiting resolution is also shown.

3.5.1.  arp add <hostid> ethernet |  ax25  <ethernet  address>  |
<ax25_address>

Add a permanent entry to the table. It will not time out as  will
an  automatically-created entry, but must be removed with the arp
drop command.

3.5.2.  arp publish <hostid> ethernet | ax25 <ethernet address> |
<ax25_address>

This command is similar to the arp add command, but  system  will
also respond to any ARP request it sees on the network that seeks
the specified address.  Use this feature with great care.

3.5.3.  arp drop <hostid> ax25 | ethernet

Remove the specified entry from the ARP table.

3.5.4.  arp flush

Drop all automatically-created entries in  the  ARP  table.  Per-
manent entries are not affected.

3.6.  asystat

Display statistics on attached asynchronous communications inter-
faces  (8250  or  16550A), if any. The display for each port con-
sists of three lines. The first line gives the port label and the
configuration  flags; these indicate whether the port is a 16550A
chip, the trigger character if any, whether CTS flow  control  is
enabled,  whether  RLSD (carrier detect) line control is enabled,
and the speed in bits per second.  (Receiving the trigger charac-
ter causes the driver to signal upper layer software that data is
ready; it is automatically set to the appropriate frame end char-
acter for SLIP, PPP and NRS lines.)

The second line of the status display shows receiver  (RX)  event
counts:  the total number of receive interrupts, received charac-
ters, receiver overruns (lost characters) and the  receiver  high
water mark.  The high water mark is the maximum number of charac-
ters ever read from the device during a single interrupt. This is
useful  for  monitoring  system  interrupt  latency margins as it
shows how close the port hardware has come to overflowing due  to
the  inability  of  the CPU to respond to a receiver interrupt in
time. 8250 chips have no FIFO, so the high water mark  cannot  go
higher  than  2  before overruns occur. The 16550A chip, however,
has a 16-byte receive FIFO which the software programs to  inter-
rupt  the  CPU when the FIFO is one-quarter full.  The high water
mark should typically be 4 or 5 when a  16550A  is  used;  higher
values  indicate  that  the  CPU  has  at least once been slow to
respond to a receiver interrupt.



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                           - 6 -


When the 16550A is  used,  a  count  of  FIFO  timeouts  is  also
displayed  on  the  RX status line. These are generated automati-
cally by the 16550A when three character  intervals  go  by  with
more  than  0  but  less than 4 characters in the FIFO. Since the
characters that make up a SLIP or NRS frame are normally sent  at
full  line speed, this count will usually be a lower bound on the
number of frames received on the port, as only the last  fragment
of a frame generally results in a timeout (and then only when the
frame is not a multiple of 4 bytes long.)

Finally, the software fifo  overruns  and  high  water  mark  are
displayed.  These indicate whether the <bufsize> parameter on the
attach command needs to be  adjusted  (see  the  Attach  Commands
chapter).

The third line shows transmit (TX) statistics, including a  total
count  of transmit interrupts, transmitted characters, the length
of the transmit queue in bytes, the number of status  interrupts,
and the number of THRE timeouts.  The status interrupt count will
be zero unless CTS flow control or RLSD  line  control  has  been
enabled.   The  THRE  timeout  is a stopgap measure to catch lost
transmit interrupts, which seem to happen when there is a lot  of
activity (ideally, this will be zero).

3.7.  attach <hw type> ...

Configure  and  attach  a  hardware  interface  to  the   system.
Detailed  instructions for each driver are in the Attach Commands
chapter.  An easy way to  obtain  a  summary  of  the  parameters
required  for a given device is to issue a partial attach command
(eg. attach packet).  This produces a usage  message  giving  the
complete command format.

3.8.  ax25 ...

These commands are used to control the AX.25 amateur  radio  link
level protocol.

3.8.1.  ax25 blimit [<count>]

Display or set the AX25 retransmission  backoff  limit.  Normally
each successive AX25 retransmission is delayed by twice the value
of the previous interval; this is called binary exponential back-
off.   When  the backoff reaches the blimit setting it is held at
that value, which defaults to 30.  To prevent the possibility  of
"congestive  collapse"  on a loaded channel, blimit should be set
at least as high as the number of stations sharing  the  channel.
Note  that this is applicable only on actual AX25 connections; UI
frames will never be retransmitted by the AX25 layer.

3.8.2.  ax25 dest

Display the AX25 destination monitoring database.  Each  callsign
seen in the destination field of an AX25 frame is displayed (most



                        June 7, 1991