sercd: RFC 2217 Serial Communications Daemon
============================================

Introduction
------------

sercd is a serial port redirector that is compliant with the RFC 2217
"Telnet Com Port Control Option" protocol. This protocol lets you
share a serial port through the network. RFC 2217 was orginally
implemented in Cisco Terminal Servers in IOS version 11.x.

RFC 2217 clients include:

 * C-Kermit 8.0 for Unix, VMS, and QNX
   http://www.columbia.edu/kermit/ckermit.html

 * DialOut/IP V2, a commercial Windows 95/98/NT client. 
   www.tacticalsoftware.com

 * cyclades-serial-client
   http://www.lysator.liu.se/~astrand/projects/cyclades-serial-client/

 * HW Virtual Serial Port (for Windows 2000 and XP)
   http://www.hw-group.com/products/hw_vsp/index_en.html

   Note: One person has reported success with this client, but others
   have reported that client does NOT work correctly with sercd; nor
   with other RFC2217 servers. As far as I can tell, the problem lies
   in HW VSP; not in sercd. 

* Java NVTCom library
  http://www.viara.cn/en/nvtcom.htm

* Lantronix Com Port Redirector
  http://www.lantronix.com/device-networking/utilities-tools/com-port-redirector.html

* Fabulatech Serial Port Redirector
  http://www.serial-port-redirector.com



Other known RFC2217 servers:

* Extended socat
  http://www.dest-unreach.org/socat/contrib/socat-rfc2217.html 
  (also a client, apparently)

* Java TS
  http://www.viara.cn/en/jts.htm

* ser2net
  http://ser2net.sourceforge.net/

* VPortPro
  http://www.vportpro.com


Compilation
-----------

Do "make". 


Command line parameters
-----------------------

sercd takes three mandatory parameters and two optional parameters, 
one at the beginning and one at the end of the command line.

The mandatory parameters are:

Log level: 0 to 7, 0 being no log and 7 being debug log. sercd uses
the standard syslog facility for logging. Look at the syslog(3) man
page for further information on the syslog facility and log levels.

Device: the device complete path. Something like /dev/ttyS0 or /dev/modem.

Lock file: the file to use for inter-application locking. This must
match the one used by other applications, usually something like
/var/lock/LCK..ttyS0 or /var/lock/LCK..modem for the devices considered
above. sercd supports only HDB style (i.e. ASCII) file locking.

The optional parameters are:

- At the beginning of the command line:

A -i option enables compatibility with the Cisco IOS wich has a small
bug in the handling of the inbound flow control.

- At the end of the command line:

Poll interval: sercd checks for line state changes (DSR/CTS/DCD/RNG)
on every single char sent on the serial line. To prevent losing state
change when there's no traffic on the serial port, sercd also polls
the line state when there's no traffic on the line for more than the
poll interval. The value is in milliseconds and the default is 100
milliseconds. Setting it to 0 disables the polling feature. This is
also the maximum time data is left in the output buffer before sending
it, even if the buffer isn't completely full, so the polling interval
globally sets the maximum latency of any sercd action.


Installation
-------------

Put the executable in a suitable directory (/usr/sbin or
/usr/local/sbin).  sercd is designed to run under inetd control. The
line in the inetd configuration file (usually /etc/inetd.conf) should
look like this:

sredir          stream  tcp     nowait  root    /usr/sbin/tcpd /usr/sbin/sercd -i 5 /dev/modem /var/lock/LCK..modem

Of course, "sredir" should be defined in the services file (usually
/etc/services, usual values start at port 7000) and "/dev/modem"
should be substituted by the appropriate device. The lock and the
device name must match for correct interlocking with other
programs. The tcpd program is the usual tcp wrapper that's not needed
for correct operation but is useful for security purposes. Don't
forget to send a SIGHUP to the inetd daemon in order to force reload
of the configuration file. The initial -i paramater is optional and
enables compatibility with the Cisco IOS.


Bugs
----

Report bugs using Bugzilla. A link is available from the sercd home page. 


Outstanding issues
------------------

The RFC 2217 protocol is a rather complete protocol that provides a lot
of features. Most of them are implemented in sercd but some seldom
used options are missing:

 * does not properly check implement BREAK handling.  Need to figure
   out how to turn a BREAK on and then off based upon receipt of 
   COM-PORT Subnegotiations

 * does not properly use select to handle input, output and
   errors on all devices.

 * Lack of login processing

 * Lack of Telnet START_TLS to protect the data stream

 * Lack of Telnet AUTHENTICATION

 * LineState processing is not implemented

 * The code probably won't compile on most versions of Unix due to the
   highly platform dependent nature of the serial apis.
   The program has been tested only under Linux 2.0.36, gcc 2.7.2.1,
   libc5 and RedHat 5.2/7.1 but should work on any POSIX compliant 
   operating system.

 * Use of polling to notify change on the serial control lines is not
   desireable.  Anyone know of an event driven mechanism for discovering
   modem and line signal changes?



Implementation Notes
--------------------

 * Setting the port to 1.5 stop bits isn't supported. When requested,
   1 stop bit is set and returned as the response value.

 * Setting the parity to mark or space isn't supported.  When requested,
   Parity is set to None and returned as the response value.

 * DTR, DSR and DCD flow control isn't supported.  When requested, 
   flow control is disabled and "no flow control" is returned as the
   response value.

 * RFC 2217 supports the ability for the Input and Output Parity and
   Flow control settings to be independent of each other.  This is not
   supported by the majority of serial port UARTs available on the market
   and is not supported by sercd.  sercd ignores all requests to set
   the Input Parity and Flow control.  The Output values are used for 
   both input and output as specified in the RFC.

 * DialOut/IP V2 version 2.0.0 has a known bug that prevent it from 
   running at 115200 baud. If you ask for 115200 baud it sets the serial
   line at 9600 baud. It also has some other small bugs that cause it to
   crash or fail under some rare circumstances. Use Version 2.0.2 or 
   higher to avoid this problem.


Credits
-------

sercd is based on sredird, which got help and support from several
people and organizations:


Thanks to David Yon and Mike Krueger of Tactical Software for their help
in the sredird developing process. Thanks also to Tactical Software for
its wonderful DialOut/IP package and for choosing an open standard for it.
Tactical Software provides also some other useful communication tools
and an RFC 2217 compliant server for the Windows environment. Browse
http://www.tacticalsoftware.com for more information.


Implementation of the Telnet State Machines, corrections to RFC 2217,
testing against various clients and comparison to hardware implementations
in Cisco IOS software was performed by the Kermit Project at Columbia 
University.  The Kermit Project provides the only known client implementation
of RFC 2217 for Unix in its C-Kermit and Kermit 95 Telnet clients.  See
http://www.columbia.edu/kermit/ for further details.


Resources
---------

* The freshmet.net listing:

  http://freshmeat.net/projects/sercd


* The sercd homepage:
  
  http://www.lysator.liu.se/~astrand/projects/sercd/


--
Peter Åstrand <[email protected]>