[KLUG Programming] tcp connection is alive

[Adam Tauno Williams]

> I'm using netcat in a perl script with an open2 function call.  All is
> working very well with netcat as long as connections are properly
> completed.  All connections were properly completed until I started
> using a 3com ethernet client bridge and the machine the bridge is
> connected to is turned off.  (the whole fin ack connection completion
> thing)

So don't turn it off! :)

> So here is my question.  I would like the vocabulary to start searching
> for how I can modify netcat to perform a syn/ack or some type of
> checking to see if the host at the other end is alive and answering. 
> Then if the connection is not alive, I want netcat to close.

You want to enable some type of "keepalive".  TCP support this.
http://www.itprc.com/tcpipfaq/faq-2.htm#crash-detection

Here is an interesting chunk from a text file I have -

  2.8.  Why does it take so long to detect that the peer died?

  From Andrew Gierth (andrew@erlenstar.demon.co.uk):

  Because by default, no packets are sent on the TCP connection unless
  there is data to send or acknowledge.

  So, if you are simply waiting for data from the peer, there is no way
  to tell if the peer has silently gone away, or just isn't ready to
  send any more data yet. This can be a problem (especially if the peer
  is a PC, and the user just hits the Big Switch...).

  One solution is to use the SO_KEEPALIVE option. This option enables
  periodic probing of the connection to ensure that the peer is still
  present.  BE WARNED: the default timeout for this option is AT LEAST 2
  HOURS.  This timeout can often be altered (in a system-dependent
  fashion) but not normally on a per-connection basis (AFAIK).

  RFC1122 specifies that this timeout (if it exists) must be
  configurable.  On the majority of Unix variants, this configuration
  may only be done globally, affecting all TCP connections which have
  keepalive enabled. The method of changing the value, moreover, is
  often difficult and/or poorly documented, and in any case is different
  for just about every version in existence.

  If you must change the value, look for something resembling
  tcp_keepidle in your kernel configuration or network options
  configuration.

  If you're sending to the peer, though, you have some better
  guarantees; since sending data implies receiving ACKs from the peer,
  then you will know after the retransmit timeout whether the peer is
  still alive. But the retransmit timeout is designed to allow for
  various contingencies, with the intention that TCP connections are not
  dropped simply as a result of minor network upsets. So you should
  still expect a delay of several minutes before getting notification of
  the failure.

  The approach taken by most application protocols currently in use on
  the Internet (e.g. FTP, SMTP etc.) is to implement read timeouts on
  the server end; the server simply gives up on the client if no
  requests are received in a given time period (often of the order of 15
  minutes). Protocols where the connection is maintained even if idle
  for long periods have two choices:

  1. use SO_KEEPALIVE

  2. use a higher-level keepalive mechanism (such as sending a null
     request to the server every so often).

  2.9.  What are the pros/cons of select(), non-blocking I/O and SIGIO?

  Using non-blocking I/O means that you have to poll sockets to see if
  there is data to be read from them.  Polling should usually be avoided
  since it uses more CPU time than other techniques.

  Using SIGIO allows your application to do what it does and have the
  operating system tell it (with a signal) that there is data waiting
  for it on a socket.  The only drawback to this soltion is that it can
  be confusing, and if you are dealing with multiple sockets you will
  have to do a select() anyway to find out which one(s) is ready to be
  read.

  Using select() is great if your application has to accept data from
  more than one socket at a time since it will block until any one of a
  number of sockets is ready with data.  One other advantage to select()
  is that you can set a time-out value after which control will be
  returned to you whether any of the sockets have data for you or not.