Re: multiple loopback devices?

[Karl Hammar <karl@kalle.csb.ki.se>]

The reason for the "invalid connection" message is probably due
to how nc handles incoming connections.

Doing

  nc -l -p 1000 127.0.0.1

makes it bind to port 1000 on any attached interface, and it only
accepts connections from source address 127.0.0.1.

It will not bind to port 127.0.0.1 only as one could assume.
From /usr/doc/netcat/README page ~188:

  Listen mode will cause netcat to wait for an inbound connection, and then the
  same data transfer happens.  Thus, you can do "nc -l -p 1234 < filename" and
  when someone else connects to your port 1234, the file is sent to them whether
  they wanted it or not.  Listen mode is generally used along with a local port
  argument -- this is required for UDP mode, while TCP mode can have the system
  assign one and tell you what it is if -v is turned on.  If you specify a target
  host and optional port in listen mode, netcat will accept an inbound connection
  only from that host and if you specify one, only from that foreign source port.
  In verbose mode you'll be informed about the inbound connection, including what
  address and port it came from, and since listening on "any" applies to several
  possibilities, which address it came *to* on your end.  If the system supports
  IP socket options, netcat will attempt to retrieve any such options from an
  inbound connection and print them out in hex.

Doing a:

  $ netstat -tln
  Active Internet connections (only servers)
  Proto Recv-Q Send-Q Local Address           Foreign Address         State      
  tcp        0      0 0.0.0.0:1000            0.0.0.0:*               LISTEN      
  ...

verifies the behaviour.

Regards,
/Karl

-----------------------------------------------------------------------
Karl Hammar                    Aspö Data           karl@kalle.csb.ki.se
Lilla Aspö 2340             +46  173 140 57                    Networks
S-742 94 Östhammar         +46  10 270 26 67                  Computers
Sweden                                                       Consulting
-----------------------------------------------------------------------



From: bronson@rinspin.com (Scott Bronson)
Subject: Re: multiple loopback devices?
Date: Fri, 13 Apr 2001 20:59:43 -0700

> On Thu, Apr 05, 2001 at 12:57:25PM +0200, Karl Hammar wrote:
> > Something like:
> >    # ifconfig lo:1 127.0.0.2
> 
> _Exactly_ like that.  Thanks, Karl!
> 
> One question...
> 
> lo        Link encap:Local Loopback  
>           inet addr:127.0.0.1  Mask:255.255.255.0
>           UP LOOPBACK RUNNING  MTU:3904  Metric:1
>           RX packets:203 errors:0 dropped:0 overruns:0 frame:0
>           TX packets:203 errors:0 dropped:0 overruns:0 carrier:0
>           collisions:0 txqueuelen:0 
>           RX bytes:39674 (38.7 Kb)  TX bytes:39674 (38.7 Kb)
> 
> lo:1      Link encap:Local Loopback  
>           inet addr:127.1.0.1  Mask:255.255.255.0
>           UP LOOPBACK RUNNING  MTU:3904  Metric:1
> 
> The following works:
> 
> (term 1) elle:/home/bronson# nc -l -p 1000 127.1.0.1
> (term 2) bronson@elle:~$ nc 127.1.0.1 1000
> 
> But this error baffles me:
> 
> (term 1) elle:/home/bronson# nc -l -p 1000 127.1.0.1
> (term 2) bronson@elle:~$ nc 127.0.0.1 1000
> (term 1) invalid connection to [127.0.0.1] from (UNKNOWN) [127.0.0.1] 2254
> 
> So, even though they're separate addresses and netmasks, they still
> conflict with each other somehow.  Am I making a silly mistake here?
> Thanks,
> 
> 	- Scott
...