RE: For Scott Klement: Re: get host IP address

When you say that the router is running dhcp, do you mean that the
router is running NAT? If you asked the user what their IP address is,
would they say something like 192.168.1.5? If that is the case, even if
you could get it, there is nothing you could do with it.

Chris

-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx
[mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of Shannon O'Donnell
Sent: Thursday, September 01, 2005 2:23 PM
To: midrange-l@xxxxxxxxxxxx
Subject: For Scott Klement: Re: get host IP address

Hey Scott,

i'm guessing that the answer to this is "Of course  not!"... But, is
there any way to retrieve/know the ip adress of a client workstation
that is behind a router which uses dhcp?

I have a situation where calling qdcrevd does not work to return the
actual pc ip address because the workstation is communicating with the
iseries first through a local router which has dhcp and its own firewall
and then across the internet to the iseries.

-----Original Message-----
    From: "Scott Klement"<midrange-l@xxxxxxxxxxxxxxxx>
    Sent: 9/1/05 12:06:43 PM
    To: "Midrange Systems Technical Discussion"<midrange-l@xxxxxxxxxxxx>
    Subject: Re: get host IP address
    
    > I've gone through the archives and it seems that the recommended
way to get
    > local host IP address is via gethostbyname function call.
    
    Not this again!
    
    gethostbyname() looks up an IP address for an internet domain name.
It 
    searches your local "HOSTS TABLE" and it also searches DNS.  Whether
or 
    not this will actually give you your "local host IP address" is
entirely 
    dependent on whether you have a domain name that can be looked up to
find 
    it in DNS.
    
    It's not a smart way to approach the problem, and unless you're in
control 
    of how DNS is set up. you shouldn't expect it to work.
    
    Furthermore, you don't assign an IP address to a host. You assign an
IP 
    address to a network interface.  Sometimes, you even assign more
than one 
    IP address to a network interface.
    
    Consequently, the "right way" to find out the IP address really
depends on 
    what you want to use it for.
    
    Consider the scenario of an iSeries system (actually, this is true
for all 
    computers, not just the iSeries) who is connected to two different 
    ethernet LANs, plus a dial-up to the internet.  There are a minimum
of 4 
    different IP addresses assigned to this iSeries:
    
    
    127.0.0,1 -- this IP address is always present, and it's used when
one
                  program on the system wants to communicate with
another one
                  on the same system.
    
    
    One is associated with the dial-up (Point to Point) interface for
talking
    to the internet, and will most likely change with each connection.
    
    One is associated with each of the LAN interfaces.
    
    
    So now you have 4 IP addresses?  which one is the "Right one"?
    
    Well, if you're talking to another program on the same system (such
as 
    using FTP to move a file from one place to another, or to print a
stream 
    file from a batch job) it's safe to hardcode 127.0.0.1.  The nice
thing 
    about this interface is that it's very fast, very secure (since it's
only 
    accessible from within the same system) and it'll always be there
with the 
    same address on every computer that your software gets installed on.
    
    If you're trying to "talk-back" to a 5250 client that has connected,

    you'll need to find out which network interface they've come in
through. 
    The QDCRDEVD API is able to tell you the IP address of the "local"
network 
    interface that they used.
    
    If you're writing a program that communicates over a TCP/IP network,
the 
    getsockname() API can be used to find out the IP address of the
interface 
    that the system routed your request through.
    
    If you're simplying trying to look this up for the purposes of
printing it 
    on a report or keeping track of it for auditing purposes or
something like 
    that, you'll want to keep track of ALL of the IP addresses on the
system.
    
    I wrote a demonstration program that lists all of the IP addresses
on the 
    system (though, you could modify it to only list a particular
network 
    interface if you wanted to) that you can download from the following
link:
    
 
http://www.iseriesnetwork.com/noderesources/code/clubtechcode/DspIpIfc.z
ip
    
    
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