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.zip
    
    
    -- 
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