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  3. May 2016

RE: Multiple TCPIP Interfaces... how does the system choose which one to use?

Rob, you should look at TCP/IP routes (CFGTCP option 2/ADDTCPRTE),
specifically "Preferred binding interface (BINDIFC)." From what I
understand, when you open a socket to an IP address, the system will look
through the route list to determine which gateway to send it through (and
if there's a preferred interface it will use that interface for the
route).



"MIDRANGE-L" <midrange-l-bounces@xxxxxxxxxxxx> wrote on 05/09/2016
12:53:11 PM:

From: Rob Berendt <rob@xxxxxxxxx>
To: Midrange Systems Technical Discussion <midrange-l@xxxxxxxxxxxx>
Date: 05/09/2016 12:53 PM
Subject: RE: Multiple TCPIP Interfaces... how does the system choose
which one to use?
Sent by: "MIDRANGE-L" <midrange-l-bounces@xxxxxxxxxxxx>

Keep in mind that IBM i can be both a client and a server.
When using it as a client many applications do not allow you to specify
which IP address to bind to. For example, if I am on my IBM i command
line and I type in
FTP "delivery01-bld.dhe.ibm.com"
it could go out on any of the interfaces listed at CFGTCP, option 1. And

if you have a few, like this sample from one of our lpars:
Internet Subnet Line Line
Address Mask Description Type
10.17.6.34 255.255.254.0 LANLIN *ELAN
10.17.6.35 255.255.254.0 LANLIN *ELAN
10.17.6.36 255.255.254.0 LANLIN *ELAN
10.17.6.37 255.255.254.0 LANLIN *ELAN
10.17.6.38 255.255.254.0 LANLIN *ELAN
10.17.6.39 255.255.254.0 LANLIN *ELAN
10.17.6.40 255.255.254.0 LANLIN *ELAN
10.17.6.41 255.255.254.0 LANLIN *ELAN
10.17.6.48 255.255.254.0 LANLIN *ELAN
127.0.0.1 255.0.0.0 *LOOPBACK *NONE

That can be problematic for some network conditions.

Some applications, such as http and smtp allow you to specify "bind
specific". But most of these are server specific, and even some server
specific ones do not allow you to specify bind specific.

Now, if I execute that IBM i ftp command,
and then from another session
do a netstat *cnn
I will see:
Connection identification:
Remote host name . . . . . . . . . . . . . . : dispby-101.boulder.ibm


Remote internet address . . . . . . . . . . : 170.225.15.101
Remote port . . . . . . . . . . . . . . . . : ftp-control
Local host name . . . . . . . . . . . . . . . : GDSSALES.DEKKO.COM
Local internet address . . . . . . . . . . : 10.17.6.37
Local port . . . . . . . . . . . . . . . . : 52481
Associated user profile . . . . . . . . . . . : ROB

Why did it go out my '37 address? Random selection?

We were having a real issue when trying to ftp new barcode setups from
IBM
i to certain printers. Our network guy was trying to be very specific
who
could ftp to a printer. This stuff caused us no end of grief.

We try to be nice to him. He hit it big in the dot com bubble and works

as a hobby. His other hobbies include collecting .50 caliber automatic
weapons. Carries a flip phone instead of a smart phone so the man can't

track him as much.

Rob Berendt
--
IBM Certified System Administrator - IBM i 6.1
Group Dekko
Dept 1600
Mail to: 2505 Dekko Drive
Garrett, IN 46738
Ship to: Dock 108
6928N 400E
Kendallville, IN 46755
http://www.dekko.com





From: "Mark Murphy/STAR BASE Consulting Inc."
<mmurphy@xxxxxxxxxxxxxxx>
To: Midrange Systems Technical Discussion <midrange-l@xxxxxxxxxxxx>
Date: 05/09/2016 01:30 PM
Subject: RE: Multiple TCPIP Interfaces... how does the system
choose which one to use?
Sent by: "MIDRANGE-L" <midrange-l-bounces@xxxxxxxxxxxx>



Does the server choose, or does the client have to choose. What do you
mean by the client does not specifically bind to an IP? Is the server
pushing to the client or is the client requesting via a broadcast?
Otherwise, the client has to send the request to a specific IP address
(and port for that matter). Am I missing something about how IP works?

Mark Murphy
STAR BASE Consulting, Inc.
mmurphy@xxxxxxxxxxxxxxx


-----"Steinmetz, Paul" <PSteinmetz@xxxxxxxxxx> wrote: -----
To: "'Midrange Systems Technical Discussion'" <midrange-l@xxxxxxxxxxxx>
From: "Steinmetz, Paul" <PSteinmetz@xxxxxxxxxx>
Date: 05/05/2016 12:26PM
Subject: RE: Multiple TCPIP Interfaces... how does the system choose
which
one to use?


Brad,

We have two interfaces and had a similar issue.
We created static routes to resolve this issue.

2. Work with TCP/IP routes

Paul

-----Original Message-----
From: MIDRANGE-L [mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of
Bradley Stone
Sent: Thursday, May 05, 2016 12:12 PM
To: Midrange Systems Technical Discussion
Subject: Multiple TCPIP Interfaces... how does the system choose which
one
to use?

Assume you have 2 TCPIP interfaces set up on your IBM i:

192.168.1.10
and
192.168.1.11

When a client application doesn't specifically bind to a specific TCPIP
interface, how does the system choose which one is used?

In this case a client said for years it would use the right one, but one

day it just "decided" to switch to the second interface causing problems

since only the first interface was set up by a firewall to allow
outgoing
requests.

I have since explained to them how to set our software to specifically
bind to the correct IP, but this question got me wondering how the
system
chooses which interface when there is no specific binding going on.

Thanks!

Brad
www.bvstools.com
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