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<h1>Hercules version 3: TCP/IP networking with Hercules</h1>
<p>
This page describes how to set up TCP/IP connectivity between
a Hercules machine and the outside world.
<p>
Since Hercules runs as a user process under the control of a driving
system (usually Linux/x86 or Windows), it does not have direct
access to the driving system's network adapter. This presents a
problem in establishing connectivity between the network and the
TCP/IP stack of an operating system running under Hercules.
<p>
But thanks to a technique originally demonstrated by
Willem Konynenberg, it is possible to establish a virtual
point-to-point link between the TCP/IP stack running under
Hercules and the TCP/IP stack of the driving system. The
driving system is then used as a router to pass IP frames
between the Hercules TCP/IP stack and the rest of the network,
as shown in the following diagram:
<p>
<code><pre>
+--------------------------------+
| Linux/x86 Driving System |
| |
+-------------+ |
| Hercules | +--------+
|-------------| | eth0 |
| OS/390 | TCP/IP ------------------> Network
| TCP/IP | |10.1.2.1|
|-------------| | +--------+
| CTCA | | |
|192.168.200.1| | |
+------|------+ 192.168.200.2 |
| /dev/tun tun0 |
| | | |
| +----------------+ |
| Virtual CTC link |
| |
+--------------------------------+
</pre></code>
<p>
The virtual CTC link is provided by the Universal TUN/TAP driver
developed by Maxim Krasnyansky. This driver creates a tunnel
which appears to Hercules as a character device (/dev/tun0 or
/dev/net/tun) and appears to the driving system as a virtual
network interface (tun0).
The Hercules 3088 driver makes the tun device appear as
a CTCA (Channel to Channel Adapter) to the S/390 operating system
running under Hercules. Each end of the link has its own IP
address which is distinct from the IP address of the driving
system's real network adapter.
<p>
The Universal TUN/TAP driver is currently only available for Linux,
Solaris, and FreeBSD. However, similar functionality may be
obtained on Windows 98/Me/2000/XP systems (but not Windows NT) via the
<a href="http://www.softdevlabs.com/Hercules/ctci-w32-index.html">
CTCI-W32</a> protocol.
<h2>Installing the TUN/TAP Driver (Linux 2.2)</h2>
<p>
The Linux 2.2 kernel does not include the TUN/TAP driver, so you
need to install it using the following procedure:
<ol>
<li>Download the file tun-1.0-1.i386.rpm from
<a href="http://vtun.sourceforge.net/tun">http://vtun.sourceforge.net/tun</a>
<li>Install the driver using the following commands:
<code>
<br>su </code>(enter the root password when prompted)<code>
<br>rpm -ivh tun-1.0-1.i386.rpm
<br>chgrp <em>xxxxx</em> /dev/tun*
</code>(where <em>xxxxx</em> is the group under which
you run Hercules)<code>
<br>chmod g+w /dev/tun*
<br>chmod o-r /dev/tun*
</code>
<li>Edit the file <b><i>/etc/modules.conf</i></b> (it is called
<b><i>/etc/conf.modules</i></b> in some distributions)
and add the following line:
<br><code>alias char-major-90 tun</code>
<br>This causes the TUN/TAP driver to be loaded automatically
when a /dev/tun* device is opened by Hercules.
</ol>
<h2>Installing the TUN/TAP Driver (Linux 2.4)</h2>
<p>
The TUN/TAP driver is delivered as part of the Linux 2.4 kernel, and
if you are using one of the popular Linux distributions you will find
that the TUN/TAP driver is already installed. If not, then you must
rebuild the kernel with the configuration option CONFIG_TUN=m specified.
<p>
Note that the version of TUN/TAP in Linux 2.4 differs from the earlier
version in that it allows access to all TUN interfaces (tun0, tun1, etc)
through a single character device /dev/net/tun, instead of defining
multiple devices /dev/tun0, /dev/tun1, etc.
<p>
The procedure for completing the TUN/TAP setup for Linux 2.4 is shown below.
<ol>
<li>Use these commands to create the TUN device:
<code>
<br>su </code>(enter the root password when prompted)<code>
<br>mkdir /dev/net
<br>mknod /dev/net/tun c 10 200
<br>chgrp <em>xxxxx</em> /dev/net/tun
</code>(where <em>xxxxx</em> is the group under which
you run Hercules)<code>
<br>chmod g+rw /dev/net/tun
<br>chmod o-rw /dev/net/tun
</code>
<li>Edit the file <b><i>/etc/modules.conf</i></b> (it is called
<b><i>/etc/conf.modules</i></b> in some distributions)
and add the following line:
<br><code>alias char-major-10-200 tun</code>
<br>This causes the TUN/TAP driver to be loaded automatically
when the /dev/net/tun device is opened by Hercules.
</ol>
<h2>Installing the TUN/TAP Driver (Linux 2.6)</h2>
<p>
For distributions based on the Linux 2.6 kernel you will probably find that the
TUN/TAP driver is already installed and the /dev/net/tun device is already
defined. If not, then follow the procedure for Linux 2.4 as descibed above.
<p>
You will certainly need to alter the permissions on the /dev/net/tun device
to allow Hercules to open it.
<ul>
<li>Use the following commands to set the necessary permissions:
<code>
<br>su </code>(enter the root password when prompted)<code>
<br>chown root:<em>xxxxx</em> /dev/net/tun
</code>(where <em>xxxxx</em> is the group under which
you run Hercules)<code>
<br>chmod 0660 /dev/net/tun
</code>
<li><i>Additional notes from Greg Smith:</i>
<ol>
<li>I find on my Fedora Core 6 system that I have to add the above 2 commands
to <b><i>/etc/rc.local</i></b>
and update <b><i>/etc/udev/rules.d/50-udev.rules</i></b> replacing
<br><code>KERNEL=="tun", NAME="net/%k"</code>
<br>by
<br><code>KERNEL=="tun", NAME="net/%k", GROUP="<em>xxxxx</em>", MODE="0660"</code>
<li>In the hercules log you should see /dev/net/tun0 opened. I get a couple
of error messages about SIOCDIFADDR and SIOCSIFHWADDR ioctl's failing
but these can be ignored (maybe I'll fix that soon).
</ol>
</ul>
<p>
In Linux 2.6 the file <b><i>/etc/modules.conf</i></b> no longer exists,
instead there is a file called <b><i>/etc/modprobe.conf</i></b>.
TUN/TAP will usually work, however, without any change to the modprobe
configuration.
<h2><a name="hercifc">Configuring the TUN interface</a></h2>
<p>
The tun0 network interface in the driving system must be configured
as a point-to-point link. The design of the TUN/TAP driver does not
allow the interface to be statically configured like a regular network
interface — the tun0 interface does not exist until Hercules
opens the TUN device. For this reason, Hercules provides a special
program called hercifc to configure the tun0 network interface. This
program is launched automatically by Hercules 3088 CTC device
initialization.
<p>
To allow the hercifc program to issue the necessary configuration
commands, you must ensure that hercifc is installed with setuid root
file permissions. When Hercules is built with the
configuration option --enable-setuid-hercifc, <b><i>make install</i></b>
will install hercifc in /usr/local/bin with setuid root permissions.
<i>Note: Unrestricted access to the hercifc program could present a
potential security exposure, so you will want to ensure that hercifc
can be executed only by the group which is authorized to run Hercules.</i>
The following commands alter the file permissions to ensure that only
users in a trusted group can execute hercifc:
<code>
<br>su </code>(enter the root password when prompted)<code>
<br>chgrp <em>xxxxx</em> /usr/local/bin/hercifc
</code>(where <em>xxxxx</em> is the group under which
you run Hercules)<code>
<br>chmod 4750 /usr/local/bin/hercifc
<br>exit
</code>
<h2>Enabling IP forwarding</h2>
<p>
You must ensure that your kernel is enabled for IP forwarding.
Popular Linux distributions usually have a configuration option
to enable IP forwarding or routing:
<ul>
<li>For RedHat, specify <code>net.ipv4.ip_forward=1</code> in
the <i><b>/etc/sysctl.conf</b></i> file.
<li>For SuSE, specify <code>IP_FORWARD="yes"</code> in
the <i><b>/etc/rc.config</b></i> file.
<li>If you cannot find this option in your distribution, the following
command should work on any Linux 2.x kernel:
<br><code>echo "1" > /proc/sys/net/ipv4/ip_forward</code>
</ul>
<h2>Defining a route to Hercules TCP/IP</h2>
<p>
Client systems which connect to TCP/IP applications running in
the Hercules machine need to have a routing entry which defines
the driving system as the gateway into the Hercules system. An
example route definition for a Unix client system is shown below:
<code><pre>
route add 192.168.200.1 gw 10.1.2.1
</pre></code>
<p>
For a Windows client, go to Settings -> Control Panel -> Network ->
Configuration -> TCP/IP -> Properties -> Gateway
and add the driving system's IP address to the list of gateways.
Alternatively, enter a route command such as:
<code><pre>
route add 192.168.200.0 mask 255.255.255.0 10.1.2.1 metric 1
</pre></code>
<p>
If you want to avoid having to update client systems, another way
is to add an appropriate routing entry to your default gateway router.
<h2>Defining the link in Hercules</h2>
<p>
You must define a CTC device pair in the Hercules configuration
file. The second device must bear the same definition as the 1st
instance and be at device number + 1. The 1st device number must be even.
Devices should preferably be grouped (furthermore, it makes the
configuration file easier to read).
<p>
<code>
<br>0E20.2 CTCI 192.168.200.1 192.168.200.2
<br>or
<br>0E20,0E21 CTCI 192.168.200.1 192.168.200.2
<br>or
<br>0E20-0E21 CTCI 192.168.200.1 192.168.200.2
</code>
<p>Check <A HREF="hercconf.html#device_definitions">Device Definition Statement syntax </A> for an explanation of device grouping.
<p>
Two IP addresses must be assigned, one for the driving system's
end of the link, and one for the Hercules end of the link. For
this example I have chosen 192.168.200.1 for the Hercules IP
address, and 192.168.200.2 for the driving system's IP address.
Since this is a point-to-point link, any addresses may be chosen,
provided that the network part of the address (192.168.200 in
this example) does not conflict with any existing network
addresses used in your IP network.
<h2>Configuring the Hercules TCP/IP stack</h2>
<h4>TCP/IP for VSE</h4>
<p>
This is an example of the configuration statements which you need
to include in the IPINIT00.L member of PRD1.BASE:
<pre>
SET IPADDR = 192.168.200.001
SET MASK = 255.255.255.000
DEFINE LINK,ID=CTCE20,TYPE=CTCA,DEV=(E20,E21),MTU=1500
DEFINE ROUTE,ID=LINUX,LINKID=CTCE20,IPADDR=0.0.0.0
</pre>
<p>
The CTC devices should be defined to VSE using the following
statements in the $IPLxxx.PROC procedure in IJSYSRS.SYSLIB:
<pre>
ADD E20:E21,CTCA,EML
</pre>
<h4>TCP/IP for OS/390 or VM/ESA</h4>
<p>
This is an example of the configuration statements which you need
to include in the TCPIP.PROFILE.TCPIP dataset (OS/390), or in the
PROFILE TCPIP file on TCPMAINT 198 (VM):
<pre>
DEVICE CTCDEV1 CTC E20
LINK CTCLINK1 CTC 0 CTCDEV1
HOME 192.168.200.1 CTCLINK1
GATEWAY
; Network First Hop Link Name Size Subnet Mask Subnet Value
192.168.200.2 = CTCLINK1 1500 HOST
DEFAULTNET 192.168.200.2 CTCLINK1 1500 0
START CTCDEV1
</pre>
<p>
For OS/390, the CTC devices need to be defined as device type 3088 in
the IODF. Use the <code>D U,CTC</code> command to find out which 3088
addresses are defined in your IODF.
<p>
For VM, the CTC devices must be attached to the TCPIP virtual machine.
<p>
Because TCP/IP uses long running channel programs, the missing interrupt
handler should be disabled for the CTC devices.
For OS/390, add this statement in PARMLIB member IECIOS00:
<pre>
MIH TIME=00:00,DEV=(E20-E21)
</pre>
<p>
For VM, add this command to the PROFILE EXEC file of OPERATOR 191:
<pre>
'CP SET MITIME 0E20-0E21 OFF'
</pre>
<h4>Linux for S/390</h4>
<p>
This is an example of the network definitions which you need
in a Linux/390 system running under Hercules:
<pre>
ifconfig ctc0 192.168.200.1 pointopoint 192.168.200.2 mtu 1500
route add defaultroute gw 192.168.200.2
</pre>
<p>
Linux/390 will autodetect the CTC devices E20 and E21
at startup and will assign the interface name ctc0.
<h2>What to do if TUN/TAP doesn't work</h2>
<p>
Check the following (thanks to Richard Higson for this checklist):
<ol>
<li>Enter the command <code>ls -l /dev/tun0 /dev/net/tun</code>.
<br>For Linux 2.4, the response should be:
<code>
<br>ls: /dev/tun0: No such file or directory
<br>crw-rw---- 1 root <i>xxxxx</i> 10, 200 Sep 13 07:06 /dev/net/tun
</code>
<br>For Linux 2.2, the response should be:
<code>
<br>crw-rw---- 1 root <i>xxxxx</i> 90, 0 Feb 3 2001 /dev/tun0
<br>ls: /dev/net/tun: No such file or directory
</code>
<br>(<i>xxxxx</i> should be the group under which you run Hercules).
<li><code>ls -l /usr/local/bin/hercifc</code> should show
<code>
<br> -rwsr-x--- 1 root <i>xxxxx</i> 17333 Dec 31 20:55 /usr/local/bin/hercifc
</code>
<br>(<i>xxxxx</i> should be the group under which you run Hercules).
<li>When hercules comes up, and <i>before</i> IPLing your favorite OS, verify
that you have your underlying network stuff up and ready to roar:
<code><pre>
[root]# <b>ifconfig</b>
eth0 Link encap:Ethernet HWaddr 00:12:34:56:78:9A
inet addr:10.1.2.1 Bcast:10.255.255.255 Mask:255.0.0.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
tun0 Link encap:Point-to-Point Protocol
inet addr:192.168.200.2 P-t-P:192.168.200.1 Mask:255.255.255.0
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
[root]# <b>netstat -in</b>
Kernel Interface table
Iface MTU Met RX-OK RX-ERR RX-DRP RX-OVR TX-OK TX-ERR TX-DRP TX-OVR Flg
eth0 1500 0 201 0 0 0 196 0 0 0 BMRU
tun0 1500 0 0 0 0 0 0 0 0 0 MOPRU
[root]# <b>netstat -rn</b>
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
10.0.0.0 0.0.0.0 255.0.0.0 U 40 0 0 eth0
192.168.200.0 0.0.0.0 255.255.255.0 U 40 0 0 tun0
127.0.0.0 0.0.0.0 255.0.0.0 U 40 0 0 lo
0.0.0.0 10.1.0.1 0.0.0.0 UG 40 0 0 eth0</pre></code>
<li><code>`cat /proc/sys/net/ipv4/ip_forward`</code> should show "1".
If it doesn't, your L386 won't forward (==route) packets at all.
<li>Is the TUN/TAP driver loaded?
<ol>
<li>TUN/TAP compiled into the kernel (<code>`make menuconfig`</code>)
look for <code>"CONFIG_TUN=m"</code> in <i><b>/usr/src/linux</i></b>
<li><code>`lsmod`</code> after starting hercules should show
<code>tun 3456 2 (autoclean)</code>
</ol>
<li>Look for
<code>
<br>Dec 14 16:47:19 wie kernel: Universal TUN/TAP device driver 1.3 (C)1999-2000
<br>Maxim Krasnyansky
</code>
<br>in syslog after starting hercules
</ol>
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