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<!--     syntheth.sgml                                               -->
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<!--     Synthetic target ethernet support package.                  -->
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<!-- Author(s):   bartv                                              -->
<!-- Contact(s):  bartv                                              -->
<!-- Date:        2002/08/20                                         -->
<!-- Version:     0.01                                               -->
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<part id="devs-eth-synth-ecosynth-ref">
<!-- reference id="devs-eth-synth-ecosynth-ref" -->
  <title>Synthetic Target Ethernet Driver</title>

<refentry id="devs-eth-synth-ecosynth">
  <refmeta>
    <refentrytitle>Synthetic Target Ethernet Driver</refentrytitle>
  </refmeta>
  <refnamediv>
    <refname>Synthetic Target Ethernet Support</refname>
    <refpurpose>Allow synthetic target applications to perform ethernet I/O</refpurpose> 
  </refnamediv>

  <refsect1><title>Overview</title>
    <para>
The synthetic target ethernet package can provide up to four network
devices, <varname>eth0</varname> to <varname>eth3</varname>. These can
be used directly by the eCos application or, more commonly, by a
TCP/IP stack that is linked with the eCos application. Each eCos
device can be mapped on to a real Linux network device. For example,
if the Linux PC has two ethernet cards and <varname>eth1</varname> is
not currently being used by Linux itself, then one of the eCos devices
can be mapped on to this Linux device. Alternatively, it is possible
to map some or all of the eCos devices on to the ethertap support
provided by the Linux kernel.
    </para>
    <para>
The ethernet package depends on the I/O auxiliary provided by the
synthetic target architectural HAL package. During initialization the
eCos application will attempt to instantiate the desired devices, by
sending a request to the auxiliary. This will load a Tcl script
<filename>ethernet.tcl</filename> that is responsible for handling the
instantiation request and subsequent I/O operations, for example
transmitting an ethernet packet. However, some of the low-level I/O
operations cannot conveniently be done by a Tcl script so
<filename>ethernet.tcl</filename> will actually run a separate program
<command>rawether</command> to interact with the Linux network device. 
    </para>
    <informalfigure PgWide=1>
      <mediaobject>
        <imageobject>
          <imagedata fileref="overview.png" Scalefit=1 Align="Center">
        </imageobject>
      </mediaobject>
    </informalfigure>
    <para>
On the target-side there are configuration options to control which
network devices should be present. For many applications a single
device will be sufficient, but if the final eCos application is
something like a network bridge then the package can support multiple
devices. On the host-side each eCos network device needs to be mapped
on to a Linux one, either a real ethernet device or an ethertap
device. This is handled by an entry in the target definition file:
    </para>
    <programlisting>
synth_device ethernet {
    eth0 real eth1
    eth1 ethertap tap3 00:01:02:03:FE:05
    &hellip;
}
</programlisting>
    <para>
The ethernet package also comes with support for packet logging,
and provides various facilities for use by user Tcl scripts.
    </para>
  </refsect1>

  <refsect1 id="devs-eth-ecosynth-install"><title>Installation</title>
    <para>
Before a synthetic target eCos application can access ethernet devices
it is necessary to build and install host-side support. The relevant
code resides in the <filename class="directory">host</filename>
subdirectory of the synthetic target ethernet package, and building it
involves the standard <command>configure</command>,
<command>make</command> and <command>make install</command> steps.
The build involves a new executable <command>rawether</command> which
must be able to access a raw Linux network device. This is achieved by
installing it suid root, so the <command>make install</command> step
has to be run with superuser privileges.
    </para>
    <caution><para>
Installing <command>rawether</command> suid root introduces a
potential security problem. Although normally
<command>rawether</command> is executed only by the I/O auxiliary,
theoretically it can be run by any program. Effectively it gives any
user the ability to monitor all ethernet traffic and to inject
arbitrary packets into the network. Also, as with any suid root
programs there may be as yet undiscovered exploits. Users and system
administrators should consider the risks before running <command>make
install</command>. 
    </para></caution>
    <para>
There are two main ways of building the host-side software. It is
possible to build both the generic host-side software and all
package-specific host-side software, including the ethernet support,
in a single build tree. This involves using the
<command>configure</command> script at the toplevel of the eCos
repository. For more information on this, see the
<filename>README.host</filename> file at the top of the repository.
Note that if you have an existing build tree which does not include
the synthetic target ethernet support then it will be necessary to
rerun the toplevel configure script: the search for appropriate
packages happens at configure time.
    </para>
    <para>
The alternative is to build just the host-side for this package.
This requires a separate build directory, building directly in the
source tree is disallowed. The <command>configure</command> options
are much the same as for a build from the toplevel, and the
<filename>README.host</filename> file can be consulted for more
details. It is essential that the ethernet support be configured with
the same <option>--prefix</option> option as other eCos host-side
software, especially the I/O auxiliary provided by the architectural
synthetic target HAL package, otherwise the I/O auxiliary will be
unable to locate the ethernet support.
    </para>
  </refsect1>

  <refsect1 id="devs-eth-ecosynth-options"><title>Target-side Configuration Options</title>
    <para>
The target-side code can be configured to support up to four ethernet
devices, <varname>eth0</varname> to <varname>eth3</varname>. By
default <varname>eth0</varname> is enabled if the configuration
includes a TCP/IP stack, otherwise it is disabled. The other three
devices are always disabled by default. If any of the devices are
enabled then there will also be the usual configuration options
related to building this package. Other options related to network
devices, for example whether or not to use DHCP, are provided by
the generic network device package.
    </para>
  </refsect1>

  <refsect1 id="devs-eth-ecosynth-real"><title>Real Ethernet</title>
    <para>
One obvious way of providing a synthetic target eCos application with
ethernet I/O is to use a real ethernet device in the PC: transmitted
packets go out on a real network, and packets on the network addressed
to the right MAC address are passed on to eCos. This way synthetic
target networking behaves just like networking on a real target with
ethernet hardware. For example, if there is a DHCP server anywhere on
the network then eCos will be able to contact it during networking
startup and get hold of IP address information.
    </para>
    <para>
Configuring the ethernet support to use a real ethernet device
requires a simple entry in the target definition file:
    </para>
    <programlisting>
synth_device ethernet {
    &lt;eCos device&gt; real &lt;linux device&gt;
    &hellip;
}
</programlisting>
    <para>
For example, to map the eCos network device <varname>eth0</varname> to
the Linux device <varname>eth1</varname>:
    </para>
    <programlisting>
synth_device ethernet {
    eth0 real eth1
    &hellip;
}
</programlisting>
    <para>
It is not possible for an ethernet device to be shared by both the
eCos TCP/IP stack and the Linux one: there would be no simple way to
work out which stack incoming packets are intended for. In theory
it might be possible to do some demultiplexing using distinct IP
addresses, but it would be impossible to support some functionality
such as DHCP. Therefore the <command>rawether</command> program will
refuse to access any ethernet device already in use. On a typical
Linux system <varname>eth0</varname> will be used for Linux
networking, and the PC will have to be equipped with additional
ethernet devices for use by eCos.
    </para>
    <para>
The <command>rawether</command> program will access the hardware via
the appropriate Linux device driver, so it is important that the
system is set up such that the relevant module will be automatically
loaded or is already loaded. The details of this will depend on the
installed distribution and version, but typically it will involve an
entry in <filename>/etc/modules.conf</filename>.
    </para>
  </refsect1>

  <refsect1 id="devs-eth-ecosynth-ethertap"><title>Ethertap</title>
    <para>
The Linux kernel's ethertap facility provides a virtual network
interface. A Linux application, for example the
<command>rawether</command> program, can open a special character
device <filename>/dev/net/tun</filename>, perform various
<function>ioctl</function> calls, and then <filename>write</filename>
and <filename>read</filename> ethernet packets. When the device is
opened the Linux kernel automatically creates a new network interface,
for example <varname>tap0</varname>. The Linux TCP/IP stack can be
made to use this network interface like any other interface, receiving
and transmitting ethernet packets. The net effect is a virtual network
connecting just the Linux and eCos TCP/IP stacks, with no other nodes
attached. By default all traffic remains inside this virtual network
and is never forwarded to a real network.
    </para>
    <para>
Support for the ethertap facility may or may not be provided
automatically, depending on your Linux distribution and version. If
your system does not have a device <filename>/dev/net/tun</filename>
or a module <filename>tun.o</filename> then the appropriate kernel
documentation should be consulted, for example
<filename>/usr/src/linux-2.4/Documentation/networking/tuntap.txt</filename>.
If you are using an old Linux kernel then the ethertap functionality
may be missing completely. When the <command>rawether</command>
program is configured and built, the <command>configure</command>
script will check for a file <filename
class="headerfile">/usr/include/linux/if_tun.h</filename>. If that
file is missing then <command>rawether</command> will be built without
ethertap functionality, and only real ethernet interfaces will be
supported.
    </para>
    <para>
The target definition file is used to map eCos network devices on to
ethertap devices. The simplest usage is:
    </para>
    <programlisting>
synth_device ethernet {
    eth0 ethertap
    &hellip;
}
</programlisting>
    <para>
The Linux kernel will automatically allocate the next available tap
network interface. Usually this will be <varname>tap0</varname> but if
other software is using the ethertap facility, for example to
implement a VPN, then a different number may be allocated. Usually it
will be better to specify the particular tap device that should be
used for each eCos device, for example:
    </para>
    <programlisting>
synth_device ethernet {
    eth0 ethertap tap3
    eth1 ethertap tap4
    &hellip;
}
</programlisting>
    <para>
The user now knows exactly which eCos device is mapped onto which
Linux device, avoiding much potential confusion. Because the virtual
devices are emulated ethernet devices, they require MAC addresses.
There is no physical hardware to provide these addresses, so normally
MAC addresses will be invented. That means that each time the eCos
application is run it will have different MAC addresses, which makes
it more difficult to compare the results of different runs. To get
more deterministic behaviour it is possible to specify the MAC
addresses in the target definition file:
    </para>
    <programlisting>
synth_device ethernet {
    eth0 ethertap tap3 00:01:02:03:FE:05
    eth1 ethertap tap4 00:01:02:03:FE:06
    &hellip;
}
</programlisting>
    <para>
During the initialization phase the eCos application will instantiate
the various network devices. This will cause the I/O auxiliary to load
the <filename>ethernet.tcl</filename> script and spawn
<command>rawether</command> processes, which in turn will
<function>open</function> <filename>/dev/net/tun</filename> and
perform the appropriate <filename>ioctl</filename> calls. On the Linux
side there will now be new network interfaces such as
<varname>tap3</varname>, and these can be configured like any other
network interface using commands such as <command>ifconfig</command>.
In addition, if the Linux system is set up with hotplug support then
it may be possible to arrange for the network interface to become
active automatically. On a Red Hat Linux system this would require
files such as
<filename>/etc/sysconfig/network-scripts/ifcfg-tap3</filename>,
containing data like:
    </para>
    <programlisting>
DEVICE="tap3"
BOOTPROTO="none"
BROADCAST=10.2.2.255
IPADDR="10.2.2.1"
NETMASK="255.255.255.0"
NETWORK=10.2.2.0
ONBOOT="no"
</programlisting>
    <para>
This gives the Linux interface the address <literal>10.2.2.1</literal>
on the network <literal>10.2.2.0</literal>. The eCos network device
should be configured with a compatible address. One way of doing this
would be to enable <varname>CYGHWR_NET_DRIVER_ETH0_ADDRS</varname>,
set <varname>CYGHWR_NET_DRIVER_ETH0_ADDRS_IP</varname> to
<literal>10.2.2.2</literal>, and similarly update the
<varname>NETMASK</varname>, <varname>BROADCAST</varname>,
<varname>GATEWAY</varname> and <varname>SERVER</varname> configuration
options. 
    </para>
    <para>
It should be noted that the ethertap facility provides a virtual
network, and any packets transmitted by the eCos application will
not appear on a real network. Therefore usually there will no
accessible DHCP server, and eCos cannot use DHCP or BOOTP to obtain IP
address information. Instead the eCos configuration should use manual
or static addresses.
    </para>
    <para>
When <command>rawether</command> exits, the tap interface is removed
by the kernel. By adding the parameter persistent
<command>rawether</command> will set the persistent flag on the tap
device.
    </para>
    <programlisting>
synth_device ethernet {
    eth0 ethertap tap3 00:01:02:03:FE:05
    eth1 ethertap tap4 00:01:02:03:FE:06 persistent
    &hellip;
}
</programlisting>
    <para>
With this flag set the kernel will not remove the interface when
<command>rawether</command> exits. This means applications such as
<command>dhcpd</command>, <command>radvd</command>, and
<command>tcpdump</command> will continue to run on the interface
between invocations of synthetic targets. As a result the target can
dynamically obtain its IP addresses from these daemons. Note it is a
good idea to specify a MAC address otherwise a different random MAC
address will be used each time and the dhcpd daemon will not be able
to reissue the same IP address.
    </para>
    <para>
Host daemons like dhcpd, ntpd, radvd etc are started at boot
time. Since the tap device does not exists at this point in time it is
not possible for these daemons to bind to the tap device. A simple
solution is to use the program
<command>install/bin/mktap</command>. This takes one parameter, the
name of the tap device it should create. eg,
<literal>tap3</literal>. 
    </para>
    <para>
An alternative approach would be to set up the Linux box as a network
bridge, using commands like <command>brctl</command> to connect the
virtual network interface <varname>tap3</varname> to a physical
network interface such as <varname>eth0</varname>. Any packets sent by
the eCos application will get forwarded automatically to the real
network, and some packets on the real network will get forwarded over
the virtual network to the eCos application. Note that the eCos
application might also get some packets that were not intended for it,
but usually those will just be discarded by the eCos TCP/IP stack. The
exact details of setting up a network bridge are left as an exercise
to the reader.
    </para>
  </refsect1>

  <refsect1 id="devs-eth-ecosynth-logging"><title>Packet Logging</title>
    <para>
The ethernet support comes with support for logging the various
packets that are transferred, including a simple protocol analyser.
This generates simple text output using the filter mechanisms provided
by the I/O auxiliary, so it is possible to control the appearance and
visibility of different types of output. For example the user might
want to see IPv4 headers and all ICMPv4 and ARP operations, but not
TCP headers or any of the packet data.
    </para>
    <para>
The protocol analyser is not intended to be a fully functional
analyser with knowledge of many different TCP/IP protocols, advanced
search facilities, graphical traffic displays, and so on.
Functionality like that is already provided by other tools such as
<application>ethereal</application> and
<application>tcpdump</application>. Achieving similar levels of
functionality would require a lot of work, for very little gain. It is
still useful to have some protocol analysis functionality available
because the output will be interleaved with other output, for example
<filename>printf</filename> calls from the application. That may make
it easier to understand the sequence of events.
    </para>
    <para>
One problem with logging ethernet traffic is that it can involve very
large amounts of data. If the application is expected to run for a
long time or is very I/O intensive then it is easy to end up with many
megabytes. When running in graphical mode all the logging data will be
held in memory, even data that is not currently visible. At some point
the system will begin to run low on memory and performance will
suffer. To avoid problems, the ethernet script maintains a flag that
controls whether or not packet logging is active. The default is to
run with logging disabled, but this can be changed in the target
definition file:
    </para>
    <programlisting>
synth_device ethernet {
    &hellip;
    logging 1
}
</programlisting>
    <para>
The ethernet script will add a toolbar button that allows this flag to
be changed at run-time, allowing the user to capture traffic for
certain periods of time while the application continues running.
    </para>
    <para>
The target definition file can contain the following entries for the
various packet logging filters:
    </para>
    <programlisting width=72>
synth_device ethernet {
    &hellip;
    filter ether  -hide 0 -background LightBlue -foreground "#000080"
    filter arp    -hide 0 -background LightBlue -foreground "#000050"
    filter ipv4   -hide 0 -background LightBlue -foreground "#000040"
    filter ipv6   -hide 1 -background LightBlue -foreground "#000040"
    filter icmpv4 -hide 0 -background LightBlue -foreground "#000070"
    filter icmpv6 -hide 1 -background LightBlue -foreground "#000070"
    filter udp    -hide 0 -background LightBlue -foreground "#000030"
    filter tcp    -hide 0 -background LightBlue -foreground "#000020"
    filter hexdata   -hide 1 -background LightBlue -foreground "#000080"
    filter asciidata -hide 1 -background LightBlue -foreground "#000080"
}
</programlisting>
    <para>
All output will show the eCos network device, for example
<literal>eth0</literal>, and the direction relative to the eCos
application. Some of the filters will show packet headers, for example
<literal>ether</literal> gives details of the ethernet packet header
and <literal>tcp</literal> gives information about TCP headers such as
whether or not the SYN flag is set. The TCP and UDP filters will also
show source and destination addresses, using numerical addresses and
if possible host names. However, host names will only be shown if the
host appears in <filename>/etc/hosts</filename>: doing full DNS
lookups while the data is being captured would add significantly to
complexity and overhead. The <literal>hexdata</literal> and
<literal>asciidata</literal> filters show the remainder of the packets
after the ethernet, IP and TCP or UDP headers have been stripped.
    </para>
    <para>
Some of the filters will provide raw dumps of some of the packet data.
Showing up to 1500 bytes of data for each packet would be expensive,
and often the most interesting information is near the start of the
packet. Therefore it is possible to set a limit on the number of bytes
that will be shown using the target definition file. The default limit
is 64 bytes.
    </para>
    <programlisting width=72>
synth_device ethernet {
    &hellip;
    max_show 128
}
</programlisting>
  </refsect1>

  <refsect1 id="devs-eth-ecosynth-gui"><title>User Interface Additions</title>
    <para>
When running in graphical mode the ethernet script extends the user
interface in two ways: a button is added to the toolbar so that users
can enable or disable packet logging; and an entry is added to the
<guimenu>Help</guimenu> menu for the ethernet-specific documentation.
    </para>
  </refsect1>

  <refsect1 id="devs-eth-ecosynth-args"><title>Command Line Arguments</title>
    <para>
The synthetic target ethernet support does not use any command line
arguments. All configuration is handled through the target definition
file. 
    </para>
  </refsect1>

  <refsect1 id="devs-eth-ecosynth-hooks"><title>Hooks</title>
    <para>
The ethernet support defines two hooks that can be used by other
scripts, especially user scripts: <literal>ethernet_tx</literal> and
<literal>ethernet_rx</literal>. The tx hook is called whenever eCos
tries to transmit a packet. The rx hook is called whenever an incoming
packet is passed to the eCos application. Note that this may be a
little bit after the packet was actually received by the I/O auxiliary
since it can buffer some packets. Both hooks are called with two
arguments, the name of the network device and the packet being
transferred. Typical usage might look like:
    </para>
    <programlisting>
  proc my_tx_hook { arg_list } {
    set dev [lindex $arg_list 0]
    incr ::my_ethernet_tx_packets($dev)
    incr ::my_ethernet_tx_bytes($dev) [string length [lindex $arg_list 1]]
  }
  proc my_rx_hook { arg_list } {
    set dev [lindex $arg_list 0]
    incr ::my_ethernet_rx_packets($dev)
    incr ::my_ethernet_rx_bytes($dev) [string length [lindex $arg_list 1]]
  }
  synth::hook_add "ethernet_tx" my_tx_hook
  synth::hook_add "ethernet_rx" my_rx_hook
</programlisting>
    <para>
The global arrays <varname>my_ethernet_tx_packets</varname> etc. will
now be updated whenever there is ethernet traffic. Other code,
probably running at regular intervals by use of the Tcl
<command>after</command> procedure, can then use this information to
update a graphical monitor of some sort.
    </para>  
  </refsect1>

  <refsect1 id="devs-eth-ecosynth-tcl"><title>Additional Tcl Procedures</title>
    <para>
The ethernet support provides one additional Tcl procedure that can be
used by other scripts;
    </para>
    <programlisting>
ethernet::devices_get_list    
</programlisting>
    <para>
This procedure returns a list of the ethernet devices that have been
instantiated, for example <literal>{eth0 eth1}</literal>.
    </para>
  </refsect1>

</refentry>
</part>
<!-- /reference -->