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<html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>nexp</title><link rel="stylesheet" href="nexp.css" type="text/css"><meta name="generator" content="DocBook XSL Stylesheets V1.70.1"></head><body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"><div class="refentry" lang="en"><a name="nexp"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>nexp &#8212; 
      A framework for crafting network packets and processing responses
    </p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><div class="cmdsynopsis"><p><code class="command">nexp</code>  [<code class="option">-V</code>] [<code class="option">-v</code>] [<code class="option">-t</code>] [<code class="option">-c</code> <em class="replaceable"><code>cmd</code></em>] [<code class="option">-s</code> <em class="replaceable"><code>seed</code></em>] [<code class="option">-h</code>] [<em class="replaceable"><code>cmdfile</code></em>] [<em class="replaceable"><code>args</code></em>]</p></div></div><div class="refsect1" lang="en"><a name="id2487792"></a><h2>INTRODUCTION</h2><p>
      <span><strong class="command">Network Expect</strong></span> (<span><strong class="command">nexp</strong></span>) is a
      framework that allows to easily build tools that can interact
      with network traffic. Following a script, traffic can be
      injected into the network, and decisions can be taken, and acted
      upon, based on received network traffic. An interpreted language
      provides branching and high-level control structures to direct
      the interaction with the network.
    </p><p>
      <span><strong class="command">Network Expect</strong></span> was heavily influenced and
      inspired on the Expect program written by Don Libes, which
      allows to "talk" to interactive programs in a scripted
      fashion. Because of this, you will find a lot of similarities
      between commands in <span><strong class="command">Network Expect</strong></span> and
      commands in Don Libes' Expect. If you are a regular Expect user,
      it should not be very difficult to start writing
      <span><strong class="command">Network Expect</strong></span> scripts because the basics are
      the same.
    </p><p>
      In Don Libes' Expect, scripts can send data to a process just as
      if a user were interactively typing commands. Then, the script
      would read the responses send by the application and take
      decisions accordingly. In <span><strong class="command">Network Expect</strong></span>'s
      case, a script could send traffic to a network device and then
      take decisions based on the received network traffic. The type
      of things that <span><strong class="command">Network Expect</strong></span> can do are
      usually very low level network operations, which usually require
      writing a custom application in a language like C.
    </p><p>
      <span><strong class="command">Network Expect</strong></span>'s philosophy is based on the
      observation that network applications always operate on an
      action-reaction principle in which something is sent to an
      application running on a remote host and a response is then
      expected.
    </p><p>
      Some of the things that <span><strong class="command">Network Expect</strong></span> can do
      include:
    </p><div class="itemizedlist"><ul type="disc"><li><p>
	  Generate arbitrary network traffic and inject it into a
	  network at layer 2 or layer 3.
	</p><p>
	  A wide range of protocols is supported, including IP version
	  6 as well as protocol options like IPv4, IPv6 and TCP
	  options, something that regular tools don't offer. For
	  example, <span><strong class="command">Network Expect</strong></span> supports TCP MD5
	  signatures (RFC 2385).
	</p><p>
	  This <span><strong class="command">Network Expect</strong></span> functionality is very
	  similar to the functionality provided by several packet
	  crafting and forging open source tools like Nemesis, Packit,
	  hping, Scapy, and others.
	</p></li><li><p>
	  Listen for network traffic and take decisions based on the
	  type of traffic received.
	</p></li><li><p>
	  Open a sniffer trace in PCAP format and replay it after
	  changing some values in the original packet capture.
	</p></li><li><p>
	  Emulate network protocols to see how they interact with
	  other speakers of that protocol. For example, emulating a
	  TCP server to investigate approaches to randomization of TCP
	  Initial Sequence Numbers (ISN) can be easily done in
	  <span><strong class="command">Network Expect</strong></span>.
	</p></li></ul></div></div><div class="refsect1" lang="en"><a name="id2487934"></a><h2>USAGE</h2><p>
      <span><strong class="command">Network Expect</strong></span> reads cmdfile for a list of
      commands to execute. <span><strong class="command">Network Expect</strong></span> may also
      be invoked implicitly on systems which support the #!  notation
      by marking the script executable, and making the first line in
      your script:
    </p><pre class="programlisting">
#!/usr/bin/nexp -f</pre><p>
      Of course, the path must accurately describe where
      <span><strong class="command">Network Expect</strong></span> lives. /usr/bin is just an
      example.
    </p><p>
      The <code class="option">-c</code> flag prefaces a command to be executed
      before any in the script.  The command should be quoted to
      prevent being broken up by the shell.  This option may be used
      multiple times.  Multiple commands may be executed with a single
      <code class="option">-c</code> by separating them with semicolons.
      Commands are executed in the order they appear.
    </p><p>
      <code class="option">-V</code> causes <span><strong class="command">Network Expect</strong></span> to
       print its version number and exit.
    </p><p>
      The <code class="option">-s</code> flag allows to specify a random seed
      that will cause predicatibility of pseudo-random numbers
      generated by <span><strong class="command">Network Expect</strong></span> during execution
      of a script. In cases where <span><strong class="command">Network Expect</strong></span> is
      used as a protocol fuzzer, this option is useful to be able to
      re-generate a specific test case.
    </p><p>
      <code class="option">-v</code> increases the verbosity level. Some commands
       display additional information when the verbosity level is
       higher.
    </p><p>
      The <code class="option">-t</code> flag changes the display format used by
      commands that display dates or generate strings that represent
      dates.
    </p><p>
      Optional args are constructed into a list and stored in the
      variable named argv.  argc is initialized to the length of argv.
    </p><p>
      argv0 is defined to be the name of the script (or binary if no
      script is used).  For example, the following prints out the name
      of the script and the first three arguments:
    </p><pre class="programlisting">
puts "$argv0 [lrange $argv 0 2]"</pre></div><div class="refsect1" lang="en"><a name="id2488044"></a><h2>NETWORK LISTENERS AND SPEAKERS</h2><p>
      An integral part of <span><strong class="command">Network Expect</strong></span> is the
      concept of network <span class="emphasis"><em>listeners</em></span> and network
      <span class="emphasis"><em>speakers</em></span>, which are the <span><strong class="command">Network
      Expect</strong></span> equivalent to spawned processes in Don Libes'
      <span><strong class="command">Expect</strong></span> world. In <span><strong class="command">Expect</strong></span>,
      the <span><strong class="command">send</strong></span> command sends data to a spawned
      process, and the <span><strong class="command">expect</strong></span> command waits for a
      specific pattern in the data received from a spawned process.
    </p><p>
      In <span><strong class="command">Network Expect</strong></span>, the command to send data
      to the network, called <span><strong class="command">send_network</strong></span>, uses a
      <span class="emphasis"><em>speaker</em></span> that specifies how the traffic will
      be injected. <span><strong class="command">Network Expect</strong></span> speakers can
      specify IPv4 or IPv6 sockets, in which case packets will be
      injected at layer 3 and will be routed by the operating system
      kernel. <span><strong class="command">Network Expect</strong></span> speakers can also
      specify a physical interface in which case the packet will be
      injected at layer 2. And finally, <span><strong class="command">Network
      Expect</strong></span> speakers can specify a PCAP file (also known as
      a "savefile") in which case packets will be written to this file
      instead of injected to the network.
    </p><p>
      <span><strong class="command">Network Expect</strong></span> listeners, on the other hand,
      specify where packets will be read from. Listeners can be
      associated with either a physical interface, or with a PCAP
      file. In either case an optional PCAP filter can be associated
      with the listener to limit the type of packets the listener will
      return. When reading from a PCAP file the inter-packet delay
      between packets can be kept, or packets can be read at full
      speed.
    </p><p>
      Both listeners and speakers are created with the
      <span><strong class="command">Network Expect</strong></span> command
      <span><strong class="command">spawn_network</strong></span>, just as in Don Libes'
      <span><strong class="command">Expect</strong></span> spawned processes are created with
      the <span><strong class="command">spawn</strong></span> command.
    </p><p>
      The way to specify network listeners and speakers in commands
      that require them is the same regardless of the command. Network
      listeners are always specified using the <span class="emphasis"><em>-i</em></span> switch (think
      <span class="emphasis"><em>i</em></span>nput) followed by the name of the
      listener, and network speakers are specified using the <span class="emphasis"><em>-o</em></span> switch (think
      <span class="emphasis"><em>o</em></span>utput) followed by the name of the
      speaker.
    </p><p>
      Network isteners and speakers are created using the
      <span><strong class="command">spawn_network</strong></span> command. Just as in Don Libes'
      <span><strong class="command">Expect</strong></span> one cannot choose the spawn ID
      returned by the <span><strong class="command">spawn</strong></span> command, in
      <span><strong class="command">Network Expect</strong></span> it is not possible to choose
      the name of a network listener or speaker. However, one can
      assign the network listener or speaker name to a variable and
      use that variable whenever a network listener or speaker needs
      to be specified.
    </p><p>
      What follows are a few examples of creation of network listeners
      and speakers:
    </p><pre class="programlisting">
spawn_network -i eth1 icmp and src host 172.16.1.1</pre><p>
      This command creates a network listener on interface eth1 and
      assigns the filter "icmp and src host 172.16.1.1", i.e. "listen
      only for ICMP messages coming from host 172.16.1.1". Since the
      <span class="emphasis"><em>-o</em></span> switch has not been specified
      this command will not create a network speaker.
    </p><pre class="programlisting">
spawn_network -o eth0 -r /tmp/packets.pcap tcp and host 172.16.1.1</pre><p>
      This command creates a network speaker on interface eth0 and a
      network listener that will read from the PCAP file
      "/tmp/packets.pcap" TCP segments to or from the host 172.16.1.1.
    </p><pre class="programlisting">
spawn_network -nolistener -6</pre><p>
      This will create a network speaker for injecting IPv6 packets at
      layer 3. Since the <span class="emphasis"><em>-nolistener</em></span>
      switch has been specified, this command only creates a network
      speaker and no listener.
    </p><pre class="programlisting">
spawn_network -nolistener -w /tmp/mypackets.pcap</pre><p>
      This only creates a speaker that will write packets to the PCAP
      file "/tmp/mypackets.pcap". Note that when writing packets to a
      PCAP file, injection implicitley takes place at layer 2, and
      using an Ethernet header. This must be taken into consideration
      when specifying the packet to send using the
      <span><strong class="command">send_network</strong></span> command.
    </p><p>
      <span><strong class="command">Network Expect</strong></span> listeners and speakers play a
      very important role in the operation of <span><strong class="command">Network
      Expect</strong></span> so becoming confortable with them is key to
      understanding <span><strong class="command">Network Expect</strong></span>'s philosophy.
    </p></div><div class="refsect1" lang="en"><a name="id2488298"></a><h2>NUMERIC SPECIFICATIONS</h2><p>When defining packets, <span><strong class="command">Network Expect</strong></span>
      allows to specify values for most fields in protocol headers
      using a syntax that gives great flexibility. This syntax allows
      to make the value of a field <span class="emphasis"><em>change</em></span> with
      each packet that is created. The syntax is better presented with
      an example. Suppose that we have an hypothetical command-line
      switch <span class="emphasis"><em>-z</em></span> that is used to specify a 16-bit
      value (please note that the same syntax is used for 8, 16 and
      32-bit quantities) in the header of certain protocol.
    </p><div class="itemizedlist"><ul type="disc"><li><p>
	  <span class="emphasis"><em>-z telnet</em></span> (fixed): the generated value
	  will always be 23, telnet's port number.
	</p></li><li><p>
	  <span class="emphasis"><em>-z 23</em></span> (fixed): the generated value will
	  always be 23.
	</p></li><li><p>
	  <span class="emphasis"><em>-z 23+</em></span> (increment): the generated value
	  will be 23 initially, and will be incremented by one with
	  each successive packet.
	</p></li><li><p>
	  <span class="emphasis"><em>-z 23-</em></span> (increment): the generated value will
	  be 23 initially, and will be decremented by one with each
	  successive packet.
	</p></li><li><p>
	  <span class="emphasis"><em>-z 23+5</em></span> (decrement): the generated
	  value will be 23 initially, and will be incremented by 5
	  with each successive packet.
	</p></li><li><p>
	  <span class="emphasis"><em>-z 23-5</em></span> (decrement): the generated
	  value will be 23 initially, and will be decremented by 5
	  with each successive packet.
	</p></li><li><p>
	  <span class="emphasis"><em>-z 23:25</em></span> (range): the generated value
	  will start with 23, will be incremented by one until it
	  reaches 25, and then will go back to 23.
	</p></li><li><p>
	  <span class="emphasis"><em>-z 25:23</em></span> (range): the generated value
	  will start with 25, will be decremented by one until it
	  reaches 23, and then will go back to 25.
	</p></li><li><p>
	  <span class="emphasis"><em>-z random</em></span>: the generated value will be
	  a random number in each successive packet.
	</p></li></ul></div></div><div class="refsect1" lang="en"><a name="id2488435"></a><h2>COMMANDS</h2><p>
      <span><strong class="command">Network Expect</strong></span> uses Tcl (Tool Command
      Language).  Tcl provides control flow (e.g., if, for, break),
      expression evaluation and several other features such as
      recursion, procedure definition, etc.  Commands used here but
      not defined (e.g., set, if, exec) are Tcl commands (see
      tcl(3)). <span><strong class="command">Network Expect</strong></span>
      introduces additional commands, described below.  Unless
      otherwise specified, commands return the empty string.
    </p><p>
      Commands are listed alphabetically so that they can be quickly
      located.  However, new users may find it easier to start by
      reading the descriptions of <span><strong class="command">spawn_network</strong></span>,
      <span><strong class="command">send_network</strong></span>,
      <span><strong class="command">expect_network</strong></span>, and
      <span><strong class="command">send_expect</strong></span>, in that order.
    </p><div class="variablelist"><dl><dt><span class="term">
	  <span><strong class="command">barray</strong></span> <span class="emphasis"><em>new | length | delete |
	  examine | dump | slice | cmp | string
	  &lt;args&gt;</em></span>
	</span></dt><dd><p>
	    The <span><strong class="command">barray</strong></span> command is used to perform
	    management of variables of type <code class="varname">barray</code>
	    (byte array.)
	  </p><p>
	    <span><strong class="command">barray new &lt;payload spec&gt;</strong></span> will
	    create and return a new <code class="varname">barray</code>
	    variable.
	  </p><p>
	    <span><strong class="command">barray length &lt;barray variable&gt;</strong></span>
	    will return the number of bytes that the barray variable
	    uses.
	  </p><p>
	    <span><strong class="command">barray delete &lt;barray variable&gt;</strong></span>
	    will delete a <code class="varname">barray</code> variable.
	  </p><p>
	    <span><strong class="command">barray examine &lt;barray variable&gt;
	    [/&lt;FMT&gt;] [&lt;offset&gt;]</strong></span> allows to
	    inspect the contents of the specified
	    <code class="varname">barray</code> variable, starting at the
	    optional <code class="option">offset</code>, and using the format
	    optionally specified with
	    <code class="option">/FMT</code>. <code class="option">/FMT</code> can include
	    an optional count followed by the display format, '&lt;'
	    or '&gt;' to specify little endian or big endian, and the
	    size of each element to display. Display formats are
	    strings ('s'), octal ('o'), hexadecimal ('x'), signed
	    decimal ('d'), and unsigned decimal ('u'). Sizes are byte
	    ('b'), half-word ('h'), and word ('w').
	  </p><p>
	    <span><strong class="command">barray dump &lt;barray variable&gt;</strong></span>
	    produces a hexadecimal dump of the specified
	    <code class="varname">barray</code> variable.
	  </p><p>
	    <span><strong class="command">barray slice &lt;barray variable&gt; &lt;slice
	    spec&gt;</strong></span> returns a slice of the specified
	    <code class="varname">barray</code> variable. <code class="option">slice
	    spec</code> must be in the format &lt;[start]:[end]&gt;
	    where <code class="option">start</code> and <code class="option">end</code> are
	    offset into the <code class="varname">barray</code> variable. If
	    <code class="option">start</code> is ommited the start offset is 0,
	    and if <code class="option">end</code> is ommited the end offset is
	    the end of the <code class="varname">barray</code> variable.
	  </p><p>
	    <span><strong class="command">barray cmp &lt;barray variable 1&gt; &lt;barray
	    variable 2&gt;</strong></span> compares two
	    <code class="varname">barray</code> variables and returns an integer
	    less than, equal to, or greater than zero if the first
	    <code class="option">n</code> bytes of <code class="varname">barray variable
	    1</code> are found, respectively, to be less than, to
	    match, or be greater than the first <code class="option">n</code>
	    bytes of <code class="varname">barray variable
	    2</code>. <code class="option">n</code> is calculated to be the
	    minimum of the lengths of both <code class="varname">barray</code>
	    variables.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">close_network</strong></span>
	  <span class="emphasis"><em>&lt;listener/speaker name&gt;</em></span>
	</span></dt><dd><p>
	    This command closes the network listener and/or speaker
	    referenced by <span class="emphasis"><em>&lt;listener/speaker
	    name&gt;</em></span>. Closing a network listener is not a
	    very important thing to do since the operation just
	    releases system resources used by the listener. However,
	    closing a network speaker is a very important, especially
	    when the speaker is associated with a PCAP file since
	    closing the speaker closes the associated PCAP file.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">expect_network</strong></span> <span class="emphasis"><em>[-timeout
	  &lt;timeout&gt;]</em></span> <span class="emphasis"><em>[-i &lt;listener
	  ID&gt;] {condition1} {body1}</em></span> ... <span class="emphasis"><em>[-i
	  &lt;listener ID&gt;] {conditionN} {bodyN}</em></span>
	</span></dt><dd><p>
	    The <span><strong class="command">expect_network</strong></span> command waits for
	    network traffic from one or more network listeners
	    (specified with the <code class="option">-i</code> option) until a
	    condition evaluates to true, until a specified time period
	    has passed, or until an end-of-file is seen (when the
	    listener is associated with a PCAP file.)
	  </p><p>
	    Conditions from the most recent
	    <span><strong class="command">expect_network_before</strong></span> command are
	    implicitly used before any other conditions. Conditions
	    from the most recent
	    <span><strong class="command">expect_network_after</strong></span> command are
	    implicitly used after any other conditions.
	  </p><p>
	    Conditions are regular Tcl expressions, and bodies are
	    sets of Tcl commands. If the final body is empty, it may
	    be omitted.
	  </p><p>
	    If the arguments to the entire
	    <span><strong class="command">expect_network</strong></span> statement require more
	    than one line, all the arguments may be "braced" into one
	    so as to avoid terminating each line with a backslash.  In
	    this one case, the usual Tcl substitutions will occur
	    despite the braces.
	  </p><p>
	    If a condition is the keyword <code class="varname">eof</code>, the
	    corresponding body is executed upon end-of-file (this is
	    only meaningful when a listener is reading from a PCAP
	    file, not on a live capture from an interface.)  If a
	    condition is the keyword <code class="varname">timeout</code>, the
	    corresponding body is executed upon timeout.  If no
	    <code class="varname">timeout</code> keyword is used, an implicit
	    null action is executed upon timeout.  The default timeout
	    period is 0.5 seconds but may be set, for example to 30,
	    by the command "set timeout 30".  An infinite timeout may
	    be designated by the value -1. 
	  </p><p>
	    If a condition is true, then the corresponding body is
	    executed.  <span><strong class="command">expect_network</strong></span> returns the
	    result of the body (or the empty string if no condition
	    was true.)  In the event that multiple conditions match,
	    the one appearing first is used to select a body.
	  </p><p>
	    Each time a new packet arrives, it is decoded and the
	    conditions are evaluated in the order they are listed.
	    When a packet is decoded, the values of the different
	    fields in the packet are made available to Tcl scripts via
	    Tcl variables. Scripts can use these values in an
	    condition for a <span><strong class="command">expect_network</strong></span> command.
	  </p><p>
	    In the following example, a listener for ARP requests on
	    interface eth0 is created and then an
	    <span><strong class="command">expect_network</strong></span> command is used to wait
	    for actual ARP requests and respond to them with an ARP
	    reply:
	  </p><pre class="programlisting">
# Spawn a listener for ARP requests
spawn_network -i eth0 host 192.168.1.1 and {arp[6:2]} == 1

expect_network {1} {
    # Received an ARP request, send ARP reply
    send_network -o eth0 \
        ether(src = $mymac, dst = $arp(sha) )/ \
        arp-reply(tha = $arp(sha), tip = $arp(sip), \
                  sha = $mymac, sip = 192.168.1.1)
    nexp_continue
}
	  </pre><p>
	    The <code class="option">-timeout</code> flag causes the current
	    <span><strong class="command">expect_network</strong></span> command to use the
	    following value as a timeout instead of using the value of
	    the <code class="varname">timeout</code> variable.
	  </p><p>
	    Actions such as <span><strong class="command">break</strong></span> and
	    <span><strong class="command">continue</strong></span> cause control structures
	    (i.e., <span><strong class="command">for</strong></span>, <span><strong class="command">proc</strong></span>) to
	    behave in the usual way.  The command
	    <span><strong class="command">nexp_continue</strong></span> allows
	    <span><strong class="command">expect_network</strong></span> itself to continue
	    executing rather than returning as it normally would.
	  </p><p>
	    This is useful for avoiding explicit loops or repeated
	    <span><strong class="command">expect_network</strong></span> statements.  The
	    following example is part of a fragment to respond to ICMP
	    echo and ARP requests.  The
	    <span><strong class="command">nexp_continue</strong></span> avoids having to write a
	    second <span><strong class="command">expect_network</strong></span> statement (to
	    look for the requests again.)
	  </p><pre class="programlisting">
# Spawn a listener for ARP requests
spawn_network -i $interface host $myip and {arp[6:2]} == 1
set arpl $listener_id

# Spawn a listener for ICMP messages sent to us
spawn_network -i $interface icmp and dst host $myip
set icmpl $listener_id

expect_network -i $arpl {1} {
    # Received an ARP request, send ARP reply
    send_network -o $interface \
        ether(src = $mymac, dst = $arp(sha) )/ \
        arp-reply(tha = $arp(sha), tip = $arp(sip), \
                  sha = $mymac, sip = $myip)
    nexp_continue
} -i $icmpl {$icmp(type) == 8} {
    # Received ICMP echo request, send echo reply
    send_network -o ip \
        ip(src = $myip, dst = $ip(src) )/ \
        icmp-echoreply(id = $icmp(id), seq = $icmp(seq) )/ \
	raw($raw)
    nexp_continue
}
	  </pre></dd><dt><span class="term">
	  <span><strong class="command">expect_network_after</strong></span>
	  <span class="emphasis"><em>[expect_args]</em></span>
	</span></dt><dd><p>
	    works identically to the
	    <span><strong class="command">expect_network_before</strong></span> except that if
	    conditions from both <span><strong class="command">expect_network</strong></span> and
	    <span><strong class="command">expect_network_after</strong></span> evaluate to true,
	    the <span><strong class="command">expect_network</strong></span> conditions is used.
	    See the <span><strong class="command">expect_network_before</strong></span> command
	    for more information.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">expect_network_background</strong></span>
	  <span class="emphasis"><em>[expect_args]</em></span>
	</span></dt><dd><p>
	    takes the same arguments as
	    <span><strong class="command">expect_network</strong></span>, however it returns
	    immediately.  Conditions are evaluated whenever a new
	    packet arrives.  The expression <code class="varname">timeout</code>
	    and <code class="varname">default</code> are meaningless to
	    <span><strong class="command">expect_network_background</strong></span> and are
	    silently discarded.  Otherwise, the
	    <span><strong class="command">expect_network_background</strong></span> command uses
	    <span><strong class="command">expect_network_before</strong></span> and
	    <span><strong class="command">expect_network_after</strong></span> conditions just
	    like <span><strong class="command">expect_network</strong></span> does.
	  </p><p>
	    Please note that if a condition of the
	    <span><strong class="command">expect_network_background</strong></span> command
	    evaluates to true, the command will not continue to listen
	    for traffic unless the body includes a
	    <span><strong class="command">nexp_continue</strong></span> command that forces
	    <span><strong class="command">expect_network_background</strong></span> to continue
	    executing.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">expect_network_before</strong></span>
	  <span class="emphasis"><em>[expect_args]</em></span>
	</span></dt><dd><p>
	    takes the same arguments as
	    <span><strong class="command">expect_network</strong></span>, however it returns
	    immediately.  Condition-action pairs from the most recent
	    <span><strong class="command">expect_network_before</strong></span> with the same
	    network listener ID are implicitly added to any following
	    <span><strong class="command">expect_network</strong></span> commands. If a condition
	    evaluates to true, it is treated as if it had been
	    specified in the <span><strong class="command">expect_network</strong></span> command
	    itself, and the associated body is executed in the context
	    of the <span><strong class="command">expect_network</strong></span> command.  If
	    conditions from both
	    <span><strong class="command">expect_network_before</strong></span> and
	    <span><strong class="command">expect_network</strong></span> can evaluate to true,
	    the <span><strong class="command">expect_network_before</strong></span> condition is
	    used.
	  </p><p>
	    Unless overridden by a <code class="option">-i</code> flag,
	    <span><strong class="command">expect_network_before</strong></span> conditions are
	    evaluated against the network listener ID defined at the
	    time that the <span><strong class="command">expect_network_before</strong></span>
	    command was executed (not when its condition evaluated to
	    true.)
	  </p><p>
	    The <code class="option">-info</code> flag causes
	    <span><strong class="command">expect_network_before</strong></span> to return the
	    current specifications of what conditions will be
	    evaluated.  By default, it reports on the current network
	    listener.  An optional network listener ID specification
	    may be given for information on that network listener.
	  </p><p>
	    Instead of a network listener specification, the flag
	    <code class="option">-all</code> will cause <code class="option">-info</code> to
	    report on all network listeners.
	  </p><p>
	    The output of the <code class="option">-info</code> flag can be
	    reused as the argument to
	    <span><strong class="command">expect_network_before</strong></span>.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">iflist</strong></span>
	  <span class="emphasis"><em>[&lt;-refresh&gt;]</em></span>
	</span></dt><dd><p>
	    <span><strong class="command">iflist</strong></span> returns a list that contains the
	    name of all interfaces in the system. This list is built
	    when <span><strong class="command">Network Expect</strong></span> starts. If the
	    optional argument <code class="option">-refresh</code> is specified
	    then the list is refreshed before returning it.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">nexp_continue</strong></span>
	</span></dt><dd><p>
	    The command <span><strong class="command">nexp_continue</strong></span> allows expect
	    itself to continue executing rather than returning as it
	    normally would.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">nexp_version</strong></span> <span class="emphasis"><em>[[-exit]
	  &lt;version&gt;]</em></span>
	</span></dt><dd><p>
             is useful for assuring that the script is compatible with
             the current version of <span><strong class="command">Network Expect</strong></span>.
	  </p><p>
             With no  arguments,  the  current version of Expect is returned.
             This version may then be encoded in your script.  If you actually
             know  that you are not using features of recent versions, you can
             specify an earlier version.
	  </p><p>
	    Versions consist of two numbers separated by dots.  First
	    is the major number.  Scripts written for versions of
	    <span><strong class="command">Network Expect</strong></span> with a different major
	    number will almost certainly not work.
	    <span><strong class="command">nexp_version</strong></span> returns an error if the
	    major numbers do not match.
	  </p><p>
	    Second is the minor number.  Scripts written for a version
	    with a greater minor number than the current version may
	    depend upon some new feature and might not run.
	    <span><strong class="command">nexp_version</strong></span> returns an error if the
	    major numbers match, but the script minor number is
	    greater than that of the running <span><strong class="command">Network
	    Expect</strong></span>.
	  </p><p>
             With the -exit flag, Expect prints an error and exits if
             the version is out of date.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">outif</strong></span> <span class="emphasis"><em>&lt;target&gt;</em></span>
	</span></dt><dd><p>
	    The <span><strong class="command">outif</strong></span> command returns the outgoing
	    interface that would be used to reach
	    <code class="option">target</code>. <code class="option">target</code> can be an
	    IP address or host name.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">packet</strong></span> <span class="emphasis"><em>decode | hash | data | dump
	  | ts | tdelta &lt;args&gt;</em></span>
	</span></dt><dd><p>
	    The <span><strong class="command">packet</strong></span> command allows to manage
	    variables of type <code class="varname">packet</code>.
	  </p><p>
	    <span><strong class="command">packet decode &lt;packet variable&gt;</strong></span>
	    will decode the specified  <code class="varname">packet</code>
	    variable, just as if the packet was received during the
	    execution of a <span><strong class="command">expect_network</strong></span> command.
	  </p><p>
	    <span><strong class="command">packet hash &lt;packet variable&gt;</strong></span>
	    calculates a hash of the specified
	    <code class="varname">packet</code> variable.
	  </p><p>
	    <span><strong class="command">packet data &lt;packet variable&gt;</strong></span>
	    returns a <code class="varname">barray</code> variable that contains
	    all of the packet's data.
	  </p><p>
	    <span><strong class="command">packet dump &lt;packet variable&gt;</strong></span>
	    displays an hexadecimal dump (on standard output) of the
	    <code class="varname">packet</code>'s data.
	  </p><p>
	    <span><strong class="command">packet ts &lt;packet variable&gt;</strong></span> will
	    return a <code class="varname">timeval</code> variable that
	    corresponds to the timestamp associated with the specified
	    <code class="varname">packet</code> variable.
	  </p><p>
	    <span><strong class="command">packet tdelta &lt;packet variable 1&gt;
	    &lt;packet variable 2&gt;</strong></span> calculates the time
	    delta between the <code class="varname">timestamp</code> associated
	    with &lt;packet variable 1&gt; and the
	    <code class="varname">timestamp</code> associated with &lt;packet
	    variable 2&gt;.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">pdu</strong></span> <span class="emphasis"><em>&lt;new | delete | dup |
	  append | count | build | list&gt; &lt;args&gt;</em></span>
	</span></dt><dd><p>
	    The <span><strong class="command">pdu</strong></span> command is used to manage
	    variables of type <code class="varname">pdu</code> (Protocol Data
	    Unit.)
	  </p><p>
	    <span><strong class="command">pdu new &lt;PDU definition&gt;</strong></span> will
	    create and return a new <code class="varname">pdu</code>
	    variable. The PDU definition is the same as used in other
	    <span><strong class="command">Network Expect</strong></span> commands like
	    <span><strong class="command">send_network</strong></span> and
	    <span><strong class="command">send_expect</strong></span>.
	  </p><p>
	    <span><strong class="command">pdu delete &lt;pdu variable&gt;</strong></span> will
	    delete a <code class="varname">pdu</code> variable.
	  </p><p>
	    <span><strong class="command">pdu dup &lt;pdu variable&gt;</strong></span> will
	    duplicate a <code class="varname">pdu</code> variable.
	  </p><p>
	    <span><strong class="command">pdu append &lt;pdu variable 1&gt; &lt;pdu
	    variable 2&gt;</strong></span> will append the
	    <code class="varname">pdu</code> variable <code class="varname">pdu variable
	    2</code> to the <code class="varname">pdu</code> variable
	    <code class="varname">pdu variable 1</code>. The result is
	    <code class="varname">pdu variable 1</code>.
	  </p><p>
	    <span><strong class="command">pdu count &lt;pdu variable&gt;</strong></span> returns
	    the number of packets that would be generated if that PDU
	    were to be sent using the <span><strong class="command">send_network</strong></span>
	    command, for example. The number of packets depends on the
	    numeric specifications used in the definition of the PDU.
	  </p><p>
	    <span><strong class="command">pdu build &lt;pdu variable&gt;</strong></span> builds
	    the specified <code class="varname">pdu</code> variable and returns
	    a variable of type <code class="varname">packet</code>.
	  </p><p>
	    <span><strong class="command">pdu list</strong></span> displays the list of PDUs that
	    <span><strong class="command">Network Expect</strong></span> knows about, i.e. the
	    PDUs that can be created via certain <span><strong class="command">Network
	    Expect</strong></span> commands like
	    <span><strong class="command">send_network</strong></span>,
	    <span><strong class="command">send_expect</strong></span>, and <span><strong class="command">pdu
	    new</strong></span>.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">random</strong></span> <span class="emphasis"><em>[x:y | number x:y | ip
	  [a.b.c.d/nn | a.b.c.d:e.f.g.h] | mac]</em></span>
	</span></dt><dd><p>
	    The <span><strong class="command">random</strong></span> command can be used to
	    obtain a variety of random objects. Current supported
	    objects are numbers, IPv4 addresses, and MAC addresses. It
	    is possible to obtain a random number or IP address from
	    within a range. In the case of numbers the range is
	    specified using the notation x:y. In the case of IP
	    addresses the range can be specified using the IP
	    address/netmask bits notation, or using the
	    a.b.c.d:e.f.g.h notation. If no object is specified then a
	    random number is returned.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">send_expect</strong></span> <span class="emphasis"><em>[-i &lt;listener
	  ID&gt;] [-o &lt;speaker ID&gt;] [-timeout &lt;timeout&gt;]
	  [-n &lt;number of tries&gt;] &lt;PDU definition&gt;</em></span>
	</span></dt><dd><p>
	    This command sends a number of packets to a target or list
	    of targets and then waits for responses. After responses
	    are received, the command matches sent packets with
	    received packets. This command was inspired by
	    the <span><strong class="command">sr()</strong></span> family of commands in
	    <span><strong class="command">Scapy</strong></span>, the packet manipulation tool by
	    Philippe Biondi.
	  </p><p>
	    The <code class="option">-i</code> flag specifies what listener to
	    use to read responses. It is possible to specify multiple
	    listeners by using this option multiple times.
	  </p><p>
	    The <code class="option">-o</code> flag specifies what speaker to use
	    when injecting the stimulus.
	  </p><p>
	    <code class="option">-timeout</code> specifies how long to wait,
	    after all packets have been sent, for answers to the
	    injected stimulus. The timeout is specified in seconds and
	    the default is 1 second.
	  </p><p>
	    <code class="option">-n</code> specifies how many times to retry
	    sending the stimulus if not all sent packets have a
	    corresponding received answer.
	  </p><p>
	    The definition of the PDU to send is the same as it used
	    in the <span><strong class="command">send_network</strong></span> command.
	  </p><p>
	    The <span><strong class="command">send_expect</strong></span> command creates three
	    lists: the list of sent packets and the corresponding list
	    of matching answers, and the list of unanswered packets. A
	    script can then use these lists, decode packets, and
	    present some useful information. For example, the
	    following code snippet sends ICMP echo requests to all hosts
	    in a network a displays which hosts replied:
	  </p><pre class="programlisting">
set network 192.168.1.1-192.168.1.254

# Spawn a listener. We don't really have to specify a filter,
# like in "spawn_network {icmp[icmptype] == icmp-echoreply}"
# because the send_expect command will intelligently match
# injected stimulus (ICMP echo requests) with received
# answers (ICMP echo replies). A filter means less work for
# the send_expect command, but other than that it adds nothing.
spawn_network

send_expect -n 2 -4 -D $network -icmp-echo random:random \
    -payload "12345678901234567890" -delay .001

puts "\n[llength $_(received)] hosts sent echo-replies back:\n"

foreach r $_(received) s $_(sent) {
    packet decode r
    puts [format "$pdu(1,tot_len) bytes from $ip(src): ttl=$ip(ttl) time=%.3f ms" [expr [packet tdelta r s]*1000] ]
}
	  </pre></dd><dt><span class="term">
	  <span><strong class="command">send_network</strong></span> <span class="emphasis"><em>[options] &lt;PDU
	  definition&gt;</em></span>
	</span></dt><dd><p>
	    This command allows the creation custom TCP/IP packets
	    based on packet definitions provided by the user through
	    command-line arguments passed to the program, through a
	    command file specified in the command-line, or through a
	    mix of these two methods. <span><strong class="command">send_network</strong></span>
	    refers to instances of protocols in the TCP/IP protocol
	    suite as <span class="emphasis"><em>protocol data units</em></span>, or PDUs
	    for short. A protocol data unit always has a header (think
	    of the IP version 4 header, or the TCP header), may or may
	    not have options (think of the TCP options, or IP version
	    6 extension headers), and may or may not have a
	    payload. Due to the extensive use of encapsulation in the
	    TCP/IP protocol suite, the payload of a PDU can be another
	    payload. For example, an Ethernet frame can have as its
	    payload an IP version 4 packet, which in itself is another
	    PDU. Then this IP version 4 packet can have as its payload
	    a TCP segment, which is another PDU, and so on.
	  </p><p>
	    When defining PDUs that <span><strong class="command">send_network</strong></span>
	    will create you start by creating the definitions for PDUs
	    that are closer to the physical layer, and then move up
	    the protocol stack until you reach, in some cases, and if
	    that is what you want, the application layer. To implement
	    this approach through a command-line interface (CLI) you
	    start by entering the lower-level PDUs closer to the
	    beginning of the command-line, or, if you are reading your
	    packet definitions from a file, by entering definitions
	    for lower-level PDUs at the beginning of the file. After
	    you have defined a certain PDU you cannot define another
	    PDU that is lower than the previous in the protocol
	    stack. For example, you cannot define a TCP segment and
	    then create an Ethernet frame as the segment's payload,
	    just because that does not make sense (defining a TCP
	    segment and then a BGP message does make sense, for
	    example.) For this reason, order does matter in the
	    command-line (or in the file, if defining PDUs in a file)
	    when creating the PDUs.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">sleep</strong></span> <span class="emphasis"><em>&lt;seconds&gt;</em></span>
	</span></dt><dd><p>
             causes the script to sleep for the given number of
             seconds. <code class="option">seconds</code> may be a decimal
             number.  Interrupts are processed while <span><strong class="command">Network
             Expect</strong></span> sleeps.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">spawn_network</strong></span>
	  <span class="emphasis"><em>[-nolistener]</em></span>
	  <span class="emphasis"><em>[-fullspeed]</em></span>
	  <span class="emphasis"><em>[-info]</em></span> <span class="emphasis"><em>[-i &lt;input
	  interface&gt;]</em></span> <span class="emphasis"><em>[-o &lt;output
	  interface&gt;]</em></span> <span class="emphasis"><em>[-r &lt;input PCAP
	  file&gt;]</em></span> <span class="emphasis"><em>[-p]</em></span> <span class="emphasis"><em>[-s
	  &lt;snaplen&gt;]</em></span> <span class="emphasis"><em>[-w &lt;output PCAP
	  file&gt;]</em></span> <span class="emphasis"><em>[-hexdump]</em></span>
	  <span class="emphasis"><em>[-stdout]</em></span> <span class="emphasis"><em>[-4]</em></span>
	  <span class="emphasis"><em>[-6]</em></span> <span class="emphasis"><em>[&lt;PCAP
	  filter&gt;]</em></span>
	</span></dt><dd><p>
	    The <span><strong class="command">spawn_network</strong></span> command is used to
	    find information about existing listeners and speakers or
	    to create network speakers and network listeners.
	  </p><p>
	    To find information about existing network listeners and
	    speakers the <span><strong class="command">spawn_network</strong></span> command
	    needs to be invoked with only the <code class="option">-info</code>
	    option.
	  </p><p>
	    Listeners are created by using the options
	    <code class="option">-i</code> or
	    <code class="option">-r</code>. <code class="option">-i</code> specifies an
	    interface to listen for traffic on, and
	    <code class="option">-r</code> specifies the use of a PCAP file for
	    reading instead of reading live traffic from an
	    interface. In both cases, an optional <code class="option">PCAP
	    filter</code> can be specified to limit the type of
	    traffic that will be read. If <code class="option">-i</code> is not
	    used then <span><strong class="command">Network Expect</strong></span> will try to
	    find a suitable interface. <code class="option">-fullspeed</code>
	    causes reading from a PCAP file at full speed, without
	    preserving the inter-packet delay present in the
	    savefile. If this option is not specified then the
	    inter-packet delay present in the savefile will not be
	    preserved.
	  </p><p>
	    Speakers are created using one of <code class="option">-o</code>,
	    <code class="option">-w</code>, <code class="option">hexdump</code>,
	    <code class="option">stdout</code>, <code class="option">-4</code>, or
	    <code class="option">-6</code>. <code class="option">-o</code> specifies the use
	    of an interface for injecting traffic into the
	    network. This implies the use of layer 2 injection in
	    which the Ethernet header is specified by the user.
	    <code class="option">-w</code> specifies that all traffic will be
	    sent to the PCAP file <code class="option">PCAP file</code>. This
	    option requires the use of layer 2 injection, i.e. the
	    Ethernet header must be
	    included. <code class="option">-hexdump</code> specifies that all
	    packets be sent to standard output in hexadecimal format.
	    <code class="option">-stdout</code> causes all packets to be sent to
	    standard output in raw format. The <code class="option">-4</code> and
	    <code class="option">-6</code> options specify the creation of layer
	    3 speakers to inject IPv4 and IPv6 traffic
	    respectively. In this case all routing decisions are
	    performed by the kernel.
	  </p><p>
	    The <code class="option">-p</code> and <code class="option">-s</code> options
	    are only meaningful for listener
	    creation. <code class="option">-p</code> specifies that the interface
	    be not put in promiscuous mode when creating a listener
	    that will listen on an interface. <code class="option">-s</code>
	    specifies the snapshot length of captured packets. These
	    two options are equivalent to the
	    tcpdump(8) options of the same names.
	  </p><p>
	    Note that if no options to create a speaker are specified,
	    i.e.  <code class="option">-o</code>, <code class="option">-w</code>,
	    <code class="option">hexdump</code>, <code class="option">stdout</code>,
	    <code class="option">-4</code>, or <code class="option">-6</code>, the default
	    action is to only create a listener. If only a speaker is
	    desired one of the options to create a speaker must be
	    specified and the <code class="option">-nolistner</code> option must
	    be used.
	  </p><p>
	    As an example, the follow command creates a listener on
	    interface eth0 for ARP requests for the MAC address of
	    host 192.168.1.1:
	  </p><pre class="programlisting">
spawn_network -i eth0 host 192.168.1.1 and {arp[6:2]} == 1
	  </pre><p>
	    Note that curly braces are necessary around "arp[6:2]"
	    because brackets have special meaning in Tcl.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">system</strong></span> <span class="emphasis"><em>&lt;system command&gt;
	  [&lt;args&gt;]</em></span>
	</span></dt><dd><p>
	    The <span><strong class="command">system</strong></span> command allows to run
	    arbitrary system commands from within a <span><strong class="command">Network
	    Expect</strong></span> script.
	  </p><p>
	    Please note that this command does no input validation at
	    all, which means that it is very insecure. For example
	    "system {ls;/bin/sh}" will give you a shell. If nexp is
	    run as roon then the shell will be a root shell.
	  </p><p>
	    The <span><strong class="command">exec</strong></span> command in the standard Tcl
	    distribution is a much better alternative to the
	    <span><strong class="command">Network Expect</strong></span>
	    <span><strong class="command">system</strong></span> command. Please see the Tcl
	    documentation for the <span><strong class="command">exec</strong></span> command for
	    additional information.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">timeval</strong></span> <span class="emphasis"><em>new | delta
	  &lt;args&gt;</em></span>
	</span></dt><dd><p>
	    The <span><strong class="command">timeval</strong></span> command allows to manage
	    variables of type <code class="varname">timeval</code>.
	  </p><p>
	    <span><strong class="command">timeval new;</strong></span> returns a
	    <code class="varname">timeval</code> variable that corresponds to
	    the current system time.
	  </p><p>
	    <span><strong class="command">timeval tdelta &lt;timeval variable 1&gt;
	    &lt;timeval variable 2&gt;</strong></span> calculates the time
	    delta between the <code class="varname">timeval variable 2</code> and
	    <code class="varname">timeval variable 1</code>.
	  </p></dd><dt><span class="term">
	  <span><strong class="command">txdelta</strong></span> <span class="emphasis"><em>&lt;speaker ID&gt;</em></span>
	</span></dt><dd><p>
	    Every time a packet is sent, the time the packet was sent
	    is saved to the network speaker that was used to send
	    that packet. The <span><strong class="command">txdelta</strong></span> (short for
	    "transmission delta") command returns the number of
	    seconds that have elapsed since the last packet that was
	    sent via the specified <code class="option">speaker ID</code>.
	  </p><p>
	    The following example provides the foundation for a very
	    simple ping program, and shows how the
	    <span><strong class="command">txdelta</strong></span> command can be used:
	  </p><pre class="programlisting">
for {set id 0; set seq 0} {1} {incr seq} {
    send_network -4 -D $target -icmp-echo $id:$seq -payload "12345678901234567890"

    expect_network -timeout 1 {$icmp(type) == 0 &amp;&amp; $icmp(id) == $id} {
        puts [format "$pdu(2,tot_len) bytes from $ip(src): icmp_seq=$seq ttl=$ip(ttl) time=%.3f ms" [expr [txdelta ip]*1000 ] ]
        sleep [expr 1.0 - [txdelta ip] ]
    }
}
	  </pre></dd></dl></div></div><div class="refsect1" lang="en"><a name="id2490463"></a><h2>SPECIAL VARIABLES</h2><p>
      Special variables.
    </p></div><div class="refsect1" lang="en"><a name="id2490475"></a><h2>EXAMPLES</h2><p>
      The <code class="filename">examples</code> directory in
      the <span><strong class="command">Network Expect</strong></span> distribution contains
      plenty of examples that should provide a very good idea of how
      to use <span><strong class="command">Network Expect</strong></span>. A few selected
      examples have been selected for this manual page.
    </p><div class="orderedlist"><ol type="1"><li><p>
	  The following code snippet performs a TCP three-way
	  handshake. Everything is done by hand, which allows to play
	  with TCP initial sequence numbers (ISNs), window sizes, etc.
	  The code is a bit more complex that necessary because an
	  effort is made to handle error coditions (timeout, remote host
	  not listening on the destination port, strange combination of
	  TCP flags received in response to our initial SYN.)
	</p><pre class="programlisting">
# Some useful constants
set SYN 0x02
set RST 0x04
set ACK 0x10

set retries 3
set isn [random]
set myip 192.168.1.2
set target 192.168.1.1
set sport [random 20000:65535]
set dport 21
set window 4096

# Spawn a listener for TCP segments coming from the FTP server to us
spawn_network -i $interface "tcp and src host $target and dst host $myip and src port $dport and dst port $sport"


# Send TCP SYN
send_network ip(src = $myip, dst = $target)/ \
             tcp(src = $sport, dst = $dport, window = $window, \
                 syn, seq = $isn, ack-seq = 0)

# Wait for response from the server
expect_network {$tcp(flags) == ($SYN | $ACK)} {
    # Got a SYN+ACK so we need to send the final segment of the
    # 3-way HS
    send_network ip(dst = $myip, dst = $target)/ \
                 tcp(dst = $tcp(dstport), dst = $tcp(srcport), \
                     window = $window, ack, seq = $tcp(ack), \
                     ack-seq = [expr $tcp(seq) + 1])
} {$tcp(flags) &amp; $RST} {
    puts "Connection refused"
    exit 1
} {1} {
    # Any other weird combination of TCP flags we respond to
    # with a RST
    send_network ip(src = $myip, dst = $target)/ \
                 tcp(src = $tcp(dstport), dst = $tcp(srcport), \
                     rst)
    exit 1
} timeout {
    # Our SYN got lost in transit or it was filtered - perform
    # exponential backoff and retransmit the SYN...
    if {$retries &gt; 0} {
        incr retries -1
        set timeout [expr $timeout*2]
        puts "SYN timeout, increasing timeout to $timeout"
        send_network ip(src = $myip, dst =  $target)/ \
                     tcp(src = $sport, dst = $dport, \
                         window = $window, syn, seq = $isn, \
                         ack-seq = 0)
        nexp_continue
    } else {
        puts "Connection timed out"
        exit 1
    }
}

# TCP connection has been established. Now do something...
	</pre></li><li><p>
	  This illustrates how to create a phantom host on the network
	  that can respond to ICMP echo requests, and therefore, to
	  ARP requests as well.
	</p><pre class="programlisting">
set interface eth0
set myip 192.168.1.1
set mymac [random mac]

# Spawn a listener for ARP requests
spawn_network -i $interface host $myip and {arp[6:2]} == 1
set arpl $listener_id

# Spawn a listener for ICMP messages sent to us
spawn_network -i $interface icmp and dst host $myip
set icmpl $listener_id

expect_network_background -i $arpl {1} {
    # Received an ARP request, send ARP reply
    send_network -o $interface \
        ether(src = $mymac, dst = $arp(sha) )/ \
        arp-reply(tha = $arp(sha), tip = $arp(sip), \
                  sha = $mymac, sip = $myip)
    nexp_continue
} -i $icmpl {$icmp(type) == 8} {
    # Received ICMP echo request, send echo reply
    send_network -o ip \
        ip(src = $myip, dst = $ip(src) )/ \
        icmp-echoreply(id =  $icmp(id), seq = $icmp(seq) )/ \
        raw($raw)
    nexp_continue
}
	</pre></li><li><p>
	  This example how to create a very simple traceroute program
	  that uses TCP probes to port 80 of the target host. It uses
	  the <span><strong class="command">send_expect</strong></span> command.
	</p><pre class="programlisting">
set target "www.example.com"
set ttlrange "1:30"
set interface [outif $target]

# Spawn a listener. We don't really have to specify a filter because the
# send_expect command will intelligently match injected stimulus with
# received answers.
spawn_network -i $interface

send_expect -tries 2  -delay 0.001 \
    ip(id = random, dst = $target, ttl = $ttlrange)/ \
    tcp(src = random, dst = 80, syn)

foreach r $_(received) s $_(sent) {
    packet decode r
    set source $ip(src)
    set pdu_type $pdu(1,type)

    packet decode s

    puts [format "$ip(ttl) $source %.3f ms $pdu_type" [expr [packet tdelta r s]*1000] ]
}
	</pre></li><li><p>
	  This shows how to perform an ARP scan using regular
	  <span><strong class="command">send_network</strong></span> and
	  <span><strong class="command">expect_network</strong></span> commands:
	</p><pre class="programlisting">
set interface eth0
set network "$iface($interface,ip)/$iface($interface,netmask)"

set arprequest [pdu new -o $interface
    ether(dst = BROADCAST)/ \
    arp-request(tha = BROADCAST, tip = $network, \
                sha = $iface($interface,hw_addr), \
                sip = $iface($interface,ip) ) ]

# Spawn a listener for ARP replies
spawn_network -i $interface {arp[6:2]} == 2

for {set i 0} {$i &lt; [pdu count arprequest]} {incr i} {
    # Send ARP request
    send_network -count 1 arprequest

    # Read ARP reply
    expect_network -timeout .05 {1} {
        puts "$arp(sip) is at $arp(sha)"
    }
}
	</pre></li><li><p>
	  This example shows how to do an ARP scan but in a more
	  efficient manner using the <span><strong class="command">send_expect</strong></span>
	  command:
	</p><pre class="programlisting">
set interface eth0
set network "$iface($interface,ip)/$iface($interface,netmask)"

# Spawn a listener for ARP replies
spawn_network -i $interface {arp[6:2]} == 2

send_expect -o $interface -delay 0.001 -tries 2 \
    ether(dst = BROADCAST)/ \
    arp-request(tha = BROADCAST, tip = $network, \
	        sha = $iface($interface,hw_addr),
                sip = $iface($interface,ip) )

puts "\nFound [llength $_(received)] hosts alive:\n"

foreach r $_(received) {
    packet decode r
    puts "$arp(sip) is at $arp(sha)"
}
	</pre></li></ol></div></div><div class="refsect1" lang="en"><a name="id2490680"></a><h2>BUGS</h2><p>
      <span><strong class="command">Network Expect</strong></span> does not run very well under
      the Solaris operating system because in that operating system
      select() does not seem to work well with packet capture file
      descriptors (select() returns when there is no data ready to be
      read.)
    </p><p>
      <span><strong class="command">Network Expect</strong></span> does not work at all in
      Microsoft Windows because select() does not work at all with
      packet capture file descriptors (pcap_get_selectable_fd() does
      not exist under Microsoft Windows.)
    </p><p>
      Error-checking is almost non-existant.
    </p><p>
      These isn't input validation for the numeric specifications.
    </p><p>
      The parser of PDU definitions (and therefore the number of
      tokens that the parser handles) has become a big, wild
      beast. It's very easy to add new tokens and PDU definitions are
      elegant and pretty, but the parser is huge. Guess can't have
      everything.
    </p></div><div class="refsect1" lang="en"><a name="id2490724"></a><h2>VERSION</h2><p>This man page is correct for version 1.0 of <span><strong class="command">Network
      Expect</strong></span>.
    </p></div><div class="refsect1" lang="en"><a name="id2490739"></a><h2>SEE ALSO</h2><p>
      nexp-numspec(1),
      nexp-payload(1),
      nexp-ether(5),
      nexp-gre(5),
      nexp-ip(5),
      nexp-mpls(5),
      expect(1)
    </p></div><div class="refsect1" lang="en"><a name="id2490774"></a><h2>AUTHOR</h2><p>
      <span><strong class="command">Network Expect</strong></span> was written by Eloy Paris
      &lt;peloy@netexpect.org&gt;. However, <span><strong class="command">Network
      Expect</strong></span> borrows ideas from lots of Open Source tools
      like Nemesis, Packit, hping, Expect, and Scapy. The
      <span><strong class="command">Network Expect</strong></span> author is indebted to the
      authors of these tools for their contribution.
    </p><p>
      This man page was written by Eloy Paris although it borrows
      heavily from Expect's manual page.
    </p></div></div></body></html>