Adding new protocol (or more correctly: a new layer) in Scapy is very easy. All the magic is in the fields. If the fields you need are already there and the protocol is not too brain-damaged, this should be a matter of minutes.
A layer is a subclass of the Packet class. All the logic behind layer manipulation is hold by the Packet class and will be inherited. A simple layer is compounded by a list of fields that will be either concatenated when assembling the layer or dissected one by one when disassembling a string. The list of fields is held in an attribute named fields_desc. Each field is an instance of a field class:
class Disney(Packet):
name = "DisneyPacket "
fields_desc=[ ShortField("mickey",5),
XByteField("minnie",3) ,
IntEnumField("donald" , 1 ,
{ 1: "happy", 2: "cool" , 3: "angry" } ) ]
In this example, our layer has three fields. The first one is an 2 byte integer field named mickey and whose default value is 5. The second one is a 1 byte integer field named minnie and whose default value is 3. The difference between a vanilla ByteField and a XByteField is only the fact that the prefered human representation of the field’s value is in hexadecimal. The last field is a 4 byte integer field named donald. It is different from a vanilla IntField by the fact that some of the possible values of the field have litterate representations. For example, if it is worth 3, the value will be displayed as angry. Moreover, if the “cool” value is assigned to this field, it will understand that it has to take the value 2.
If your protocol is as simple as this, it is ready to use:
>>> d=Disney(mickey=1)
>>> ls(d)
mickey : ShortField = 1 (5)
minnie : XByteField = 3 (3)
donald : IntEnumField = 1 (1)
>>> d.show()
###[ Disney Packet ]###
mickey= 1
minnie= 0x3
donald= happy
>>> d.donald="cool"
>>> str(d)
’\x00\x01\x03\x00\x00\x00\x02’
>>> Disney( )
<Disney mickey=1 minnie=0x3 donald=cool |>
This chapter explains how to build a new protocol within Scapy. There are two main objectives:
Before digging into dissection itself, let us look at how packets are organized.
>>> p = IP()/TCP()/"AAAA"
>>> p
<IP frag=0 proto=TCP |<TCP |<Raw load='AAAA' |>>>
>>> p.summary()
'IP / TCP 127.0.0.1:ftp-data > 127.0.0.1:www S / Raw'
We are interested in 2 “inside” fields of the class Packet:
And here is the main “trick”. You do not care about packets, only about layers, stacked one after the other.
One can easily access a layer by its name: p[TCP] returns the TCP and followings layers. This is a shortcut for p.getlayer(TCP).
Note
There is an optional argument (nb) which returns the nb th layer of required protocol.
Let’s put everything together now, playing with the TCP layer:
>>> tcp=p[TCP]
>>> tcp.underlayer
<IP frag=0 proto=TCP |<TCP |<Raw load='AAAA' |>>>
>>> tcp.payload
<Raw load='AAAA' |>
As expected, tcp.underlayer points to the beginning of our IP packet, and tcp.payload to its payload.
VERY EASY! A layer is mainly a list of fields. Let’s look at UDP definition:
class UDP(Packet):
name = "UDP"
fields_desc = [ ShortEnumField("sport", 53, UDP_SERVICES),
ShortEnumField("dport", 53, UDP_SERVICES),
ShortField("len", None),
XShortField("chksum", None), ]
And you are done! There are many fields already defined for convenience, look at the doc``^W`` sources as Phil would say.
So, defining a layer is simply gathering fields in a list. The goal is here to provide the efficient default values for each field so the user does not have to give them when he builds a packet.
The main mechanism is based on the Field structure. Always keep in mind that a layer is just a little more than a list of fields, but not much more.
So, to understanding how layers are working, one needs to look quickly at how the fields are handled.
A field should be considered in different states:
i (nternal) : this is the way Scapy manipulates it.
on the network.
h (uman) : how the packet is displayed to our human eyes.
This explains the mysterious methods i2h(), i2m(), m2i() and so on available in each field: they are conversion from one state to another, adapted to a specific use.
Other special functions:
However, all these are “low level” functions. The functions adding or extracting a field to the current layer are:
addfield(self, pkt, s, val): copy the network representation of field val (belonging to layer pkt) to the raw string packet s:
class StrFixedLenField(StrField):
def addfield(self, pkt, s, val):
return s+struct.pack("%is"%self.length,self.i2m(pkt, val))
getfield(self, pkt, s): extract from the raw packet s the field value belonging to layer pkt. It returns a list, the 1st element is the raw packet string after having removed the extracted field, the second one is the extracted field itself in internal representation:
class StrFixedLenField(StrField):
def getfield(self, pkt, s):
return s[self.length:], self.m2i(pkt,s[:self.length])
When defining your own layer, you usually just need to define some *2*() methods, and sometimes also the addfield() and getfield().
There is way to represent integers on a variable length quantity often used in protocols, for instance when dealing with signal processing (e.g. MIDI).
Each byte of the number is coded with the MSB set to 1, except the last byte. For instance, 0x123456 will be coded as 0xC8E856:
def vlenq2str(l):
s = []
s.append( hex(l & 0x7F) )
l = l >> 7
while l>0:
s.append( hex(0x80 | (l & 0x7F) ) )
l = l >> 7
s.reverse()
return "".join(map( lambda(x) : chr(int(x, 16)) , s))
def str2vlenq(s=""):
i = l = 0
while i<len(s) and ord(s[i]) & 0x80:
l = l << 7
l = l + (ord(s[i]) & 0x7F)
i = i + 1
if i == len(s):
warning("Broken vlenq: no ending byte")
l = l << 7
l = l + (ord(s[i]) & 0x7F)
return s[i+1:], l
We will define a field which computes automatically the length of a associated string, but used that encoding format:
class VarLenQField(Field):
""" variable length quantities """
def __init__(self, name, default, fld):
Field.__init__(self, name, default)
self.fld = fld
def i2m(self, pkt, x):
if x is None:
f = pkt.get_field(self.fld)
x = f.i2len(pkt, pkt.getfieldval(self.fld))
x = vlenq2str(x)
return str(x)
def m2i(self, pkt, x):
if s is None:
return None, 0
return str2vlenq(x)[1]
def addfield(self, pkt, s, val):
return s+self.i2m(pkt, val)
def getfield(self, pkt, s):
return str2vlenq(s)
And now, define a layer using this kind of field:
class FOO(Packet):
name = "FOO"
fields_desc = [ VarLenQField("len", None, "data"),
StrLenField("data", "", "len") ]
>>> f = FOO(data="A"*129)
>>> f.show()
###[ FOO ]###
len= 0
data= 'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA'
Here, len is not yet computed and only the default value are displayed. This is the current internal representation of our layer. Let’s force the computation now:
>>> f.show2()
###[ FOO ]###
len= 129
data= 'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA'
The method show2() displays the fields with their values as they will be sent to the network, but in a human readable way, so we see len=129. Last but not least, let us look now at the machine representation:
>>> str(f)
'\x81\x01AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA'
The first 2 bytes are \x81\x01, which is 129 in this encoding.
Layers are only list of fields, but what is the glue between each field, and after, between each layer. These are the mysteries explain in this section.
The core function for dissection is Packet.dissect():
def dissect(self, s):
s = self.pre_dissect(s)
s = self.do_dissect(s)
s = self.post_dissect(s)
payl,pad = self.extract_padding(s)
self.do_dissect_payload(payl)
if pad and conf.padding:
self.add_payload(Padding(pad))
When called, s is a string containing what is going to be dissected. self points to the current layer.
>>> p=IP("A"*20)/TCP("B"*32)
WARNING: bad dataofs (4). Assuming dataofs=5
>>> p
<IP version=4L ihl=1L tos=0x41 len=16705 id=16705 flags=DF frag=321L ttl=65 proto=65 chksum=0x4141
src=65.65.65.65 dst=65.65.65.65 |<TCP sport=16962 dport=16962 seq=1111638594L ack=1111638594L dataofs=4L
reserved=2L flags=SE window=16962 chksum=0x4242 urgptr=16962 options=[] |<Raw load='BBBBBBBBBBBB' |>>>
Packet.dissect() is called 3 times:
For a given layer, everything is quite straightforward:
pre_dissect() is called to prepare the layer.
do_dissect() perform the real dissection of the layer.
post_dissection() is called when some updates are needed on the dissected inputs (e.g. deciphering, uncompressing, ... )
extract_padding() is an important function which should be called by every layer containing its own size, so that it can tell apart in the payload what is really related to this layer and what will be considered as additional padding bytes.
do_dissect_payload() is the function in charge of dissecting the payload (if any). It is based on guess_payload_class() (see below). Once the type of the payload is known, the payload is bound to the current layer with this new type:
def do_dissect_payload(self, s):
cls = self.guess_payload_class(s)
p = cls(s, _internal=1, _underlayer=self)
self.add_payload(p)
At the end, all the layers in the packet are dissected, and glued together with their known types.
The method with all the magic between a layer and its fields is do_dissect(). If you have understood the different representations of a layer, you should understand that “dissecting” a layer is building each of its fields from the machine to the internal representation.
Guess what? That is exactly what do_dissect() does:
def do_dissect(self, s):
flist = self.fields_desc[:]
flist.reverse()
while s and flist:
f = flist.pop()
s,fval = f.getfield(self, s)
self.fields[f] = fval
return s
So, it takes the raw string packet, and feed each field with it, as long as there are data or fields remaining:
>>> FOO("\xff\xff"+"B"*8)
<FOO len=2097090 data='BBBBBBB' |>
When writing FOO("\xff\xff"+"B"*8), it calls do_dissect(). The first field is VarLenQField. Thus, it takes bytes as long as their MSB is set, thus until (and including) the first ‘B‘. This mapping is done thanks to VarLenQField.getfield() and can be cross-checked:
>>> vlenq2str(2097090)
'\xff\xffB'
Then, the next field is extracted the same way, until 2097090 bytes are put in FOO.data (or less if 2097090 bytes are not available, as here).
If there are some bytes left after the dissection of the current layer, it is mapped in the same way to the what the next is expected to be (Raw by default):
>>> FOO("\x05"+"B"*8)
<FOO len=5 data='BBBBB' |<Raw load='BBB' |>>
Hence, we need now to understand how layers are bound together.
One of the cool features with Scapy when dissecting layers is that is try to guess for us what the next layer is. The official way to link 2 layers is using bind_layers():
For instance, if you have a class HTTP, you may expect that all the packets coming from or going to port 80 will be decoded as such. This is simply done that way:
bind_layers( TCP, HTTP, sport=80 )
bind_layers( TCP, HTTP, dport=80 )
That’s all folks! Now every packet related to port 80 will be associated to the layer HTTP, whether it is read from a pcap file or received from the network.
Sometimes, guessing the payload class is not as straightforward as defining a single port. For instance, it can depends on a value of a given byte in the current layer. The 2 needed methods are:
For instance, decoding 802.11 changes depending on whether it is ciphered or not:
class Dot11(Packet):
def guess_payload_class(self, payload):
if self.FCfield & 0x40:
return Dot11WEP
else:
return Packet.guess_payload_class(self, payload)
Several comments are needed here:
Most of the time, defining a method guess_payload_class() is not a necessity as the same result can be obtained from bind_layers().
If you do not like Scapy’s behavior for a given layer, you can either change or disable it through the call to split_layer(). For instance, if you do not want UDP/53 to be bound with DNS, just add in your code: `` split_layers(UDP, DNS, sport=53) `` Now every packet with source port 53 will not be handled as DNS, but whatever you specify instead.
In fact, each layer has a field payload_guess. When you use the bind_layers() way, it adds the defined next layers to that list.
>>> p=TCP()
>>> p.payload_guess
[({'dport': 2000}, <class 'scapy.Skinny'>), ({'sport': 2000}, <class 'scapy.Skinny'>), ... )]
Then, when it needs to guess the next layer class, it calls the default method Packet.guess_payload_class(). This method runs through each element of the list payload_guess, each element being a tuple:
So, the default guess_payload_class() tries all element in the list, until one matches. If no element are found, it then calls default_payload_class(). If you have redefined this method, then yours is called, otherwise, the default one is called, and Raw type is returned.
Packet.guess_payload_class()
Building a packet is as simple as building each layer. Then, some magic happens to glue everything. Let’s do magic then.
First thing to establish: what does “build” mean? As we have seen, a layer can be represented in different ways (human, internal, machine). Building means going to the machine format.
Second thing to understand is ‘’when’’ a layer is built. Answer is not that obvious, but as soon as you need the machine representation, the layers are built: when the packet is dropped on the network or written to a file, when it is converted as a string, ... In fact, machine representation should be regarded as a big string with the layers appended altogether.
>>> p = IP()/TCP()
>>> hexdump(p)
0000 45 00 00 28 00 01 00 00 40 06 7C CD 7F 00 00 01 E..(....@.|.....
0010 7F 00 00 01 00 14 00 50 00 00 00 00 00 00 00 00 .......P........
0020 50 02 20 00 91 7C 00 00 P. ..|..
In fact, using str() rather than show2() or any other method is not a random choice as all the functions building the packet calls Packet.__str__(). However, __str__() calls another method: build():
def __str__(self):
return self.__iter__().next().build()
What is important also to understand is that usually, you do not care about the machine representation, that is why the human and internal representations are here.
So, the core method is build() (the code has been shortened to keep only the relevant parts):
def build(self,internal=0):
pkt = self.do_build()
pay = self.build_payload()
p = self.post_build(pkt,pay)
if not internal:
pkt = self
while pkt.haslayer(Padding):
pkt = pkt.getlayer(Padding)
p += pkt.load
pkt = pkt.payload
return p
So, it starts by building the current layer, then the payload, and post_build() is called to update some late evaluated fields (like checksums). Last, the padding is added to the end of the packet.
Of course, building a layer is the same as building each of its fields, and that is exactly what do_build() does.
The building of each field of a layer is called in Packet.do_build():
def do_build(self):
p=""
for f in self.fields_desc:
p = f.addfield(self, p, self.getfieldval(f))
return p
The core function to build a field is addfield(). It takes the internal view of the field and put it at the end of p. Usually, this method calls i2m() and returns something like p.self.i2m(val) (where val=self.getfieldval(f)).
If val is set, then i2m() is just a matter of formatting the value the way it must be. For instance, if a byte is expected, struct.pack("B", val) is the right way to convert it.
However, things are more complicated if val is not set, it means no default value was provided earlier, and thus the field needs to compute some “stuff” right now or later.
“Right now” means thanks to i2m(), if all pieces of information is available. For instance, if you have to handle a length until a certain delimiter.
Ex: counting the length until a delimiter
class XNumberField(FieldLenField):
def __init__(self, name, default, sep="\r\n"):
FieldLenField.__init__(self, name, default, fld)
self.sep = sep
def i2m(self, pkt, x):
x = FieldLenField.i2m(self, pkt, x)
return "%02x" % x
def m2i(self, pkt, x):
return int(x, 16)
def addfield(self, pkt, s, val):
return s+self.i2m(pkt, val)
def getfield(self, pkt, s):
sep = s.find(self.sep)
return s[sep:], self.m2i(pkt, s[:sep])
In this example, in i2m(), if x has already a value, it is converted to its hexadecimal value. If no value is given, a length of “0” is returned.
The glue is provided by Packet.do_build() which calls Field.addfield() for each field in the layer, which in turn calls Field.i2m(): the layer is built IF a value was available.
A default value for a given field is sometimes either not known or impossible to compute when the fields are put together. For instance, if we used a XNumberField as defined previously in a layer, we expect it to be set to a given value when the packet is built. However, nothing is returned by i2m() if it is not set.
The answer to this problem is Packet.post_build().
When this method is called, the packet is already built, but some fields still need to be computed. This is typically what is required to compute checksums or lengths. In fact, this is required each time a field’s value depends on something which is not in the current
So, let us assume we have a packet with a XNumberField, and have a look to its building process:
class Foo(Packet):
fields_desc = [
ByteField("type", 0),
XNumberField("len", None, "\r\n"),
StrFixedLenField("sep", "\r\n", 2)
]
def post_build(self, p, pay):
if self.len is None and pay:
l = len(pay)
p = p[:1] + hex(l)[2:]+ p[2:]
return p+pay
When post_build() is called, p is the current layer, pay the payload, that is what has already been built. We want our length to be the full length of the data put after the separator, so we add its computation in post_build().
>>> p = Foo()/("X"*32)
>>> p.show2()
###[ Foo ]###
type= 0
len= 32
sep= '\r\n'
###[ Raw ]###
load= 'XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX'
len is correctly computed now:
>>> hexdump(str(p))
0000 00 32 30 0D 0A 58 58 58 58 58 58 58 58 58 58 58 .20..XXXXXXXXXXX
0010 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 58 XXXXXXXXXXXXXXXX
0020 58 58 58 58 58 XXXXX
And the machine representation is the expected one.
As we have previously seen, the dissection mechanism is built upon the links between the layers created by the programmer. However, it can also be used during the building process.
In the layer Foo(), our first byte is the type, which defines what comes next, e.g. if type=0, next layer is Bar0, if it is 1, next layer is Bar1, and so on. We would like then this field to be set automatically according to what comes next.
class Bar1(Packet):
fields_desc = [
IntField("val", 0),
]
class Bar2(Packet):
fields_desc = [
IPField("addr", "127.0.0.1")
]
If we use these classes with nothing else, we will have trouble when dissecting the packets as nothing binds Foo layer with the multiple Bar* even when we explicitly build the packet through the call to show2():
>>> p = Foo()/Bar1(val=1337)
>>> p
<Foo |<Bar1 val=1337 |>>
>>> p.show2()
###[ Foo ]###
type= 0
len= 4
sep= '\r\n'
###[ Raw ]###
load= '\x00\x00\x059'
Problems:
In order to understand what we should have done to obtain the proper behavior, we must look at how the layers are assembled. When two independent packets instances Foo() and Bar1(val=1337) are compounded with the ‘/’ operator, it results in a new packet where the two previous instances are cloned (i.e. are now two distinct objects structurally different, but holding the same values):
def __div__(self, other):
if isinstance(other, Packet):
cloneA = self.copy()
cloneB = other.copy()
cloneA.add_payload(cloneB)
return cloneA
elif type(other) is str:
return self/Raw(load=other)
The right hand side of the operator becomes the payload of the left hand side. This is performed through the call to add_payload(). Finally, the new packet is returned.
Note: we can observe that if other isn’t a Packet but a string, the Raw class is instantiated to form the payload. Like in this example:
>>> IP()/"AAAA"
<IP |<Raw load='AAAA' |>>
Well, what add_payload() should implement? Just a link between two packets? Not only, in our case this method will appropriately set the correct value to type.
Instinctively we feel that the upper layer (the right of ‘/’) can gather the values to set the fields to the lower layer (the left of ‘/’). Like previously explained, there is a convenient mechanism to specify the bindings in both directions between two neighbouring layers.
Once again, these information must be provided to bind_layers(), which will internally call bind_top_down() in charge to aggregate the fields to overload. In our case what we needs to specify is:
bind_layers( Foo, Bar1, {'type':1} )
bind_layers( Foo, Bar2, {'type':2} )
Then, add_payload() iterates over the overload_fields of the upper packet (the payload), get the fields associated to the lower packet (by its type) and insert them in overloaded_fields.
For now, when the value of this field will be requested, getfieldval() will return the value inserted in overloaded_fields.
The fields are dispatched between three dictionaries:
fields: fields whose the value have been explicitly set, like pdst in TCP (pdst='42')
overloaded_fields: overloaded fields
are initialized according to fields_desc by the constructor by calling init_fields() ).
In the following code we can observe how a field is selected and its value returned:
def getfieldval(self, attr):
for f in self.fields, self.overloaded_fields, self.default_fields:
if f.has_key(attr):
return f[attr]
return self.payload.getfieldval(attr)
Fields inserted in fields have the higher priority, then overloaded_fields, then finally default_fields. Hence, if the field type is set in overloaded_fields, its value will be returned instead of the value contained in default_fields.
We are now able to understand all the magic behind it!
>>> p = Foo()/Bar1(val=0x1337)
>>> p
<Foo type=1 |<Bar1 val=4919 |>>
>>> p.show()
###[ Foo ]###
type= 1
len= 4
sep= '\r\n'
###[ Bar1 ]###
val= 4919
Our 2 problems have been solved without us doing much: so good to be lazy :)
Last but not least, it is very useful to understand when each function is called when a packet is built:
>>> hexdump(str(p))
Packet.str=Foo
Packet.iter=Foo
Packet.iter=Bar1
Packet.build=Foo
Packet.build=Bar1
Packet.post_build=Bar1
Packet.post_build=Foo
As you can see, it first runs through the list of each field, and then build them starting from the beginning. Once all layers have been built, it then calls post_build() starting from the end.
Here’s a list of fields that Scapy supports out of the box:
Legend:
ByteField
XByteField
ShortField
LEShortField
XShortField
X3BytesField # three bytes (in hexad
IntField
SignedIntField
LEIntField
LESignedIntField
XIntField
LongField
XLongField
LELongField
IEEEFloatField
IEEEDoubleField
BCDFloatField # binary coded decimal
BitField
XBitField
BitFieldLenField # BitField specifying a length (used in RTP)
FlagsField
FloatField
Possible field values are taken from a given enumeration (list, dictionary, ...) e.g.:
ByteEnumField("code", 4, {1:"REQUEST",2:"RESPONSE",3:"SUCCESS",4:"FAILURE"})
EnumField(name, default, enum, fmt = "H")
CharEnumField
BitEnumField
ShortEnumField
LEShortEnumField
ByteEnumField
IntEnumField
SignedIntEnumField
LEIntEnumField
XShortEnumField
StrField(name, default, fmt="H", remain=0, shift=0)
StrLenField(name, default, fld=None, length_from=None, shift=0):
StrFixedLenField
StrNullField
StrStopField
FieldList(name, default, field, fld=None, shift=0, length_from=None, count_from=None)
# A list assembled and dissected with many times the same field type
# field: instance of the field that will be used to assemble and disassemble a list item
# length_from: name of the FieldLenField holding the list length
FieldLenField # holds the list length of a FieldList field
LEFieldLenField
LenField # contains len(pkt.payload)
PacketField # holds packets
PacketLenField # used e.g. in ISAKMP_payload_Proposal
PacketListField
This is about how fields that have a variable length can be handled with Scapy. These fields usually know their length from another field. Let’s call them varfield and lenfield. The idea is to make each field reference the other so that when a packet is dissected, varfield can know its length from lenfield when a packet is assembled, you don’t have to fill lenfield, that will deduce its value directly from varfield value.
Problems arise whe you realize that the relation between lenfield and varfield is not always straightforward. Sometimes, lenfield indicates a length in bytes, sometimes a number of objects. Sometimes the length includes the header part, so that you must substract the fixed header length to deduce the varfield length. Sometimes the length is not counted in bytes but in 16bits words. Sometimes the same lenfield is used by two different varfields. Sometimes the same varfield is referenced by two lenfields, one in bytes one in 16bits words.
First, a lenfield is declared using FieldLenField (or a derivate). If its value is None when assembling a packet, its value will be deduced from the varfield that was referenced. The reference is done using either the length_of parameter or the count_of parameter. The count_of parameter has a meaning only when varfield is a field that holds a list (PacketListField or FieldListField). The value will be the name of the varfield, as a string. According to which parameter is used the i2len() or i2count() method will be called on the varfield value. The returned value will the be adjusted by the function provided in the adjust parameter. adjust will be applied on 2 arguments: the packet instance and the value returned by i2len() or i2count(). By default, adjust does nothing:
adjust=lambda pkt,x: x
For instance, if the_varfield is a list
FieldLenField("the_lenfield", None, count_of="the_varfield")
or if the length is in 16bits words:
FieldLenField("the_lenfield", None, length_of="the_varfield", adjust=lambda pkt,x:(x+1)/2)
A varfield can be: StrLenField, PacketLenField, PacketListField, FieldListField, ...
For the two firsts, whe a packet is being dissected, their lengths are deduced from a lenfield already dissected. The link is done using the length_from parameter, which takes a function that, applied to the partly dissected packet, returns the length in bytes to take for the field. For instance:
StrLenField("the_varfield", "the_default_value", length_from = lambda pkt: pkt.the_lenfield)
or
StrLenField("the_varfield", "the_default_value", length_from = lambda pkt: pkt.the_lenfield-12)
For the PacketListField and FieldListField and their derivatives, they work as above when they need a length. If they need a number of elements, the length_from parameter must be ignored and the count_from parameter must be used instead. For instance:
FieldListField("the_varfield", ["1.2.3.4"], IPField("", "0.0.0.0"), count_from = lambda pkt: pkt.the_lenfield)
class TestSLF(Packet):
fields_desc=[ FieldLenField("len", None, length_of="data"),
StrLenField("data", "", length_from=lambda pkt:pkt.len) ]
class TestPLF(Packet):
fields_desc=[ FieldLenField("len", None, count_of="plist"),
PacketListField("plist", None, IP, count_from=lambda pkt:pkt.len) ]
class TestFLF(Packet):
fields_desc=[
FieldLenField("the_lenfield", None, count_of="the_varfield"),
FieldListField("the_varfield", ["1.2.3.4"], IPField("", "0.0.0.0"),
count_from = lambda pkt: pkt.the_lenfield) ]
class TestPkt(Packet):
fields_desc = [ ByteField("f1",65),
ShortField("f2",0x4244) ]
def extract_padding(self, p):
return "", p
class TestPLF2(Packet):
fields_desc = [ FieldLenField("len1", None, count_of="plist",fmt="H", adjust=lambda pkt,x:x+2),
FieldLenField("len2", None, length_of="plist",fmt="I", adjust=lambda pkt,x:(x+1)/2),
PacketListField("plist", None, TestPkt, length_from=lambda x:(x.len2*2)/3*3) ]
Test the FieldListField class:
>>> TestFLF("\x00\x02ABCDEFGHIJKL")
<TestFLF the_lenfield=2 the_varfield=['65.66.67.68', '69.70.71.72'] |<Raw load='IJKL' |>>
Emph # Wrapper to emphasize field when printing, e.g. Emph(IPField("dst", "127.0.0.1")),
ActionField
ConditionalField(fld, cond)
# Wrapper to make field 'fld' only appear if
# function 'cond' evals to True, e.g.
# ConditionalField(XShortField("chksum",None),lambda pkt:pkt.chksumpresent==1)
PadField(fld, align, padwith=None)
# Add bytes after the proxified field so that it ends at
# the specified alignment from its beginning
IPField
SourceIPField
IPoptionsField
TCPOptionsField
MACField
DestMACField(MACField)
SourceMACField(MACField)
ARPSourceMACField(MACField)
ICMPTimeStampField
Dot11AddrMACField
Dot11Addr2MACField
Dot11Addr3MACField
Dot11Addr4MACField
Dot11SCField
DNSStrField
DNSRRCountField
DNSRRField
DNSQRField
RDataField
RDLenField
ASN1F_element
ASN1F_field
ASN1F_INTEGER
ASN1F_enum_INTEGER
ASN1F_STRING
ASN1F_OID
ASN1F_SEQUENCE
ASN1F_SEQUENCE_OF
ASN1F_PACKET
ASN1F_CHOICE
NetBIOSNameField # NetBIOS (StrFixedLenField)
ISAKMPTransformSetField # ISAKMP (StrLenField)
TimeStampField # NTP (BitField)