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Commit a7bf04f3 authored by Dirk Loss's avatar Dirk Loss
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Added ASN.1, SNMP and Automata sections 

This was taken from Phil's articles on the Wiki.
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**************
Advanced usage
**************
ASN.1 and SNMP
==============
What is ASN.1?
--------------
.. note::
This is only my view on ASN.1, explained as simply as possible. For more theoretical or academic views, I'm sure you'll find better on the Internet.
ASN.1 is a notation whose goal is to specify formats for data exchange. It is independant of the way data is encoded. Data encoding is specified in Encoding Rules.
The most used encoding rules are BER (Basic Encoding Rules) and DER (Distinguished Encoding Rules). Both look the same, but the latter is specified to guarantee uniqueness of encoding. This property is quite interesting when speaking about cryptography, hashes and signatures.
ASN.1 provides basic objects: integers, many kinds of strings, floats, booleans, containers, etc. They are grouped in the so called Universal class. A given protocol can provide other objects which will be grouped in the Context class. For example, SNMP defines PDU_GET or PDU_SET objects. There are also the Application and Private classes.
Each of theses objects is given a tag that will be used by the encoding rules. Tags from 1 are used for Universal class. 1 is boolean, 2 is integer, 3 is a bit string, 6 is an OID, 48 is for a sequence. Tags from the ``Context`` class begin at 0xa0. When encountering an object tagged by 0xa0, we'll need to know the context to be able to decode it. For example, in SNMP context, 0xa0 is a PDU_GET object, while in X509 context, it is a container for the certificate version.
Other objects are created by assembling all those basic brick objects. The composition is done using sequences and arrays (sets) of previously defined or existing objects. The final object (an X509 certificate, a SNMP packet) is a tree whose non-leaf nodes are sequences and sets objects (or derived context objects), and whose leaf nodes are integers, strings, OID, etc.
Scapy and ASN.1
---------------
Scapy provides a way to easily encode or decode ASN.1 and also program those encoders/decoders. It is quite more lax than what an ASN.1 parser should be, and it kind of ignores constraints. It won't replace neither an ASN.1 parser nor an ASN.1 compiler. Actually, it has been written to be able to encode and decode broken ASN.1. It can handle corrupted encoded strings and can also create those.
ASN.1 engine
^^^^^^^^^^^^
Note: many of the classes definitions presented here use metaclasses. If you don't look precisely at the source code and you only rely on my captures, you may think they sometimes exhibit a kind of magic behaviour.
``
Scapy ASN.1 engine provides classes to link objects and their tags. They inherit from the ``ASN1_Class``. The first one is ``ASN1_Class_UNIVERSAL``, which provide tags for most Universal objects. Each new context (``SNMP``, ``X509``) will inherit from it and add its own objects.
::
class ASN1_Class_UNIVERSAL(ASN1_Class):
name = "UNIVERSAL"
# [...]
BOOLEAN = 1
INTEGER = 2
BIT_STRING = 3
# [...]
class ASN1_Class_SNMP(ASN1_Class_UNIVERSAL):
name="SNMP"
PDU_GET = 0xa0
PDU_NEXT = 0xa1
PDU_RESPONSE = 0xa2
class ASN1_Class_X509(ASN1_Class_UNIVERSAL):
name="X509"
CONT0 = 0xa0
CONT1 = 0xa1
# [...]
All ASN.1 objects are represented by simple Python instances that act as nutshells for the raw values. The simple logic is handled by ``ASN1_Object`` whose they inherit from. Hence they are quite simple::
class ASN1_INTEGER(ASN1_Object):
tag = ASN1_Class_UNIVERSAL.INTEGER
class ASN1_STRING(ASN1_Object):
tag = ASN1_Class_UNIVERSAL.STRING
class ASN1_BIT_STRING(ASN1_STRING):
tag = ASN1_Class_UNIVERSAL.BIT_STRING
These instances can be assembled to create an ASN.1 tree::
>>> x=ASN1_SEQUENCE([ASN1_INTEGER(7),ASN1_STRING("egg"),ASN1_SEQUENCE([ASN1_BOOLEAN(False)])])
>>> x
<ASN1_SEQUENCE[[<ASN1_INTEGER[7]>, <ASN1_STRING['egg']>, <ASN1_SEQUENCE[[<ASN1_BOOLEAN[False]>]]>]]>
>>> x.show()
# ASN1_SEQUENCE:
<ASN1_INTEGER[7]>
<ASN1_STRING['egg']>
# ASN1_SEQUENCE:
<ASN1_BOOLEAN[False]>
Encoding engines
^^^^^^^^^^^^^^^^^
As with the standard, ASN.1 and encoding are independent. We have just seen how to create a compounded ASN.1 object. To encode or decode it, we need to choose an encoding rule. Scapy provides only BER for the moment (actually, it may be DER. DER looks like BER except only minimal encoding is authorised which may well be what I did). I call this an ASN.1 codec.
Encoding and decoding are done using class methods provided by the codec. For example the ``BERcodec_INTEGER`` class provides a ``.enc()`` and a ``.dec()`` class methods that can convert between an encoded string and a value of their type. They all inherit from BERcodec_Object which is able to decode objects from any type::
>>> BERcodec_INTEGER.enc(7)
'\x02\x01\x07'
>>> BERcodec_BIT_STRING.enc("egg")
'\x03\x03egg'
>>> BERcodec_STRING.enc("egg")
'\x04\x03egg'
>>> BERcodec_STRING.dec('\x04\x03egg')
(<ASN1_STRING['egg']>, '')
>>> BERcodec_STRING.dec('\x03\x03egg')
Traceback (most recent call last):
File "<console>", line 1, in ?
File "/usr/bin/scapy", line 2099, in dec
return cls.do_dec(s, context, safe)
File "/usr/bin/scapy", line 2178, in do_dec
l,s,t = cls.check_type_check_len(s)
File "/usr/bin/scapy", line 2076, in check_type_check_len
l,s3 = cls.check_type_get_len(s)
File "/usr/bin/scapy", line 2069, in check_type_get_len
s2 = cls.check_type(s)
File "/usr/bin/scapy", line 2065, in check_type
(cls.__name__, ord(s[0]), ord(s[0]),cls.tag), remaining=s)
BER_BadTag_Decoding_Error: BERcodec_STRING: Got tag [3/0x3] while expecting <ASN1Tag STRING[4]>
### Already decoded ###
None
### Remaining ###
'\x03\x03egg'
>>> BERcodec_Object.dec('\x03\x03egg')
(<ASN1_BIT_STRING['egg']>, '')
ASN.1 objects are encoded using their ``.enc()`` method. This method must be called with the codec we want to use. All codecs are referenced in the ASN1_Codecs object. str() can also be used. In this case, the default codec (``conf.ASN1_default_codec``) will be used.
::
>>> x.enc(ASN1_Codecs.BER)
'0\r\x02\x01\x07\x04\x03egg0\x03\x01\x01\x00'
>>> str(x)
'0\r\x02\x01\x07\x04\x03egg0\x03\x01\x01\x00'
>>> xx,remain = BERcodec_Object.dec(_)
>>> xx.show()
# ASN1_SEQUENCE:
<ASN1_INTEGER[7L]>
<ASN1_STRING['egg']>
# ASN1_SEQUENCE:
<ASN1_BOOLEAN[0L]>
>>> remain
''
By default, decoding is done using the ``Universal`` class, which means objects defined in the ``Context`` class will not be decoded. There is a good reason for that: the decoding depends on the context!
::
>>> cert="""
... MIIF5jCCA86gAwIBAgIBATANBgkqhkiG9w0BAQUFADCBgzELMAkGA1UEBhMC
... VVMxHTAbBgNVBAoTFEFPTCBUaW1lIFdhcm5lciBJbmMuMRwwGgYDVQQLExNB
... bWVyaWNhIE9ubGluZSBJbmMuMTcwNQYDVQQDEy5BT0wgVGltZSBXYXJuZXIg
... Um9vdCBDZXJ0aWZpY2F0aW9uIEF1dGhvcml0eSAyMB4XDTAyMDUyOTA2MDAw
... MFoXDTM3MDkyODIzNDMwMFowgYMxCzAJBgNVBAYTAlVTMR0wGwYDVQQKExRB
... T0wgVGltZSBXYXJuZXIgSW5jLjEcMBoGA1UECxMTQW1lcmljYSBPbmxpbmUg
... SW5jLjE3MDUGA1UEAxMuQU9MIFRpbWUgV2FybmVyIFJvb3QgQ2VydGlmaWNh
... dGlvbiBBdXRob3JpdHkgMjCCAiIwDQYJKoZIhvcNAQEBBQADggIPADCCAgoC
... ggIBALQ3WggWmRToVbEbJGv8x4vmh6mJ7ouZzU9AhqS2TcnZsdw8TQ2FTBVs
... RotSeJ/4I/1n9SQ6aF3Q92RhQVSji6UI0ilbm2BPJoPRYxJWSXakFsKlnUWs
... i4SVqBax7J/qJBrvuVdcmiQhLE0OcR+mrF1FdAOYxFSMFkpBd4aVdQxHAWZg
... /BXxD+r1FHjHDtdugRxev17nOirYlxcwfACtCJ0zr7iZYYCLqJV+FNwSbKTQ
... 2O9ASQI2+W6p1h2WVgSysy0WVoaP2SBXgM1nEG2wTPDaRrbqJS5Gr42whTg0
... ixQmgiusrpkLjhTXUr2eacOGAgvqdnUxCc4zGSGFQ+aJLZ8lN2fxI2rSAG2X
... +Z/nKcrdH9cG6rjJuQkhn8g/BsXS6RJGAE57COtCPStIbp1n3UsC5ETzkxml
... J85per5n0/xQpCyrw2u544BMzwVhSyvcG7mm0tCq9Stz+86QNZ8MUhy/XCFh
... EVsVS6kkUfykXPcXnbDS+gfpj1bkGoxoigTTfFrjnqKhynFbotSg5ymFXQNo
... Kk/SBtc9+cMDLz9l+WceR0DTYw/j1Y75hauXTLPXJuuWCpTehTacyH+BCQJJ
... Kg71ZDIMgtG6aoIbs0t0EfOMd9afv9w3pKdVBC/UMejTRrkDfNoSTllkt1Ex
... MVCgyhwn2RAurda9EGYrw7AiShJbAgMBAAGjYzBhMA8GA1UdEwEB/wQFMAMB
... Af8wHQYDVR0OBBYEFE9pbQN+nZ8HGEO8txBO1b+pxCAoMB8GA1UdIwQYMBaA
... FE9pbQN+nZ8HGEO8txBO1b+pxCAoMA4GA1UdDwEB/wQEAwIBhjANBgkqhkiG
... 9w0BAQUFAAOCAgEAO/Ouyuguh4X7ZVnnrREUpVe8WJ8kEle7+z802u6teio0
... cnAxa8cZmIDJgt43d15Ui47y6mdPyXSEkVYJ1eV6moG2gcKtNuTxVBFT8zRF
... ASbI5Rq8NEQh3q0l/HYWdyGQgJhXnU7q7C+qPBR7V8F+GBRn7iTGvboVsNIY
... vbdVgaxTwOjdaRITQrcCtQVBynlQboIOcXKTRuidDV29rs4prWPVVRaAMCf/
... drr3uNZK49m1+VLQTkCpx+XCMseqdiThawVQ68W/ClTluUI8JPu3B5wwn3la
... 5uBAUhX0/Kr0VvlEl4ftDmVyXr4m+02kLQgH3thcoNyBM5kYJRF3p+v9WAks
... mWsbivNSPxpNSGDxoPYzAlOL7SUJuA0t7Zdz7NeWH45gDtoQmy8YJPamTQr5
... O8t1wswvziRpyQoijlmn94IM19drNZxDAGrElWe6nEXLuA4399xOAU++CrYD
... 062KRffaJ00psUjf5BHklka9bAI+1lHIlRcBFanyqqryvy9lG2/QuRqT9Y41
... xICHPpQvZuTpqP9BnHAqTyo5GJUefvthATxRCC4oGKQWDzH9OmwjkyB24f0H
... hdFbP9IcczLd+rn4jM8Ch3qaluTtT4mNU0OrDhPAARW0eTjb/G49nlG2uBOL
... Z8/5fNkiHfZdxRwBL5joeiQYvITX+txyW/fBOmg=
... """.decode("base64")
>>> (dcert,remain) = BERcodec_Object.dec(cert)
Traceback (most recent call last):
File "<console>", line 1, in ?
File "/usr/bin/scapy", line 2099, in dec
return cls.do_dec(s, context, safe)
File "/usr/bin/scapy", line 2094, in do_dec
return codec.dec(s,context,safe)
File "/usr/bin/scapy", line 2099, in dec
return cls.do_dec(s, context, safe)
File "/usr/bin/scapy", line 2218, in do_dec
o,s = BERcodec_Object.dec(s, context, safe)
File "/usr/bin/scapy", line 2099, in dec
return cls.do_dec(s, context, safe)
File "/usr/bin/scapy", line 2094, in do_dec
return codec.dec(s,context,safe)
File "/usr/bin/scapy", line 2099, in dec
return cls.do_dec(s, context, safe)
File "/usr/bin/scapy", line 2218, in do_dec
o,s = BERcodec_Object.dec(s, context, safe)
File "/usr/bin/scapy", line 2099, in dec
return cls.do_dec(s, context, safe)
File "/usr/bin/scapy", line 2092, in do_dec
raise BER_Decoding_Error("Unknown prefix [%02x] for [%r]" % (p,t), remaining=s)
BER_Decoding_Error: Unknown prefix [a0] for ['\xa0\x03\x02\x01\x02\x02\x01\x010\r\x06\t*\x86H...']
### Already decoded ###
[[]]
### Remaining ###
'\xa0\x03\x02\x01\x02\x02\x01\x010\r\x06\t*\x86H\x86\xf7\r\x01\x01\x05\x05\x000\x81\x831\x0b0\t\x06\x03U\x04\x06\x13\x02US1\x1d0\x1b\x06\x03U\x04\n\x13\x14AOL Time Warner Inc.1\x1c0\x1a\x06\x03U\x04\x0b\x13\x13America Online Inc.1705\x06\x03U\x04\x03\x13.AOL Time Warner Root Certification Authority 20\x1e\x17\r020529060000Z\x17\r370928234300Z0\x81\x831\x0b0\t\x06\x03U\x04\x06\x13\x02US1\x1d0\x1b\x06\x03U\x04\n\x13\x14AOL Time Warner Inc.1\x1c0\x1a\x06\x03U\x04\x0b\x13\x13America Online Inc.1705\x06\x03U\x04\x03\x13.AOL Time Warner Root Certification Authority 20\x82\x02"0\r\x06\t*\x86H\x86\xf7\r\x01\x01\x01\x05\x00\x03\x82\x02\x0f\x000\x82\x02\n\x02\x82\x02\x01\x00\xb47Z\x08\x16\x99\x14\xe8U\xb1\x1b$k\xfc\xc7\x8b\xe6\x87\xa9\x89\xee\x8b\x99\xcdO@\x86\xa4\xb6M\xc9\xd9\xb1\xdc<M\r\x85L\x15lF\x8bRx\x9f\xf8#\xfdg\xf5$:h]\xd0\xf7daAT\xa3\x8b\xa5\x08\xd2)[\x9b`O&\x83\xd1c\x12VIv\xa4\x16\xc2\xa5\x9dE\xac\x8b\x84\x95\xa8\x16\xb1\xec\x9f\xea$\x1a\xef\xb9W\\\x9a$!,M\x0eq\x1f\xa6\xac]Et\x03\x98\xc4T\x8c\x16JAw\x86\x95u\x0cG\x01f`\xfc\x15\xf1\x0f\xea\xf5\x14x\xc7\x0e\xd7n\x81\x1c^\xbf^\xe7:*\xd8\x97\x170|\x00\xad\x08\x9d3\xaf\xb8\x99a\x80\x8b\xa8\x95~\x14\xdc\x12l\xa4\xd0\xd8\xef@I\x026\xf9n\xa9\xd6\x1d\x96V\x04\xb2\xb3-\x16V\x86\x8f\xd9 W\x80\xcdg\x10m\xb0L\xf0\xdaF\xb6\xea%.F\xaf\x8d\xb0\x8584\x8b\x14&\x82+\xac\xae\x99\x0b\x8e\x14\xd7R\xbd\x9ei\xc3\x86\x02\x0b\xeavu1\t\xce3\x19!\x85C\xe6\x89-\x9f%7g\xf1#j\xd2\x00m\x97\xf9\x9f\xe7)\xca\xdd\x1f\xd7\x06\xea\xb8\xc9\xb9\t!\x9f\xc8?\x06\xc5\xd2\xe9\x12F\x00N{\x08\xebB=+Hn\x9dg\xddK\x02\xe4D\xf3\x93\x19\xa5\'\xceiz\xbeg\xd3\xfcP\xa4,\xab\xc3k\xb9\xe3\x80L\xcf\x05aK+\xdc\x1b\xb9\xa6\xd2\xd0\xaa\xf5+s\xfb\xce\x905\x9f\x0cR\x1c\xbf\\!a\x11[\x15K\xa9$Q\xfc\xa4\\\xf7\x17\x9d\xb0\xd2\xfa\x07\xe9\x8fV\xe4\x1a\x8ch\x8a\x04\xd3|Z\xe3\x9e\xa2\xa1\xcaq[\xa2\xd4\xa0\xe7)\x85]\x03h*O\xd2\x06\xd7=\xf9\xc3\x03/?e\xf9g\x1eG@\xd3c\x0f\xe3\xd5\x8e\xf9\x85\xab\x97L\xb3\xd7&\xeb\x96\n\x94\xde\x856\x9c\xc8\x7f\x81\t\x02I*\x0e\xf5d2\x0c\x82\xd1\xbaj\x82\x1b\xb3Kt\x11\xf3\x8cw\xd6\x9f\xbf\xdc7\xa4\xa7U\x04/\xd41\xe8\xd3F\xb9\x03|\xda\x12NYd\xb7Q11P\xa0\xca\x1c\'\xd9\x10.\xad\xd6\xbd\x10f+\xc3\xb0"J\x12[\x02\x03\x01\x00\x01\xa3c0a0\x0f\x06\x03U\x1d\x13\x01\x01\xff\x04\x050\x03\x01\x01\xff0\x1d\x06\x03U\x1d\x0e\x04\x16\x04\x14Oim\x03~\x9d\x9f\x07\x18C\xbc\xb7\x10N\xd5\xbf\xa9\xc4 (0\x1f\x06\x03U\x1d#\x04\x180\x16\x80\x14Oim\x03~\x9d\x9f\x07\x18C\xbc\xb7\x10N\xd5\xbf\xa9\xc4 (0\x0e\x06\x03U\x1d\x0f\x01\x01\xff\x04\x04\x03\x02\x01\x860\r\x06\t*\x86H\x86\xf7\r\x01\x01\x05\x05\x00\x03\x82\x02\x01\x00;\xf3\xae\xca\xe8.\x87\x85\xfbeY\xe7\xad\x11\x14\xa5W\xbcX\x9f$\x12W\xbb\xfb?4\xda\xee\xadz*4rp1k\xc7\x19\x98\x80\xc9\x82\xde7w^T\x8b\x8e\xf2\xeagO\xc9t\x84\x91V\t\xd5\xe5z\x9a\x81\xb6\x81\xc2\xad6\xe4\xf1T\x11S\xf34E\x01&\xc8\xe5\x1a\xbc4D!\xde\xad%\xfcv\x16w!\x90\x80\x98W\x9dN\xea\xec/\xaa<\x14{W\xc1~\x18\x14g\xee$\xc6\xbd\xba\x15\xb0\xd2\x18\xbd\xb7U\x81\xacS\xc0\xe8\xddi\x12\x13B\xb7\x02\xb5\x05A\xcayPn\x82\x0eqr\x93F\xe8\x9d\r]\xbd\xae\xce)\xadc\xd5U\x16\x800\'\xffv\xba\xf7\xb8\xd6J\xe3\xd9\xb5\xf9R\xd0N@\xa9\xc7\xe5\xc22\xc7\xaav$\xe1k\x05P\xeb\xc5\xbf\nT\xe5\xb9B<$\xfb\xb7\x07\x9c0\x9fyZ\xe6\xe0@R\x15\xf4\xfc\xaa\xf4V\xf9D\x97\x87\xed\x0eer^\xbe&\xfbM\xa4-\x08\x07\xde\xd8\\\xa0\xdc\x813\x99\x18%\x11w\xa7\xeb\xfdX\t,\x99k\x1b\x8a\xf3R?\x1aMH`\xf1\xa0\xf63\x02S\x8b\xed%\t\xb8\r-\xed\x97s\xec\xd7\x96\x1f\x8e`\x0e\xda\x10\x9b/\x18$\xf6\xa6M\n\xf9;\xcbu\xc2\xcc/\xce$i\xc9\n"\x8eY\xa7\xf7\x82\x0c\xd7\xd7k5\x9cC\x00j\xc4\x95g\xba\x9cE\xcb\xb8\x0e7\xf7\xdcN\x01O\xbe\n\xb6\x03\xd3\xad\x8aE\xf7\xda\'M)\xb1H\xdf\xe4\x11\xe4\x96F\xbdl\x02>\xd6Q\xc8\x95\x17\x01\x15\xa9\xf2\xaa\xaa\xf2\xbf/e\x1bo\xd0\xb9\x1a\x93\xf5\x8e5\xc4\x80\x87>\x94/f\xe4\xe9\xa8\xffA\x9cp*O*9\x18\x95\x1e~\xfba\x01<Q\x08.(\x18\xa4\x16\x0f1\xfd:l#\x93 v\xe1\xfd\x07\x85\xd1[?\xd2\x1cs2\xdd\xfa\xb9\xf8\x8c\xcf\x02\x87z\x9a\x96\xe4\xedO\x89\x8dSC\xab\x0e\x13\xc0\x01\x15\xb4y8\xdb\xfcn=\x9eQ\xb6\xb8\x13\x8bg\xcf\xf9|\xd9"\x1d\xf6]\xc5\x1c\x01/\x98\xe8z$\x18\xbc\x84\xd7\xfa\xdcr[\xf7\xc1:h'
The ``Context`` class must be specified::
>>> (dcert,remain) = BERcodec_Object.dec(cert, context=ASN1_Class_X509)
>>> dcert.show()
# ASN1_SEQUENCE:
# ASN1_SEQUENCE:
# ASN1_X509_CONT0:
<ASN1_INTEGER[2L]>
<ASN1_INTEGER[1L]>
# ASN1_SEQUENCE:
<ASN1_OID['.1.2.840.113549.1.1.5']>
<ASN1_NULL[0L]>
# ASN1_SEQUENCE:
# ASN1_SET:
# ASN1_SEQUENCE:
<ASN1_OID['.2.5.4.6']>
<ASN1_PRINTABLE_STRING['US']>
# ASN1_SET:
# ASN1_SEQUENCE:
<ASN1_OID['.2.5.4.10']>
<ASN1_PRINTABLE_STRING['AOL Time Warner Inc.']>
# ASN1_SET:
# ASN1_SEQUENCE:
<ASN1_OID['.2.5.4.11']>
<ASN1_PRINTABLE_STRING['America Online Inc.']>
# ASN1_SET:
# ASN1_SEQUENCE:
<ASN1_OID['.2.5.4.3']>
<ASN1_PRINTABLE_STRING['AOL Time Warner Root Certification Authority 2']>
# ASN1_SEQUENCE:
<ASN1_UTC_TIME['020529060000Z']>
<ASN1_UTC_TIME['370928234300Z']>
# ASN1_SEQUENCE:
# ASN1_SET:
# ASN1_SEQUENCE:
<ASN1_OID['.2.5.4.6']>
<ASN1_PRINTABLE_STRING['US']>
# ASN1_SET:
# ASN1_SEQUENCE:
<ASN1_OID['.2.5.4.10']>
<ASN1_PRINTABLE_STRING['AOL Time Warner Inc.']>
# ASN1_SET:
# ASN1_SEQUENCE:
<ASN1_OID['.2.5.4.11']>
<ASN1_PRINTABLE_STRING['America Online Inc.']>
# ASN1_SET:
# ASN1_SEQUENCE:
<ASN1_OID['.2.5.4.3']>
<ASN1_PRINTABLE_STRING['AOL Time Warner Root Certification Authority 2']>
# ASN1_SEQUENCE:
# ASN1_SEQUENCE:
<ASN1_OID['.1.2.840.113549.1.1.1']>
<ASN1_NULL[0L]>
<ASN1_BIT_STRING['\x000\x82\x02\n\x02\x82\x02\x01\x00\xb47Z\x08\x16\x99\x14\xe8U\xb1\x1b$k\xfc\xc7\x8b\xe6\x87\xa9\x89\xee\x8b\x99\xcdO@\x86\xa4\xb6M\xc9\xd9\xb1\xdc<M\r\x85L\x15lF\x8bRx\x9f\xf8#\xfdg\xf5$:h]\xd0\xf7daAT\xa3\x8b\xa5\x08\xd2)[\x9b`O&\x83\xd1c\x12VIv\xa4\x16\xc2\xa5\x9dE\xac\x8b\x84\x95\xa8\x16\xb1\xec\x9f\xea$\x1a\xef\xb9W\\\x9a$!,M\x0eq\x1f\xa6\xac]Et\x03\x98\xc4T\x8c\x16JAw\x86\x95u\x0cG\x01f`\xfc\x15\xf1\x0f\xea\xf5\x14x\xc7\x0e\xd7n\x81\x1c^\xbf^\xe7:*\xd8\x97\x170|\x00\xad\x08\x9d3\xaf\xb8\x99a\x80\x8b\xa8\x95~\x14\xdc\x12l\xa4\xd0\xd8\xef@I\x026\xf9n\xa9\xd6\x1d\x96V\x04\xb2\xb3-\x16V\x86\x8f\xd9 W\x80\xcdg\x10m\xb0L\xf0\xdaF\xb6\xea%.F\xaf\x8d\xb0\x8584\x8b\x14&\x82+\xac\xae\x99\x0b\x8e\x14\xd7R\xbd\x9ei\xc3\x86\x02\x0b\xeavu1\t\xce3\x19!\x85C\xe6\x89-\x9f%7g\xf1#j\xd2\x00m\x97\xf9\x9f\xe7)\xca\xdd\x1f\xd7\x06\xea\xb8\xc9\xb9\t!\x9f\xc8?\x06\xc5\xd2\xe9\x12F\x00N{\x08\xebB=+Hn\x9dg\xddK\x02\xe4D\xf3\x93\x19\xa5\'\xceiz\xbeg\xd3\xfcP\xa4,\xab\xc3k\xb9\xe3\x80L\xcf\x05aK+\xdc\x1b\xb9\xa6\xd2\xd0\xaa\xf5+s\xfb\xce\x905\x9f\x0cR\x1c\xbf\\!a\x11[\x15K\xa9$Q\xfc\xa4\\\xf7\x17\x9d\xb0\xd2\xfa\x07\xe9\x8fV\xe4\x1a\x8ch\x8a\x04\xd3|Z\xe3\x9e\xa2\xa1\xcaq[\xa2\xd4\xa0\xe7)\x85]\x03h*O\xd2\x06\xd7=\xf9\xc3\x03/?e\xf9g\x1eG@\xd3c\x0f\xe3\xd5\x8e\xf9\x85\xab\x97L\xb3\xd7&\xeb\x96\n\x94\xde\x856\x9c\xc8\x7f\x81\t\x02I*\x0e\xf5d2\x0c\x82\xd1\xbaj\x82\x1b\xb3Kt\x11\xf3\x8cw\xd6\x9f\xbf\xdc7\xa4\xa7U\x04/\xd41\xe8\xd3F\xb9\x03|\xda\x12NYd\xb7Q11P\xa0\xca\x1c\'\xd9\x10.\xad\xd6\xbd\x10f+\xc3\xb0"J\x12[\x02\x03\x01\x00\x01']>
# ASN1_X509_CONT3:
# ASN1_SEQUENCE:
# ASN1_SEQUENCE:
<ASN1_OID['.2.5.29.19']>
<ASN1_BOOLEAN[-1L]>
<ASN1_STRING['0\x03\x01\x01\xff']>
# ASN1_SEQUENCE:
<ASN1_OID['.2.5.29.14']>
<ASN1_STRING['\x04\x14Oim\x03~\x9d\x9f\x07\x18C\xbc\xb7\x10N\xd5\xbf\xa9\xc4 (']>
# ASN1_SEQUENCE:
<ASN1_OID['.2.5.29.35']>
<ASN1_STRING['0\x16\x80\x14Oim\x03~\x9d\x9f\x07\x18C\xbc\xb7\x10N\xd5\xbf\xa9\xc4 (']>
# ASN1_SEQUENCE:
<ASN1_OID['.2.5.29.15']>
<ASN1_BOOLEAN[-1L]>
<ASN1_STRING['\x03\x02\x01\x86']>
# ASN1_SEQUENCE:
<ASN1_OID['.1.2.840.113549.1.1.5']>
<ASN1_NULL[0L]>
<ASN1_BIT_STRING['\x00;\xf3\xae\xca\xe8.\x87\x85\xfbeY\xe7\xad\x11\x14\xa5W\xbcX\x9f$\x12W\xbb\xfb?4\xda\xee\xadz*4rp1k\xc7\x19\x98\x80\xc9\x82\xde7w^T\x8b\x8e\xf2\xeagO\xc9t\x84\x91V\t\xd5\xe5z\x9a\x81\xb6\x81\xc2\xad6\xe4\xf1T\x11S\xf34E\x01&\xc8\xe5\x1a\xbc4D!\xde\xad%\xfcv\x16w!\x90\x80\x98W\x9dN\xea\xec/\xaa<\x14{W\xc1~\x18\x14g\xee$\xc6\xbd\xba\x15\xb0\xd2\x18\xbd\xb7U\x81\xacS\xc0\xe8\xddi\x12\x13B\xb7\x02\xb5\x05A\xcayPn\x82\x0eqr\x93F\xe8\x9d\r]\xbd\xae\xce)\xadc\xd5U\x16\x800\'\xffv\xba\xf7\xb8\xd6J\xe3\xd9\xb5\xf9R\xd0N@\xa9\xc7\xe5\xc22\xc7\xaav$\xe1k\x05P\xeb\xc5\xbf\nT\xe5\xb9B<$\xfb\xb7\x07\x9c0\x9fyZ\xe6\xe0@R\x15\xf4\xfc\xaa\xf4V\xf9D\x97\x87\xed\x0eer^\xbe&\xfbM\xa4-\x08\x07\xde\xd8\\\xa0\xdc\x813\x99\x18%\x11w\xa7\xeb\xfdX\t,\x99k\x1b\x8a\xf3R?\x1aMH`\xf1\xa0\xf63\x02S\x8b\xed%\t\xb8\r-\xed\x97s\xec\xd7\x96\x1f\x8e`\x0e\xda\x10\x9b/\x18$\xf6\xa6M\n\xf9;\xcbu\xc2\xcc/\xce$i\xc9\n"\x8eY\xa7\xf7\x82\x0c\xd7\xd7k5\x9cC\x00j\xc4\x95g\xba\x9cE\xcb\xb8\x0e7\xf7\xdcN\x01O\xbe\n\xb6\x03\xd3\xad\x8aE\xf7\xda\'M)\xb1H\xdf\xe4\x11\xe4\x96F\xbdl\x02>\xd6Q\xc8\x95\x17\x01\x15\xa9\xf2\xaa\xaa\xf2\xbf/e\x1bo\xd0\xb9\x1a\x93\xf5\x8e5\xc4\x80\x87>\x94/f\xe4\xe9\xa8\xffA\x9cp*O*9\x18\x95\x1e~\xfba\x01<Q\x08.(\x18\xa4\x16\x0f1\xfd:l#\x93 v\xe1\xfd\x07\x85\xd1[?\xd2\x1cs2\xdd\xfa\xb9\xf8\x8c\xcf\x02\x87z\x9a\x96\xe4\xedO\x89\x8dSC\xab\x0e\x13\xc0\x01\x15\xb4y8\xdb\xfcn=\x9eQ\xb6\xb8\x13\x8bg\xcf\xf9|\xd9"\x1d\xf6]\xc5\x1c\x01/\x98\xe8z$\x18\xbc\x84\xd7\xfa\xdcr[\xf7\xc1:h']>
ASN.1 layers
^^^^^^^^^^^^
While this may be nice, it's only an ASN.1 encoder/decoder. Nothing related to Scapy yet.
ASN.1 fields
~~~~~~~~~~~~
Scapy provides ASN.1 fields. They will wrap ASN.1 objects and provide the necessary logic to bind a field name to the value. ASN.1 packets will be described as a tree of ASN.1 fields. Then each field name will be made available as a normal ``Packet`` object, in a flat flavor (ex: to access the version field of a SNMP packet, you don't need to know how many containers wrap it).
Each ASN.1 field is linked to an ASN.1 object through its tag.
ASN.1 packets
~~~~~~~~~~~~~
ASN.1 packets inherit from the Packet class. Instead of a ``fields_desc`` list of fields, they define ``ASN1_codec`` and ``ASN1_root`` attributes. The first one is a codec (for example: ``ASN1_Codecs.BER``), the second one is a tree compounded with ASN.1 fields.
A complete example: SNMP
------------------------
SNMP defines new ASN.1 objects. We need to define them::
class ASN1_Class_SNMP(ASN1_Class_UNIVERSAL):
name="SNMP"
PDU_GET = 0xa0
PDU_NEXT = 0xa1
PDU_RESPONSE = 0xa2
PDU_SET = 0xa3
PDU_TRAPv1 = 0xa4
PDU_BULK = 0xa5
PDU_INFORM = 0xa6
PDU_TRAPv2 = 0xa7
These objects are PDU, and are in fact new names for a sequence container (this is generally the case for context objects: they are old containers with new names). This means creating the corresponding ASN.1 objects and BER codecs is simplistic::
class ASN1_SNMP_PDU_GET(ASN1_SEQUENCE):
tag = ASN1_Class_SNMP.PDU_GET
class ASN1_SNMP_PDU_NEXT(ASN1_SEQUENCE):
tag = ASN1_Class_SNMP.PDU_NEXT
# [...]
class BERcodec_SNMP_PDU_GET(BERcodec_SEQUENCE):
tag = ASN1_Class_SNMP.PDU_GET
class BERcodec_SNMP_PDU_NEXT(BERcodec_SEQUENCE):
tag = ASN1_Class_SNMP.PDU_NEXT
# [...]
Metaclasses provide the magic behind the fact that everything is automatically registered and that ASN.1 objects and BER codecs can find each other.
The ASN.1 fields are also trivial::
class ASN1F_SNMP_PDU_GET(ASN1F_SEQUENCE):
ASN1_tag = ASN1_Class_SNMP.PDU_GET
class ASN1F_SNMP_PDU_NEXT(ASN1F_SEQUENCE):
ASN1_tag = ASN1_Class_SNMP.PDU_NEXT
# [...]
Now, the hard part, the ASN.1 packet::
SNMP_error = { 0: "no_error",
1: "too_big",
# [...]
}
SNMP_trap_types = { 0: "cold_start",
1: "warm_start",
# [...]
}
class SNMPvarbind(ASN1_Packet):
ASN1_codec = ASN1_Codecs.BER
ASN1_root = ASN1F_SEQUENCE( ASN1F_OID("oid","1.3"),
ASN1F_field("value",ASN1_NULL(0))
)
class SNMPget(ASN1_Packet):
ASN1_codec = ASN1_Codecs.BER
ASN1_root = ASN1F_SNMP_PDU_GET( ASN1F_INTEGER("id",0),
ASN1F_enum_INTEGER("error",0, SNMP_error),
ASN1F_INTEGER("error_index",0),
ASN1F_SEQUENCE_OF("varbindlist", [], SNMPvarbind)
)
class SNMPnext(ASN1_Packet):
ASN1_codec = ASN1_Codecs.BER
ASN1_root = ASN1F_SNMP_PDU_NEXT( ASN1F_INTEGER("id",0),
ASN1F_enum_INTEGER("error",0, SNMP_error),
ASN1F_INTEGER("error_index",0),
ASN1F_SEQUENCE_OF("varbindlist", [], SNMPvarbind)
)
# [...]
class SNMP(ASN1_Packet):
ASN1_codec = ASN1_Codecs.BER
ASN1_root = ASN1F_SEQUENCE(
ASN1F_enum_INTEGER("version", 1, {0:"v1", 1:"v2c", 2:"v2", 3:"v3"}),
ASN1F_STRING("community","public"),
ASN1F_CHOICE("PDU", SNMPget(),
SNMPget, SNMPnext, SNMPresponse, SNMPset,
SNMPtrapv1, SNMPbulk, SNMPinform, SNMPtrapv2)
)
def answers(self, other):
return ( isinstance(self.PDU, SNMPresponse) and
( isinstance(other.PDU, SNMPget) or
isinstance(other.PDU, SNMPnext) or
isinstance(other.PDU, SNMPset) ) and
self.PDU.id == other.PDU.id )
# [...]
bind_layers( UDP, SNMP, sport=161)
bind_layers( UDP, SNMP, dport=161)
That wasn't that much difficult. If you think that can't be that short to implement SNMP encoding/decoding and that I may may have cut too much, just look at the complete source code.
Now, how to use it? As usual::
>>> a=SNMP(version=3, PDU=SNMPget(varbindlist=[SNMPvarbind(oid="1.2.3",value=5),
... SNMPvarbind(oid="3.2.1",value="hello")]))
>>> a.show()
###[ SNMP ]###
version= v3
community= 'public'
\PDU\
|###[ SNMPget ]###
| id= 0
| error= no_error
| error_index= 0
| \varbindlist\
| |###[ SNMPvarbind ]###
| | oid= '1.2.3'
| | value= 5
| |###[ SNMPvarbind ]###
| | oid= '3.2.1'
| | value= 'hello'
>>> hexdump(a)
0000 30 2E 02 01 03 04 06 70 75 62 6C 69 63 A0 21 02 0......public.!.
0010 01 00 02 01 00 02 01 00 30 16 30 07 06 02 2A 03 ........0.0...*.
0020 02 01 05 30 0B 06 02 7A 01 04 05 68 65 6C 6C 6F ...0...z...hello
>>> send(IP(dst="1.2.3.4")/UDP()/SNMP())
.
Sent 1 packets.
>>> SNMP(str(a)).show()
###[ SNMP ]###
version= <ASN1_INTEGER[3L]>
community= <ASN1_STRING['public']>
\PDU\
|###[ SNMPget ]###
| id= <ASN1_INTEGER[0L]>
| error= <ASN1_INTEGER[0L]>
| error_index= <ASN1_INTEGER[0L]>
| \varbindlist\
| |###[ SNMPvarbind ]###
| | oid= <ASN1_OID['.1.2.3']>
| | value= <ASN1_INTEGER[5L]>
| |###[ SNMPvarbind ]###
| | oid= <ASN1_OID['.3.2.1']>
| | value= <ASN1_STRING['hello']>
Resolving OID from a MIB
------------------------
About OID objects
^^^^^^^^^^^^^^^^^
OID objects are created with an ``ASN1_OID`` class::
>>> o1=ASN1_OID("2.5.29.10")
>>> o2=ASN1_OID("1.2.840.113549.1.1.1")
>>> o1,o2
(<ASN1_OID['.2.5.29.10']>, <ASN1_OID['.1.2.840.113549.1.1.1']>)
Loading a MIB
^^^^^^^^^^^^^
Scapy can parse MIB files and become aware of a mapping between an OID and its name::
>>> load_mib("mib/*")
>>> o1,o2
(<ASN1_OID['basicConstraints']>, <ASN1_OID['rsaEncryption']>)
The MIB files I've used are attached to this page.
Scapy's MIB database
^^^^^^^^^^^^^^^^^^^^
All MIB information is stored into the conf.mib object. This object can be used to find the OID of a name
::
>>> conf.mib.sha1_with_rsa_signature
'1.2.840.113549.1.1.5'
or to resolve an OID::
>>> conf.mib._oidname("1.2.3.6.1.4.1.5")
'enterprises.5'
It is even possible to graph it::
>>> conf.mib._make_graph()
Automata
========
Scapy enables to create esaily network automata. Scapy does not stick to a specific model like Moore or Mealy automata. It provides a flexible way for you to choose you way to go.
An automaton in Scapy is deterministic. It has different states. A start state and some end and error states. There are transitions from one state to another. Transitions can be transitions on a specific condition, transitions on the reception of a specific packet or transitions on a timeout. When a transition is taken, one or more actions can be run. An action can be bound to many transitions. Parameters can be passed from states to transitions and from transitions to states and actions.
From a programmer's point of view, states, transitions and actions are methods from an Automaton subclass. They are decorated to provide meta-information needed in order for the automaton to work.
First example
-------------
Let's begin with a simple example. I take the convention to write states with capitals, but anything valid with python syntax would work as well.
::
class HelloWorld(Automaton):
@ATMT.state(initial=1)
def BEGIN(self):
print "State=BEGIN"
@ATMT.condition(BEGIN)
def wait_for_nothing(self):
print "Wait for nothing..."
raise self.END()
@ATMT.action(wait_for_nothing)
def on_nothing(self):
print "Action on 'nothing' condition"
@ATMT.state(final=1)
def END(self):
print "State=END"
In this example, we can see 3 decorators:
* ATMT.state that is used to indicate that a method is a state, and that can have initial, final and error optional arguments set to non-zero for special states.
* ATMT.condition that indicate a method to be run when the automaton state reaches the indicated state. The argument is the name of the method representing that state
* ATMT.action binds a method to a transition and is run when the transition is taken.
Running this example gives the following result::
>>> a=HelloWorld()
>>> a.run()
State=BEGIN
Wait for nothing...
Action on 'nothing' condition
State=END
This simple automaton can be described with the following graph:
The graph can be automatically drawn from the code with::
>>> HelloWorld.graph()
Changing states
---------------
The ``ATMT.state`` decorator transforms a method into a function that returns an exception. If you raise that exception, the automaton state will be changed. If the change occurs in a transition, actions bound to this transition will be called. The parameters given to the function replacing the method will be kept and finally delivered to the method. The exception has a method action_parameters that can be called before it is raised so that it will store parameters to be delivered to all actions bound to the current transition.
As an example, let's consider the following state::
@ATMT.state()
def MY_STATE(self, param1, param2):
print "state=MY_STATE. param1=%r param2=%r" % (param1, param2)
This state will be reached with the following code::
@ATMT.receive_condition(ANOTHER_STATE)
def received_ICMP(self, pkt):
if ICMP in pkt:
raise self.MY_STATE("got icmp", pkt[ICMP].type)
Let's suppose we want to bind an action to this transition, that will also need some parameters::
@ATMT.action(received_ICMP)
def on_ICMP(self, icmp_type, icmp_code):
self.retaliate(icmp_type, icmp_code)
The condition should become::
@ATMT.receive_condition(ANOTHER_STATE)
def received_ICMP(self, pkt):
if ICMP in pkt:
raise self.MY_STATE("got icmp", pkt[ICMP].type).action_parameters(pkt[ICMP].type, pkt[ICMP].code)
Real example
------------
Here is a real example take from Scapy. It implements a TFTP client that can issue read requests.
::
class TFTP_read(Automaton):
def parse_args(self, filename, server, sport = None, port=69, **kargs):
Automaton.parse_args(self, **kargs)
self.filename = filename
self.server = server
self.port = port
self.sport = sport
def master_filter(self, pkt):
return ( IP in pkt and pkt[IP].src == self.server and UDP in pkt
and pkt[UDP].dport == self.my_tid
and (self.server_tid is None or pkt[UDP].sport == self.server_tid) )
# BEGIN
@ATMT.state(initial=1)
def BEGIN(self):
self.blocksize=512
self.my_tid = self.sport or RandShort()._fix()
bind_bottom_up(UDP, TFTP, dport=self.my_tid)
self.server_tid = None
self.res = ""
self.l3 = IP(dst=self.server)/UDP(sport=self.my_tid, dport=self.port)/TFTP()
self.last_packet = self.l3/TFTP_RRQ(filename=self.filename, mode="octet")
self.send(self.last_packet)
self.awaiting=1
raise self.WAITING()
# WAITING
@ATMT.state()
def WAITING(self):
pass
@ATMT.receive_condition(WAITING)
def receive_data(self, pkt):
if TFTP_DATA in pkt and pkt[TFTP_DATA].block == self.awaiting:
if self.server_tid is None:
self.server_tid = pkt[UDP].sport
self.l3[UDP].dport = self.server_tid
raise self.RECEIVING(pkt)
@ATMT.action(receive_data)
def send_ack(self):
self.last_packet = self.l3 / TFTP_ACK(block = self.awaiting)
self.send(self.last_packet)
@ATMT.receive_condition(WAITING, prio=1)
def receive_error(self, pkt):
if TFTP_ERROR in pkt:
raise self.ERROR(pkt)
@ATMT.timeout(WAITING, 3)
def timeout_waiting(self):
raise self.WAITING()
@ATMT.action(timeout_waiting)
def retransmit_last_packet(self):
self.send(self.last_packet)
# RECEIVED
@ATMT.state()
def RECEIVING(self, pkt):
recvd = pkt[Raw].load
self.res += recvd
self.awaiting += 1
if len(recvd) == self.blocksize:
raise self.WAITING()
raise self.END()
# ERROR
@ATMT.state(error=1)
def ERROR(self,pkt):
split_bottom_up(UDP, TFTP, dport=self.my_tid)
return pkt[TFTP_ERROR].summary()
#END
@ATMT.state(final=1)
def END(self):
split_bottom_up(UDP, TFTP, dport=self.my_tid)
return self.res
It can be run like this, for instance::
>>> TFTP_read("my_file", "192.168.1.128").run()
Detailed documentation
----------------------
Decorators
^^^^^^^^^^
Decorator for states
~~~~~~~~~~~~~~~~~~~~
States are methods decorated by the result of the ``ATMT.state`` function. It can take 3 optional parameters, ``initial``, ``final`` and ``error``, that, when set to ``True``, indicate that the state is an initial, final or error state.
::
class Example(Automaton):
@ATMT.state(initial=1)
def BEGIN(self):
pass
@ATMT.state()
def SOME_STATE(self)
pass
@ATMT.state(final=1)
def END(self):
return "Result of the automaton: 42"
@ATMT.state(error=1)
def ERROR(self):
return "Partial result, or explanation"
# [...]
Decorators for transitions
~~~~~~~~~~~~~~~~~~~~~~~~~~
Transitions are methods decorated by the result of one of ``ATMT.condition``, ``ATMT.receive_condition``, ``ATMT.timeout``. They all take as argument the state method they are related to. ATMT.timeout also have a mandatory ``timeout`` parameter to provide the timeout value in seconds. ``ATMT.condition`` and ``ATMT.receive_condition`` have an optional ``prio`` parameter so that the order in which conditions are evaluated can be forced. Default priority is 0. Transitions with the same priority level are called in an undetermined order.
When the automaton switches to a given state, the state's method is executed. Then transitions methods are called at specific moments until one triggers a new state (something like ``raise self.MY_NEW_STATE()``). First, right after the state's method returns, the ``ATMT.condition`` decorated methods are run by growing prio. Then each time a packet is received and accepted by the master filter all ``ATMT.receive_condition`` decorated hods are called by growing prio. When a timeout is reached since the time we entered into the current space, the corresponding ``ATMT.timeout`` decorated method is called.
::
class Example(Automaton):
@ATMT.state()
def WAITING(self):
pass
@ATMT.condition(WAITING)
def it_is_raining(self):
if not self.have_umbrella:
raise self.ERROR_WET()
@ATMT.receive_condition(WAITING, prio=1)
def it_is_ICMP(self, pkt):
if ICMP in pkt:
raise self.RECEIVED_ICMP(pkt)
@ATMT.receive_condition(WAITING, prio=2)
def it_is_IP(self, pkt):
if IP in pkt:
raise self.RECEIVED_IP(pkt)
@ATMT.timeout(WAITING, 10.0)
def waiting_timeout(self):
raise self.ERROR_TIMEOUT()
Decorator for actions
~~~~~~~~~~~~~~~~~~~~~
Actions are methods that are decorated by the return of ``ATMT.action`` function. This function takes the transition method it is bound to as first parameter and an optionnal priority ``prio`` as a second parameter. Default priority is 0. An action method can be decorated many times to be bound to many transitions.
::
class Example(Automaton):
@ATMT.state(initial=1)
def BEGIN(self):
pass
@ATMT.state(final=1)
def END(self):
pass
@ATMT.condition(BEGIN, prio=1)
def maybe_go_to_end(self):
if random() > 0.5:
raise self.END()
@ATMT.condition(BEGIN, prio=2)
def certainly_go_to_end(self):
raise self.END()
@ATMT.action(maybe_go_to_end)
def maybe_action(self):
print "We are lucky..."
@ATMT.action(certainly_go_to_end)
def certainly_action(self):
print "We are not lucky..."
@ATMT.action(maybe_go_to_end, prio=1)
@ATMT.action(certainly_go_to_end, prio=1)
def always_action(self):
print "This wasn't luck!..."
The two possible outputs are::
>>> a=Example()
>>> a.run()
We are not lucky...
This wasn't luck!...
>>> a.run()
We are lucky...
This wasn't luck!...
Methods to overload
^^^^^^^^^^^^^^^^^^^
Two methods are hooks to be overloaded.
The ``parse_args()`` method is called with arguments given at ``__init__()`` and ``run()``. Use that to parametrize the behaviour of your automaton.
The ``master_filter()` method is called each time a packet is sniffed and decides if it is interesting for the automaton. When working on a specific protocol, this is where you will ensure the packet belongs to the connection you are being part of, so that you do not need to make all the sanity checks in each transition.
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