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一、包
包(Packet)是TCP/IP协议通信传输中的数据单位，一般也称“数据包”。其主要由“目的IP地址”、“源IP地址”、“净载数据”等部分构成，包括包头和包体，包头是固定长度，包体的长度不定，各字段长度固定，双方的请求数据包和应答数据包的包头结构是一致的，不同的是包体的定义。 数据包的结构与我们平常写信非常类似，目的IP地址是说明这个数据包是要发给谁的，相当于收信人地址；源IP地址是说明这个数据包是发自哪里的，相当于发信人地址；而净载数据相当于信件的内容。包沿着不同的路径在一个或多个网络中传输，并且在目的地重新组合。
二、常见的几个关键字
ICMP:Internet Control Message Protocol（Internet控制报文协议）的缩写。它是TCP/IP协议族的一个子协议，用于在IP主机、路由器之间传递控制消息。控制消息是指网络通不通、主机是否可达、路由是否可用等网络本身的消息。这些控制消息虽然并不传输用户数据，但是对于用户数据的传递起着重要的作用。
DST:目的地址
SRC:源地址
TTL：(Time To Live ) 生存时间,指定数据包被路由器丢弃之前允许通过的网段数量。TTL是IP协议包中的一个值，它告诉网络，数据包在网络中的时间是否太长而应被丢弃。有很多原因使包在一定时间内不能被传递到目的地。解决方法就是在一段时间后丢弃这个包，然后给发送者一个报文，由发送者决定是否要重发。TTL的初值通常是系统缺省值，是包头中的8位的域。TTL的最初设想是确定一个时间范围，超过此时间就把包丢弃。由于每个路由器都至少要把TTL域减一，TTL通常表示包在被丢弃前最多能经过的路由器个数。当记数到0时，路由器决定丢弃该包，并发送一个ICMP报文给最初的发送者。
三、scapy中常用的几个命令
1、ls():作用也是list show，可以显示所有支持的数据包对象。ls()可以不带参数，也可以带参数，参数可是任何一个具体的包。下面列出了一部分结果：
>>> from scapy.all import * WARNING: No route found for IPv6 destination :: (no default route?) >>> ls() ARP : ARP ASN1_Packet : None BOOTP : BOOTP CookedLinux : cooked linux DHCP : DHCP options DHCP6 : DHCPv6 Generic Message) DHCP6OptAuth : DHCP6 Option - Authentication DHCP6OptBCMCSDomains : DHCP6 Option - BCMCS Domain Name List DHCP6OptBCMCSServers : DHCP6 Option - BCMCS Addresses List DHCP6OptClientFQDN : DHCP6 Option - Client FQDN DHCP6OptClientId : DHCP6 Client Identifier Option DHCP6OptDNSDomains : DHCP6 Option - Domain Search List option DHCP6OptDNSServers : DHCP6 Option - DNS Recursive Name Server DHCP6OptElapsedTime : DHCP6 Elapsed Time Option DHCP6OptGeoConf :
列出TCP的所有对象：
>>> from scapy.all import *
WARNING: No route found for IPv6 destination :: (no default route?)
>>> ls(TCP)
sport : ShortEnumField = (20)
dport : ShortEnumField = (80)
seq : IntField = (0)
ack : IntField = (0)
dataofs : BitField = (None)
reserved : BitField = (0)
flags : FlagsField = (2)
window : ShortField = (8192)
chksum : XShortField = (None)
urgptr : ShortField = (0)
options : TCPOptionsField = ({})
列出任意包的情况如：
>>> a=IP(ttl=5)
>>> a.src
'127.0.0.1'
>>> a
<IP ttl=5 |>
>>> a.dst
'127.0.0.1'
>>> a.dst="192.168.0.1"
>>> a
<IP ttl=5 dst=192.168.0.1 |>
>>> ls(a)
version : BitField = 4 (4)
ihl : BitField = None (None)
tos : XByteField = 0 (0)
len : ShortField = None (None)
id : ShortField = 1 (1)
flags : FlagsField = 0 (0)
frag : BitField = 0 (0)
ttl : ByteField = 5 (64)
proto : ByteEnumField = 0 (0)
chksum : XShortField = None (None)
src : Emph = '27.214.7.85' (None)
dst : Emph = '192.168.0.1' ('127.0.0.1')
options : PacketListField = [] ([])
>>>
2、lsc()列出所有函数。如：
>>> lsc() arpcachepoison : Poison target's cache with (your MAC,victim's IP) couple arping : Send ARP who-has requests to determine which hosts are up bind_layers : Bind 2 layers on some specific fields' values corrupt_bits : Flip a given percentage or number of bits from a string corrupt_bytes : Corrupt a given percentage or number of bytes from a string defrag : defrag(plist) -> ([not fragmented], [defragmented], defragment : defrag(plist) -> plist defragmented as much as possible dyndns_add : Send a DNS add message to a nameserver for "name" to have a new "rdata" dyndns_del : Send a DNS delete message to a nameserver for "name" etherleak : Exploit Etherleak flaw fragment : Fragment a big IP datagram fuzz : Transform a layer into a fuzzy layer by replacing some default values by random objects getmacbyip : Return MAC address corresponding to a given IP address hexdiff : Show differences between 2 binary strings hexdump : -- hexedit : -- is_promisc : Try to guess if target is in Promisc mode. The target is provided by its ip. linehexdump : -- ls : List available layers, or infos on a given layer promiscping : Send ARP who-has requests to determine which hosts are in promiscuous mode rdpcap : Read a pcap file and return a packet list send : Send packets at layer 3 sendp : Send packets at layer 2 sendpfast : Send packets at layer 2 using tcpreplay for performance sniff : Sniff packets split_layers : Split 2 layers previously bound sr : Send and receive packets at layer 3 sr1 : Send packets at layer 3 and return only the first answer srbt : send and receive using a bluetooth socket srbt1 : send and receive 1 packet using a bluetooth socket srflood : Flood and receive packets at layer 3 srloop : Send a packet at layer 3 in loop and print the answer each time srp : Send and receive packets at layer 2 srp1 : Send and receive packets at layer 2 and return only the first answer srpflood : Flood and receive packets at layer 2 srploop : Send a packet at layer 2 in loop and print the answer each time traceroute : Instant TCP traceroute tshark : Sniff packets and print them calling pkt.show(), a bit like text wireshark wireshark : Run wireshark on a list of packets wrpcap : Write a list of packets to a pcap file
3、hide_defaults()方法，用来删除一些用户提供的那些和default value相同的项目
>>> a=IP()/TCP() >>> b=IP(str(a)) >>> b <IP version=4L ihl=5L tos=0x0 len=40 id=1 flags= frag=0L ttl=64 proto=tcp chksum=0x7ccd src=127.0.0.1 dst=127.0.0.1 options=[] |<TCP sport=ftp_data dport=http seq=0 ack=0 dataofs=5L reserved=0L flags=S window=8192 chksum=0x917c urgptr=0 |>> >>> b.hide_defaults() >>> b <IP ihl=5L len=40 frag=0 proto=tcp chksum=0x7ccd src=127.0.0.1 |<TCP dataofs=5L chksum=0x917c |>>
4、display():display()方法可以简单查看当前packet的各个参数的取值情况,例子见下。
5、sprintf:输出某一层某个参数的取值，如果不存在就输出？？,具体的format是：%[[mt][r],][layer[:nb].]field%,参数的具体信息请参看《Security Power Tools》146页或者http://wikicode.net。例：
>>> a=IP()/TCP()
>>> b=IP(str(a))
>>> b
<IP version=4L ihl=5L tos=0x0 len=40 id=1 flags= frag=0L ttl=64 proto=tcp chksum=0x7ccd src=127.0.0.1 dst=127.0.0.1 options=[] |<TCP sport=ftp_data dport=http seq=0 ack=0 dataofs=5L reserved=0L flags=S window=8192 chksum=0x917c urgptr=0 |>>
>>> b.hide_defaults()
>>> b
<IP ihl=5L len=40 frag=0 proto=tcp chksum=0x7ccd src=127.0.0.1 |<TCP dataofs=5L chksum=0x917c |>>
>>> a.sprintf("%IP.gabuzomeu%")
'??'
四、创建包
scapy的包创建是按照网络接口层，互联网层，传输层，应用层四层参考模型来完成，各个层都有各自的创建函数，比如IP（），TCP（），UDP()等等，不同层之间通过“/”来连接。例如 ，接前面a的例子：
例1
>>> a=IP(ttl=5) >>> a.src '127.0.0.1' >>> a <IP ttl=5 |> >>> a.dst '127.0.0.1' >>> a.dst="192.168.0.1" >>> a <IP ttl=5 dst=192.168.0.1 |> >>> packet1=a >>> packet1 <IP ttl=5 dst=192.168.0.1 |> 例2
>>> packet2=IP(dst="192.168.0.1")/TCP(dport=80) 例3
>>> packet3=IP(dst="www.baidu.com")/ICMP() >>> packet3 <IP frag=0 proto=icmp dst=Net('www.baidu.com') |<ICMP |>> >>> ls(packet3) version : BitField = 4 (4) ihl : BitField = None (None) tos : XByteField = 0 (0) len : ShortField = None (None) id : ShortField = 1 (1) flags : FlagsField = 0 (0) frag : BitField = 0 (0) ttl : ByteField = 64 (64) proto : ByteEnumField = 1 (0) chksum : XShortField = None (None) src : Emph = '27.214.7.85' (None) dst : Emph = Net('www.baidu.com') ('127.0.0.1') options : PacketListField = [] ([]) -- type : ByteEnumField = 8 (8) code : MultiEnumField = 0 (0) chksum : XShortField = None (None) id : ConditionalField = 0 (0) seq : ConditionalField = 0 (0) ts_ori : ConditionalField = 4842323 (4842323) ts_rx : ConditionalField = 4842323 (4842323) ts_tx : ConditionalField = 4842323 (4842323) gw : ConditionalField = '0.0.0.0' ('0.0.0.0') ptr : ConditionalField = 0 (0) reserved : ConditionalField = 0 (0) addr_mask : ConditionalField = '0.0.0.0' ('0.0.0.0') unused : ConditionalField = 0 (0) 例4
>>> target="www.baidu.com/30" >>> ip=IP(dst=target) >>> ip <IP dst=Net('www.baidu.com/30') |> >>> ls(ip) version : BitField = 4 (4) ihl : BitField = None (None) tos : XByteField = 0 (0) len : ShortField = None (None) id : ShortField = 1 (1) flags : FlagsField = 0 (0) frag : BitField = 0 (0) ttl : ByteField = 64 (64) proto : ByteEnumField = 0 (0) chksum : XShortField = None (None) src : Emph = '27.214.7.85' (None) dst : Emph = Net('www.baidu.com/30') ('127.0.0.1') options : PacketListField = [] ([])
>>> IP().display()
###[ IP ]###
version = 4
ihl = None
tos = 0x0
len = None
id = 1
flags =
frag = 0
ttl = 64
proto = ip
chksum = None
src = 127.0.0.1
dst = 127.0.0.1
options
>>> TCP().display()
###[ TCP ]###
sport = ftp_data
dport = http
seq = 0
ack = 0
dataofs = None
reserved = 0
flags = S
window = 8192
chksum = None
urgptr = 0
options = {}
这里的display()方法可以简单查看当前packet的各个参数的取值情况.
五、包的结构
在Scapy中，scapy为各个层都写了类，使用时，只需要将其实例化，调用类的方法或者改变类的参数取值。如IP（）没有传给它参数，那么它的参数就是默认的，如果传了就覆盖了默认值：
>>> a=IP() >>> a.display() ###[ IP ]### version = 4 ihl = None tos = 0x0 len = None id = 1 flags = frag = 0 ttl = 64 proto = ip chksum = None src = 127.0.0.1 dst = 127.0.0.1 options >>> a=IP(dst="192.168.0.1") >>> a.display() ###[ IP ]### version = 4 ihl = None tos = 0x0 len = None id = 1 flags = frag = 0 ttl = 64 proto = ip chksum = None src = 27.214.7.** //（本机IP） dst = 192.168.0.1 options
注意比较这两次display()的不同，第一次是默认值，第二次传入了“192.168.0.1”。
"/"用来连接各层，如IP（）/TCP（）。如：
>>> IP() <IP |> >>> IP()/TCP() <IP frag=0 proto=tcp |<TCP |>> >>> Ether()/IP()/TCP() <Ether type=0x800 |<IP frag=0 proto=tcp |<TCP |>>> >>> IP()/TCP()/"GET / HTTP/1.0 " <IP frag=0 proto=tcp |<TCP |<Raw load='GET / HTTP/1.0 ' |>>> >>> Ether()/IP()/IP()/UDP() <Ether type=0x800 |<IP frag=0 proto=ipencap |<IP frag=0 proto=udp |<UDP |>>>> >>> IP(proto=55,ttl=10)/TCP() <IP frag=0 ttl=10 proto=55 |<TCP |>> 具体的参数传递过程，在scapy文档中提供了图表，如下：
