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Snoopy reads packets from a packet–source (default /net/ether0),
matches them to a filter (by default anything matches), and writes
matching packets to standard output either in human readable form
(default) or in a binary trace format that can be later read by
snoopy. Packet–source can be the name of an
Ethernet (e.g., /net/ether0), an interface (e.g., /net/ipifc/0),
or a file of captured packets.
The human readable format consists of multiple lines per packet.
The first line contains the milliseconds since the trace was started.
Subsequent ones are indented with a tab and each contains the
dump of a single protocol header. The last line contains the dump
of any contained data. For example, a BOOTP packet
would look like:
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324389 ms
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ether(s=0000929b1b54 d=ffffffffffff pr=0800 ln=342)
ip(s=135.104.9.62 d=255.255.255.255 id=5099 frag=0000...
udp(s=68 d=67 ck=d151 ln= 308)
bootp(t=Req ht=1 hl=16 hp=0 xid=217e5f27 sec=0 fl=800...
dhcp(t=Request clientid=0152415320704e7266238ebf01030...
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The binary format consists of:
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2 bytes of packet length, msb first
8 bytes of nanosecond time, msb first
the packet
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Filters are expressions specifying protocols to be traced and
specific values for fields in the protocol headers. The grammar
is:
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expr: protocol
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| field '=' value
| field '!=' value
| protocol '(' expr ')'
| '(' expr ')'
| expr '||' expr
| expr '&&' expr
| '!' expr
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The values for protocol and field can be obtained using the –?
option. With no arguments, it lists the known protocols. Otherwise
it prints, for each protocol specified, which subprotocols it
can multiplex to, and which fields can be used for filtering.
For example, the listing for ethernet is currently:
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ether's filter attributes:
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s – source address
d – destination address
a – source|destination address
sd – source|destination address
t – type
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ether's subprotos:
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0x0800 ip 0x8863 pppoe_disc
0x0806 arp 0x8864 pppoe_sess
0x0806 rarp 0x888e eapol
0x86dd ip6
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The format of value depends on context. In general, ethernet addresses
are entered as a string of hex digits; IP numbers in the canonical
`.' format for v4 and `:' format for v6; and ports in decimal.
Snoopy's options are:
–C compute the correct checksum for each packet; on mismatch, add
a field !ck=xxxx where xxxx is the correct checksum.
–D output will be a binary trace file in Unix pcap format.
–d output will be a binary trace file.
–t input is a binary trace file as generated with the –d option.
–p do not enter promiscuous mode. Only packets to this interface
will be seen.
–s force one output line per packet. The default is multiline.
–M discard all but the first m bytes of each packet. The default
is to keep the entire packet. This option is most useful when
writing packets to a file with the –d option.
–N dump n data bytes per packet. The default is 32.
–f use filter–expression to filter the packet stream. The default
is to match all packets.
–h assume the first header per packet to be of the first–header protocol.
The default is ether.
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