snoopy – spy on network packets


snoopy [ -CDdpst ] [ -M m ] [ -N n ] [ -f filter-expression ] [ -h first-header ] [ packet-source ]

snoopy -? [ proto... ]


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:


324389 ms
  	ether(s=0000929b1b54 d=ffffffffffff pr=0800 ln=342)
  	ip(s= d= 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...

The binary format consists of:

2 bytes of packet length, msb first  

8 bytes of nanosecond time, msb first  

the packet  

Filters are expressions specifying protocols to be traced and specific values for fields in the protocol headers. The grammar is:


expr:	  protocol
	| field '=' value
	| field '!=' value
	| protocol '(' expr ')'
	| '(' expr ')'
	| expr '||' expr
	| expr '&&' expr
	| '!' expr

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:


ether's filter attributes:
  s  - source address
  d  - destination address
  a  - source|destination address
  sd - source|destination address
  t  - type
ether's subprotos:
  0x0800 ip		  0x8863 pppoe_disc
  0x0806 arp		  0x8864 pppoe_sess
  0x0806 rarp		  0x888e eapol
  0x86dd ip6

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:


compute the correct checksum for each packet; on mismatch, add a field !ck=xxxx where xxxx is the correct checksum.  


output will be a binary trace file in Unix pcap format.  


output will be a binary trace file.  


input is a binary trace file as generated with the -d option.  


do not enter promiscuous mode. Only packets to this interface will be seen.  


force one output line per packet. The default is multiline.  


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.  


dump n data bytes per packet. The default is 32.  


use filter-expression to filter the packet stream. The default is to match all packets.  


assume the first header per packet to be of the first-header protocol. The default is ether.  


To display only BOOTP and ARP packets:


% snoopy -f 'arp || bootp'
after optimize: ether(arp || ip(udp(bootp)))

The first line of output shows the completed filter expression. Snoopy will fill in other protocols as necessary to complete the filter and then optimize to remove redundant comparisons.

To save all packets between to and later display those to/from TCP port 80:


% ramfs
% snoopy -df 'ip(s= && d= ||\\
	ip(s= && d=' > /tmp/quux
<interrupt from the keyboard>
% snoopy -tf 'tcp(sd=80)' /tmp/quux



Ethernet device  




Snoopy only dumps ethernet packets, because there’s no device to get IP packets without a media header.