SNOOP(8) Maintenance Procedures SNOOP(8)

NAME

snoop - capture and inspect network packets

SYNOPSIS

snoop [-afqrCDINPSvV] [-t [r | a | d]] [-c maxcount]
[-d device] [-i filename] [-n filename] [-o filename]
[-p first [, last]] [-s snaplen] [-x offset [, length]]
[expression]

DESCRIPTION

From a datalink or IP interface, snoop captures packets and displays
their contents. If the datalink or IP interface is not specified, snoop
will pick a datalink to use, giving priority to datalinks that have been
plumbed for IP traffic. snoop uses the pfmod(4M) and bufmod(4M) STREAMS
modules to provide efficient capture of packets from the network.
Captured packets can be displayed as they are received or saved to a file
(which is RFC 1761-compliant) for later inspection.


snoop can display packets in a single-line summary form or in verbose
multi-line forms. In summary form, with the exception of certain VLAN
packets, only the data pertaining to the highest level protocol is
displayed. If a packet has a VLAN header and its VLAN ID is non-zero,
then snoop will show that the packet is VLAN tagged. For example, an NFS
packet will have only NFS information displayed. Except for VLAN
information under the condition just described, the underlying RPC, UDP,
IP, and Ethernet frame information is suppressed, but can be displayed if
either of the verbose options are chosen.


In the absence of a name service, such as LDAP or NIS, snoop displays
host names as numeric IP addresses.


snoop requires an interactive interface.

OPTIONS

-C

List the code generated from the filter expression for either the
kernel packet filter, or snoop's own filter.


-D

Display number of packets dropped during capture on the summary line.


-N

Create an IP address-to-name file from a capture file. This must be
set together with the -i option that names a capture file. The
address-to-name file has the same name as the capture file with
.names appended. This file records the IP address to hostname mapping
at the capture site and increases the portability of the capture
file. Generate a .names file if the capture file is to be analyzed
elsewhere. Packets are not displayed when this flag is used.


-I interface

Capture IP packets from the network using the IP interface specified
by interface, for example, lo0. The ifconfig(8) command can be used
to list available IP interfaces. The -I and -d options are mutually
exclusive.


-P

Capture packets in non-promiscuous mode. Only broadcast, multicast,
or packets addressed to the host machine will be seen.


-S

Display size of the entire link layer frame in bytes on the summary
line.


-V

Verbose summary mode. This is halfway between summary mode and
verbose mode in degree of verbosity. Instead of displaying just the
summary line for the highest level protocol in a packet, it displays
a summary line for each protocol layer in the packet. For instance,
for an NFS packet it will display a line each for the ETHER, IP, UDP,
RPC and NFS layers. Verbose summary mode output may be easily piped
through grep to extract packets of interest. For example, to view
only RPC summary lines, enter the following: example# snoop -i
rpc.cap -V | grep RPC


-a

Listen to packets on /dev/audio (warning: can be noisy).


-c maxcount

Quit after capturing maxcount packets. Otherwise keep capturing until
there is no disk space left or until interrupted with Control-C.


-d datalink

Capture link-layer packets from the network using the DLPI datalink
specified by datalink, for example, bge0 or net0. The dladm(8) show-
link subcommand can be used to list available datalinks. The -d and
-I options are mutually exclusive.


-f

Ignore any errors when enabling promiscuous mode. Normally any error
when enabling promiscuous mode on a datalink or IP interface is fatal
and causes snoop to exit.


-i filename

Display packets previously captured in filename. Without this option,
snoop reads packets from the network interface. If a filename.names
file is present, it is automatically loaded into the snoop IP
address-to-name mapping table (See -N flag).


-n filename

Use filename as an IP address-to-name mapping table. This file must
have the same format as the /etc/hosts file (IP address followed by
the hostname).


-o filename

Save captured packets in filename as they are captured. (This
filename is referred to as the "capture file".) The format of the
capture file is RFC 1761-compliant. During packet capture, a count of
the number of packets saved in the file is displayed. If you wish
just to count packets without saving to a file, name the file
/dev/null.


-p first [ , last ]

Select one or more packets to be displayed from a capture file. The
first packet in the file is packet number 1.


-q

When capturing network packets into a file, do not display the packet
count. This can improve packet capturing performance.


-r

Do not resolve the IP address to the symbolic name. This prevents
snoop from generating network traffic while capturing and displaying
packets. However, if the -n option is used, and an address is found
in the mapping file, its corresponding name will be used.


-s snaplen

Truncate each packet after snaplen bytes. Usually the whole packet is
captured. This option is useful if only certain packet header
information is required. The packet truncation is done within the
kernel giving better utilization of the streams packet buffer. This
means less chance of dropped packets due to buffer overflow during
periods of high traffic. It also saves disk space when capturing
large traces to a capture file. To capture only IP headers (no
options) use a snaplen of 34. For UDP use 42, and for TCP use 54. You
can capture RPC headers with a snaplen of 80 bytes. NFS headers can
be captured in 120 bytes.


-t [ r | a | d ]

Time-stamp presentation. Time-stamps are accurate to within 4
microseconds. The default is for times to be presented in d (delta)
format (the time since receiving the previous packet). Option a
(absolute) gives wall-clock time. Option r (relative) gives time
relative to the first packet displayed. This can be used with the -p
option to display time relative to any selected packet.


-v

Verbose mode. Print packet headers in lots of detail. This display
consumes many lines per packet and should be used only on selected
packets.


-xoffset [ , length]

Display packet data in hexadecimal and ASCII format. The offset and
length values select a portion of the packet to be displayed. To
display the whole packet, use an offset of 0. If a length value is
not provided, the rest of the packet is displayed.

OPERANDS

expression

Select packets either from the network or from a capture file. Only
packets for which the expression is true will be selected. If no
expression is provided it is assumed to be true.

Given a filter expression, snoop generates code for either the kernel
packet filter or for its own internal filter. If capturing packets
with the network interface, code for the kernel packet filter is
generated. This filter is implemented as a streams module, upstream
of the buffer module. The buffer module accumulates packets until it
becomes full and passes the packets on to snoop. The kernel packet
filter is very efficient, since it rejects unwanted packets in the
kernel before they reach the packet buffer or snoop. The kernel
packet filter has some limitations in its implementation; it is
possible to construct filter expressions that it cannot handle. In
this event, snoop tries to split the filter and do as much filtering
in the kernel as possible. The remaining filtering is done by the
packet filter for snoop. The -C flag can be used to view generated
code for either the packet filter for the kernel or the packet filter
for snoop. If packets are read from a capture file using the -i
option, only the packet filter for snoop is used.

A filter expression consists of a series of one or more boolean
primitives that may be combined with boolean operators (AND, OR, and
NOT). Normal precedence rules for boolean operators apply. Order of
evaluation of these operators may be controlled with parentheses.
Since parentheses and other filter expression characters are known to
the shell, it is often necessary to enclose the filter expression in
quotes. Refer to for information about setting up more efficient
filters.

The primitives are:

host hostname

True if the source or destination address is that of hostname.
The hostname argument may be a literal address. The keyword host
may be omitted if the name does not conflict with the name of
another expression primitive. For example, pinky selects packets
transmitted to or received from the host pinky, whereas pinky and
dinky selects packets exchanged between hosts pinky AND dinky.

The type of address used depends on the primitive which precedes
the host primitive. The possible qualifiers are inet, inet6,
ether, or none. These three primitives are discussed below.
Having none of the primitives present is equivalent to "inet host
hostname or inet6 host hostname". In other words, snoop tries to
filter on all IP addresses associated with hostname.


inet or inet6

A qualifier that modifies the host primitive that follows. If it
is inet, then snoop tries to filter on all IPv4 addresses
returned from a name lookup. If it is inet6, snoop tries to
filter on all IPv6 addresses returned from a name lookup.


ipaddr, atalkaddr, or etheraddr

Literal addresses, IP dotted, AppleTalk dotted, and Ethernet
colon are recognized. For example,

o "172.16.40.13" matches all packets with that IP

o "2::9255:a00:20ff:fe73:6e35" matches all packets with
that IPv6 address as source or destination;

o "65281.13" matches all packets with that AppleTalk
address;

o "8:0:20:f:b1:51" matches all packets with the Ethernet
address as source or destination.
An Ethernet address beginning with a letter is interpreted as a
hostname. To avoid this, prepend a zero when specifying the
address. For example, if the Ethernet address is
aa:0:45:23:52:44, then specify it by add a leading zero to make
it 0aa:0:45:23:52:44.


from or src

A qualifier that modifies the following host, net, ipaddr,
atalkaddr, etheraddr, port or rpc primitive to match just the
source address, port, or RPC reply.


to or dst

A qualifier that modifies the following host, net, ipaddr,
atalkaddr, etheraddr, port or rpc primitive to match just the
destination address, port, or RPC call.


ether

A qualifier that modifies the following host primitive to resolve
a name to an Ethernet address. Normally, IP address matching is
performed. This option is not supported on media such as IPoIB
(IP over InfiniBand).


ethertype number

True if the Ethernet type field has value number. If number is
not 0x8100 (VLAN) and the packet is VLAN tagged, then the
expression will match the encapsulated Ethernet type.


ip, ip6, arp, rarp, pppoed, pppoes

True if the packet is of the appropriate ethertype.


vlan

True if the packet has ethertype VLAN and the VLAN ID is not
zero.


vlan-id id

True for packets of ethertype VLAN with the id id.


pppoe

True if the ethertype of the packet is either pppoed or pppoes.


broadcast

True if the packet is a broadcast packet. Equivalent to
ether[2:4] = 0xffffffff for Ethernet. This option is not
supported on media such as IPoIB (IP over InfiniBand).


multicast

True if the packet is a multicast packet. Equivalent to "ether[0]
& 1 = 1" on Ethernet. This option is not supported on media such
as IPoIB (IP over InfiniBand).


bootp, dhcp

True if the packet is an unfragmented IPv4 UDP packet with either
a source port of BOOTPS (67) and a destination port of BOOTPC
(68), or a source port of BOOTPC (68) and a destination of BOOTPS
(67).


dhcp6

True if the packet is an unfragmented IPv6 UDP packet with either
a source port of DHCPV6-SERVER (547) and a destination port of
DHCPV6-CLIENT (546), or a source port of DHCPV6-CLIENT (546) and
a destination of DHCPV6-SERVER (547).


apple

True if the packet is an Apple Ethertalk packet. Equivalent to
"ethertype 0x809b or ethertype 0x80f3".


decnet

True if the packet is a DECNET packet.


greater length

True if the packet is longer than length.


less length

True if the packet is shorter than length.


udp, tcp, icmp, icmp6, ah, esp

True if the IP or IPv6 protocol is of the appropriate type.


net net

True if either the IP source or destination address has a network
number of net. The from or to qualifier may be used to select
packets for which the network number occurs only in the source or
destination address.


port port

True if either the source or destination port is port. The port
may be either a port number or name from /etc/services. The tcp
or udp primitives may be used to select TCP or UDP ports only.
The from or to qualifier may be used to select packets for which
the port occurs only as the source or destination.


rpc prog [ , vers [ , proc ] ]

True if the packet is an RPC call or reply packet for the
protocol identified by prog. The prog may be either the name of
an RPC protocol from /etc/rpc or a program number. The vers and
proc may be used to further qualify the program version and
procedure number, for example, rpc nfs,2,0 selects all calls and
replies for the NFS null procedure. The to or from qualifier may
be used to select either call or reply packets only.


zone zoneid

True if zoneid matches either the source or destination zoneid of
a packet received on an ipnet device.


ldap

True if the packet is an LDAP packet on port 389.


gateway host

True if the packet used host as a gateway, that is, the Ethernet
source or destination address was for host but not the IP
address. Equivalent to "ether host host and not host host".


nofrag

True if the packet is unfragmented or is the first in a series of
IP fragments. Equivalent to ip[6:2] & 0x1fff = 0.


expr relop expr

True if the relation holds, where relop is one of >, <, >=, <=,
=, !=, and expr is an arithmetic expression composed of numbers,
packet field selectors, the length primitive, and arithmetic
operators +, -, *, &, |, ^, and %. The arithmetic operators
within expr are evaluated before the relational operator and
normal precedence rules apply between the arithmetic operators,
such as multiplication before addition. Parentheses may be used
to control the order of evaluation. To use the value of a field
in the packet use the following syntax:

base[expr [: size ] ]


where expr evaluates the value of an offset into the packet from
a base offset which may be ether, ip, ip6, udp, tcp, or icmp. The
size value specifies the size of the field. If not given, 1 is
assumed. Other legal values are 2 and 4. For example,

ether[0] & 1 = 1

is equivalent to multicast

ether[2:4] = 0xffffffff

is equivalent to broadcast.

ip[ip[0] & 0xf * 4 : 2] = 2049

is equivalent to udp[0:2] = 2049

ip[0] & 0xf > 5

selects IP packets with options.

ip[6:2] & 0x1fff = 0

eliminates IP fragments.

udp and ip[6:2]&0x1fff = 0 and udp[6:2] != 0

finds all packets with UDP checksums.

The length primitive may be used to obtain the length of the
packet. For instance "length > 60" is equivalent to "greater 60",
and "ether[length - 1]" obtains the value of the last byte in a
packet.


and

Perform a logical AND operation between two boolean values. The
AND operation is implied by the juxtaposition of two boolean
expressions, for example "dinky pinky" is the same as "dinky AND
pinky".


or or ,

Perform a logical OR operation between two boolean values. A
comma may be used instead, for example, "dinky,pinky" is the same
as "dinky OR pinky".


not or !

Perform a logical NOT operation on the following boolean value.
This operator is evaluated before AND or OR.


slp

True if the packet is an SLP packet.


sctp

True if the packet is an SCTP packet.


ospf

True if the packet is an OSPF packet.

EXAMPLES

Example 1: Using the snoop Command

Capture all packets and display them as they are received:


example# snoop


Capture packets with host funky as either the source or destination and
display them as they are received:


example# snoop funky


Capture packets between funky and pinky and save them to a file. Then
inspect the packets using times (in seconds) relative to the first
captured packet:


example# snoop -o cap funky pinky
example# snoop -i cap -t r | more


To look at selected packets in another capture file:


example# snoop -i pkts -p 99,108
99 0.0027 boutique -> sunroof NFS C GETATTR FH=8E6
100 0.0046 sunroof -> boutique NFS R GETATTR OK
101 0.0080 boutique -> sunroof NFS C RENAME FH=8E6C MTra00192 to .nfs08
102 0.0102 marmot -> viper NFS C LOOKUP FH=561E screen.r.13.i386
103 0.0072 viper -> marmot NFS R LOOKUP No such file or directory
104 0.0085 bugbomb -> sunroof RLOGIN C PORT=1023 h
105 0.0005 kandinsky -> sparky RSTAT C Get Statistics
106 0.0004 beeblebrox -> sunroof NFS C GETATTR FH=0307
107 0.0021 sparky -> kandinsky RSTAT R
108 0.0073 office -> jeremiah NFS C READ FH=2584 at 40960 for 8192


To look at packet 101 in more detail:


example# snoop -i pkts -v -p101
ETHER: ----- Ether Header -----
ETHER:
ETHER: Packet 101 arrived at 16:09:53.59
ETHER: Packet size = 210 bytes
ETHER: Destination = 8:0:20:1:3d:94, Sun
ETHER: Source = 8:0:69:1:5f:e, Silicon Graphics
ETHER: Ethertype = 0800 (IP)
ETHER:
IP: ----- IP Header -----
IP:
IP: Version = 4, header length = 20 bytes
IP: Type of service = 00
IP: ..0. .... = routine
IP: ...0 .... = normal delay
IP: .... 0... = normal throughput
IP: .... .0.. = normal reliability
IP: Total length = 196 bytes
IP: Identification 19846
IP: Flags = 0X
IP: .0.. .... = may fragment
IP: ..0. .... = more fragments
IP: Fragment offset = 0 bytes
IP: Time to live = 255 seconds/hops
IP: Protocol = 17 (UDP)
IP: Header checksum = 18DC
IP: Source address = 172.16.40.222, boutique
IP: Destination address = 172.16.40.200, sunroof
IP:
UDP: ----- UDP Header -----
UDP:
UDP: Source port = 1023
UDP: Destination port = 2049 (Sun RPC)
UDP: Length = 176
UDP: Checksum = 0
UDP:
RPC: ----- SUN RPC Header -----
RPC:
RPC: Transaction id = 665905
RPC: Type = 0 (Call)
RPC: RPC version = 2
RPC: Program = 100003 (NFS), version = 2, procedure = 1
RPC: Credentials: Flavor = 1 (Unix), len = 32 bytes
RPC: Time = 06-Mar-90 07:26:58
RPC: Hostname = boutique
RPC: Uid = 0, Gid = 1
RPC: Groups = 1
RPC: Verifier : Flavor = 0 (None), len = 0 bytes
RPC:
NFS: ----- SUN NFS -----
NFS:
NFS: Proc = 11 (Rename)
NFS: File handle = 000016430000000100080000305A1C47
NFS: 597A0000000800002046314AFC450000
NFS: File name = MTra00192
NFS: File handle = 000016430000000100080000305A1C47
NFS: 597A0000000800002046314AFC450000
NFS: File name = .nfs08
NFS:


To view just the NFS packets between sunroof and boutique:


example# snoop -i pkts rpc nfs and sunroof and boutique
1 0.0000 boutique -> sunroof NFS C GETATTR FH=8E6C
2 0.0046 sunroof -> boutique NFS R GETATTR OK
3 0.0080 boutique -> sunroof NFS C RENAME FH=8E6C MTra00192 to .nfs08


To save these packets to a new capture file:


example# snoop -i pkts -o pkts.nfs rpc nfs sunroof boutique


To view encapsulated packets, there will be an indicator of
encapsulation:


example# snoop ip-in-ip
sunroof -> boutique ICMP Echo request (1 encap)


If -V is used on an encapsulated packet:


example# snoop -V ip-in-ip
sunroof -> boutique ETHER Type=0800 (IP), size = 118 bytes
sunroof -> boutique IP D=172.16.40.222 S=172.16.40.200 LEN=104, ID=27497
sunroof -> boutique IP D=10.1.1.2 S=10.1.1.1 LEN=84, ID=27497
sunroof -> boutique ICMP Echo request

Example 2: Setting Up A More Efficient Filter

To set up a more efficient filter, the following filters should be used
toward the end of the expression, so that the first part of the
expression can be set up in the kernel: greater, less, port, rpc, nofrag,
and relop. The presence of OR makes it difficult to split the filtering
when using these primitives that cannot be set in the kernel. Instead,
use parentheses to enforce the primitives that should be OR'd.


To capture packets between funky and pinky of type tcp or udp on port 80:


example# snoop funky and pinky and port 80 and tcp or udp


Since the primitive port cannot be handled by the kernel filter, and
there is also an OR in the expression, a more efficient way to filter is
to move the OR to the end of the expression and to use parentheses to
enforce the OR between tcp and udp:


example# snoop funky and pinky and (tcp or udp) and port 80

EXIT STATUS

0
Successful completion.


1
An error occurred.

FILES

/dev/audio
Symbolic link to the system's primary audio device.


/dev/null
The null file.


/etc/hosts
Host name database.


/etc/rpc
RPC program number data base.


/etc/services
Internet services and aliases.

SEE ALSO

ipnet(4D), audio(4I), bufmod(4M), pfmod(4M), dlpi(4P), hosts(5), rpc(5),
services(5), attributes(7), dladm(8), ifconfig(8), netstat(8)


Callaghan, B. and Gilligan, R. RFC 1761, Snoop Version 2 Packet Capture
File Format. Network Working Group. February 1995.

WARNINGS

The processing overhead is much higher for real-time packet
interpretation. Consequently, the packet drop count may be higher. For
more reliable capture, output raw packets to a file using the -o option
and analyze the packets offline.


Unfiltered packet capture imposes a heavy processing load on the host
computer, particularly if the captured packets are interpreted real-time.
This processing load further increases if verbose options are used. Since
heavy use of snoop may deny computing resources to other processes, it
should not be used on production servers. Heavy use of snoop should be
restricted to a dedicated computer.


snoop does not reassemble IP fragments. Interpretation of higher level
protocol halts at the end of the first IP fragment.


snoop may generate extra packets as a side-effect of its use. For example
it may use a network name service to convert IP addresses to host names
for display. Capturing into a file for later display can be used to
postpone the address-to-name mapping until after the capture session is
complete. Capturing into an NFS-mounted file may also generate extra
packets.


Setting the snaplen (-s option) to small values may remove header
information that is needed to interpret higher level protocols. The exact
cutoff value depends on the network and protocols being used. For NFS
Version 2 traffic using UDP on 10 Mb/s Ethernet, do not set snaplen less
than 150 bytes. For NFS Version 3 traffic using TCP on 100 Mb/s Ethernet,
snaplen should be 250 bytes or more.


snoop requires information from an RPC request to fully interpret an RPC
reply. If an RPC reply in a capture file or packet range does not have a
request preceding it, then only the RPC reply header will be displayed.


March 22, 2021 SNOOP(8)