snoop captures packets from the network and displays their contents. snoop uses both the network packet filter and streams buffer 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, only the data pertaining to the highest level protocol is displayed. For example, an NFS packet will have only NFS information displayed. The underlying RPC, UDP, IP, and ethernet frame information is suppressed but can be displayed if either of the verbose options are chosen.

snoop requires an interactive interface.

-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.

-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 ethernet 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 left or until interrupted with Control-C.

-d device

Receive packets from the network using the interface specified by device, for example, le0 or hme0. The program netstat(1M), when invoked with the -i flag, lists all the interfaces that a machine has. Normally, snoop will automatically choose the first non-loopback interface it finds.

-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.

-x offset [ , 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.

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 Setting Up A More Efficient Filter 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,

• "129.144.40.13" matches all packets with that IP ;
• "2::9255:a00:20ff:fe73:6e35" matches all packets with that IPv6 address as source or destination;
• "65281.13" matches all packets with that AppleTalk address;
• "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.

ethertype number

True if the ethernet type field has value number. Equivalent to "ether[12:2] = number".

ip, ip6, arp, rarp, pppoed, pppoes

True if the packet is of the appropriate ethertype.

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".

multicast

True if the packet is a multicast packet. Equivalent to "ether[0] & 1 = 1".

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.

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, 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 a SCTP packet.

0

Successful completion.

1

An error occurred.

/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 attributes(5) for descriptions of the following attributes:

netstat(1M), hosts(4), rpc(4), services(4), attributes(5), audio(7I), bufmod(7M), dlpi(7P), le(7D), pfmod(7M), tun(7M)

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

The processing overhead is much higher for realtime 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 off-line.

Unfiltered packet capture imposes a heavy processing load on the host computer, particularly if the captured packets are interpreted realtime. 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 (NIS or NIS+) 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.