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     snoop - capture and inspect network packets

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

     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 pack-
     ets  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.


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


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


         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  cap-
         ture  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.


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


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


         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


         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 device

         Receive packets from the  network  using  the  interface
         specified by device, for example, eri0 or hme0. The pro-
         gram 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  inter-
         face it finds.

     -i filename

         Display packets previously captured in filename. Without
         this option, snoop reads packets from the network inter-
         face.  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

     -p first [ , last ]

         Select one or more packets to be displayed from  a  cap-
         ture file. The first packet in the file is packet number


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


         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 over-
         flow 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.


         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.


         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 cap-
         ture 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 informa-
         tion 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 expres-
             sion 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    ""  matches  all  packets  with
                      that IP

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

                 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 destina-
             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

         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

         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


             A qualifier that modifies the following host  primi-
             tive  to resolve a name to an Ethernet address. Nor-
             mally, IP address matching is performed. This option
             is  not  supported  on  media such as IPoIB (IP over

         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  encapsu-
             lated Ethernet type.

         ip, ip6, arp, rarp, pppoed, pppoes

             True if the packet is of the appropriate ethertype.


             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.


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


             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


             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

         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).


             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 desti-
             nation of DHCPV6-SERVER (547).


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


             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  appropri-
             ate type.

         net net

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

         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.


             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".


             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 expres-
             sion 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 arith-
             metic operators, such as multiplication before addi-
             tion. 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.


             Perform a logical AND operation between two  boolean
             values. The AND operation is implied by the juxtapo-
             sition  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.


             True if the packet is an SLP packet.


             True if the packet is an SCTP packet.


             True if the packet is an OSPF packet.

     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 des-
     tination 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:  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)
       IP:   ----- IP Header -----
       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 =, boutique
       IP:   Destination address =, sunroof
       UDP:  ----- UDP Header -----
       UDP:  Source port = 1023
       UDP:  Destination port = 2049 (Sun RPC)
       UDP:  Length = 176
       UDP:  Checksum = 0
       RPC:  ----- SUN RPC Header -----
       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
       NFS:  ----- SUN 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

     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

       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= S= LEN=104, ID=27497
       sunroof -> boutique  IP  D= S= 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

     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

     0    Successful completion.

     1    An error occurred.

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

     /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  attri-

    |       ATTRIBUTE TYPE        |       ATTRIBUTE VALUE       |
    | Availability                | SUNWrcmdc                   |

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

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

     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 frag-

     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 post-
     pone 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.

Man pages from Solaris 10 Update 8. See docs.sun.com and www.oracle.com for further documentation and Solaris information.