snoop(1M)
snoop - capture and inspect network packets
As shipped in IRIX 6.5.19. Last changed in IRIX 6.5.19.
NAME snoop - capture and inspect network packets SYNOPSIS snoop [ -aqrPDSvVNC ] [ -d device ] [ -s snaplen ] [ -c maxcount ] [ -i filename ] [ -o filename ] [ -n filename ] [ -t [ r | a | d ] ] [ -p first [ , last ] ] [ -x offset [ , length ] ] [ expression ] DESCRIPTION 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 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. OPTIONS -a Listen to packets on /dev/audio (warning: can be noisy). -P Capture packets in non-promiscuous mode. Only broadcast, multicast, or packets addressed to the host machine will be seen. -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. -d device Receive packets from the network using the interface specified by device. Usually ec0. 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. -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. -c maxcount Quit after capturing maxcount packets. Otherwise keep capturing until there is no disk left or until interrupted with CTRL-C. -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 snoop's IP address-to-name mapping table (See -N flag below). -o filename Save captured packets in filename as they are captured. 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. -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). -D Display number of packets dropped during capture on the summary line. -S Display size of the entire ethernet frame in bytes on the summary line. -t [ r | a | d ] Time-stamp presentation. Time-stamps are accurate to within a few 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. -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: example# snoop -i rpc.cap -V | grep RPC -p first [ , last ] Select one or more packets to be displayed from a capture file. The first packet in the file is packet #1. See WARNINGS for an example of how this can cause RPC reply packets to be printed in a confusing way. -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. -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. -C List the code generated from the filter expression for either the kernel packet filter, or snoop's own filter. 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 generates code for its own filter. The -C flag can be used to view generated code for either the kernel's or snoop's own packet filter. If packets are read from a capture file using the -i option, only snoop's packet filter 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 the filter expression in quotes. The primitives are: host hostname True if the source or destination address is that of hostname. The keyword host may be omitted if the name does not conflict with the name of another expression primitive e.g. "pinky" selects packets transmitted to or received from the host pinky whereas "pinky and dinky" selects packets exchanged between hosts pinky AND dinky. Normally the IP address is used. With the ether qualifier the ethernet address is used, for instance, "ether pinky". 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 or etheraddr Literal addresses, both IP dotted and ethernet colon are recognized. For example, "129.144.40.13" matches all packets with that IP address as source or destination, and similarly, "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, 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, 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 True if the packet is of the appropriate ethertype. 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 0x803f". 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. 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. Examples: "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 . EXAMPLES 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#snoopfunky 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 Look at selected packets in another capture file: example$ snoop -i pkts -p99,108 99 0.0027 boutique -> sunroof NFS C GETATTR FH=8E6C 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 Packet 101 Looks interesting. Take a look 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 = 129.144.40.222, boutique IP: Destination address = 129.144.40.200, 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: 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 Save these packets to a new capture file: $ snoop -i pkts -o pkts.nfs rpc nfs sunroof boutique EXIT STATUS Unless snoop receives an error signal, its Exit Status is zero. All abnormal exits return 1. WARNINGS 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 required for packet interpretation for higher level protocols. For complete NFS interpretation do not set snaplen less than 120 bytes. snoop requires information from an RPC request to fully interpret an RPC reply. If an RPC reply in a capture file or packet range (specified by the -p option) does not have a request preceding it, then only the RPC reply header will be displayed, and not the higher protocol. For example an NFS trace will look like this example$ snoop -i snoop.dat -p 2,5 2 0.000114 coke -> oops NFS C GETATTR3 FH=9D27 3 0.000798 oops -> coke NFS R GETATTR3 OK 4 1.227730 coke -> oops NFS C GETATTR3 FH=9D27 5 0.000839 oops -> coke NFS R GETATTR3 OK but when the -p option is used to skip the request in packet 2, the matching reply in packet 3 is displayed as RPC not NFS: example$ snoop -i snoop.dat -p 3,5 3 0.000000 oops -> coke RPC R XID=835404387 Success 4 1.227730 coke -> oops NFS C GETATTR3 FH=9D27 5 0.000839 oops -> coke NFS R GETATTR3 OK NOTES snoop requires an interactive interface.