MirBSD manpage: tcpdump(8)

TCPDUMP(8)               BSD System Manager's Manual                TCPDUMP(8)


     tcpdump - dump traffic on a network


     tcpdump [-adefLlNnOopqStvXx] [-c count] [-E [espalg:]espkey] [-F file]
             [-i interface] [-r file] [-s snaplen] [-T type] [-w file]
             [-y datalinktype] [expression]


     tcpdump prints out the headers of packets on a network interface that
     match the boolean expression. You must have read access to /dev/bpf*.

     The options are as follows:

     -a        Attempt to convert network and broadcast addresses to names.

     -c count  Exit after receiving count packets.

     -d        Dump the compiled packet-matching code in a human readable form
               to standard output and stop.

     -dd       Dump packet-matching code as a C program fragment.

     -ddd      Dump packet-matching code as decimal numbers preceded with a

     -E [espalg:]espkey
               Try to decrypt RFC 2406 ESP (Encapsulating Security Payload)
               traffic using the specified hex key espkey. Supported algo-
               rithms for espalg are: aes128, aes128-hmac96, blowfish,
               blowfish-hmac96, cast, cast-hmac96, des3, des3-hmac96, des and
               des-hmac96. The algorithm defaults to aes128-hmac96. This op-
               tion should be used for debugging only, since the key will show
               up in ps(1) output.

     -e        Print the link-level header on each dump line.

     -F file   Use file as input for the filter expression. Any additional ex-
               pressions given on the command line are ignored.

     -f        Print "foreign" internet addresses numerically rather than sym-
               bolically. This option is intended to get around serious brain
               damage in Sun's yp server - usually it hangs forever translat-
               ing non-local internet numbers.

     -i interface
               Listen on interface. If unspecified, tcpdump searches the sys-
               tem interface list for the lowest numbered, configured "up" in-
               terface (excluding loopback). Ties are broken by choosing the
               earliest match.

     -L        List the supported data link types for the interface and exit.

     -l        Make stdout line buffered. Useful if you want to see the data
               while capturing it. For example:

                     # tcpdump -l | tee dat
                     # tcpdump -l > dat & tail -f dat

     -N        Do not print domain name qualification of host names. For exam-
               ple, if you specify this flag then tcpdump will print "nic" in-
               stead of "nic.ddn.mil".

     -n        Do not convert addresses (host addresses, port numbers, etc.)
               to names.

     -O        Do not run the packet-matching code optimizer. This is useful
               only if you suspect a bug in the optimizer.

     -o        Print a guess of the possible operating system(s) of hosts that
               sent TCP SYN packets. See pf.os(5) for a description of the
               passive operating system fingerprints.

     -p        Do not put the interface into promiscuous mode. The interface
               might be in promiscuous mode for some other reason; hence, -p
               cannot be used as an abbreviation for "ether host "{local-hw-
               addr}"" or "ether broadcast".

     -q        Quick (quiet?) output. Print less protocol information so out-
               put lines are shorter.

     -r file   Read packets from a file which was created with the -w option.
               Standard input is used if file is '-'.

     -S        Print absolute, rather than relative, TCP sequence numbers.

     -s snaplen
               Analyze at most the first snaplen bytes of data from each pack-
               et rather than the default of 96. 96 bytes is adequate for IP,
               ICMP, TCP, and UDP, but may truncate protocol information from
               name server and NFS packets (see below). Packets truncated be-
               cause of a limited snaplen are indicated in the output with
               "[|proto]", where proto is the name of the protocol level at
               which the truncation has occurred. Taking larger snapshots both
               increases the amount of time it takes to process packets and,
               effectively, decreases the amount of packet buffering. This may
               cause packets to be lost. You should limit snaplen to the smal-
               lest number that will capture the protocol information you're
               interested in.

     -T type   Force packets selected by expression to be interpreted as the
               specified type. Currently known types are vrrp (Virtual Router
               Redundancy protocol), cnfp (Cisco NetFlow protocol), rpc
               (Remote Procedure Call), rtp (Real-Time Applications protocol),
               rtcp (Real-Time Applications control protocol), sack (RFC 2018
               TCP Selective Acknowledgements Options), tcp (Transmission
               Control Protocol), vat (Visual Audio Tool), and wb (distributed
               White Board).

     -t        Do not print a timestamp on each dump line.

     -tt       Print an unformatted timestamp on each dump line.

     -ttt      Print day and month in timestamp.

     -tttt     Print timestamp difference between packets.

     -ttttt    Print timestamp difference since the first packet.

     -v        (Slightly more) verbose output. For example, the time to live
               (TTL) and type of service (ToS) information in an IP packet are

     -vv       Even more verbose output. For example, additional fields are
               printed from NFS reply packets.

     -w file   Write the raw packets to file rather than parsing and printing
               them out. They can be analyzed later with the -r option. Stan-
               dard output is used if file is '-'.

     -X        Like -x but dumps the packet in emacs-hexl like format.

     -x        Print each packet (minus its link-level header) in hex. The
               smaller of the entire packet or snaplen bytes will be printed.

     -y datalinktype
               Set the data link type to use while capturing to datalinktype.

     expression selects which packets will be dumped. If no expression is
     given, all packets on the net will be dumped. Otherwise, only packets
     satisfying expression will be dumped.

     The expression consists of one or more primitives. Primitives usually
     consist of an id (name or number) preceded by one or more qualifiers.
     There are three different kinds of qualifiers:

     type   Specify which kind of address component the id name or number
            refers to. Possible types are host, net and port. E.g., "host
            foo", "net 128.3", "port 20". If there is no type qualifier, host
            is assumed.

     dir    Specify a particular transfer direction to and/or from id. Possi-
            ble directions are src, dst, src or dst, and src and dst. E.g.,
            "src foo", "dst net 128.3", "src or dst port ftp-data". If there
            is no dir qualifier, src or dst is assumed. For null link layers
            (i.e., point-to-point protocols such as SLIP (Serial Line Internet
            Protocol) or the pflog(4) header), the inbound and outbound qual-
            ifiers can be used to specify a desired direction.

     proto  Restrict the match to a particular protocol. Possible protocols
            are: ah, arp, atalk, decnet, esp, ether, fddi, icmp, icmp6, igmp,
            igrp, ip, ip6, lat, mopdl, moprc, pim, rarp, sca, tcp, and udp.
            E.g., "ether src foo", "arp net 128.3", "tcp port 21". If there is
            no protocol qualifier, all protocols consistent with the type are
            assumed. E.g., "src foo" means "(ip or arp or rarp) src foo"
            (except the latter is not legal syntax); "net bar" means
            "(ip or arp or rarp) net bar"; and "port 53" means
            "(TCP or UDP) port 53".

            fddi is actually an alias for ether; the parser treats them ident-
            ically as meaning
            "the data link level used on the specified network interface".
            FDDI (Fiber Distributed Data Interface) headers contain Ethernet-
            like source and destination addresses, and often contain
            Ethernet-like packet types, so you can filter on these FDDI fields
            just as with the analogous Ethernet fields. FDDI headers also con-
            tain other fields, but you cannot name them explicitly in a filter

     In addition to the above, there are some special primitive keywords that
     don't follow the pattern: gateway, broadcast, less, greater, and arith-
     metic expressions. All of these are described below.

     More complex filter expressions are built up by using the words and, or,
     and not to combine primitives e.g.,
     "host foo and not port ftp and not port ftp-data". To save typing, ident-
     ical qualifier lists can be omitted e.g., "tcp dst port ftp or ftp-data
     or domain" is exactly the same as "tcp dst port ftp or tcp dst port ftp-
     data or tcp dst port domain".

     Allowable primitives are:

     dst host host      True if the IP destination field of the packet is
                        host, which may be either an address or a name.

     src host host      True if the IP source field of the packet is host.

     host host          True if either the IP source or destination of the
                        packet is host.

                        Any of the above host expressions can be prepended
                        with the keywords, ip, arp, or rarp as in:

                              ip host host

                        which is equivalent to:

                              ether proto ip and host host

                        If host is a name with multiple IP addresses, each ad-
                        dress will be checked for a match.

     ether dst ehost    True if the Ethernet destination address is ehost.
                        ehost may be either a name from /etc/ethers or a
                        number (see ethers(3) for a numeric format).

     ether src ehost    True if the Ethernet source address is ehost.

     ether host ehost   True if either the Ethernet source or destination ad-
                        dress is ehost.

     gateway host       True if the packet used host as a gateway; i.e., the
                        Ethernet source or destination address was host but
                        neither the IP source nor the IP destination was host.
                        host must be a name and must be found in both
                        /etc/hosts and /etc/ethers. An equivalent expression

                              ether host ehost and not host host

                        which can be used with either names or numbers for

     dst net net        True if the IP destination address of the packet has a
                        network number of net. net may be either a name from
                        /etc/networks or a network number (see networks(5) for

     src net net        True if the IP source address of the packet has a net-
                        work number of net.

     net net            True if either the IP source or destination address of
                        the packet has a network number of net.

     dst port port      True if the packet is IP/TCP or IP/UDP and has a des-
                        tination port value of port. The port can be a number
                        or name from services(5) (see tcp(4) and udp(4)). If a
                        name is used, both the port number and protocol are
                        checked. If a number or ambiguous name is used, only
                        the port number is checked; e.g., "dst port 513" will
                        print both TCP/login traffic and UDP/who traffic, and
                        "dst port domain" will print both TCP/domain and
                        UDP/domain traffic.

     src port port      True if the packet has a source port value of port.

     port port          True if either the source or destination port of the
                        packet is port.

                        Any of the above port expressions can be prepended
                        with the keywords tcp or udp, as in:

                              tcp src port port

                        which matches only TCP packets whose source port is

     less length        True if the packet has a length less than or equal to
                        length. This is equivalent to:

                              len <= length

     greater length     True if the packet has a length greater than or equal
                        to length. This is equivalent to:

                              len >= length

     ip proto proto     True if the packet is an IP packet (see ip(4)) of pro-
                        tocol type proto. proto can be a number or name from
                        protocols(5), such as icmp, udp, or tcp. These iden-
                        tifiers are also keywords and must be escaped using a
                        backslash character ('\').

     ether broadcast    True if the packet is an Ethernet broadcast packet.
                        The ether keyword is optional.

     ip broadcast       True if the packet is an IP broadcast packet. It
                        checks for both the all-zeroes and all-ones broadcast
                        conventions and looks up the local subnet mask.

     ether multicast    True if the packet is an Ethernet multicast packet.
                        The ether keyword is optional. This is shorthand for
                        "ether[0] & 1 != 0".

     ip multicast       True if the packet is an IP multicast packet.

     ether proto proto  True if the packet is of ether type proto. proto can
                        be a number or name from protocols(5), such as ip,
                        arp, or rarp. These identifiers are also keywords and
                        must be escaped using a backslash character ('\'). In
                        the case of FDDI (e.g., "fddi protocol arp"), the pro-
                        tocol identification comes from the 802.2 Logical Link
                        Control (LLC) header, which is usually layered on top
                        of the FDDI header. tcpdump assumes, when filtering on
                        the protocol identifier, that all FDDI packets include
                        an LLC header, and that the LLC header is in so-called
                        SNAP format.

     decnet src host    True if the DECNET source address is host, which may
                        be an address of the form "10.123", or a DECNET host
                        name. DECNET host name support is only available on
                        systems that are configured to run DECNET.

     decnet dst host    True if the DECNET destination address is host.

     decnet host host   True if either the DECNET source or destination ad-
                        dress is host.

     ifname interface   True if the packet was logged as coming from the
                        specified interface (applies only to packets logged by

     on interface       Synonymous with the ifname modifier.

     rnr num            True if the packet was logged as matching the speci-
                        fied PF rule number in the main ruleset (applies only
                        to packets logged by pf(4)).

     rulenum num        Synonymous with the rnr modifier.

     reason code        True if the packet was logged with the specified PF
                        reason code. The known codes are: match, bad-offset,
                        fragment, bad-timestamp, short, normalize, and memory
                        (applies only to packets logged by pf(4)).

     rset name          True if the packet was logged as matching the speci-
                        fied PF ruleset name of an anchored ruleset (applies
                        only to packets logged by pf(4)).

     ruleset name       Synonymous with the rset modifier.

     srnr num           True if the packet was logged as matching the speci-
                        fied PF rule number of an anchored ruleset (applies
                        only to packets logged by pf(4)).

     subrulenum num     Synonymous with the srnr modifier.

     action act         True if PF took the specified action when the packet
                        was logged. Known actions are: pass, and block (ap-
                        plies only to packets logged by pf(4)).

     atalk, ip, ip6, arp, decnet, lat, moprc, mopdl, rarp, sca
                        Abbreviations for: ether proto p where p is one of the
                        above protocols. tcpdump does not currently know how
                        to parse lat, moprc, or mopdl.

     ah, esp, icmp, icmp6, igmp, igrp, pim, tcp, udp
                        Abbreviations for: ip proto p where p is one of the
                        above protocols.

     expr relop expr    True if the relation holds, where relop is one of '>',
                        '<', '>=', '<=', '=', '!=', and expr is an arithmetic
                        expression composed of integer constants (expressed in
                        standard C syntax), the normal binary operators ('+',
                        '-', '*', '/', '&', '|'), a length operator, and spe-
                        cial packet data accessors. To access data inside the
                        packet, use the following syntax:


                        proto is one of ether, fddi, ip, arp, rarp, tcp, udp,
                        or icmp, and indicates the protocol layer for the in-
                        dex operation. The byte offset, relative to the indi-
                        cated protocol layer, is given by expr. size is op-
                        tional and indicates the number of bytes in the field
                        of interest; it can be either one, two, or four, and
                        defaults to one. The length operator, indicated by the
                        keyword len, gives the length of the packet.

                        For example, "ether[0] & 1 != 0" catches all multicast
                        traffic. The expression "ip[0] & 0xf != 5" catches all
                        IP packets with options. The expression "ip[6:2] &
                        0x1fff = 0" catches only unfragmented datagrams and
                        frag zero of fragmented datagrams. This check is im-
                        plicitly applied to the tcp and udp index operations.
                        For instance, "tcp[0]" always means the first byte of
                        the TCP header, and never means the first byte of an
                        intervening fragment.

     Primitives may be combined using a parenthesized group of primitives and
     operators. Parentheses are special to the shell and must be escaped. Al-
     lowable primitives and operators are:

           Negation ("!" or "not")

           Concatenation ("&&" or "and")

           Alternation ("||" or "or")

     Negation has highest precedence. Alternation and concatenation have equal
     precedence and associate left to right. Explicit and tokens, not juxtapo-
     sition, are now required for concatenation.

     If an identifier is given without a keyword, the most recent keyword is
     assumed. For example,

           not host vs and ace

     is short for

           not host vs and host ace

     which should not be confused with

           not (host vs or ace)

     Expression arguments can be passed to tcpdump as either a single argument
     or as multiple arguments, whichever is more convenient. Generally, if the
     expression contains shell metacharacters, it is easier to pass it as a
     single, quoted argument. Multiple arguments are concatenated with spaces
     before being parsed.


     To print all packets arriving at or departing from sundown:

           # tcpdump host sundown

     To print traffic between helios and either hot or ace (the expression is
     quoted to prevent the shell from mis-interpreting the parentheses):

           # tcpdump 'host helios and (hot or ace)'

     To print all IP packets between ace and any host except helios:

           # tcpdump ip host ace and not helios

     To print all traffic between local hosts and hosts at Berkeley:

           # tcpdump net ucb-ether

     To print all FTP traffic through internet gateway snup:

           # tcpdump 'gateway snup and (port ftp or ftp-data)'

     To print traffic neither sourced from nor destined for local hosts (if
     you gateway to one other net, this stuff should never make it onto your
     local net):

           # tcpdump ip and not net localnet

     To print the start and end packets (the SYN and FIN packets) of each TCP
     connection that involves a non-local host:

           # tcpdump 'tcp[13] & 3 != 0 and not src and dst net localnet'

     To print IP packets longer than 576 bytes sent through gateway snup:

           # tcpdump 'gateway snup and ip[2:2] > 576'

     To print IP broadcast or multicast packets that were not sent via Ether-
     net broadcast or multicast:

           # tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'

     To print all ICMP packets that are not echo requests/replies (i.e., not
     ping packets):

           # tcpdump 'icmp[0] != 8 and icmp[0] != 0'

     To print and decrypt all ESP packets with SPI 0x00001234:

           # tcpdump -E des3-hmac96:ab...def 'ip[20:4] = 0x00001234'


     The output of tcpdump is protocol dependent. The following gives a brief
     description and examples of most of the formats.

Link Level Headers

     If the -e option is given, the link level header is printed out. On Eth-
     ernets, the source and destination addresses, protocol, and packet length
     are printed.

     On the packet filter logging interface pflog(4), logging reason (rule
     match, bad-offset, fragment, bad-timestamp, short, normalize, memory),
     action taken (pass/block), direction (in/out) and interface information
     are printed out for each packet.

     On FDDI networks, the -e option causes tcpdump to print the frame control
     field, the source and destination addresses, and the packet length. The
     frame control field governs the interpretation of the rest of the packet.
     Normal packets (such as those containing IP datagrams) are "async" pack-
     ets, with a priority value between 0 and 7; for example, async4. Such
     packets are assumed to contain an 802.2 Logical Link Control (LLC) pack-
     et; the LLC header is printed if it is not an ISO datagram or a so-called
     SNAP packet.

     The following description assumes familiarity with the SLIP compression
     algorithm described in RFC 1144.

     On SLIP links, a direction indicator ('I' for inbound, 'O' for outbound),
     packet type, and compression information are printed out. The packet type
     is printed first. The three types are ip, utcp, and ctcp. No further link
     information is printed for IP packets. For TCP packets, the connection
     identifier is printed following the type. If the packet is compressed,
     its encoded header is printed out. The special cases are printed out as
     *S+n and *SA+n, where n is the amount by which the sequence number (or
     sequence number and ack) has changed. If it is not a special case, zero
     or more changes are printed. A change is indicated by 'U' (urgent
     pointer), 'W' (window), 'A' (ack), 'S' (sequence number), and 'I' (packet
     ID), followed by a delta (+n or -n), or a new value (=n). Finally, the
     amount of data in the packet and compressed header length are printed.

     For example, the following line shows an outbound compressed TCP packet,
     with an implicit connection identifier; the ack has changed by 6, the se-
     quence number by 49, and the packet ID by 6; there are 3 bytes of data
     and 6 bytes of compressed header:

           O ctcp * A +6 S +49 I +6 3 (6)

ARP/RARP Packets

     arp/rarp output shows the type of request and its arguments. The format
     is intended to be self-explanatory. Here is a short sample taken from the
     start of an rlogin from host rtsg to host csam:

           arp who-has csam tell rtsg
           arp reply csam is-at CSAM

     In this example, Ethernet addresses are in caps and internet addresses in
     lower case. The first line says that rtsg sent an arp packet asking for
     the Ethernet address of internet host csam. csam replies with its Ether-
     net address CSAM.

     This would look less redundant if we had done tcpdump -n:

           arp who-has tell
           arp reply is-at 02:07:01:00:01:c4

     If we had done tcpdump -e, the fact that the first packet is broadcast
     and the second is point-to-point would be visible:

           RTSG Broadcast 0806 64: arp who-has csam tell rtsg
           CSAM RTSG 0806 64: arp reply csam is-at CSAM

     For the first packet this says the Ethernet source address is RTSG, the
     destination is the Ethernet broadcast address, the type field contained
     hex 0806 (type ETHER_ARP) and the total length was 64 bytes.

TCP Packets

     The following description assumes familiarity with the TCP protocol
     described in RFC 793. If you are not familiar with the protocol, neither
     this description nor tcpdump will be of much use to you.

     The general format of a TCP protocol line is:

           src > dst: flags src-os data-seqno ack window urgent options

     src and dst are the source and destination IP addresses and ports. flags
     is some combination of 'S' (SYN), 'F' (FIN), 'P' (PUSH), or 'R' (RST),
     'W' (congestion Window reduced), 'E' (ecn ECHO) or a single '.' (no
     flags). src-os will list a guess of the source host's operating system if
     the -o command line flag was passed to tcpdump. data-seqno describes the
     portion of sequence space covered by the data in this packet (see example
     below). ack is the sequence number of the next data expected by the other
     end of this connection. window is the number of bytes of receive buffer
     space available at the other end of this connection. urg indicates there
     is urgent data in the packet. options are TCP options enclosed in angle
     brackets e.g., <mss 1024>.

     src, dst and flags are always present. The other fields depend on the
     contents of the packet's TCP protocol header and are output only if ap-

     Here is the opening portion of an rlogin from host rtsg to host csam.

       rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024>
       csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
       rtsg.1023 > csam.login: . ack 1 win 4096
       rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096
       csam.login > rtsg.1023: . ack 2 win 4096
       rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096
       csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077
       csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1
       csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1

     The first line says that TCP port 1023 on rtsg sent a packet to port lo-
     gin on host csam. The 'S' indicates that the SYN flag was set. The packet
     sequence number was 768512 and it contained no data. The notation is
     'first:last(nbytes)' which means sequence numbers first up to but not in-
     cluding last which is nbytes bytes of user data. There was no piggy-
     backed ack, the available receive window was 4096 bytes and there was a
     max-segment-size option requesting an mss of 1024 bytes.

     Csam replies with a similar packet except it includes a piggy-backed ack
     for rtsg's SYN. Rtsg then acks csam's SYN. The '.' means no flags were
     set. The packet contained no data so there is no data sequence number.
     The ack sequence number is a 32-bit integer. The first time tcpdump sees
     a TCP connection, it prints the sequence number from the packet. On sub-
     sequent packets of the connection, the difference between the current
     packet's sequence number and this initial sequence number is printed.
     This means that sequence numbers after the first can be interpreted as
     relative byte positions in the connection's data stream
     (with the first data byte each direction being 1). -S will override this
     feature, causing the original sequence numbers to be output.

     On the 6th line, rtsg sends csam 19 bytes of data
     (bytes 2 through 20 in the rtsg -> csam side of the connection). The PUSH
     flag is set in the packet. On the 7th line, csam says it's received data
     sent by rtsg up to but not including byte 21. Most of this data is ap-
     parently sitting in the socket buffer since csam's receive window has
     gotten 19 bytes smaller. Csam also sends one byte of data to rtsg in this
     packet. On the 8th and 9th lines, csam sends two bytes of urgent, pushed
     data to rtsg.

UDP Packets

     UDP format is illustrated by this rwho packet:

           actinide.who > broadcast.who: udp 84

     This says that port who on host actinide sent a UDP datagram to port who
     on host broadcast, the Internet broadcast address. The packet contained
     84 bytes of user data.

     Some UDP services are recognized (from the source or destination port
     number) and the higher level protocol information printed. In particular,
     Domain Name service requests (RFC 1034/1035) and Sun RPC calls (RFC 1050)
     to NFS.

UDP Name Server Requests

     The following description assumes familiarity with the Domain Service
     protocol described in RFC 1035. If you are not familiar with the proto-
     col, the following description will appear to be written in Greek.

     Name server requests are formatted as

           src > dst: id op? flags qtype qclass name (len)

     For example:

           h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)

     Host h2opolo asked the domain server on helios for an address record
     (qtype=A) associated with the name ucbvax.berkeley.edu. The query id was
     3. The '+' indicates the recursion desired flag was set. The query length
     was 37 bytes, not including the UDP and IP protocol headers. The query
     operation was the normal one (Query) so the op field was omitted. If op
     had been anything else, it would have been printed between the 3 and the
     '+'. Similarly, the qclass was the normal one (C_IN) and was omitted. Any
     other qclass would have been printed immediately after the A.

     A few anomalies are checked and may result in extra fields enclosed in
     square brackets: if a query contains an answer, name server or authority
     section, ancount, nscount, or arcount are printed as "[na]", "[nn]", or
     "[nau]" where n is the appropriate count. If any of the response bits are
     set (AA, RA or rcode) or any of the "must be zero" bits are set in bytes
     two and three, "[b2&3=x]" is printed, where x is the hex value of header
     bytes two and three.

UDP Name Server Responses

     Name server responses are formatted as

           src > dst: id op rcode flags a / n / au type class data (len)

     For example:

           helios.domain > h2opolo.1538: 3 3/3/7 A (273)
           helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)

     In the first example, helios responds to query id 3 from h2opolo with 3
     answer records, 3 name server records and 7 authority records. The first
     answer record is type A (address and its data is internet) address The total size of the response was 273 bytes, excluding UDP
     and IP headers. The op (Query) and rcode (NoError) were omitted, as was
     the class (C_IN) of the A record.

     In the second example, helios responds to query op 2 with an rcode of
     non-existent domain (NXDomain) with no answers, one name server and no
     authority records. The '*' indicates that the authoritative answer bit
     was set. Since there were no answers, no type, class or data were print-

     Other flag characters that might appear are '-' (recursion available, RA,
     not set) and '|' (truncated message, TC, set). If the question section
     doesn't contain exactly one entry, "[nq]" is printed.

     Name server requests and responses tend to be large and the default
     snaplen of 96 bytes may not capture enough of the packet to print. Use
     the -s flag to increase the snaplen if you need to seriously investigate
     name server traffic. "-s 128" has worked well for me.

NFS Requests and Replies

     Sun NFS (Network Filesystem) requests and replies are printed as:

           src.xid > dst.nfs: len op args

           src.nfs > dst.xid: reply stat len op results

           sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
           wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
           sushi.201b > wrl.nfs:
                144 lookup fh 9,74/4096.6878 "xcolors"
           wrl.nfs > sushi.201b:
                reply ok 128 lookup fh 9,74/4134.3150

     In the first line, host sushi sends a transaction with ID 6709 to wrl.
     The number following the src host is a transaction ID, not the source
     port. The request was 112 bytes, excluding the UDP and IP headers. The op
     was a readlink (read symbolic link) on fh ("file handle")
     21,24/10.731657119. If one is lucky, as in this case, the file handle can
     be interpreted as a major,minor device number pair, followed by the inode
     number and generation number. Wrl replies with a stat of ok and the con-
     tents of the link.

     In the third line, sushi asks wrl to look up the name "xcolors" in direc-
     tory file 9,74/4096.6878. The data printed depends on the operation type.
     The format is intended to be self-explanatory if read in conjunction with
     an NFS protocol spec.

     If the -v (verbose) flag is given, additional information is printed. For

           sushi.1372a > wrl.nfs:
                148 read fh 21,11/12.195 8192 bytes @ 24576
           wrl.nfs > sushi.1372a:
                reply ok 1472 read REG 100664 ids 417/0 sz 29388

     -v also prints the IP header TTL, ID, and fragmentation fields, which
     have been omitted from this example. In the first line, sushi asks wrl to
     read 8192 bytes from file 21,11/12.195, at byte offset 24576. Wrl replies
     with a stat of ok; the packet shown on the second line is the first frag-
     ment of the reply, and hence is only 1472 bytes long. The other bytes
     will follow in subsequent fragments, but these fragments do not have NFS
     or even UDP headers and so might not be printed, depending on the filter
     expression used. Because the -v flag is given, some of the file attri-
     butes (which are returned in addition to the file data) are printed: the
     file type ('REG', for regular file), the file mode (in octal), the UID
     and GID, and the file size.

     If the -v flag is given more than once, even more details are printed.

     NFS requests are very large and much of the detail won't be printed un-
     less snaplen is increased. Try using "-s 192" to watch NFS traffic.

     NFS reply packets do not explicitly identify the RPC operation. Instead,
     tcpdump keeps track of "recent" requests, and matches them to the replies
     using the xid (transaction ID). If a reply does not closely follow the
     corresponding request, it might not be parsable.

KIP AppleTalk (DDP in UDP)

     AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated
     and dumped as DDP packets (i.e., all the UDP header information is
     discarded). The file /etc/atalk.names is used to translate AppleTalk net
     and node numbers to names. Lines in this file have the form

           number            name
           1.254             ether
           16.1              icsd-net
           1.254.110         ace

     The first two lines give the names of AppleTalk networks. The third line
     gives the name of a particular host (a host is distinguished from a net
     by the 3rd octet in the number; a net number must have two octets and a
     host number must have three octets). The number and name should be
     separated by whitespace (blanks or tabs). The /etc/atalk.names file may
     contain blank lines or comment lines (lines starting with a '#').

     AppleTalk addresses are printed in the form

           net.host. port

     For example:

  > icsd-net.112.220
           office.2 > icsd-net.112.220
           jssmag.149.235 > icsd-net.2

     If /etc/atalk.names doesn't exist or doesn't contain an entry for some
     AppleTalk host/net number, addresses are printed in numeric form. In the
     first example, NBP (DDP port 2) on net 144.1 node 209 is sending to what-
     ever is listening on port 220 of net icsd-net node 112. The second line
     is the same except the full name of the source node is known ("office").
     The third line is a send from port 235 on net jssmag node 149 to broad-
     cast on the icsd-net NBP port. The broadcast address (255) is indicated
     by a net name with no host number; for this reason it is a good idea to
     keep node names and net names distinct in /etc/atalk.names.

     NBP (name binding protocol) and ATP (AppleTalk transaction protocol)
     packets have their contents interpreted. Other protocols just dump the
     protocol name (or number if no name is registered for the protocol) and
     packet size.

     NBP packets are formatted like the following examples:

     icsd-net.112.220 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*"
     jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250
     techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186

     The first line is a name lookup request for laserwriters sent by net
     icsdi-net host 112 and broadcast on net jssmag. The nbp ID for the lookup
     is 190. The second line shows a reply for this request (note that it has
     the same ID) from host jssmag.209 saying that it has a laserwriter
     resource named RM1140 registered on port 250. The third line is another
     reply to the same request saying host techpit has laserwriter techpit re-
     gistered on port 186.

     ATP packet formatting is demonstrated by the following example:

           jssmag.209.165 > helios.132: atp-req  12266<0-7> 0xae030001
           helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000
           helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000
           helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000
           helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
           helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000
           helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
           helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000
           helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000
           jssmag.209.165 > helios.132: atp-req  12266<3,5> 0xae030001
           helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000
           helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000
           jssmag.209.165 > helios.132: atp-rel  12266<0-7> 0xae030001
           jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002

     Jssmag.209 initiates transaction ID 12266 with host helios by requesting
     up to 8 packets (the"<0-7>"). The hex number at the end of the line is
     the value of the userdata field in the request.

     Helios responds with 8 512-byte packets. The ":n" following the transac-
     tion ID gives the packet sequence number in the transaction and the
     number in parentheses is the amount of data in the packet, excluding the
     ATP header. The '*' on packet 7 indicates that the EOM bit was set.

     Jssmag.209 then requests that packets 3 & 5 be retransmitted. Helios
     resends them then jssmag.209 releases the transaction. Finally,
     jssmag.209 initiates the next request. The '*' on the request indicates
     that XO (exactly once) was not set.

IP Fragmentation

     Fragmented Internet datagrams are printed as

           (frag id : size @ offset [+])

     A '+' indicates there are more fragments. The last fragment will have no

     id is the fragment ID. size is the fragment size (in bytes) excluding the
     IP header. offset is this fragment's offset (in bytes) in the original

     The fragment information is output for each fragment. The first fragment
     contains the higher level protocol header and the fragment info is print-
     ed after the protocol info. Fragments after the first contain no higher
     level protocol header and the fragment info is printed after the source
     and destination addresses. For example, here is part of an FTP from
     arizona.edu to lbl-rtsg.arpa over a CSNET connection that doesn't appear
     to handle 576 byte datagrams:

           arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+)
           arizona > rtsg: (frag 595a:204@328)
           rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560

     There are a couple of things to note here: first, addresses in the 2nd
     line don't include port numbers. This is because the TCP protocol infor-
     mation is all in the first fragment and we have no idea what the port or
     sequence numbers are when we print the later fragments. Second, the TCP
     sequence information in the first line is printed as if there were 308
     bytes of user data when, in fact, there are 512 bytes
     (308 in the first frag and 204 in the second). If you are looking for
     holes in the sequence space or trying to match up acks with packets, this
     can fool you.

     A packet with the IP don't fragment flag is marked with a trailing


     By default, all output lines are preceded by a timestamp. The timestamp
     is the current clock time in the form hh:mm:ss.frac and is as accurate as
     the kernel's clock. The timestamp reflects the time the kernel first saw
     the packet. No attempt is made to account for the time lag between when
     the Ethernet interface removed the packet from the wire and when the ker-
     nel serviced the "new packet" interrupt.


     ethers(3), pcap(3), bpf(4), ip(4), pf(4), pflog(4), tcp(4), udp(4),
     networks(5), pf.os(5), protocols(5), services(5)

     Transmission Control Protocol, RFC 793, September 1981.

     Domain Names - Concepts and Facilities, RFC 1034, November 1987.

     Domain Names - Implementation and Specification, RFC 1035, November 1987.

     RPC: Remote Procedure Call, RFC 1050, April 1988.

     Compressing TCP/IP Headers for Low-Speed Serial Links, RFC 1144, February

     TCP Selective Acknowledgement Options, RFC 2018, October 1996.

     IP Encapsulating Security Payload (ESP), RFC 2406, November 1998.


     Van Jacobson <van@ee.lbl.gov>,
     Craig Leres <leres@ee.lbl.gov>, and
     Steven McCanne <mccanne@ee.lbl.gov>, all of the Lawrence Berkeley Labora-
     tory, University of California, Berkeley, CA.


     This utility is no longer part of MirBSD.


     Please send bug reports to <tcpdump@ee.lbl.gov> or <libpcap@ee.lbl.gov>.

     Some attempt should be made to reassemble IP fragments, or at least to
     compute the right length for the higher level protocol.

     Name server inverse queries are not dumped correctly: The (empty) ques-
     tion section is printed rather than the real query in the answer section.
     Some believe that inverse queries are themselves a bug and prefer to fix
     the program generating them rather than tcpdump.

     Apple Ethertalk DDP packets could be dumped as easily as KIP DDP packets
     but aren't. Even if we were inclined to do anything to promote the use of
     Ethertalk (we aren't, LBL doesn't allow Ethertalk on any of its networks
     so we'd have no way of testing this code).

     A packet trace that crosses a daylight saving time change will give
     skewed time stamps (the time change is ignored).

     Filter expressions that manipulate FDDI headers assume that all FDDI
     packets are encapsulated Ethernet packets. This is true for IP, ARP, and
     DECNET Phase IV, but is not true for protocols such as ISO CLNS. There-
     fore, the filter may inadvertently accept certain packets that do not
     properly match the filter expression.

MirBSD #10-current               May 25, 1999                               14

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