Tcpdump

TCPDUMP(1)                                            TCPDUMP(1)

NAME tcpdump - dump traffic on a network

SYNOPSIS tcpdump [ -adeflnNOpqStvx ] [ -c count ] [ -F file ] [ -i interface ] [ -r file ] [ -s snaplen ] [ -T type ] [ -w file ] [ expression ]

DESCRIPTION Tcpdump prints  out  the  headers of packets on a network interface that match the boolean expression.

Under SunOS with nit or bpf: To run tcpdump you must have read access to /dev/nit or /dev/bpf*. Under Solaris with dlpi: You must have read access  to  the  network  pseudo device, e.g. /dev/le. Under HP-UX with dlpi: You must be      root or it must be installed setuid to root. Under IRIX with snoop:  You  must  be  root  or it must be installed setuid to root. Under Linux: You must be root or it must be installed  setuid  to  root. Under Ultrix and Digital UNIX: Once the super-user  has  enabled  promiscuous-mode operation using  pfconfig(8),  any  user may run tcpdump. Under BSD: You must have read access to /dev/bpf*.

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

-c    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 (pre-              ceded with a count).

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

-f    Print  `foreign'  internet  addresses   numerically rather than  symbolically (this option is intended              to get around serious  brain  damage  in  Sun's  yp              server -- usually it hangs forever translating non-              local internet numbers).

-F    Use file as input for the  filter  expression. An             additional  expression given on the command line is              ignored.

-i    Listen  on  interface. If unspecified,   tcpdump searches the  system interface list for the lowest numbered, configured up interface (excluding loop-              back). Ties are  broken by choosing the earliest match.

-l    Make stdout line buffered. Useful if you want  to              see the data while capturing it. E.g., ``tcpdump -l  |  tee  dat''  or  ``tcpdump  -l   > dat &  tail  -f  dat''.

-n    Don't convert addresses (i.e., host addresses, port              numbers, etc.) to names.

-N    Don't  print  domain  name  qualification  of  host names. E.g., if you give this flag  then  tcpdump will print ``nic instead of ``nic.ddn.mil.

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

-p    Don't  put  the  interface  into  promiscuous mode. Note that 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 infor- mation so output lines are shorter.

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

-s    Snarf snaplen bytes of data from each packet rather than the default of 68 (with SunOS's NIT, the mini-             mum is actually 96). 68 bytes is adequate for IP, ICMP, TCP and UDP but may truncate protocol infor- mation from  name  server  and  NFS  packets  (see              below). Packets truncated  because  of a limited snapshot are  indicated   in   the   output   with ``[|proto]'', where proto is the name of the proto- col level at which the  truncation  has  occurred. Note that  taking  larger snapshots both increases the amount of time it takes to process packets and, effectively, decreases the amount of packet buffer- ing. This may cause  packets  to  be  lost. You should limit  snaplen  to the smallest number that will capture the protocol information you're inter- ested in.

-T    Force packets selected by "expression" to be inter- preted the specified type. Currently known  types are rpc  (Remote  Procedure  Call), rtp (Real-Time              Applications protocol),  rtcp  (Real-Time  Applica-              tions  control  protocol), vat (Visual Audio Tool), and wb (distributed White Board).

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

-t    Don't print a timestamp on each dump line.

-tt   Print an unformatted timestamp on each dump line.

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

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

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

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

expression selects which  packets  will  be  dumped. If no              expression is given, all packets on the net will be              dumped. Otherwise, only packets for which expres- sion is `true' will be dumped.

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

type  qualifiers  say  what  kind  of thing 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   qualifiers  specify  a  particular  transfer direction to  and/or  from  id. Possible 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 pro-                    tocols such as slip) the  inbound  and  out- bound qualifiers  can  be used to specify a                     desired direction.

proto qualifiers restrict the match to a  particu- lar protocol. Possible protos are: ether, fddi, ip,  arp,  rarp,  decnet,  lat,  sca, moprc, mopdl, iso, esis, isis, tcp and udp. E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is no proto 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 identically  as  meaning  ``the data link  level  used  on  the  specified network interface.'' FDDI  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 Ether- net fields. FDDI headers  also  contain   other fields, but  you  cannot name them explicitly in a              filter expression.]

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

More complex  filter  expressions  are built up by              using the words and, or and not to  combine  primi- tives. E.g., `host  foo and not port ftp and not port ftp-data'. To save typing, identical  quali- fier 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 address 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(3N) for                     numeric format).

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

ether host ehost True if either the ethernet source or desti- nation address 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 is                          ether host ehost and not host host                     which  can be used with either names or num-                     bers for host / ehost.)

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(4)   for                     details).

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

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

net net mask mask True if the IP address matches net with the specific netmask. May be qualified with src or dst.

net net/len True if the IP address matches net a netmask len bits wide. May be qualified with src or                    dst.

dst port port True if  the packet is ip/tcp or ip/udp and has a destination port value of port. The port can  be  a  number  or  a name used in                     /etc/services (see tcp(4P) and udp(4P)). 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 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 port.

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 protocol True if the packet is  an  ip  packet  (see                     ip(4P)) of protocol type protocol. Protocol can be a number or one of the  names  icmp, igrp, udp, nd, or tcp. Note that the iden- tifiers tcp, udp, and icmp are also keywords and must be escaped via backslash, which is \ in the C-shell.

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 protocol True if  the packet is of ether type proto- col. Protocol can be a  number  or  a  name like ip,  arp, or rarp. Note these identi- fiers are also keywords and must be escaped via backslash. [In the case of FDDI (e.g., `fddi protocol arp'),  the  protocol 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                     Ultrix  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 destina- tion address is host.

ip, arp, rarp, decnet, iso Abbreviations for: ether proto p                    where p is one of the above protocols.

lat, moprc, mopdl Abbreviations for: ether proto p                    where p is one of the above protocols. Note that tcpdump does not currently know how to                     parse these protocols.

tcp, udp, icmp Abbreviations for: ip proto p                    where p is one of the above protocols.

esis, isis Abbreviations for: iso proto p                    where p is one of the above protocols. Note that tcpdump does an incomplete job of pars- ing these 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 special packet data accessors. To access data inside  the packet, use the following syntax: proto [ expr : size ] Proto is one of ether, fddi, ip, arp, rarp, tcp, udp, or icmp, and indicates the proto- col layer for the index operation. The byte offset, relative to the indicated  protocol layer, is  given by expr. Size is optional 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 implicitly 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).

Negation (`!' or `not').

Concatenation (`&&' or `and').

Alternation (`||' or `or').

Negation has highest precedence. Alternation and concatenation have  equal precedence and associate left to right. Note that explicit and tokens, not juxtaposition, 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.

EXAMPLES To print  all  packets arriving at or departing from sun- down: tcpdump host sundown

To print traffic between helios and either hot or ace: 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: (note that  the expression is quoted to prevent the shell       from (mis-)interpreting the parentheses): 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 pack-       ets)  of  each  TCP conversation 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 ethernet 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'

OUTPUT FORMAT The output of tcpdump is protocol dependent. The follow- ing 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 ethernets, the  source  and  destination addresses, protocol, and packet length are printed.

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 data- grams) are `async' packets, with a priority value between       0  and 7; for example, `async4'.  Such packets are assumed       to contain an 802.2 Logical Link Control (LLC) packet; the       LLC  header  is  printed if it is not an ISO datagram or a       so-called SNAP packet.

(N.B.: 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 informa- tion 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 spe- cial 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  com- pressed TCP  packet,  with an implicit connection identi- fier; the ack has changed by 6, the sequence 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  argu- ments. 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 The first  line  says that rtsg sent an arp packet asking for the ethernet address of  internet  host  csam. Csam replies with its ethernet address (in this example, ether-      net addresses are in caps and internet addresses in  lower       case).

This would look less redundant if we had done tcpdump -n: arp who-has 128.3.254.6 tell 128.3.254.68 arp reply 128.3.254.6 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

(N.B.: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 data-seqno ack window urgent options Src and  dst  are the source and destination IP addresses and ports. Flags are some  combination  of  S  (SYN),  F       (FIN),  P  (PUSH)  or  R (RST) or a single `.' (no flags). Data-seqno describes the portion of sequence space covered by the  data  in this packet (see example below). Ack is      sequence number of the next data expected the other direc- tion on this connection. Window is the number of bytes of      receive buffer space available the other direction on this connection. Urg indicates there is `urgent' data in the packet. Options are tcp options enclosed in angle brack- ets (e.g., ).

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 appropriate.

Here is the opening portion of an rlogin from host rtsg to      host csam. rtsg.1023 > csam.login: S 768512:768512(0) win 4096 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 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  login on 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 including 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  con- tained no data so there is no data sequence number. Note that the ack sequence number is a small integer (1). The first time  tcpdump  sees a tcp `conversation', it prints the sequence number from the packet. On subsequent pack- ets of  the conversation, the difference between the cur- rent 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  conversation's  data stream (with the first data byte       each direction being `1'). `-S' will override this  fea- ture, 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 conversation). 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 includ- ing byte 21. Most of this data is apparently 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.

If the snapshot was small enough that tcpdump didn't cap- ture the  full  TCP  header, it interprets as much of the header as it can and then reports ``[|tcp]'' to  indicate the remainder  could  not  be interpreted. If the header contains a bogus option (one with a length that's  either       too  small  or  beyond  the  end  of  the header), tcpdump reports it as ``[bad opt]'' and does  not  interpret  any further options (since it's impossible to tell where they       start). If the header length indicates options are  pre- sent but the IP datagram length is not long enough for the options to actually be there, tcpdump reports it as ``[bad hdr length]''.

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 data- gram 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 des-       tination port number) and the higher level protocol infor- mation  printed. In particular,  Domain  Name  service requests (RFC-1034/1035) and Sun RPC calls (RFC-1050)  to       NFS.

UDP Name Server Requests

(N.B.:The following  description assumes familiarity with       the Domain Service protocol described in RFC-1035.  If you       are not familiar with the protocol, the following descrip-       tion will appear to be written in greek.)

Name server requests are formatted as             src > dst: id op? flags qtype qclass name (len) 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 ucb- vax.berkeley.edu. The query id was `3'. The `+'  indi- cates 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 the 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 omitted. Any other qclass would have been printed immedi- ately 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) helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (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 128.32.137.3. The total size of the response was 273  bytes,  excluding UDP and  IP  headers. The op (Query) and response code (NoError) were omitted, as was the class (C_IN) of the  A       record.

In the  second example, helios responds to query 2 with a       response code 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 printed.

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.

Note that  name  server requests and responses tend to be       large and the default snaplen of 68 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  File  System) 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 (note that 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 operation was a readlink (read symbolic link) on file han- dle (fh) 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 gen-       eration number.)  Wrl replies `ok' with  the  contents  of       the link.

In the  third  line,  sushi  asks  wrl to lookup the name `xcolors' in directory file 9,74/4096.6878. Note that 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 example:

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 `ok'; the packet shown on the second line  is  the  first fragment 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 attributes (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.

Note that NFS requests are very large  and  much  of  the detail won't be printed unless snaplen is increased. Try using `-s 192' to watch NFS traffic.

NFS reply packets do not explicitly identify the RPC oper- ation. Instead,  tcpdump  keeps  track  of  ``recent'' requests, and matches them to the replies using the trans- action ID. If a reply does not closely follow the corre- sponding 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  sepa- rated   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

144.1.209.2 > icsd-net.112.220 office.2 > icsd-net.112.220 jssmag.149.235 > icsd-net.2 (If the /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  whatever  is       listening  on  port  220 of net icsd 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 broadcast on  the  icsd-net NBP port  (note that the broadcast address (255) is indi-       cated by a net name with no host number - for this  reason       it's 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 icsd 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" registered 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 num- ber at the end of the line is the value of the `userdata' field in the request.

Helios responds with 8 512-byte  packets. The `:digit' following the  transaction  id  gives the packet sequence number in the transaction and the number in parens 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 retransmit- ted. 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+) (frag id:size@offset) (The first form indicates there are more fragments.   The       second indicates this is the last fragment.)

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 datagram.

The fragment information is output for each fragment. The first fragment contains the higher level protocol  header and the  frag  info  is  printed after the protocol info. Fragments after the first contain no higher level protocol header and  the frag 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 connec- tion 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 information 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 (DF).

Timestamps

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 kernel serviced the `new packet' interrupt.

SEE ALSO bpf(4), pcap(3)

AUTHORS Van Jacobson, Craig Leres and Steven McCanne, all of  the Lawrence Berkeley National Laboratory, University of Cali- fornia, Berkeley, CA.

The current version is available via anonymous ftp:

ftp://ftp.ee.lbl.gov/tcpdump.tar.Z

BUGS Please send bug reports to tcpdump@ee.lbl.gov.

NIT doesn't let you watch your own outbound traffic,  BPF will. We recommend that you use the latter.

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) question section is printed rather than real query in the answer section. Some believe that inverse queries are themselves a bug and prefer to fix the program gener- ating 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      would have no way of testing this code.

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

Filters 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. Therefore, the fil- ter may  inadvertently accept certain packets that do not properly match the filter expression.

30 June 1997                        1