Internet-Technologie

Prof. Jürgen Plate

Manualpage `tcpdump`

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

NAME
tcpdump - dump traffic on a network

SYNOPSIS
tcpdump [-adeflnNOpqStvxX] [-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. 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
count.

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

-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 translating
non-local internet numbers.

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

-i interface
Listen on interface. If unspecified, tcpdump searches the system
interface list for the lowest numbered, configured ``up'' inter-
face (excluding loopback). Ties are broken by choosing the ear-
liest 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 Do not convert addresses (i.e., host addresses, port numbers,
etc.) to names.

-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''.

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

-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 output
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 snaplen
Analyze at most the first snaplen bytes of data from each packet
rather than the default of 68. 68 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, ef-
fectively, decreases the amount of packet buffering. This may
cause packets to be lost. You should limit snaplen to the small-
est number that will capture the protocol information you're in-
terested in.

-T type
Force packets selected by expression to be interpreted as the
specified type. Currently known types are cnfp (Cisco NetFlow
protocol), rpc (Remote Procedure Call), rtp (Real-Time
Applications protocol), rtcp (Real-Time Applications control
protocol), sack (RFC 2018 No Selective Acknowledgements), vat
(Visual Audio Tool), and wb (distributed White Board).

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

-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.

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

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

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.
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
protocols such as SLIP or the pflog header) the inbound
and outbound qualifiers can be used to specify a desired
direction.

proto Restrict the match to a particular protocol. Possible
protocols are: ether, fddi, ip, arp, rarp, decnet, lat,
moprc, mopdl, 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 as-
sumed. 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 Ethernet
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 key-
words 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 primitives. e.g., ``host foo and not
port ftp and not port ftp-data''. To save typing, identical qual-
ifier 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(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 destina-
tion address is ehost.

gateway host True if the packet used host as a gateway;
i.e., the Ethernet source or destination ad-
dress 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 numbers
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(5) 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.

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(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 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 proto True if the packet is an IP packet (see ip(4))
of protocol type proto. proto can be a number
or one of the names icmp, udp, nd, or tcp. The
identifiers tcp, udp, and icmp are also shell
keywords and must be escaped.

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 a name like ip, arp,
or rarp. These identifiers are also shell key-
words and must be escaped. In the case of
FDDI (e.g., ``fddi protocol arp''), the proto-
col identification comes from the 802.2 Logi-
cal Link Control (LLC) header, which is usual-
ly layered on top of the FDDI header. tcpdump
assumes, when filtering on the protocol iden-
tifier, 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 destina-
tion address is host.

ifname interface True if the packet was logged as coming from
the specified interface (applies only to pack-
ets logged by pf(4)).

on interface Synonymous with the ifname modifier.

rnr num True if the packet was logged as matching the
specified PF rule number (applies only to
packets logged by pf(4)).

rulenum num Synonomous with the rnr modifier.

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

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

ip, arp, rarp, decnet, lat, moprc, mopdl
Abbreviations for:

ether proto p

where p is one of the above protocols. tcp-
dump does not currently know how to parse lat,
moprc, or mopdl.

tcp, udp, icmp 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 inte-
ger constants (expressed in standard C syn-
tax), 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 protocol
layer for the index operation. The byte off-
set, relative to the indicated protocol layer,
is given by expr. size is optional and indi-
cates the number of bytes in the field of in-
terest; it can be either one, two, or four,
and defaults to one. The length operator, in-
dicated 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 op-
tions. 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 index op-
erations. 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 primi-
tives and operators. Parentheses are special to the shell and
must be escaped. Allowed 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 juxtaposition, are now required for concatenation.

If an identifier is given without a keyword, the most recent key-
word 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 argu-
ments are concatenated with spaces before being parsed.

EXAMPLES
To print all packets arriving at or departing from sundown:

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:

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

The expression is quoted to prevent the shell from mis-interpreting
the parentheses.

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

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 '

OUTPUT FORMAT
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, logging reason (rule match,
bad-offset, fragment, short, normalize, memory), action taken
(pass/block), direction (in/out) and interface information is 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''
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) 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 se-
quence 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). Fi-
nally, 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 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

The following description assumes familiarity with the TCP protocol de-
scribed 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
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). 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-
propriate.

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
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 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 apparently sitting in
the socket buffer since csam's receive window has gotten 19 bytes small-
er. 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 particu-
lar, 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)

e.g.,

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)

e.g.,

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 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 a rcode of non-
existent domain (NXDomain) with no answers, one name server and no au-
thority 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.

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.
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 contents of the link.

In the third line, sushi asks wrl to lookup the name ``xcolors'' in di-
rectory file 9,74/4096.6878. The data printed depends on the operation
type. The format is intended to be self-explanatory if read in conjunc-
tion 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 with a stat of ok; the packet shown on the second line is the
first fragment of the reply, and hence is only 1472 bytes long. The oth-
er 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.

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 fol-
low 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 sepa-
rated by whitespace (blanks or tabs). The /etc/atalk.names file may con-
tain blank lines or comment lines (lines starting with a `#').

AppleTalk addresses are printed in the form

net.host. port

e.g.,

144.1.209.2 > 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 broadcast 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 ics-
di-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 re-
source 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 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 trans-
action 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 re-
sends 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 origi-
nal datagram.

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 ari-
zona.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
``(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 ker-
nel serviced the ``new packet'' interrupt.

Internet-Technologie

Manualpage tcpdump

Manualpage `tcpdump`