tcpdump - dump traffic on a network
tcpdump [-adeflnNoOpqStvxX] [-c count] [-E [espalg:]espkey]
[-i interface] [-r file] [-s snaplen] [-T type] [-w
tcpdump prints out the headers of packets on a network interface that
match the boolean expression. You must have read access to
The options are as follows:
-a Attempt to convert network and broadcast addresses
-c count Exit after receiving count packets.
-d Dump the compiled packet-matching code in a human
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 Print the link-level header on each dump line.
Try to decrypt RFC 2406 ESP (Encapsulating Security Payload)
traffic using the specified hex key espkey. Supported algorithms
for espalg are: aes128, aes128-hmac96,
blowfish-hmac96, cast, cast-hmac96, des3,
des3-hmac96, des and
des-hmac96. The algorithm defaults to
aes128-hmac96. This option
should be used for debugging only, since the
key will show
up in ps(1) output.
-f Print ``foreign'' internet addresses numerically
symbolically. This option is intended to get
brain damage in Sun's yp server -- usually it
translating non-local internet numbers.
-F file Use file as input for the filter expression. Any
expressions given on the command line are ignored.
Listen on interface. If unspecified, tcpdump
searches the system
interface list for the lowest numbered, configured ``up''
interface (excluding loopback). Ties are broken
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
# tcpdump -l > dat & tail -f dat
-n Do not convert addresses (host addresses, port
-N Do not print domain name qualification of host
names. For example,
if you specify this flag then tcpdump will
instead of ``nic.ddn.mil''.
-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.
-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.
might be in promiscuous mode for some other reason; hence, -p
cannot be used as an abbreviation for ``ether host
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 `-'.
Analyze at most the first snaplen bytes of data
from each packet
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
of a limited snaplen are indicated in the
``[|proto]'', where proto is the name of the protocol level at
which the truncation has occurred. Taking larger
both increases the amount of time it takes to process packets
and, effectively, decreases the amount of packet
This may cause packets to be lost. You should
limit snaplen to
the smallest number that will capture the protocol
you're interested in.
-S Print absolute, rather than relative, TCP sequence
-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.
-T type Force packets selected by expression to be interpreted as the
specified type. Currently known types are vrrp
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), vat (Visual Audio
Tool), and wb (distributed White Board).
-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
printed from NFS reply packets.
-w file Write the raw packets to file rather than parsing
them out. They can be analyzed later with the -r
Standard output is used if file is `-'.
-x Print each packet (minus its link-level header) in
smaller of the entire packet or snaplen bytes will
-X Like -x but dumps the packet in emacs-hexl like
expression selects which packets will be dumped. If no
given, all packets on the net will be dumped. Otherwise,
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
There are three different kinds of qualifiers:
type Specify which kind of address component the id name
refers to. Possible types are host, net and port.
foo'', ``net 128.3'', ``port 20''. If there is no
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
``src foo'', ``dst net 128.3'', ``src or dst port
there is no dir qualifier, src or dst is assumed.
For null link
layers (i.e., point-to-point protocols such as SLIP
Internet Protocol) or the pflog(4) header), the
outbound qualifiers can be used to specify a desired
proto Restrict the match to a particular protocol. Possible protocols
are: arp, decnet, ether, fddi, ip, lat, mopdl, moprc,
and udp. E.g., ``ether src foo'', ``arp net 128.3'',
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
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
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
these FDDI fields just as with the analogous Ethernet
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
don't follow the pattern: gateway, broadcast, less, greater,
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 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
Allowable primitives are:
dst host host True if the IP destination field of the
host, which may be either an address or a
src host host True if the IP source field of the packet
host host True if either the IP source or destination of the
packet is host.
Any of the above host expressions can be
with the keywords, ip, arp, or rarp as
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
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
ether host ehost True if either the Ethernet source or
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
/etc/hosts and /etc/ethers. An equivalent expression
ether host ehost and not host host
which can be used with either names or
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
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
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;
port 513'' will print both TCP/login
UDP/who traffic, and ``dst port domain''
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
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 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
checks for both the all-zeroes and allones broadcast
conventions and looks up the local subnet
ether multicast True if the packet is an Ethernet multicast packet.
The ether keyword is optional. This is
``ether & 1 != 0''.
ip multicast True if the packet is an IP multicast
ether proto proto True if the packet is of ether type
proto. proto can
be a number or a name like ip, arp, or
identifiers are also shell keywords and
must be escaped.
In the case of FDDI (e.g., ``fddi
arp''), the protocol identification comes
802.2 Logical Link Control (LLC) header,
which is usually
layered on top of the FDDI header.
when filtering on the protocol
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
systems that are configured to run DECNET.
decnet dst host True if the DECNET destination address is
decnet host host True if either the DECNET source or destination address
ifname interface True if the packet was logged as coming
from the specified
interface (applies only to packets
on interface Synonymous with the ifname modifier.
rnr num True if the packet was logged as matching
PF rule number in the main ruleset
to packets logged by pf(4)).
rulenum num Synonymous with the rnr modifier.
reason code True if the packet was logged with the
reason code. The known codes are: match,
fragment, bad-timestamp, short,
normalize, and memory
(applies only to packets logged by
rset name True if the packet was logged as matching
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
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
was logged. Known actions are: pass, and
only to packets logged by pf(4)).
ip, arp, rarp, decnet, lat, moprc, mopdl
ether proto p
where p is one of the above protocols.
not currently know how to parse lat,
moprc, or mopdl.
tcp, udp, icmp Abbreviations for: ip proto p where p is
one of the
expr relop expr True if the relation holds, where relop
is one of `>',
`<', `>=', `<=', `=', `!=', and expr is
expression composed of integer constants
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 is one of ether, fddi, ip, arp,
rarp, tcp, udp,
or icmp, and indicates the protocol 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,
the keyword len, gives the length of the
For example, ``ether & 1 != 0'' catches all multicast
traffic. The expression ``ip &
0xf != 5''
catches all IP packets with options. The
``ip[6:2] & 0x1fff = 0'' catches only unfragmented
datagrams and frag zero of fragmented
check is implicitly applied to the tcp
and udp index
operations. For instance, ``tcp'' always means the
first byte of the TCP header, and never
first byte of an intervening fragment.
Primitives may be combined using a parenthesized group of
operators. Parentheses are special to the shell and must be
Allowable 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
juxtaposition, are now required for concatenation.
If an identifier is given without a keyword, the most recent
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
or as multiple arguments, whichever is more convenient.
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
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
you gateway to one other net, this stuff should never make
it onto your
# 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 & 3 != 0 and not src and dst net
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 & 1 = 0 and ip >= 224'
To print all ICMP packets that are not echo requests/replies
# tcpdump 'icmp != 8 and icmp != 0'
To print and decrypt all ESP packets with SPI 0x00001234:
# tcpdump -E des3-hmac96:ab...def 'ip[20:4] =
The output of tcpdump is protocol dependent. The following
gives a brief
description and examples of most of the formats.
Link Level Headers [Toc] [Back]
If the -e option is given, the link level header is printed
out. On Ethernets,
the source and destination addresses, protocol, and
On the packet filter logging interface pflog(4), logging
match, bad-offset, fragment, short, normalize, memory), action taken
(pass/block), direction (in/out) and interface information
out for each packet.
On FDDI networks, the -e option causes tcpdump to print the
field, the source and destination addresses, and the packet
frame control field governs the interpretation of the rest
of the packet.
Normal packets (such as those containing IP datagrams) are
packets, with a priority value between 0 and 7; for example,
Such packets are assumed to contain an 802.2 Logical Link
packet; the LLC header is printed if it is not an ISO datagram or a socalled
The following description assumes familiarity with the SLIP
algorithm described in RFC 1144.
On SLIP links, a direction indicator (`I' for inbound, `O'
packet type, and compression information are printed out.
type is printed first. The three types are ip, utcp, and
ctcp. No further
link information is printed for IP packets. For TCP
connection identifier is printed following the type. If the
compressed, its encoded header is printed out. The special
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
`U' (urgent pointer), `W' (window), `A' (ack), `S' (sequence
`I' (packet ID), followed by a delta (+n or -n), or a new
Finally, the amount of data in the packet and compressed
For example, the following line shows an outbound compressed
with an implicit connection identifier; 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 output shows the type of request and its arguments.
is intended to be self-explanatory. Here is a short sample
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
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 Ethernet
This would look less redundant if we had done tcpdump -n:
arp who-has 18.104.22.168 tell 22.214.171.124
arp reply 126.96.36.199 is-at 02:07:01:00:01:c4
If we had done tcpdump -e, the fact that the first packet is
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
hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
TCP Packets [Toc] [Back]
The following description assumes familiarity with the TCP
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
src and dst are the source and destination IP addresses and
is some combination of `S' (SYN), `F' (FIN), `P' (PUSH), or
`W' (congestion Window reduced), `E' (ecn ECHO) or a single
flags). src-os will list a guess of the source host's operating system
if the -o command line flag was passed to tcpdump.
the portion of sequence space covered by the data in this
example below). ack is the sequence number of the next data
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
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 appropriate.
Here is the opening portion of an rlogin from host rtsg to
rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss
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 login
on host csam. The `S' indicates that the SYN flag was
packet sequence number was 768512 and it contained no data.
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 piggybacked
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
a TCP connection, it prints the sequence number from the
packet. On subsequent
packets of the connection, the difference between
packet's sequence number and this initial sequence number is
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
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
the socket buffer since csam's receive window has gotten 19
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
UDP Packets [Toc] [Back]
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
84 bytes of user data.
Some UDP services are recognized (from the source or destination port
number) and the higher level protocol information printed.
Domain Name service requests (RFC 1034/1035) and Sun
RPC calls (RFC
1050) to NFS.
UDP Name Server Requests [Toc] [Back]
The following description assumes familiarity with the Domain Service
protocol described in RFC 1035. If you are not familiar
with the protocol,
the following description will appear to be written in
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 ucbvax.berkeley.edu. The
query id was
3. The `+' indicates the recursion desired flag was set.
length was 37 bytes, not including the UDP and IP protocol
query operation was the normal one (Query) so the op field
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 few anomalies are checked and may result in extra fields
square brackets: if a query contains an answer, name server
section, ancount, nscount, or arcount are printed as
or ``[nau]'' where n is the appropriate count. If any of
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 [Toc] [Back]
Name server responses are formatted as
src > dst: id op rcode flags a / n / au type class
helios.domain > h2opolo.1538: 3 3/3/7 A 188.8.131.52
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)
184.108.40.206. The total size of the response was 273 bytes,
UDP and IP headers. The op (Query) and rcode (NoError) were
was the class (C_IN) of the A record.
In the second example, helios responds to query op 2 with an
non-existent domain (NXDomain) with no answers, one name
server and no
authority records. The `*' indicates that the authoritative
was set. Since there were no answers, no type, class or
data were printed.
Other flag characters that might appear are `-' (recursion
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
snaplen of 96 bytes may not capture enough of the packet to
the -s flag to increase the snaplen if you need to seriously
name server traffic. ``-s 128'' has worked well for me.
NFS Requests and Replies [Toc] [Back]
Sun NFS (Network File System) requests and replies are
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
port. The request was 112 bytes, excluding the UDP and IP
op was a readlink (read symbolic link) on fh (``file handle'')
21,24/10.731657119. If one is lucky, as in this case, the
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 look up the name
``xcolors'' in directory
file 9,74/4096.6878. The data printed depends on
type. The format is intended to be self-explanatory if read
with an NFS protocol spec.
If the -v (verbose) flag is given, additional information is
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
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
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 other
bytes will follow in subsequent fragments, but these
fragments do not
have NFS or even UDP headers and so might not be printed,
the filter expression used. Because the -v flag is given,
some of the
file attributes (which are returned in addition to the file
printed: the file type (`REG', for regular file), the file
octal), the UID and GID, and the file size.
If the -v flag is given more than once, even more details
NFS requests are very large and much of the detail won't be
snaplen is increased. Try using ``-s 192'' to watch
NFS reply packets do not explicitly identify the RPC operation. Instead,
tcpdump keeps track of ``recent'' requests, and matches them
replies using the xid (transaction ID). If a reply does not
the corresponding request, it might not be parsable.
KIP AppleTalk (DDP in UDP) [Toc] [Back]
AppleTalk DDP packets encapsulated in UDP datagrams are deencapsulated
and dumped as DDP packets (i.e., all the UDP header information is
discarded). The file /etc/atalk.names is used to translate
and node numbers to names. Lines in this file have the form
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
220.127.116.11 > 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 whatever
is listening on port 220 of net icsd-net node 112. The
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
149 to broadcast on the icsd-net NBP port. The broadcast
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
NBP (name binding protocol) and ATP (AppleTalk transaction
packets have their contents interpreted. Other protocols
just dump the
protocol name (or number if no name is registered for the
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:
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
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
named RM1140 registered on port 250. The third line
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>
helios.132 > jssmag.209.165: atp-resp 12266:0 (512)
helios.132 > jssmag.209.165: atp-resp 12266:1 (512)
helios.132 > jssmag.209.165: atp-resp 12266:2 (512)
helios.132 > jssmag.209.165: atp-resp 12266:3 (512)
helios.132 > jssmag.209.165: atp-resp 12266:4 (512)
helios.132 > jssmag.209.165: atp-resp 12266:5 (512)
helios.132 > jssmag.209.165: atp-resp 12266:6 (512)
helios.132 > jssmag.209.165: atp-resp*12266:7 (512)
jssmag.209.165 > helios.132: atp-req 12266<3,5>
helios.132 > jssmag.209.165: atp-resp 12266:3 (512)
helios.132 > jssmag.209.165: atp-resp 12266:5 (512)
jssmag.209.165 > helios.132: atp-rel 12266<0-7>
jssmag.209.133 > helios.132: atp-req* 12267<0-7>
Jssmag.209 initiates transaction ID 12266 with host helios
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 transaction
ID gives the packet sequence number in the transaction and the
number in parentheses is the amount of data in the packet,
ATP header. The `*' on packet 7 indicates that the EOM bit
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 [Toc] [Back]
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)
the IP header. offset is this fragment's offset (in bytes)
in the original
The fragment information is output for each fragment. The
contains the higher level protocol header and the fragment
info is printed
after the protocol info. Fragments after the first contain no higher
level protocol header and the fragment info is printed after
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
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
Timestamps [Toc] [Back]
By default, all output lines are preceded by a 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
the Ethernet interface removed the packet from the wire and
when the kernel
serviced the ``new packet'' interrupt.
ethers(3), pcap(3), bpf(4), ip(4), pf(4), pflog(4), tcp(4),
Transmission Control Protocol, RFC 793, September 1981.
Domain Names - Concepts and Facilities, RFC 1034, November
Domain Names - Implementation and Specification, RFC 1035,
RPC: Remote Procedure Call, RFC 1050, April 1988.
Compressing TCP/IP Headers for Low-Speed Serial Links, RFC
TCP Selective Acknowledgement Options, RFC 2018, October
IP Encapsulating Security Payload (ESP), RFC 2406, November
Van Jacobson <firstname.lastname@example.org>,
Craig Leres <email@example.com>, and
Steven McCanne <firstname.lastname@example.org>, all of the Lawrence
University of California, Berkeley, CA.
Please send bug reports to <email@example.com> or <firstname.lastname@example.org>.
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
section is printed rather than the real query in the
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
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
skewed time stamps (the time change is ignored).
Filter expressions that manipulate FDDI headers assume that
packets are encapsulated Ethernet packets. This is true for
IP, ARP, and
DECNET Phase IV, but is not true for protocols such as ISO
the filter may inadvertently accept certain packets
that do not
properly match the filter expression.
OpenBSD 3.6 May 25, 1999
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