TCPDUMP说明
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TCPDUMP(1) TCPDUMP(1)
3 January 2001
NAME
tcpdump - dump traffic on a network
SYNOPSIS
tcpdump [ -adeflnNOpqRStuvxX ] [ -c count ]
[ -C file_size ] [ -F file ]
[ -i interface ] [ -m module ] [ -r file ]
[ -s snaplen ] [ -T type ] [ -w file ]
[ -E algo:secret ] [ expression ]
DESCRIPTION
Tcpdump prints out the headers of packets on a network interface that
match the boolean expression. It can also be run with the -w flag,
which causes it to save the packet data to a file for later analysis,
and/or with the -b flag, which causes it to read from a saved packet
file rather than to read packets from a network interface. In all
cases, only packets that match expression will be processed by
tcpdump. Tcpdump will, if not run with the -c flag, continue
capturing packets until it is interrupted by a SIGINT signal
(generated, for example, by typing your interrupt character, typically
control-C) or a SIGTERM signal (typically generated with the kill(1)
command); if run with the -c flag, it will capture packets until it is
interrupted by a SIGINT or SIGTERM signal or the specified number of
packets have been processed. When tcpdump finishes capturing packets,
it will report counts of:
packets ``received by filter'' (the meaning of this depends on
the OS on which you're running tcpdump, and possibly on the way
the OS was configured - if a filter was specified on the command
line, on some OSes it counts packets regardless of whether they
were matched by the filter expression, and on other OSes it
counts only packets that were matched by the filter expression
and were processed by tcpdump);
packets ``dropped by kernel'' (this is the number of packets that
were dropped, due to a lack of buffer space, by the packet
capture mechanism in the OS on which tcpdump is running, if the
OS reports that information to applications; if not, it will be
reported as 0). On platforms that support the SIGINFO signal,
such as most BSDs, it will report those counts when it receives a
SIGINFO signal (generated, for example, by typing your ``status''
character, typically control-T) and will continue capturing
packets. Reading packets from a network interface may require
that you have special privileges:
Under SunOS 3.x or 4.x with
You must have read access to /dev/nit or /dev/bpf*.
Under Solaris with DLPI:
You must have read/write access to the network pseudo device,
e.g. /dev/le. On at least some versions of Solaris, however,
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this is not sufficient to allow tcpdump to capture in promiscuous
mode; on those versions of Solaris, you must be root, or tcpdump
must be installed setuid to root, in order to capture in
promiscuous mode.
Under HP-UX with DLPI:
You must be root or tcpdump must be installed setuid to root.
Under IRIX with snoop:
You must be root or tcpdump must be installed setuid to root.
Under Linux:
You must be root or tcpdump 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 capture network traffic with tcpdump.
Under BSD:
You must have read access to /dev/bpf*. Reading a saved packet
file doesn't require special privileges.
OPTIONS
-a Attempt to convert network and broadcast addresses to names.
-c Exit after receiving count packets.
-C Before writing a raw packet to a savefile, check whether the file
is currently larger than file_size and, if so, close the current
savefile and open a new one. Savefiles after the first savefile
will have the name specified with the -w flag, with a number
after it, starting at 2 and continuing upward. The units of
file_size are millions of bytes (1,000,000 bytes, not 1,048,576
bytes).
-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.
-E Use algo:secret for decrypting IPsec ESP packets. Algorithms may
be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc, cast128-cbc, or
none. The default is des-cbc. The ability to decrypt packets is
only present if tcpdump was compiled with cryptography enabled.
secret the ascii text for ESP secret key. We cannot take
arbitrary binary value at this moment. The option assumes
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RFC2406 ESP, not RFC1827 ESP. The option is only for debugging
purposes, and the use of this option with truly `secret' key is
discouraged. By presenting IPsec secret key onto command line
you make it visible to others, via ps(1) and other occasions.
-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 loopback). Ties are broken by choosing the earliest
match.
On Linux systems with 2.2 or later kernels, an interface argument
of ``any'' can be used to capture packets from all interfaces.
Note that captures on the ``any'' device will not be done in
promiscuous mode.
-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''.
-m Load SMI MIB module definitions from file module. This option
can be used several times to load several MIB modules into
tcpdump.
-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 information so output
lines are shorter.
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-R Assume ESP/AH packets to be based on old specification (RFC1825
to RFC1829). If specified, tcpdump will not print replay
prevention field. Since there is no protocol version field in
ESP/AH specification, tcpdump cannot deduce the version of ESP/AH
protocol.
-r Read packets from file (which was created with the -w option).
Standard input is used if file is ``-''.
-S Print absolute, rather than relative, TCP sequence numbers.
-s Snarf snaplen bytes of data from each packet rather than the
default of 68 (with SunOS's NIT, the minimum is actually 96). 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 because of a limited snapshot are
indicated in the output with ``[|proto]'', where proto is the
name of the protocol 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 buffering. This may cause packets to be lost.
You should limit snaplen to the smallest number that will capture
the protocol information you're interested in. Setting snaplen
to 0 means use the required length to catch whole packets.
-T Force packets selected by "expression" to be interpreted 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), snmp (Simple Network Management Protocol), vat (Visual
Audio Tool), and wb (distributed White Board).
-t Don't print a timestamp on each dump line.
-tt Print an unformatted timestamp on each dump line.
-ttt Print a delta (in micro-seconds) between current and previous
line on each dump line.
-tttt
Print a timestamp in default format proceeded by date on each
dump line. -u Print undecoded NFS handles.
-v (Slightly more) verbose output. For example, the time to live,
identification, total length and options in an IP packet are
printed. Also enables additional packet integrity checks such as
verifying the IP and ICMP header checksum.
-vv Even more verbose output. For example, additional fields are
printed from NFS reply packets, and SMB packets are fully
decoded.
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-vvv Even more verbose output. For example, telnet SB ... SE options
are printed in full. With -X telnet options are printed in hex
as well.
-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.
-X When printing hex, print ascii too. Thus if -x is also set, the
packet is printed in hex/ascii. This is very handy for analysing
new protocols. Even if -x is not also set, some parts of some
packets may be printed in hex/ascii.
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 expression is `true' 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 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
protocols such as slip) the inbound and outbound qualifiers
can be used to specify a desired direction.
proto
qualifiers restrict the match to a particular protocol.
Possible protos are: ether, fddi, tr, ip, ip6, arp, rarp,
decnet, 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
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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.
Similarly, `tr' is an alias for `ether'; the previous
paragraph's statements about FDDI headers also apply to
Token Ring headers.] 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 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 port
domain'. Allowable primitives are:
dst host host
True if the IPv4/v6 destination field of the packet is host,
which may be either an address or a name.
src host host
True if the IPv4/v6 source field of the packet is host.
host host
True if either the IPv4/v6 source or destination of the
packet is host. Any of the above host expressions can be
prepended with the keywords, ip, arp, rarp, or ip6 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 destination 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 both by the machine's host-name-
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to-IP-address resolution mechanisms (host name file, DNS,
NIS, etc.) and by the machine's host-name-to-Ethernet-
address resolution mechanism (/etc/ethers, etc.). (An
equivalent expression is
ether host ehost and not host host
which can be used with either names or numbers for host /
ehost.) This syntax does not work in IPv6-enabled
configuration at this moment.
dst net net
True if the IPv4/v6 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 IPv4/v6 source address of the packet has a
network number of net.
net net
True if either the IPv4/v6 source or destination address of
the packet has a network number of net.
net net mask netmask
True if the IP address matches net with the specific
netmask. May be qualified with src or dst. Note that this
syntax is not valid for IPv6 net.
net net/len
True if the IPv4/v6 address matches net with a netmask len
bits wide. May be qualified with src or dst.
dst port port
True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/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
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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, icmp6, igmp, igrp, pim, ah, esp, vrrp, udp, or tcp.
Note that the identifiers tcp, udp, and icmp are also
keywords and must be escaped via backslash (/), which is //
in the C-shell. Note that this primitive does not chase the
protocol header chain.
ip6 proto protocol
True if the packet is an IPv6 packet of protocol type
protocol. Note that this primitive does not chase the
protocol header chain.
ip6 protochain protocol
True if the packet is IPv6 packet, and contains protocol
header with type protocol in its protocol header chain. For
example,
ip6 protochain 6
matches any IPv6 packet with TCP protocol header in the
protocol header chain. The packet may contain, for example,
authentication header, routing header, or hop-by-hop option
header, between IPv6 header and TCP header. The BPF code
emitted by this primitive is complex and cannot be optimized
by BPF optimizer code in tcpdump, so this can be somewhat
slow.
ip protochain protocol
Equivalent to ip6 protochain protocol, but this is for IPv4.
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.
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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.
ip6 multicast
True if the packet is an IPv6 multicast packet.
ether proto protocol
True if the packet is of ether type protocol. Protocol can
be a number or one of the names ip, ip6, arp, rarp, atalk,
aarp, decnet, sca, lat, mopdl, moprc, iso, stp, ipx, or
netbeui. Note these identifiers are also keywords and must
be escaped via backslash (/).
[In the case of FDDI (e.g., `fddi protocol arp') and Token
Ring (e.g., `tr protocol arp'), for most of those protocols,
the protocol identification comes from the 802.2 Logical
Link Control (LLC) header, which is usually layered on top
of the FDDI or Token Ring header.
When filtering for most protocol identifiers on FDDI or
Token Ring, tcpdump checks only the protocol ID field of an
LLC header in so-called SNAP format with an Organizational
Unit Identifier (OUI) of 0x000000, for encapsulated
Ethernet; it doesn't check whether the packet is in SNAP
format with an OUI of 0x000000.
The exceptions are iso, for which it checks the DSAP
(Destination Service Access Point) and SSAP (Source Service
Access Point) fields of the LLC header, stp and netbeui,
where it checks the DSAP of the LLC header, and atalk, where
it checks for a SNAP-format packet with an OUI of 0x080007
and the Appletalk etype.
In the case of Ethernet, tcpdump checks the Ethernet type
field for most of those protocols; the exceptions are iso,
sap, and netbeui, for which it checks for an 802.3 frame and
then checks the LLC header as it does for FDDI and Token
Ring, atalk, where it checks both for the Appletalk etype in
an Ethernet frame and for a SNAP-format packet as it does
for FDDI and Token Ring, aarp, where it checks for the
Appletalk ARP etype in either an Ethernet frame or an 802.2
SNAP frame with an OUI of 0x000000, and ipx, where it checks
for the IPX etype in an Ethernet frame, the IPX DSAP in the
LLC header, the 802.3 with no LLC header encapsulation of
IPX, and the IPX etype in a SNAP frame.]
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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 destination address is
host.
ip, ip6, arp, rarp, atalk, aarp, decnet, iso, stp, ipx, netbeui
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.
vlan [vlan_id]
True if the packet is an IEEE 802.1Q VLAN packet. If
[vlan_id] is specified, only true is the packet has the
specified vlan_id. Note that the first vlan keyword
encountered in expression changes the decoding offsets for
the remainder of expression on the assumption that the
packet is a VLAN packet.
tcp, udp, icmp
Abbreviations for:
ip proto p or ip6 proto p
where p is one of the above protocols.
iso proto protocol
True if the packet is an OSI packet of protocol type
protocol. Protocol can be a number or one of the names
clnp, esis, or isis.
clnp, esis, isis
Abbreviations for:
iso proto p
where p is one of the above protocols. Note that tcpdump
does an incomplete job of parsing these protocols.
expr relop expr
True if the relation holds, where relop is one of >, <, >=,
<=, =, !=, and expr is an arithmetic expression composed of
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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, tr, ip, arp, rarp, tcp, udp,
icmp or ip6, and indicates the protocol layer for the index
operation. Note that tcp, udp and other upper-layer
protocol types only apply to IPv4, not IPv6 (this will be
fixed in the future). 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.
Some offsets and field values may be expressed as names
rather than as numeric values. The following protocol
header field offsets are available: icmptype (ICMP type
field), icmpcode (ICMP code field), and tcpflags (TCP flags
field).
The following ICMP type field values are available: icmp-
echoreply, icmp-unreach, icmp-sourcequench, icmp-redirect,
icmp-echo, icmp-routeradvert, icmp-routersolicit, icmp-
timxceed, icmp-paramprob, icmp-tstamp, icmp-tstampreply,
icmp-ireq, icmp-ireqreply, icmp-maskreq, icmp-maskreply.
The following TCP flags field values are available: tcp-fin,
tcp-syn, tcp-rst, tcp-push, tcp-push, tcp-ack, tcp-urg.
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
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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 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: (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 packets) of each
TCP conversation that involves a non-local host.
tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 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[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
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 ethernets, the source and destination addresses,
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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 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) packet; the LLC header is printed if it is not an ISO datagram
or a so-called SNAP packet. On Token Ring networks, the '-e' option
causes tcpdump to print the `access control' and `frame control'
fields, the source and destination addresses, and the packet length.
As on FDDI networks, packets are assumed to contain an LLC packet.
Regardless of whether the '-e' option is specified or not, the source
routing information is printed for source-routed packets. (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
information are printed out. The packet type is printed first. The
three types are ip, utcp, and ctcp. No further link information is
printed for ip packets. For TCP packets, the connection identifier is
printed following the type. If the packet is compressed, its encoded
header is printed out. The special cases are printed out as *S+n and
*SA+n, where n is the amount by which the sequence number (or sequence
number and ack) has changed. If it is not a special case, zero or
more changes are printed. A change is indicated by U (urgent
pointer), W (window), A (ack), S (sequence number), and I (packet ID),
followed by a delta (+n or -n), or a new value (=n). Finally, the
amount of data in the packet and compressed header length are printed.
For example, the following line shows an outbound compressed TCP
packet, with an implicit connection identifier; the ack has changed by
6, the 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
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
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, ethernet 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:
- 13 - Formatted: August 11, 2004
TCPDUMP(1) TCPDUMP(1)
3 January 2001
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 direction 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 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 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
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 contained 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 packets of the
conversation, 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
- 14 - Formatted: August 11, 2004
TCPDUMP(1) TCPDUMP(1)
3 January 2001
positions in the conversation'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 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
including 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 capture 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 present but the IP datagram length is not long enough for
the options to actually be there, tcpdump reports it as ``[bad hdr
length]''.
Capturing TCP packets with particular flag
There are 8 bits in the control bits section of the TCP header:
CWR | ECE | URG |
Let's assume that we want to watch packets used in establishing a TCP
connection. Recall that TCP uses a 3-way handshake protocol when it
initializes a new connection; the connection sequence with regard to
the TCP control bits is
1) Caller sends SYN
2) Recipient responds with SYN, ACK
3) Caller sends ACK
Now we're interested in capturing packets that have only the SYN bit
set (Step 1). Note that we don't want packets from step 2 (SYN-ACK),
just a plain initial SYN. What we need is a correct filter expression
for tcpdump.
Recall the structure of a TCP header without options:
0 15 31
-----------------------------------------------------------------
| source port | destination port |
-----------------------------------------------------------------
| sequence number |
-----------------------------------------------------------------
| acknowledgment number |
- 15 - Formatted: August 11, 2004
TCPDUMP(1) TCPDUMP(1)
3 January 2001
-----------------------------------------------------------------
| HL | rsvd |C|E|U|A|P|R|S|F| window size |
-----------------------------------------------------------------
| TCP checksum | urgent pointer |
-----------------------------------------------------------------
A TCP header usually holds 20 octets of data, unless options are
present. The first line of the graph contains octets 0 - 3, the
second line shows octets 4 - 7 etc.
Starting to count with 0, the relevant TCP control bits are contained
in octet 13:
0 7| 15| 23| 31
----------------|---------------|---------------|----------------
| HL | rsvd |C|E|U|A|P|R|S|F| window size |
----------------|---------------|---------------|----------------
| | 13th octet | | |
Let's have a closer look at octet no. 13:
| |
|---------------|
|C|E|U|A|P|R|S|F|
|---------------|
|7 5 3 0|
These are the TCP control bits we are interested in. We have numbered
the bits in this octet from 0 to 7, right to left, so the PSH bit is
bit number 3, while the URG bit is number 5.
Recall that we want to capture packets with only SYN set. Let's see
what happens to octet 13 if a TCP datagram arrives with the SYN bit
set in its header:
|C|E|U|A|P|R|S|F|
|---------------|
|0 0 0 0 0 0 1 0|
|---------------|
|7 6 5 4 3 2 1 0|
Looking at the control bits section we see that only bit number 1
(SYN) is set.
Assuming that octet number 13 is an 8-bit unsigned integer in network
byte order, the binary value of this octet is
00000010
and its decimal representation is
- 16 - Formatted: August 11, 2004
TCPDUMP(1) TCPDUMP(1)
3 January 2001
7 6 5 4 3 2 1 0
0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2
We're almost done, because now we know that if only SYN is set, the
value of the 13th octet in the TCP header, when interpreted as a 8-bit
unsigned integer in network byte order, must be exactly 2.
This relationship can be expressed as
tcp[13] == 2
We can use this expression as the filter for tcpdump in order to watch
packets which have only SYN set:
tcpdump -i xl0 tcp[13] == 2
The expression says "let the 13th octet of a TCP datagram have the
decimal value 2", which is exactly what we want.
Now, let's assume that we need to capture SYN packets, but we don't
care if ACK or any other TCP control bit is set at the same time.
Let's see what happens to octet 13 when a TCP datagram with SYN-ACK
set arrives:
|C|E|U|A|P|R|S|F|
|---------------|
|0 0 0 1 0 0 1 0|
|---------------|
|7 6 5 4 3 2 1 0|
Now bits 1 and 4 are set in the 13th octet. The binary value of octet
13 is
00010010
which translates to decimal
7 6 5 4 3 2 1 0
0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18
Now we can't just use 'tcp[13] == 18' in the tcpdump filter
expression, because that would select only those packets that have
SYN-ACK set, but not those with only SYN set. Remember that we don't
care if ACK or any other control bit is set as long as SYN is set.
In order to achieve our goal, we need to logically AND the binary
value of octet 13 with some other value to preserve the SYN bit. We
know that we want SYN to be set in any case, so we'll logically AND
the value in the 13th octet with the binary value of a SYN:
00010010 SYN-ACK 00000010 SYN
AND 00000010 (we want SYN) AND 00000010 (we want SYN)
-------- --------
- 17 - Formatted: August 11, 2004
TCPDUMP(1) TCPDUMP(1)
3 January 2001
= 00000010 = 00000010
We see that this AND operation delivers the same result regardless
whether ACK or another TCP control bit is set. The decimal
representation of the AND value as well as the result of this
operation is 2 (binary 00000010), so we know that for packets with SYN
set the following relation must hold true:
( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
This points us to the tcpdump filter expression
tcpdump -i xl0 'tcp[13] & 2 == 2'
Note that you should use single quotes or a backslash in the
expression to hide the AND ('&') special character from the shell.
UDP Packets UDP format is illustrated by this rwho packet:
actinide.who > broadcast.who: udp 84
This says that port who on host actinide sent a udp datagram to port
who on host broadcast, the Internet broadcast address. The packet
contained 84 bytes of user data. Some UDP services are recognized
(from the source or destination port number) and the higher level
protocol information printed. In particular, Domain Name service
requests (RFC-1034/1035) and Sun RPC calls (RFC-1050) to NFS.
UDP Name Server Requests (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 description
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 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 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 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, authority records or additional records 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
- 18 - Formatted: August 11, 2004
TCPDUMP(1) TCPDUMP(1)
3 January 2001
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 additional 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.
SMB/CIFS decoding tcpdump now includes fairly extensive SMB/CIFS/NBT
decoding for data on UDP/137, UDP/138 and TCP/139. Some primitive
decoding of IPX and NetBEUI SMB data is also done.
By default a fairly minimal decode is done, with a much more detailed
decode done if -v is used. Be warned that with -v a single SMB packet
may take up a page or more, so only use -v if you really want all the
gory details.
If you are decoding SMB sessions containing unicode strings then you
may wish to set the environment variable USE_UNICODE to 1. A patch to
auto-detect unicode srings would be welcome.
For information on SMB packet formats and what all te fields mean see
www.cifs.org or the pub/samba/specs/ directory on your favourite
samba.org mirror site. The SMB patches were written by Andrew
Tridgell (tridge@samba.org).
NFS Requests and Replies Sun NFS (Network File System) requests and
replies are printed as:
- 19 - Formatted: August 11, 2004
TCPDUMP(1) TCPDUMP(1)
3 January 2001
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
handle (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 generation 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, length, 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 operation. Instead, tcpdump keeps track of ``recent'' requests,
and matches them to the replies using the transaction ID. If a reply
does not closely follow the corresponding request, it might not be
parsable.
- 20 - Formatted: August 11, 2004
TCPDUMP(1) TCPDUMP(1)
3 January 2001
AFS Requests and Replies Transarc AFS (Andrew File System) requests
and replies are printed as:
src.sport > dst.dport: rx packet-type
src.sport > dst.dport: rx packet-type service call call-name args
src.sport > dst.dport: rx packet-type service reply call-name args
elvis.7001 > pike.afsfs:
rx data fs call rename old fid 536876964/1/1 ".newsrc.new"
new fid 536876964/1/1 ".newsrc"
pike.afsfs > elvis.7001: rx data fs reply rename
In the first line, host elvis sends a RX packet to pike. This was a
RX data packet to the fs (fileserver) service, and is the start of an
RPC call. The RPC call was a rename, with the old directory file id
of 536876964/1/1 and an old filename of `.newsrc.new', and a new
directory file id of 536876964/1/1 and a new filename of `.newsrc'.
The host pike responds with a RPC reply to the rename call (which was
successful, because it was a data packet and not an abort packet). In
general, all AFS RPCs are decoded at least by RPC call name. Most AFS
RPCs have at least some of the arguments decoded (generally only the
`interesting' arguments, for some definition of interesting). The
format is intended to be self-describing, but it will probably not be
useful to people who are not familiar with the workings of AFS and RX.
If the -v (verbose) flag is given twice, acknowledgement packets and
additional header information is printed, such as the the RX call ID,
call number, sequence number, serial number, and the RX packet flags.
If the -v flag is given twice, additional information is printed, such
as the the RX call ID, serial number, and the RX packet flags. The
MTU negotiation information is also printed from RX ack packets. If
the -v flag is given three times, the security index and service id
are printed. Error codes are printed for abort packets, with the
exception of Ubik beacon packets (because abort packets are used to
signify a yes vote for the Ubik protocol). Note that AFS requests are
very large and many of the arguments won't be printed unless snaplen
is increased. Try using `-s 256' to watch AFS traffic. AFS reply
packets do not explicitly identify the RPC operation. Instead,
tcpdump keeps track of ``recent'' requests, and matches them to the
replies using the call number and service ID. If a reply does not
closely follow the corresponding request, it might not be parsable.
KIP Appletalk (DDP in UDP) Appletalk DDP packets encapsulated in UDP
datagrams are de-encapsulated and dumped as DDP packets (i.e., all the
UDP header information is discarded). The file /etc/atalk.names is
used to translate appletalk net and node numbers to names. Lines in
this file have the form
number name
- 21 - Formatted: August 11, 2004
TCPDUMP(1) TCPDUMP(1)
3 January 2001
1.254 ether
16.1 icsd-net
1.254.110 ace
The first two lines give the names of appletalk networks. The third
line gives the name of a particular host (a host is distinguished from
a net by the 3rd octet in the number - a net number must have two
octets and a host number must have three octets.) The number and name
should be separated by whitespace (blanks or tabs). The
/etc/atalk.names file may contain blank lines or comment lines (lines
starting with a `#'). Appletalk addresses are printed in the form
net.host.port
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 indicated 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
- 22 - Formatted: August 11, 2004
TCPDUMP(1) TCPDUMP(1)
3 January 2001
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 `: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 retransmitted.
Helios resends them then jssmag.209 releases the transaction.
Finally, jssmag.209 initiates the next request. The `*' on the
request indicates that XO (`exactly once') was not set.
IP Fragmentation Fragmented Internet datagrams are printed as
(frag id:size@offset+)
(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 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
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).
- 23 - Formatted: August 11, 2004
TCPDUMP(1) TCPDUMP(1)
3 January 2001
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
traffic(1C), nit(4P), bpf(4), pcap(3)
AUTHORS
The original authors are: Van Jacobson, Craig Leres and Steven
McCanne, all of the Lawrence Berkeley National Laboratory, University
of California, Berkeley, CA. It is currently being maintained by
tcpdump.org. The current version is available via http:
http://www.tcpdump.org/
The original distribution is available via anonymous ftp:
ftp://ftp.ee.lbl.gov/tcpdump.tar.Z
IPv6/IPsec support is added by WIDE/KAME project. This program uses
Eric Young's SSLeay library, under specific configuration.
BUGS
Please send problems, bugs, questions, desirable enhancements, etc.
to:
tcpdump-workers@tcpdump.org
Please send source code contributions, etc. to:
patches@tcpdump.org
NIT doesn't let you watch your own outbound traffic, BPF will. We
recommend that you use the latter. On Linux systems with 2.0[.x]
kernels:
packets on the loopback device will be seen twice;
packet filtering cannot be done in the kernel, so that all
packets must be copied from the kernel in order to be filtered in
user mode;
all of a packet, not just the part that's within the snapshot
length, will be copied from the kernel (the 2.0[.x] packet
capture mechanism, if asked to copy only part of a packet to
userland, will not report the true length of the packet; this
would cause most IP packets to get an error from tcpdump). We
recommend that you upgrade to a 2.2 or later kernel. 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 generating them rather than
- 24 - Formatted: August 11, 2004
TCPDUMP(1) TCPDUMP(1)
3 January 2001
tcpdump. A packet trace that crosses a daylight savings time
change will give skewed time stamps (the time change is ignored).
Filter expressions that manipulate FDDI or Token Ring headers
assume that all FDDI and Token Ring packets are SNAP-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
filter may inadvertently accept certain packets that do not
properly match the filter expression. Filter expressions on
fields other than those that manipulate Token Ring headers will
not correctly handle source-routed Token Ring packets. ip6 proto
should chase header chain, but at this moment it does not. ip6
protochain is supplied for this behavior. Arithmetic expression
against transport layer headers, like tcp[0], does not work
against IPv6 packets. It only looks at IPv4 packets.
- 25 - Formatted: August 11, 2004
- TCPDUMP说明
- tcpdump 使用简要说明
- tcpdump 参数简要说明
- tcpdump 参数说明
- tcpdump参数简要说明
- tcpdump的基本参数说明
- tcpdump常用参数说明
- tcpdump说明，注意与wireshark完美结合
- tcpdump
- TCPDUMP
- tcpdump
- tcpdump
- tcpdump
- TCPDUMP
- tcpdump
- tcpdump
- tcpdump
- tcpdump
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