tcpflow(1) tcpflow 0.21 tcpflow(1)
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20 April 1999
NAME
tcpflow - TCP flow recorder
SYNOPSIS
tcpflow [-chpsv] [-b max_bytes] [-d debug_level] [-f max_fds] [-i iface
] [-r file] [expression]
DESCRIPTION
tcpflow is a program that captures data transmitted as part of TCP
connections (flows), and stores the data in a way that is convenient
for protocol analysis or debugging. A program like tcpdump(4) shows a
summary of packets seen on the wire, but usually doesn't store the
data that's actually being transmitted. In contrast, tcpflow
reconstructs the actual data streams and stores each flow in a
separate file for later analysis. tcpflow understands TCP sequence
numbers and will correctly reconstruct data streams regardless of
retransmissions or out-of-order delivery. tcpflow stores all captured
data in files that have names of the form
192.168.101.102.02345-010.011.012.013.45103
where the contents of the above file would be data transmitted from
host 192.168.101.102 port 2345, to host 10.11.12.13 port 45103.
OPTIONS
-b Max bytes per flow. Capture no more than max_bytes bytes per
flow. Any data captured for a flow beyond max_bytes from the
first byte captured will be discarded. The default is to store
an unlimited number of bytes per flow.
-c Console print. Print the contents of packets to stdout as they
are received, without storing any captured data to files (implies
-s ).
-d Debug level. Set the level of debugging messages printed to
stderr to debug_level. Higher numbers produce more messages. -d
0 causes completely silent operation. -d 1 , the default,
produces minimal status messages. -d 10 produces verbose output
equivalent to -v . Numbers higher than 10 can produce a large
amount of debugging information useful only to developers.
-f Max file descriptors used. Limit the number of file descriptors
used by tcpflow to max_fds. Higher numbers use more system
resources, but usually perform better. If the underlying
operating system supports the setrlimit() system call, the OS
will be asked to enforce the requested limit. The default is for
tcpflow to use the maximum number of file descriptors allowed by
the OS. The -v option will report how many file descriptors
tcpflow is using.
-h Help. Print usage information and exit.
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-i Interface name. Capture packets from the network interface named
iface. If no interface is specified with -i , a reasonable
default will be used by libpcap automatically.
-p No promiscuous mode. Normally, tcpflow attempts to put the
network interface into promiscuous mode before capturing packets.
The -p option tells tcpflow not to put the interface into
promiscuous mode. Note that it might already be in promiscuous
mode for some other reason.
-r Read from file. Read packets from file, which was created using
the -w option of tcpdump(1). Standard input is used if file is
``-''. Note that for this option to be useful, tcpdump's -s
option should be used to set the snaplen to the MTU of the
interface (e.g., 1500) while capturing packets.
-s Strip non-printables. Convert all non-printable characters to
the "." character before printing packets to the console or
storing them to a file.
-v Verbose operation. Verbosely describe tcpflow's operation.
Equivalent to -d 10 .
FILTERING EXPRESSIONS
The expression specified on the command-line specifies which packets
should be captured. Because tcpflow uses the the libpcap library,
tcpflow has the same powerful filtering language available as programs
such as tcpdump(1). The following part of the man
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.
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proto
qualifiers restrict the match to a particular protocol. Possible
protos are: ether, fddi, ip, arp, rarp, decnet, lat, sca, moprc,
mopdl, tcp and udp. E.g., `ether src foo', `arp net 128.3', `tcp
port 21'. If there is no proto qualifier, all protocols
consistent with the type are assumed. E.g., `src foo' means `(ip
or arp or rarp) src foo' (except the latter is not legal syntax),
`net bar' means `(ip or arp or rarp) net bar' and `port 53' means
`(tcp or udp) port 53'. [`fddi' is actually an alias for
`ether'; the parser treats them identically as meaning ``the data
link level used on the specified network interface.'' FDDI
headers contain Ethernet-like source and destination addresses,
and often contain Ethernet-like packet types, so you can filter
on these FDDI fields just as with the analogous 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' 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 IP destination field of the packet is host, which may
be either an address or a name.
src host host
True if the IP source field of the packet is host.
host host
True if either the IP source or destination of the packet is
host. Any of the above host expressions can be prepended with
the keywords, ip, arp, or rarp as in:
ip host host
which is equivalent to:
ether proto \ip and host host
If host is a name with multiple IP addresses, each address will
be checked for a match.
ether dst ehost
True if the ethernet destination address is ehost. Ehost may be
either a name from /etc/ethers or a number (see ethers(3N) for
numeric format).
ether src ehost
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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 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.
net net mask mask
True if the IP address matches net with the specific netmask.
May be qualified with src or dst.
net net/len
True if the IP address matches net a netmask len bits wide. May
be qualified with src or dst.
dst port port
True if the packet is ip/tcp or ip/udp and has a destination port
value of port. The port can be a number or a name used in
/etc/services (see tcp(4P) and udp(4P)). If a name is used, both
the port number and protocol are checked. If a number or
ambiguous name is used, only the port number is checked (e.g.,
dst port 513 will print both tcp/login traffic and udp/who
traffic, and port domain will print both tcp/domain and
udp/domain traffic).
src port port
True if the packet has a source port value of port.
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port port
True if either the source or destination port of the packet is
port. Any of the above port expressions can be prepended with
the keywords, tcp or udp, as in:
tcp src port port
which matches only tcp packets whose source port is port.
less length
True if the packet has a length less than or equal to length.
This is equivalent to:
len <= length.
greater length
True if the packet has a length greater than or equal to length.
This is equivalent to:
len >= length.
ip proto protocol
True if the packet is an ip packet (see ip(4P)) of protocol type
protocol. Protocol can be a number or one of the names icmp,
igrp, udp, nd, or tcp. Note that the identifiers tcp, udp, and
icmp are also keywords and must be escaped via backslash (\),
which is \\ in the C-shell.
ether broadcast
True if the packet is an ethernet broadcast packet. The ether
keyword is optional.
ip broadcast
True if the packet is an IP broadcast packet. It checks for both
the all-zeroes and all-ones broadcast conventions, and looks up
the local subnet mask.
ether multicast
True if the packet is an ethernet multicast packet. The ether
keyword is optional. This is shorthand for `ether[0] & 1 != 0'.
ip multicast
True if the packet is an IP multicast packet.
ether proto protocol
True if the packet is of ether type protocol. Protocol can be a
number or a name like ip, arp, or rarp. Note these identifiers
are also keywords and must be escaped via backslash (\). [In the
case of FDDI (e.g., `fddi protocol arp'), the protocol
identification comes from the 802.2 Logical Link Control (LLC)
header, which is usually layered on top of the FDDI header.
Tcpdump assumes, when filtering on the protocol identifier, that
all FDDI packets include an LLC header, and that the LLC header
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is in so-called SNAP format.]
decnet src host
True if the DECNET source address is host, which may be an
address of the form ``10.123'', or a DECNET host name. [DECNET
host name support is only available on Ultrix systems that are
configured to run DECNET.]
decnet dst host
True if the DECNET destination address is host.
decnet host host
True if either the DECNET source or destination address is host.
ip, arp, rarp, decnet
Abbreviations for:
ether proto p
where p is one of the above protocols.
lat, moprc, mopdl
Abbreviations for:
ether proto p
where p is one of the above protocols. Note that tcpdump does
not currently know how to parse these protocols.
tcp, udp, icmp
Abbreviations for:
ip proto p
where p is one of the above protocols.
expr relop expr
True if the relation holds, where relop is one of >, <, >=, <=,
=, !=, and expr is an arithmetic expression composed of integer
constants (expressed in standard C syntax), the normal binary
operators [+, -, *, /, &, |], a length operator, and special
packet data accessors. To access data inside the packet, use the
following syntax:
proto [ expr : size ]
Proto is one of ether, fddi, ip, arp, rarp, tcp, udp, or icmp,
and indicates the 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, 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
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unfragmented datagrams and frag zero of fragmented datagrams.
This check is implicitly applied to the tcp and udp index
operations. For instance, tcp[0] always means the first byte of
the TCP header, and never means the first byte of an intervening
fragment. Primitives may be combined using:
A parenthesized group of primitives and operators (parentheses
are special to the Shell and must be escaped).
Negation (`!' or `not').
Concatenation (`&&' or `and').
Alternation (`||' or `or'). Negation has highest precedence.
Alternation and concatenation have equal precedence and associate
left to right. Note that explicit and tokens, not juxtaposition,
are now required for concatenation. If an identifier is given
without a keyword, the most recent keyword is assumed. For
example,
not host vs and ace
is short for
not host vs and host ace
which should not be confused with
not ( host vs or ace )
Expression arguments can be passed to tcpdump as either a single
argument or as multiple arguments, whichever is more convenient.
Generally, if the expression contains Shell metacharacters, it is
easier to pass it as a single, quoted argument. Multiple
arguments are concatenated with spaces before being parsed.
EXAMPLES
The following part of the man To record all packets arriving at or
departing from sundown:
tcpflow host sundown
To record traffic between helios and either hot or ace:
tcpflow host helios and \( hot or ace \)
To record traffic between ace and any host except helios:
tcpflow host ace and not helios
To record all traffic between local hosts and hosts at Berkeley:
tcpflow net ucb-ether
To record all ftp traffic through internet gateway snup: (note that
the expression is quoted to prevent the shell from (mis-)interpreting
the parentheses):
tcpflow 'gateway snup and (port ftp or ftp-data)'
BUGS
Please send bug reports to jelson@circlemud.org. tcpflow currently
does not understand IP fragments. Flows containing IP fragments will
not be recorded correctly. tcpflow never frees state associated with
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flows that it records, so will grow large if used to capture a very
large number of flows (e.g., on the order of 100,000 flows or more).
There appears to be a bug in the way that Linux delivers packets to
libpcap when using the loopback interface ("localhost"). When
listening to the Linux loopback interface, selective packet filtering
is not possible; all TCP flows on the localhost interface will be
recorded.
AUTHOR
Jeremy Elson <jelson@circlemud.org> The current version of this
software is available at
http://www.circlemud.org/~jelson/software/tcpflow
SEE ALSO
tcpdump(1), nit(4P), bpf(4), pcap(3)
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