TCP UMPp

作者 elab
NAME
tcpdump - dump traffic on a network
SYNOPSIS
tcpdump [ -adeflnNOpqStvx ] [ -c count ] [ -F file ]
[ -i interface ] [ -r file ] [ -s snaplen ]
[ -T type ] [ -w file ] [ expression ]
DESCRIPTION
Tcpdump prints out the headers of packets on a network interface that match the
boolean expression.
Under SunOS with nit or bpf: To run tcpdump you must have read access to
/dev/nit or /dev/bpf*. Under Solaris with dlpi: You must have read access to the
network pseudo device, e.g. /dev/le. Under HP-UX with dlpi: You must be root or
it must be installed setuid to root. Under IRIX with snoop: You must be root or it
must be installed setuid to root. Under Linux: You must be root or it must be
installed setuid to root. Under Ultrix and Digital UNIX: Once the super-user has
enabled promiscuous-mode operation using pfconfig(8), any user may run
tcpdump. Under BSD: You must have read access to /dev/bpf*.
OPTIONS
-a
Attempt to convert network and broadcast addresses to names.
-c
Exit after receiving count packets.
-d
Dump the compiled packet-matching code in a human readable form to standard
output and stop.
-dd
Dump packet-matching code as a C program fragment.
-ddd
Dump packet-matching code as decimal numbers (preceded with a count).
-e
Print the link-level header on each dump line.
-f
Print `foreign' internet addresses numerically rather than symbolically (this option
is intended to get around serious brain damage in Sun's yp server --- usually it
hangs forever translating non-local internet numbers).
-F
Use file as input for the filter expression. An additional expression given on the
command line is ignored.
-i
Listen on interface. If unspecified, tcpdump searches the system interface list for
the lowest numbered, configured up interface (excluding loopback). Ties are
broken by choosing the earliest match.
-l
Make stdout line buffered. Useful if you want to see the data while capturing it.
E.g.,
``tcpdump -l | tee dat'' or ``tcpdump -l > dat & tail -f dat''.
-n
Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
-N
Don't print domain name qualification of host names. E.g., if you give this flag
then tcpdump will print ``nic'' instead of ``nic.ddn.mil''.
-O
Do not run the packet-matching code optimizer. This is useful only if you suspect
a bug in the optimizer.
-p
Don't put the interface into promiscuous mode. Note that the interface might be in
promiscuous mode for some other reason; hence, `-p' cannot be used as an
abbreviation for `ether host {local-hw-addr} or ether broadcast'.
-q
Quick (quiet?) output. Print less protocol information so output lines are shorter.
-r
Read packets from file (which was created with the -w option). Standard input is
used if file is ``-''.
-s
Snarf snaplen bytes of data from each packet rather than the default of 68 (with
SunOS's NIT, the 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.
-T
Force packets selected by "expression" to be interpreted the specified type.
Currently known types are rpc (Remote Procedure Call), rtp (Real-Time
Applications protocol), rtcp (Real-Time Applications control protocol), vat (Visual
Audio Tool), wb (distributed White Board), and snmp (Simple Network
Management Protocol).
-S
Print absolute, rather than relative, TCP sequence numbers.
-t
Don't print a timestamp on each dump line.
-tt
Print an unformatted timestamp on each dump line.
-v
(Slightly more) verbose output. For example, the time to live and type of service
information in an IP packet is printed.
-vv
Even more verbose output. For example, additional fields are printed from NFS
reply packets.
-w
Write the raw packets to file rather than parsing and printing them out. They can
later be printed with the -r option. Standard output is used if file is ``-''.
-x
Print each packet (minus its link level header) in hex. The smaller of the entire
packet or snaplen bytes will be printed.
expression
selects which packets will be dumped. If no expression is given, all packets on
the net will be dumped. Otherwise, only packets for which 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, 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
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(4)
for details).
src net net
True if the IP source address of the packet has a network number of net.
net net
True if either the IP source or destination address of the packet has a network
number of net.
net net mask mask
True if the IP address matches net with the specific netmask. May be qualified
with src or dst.
net net/len
True if the IP address matches net a netmask len bits wide. May be qualified with
src or dst.
dst port port
True if the packet is ip/tcp or ip/udp and has a destination port value of port. The
port can be a number or a name used in /etc/services (see tcp(4P) and udp(4P)).
If a name is used, both the port number and protocol are checked. If a number or
ambiguous name is used, only the port number is checked (e.g., dst port 513 will
print both tcp/login traffic and udp/who traffic, and port domain will print both
tcp/domain and udp/domain traffic).
src port port
True if the packet has a source port value of port.
port port
True if either the source or destination port of the packet is port. Any of the above
port expressions can be prepended with the keywords, tcp or udp, as in:
tcp src port port
which matches only tcp packets whose source port is port.
less length
True if the packet has a length less than or equal to length. This is equivalent to:
len <= length.
greater length
True if the packet has a length greater than or equal to length. This is equivalent
to:
len >= length.
ip proto protocol
True if the packet is an ip packet (see ip(4P)) of protocol type protocol. Protocol
can be a number or one of the names icmp, igrp, udp, nd, or tcp. Note that the
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 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 unfragmented datagrams and frag zero of fragmented datagrams.
This check is implicitly applied to the tcp and udp index operations. For instance,
tcp[0] always means the first byte of the TCP header, and never means the first
byte of an intervening fragment.
Primitives may be combined using:
A parenthesized group of primitives and operators (parentheses are special to
the Shell and must be escaped).
Negation (`!' or `not').
Concatenation (`&&' or `and').
Alternation (`||' or `or').
Negation has highest precedence. Alternation and concatenation have equal
precedence and associate left to right. Note that explicit and tokens, not
juxtaposition, are now required for concatenation.
If an identifier is given without a keyword, the most recent keyword is assumed.
For example,
not host vs and ace
is short for
not host vs and host ace
which should not be confused with
not ( host vs or ace )
Expression arguments can be passed to tcpdump as either a single argument or
as multiple arguments, whichever is more convenient. Generally, if the
expression contains Shell metacharacters, it is easier to pass it as a single,
quoted argument. Multiple arguments are concatenated with spaces before being
parsed.
__________________
EXAMPLES
To print all packets arriving at or departing from 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[13] & 3 != 0 and not src and dst net localnet'
To print IP packets longer than 576 bytes sent through gateway snup:
tcpdump 'gateway snup and ip[2:2] > 576'
To print IP broadcast or multicast packets that were not sent via ethernet
broadcast or multicast:
tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
To print all ICMP packets that are not echo requests/replies (i.e., not ping
packets):
tcpdump 'icmp[0] != 8 and icmp[0] != 0"
OUTPUT FORMAT
The output of tcpdump is protocol dependent. The 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, 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.
(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:
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 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]''.
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, name server or authority section,
ancount, nscount, or arcount are printed as `[na]', `[nn]' or `[nau]' where n is the
appropriate count. If any of the response bits are set (AA, RA or rcode) or any of
the `must be zero' bits are set in bytes two and three, `[b2&3=x]' is printed, where
x is the hex value of header bytes two and three.
UDP Name Server Responses
Name server responses are formatted as
src > dst: id op rcode flags a/n/au type class data (len)
helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273)
helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
In the first example, helios responds to query id 3 from h2opolo with 3 answer
records, 3 name server records and 7 authority records. The first answer record
is type A (address) and its data is internet address 128.32.137.3. The total size of
the response was 273 bytes, excluding UDP and IP headers. The op (Query) and
response code (NoError) were omitted, as was the class (C_IN) of the A record.
In the second example, helios responds to query 2 with a response code of
non-existent domain (NXDomain) with no answers, one name server and no
authority records. The `*' indicates that the authoritative answer bit was set. Since
there were no answers, no type, class or data were printed.
Other flag characters that might appear are `-' (recursion available, RA, not set)
and `|' (truncated message, TC, set). If the `question' section doesn't contain
exactly one entry, `[nq]' is printed.
Note that name server requests and responses tend to be large and the default
snaplen of 68 bytes may not capture enough of the packet to print. Use the -s flag
to increase the snaplen if you need to seriously investigate name server traffic.
`-s 128' has worked well for me.
NFS Requests and Replies
Sun NFS (Network File System) requests and replies are printed as:
src.xid > dst.nfs: len op args
src.nfs > dst.xid: reply stat len op results
sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
sushi.201b > wrl.nfs:
144 lookup fh 9,74/4096.6878 "xcolors"
wrl.nfs > sushi.201b:
reply ok 128 lookup fh 9,74/4134.3150
In the first line, host sushi sends a transaction with id 6709 to wrl (note that the
number following the src host is a transaction id, not the source port). The
request was 112 bytes, excluding the UDP and IP headers. The operation was a
readlink (read symbolic link) on file 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, 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.
KIP Appletalk (DDP in UDP)
Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated
and dumped as DDP packets (i.e., all the UDP header information is discarded).
The file /etc/atalk.names is used to translate appletalk net and node numbers to
names. Lines in this file have the form
number name
1.254 ether
16.1 icsd-net
1.254.110 ace
The first two lines give the names of appletalk networks. The third line gives the
name of a particular host (a host is distinguished from a net by the 3rd octet in the
number - a net number must have two octets and a host number must have three
octets.) The number and name should be 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
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).
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.