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.
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:
-
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]''.
Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)
There are 8 bits in the control bits section of the TCP header:
-
CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
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 |
-----------------------------------------------------------------
| 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
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)
-------- --------
= 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 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:
-
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.
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
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.
