NAME
tcpdump
—
dump traffic on a network
SYNOPSIS
tcpdump |
[-AadefILlNnOopqStvXx ]
[-B fildrop]
[-c count]
[-D direction]
[-E
[espalg:]espkey]
[-F file]
[-i interface]
[-r file]
[-s snaplen]
[-T type]
[-w file]
[-y datalinktype]
[expression] |
DESCRIPTION
tcpdump
prints out the headers of packets
on a network interface that match the boolean
expression. You must have read access to
/dev/bpf.
The options are as follows:
-A
- Print each packet in ASCII. If the
-e
option is also specified, the link-level header will be included. The smaller of the entire packet or snaplen bytes will be printed. -a
- Attempt to convert network and broadcast addresses to names.
-B
fildrop- Configure the drop action specified by fildrop to be
used when the filter expression matches a packet. The actions are:
The default action is
pass
. -c
count- Exit after receiving count packets.
-D
direction- Select packets flowing in the specified direction.
Valid directions are:
in
andout
. The default is to accept packets flowing in any direction. -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
[espalg:]espkey- Try to decrypt RFC 4835 ESP (Encapsulating Security Payload) traffic using
the specified hex key espkey. Supported algorithms
for espalg are:
aes128
,aes128-hmac96
,blowfish
,blowfish-hmac96
,cast
,cast-hmac96
,des3
,des3-hmac96
,des
anddes-hmac96
. The algorithm defaults toaes128-hmac96
. This option should be used for debugging only, since the key will show up in ps(1) output. -e
- Print the link-level header on each dump line.
-F
file- Use file as input for the filter expression. Any additional expressions given on the command line are ignored.
-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.
-I
- Print the interface on each dump line.
-i
interface- 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
- List the supported data link types for the interface and exit.
-l
- Make stdout line buffered. Useful if you want to see the data while
capturing it. For example:
or
# tcpdump -l | tee dat
# tcpdump -l > dat & tail -f dat
-N
- Do not print domain name qualification of host names. For example, if you
specify this flag then
tcpdump
will print “nic” instead of “nic.ddn.mil”. -n
- Do not convert addresses (host addresses, port numbers, etc.) to names.
-O
- Do not run the packet-matching code optimizer. This is useful only if you suspect a bug in the optimizer.
-o
- Print a guess of the possible operating system(s) of hosts that sent TCP SYN packets. See pf.os(5) for a description of the passive operating system fingerprints.
-p
- Do not put the interface into promiscuous mode. 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
file- Read packets from a 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
snaplen- Analyze at most the first snaplen bytes of data from each packet rather than the default of 116. 116 bytes is adequate for IPv6, ICMP, TCP, and UDP, but may truncate protocol information from name server and NFS packets (see below). Packets truncated because of a limited snaplen are indicated in the output with “[|proto]”, where proto is the name of the protocol level at which the truncation has occurred. 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
type- Force packets selected by expression to be
interpreted as the specified type. Currently known
types are:
cnfp
- Cisco NetFlow protocol
erspan
- Cisco Encapsulated Remote Switch Port Analyzer (ERSPAN) over GRE
gre
- Generic Routing Encapsulation over UDP
mpls
- Multiprocol Label Switching over UDP
rpc
- Remote Procedure Call
rtcp
- Real-Time Applications control protocol
rtp
- Real-Time Applications protocol
sack
- RFC 2018 TCP Selective Acknowledgements Options
tcp
- Transmission Control Protocol
tftp
- Trivial File Transfer Protocol
vat
- Visual Audio Tool
vrrp
- Virtual Router Redundancy protocol
vxlan
- Virtual eXtensible Local Area Network
wb
- distributed White Board
-t
- Do not print a timestamp on each dump line.
-tt
- Print an unformatted timestamp on each dump line.
-ttt
- Print day and month in timestamp.
-tttt
- Print timestamp difference between packets.
-ttttt
- Print timestamp difference since the first packet.
-v
- (Slightly more) verbose output. For example, the time to live (TTL) and type of service (ToS) information in an IP packet are printed.
-vv
- Even more verbose output. For example, additional fields are printed from NFS reply packets.
-w
file- Write the raw packets to file rather than parsing
and printing them out. They can be analyzed later with the
-r
option. Standard output is used if file is ‘-
’. -X
- Print each packet in hex and ASCII. If the
-e
option is also specified, the link-level header will be included. The smaller of the entire packet or snaplen bytes will be printed. -x
- Print each packet in hex. If the
-e
option is also specified, the link-level header will be included. The smaller of the entire packet or snaplen bytes will be printed. -y
datalinktype- Set the data link type to use while capturing to
datalinktype. Commonly used types include
EN10MB
,IEEE802_11
, andIEEE802_11_RADIO
. The choices applicable to a particular device can be listed using-L
.
expression selects which packets will be dumped. If no expression is given, all packets on the net will be dumped. Otherwise, only packets satisfying expression will be dumped.
The expression consists of one or more primitives. Primitives usually consist of an id (name or number) preceded by one or more qualifiers. There are three different kinds of qualifiers:
- type
- Specify which kind of address component the id name
or number refers to. Possible types are
host
,net
andport
. E.g., “host foo”, “net 128.3”, “port 20”. If there is no type qualifier,host
is assumed. - dir
- Specify a particular transfer direction to and/or from
id. Possible directions are
src
,dst
,src or dst
,src and dst
,addr1
,addr2
,addr3
, andaddr4
. 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. Theaddr1
,addr2
,addr3
, andaddr4
qualifiers are only valid for IEEE 802.11 Wireless LAN link layers. For null link layers (i.e., point-to-point protocols such as SLIP (Serial Line Internet Protocol) or the pflog(4) header), theinbound
andoutbound
qualifiers can be used to specify a desired direction. - proto
- Restrict the match to a particular protocol. Possible protocols are:
ah
,arp
,atalk
,decnet
,esp
,ether
,fddi
,icmp
,icmp6
,igmp
,igrp
,ip
,ip6
,lat
,mopdl
,moprc
,pim
,rarp
,sca
,stp
,tcp
,udp
, andwlan
. E.g., “ether src foo”, “arp net 128.3”, “tcp port 21”, “wlan addr1 0:2:3:4:5:6”. If there is no protocol qualifier, all protocols consistent with the type are assumed. E.g., “src foo” means “(ip or arp or rarp) src foo” (except the latter is not legal 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 forether
; the parser treats them identically as meaning "the data link level used on the specified network interface". FDDI (Fiber Distributed Data Interface) headers contain Ethernet-like source and destination addresses, and often contain Ethernet-like packet types, so you can 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
, orrarp
as in:ip host
hostwhich is equivalent to:
ether proto
ipand host
hostIf 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 ether_aton(3) for a 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
ehostand not host
hostwhich 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/hosts or a network number (see hosts(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.
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 name from
services(5) (see
tcp(4) and udp(4)). If a name is used, both the port number and
protocol are checked. If a number or ambiguous name is used, only the port
number is checked; e.g., “
dst port
513” will print both TCP/login traffic and UDP/who traffic, and “dst 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
orudp
, as in:tcp src port
portwhich 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
proto- True if the packet is an IP packet (see
ip(4)) of protocol type proto.
proto can be a number or name from
protocols(5), such as
icmp
,udp
, ortcp
. These identifiers are also keywords and must be escaped using a backslash character (‘\’). 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
proto- True if the packet is of ether type proto.
proto can be a number or one of the names
ip
,ip6
,arp
,rarp
,atalk
,atalkarp
,decnet
,decdts
,decdns
,lanbridge
,lat
,mopdl
,moprc
,pup
,sca
,sprite
,stp
,vexp
,vprod
, orxns
. These identifiers are also keywords and must be escaped using a backslash character (‘\’). 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 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.
ifname
interface- True if the packet was logged as coming from the specified interface (applies only to packets logged by pf(4)).
on
interface- Synonymous with the ifname modifier.
rnr
num- True if the packet was logged as matching the specified PF rule number in the main ruleset (applies only to packets logged by pf(4)).
rulenum
num- Synonymous with the rnr modifier.
reason
code- True if the packet was logged with the specified PF reason code. The known codes are: match, bad-offset, fragment, short, normalize, memory, bad-timestamp, congestion, ip-option, proto-cksum, state-mismatch, state-insert, state-limit, src-limit, and synproxy (applies only to packets logged by pf(4)).
rset
name- True if the packet was logged as matching the specified PF ruleset name of an anchored ruleset (applies only to packets logged by pf(4)).
ruleset
name- Synonymous with the rset modifier.
srnr
num- True if the packet was logged as matching the specified PF rule number of an anchored ruleset (applies only to packets logged by pf(4)).
subrulenum
num- Synonymous with the srnr modifier.
action
act- True if PF took the specified action when the packet was logged. Valid actions are: pass, block, and match (applies only to packets logged by pf(4)).
wlan addr1
ehost- True if the first IEEE 802.11 address is ehost.
wlan addr2
ehost- True if the second IEEE 802.11 address is ehost.
wlan addr3
ehost- True if the third IEEE 802.11 address is ehost.
wlan addr4
ehost- True if the fourth IEEE 802.11 address is ehost. The fourth address field is only used for WDS (Wireless Distribution System) frames.
wlan host
ehost- True if either the first, second, third, or fourth IEEE 802.11 address is ehost.
type
type- True if the IEEE 802.11 frame type matches the specified type. Valid types are: data, mgt, ctl, or a numeric value.
subtype
subtype- True if the IEEE 802.11 frame subtype matches the specified subtype. Valid subtypes are: assocreq, assocresp, reassocreq, reassocresp, probereq, proberesp, beacon, atim, disassoc, auth, deauth, data, or a numeric value.
dir
dir- True if the IEEE 802.11 frame direction matches the specified dir. Valid directions are: nods, tods, fromds, dstods, or a numeric value.
atalk
,ip
,ip6
,arp
,decnet
,lat
,moprc
,mopdl
,rarp
,sca
- Abbreviations for:
ether proto
p where p is one of the above protocols.tcpdump
does not currently know how to parselat
,moprc
, ormopdl
. ah
,esp
,icmp
,icmp6
,igmp
,igrp
,pim
,tcp
,udp
- 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
, oricmp
, 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 keywordlen
, 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 thetcp
andudp
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. Allowable primitives and operators are:
!
”
or “not
”)
Concatenation (“&&
”
or “and
”)
Alternation (“||
” or
“or
”)
Negation has highest precedence. Alternation and concatenation
have equal precedence and associate left to right. Explicit
and
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
aceis short for
not host
vs
and host
acewhich 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 (the expression is quoted to prevent the shell from misinterpreting the parentheses):
# tcpdump 'host helios and (hot or
ace)'
To print all IP packets between ace and any host except helios:
# tcpdump ip host ace and not
helios
To print all traffic between local hosts and hosts at Berkeley:
# tcpdump net ucb-ether
To print all FTP traffic through internet gateway snup:
# tcpdump 'gateway snup and (port ftp
or ftp-data)'
To print traffic neither sourced from nor destined for local network 192.168.7.0/24 (if you gateway to one other net, this stuff should never make it onto your local network):
# tcpdump ip and not net
192.168.7.0/24
To print the start and end packets (the SYN and FIN packets) of each TCP connection that involves a host that is not in local network 192.168.7.0/24:
# tcpdump 'tcp[13] & 3 != 0 and not src and dst net 192.168.7.0/24'
To print only the SYN packets of HTTP connections:
# tcpdump 'tcp[tcpflags] = tcp-syn
and port http'
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'
To print only echo request ICMP packets:
# tcpdump 'icmp[icmptype] =
icmp-echo'
To print and decrypt all ESP packets with SPI 0x00001234:
# tcpdump -E des3-hmac96:ab...def
'ip[20:4] = 0x00001234'
To print raw wireless frames passing the iwn0 interface:
# tcpdump -i iwn0 -y IEEE802_11_RADIO
-v
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 the packet filter logging interface pflog(4), logging reason (rule match, bad-offset, fragment, bad-timestamp, short, normalize, memory), action taken (pass/block), direction (in/out) and interface information are printed out for each packet.
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.
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:
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
In this example, Ethernet addresses are in caps and internet addresses in lower case. The first line says that rtsg sent an arp packet asking for the Ethernet address of internet host csam. csam replies with its Ethernet address CSAM.
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
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 and dst are the
source and destination IP addresses and ports. flags
is some combination of ‘S’ (SYN), ‘F’ (FIN),
‘P’ (PUSH), or ‘R’ (RST), ‘W’
(congestion Window reduced), ‘E’ (ecn ECHO) or a single
‘.
’ (no flags).
src-os will list a guess of the source host's
operating system if the -o
command line flag was
passed to tcpdump
. data-seqno
describes the portion of sequence space covered by the data in this packet
(see example below). ack is the sequence number of the
next data expected by the other end of this connection.
window is the number of bytes of receive buffer space
available at the other end of this connection. urg
indicates there is urgent data in the packet. options
are 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 host 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. The ack
sequence number is a 32-bit integer. The first time
tcpdump
sees a TCP connection, it prints the
sequence number from the packet. On subsequent packets of the connection,
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 connection'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 connection). The PUSH flag is set in the packet. On the 7th line, csam says it's received data sent by rtsg up to but not including byte 21. Most of this data is apparently 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.
UDP Packets
UDP format is illustrated by this rwho packet:
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
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
For example:
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 op had
been anything else, it would have been printed between the 3 and the
‘+
’. Similarly, the
qclass was the normal one (C_IN) and was omitted. Any
other qclass would have been printed immediately after
the A.
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
For example:
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 rcode (NoError) were omitted, as was the class (C_IN) of the A record.
In the second example, helios responds to query
op 2 with an rcode 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.
Name server requests and responses tend to be large and the
default snaplen of 96 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.nfs > dst.xid: reply stat len op results
sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165 wrl.nfs > sushi.6709: reply ok 40 readlink "../var" sushi.201b > wrl.nfs: 144 lookup fh 9,74/4096.6878 "xcolors" wrl.nfs > sushi.201b: reply ok 128 lookup fh 9,74/4134.3150
In the first line, host sushi sends a transaction with ID 6709 to wrl. The number following the src host is a transaction ID, not the source port. The request was 112 bytes, excluding the UDP and IP headers. The op was a readlink (read symbolic link) on fh (“file handle”) 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 with a stat of ok and the contents of the link.
In the third line, sushi asks wrl to look up the name “xcolors” in directory file 9,74/4096.6878. The data printed depends on 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 with a stat of ok; the
packet shown on the second line is the first fragment of the reply, and
hence is only 1472 bytes long. The other bytes will follow in subsequent
fragments, but these fragments do not have NFS or even UDP headers and so
might not be printed, 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.
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
xid (transaction ID). If a reply does not closely
follow the corresponding request, it might not be parsable.
IP Fragmentation
Fragmented Internet datagrams are printed as
frag
id : size @
offset [+])A ‘+
’ indicates there are
more fragments. The last fragment will have no
‘+
’.
id is the fragment ID. size is the fragment size (in bytes) 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 fragment info is printed after the protocol info. Fragments after the first contain no higher level protocol header and the fragment info is printed after 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.
IP and Protocol Checksum Offload
Some network cards support IP and/or protocol checksum offload.
Packet headers for such interfaces erroneously indicate a bad checksum,
since the checksum is not calculated until after
tcpdump
sees the packet.
SEE ALSO
ether_aton(3), pcap_open_live(3), bpf(4), ip(4), pf(4), pflog(4), tcp(4), udp(4), hosts(5), pcap-filter(5), pf.os(5), protocols(5), services(5)
STANDARDS
Transmission Control Protocol, RFC 793, September 1981.
P. Mockapetris, Domain Names – Concepts and Facilities, RFC 1034, November 1987.
P. Mockapetris, Domain Names – Implementation and Specification, RFC 1035, November 1987.
RPC: Remote Procedure Call Protocol Specification, RFC 1050, April 1988.
V. Jacobson, Compressing TCP/IP Headers for Low-Speed Serial Links, RFC 1144, February 1990.
M. Mathis, J. Mahdavi, S. Floyd, and A. Romanow, TCP Selective Acknowledgement Options, RFC 2018, October 1996.
V. Manral, Cryptographic Algorithm Implementation Requirements for Encapsulating Security Payload (ESP) and Authentication Header (AH), RFC 4835, April 2007.
AUTHORS
Van Jacobson <van@ee.lbl.gov>, Craig Leres <leres@ee.lbl.gov>, and Steven McCanne <mccanne@ee.lbl.gov>, all of the Lawrence Berkeley Laboratory, University of California, Berkeley, CA.
BUGS
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 the 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
tcpdump
.
A packet trace that crosses a daylight saving time change will give skewed time stamps (the time change is ignored).
Filter expressions that manipulate FDDI headers assume that all FDDI packets are 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.