tcpdump(8) System Manager's Manual tcpdump(8)
NAME
tcpdump - Dump traffic on a network
SYNOPSIS
/usr/sbin/tcpdump [-deflmnNOpqStvxX] [-c count] [-Ffile] [-iinterface] [-rfile] [-ssnaplen] [-wfile] expression
OPTIONS
Exits after receiving count packets. Dumps the compiled packet-matching code to standard output and stop. Prints the link-level header on
each dump line. Prints "foreign" internet addresses numerically rather than symbolically. Uses file as input for the filter expression.
Any additional expressions on the command line are ignored. Listens 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. Makes stdout
line buffered. This is useful if you want to see the data while capturing it. Enables multiline output from some protocols. This affects
most Sun RPC decoding, as those protocols are often difficult to display on a single line. Does not convert addresses (for example, host
addresses and port numbers) to names. Does not print domain name qualification of host names. For example, with the -N option, tcpdump
prints nic instead of nic.ddn.mil. Does not run the packet-matching code optimizer. This is useful only if you suspect a bug in the opti-
mizer. Does not put the interface into promiscuous mode. Note the interface might be in promiscuous mode for some other reason; there-
fore, -p cannot be used as an abbreviation for ether host {localhost} or broadcast. Quick (quiet) output. Prints less protocol informa-
tion so output lines are shorter. Reads packets from file (which was created with the -w option). Standard input is used if a hyphen (-)
is used to specify file. Displays snaplen bytes of data from each packet rather than the default of 68 (with NIT, the minimum is 96). The
default of 68 bytes is adequate for IP, ICMP, TCP, and UDP, but may truncate protocol information from name server and NFS packets (dis-
cussed later in this reference page). 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
Taking larger snapshots both increases the amount of time it takes to process packets and 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 needed protocol informa-
tion. Prints absolute, rather than relative, TCP sequence numbers. Does not print a timestamp on each dump line. Prints an unfor-
matted timestamp on each dump line. Prints slightly more verbose output. For example, the time to live and type of service infor-
mation in an IP packet is printed. If -m is also specified, Sun RPC packets sent using TCP are decoded twice: first as RPC, then as
TCP. Normally the TCP decoding is suppressed. Prints even more verbose output. For example, additional fields are printed from
NFS reply packets. Writes the raw packets to file rather than parsing and printing them. They can later be printed with the -r
option. Standard output is used if a hyphen (-) is used to specify file. Prints each packet (minus its link level header) in hexa-
decimal format. The smaller of the entire packet or snaplen bytes is printed. Prints packets in both hexadecimal and ASCII formats.
Selects the packets to dump. If no expression is given, all packets on the network are dumped. Otherwise, only packets for which
expression is "true" are dumped.
The expression consists of one or more primitives. Primitives usually consist of an id (name or number) preceded by one or more of
the following qualifiers: Defines the object to which the id name or number refers. The following types are allowed: host, net, and
port. For example:
host foo net 128.3 port 20
If no type qualifier is specified, host is the default. Specifies a particular transfer direction to or from id. The following
directions are allowed: src, dst, src or dst, and src and dst. For example:
src foo dst net 128.3 src or dst port 20 src and dst port 123
If no dir qualifier is specified, src or dst is the default. Restricts the match to a particular protocol. The following protocols
are allowed: ether, fddi, ip, ipv6, icmpv6, arp, rarp, decnet, lat, moprc, mopdl, tcp, and udp. For example:
ether src foo arp net 128.3 tcp port 21
If no proto qualifier is specified, all protocols consistent with the type are assumed. For example, 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, port 53 means (tcp or udp) port
53.
The fddi argument is 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 do not follow the pattern: gateway, broadcast, less, greater,
and arithmetic expressions. All of these are described later in this reference page.
More complex filter expressions are built up by using the words and, or, and not to combine primitives. For example:
host foo and not port ftp and not port ftp-data
To save typing, identical qualifier lists can be omitted. For example, the following two lines are treated the same:
tcp dst port ftp or ftp-data or domain tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain
Allowable primitives are: True if the IP destination field of the packet is host, which may be either an address or a name. True if the IP
source field of the packet is host. True if either the IP source or destination of the packet is host. The following keywords can precede
any of these host expressions: ip, arp, or rarp. For example, the following examples are equivalent:
ip host host ether proto ip and host host
If host is a name with multiple IP addresses, each address is checked for a match. True if the Ethernet destination address is
ehost. The ehost may be either a name from /etc/ethers or a number (see ethers(3) for numeric format). True if the Ethernet source
address is ehost. True if either the Ethernet source or destination address is ehost. True if the packet used host as a gateway.
That is, the Ethernet source or destination address was host but neither the IP source nor the IP destination was host. The host
argument must be a name and must be found in both /etc/hosts and /etc/ethers.
The following expression is equivalent:
ether host ehost and not host host
You can use either names or numbers for host and ehost. [IPv4 networks only] True if the IP destination address of the packet has
a network number of net, which may be either an address or a name. [IPv4 networks only] True if the IP source address of the
packet has a network number of net. [IPv4 networks only] True if either the IP source or destination address of the packet has a
network number of net. 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(7) and udp(7)). 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. (For example, dst port 513 prints both TCP login service traffic and
UDP who service traffic, and port domain prints both TCP/DOMAIN and UDP/DOMAIN traffic). True if the packet has a source port value
of port. True if either the source or destination port of the packet is port. The following keywords can precede any of these port
expressions: tcp or udp. For example, the following example matches only TCP packets.
tcp src port port True if the packet has a length less than or equal to length. The following example is equivalent:
len <= length True if the packet has a length greater than or equal to length. The following example is equivalent:
len >= length. True if the packet is an IP packet (see ip(7)) of protocol type protocol. The protocol can be a number or one of the
names ipv6, icmp, icmpv6, udp, nd, or tcp.
Note
The identifiers tcp, udp, and icmp are also keywords and must be escaped via backslash (), which is \ in the C-shell. True if the
packet is an Ethernet broadcast packet. The ether keyword is optional. [IPv4 networks only] 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. True if the
packet is an Ethernet multicast packet. The ether keyword is optional. This is shorthand for `ether[0] & 1 != 0'. [IPv4 networks
only] True if the packet is an IPv4 multicast packet. True if the packet is of ether type protocol. The protocol argument can be a
number or a name like ip, ipv6, arp, or rarp. Note these identifiers are also keywords and must be escaped via backslash (). (In
the case of FDDI (for example, 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. The tcpdump utility 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.) 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.) True if the DECnet destination address is host. True if either the DECnet source or
destination address is host. Abbreviations for:
ether proto p
Where p is one of the above protocols. Abbreviations for:
ether proto p
Where p is one of the above protocols.
Note
The tcpdump utility does not currently know how to parse these protocols. Abbreviations for:
ip proto p
Where p is one of the protocols listed earlier. True if the relation holds, where relop is >, <, >=, <=, =, or !=, 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]
The proto variable is one of ether, fddi, ip, arp, rarp, tcp, udp, or icmp, and indicates the protocol layer for the index opera-
tion. The byte offset, relative to the indicated protocol layer, is given by expr. The size variable 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[2:2] & 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 (and) Alternation (or)
Negation has highest precedence. Alternation and concatenation have equal precedence and associate left to right. Note that explicit and
tokens (not juxtaposition) are required for concatenation.
If an identifier is given without a keyword, the most recent keyword is assumed. For example, the following two examples are equivalent:
not host vs and ace
not host vs and host ace
However, the following example is not equivalent to the previous two:
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. Gener-
ally, if the expression contains shell metacharacters, it is easier to pass it as a single, quoted argument. Multiple arguments are con-
catenated with spaces before being parsed.
DESCRIPTION
The tcpdump utility prints out the headers of packets on a network interface that match the boolean expression. Your kernel must be con-
figured with the packetfilter option. (See packetfilter(7).) After kernel configuration, any user can invoke tcpdump once the superuser
has enabled promiscuous-mode operation using pfconfig(8).
OUTPUT FORMAT
The output of the tcpdump utility is protocol dependent. The following sections describe most of the formats and provide examples.
Link Level Headers
The -e option is used to print the link level header. On Ethernets, the source and destination addresses, protocol, and packet length are
printed.
On FDDI networks, the -e option causes the tcpdump utility 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.
Note
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.
The packet type is printed first. The three types of packets are ip, utcp, and ctcp. No further link information is printed for ip pack-
ets.
For TCP packets, the connection identifier is printed after the type. If the packet is compressed, its encoded header is printed. The spe-
cial cases are printed as *S+n and *SA+n, where n is the amount by which the sequence number (or sequence number and acknowledgement) 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
(acknowledgement), 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.
The following example shows an outbound compressed TCP packet, with an implicit connection identifier; the value of the acknowledgement 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 and RARP output shows the type of request and its arguments. The format is intended to be self explanatory. The following example is
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 indicates that host rtsg sent an ARP packet asking for the Ethernet address of Internet host csam. Host csam replies with
its Ethernet address (in this example, Ethernet addresses are uppercase and Internet addresses in lowercase).
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 you issue the tcpdump -e command, the first packet is explicitly a broadcast packet and the second is a point-to-point packet:
RTSG Broadcast 0806 64: arp who-has csam tell rtsg CSAM RTSG 0806 64: arp reply csam is-at CSAM
For the first packet, the Ethernet source address is RTSG, the destination is the broadcast address, the type field contain hex 0806 (type
ETHER_ARP) and the total length is 64 bytes.
TCP Packets
The following description assumes familiarity with the TCP protocol described in RFC 793.
The general format of a TCP protocol line is:
src > dst: flags data-seqno ack window urgent options
The fields represent the following: The destination IP addresses and ports. The destination IP addresses and ports. The sum combination
of S (SYN), F (FIN), P (PUSH) or R (RST) or a single period (.) for no flags. The portion of sequence space covered by the data in this
packet (see the following example). The sequence number of the next data expected from the other direction on this connection. The number
of bytes of receive buffer space available from the other direction on this connection. Indicates there is urgent data in the packet. The
TCP options enclosed in angle brackets. For example,
<mss 1024>
The src, dst, and flags fields are always present. The other fields depend on the contents of the packet's TCP protocol header and are out-
put only if appropriate.
The following example shows the opening portion of an rlogin session 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 indicates that TCP port 1023 on system 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.
Host csam replies with a similar packet except it includes a piggy-backed ack for the SYN sent by rtsg. Host rtsg then sends an ack reply
to the SYN sent by csam. The period (.) 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 conver-
sation's data stream (with the first data byte each direction being 1). The -S option overrides this feature, causing the original
sequence numbers to be output.
The sixth line indicates that host rtsg sends host csam 19 bytes of data (bytes 2 through 20 in the rtsg to csam side of the conversation).
The PUSH flag is set in the packet. The seventh line indicates that host csam has received data sent by host rtsg up to but not including
byte 21. Most of this data is apparently sitting in the socket buffer because the receive window on host csam is 19 bytes smaller. Host
csam also sends one byte of data to host rtsg in this packet. The eighth and ninth lines show that host csam sends two bytes of urgent,
pushed data to rtsg.
UDP Packets
The UDP format is illustrated by the following rwho packet:
actinide.who > broadcast.who: udp 84
This line of output indicates 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 partic-
ular, Domain Name service requests (RFC 1034 and RFC 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.
Name server requests are formatted as follows:
src > dst: id op? flags qtype qclass name (len)
For example:
h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu.(37)
Host h2opolo queried the domain server on host helios for an address record (qtype=A) associated with the name ucbvax.berkeley.edu. The
query ID was 3. The plus sign (+) 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 field had been anything else,
it would have been printed between the 3 and the plus sign (+). Similarly, the qclass was the normal one, C_IN, and omitted. Any other
qclass would have been printed immediately after the A.
The following 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 and 3, [b2&3=x] is printed, where x is the hex
value of header bytes 2 and 3.
UDP Name Server Responses
Name server responses are formatted as follows:
src > dst: id op rcode flags a/n/au type class data (len)
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, host helios responds to query ID 3 from host 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 is 273
bytes, excluding UDP and IP headers. The op (Query) and response code (NoError) are omitted, as is the class (C_IN) of the A record.
In the second example, host helios responds to query 2 with a response code of nonexistent domain (NXDomain) with no answers, one name
server and no authority records. The asterisk (*) indicates that the authoritative answer bit is set. Since there are no answers, no
type, class or data are printed.
Other flag characters that might appear are the minus sign (-) (recursion available, RA, not set) and vertical bar (|) (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 value of snaplen, 96 bytes, may not capture enough of the
packet to print. Use the -s option to increase the snaplen if you need to seriously investigate name server traffic.
Sun RPC Requests and Replies
Sun RPC (RFC 1057) is decoded, as are several of the protocols that use Sun RPC, listed in the following table:
---------------------------------------------------------
Name Users Description
---------------------------------------------------------
PORTMAP libc.a, portmap Maps RPC program num-
bers to TCP/UDP ports
MOUNT mount, mountd Maps file names to NFS
file handles
NLM rpc.lockd NFS remote file locking
STAT rpc.statd, rpc.lockd Remote status monitor
YP libc.a, ypserv Network Information
Services
YPBIND ypbind, ypset NIS domain manipulation
NFS UNIX Network File System
---------------------------------------------------------
Requests sent using TCP must start at the beginning of a packet to be decoded. Normally they are; however, applications that have multiple
requests outstanding (for example, NFS) may not always do this.
Replies can only be decoded if the request was found and only if they start a packet.
The general form of a RPC request and reply is as follows:
src.xid > dst.prot-v#: len call op args src.xid > dst.prot-v#: len reply op results
For example, NFS mounting a file system generates:
clnt.312dbc68 > svc.pmap-v2: 56 call getport prog "nfs" V3 prot UDP port 0 svc.312dbc68 > clnt.pmap-v2: 28 reply getport 2049
clnt.312deff8 > svc.pmap-v2: 56 call getport prog "mount" V3 prot UDP port 0 svc.312deff8 > clnt.pmap-v2: 28 reply getport 1034
clnt.312deff8 > svc.mount-v3: 124 call mount "/build" svc.312deff8 > clnt.mount-v3: 68 reply mount OSF/1 fh 8,3079/1.2 clnt.907312 >
svc.nfs-v3: 148 call getattr OSF/1 fh 8,3079/1.2 svc.907312 > clnt.nfs-v3: 112 reply getattr {dir size 1024 mtime ... }
In general, the UDP or TCP protocol information is not printed. This is generally not important for UDP; however, it can be for TCP. If
the -m and -v options are in effect, both RPC and TCP decoding are done. For example, a showmount -e srv command generates information
such as the following:
clnt.3123f473 > svc.pmap-v2: 56 call getport prog "mount" V1 prot TCP port 0
(ttl 29, id 19672) svc.3123f473 > clnt.pmap-v2: 28 reply getport 892
(ttl 30, id 31644) clnt.1032 > svc.892: S 25280000:25280000(0) win 32768 <mss 1460,nop,wscale 0>
(DF) (ttl 59, id 19674) svc.892 > clnt.1032: S 483136000:483136000(0) ack 25280001 win 33580
<mss 1460,nop,wscale 0> (ttl 60, id 31645) clnt.1032 > svc.892: . ack 1 win 33580 (DF) (ttl 59, id 19675) clnt.2f221c23 >
svc.mount-v1: 40 call return export list TCP: clnt.1032 > svc.892: P 1:45(44) ack 1 win 33580 (DF) (ttl 59, id 19676) svc.2f221c23 >
clnt.mount-v1: 184 reply export
"/usr": "client" "clnt"
"/build":
... TCP: svc.892 > clnt.1032: P 1:189(188) ack 45 win 33580 (ttl 60, id 31648) clnt.1032 > svc.892: F 45:45(0) ack 189 win 33580
(DF) (ttl 59, id 19679) svc.892 > clnt.1032: . ack 46 win 33580 (ttl 60, id 31649) svc.892 > clnt.1032: F 189:189(0) ack 46 win 33580 (ttl
60, id 31650) clnt.1032 > svc.892: . ack 190 win 33580 (DF) (ttl 59, id 19681)
The following is another NFS sample:
sushi.6709 > wrl.nfs-v2: 112 call readlink fh 21,24/10.731657119 wrl.6709 > sushi.nfs-v2: 40 reply readlink "../var" sushi.201b > wrl.nfs-
v2: 144 call lookup fh 9,74/4096.6878 "xcolors" wrl.201b > sushi.nfs-v2: 128 reply lookup fh 9,74/4134.3150
In the first line, host sushi sends a transaction with ID 6709 to host 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 operation was a readlink (read symbolic link) on file han-
dle (fh) 21,24/10.731657119. (In some cases, the file handle can be interpreted as a major and minor device number pair, followed by the
inode number and generation number.) Host wrl replies ok with the contents of the link.
In the third line, host sushi asks host wrl to look up 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 a protocol specification rpcgen
file.
If the -v (verbose) option is given, additional information is printed.
If the -v option is given more than once, more details may be printed.
Note that RPC requests are very large and much of the detail is not printed. Property list information may also be obtained using tcpdump.
For example:
alpha1.zk3.dec.com.da31fba5 > alpha2.zk3.dec.com.proplist-v3:
276 call proproc3_get OSF/1 fh 8,18434/1.4 mask:-1 11 entries
alpha2.zk3.dec.com.da31fba5 > alpha1.zk3.dec.com.proplist-v3:
296 reply proproc3_get status OK 368 bytes 11 entries
For property list calls, you can request the mask value (see the setproplist(3) reference page) and the number of property list entries.
Property list replies return the status, the number of bytes in the property list and the number of entries in property list.
Note that NFS requests are very large and much of the detail is not printed unless the value of snaplen is increased. Try using -s 192 to
watch RPC traffic.
RPC 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.
NFS and Sun are registered trademarks of Sun Microsystems, Inc.
KIP AppleTalk (DDP in UDP)
AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated and dumped as DDP packets (for example, all the UDP header informa-
tion is discarded). The file /etc/atalk.names is used to translate AppleTalk network numbers and node numbers to names. Lines in this file
have the following form:
number name
For example:
1.254 ether 16.1 icsd-net 1.254.110 ace
The first two lines provide the names of AppleTalk networks. The third line provides the name of a particular host (a host is distin-
guished from a network by the third octet in the number. (A network number must have two octets and a host number must have three octets.)
The number and name are separated by either blanks or tabs. The /etc/atalk.names file may contain blank lines or comment lines (lines
starting with a pound sign (#)).
AppleTalk addresses are printed in the following form:
net.host.port
For example:
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 file does not exist or does not contain an entry for some AppleTalk host or network number, addresses are printed
in numeric form.)
In the first example, the name binding protocol (NBP) (DDP port 2) on network 144.1 node 209 sends to whatever is listening on port 220 of
network icsd node 112. The second line is the same except the full name of the source node is known (office). The third line sends from
port 235 on network jssmag node 149 to broadcast on the icsd-net NBP port. (Note that the broadcast address(255) is indicated by a network
name with no host number. For this reason it is a good idea to keep node names and network names distinct in /etc/atalk.names).
NBP and ATP (AppleTalk transaction protocol) packets have their contents interpreted. Other protocols dump the protocol name (or number if
no name is registered for the protocol) and packet size.
NBP packets are formatted as shown in 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 shows a name lookup request for laserwriters sent by network host icsd 112 and broadcast on network jssmag. The NBP ID for
the lookup is 190. The second line shows a reply to this request (it has the same ID) from host jssmag.209 indicating that it has a laser-
writer resource named RM1140 registered on port 250. The third line shows another reply to the same request indicating 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 > jss-
mag.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
Host jssmag.209 initiates transaction ID 12266 with host helios by requesting up to eight packets (0-7). The hex number at the end of the
line is the value of the userdata field in the request.
Host helios responds with eight 512-byte packets. The :digit following the transaction ID gives the packet sequence number in the transac-
tion and the number in parenthesis is the amount of data in the packet, excluding the ATP header. The asterisk (*) on packet 7 indicates
that the EOM bit was set.
Host jssmag.209 then requests that packets 3 and 5 be retransmitted. Host helios resends them, then jssmag.209 releases the transaction.
Finally, jssmag.209 initiates the next request. The asterisk (*) on the request indicates that exactly once (XO) was not set.
AppleTalk is a registered trademark of Apple Computer, Inc.
IP Fragmentation
Fragmented Internet datagrams are printed as follows:
(frag id:size@offset+) (frag id:size@offset)
The first line indicates there are more fragments. The second indicates this is the last fragment.
The following list explains the fields: The fragment ID The fragment size (in bytes) excluding the IP header The 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 infor-
mation is printed after the protocol information. Fragments after the first contain no higher level protocol header and the fragment
information is printed after the source and destination addresses. The following example shows part of an FTP session from arizona.edu to
lbl-rtsg.arpa over a CSNET connection that does not 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 > ari-
zona.ftp-data: . ack 1536 win 2560
Note the following: Addresses in the second line do not include port numbers. This is because the TCP protocol information is in the first
fragment and we do not know what the port or sequence numbers are when we print the later fragments. TCP sequence information in the first
line is printed as if there were 308 bytes of user data; however, there are 512 bytes (308 in the first fragment and 204 in the second).
If you are looking for holes in the sequence space or trying to match up acknowledgements with packets, this can be misleading.
A packet with the IP `do not 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 following form:
hh:mm:ss.frac
It 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 difference between when the Ethernet interface removed the packet from the wire and when the kernel serviced the new packet
interrupt.
RESTRICTIONS
To watch either outbound or inbound traffic, you need to have enabled copyall mode using the pfconfig command. For example, pfconfig +c
ln0.
Name server inverse queries are not dumped correctly: The (empty) question section is printed rather than real query in the answer section.
A packet trace that crosses a daylight saving time change produces 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.
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 )
Note that to ease typing complex expressions, you can enclose expressions in single quotation marks (` ') to prevent the shell from
processing special characters. For example, the previous example could be entered as follows: 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 hosts (if your network is connected to
one other network by a gateway, the following does not produce any results on your local network): 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 nonlocal 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 or replies (that is, not ping
packets): tcpdump 'icmp[0] != 8 and icmp[0] != 0' To print all RIPv6 packets, enter: tcpdump -i ln0 -s 1500 -envv ipv6 and udp port
521 To print all IPv6 packets arriving at or departing from a host with the Ethernet address a:b:c:d:e:f, enter: tcpdump -i ln0 -s
1500 -envv ipv6 and ether host a:b:c:d:e:f
SEE ALSO
Commands: pfstat(1), nfswatch(8), pfconfig(8), tcpslice(8)
Files: bpf(7), packetfilter(7)
RFC 2080, RFC 2080, Malkin, G., Minnear, R.
tcpdump(8)
