tcpdump
dump traffic on a network
TLDR
List available network interfaces
Capture the traffic of a specific interface
Capture all TCP traffic showing contents (ASCII) in console
Capture the traffic from or to a host
Capture the traffic from a specific interface, source, destination and destination port
Capture the traffic of a network
Capture all traffic except traffic over port 22 and save to a dump file
Read from a given dump file
SYNOPSIS
tcpdump [ -AbdDefhHIJKlLnNOpqStuUvxX# ] [ -B buffer_size ] [ -c count ] [ -C file_size ] [ -G rotate_seconds ] [ -F file ] [ -i interface ] [ -j tstamp_type ] [ -m module ] [ -M secret ] [ --number ] [ -Q in|out|inout ] [ -r file ] [ -V file ] [ -s snaplen ] [ -T type ] [ -w file ] [ -W filecount ] [ -E spi@ipaddr algo:secret,... ] [ -y datalinktype ] [ -z postrotate-command ] [ -Z user ] [ --time-stamp-precision=tstamp_precision ] [ --immediate-mode ] [ --version ] [ expression ]
DESCRIPTION
Tcpdump prints out a description of the contents of packets on a net‐ work interface that match the boolean expression; the description is preceded by a time stamp, printed, by default, as hours, minutes, sec‐ onds, and fractions of a second since midnight. It can also be run with the -w flag, which causes it to save the packet data to a file for later analysis, and/or with the -r flag, which causes it to read from a saved packet file rather than to read packets from a network interface. It can also be run with the -V flag, which causes it to read a list of saved packet files. In all cases, only packets that match expression will be processed by tcpdump. Tcpdump will, if not run with the -c flag, continue capturing packets until it is interrupted by a SIGINT signal (generated, for example, by typing your interrupt character, typically control-C) or a SIGTERM sig‐ nal (typically generated with the kill(1) command); if run with the -c flag, it will capture packets until it is interrupted by a SIGINT or SIGTERM signal or the specified number of packets have been processed. When tcpdump finishes capturing packets, it will report counts of: packets ``captured'' (this is the number of packets that tcpdump has received and processed); packets ``received by filter'' (the meaning of this depends on the OS on which you're running tcpdump, and possibly on the way the OS was configured - if a filter was specified on the command line, on some OSes it counts packets regardless of whether they were matched by the filter expression and, even if they were matched by the filter expression, regardless of whether tcpdump has read and processed them yet, on other OSes it counts only packets that were matched by the filter expression regardless of whether tcpdump has read and processed them yet, and on other OSes it counts only packets that were matched by the filter ex‐ pression and were processed by tcpdump); packets ``dropped by kernel'' (this is the number of packets that were dropped, due to a lack of buffer space, by the packet capture mechanism in the OS on which tcpdump is running, if the OS reports that information to applications; if not, it will be reported as 0). On platforms that support the SIGINFO signal, such as most BSDs (in‐ cluding Mac OS X) and Digital/Tru64 UNIX, it will report those counts when it receives a SIGINFO signal (generated, for example, by typing your ``status'' character, typically control-T, although on some plat‐ forms, such as Mac OS X, the ``status'' character is not set by de‐ fault, so you must set it with stty(1) in order to use it) and will continue capturing packets. On platforms that do not support the SIG‐ INFO signal, the same can be achieved by using the SIGUSR1 signal. Reading packets from a network interface may require that you have spe‐ cial privileges; see the pcap (3PCAP) man page for details. Reading a saved packet file doesn't require special privileges.
OPTIONS
-A Print each packet (minus its link level header) in ASCII. Handy
for capturing web pages.
-b Print the AS number in BGP packets in ASDOT notation rather than
ASPLAIN notation.
-B buffer_size
--buffer-size=buffer_size
Set the operating system capture buffer size to buffer_size, in
units of KiB (1024 bytes).
-c count
Exit after receiving count packets.
-C file_size
Before writing a raw packet to a savefile, check whether the
file is currently larger than file_size and, if so, close the
current savefile and open a new one. Savefiles after the first
savefile will have the name specified with the -w flag, with a
number after it, starting at 1 and continuing upward. The units
of file_size are millions of bytes (1,000,000 bytes, not
1,048,576 bytes).
-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).
-D
--list-interfaces
Print the list of the network interfaces available on the system
and on which tcpdump can capture packets. For each network in‐
terface, a number and an interface name, possibly followed by a
text description of the interface, is printed. The interface
name or the number can be supplied to the -i flag to specify an
interface on which to capture.
This can be useful on systems that don't have a command to list
them (e.g., Windows systems, or UNIX systems lacking ifconfig
-a); the number can be useful on Windows 2000 and later systems,
where the interface name is a somewhat complex string.
The -D flag will not be supported if tcpdump was built with an
older version of libpcap that lacks the pcap_findalldevs() func‐
tion.
-e Print the link-level header on each dump line. This can be
used, for example, to print MAC layer addresses for protocols
such as Ethernet and IEEE 802.11.
-E Use spi@ipaddr algo:secret for decrypting IPsec ESP packets that
are addressed to addr and contain Security Parameter Index value
spi. This combination may be repeated with comma or newline sep‐
aration.
Note that setting the secret for IPv4 ESP packets is supported
at this time.
Algorithms may be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc,
cast128-cbc, or none. The default is des-cbc. The ability to
decrypt packets is only present if tcpdump was compiled with
cryptography enabled.
secret is the ASCII text for ESP secret key. If preceded by 0x,
then a hex value will be read.
The option assumes RFC2406 ESP, not RFC1827 ESP. The option is
only for debugging purposes, and the use of this option with a
true `secret' key is discouraged. By presenting IPsec secret
key onto command line you make it visible to others, via ps(1)
and other occasions.
In addition to the above syntax, the syntax file name may be
used to have tcpdump read the provided file in. The file is
opened upon receiving the first ESP packet, so any special per‐
missions that tcpdump may have been given should already have
been given up.
-f Print `foreign' IPv4 addresses numerically rather than symboli‐
cally (this option is intended to get around serious brain dam‐
age in Sun's NIS server — usually it hangs forever translating
non-local internet numbers).
The test for `foreign' IPv4 addresses is done using the IPv4 ad‐
dress and netmask of the interface on which capture is being
done. If that address or netmask are not available, available,
either because the interface on which capture is being done has
no address or netmask or because the capture is being done on
the Linux "any" interface, which can capture on more than one
interface, this option will not work correctly.
-F file
Use file as input for the filter expression. An additional ex‐
pression given on the command line is ignored.
-G rotate_seconds
If specified, rotates the dump file specified with the -w option
every rotate_seconds seconds. Savefiles will have the name
specified by -w which should include a time format as defined by
strftime(3). If no time format is specified, each new file will
overwrite the previous.
If used in conjunction with the -C option, filenames will take
the form of `file
EXAMPLES
To print all packets arriving at or departing from sundown: tcpdump host sundown To print traffic between helios and either hot or ace: tcpdump host helios and \( hot or ace \) To print all IP packets between ace and any host except helios: tcpdump ip host ace and not helios To print all traffic between local hosts and hosts at Berkeley: tcpdump net ucb-ether To print all ftp traffic through internet gateway snup: (note that the expression is quoted to prevent the shell from (mis-)interpreting the parentheses): tcpdump 'gateway snup and (port ftp or ftp-data)' To print traffic neither sourced from nor destined for local hosts (if you gateway to one other net, this stuff should never make it onto your local net). tcpdump ip and not net localnet To print the start and end packets (the SYN and FIN packets) of each TCP conversation that involves a non-local host. tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet' To print all IPv4 HTTP packets to and from port 80, i.e. print only packets that contain data, not, for example, SYN and FIN packets and ACK-only packets. (IPv6 is left as an exercise for the reader.) tcpdump 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)' 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 Eth‐ ernet 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[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
OUTPUT FORMAT
The output of tcpdump is protocol dependent. The following gives a brief description and examples of most of the formats. Timestamps By default, all output lines are preceded by a timestamp. The time‐ stamp 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 applied a time stamp to the packet. No attempt is made to account for the time lag between when the network interface finished receiving the packet from the network and when the kernel applied a time stamp to the packet; that time lag could include a delay between the time when the network interface finished receiving a packet from the network and the time when an interrupt was delivered to the kernel to get it to read the packet and a delay between the time when the ker‐ nel serviced the `new packet' interrupt and the time when it applied a time stamp to the packet. 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 data‐ grams) are `async' packets, with a priority value between 0 and 7; for example, `async4'. Such packets are assumed to contain an 802.2 Logi‐ cal 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 as‐ sumed 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. On 802.11 networks, the '-e' option causes tcpdump to print the `frame control' fields, all of the addresses in the 802.11 header, and the packet length. As on FDDI networks, packets are assumed to contain an LLC packet. (N.B.: The following description assumes familiarity with the SLIP com‐ pression algorithm described in RFC-1144.) On SLIP links, a direction indicator (``I'' for inbound, ``O'' for out‐ bound), 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 pack‐ ets, 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 in‐ dicated by U (urgent pointer), W (window), A (ack), S (sequence num‐ ber), 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 for‐ mat 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 Ether‐ net address of internet host csam. Csam replies with its Ethernet ad‐ dress (in this example, Ethernet addresses are in caps and internet ad‐ dresses 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. IPv4 Packets If the link-layer header is not being printed, for IPv4 packets, IP is printed after the time stamp. If the -v flag is specified, information from the IPv4 header is shown in parentheses after the IP or the link-layer header. The general for‐ mat of this information is: tos tos, ttl ttl, id id, offset offset, flags [flags], proto proto, length length, options (options) tos is the type of service field; if the ECN bits are non-zero, those are reported as ECT(1), ECT(0), or CE. ttl is the time-to-live; it is not reported if it is zero. id is the IP identification field. offset is the fragment offset field; it is printed whether this is part of a fragmented datagram or not. flags are the MF and DF flags; + is re‐ ported if MF is set, and DFP is reported if F is set. If neither are set, . is reported. proto is the protocol ID field. length is the to‐ tal length field. options are the IP options, if any. Next, for TCP and UDP packets, the source and destination IP addresses and TCP or UDP ports, with a dot between each IP address and its corre‐ sponding port, will be printed, with a > separating the source and des‐ tination. For other protocols, the addresses will be printed, with a > separating the source and destination. Higher level protocol informa‐ tion, if any, will be printed after that. For fragmented IP datagrams, the first fragment contains the higher level protocol header; fragments after the first contain no higher level protocol header. Fragmentation information will be printed only with the -v flag, in the IP header information, as described above. TCP Packets (N.B.:The following description assumes familiarity with the TCP proto‐ col described in RFC-793. If you are not familiar with the protocol, this description will not be of much use to you.) The general format of a TCP protocol line is: src > dst: Flags [tcpflags], seq data-seqno, ack ackno, win window, urg urgent, options [opts], length len Src and dst are the source and destination IP addresses and ports. Tcpflags are some combination of S (SYN), F (FIN), P (PUSH), R (RST), U (URG), W (ECN CWR), E (ECN-Echo) or `.' (ACK), or `none' if no flags are set. Data-seqno describes the portion of sequence space covered by the data in this packet (see example below). Ackno is sequence number of the next data expected the other direction on this connection. Win‐ dow 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. Opts are TCP options (e.g., mss 1024). Len is the length of payload data. Iptype, Src, dst, and flags are always present. The other fields de‐ pend 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. IP rtsg.1023 > csam.login: Flags [S], seq 768512:768512, win 4096, opts [mss 1024] IP csam.login > rtsg.1023: Flags [S.], seq, 947648:947648, ack 768513, win 4096, opts [mss 1024] IP rtsg.1023 > csam.login: Flags [.], ack 1, win 4096 IP rtsg.1023 > csam.login: Flags [P.], seq 1:2, ack 1, win 4096, length 1 IP csam.login > rtsg.1023: Flags [.], ack 2, win 4096 IP rtsg.1023 > csam.login: Flags [P.], seq 2:21, ack 1, win 4096, length 19 IP csam.login > rtsg.1023: Flags [P.], seq 1:2, ack 21, win 4077, length 1 IP csam.login > rtsg.1023: Flags [P.], seq 2:3, ack 21, win 4077, urg 1, length 1 IP csam.login > rtsg.1023: Flags [P.], seq 3:4, ack 21, win 4077, urg 1, length 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' which means `sequence numbers first up to but not includ‐ ing last.) 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 the ACK flag was set. The packet contained no data so there is no data se‐ quence number or length. 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 sit‐ ting 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 re‐ ports ``[|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 dec‐ imal 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 representa‐ tion of the AND value as well as the result of this operation is 2 (bi‐ nary 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' Some offsets and field values may be expressed as names rather than as numeric values. For example tcp[13] may be replaced with tcp[tcpflags]. The following TCP flag field values are also available: tcp-fin, tcp- syn, tcp-rst, tcp-push, tcp-act, tcp-urg. This can be demonstrated as: tcpdump -i xl0 'tcp[tcpflags] & tcp-push != 0' 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 con‐ tained 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 particu‐ lar, 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 head‐ ers. 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 ad‐ dress 128.32.137.3. The total size of the response was 273 bytes, ex‐ cluding 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. 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 Net‐ BEUI 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. For information on SMB packet formats and what all the fields mean see www.cifs.org or the pub/samba/specs/ directory on your favorite 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.sport > dst.nfs: NFS request xid xid len op args src.nfs > dst.dport: NFS reply xid xid reply stat len op results sushi.1023 > wrl.nfs: NFS request xid 26377 112 readlink fh 21,24/10.73165 wrl.nfs > sushi.1023: NFS reply xid 26377 reply ok 40 readlink "../var" sushi.1022 > wrl.nfs: NFS request xid 8219 144 lookup fh 9,74/4096.6878 "xcolors" wrl.nfs > sushi.1022: NFS reply xid 8219 reply ok 128 lookup fh 9,74/4134.3150 In the first line, host sushi sends a transaction with id 26377 to wrl. The request was 112 bytes, excluding the UDP and IP headers. The oper‐ ation 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.) In the second line, wrl replies `ok' with the same transaction id and the contents of the link. In the third line, sushi asks (using a new transaction id) wrl to lookup the name `xcolors' in directory file 9,74/4096.6878. In the fourth line, wrl sends a reply with the respective transaction id. 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. Also note that older versions of tcpdump printed NFS packets in a slightly different format: the transaction id (xid) would be printed instead of the non-NFS port number of the packet. If the -v (verbose) flag is given, additional information is printed. For example: sushi.1023 > wrl.nfs: NFS request xid 79658 148 read fh 21,11/12.195 8192 bytes @ 24576 wrl.nfs > sushi.1023: NFS reply xid 79658 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 off‐ set 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. In‐ stead, tcpdump keeps track of ``recent'' requests, and matches them to the replies using the transaction ID. If a reply does not closely fol‐ low 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 success‐ ful, 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 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 RX call ID, serial number, and the RX packet flags. The MTU ne‐ gotiation 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. In‐ stead, 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 dis‐ carded). 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 (`of‐ fice'). 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 an‐ other 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 request‐ ing 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, jss‐ mag.209 initiates the next request. The `*' on the request indicates that XO (`exactly once') was not set.
AUTHORS
The original authors are: Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley National Laboratory, University of California, Berkeley, CA. It is currently being maintained by tcpdump.org. The current version is available via http: http://www.tcpdump.org/ The original distribution is available via anonymous ftp: ftp://ftp.ee.lbl.gov/old/tcpdump.tar.Z IPv6/IPsec support is added by WIDE/KAME project. This program uses Eric Young's SSLeay library, under specific configurations.
BUGS
To report a security issue please send an e-mail to security@tcpdump.org. To report bugs and other problems, contribute patches, request a fea‐ ture, provide generic feedback etc please see the file CONTRIBUTING in the tcpdump source tree root. NIT doesn't let you watch your own outbound traffic, BPF will. We rec‐ ommend that you use the latter. On Linux systems with 2.0[.x] kernels: packets on the loopback device will be seen twice; packet filtering cannot be done in the kernel, so that all pack‐ ets must be copied from the kernel in order to be filtered in user mode; all of a packet, not just the part that's within the snapshot length, will be copied from the kernel (the 2.0[.x] packet cap‐ ture mechanism, if asked to copy only part of a packet to user‐ land, will not report the true length of the packet; this would cause most IP packets to get an error from tcpdump); capturing on some PPP devices won't work correctly. We recommend that you upgrade to a 2.2 or later kernel. 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) ques‐ tion section is printed rather than 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 savings time change will give skewed time stamps (the time change is ignored). Filter expressions on fields other than those in Token Ring headers will not correctly handle source-routed Token Ring packets. Filter expressions on fields other than those in 802.11 headers will not correctly handle 802.11 data packets with both To DS and From DS set. ip6 proto should chase header chain, but at this moment it does not. ip6 protochain is supplied for this behavior. Arithmetic expression against transport layer headers, like tcp[0], does not work against IPv6 packets. It only looks at IPv4 packets. 2 February 2017 TCPDUMP(8)
SEE ALSO
stty(1), pcap(3PCAP), bpf(4), nit(4P), pcap-savefile(5), pcap-fil‐ ter(7), pcap-tstamp(7) http://www.iana.org/assignments/media-types/application/vnd.tcp‐ dump.pcap
