Dump traffic on a network
tcpdump [-aAbdDefhHIJKlLnNOpqStuUvxX#] [ -B size ] [ -c count ]
[ -C file_size ][ -E algo:secret ] [ -F file ]
[ -g fibnum ] [ -G rotate_seconds ] [ -i interface ]
[ -j tstamptype ] [-m module] [ -M secret ] [ -Q direction ]
[ -r file ] [ -s snaplen ] [ -T type ] [ -V file ]
[ -w file ] [ -W filecount ] [ -y linktype ] [ -z command ]
[ -Z user ] [ --time-stamp-precision=tstamp_precision ]
[ --immediate-mode ] [ --version ] [ expression ]
QNX Neutrino
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 modern Windows systems, where the interface name is a somewhat complex string.
The algorithm can be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc, cast128-cbc, or none. The default is des-cbc.
The secret is the ASCII text for the ESP secret key. If preceded by 0x, then a hexadecimal value is read.
The option assumes RFC 2406 ESP, not RFC 1827 ESP. The option is only for debugging purposes, and the use of this option with a true “secret” key is discouraged. By presenting the IPsec secret key onto command line, you make it visible to others, via ps and on other occasions.
In addition to the above syntax, you can use the syntax file_name to have tcpdump read the provided file. The file is opened on receiving the first ESP packet, so any special permissions that tcpdump may have been given should already have been given up.
The test for “foreign” IPv4 addresses is done using the IPv4 address and netmask of the interface on which capture is being done. If that address or netmask isn't 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 won't work correctly.
If used in conjunction with the -C option, file names take the form file<count>.
On Linux systems with 2.2 or later kernels, you can use an interface argument of any to capture packets from all interfaces. Note that captures on the any device aren't done in promiscuous mode.
You can use an interface number as printed by that option as the interface argument.
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.
When writing to a file with the -w option, report, every 10 seconds, the number of packets captured.
When used with the -G option, the -W option limits the number of rotated dump files that get created, exiting with a status of 0 when tcpdump reaches the limit. If you set the -C option as well, tcpdump uses cyclical files per timeslice.
If you want to use a command that itself takes options or different arguments, write a shell script that takes the savefile name as the only argument, arrange the options and arguments as required, and then execute the command that you want.
The tcpdump utility prints a description of the contents of packets on a network interface that match the boolean expression. You can also run it with the -w option, which causes it to save the packet data to a file for later analysis, and/or with the -r option, which causes it to read from a saved packet file rather than to read packets from a network interface. In all cases, tcpdump processes only those packets that match expression.
For information about the expression argument, see “Expressions,” below.
The tcpdump utility, if not run with the -c option, continues capturing packets until it's interrupted by a SIGINT signal (generated, for example, by typing your interrupt character, typically Control–C) or a SIGTERM signal (typically generated with the kill command); if run with the -c option, tcpdump captures packets until it's interrupted by a SIGINT or SIGTERM signal, or the specified number of packets have been processed.
When tcpdump finishes capturing packets, it reports counts of:
You must be root in order to read packets from a network interface. Reading a saved packet file doesn't require special privileges; you just need read permission for the file.
Expressions
The expression on the command line selects which packets to dump. If no expression is given, all packets on the net will be dumped. Otherwise, only packets for which expression is true are dumped.
The expression consists of one or more primitives that usually consist of an ID (name or number) preceded by one or more qualifiers. The different kinds of qualifier are:
For some link layers, such as SLIP and the “cooked” Linux capture mode used for the any device and for some other device types, you can use the inbound and outbound qualifiers to specify a desired direction.
If there's no proto qualifier, all protocols consistent with the type are assumed. For example, src xyz means (ip or arp or rarp) src xyz (except the latter isn't legal syntax), net abc means (ip or arp or rarp) net abc, and port 53 means (tcp or udp) port 53.
The fddi protocol is actually an alias for ether; the parser treats them identically as meaning “the data link level used on the specified network interface.” FDDI headers contain Ethernet-like source and destination addresses, and often contain Ethernet-like packet types, so you can filter on these FDDI fields just as with the analogous Ethernet fields. FDDI headers also contain other fields, but you can't name them explicitly in a filter expression.
Similarly, tr and wlan are aliases for ether; the previous paragraph's statements about FDDI headers also apply to Token Ring and 802.11 wireless LAN headers. For 802.11 headers, the destination address is the DA field and the source address is the SA field; the BSSID, RA, and TA fields aren't tested.
In addition to the above, there are some special “primitive” keywords that don't follow the pattern:
and arithmetic expressions. All of these are described below.
You can build more complex filter expressions by using the words and, or, and not to combine primitives. For example, host xyz and not port ftp and not port ftp-data. To save typing, you can omit identical qualifier lists. For example, 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:
You can prepend any of the above host expressions with the keywords ip, arp, rarp, or ip6 as in:
ip host host
which is equivalent to:
ether proto \ip and host host
If host is a name with multiple IP addresses, each address is checked for a match.
ether host ehost and not host host
which can be used with either names or numbers for host / ehost.) This syntax doesn't work in IPv6-enabled configuration at this moment.
An IPv4 network number can be written as a dotted quad (e.g., 192.168.1.0), dotted triple (e.g., 192.168.1), dotted pair (e.g, 172.16), or single number (e.g., 10); the netmask is 255.255.255.255 for a dotted quad (which means that it's really a host match), 255.255.255.0 for a dotted triple, 255.255.0.0 for a dotted pair, or 255.0.0.0 for a single number.
An IPv6 network number must be written out fully; the netmask is ff:ff:ff:ff:ff:ff:ff:ff, so IPv6 “network” matches are really always host matches, and a network match requires a netmask length.
If you use a name, both the port number and protocol are checked. If you use a number or ambiguous name, only the port number is checked (e.g., dst port 513 will print both tcp/login traffic and udp/who traffic, and port domain will print both tcp/domain and udp/domain traffic).
You can prepend any of the above port or port range expressions with the keywords tcp or udp, as in:
tcp src port port
which matches only tcp packets whose source port is port.
len <= length.
len >= length.
ip6 protochain 6
matches any IPv6 packet with TCP protocol header in the protocol header chain. The packet may contain, for example, authentication header, routing header, or hop-by-hop option header, between IPv6 header and TCP header. The BPF code emitted by this primitive is complex and can't be optimized by BPF optimizer code in tcpdump, so this can be somewhat slow.
If the subnet mask of the interface on which the capture is being done isn't available, either because the interface on which capture is being done has no netmask or because the capture is being done on the Linux “any” interface, which can capture on more than one interface, this check doesn't work correctly.
In the case of FDDI (e.g., fddi protocol arp), Token Ring (e.g., tr protocol arp), and IEEE 802.11 wireless LANs (e.g., wlan protocol arp), for most of those protocols, the protocol identification comes from the 802.2 Logical Link Control (LLC) header, which is usually layered on top of the FDDI, Token Ring, or 802.11 header.
When filtering for most protocol identifiers on FDDI, Token Ring, or 802.11, tcpdump checks only the protocol ID field of an LLC header in so-called SNAP format with an Organizational Unit Identifier (OUI) of 0x000000, for encapsulated Ethernet; it doesn't check whether the packet is in SNAP format with an OUI of 0x000000. The exceptions are:
In the case of Ethernet, tcpdump checks the Ethernet type field for most of those protocols. The exceptions are:
ether proto p
where p is one of the above protocols.
ether proto p
where p is one of the above protocols. Note that tcpdump doesn't currently know how to parse these protocols.
If the specified wlan_type is mgt, then valid wlan_subtypes are assoc-req, assoc-resp, reassoc-req, reassoc-resp, probe-req, probe-resp, beacon, atim, disassoc, auth, and deauth.
If the specified wlan_type is ctl, then valid wlan_subtypes are ps-poll, rts, cts, ack, cf-end, and cf-end-ack.
If the specified wlan_type is data, then valid wlan_subtypes are data, data-cf-ack, data-cf-poll, data-cf-ack-poll, null, cf-ack, cf-poll, cf-ack-poll, qos-data, qos-data-cf-ack, qos-data-cf-poll, qos-data-cf-ack-poll, qos, qos-cf-poll, and qos-cf-ack-poll.
For example:
vlan 100 && vlan 200
filters on VLAN 200 encapsulated within VLAN 100, and:
vlan && vlan 300 && ip
filters IPv4 protocols encapsulated in VLAN 300 encapsulated within any higher order VLAN.
For example:
mpls 100000 && mpls 1024
filters packets with an outer label of 100000 and an inner label of 1024, and:
mpls && mpls 1024 && host 192.9.200.1
filters packets to or from 192.9.200.1 with an inner label of 1024 and any outer label.
For example:
pppoes && ip
filters IPv4 protocols encapsulated in PPPoE.
ip proto p or ip6 proto p
where p is one of the above protocols.
iso proto p
where p is one of the above protocols.
proto [expr : size ]
where proto is one of ether, fddi, tr, wlan, ppp, slip, link, ip, arp, rarp, tcp, udp, icmp, ip6, or radio, and indicates the protocol layer for the index operation (ether, fddi, wlan, tr, ppp, slip, and link all refer to the link layer; radio refers to the “radio header” added to some 802.11 captures).
The byte offset, relative to the indicated protocol layer, is given by expr. The size is optional and indicates the number of bytes in the field of interest; it can be 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 IPv4 packets with options. The expression ip[6:2] & 0x1fff = 0 catches only unfragmented IPv4 datagrams and frag zero of fragmented IPv4 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.
Some offsets and field values may be expressed as names rather than as numeric values. The following protocol header field offsets are available: icmptype (ICMP type field), icmpcode (ICMP code field), and tcpflags (TCP flags field).
The following ICMP type field values are available: icmp-echoreply, icmp-unreach, icmp-sourcequench, icmp-redirect, icmp-echo, icmp-routeradvert, icmp-routersolicit, icmp-timxceed, icmp-paramprob, icmp-tstamp, icmp-tstampreply, icmp-ireq, icmp-ireqreply, icmp-maskreq, and icmp-maskreply.
The following TCP flags field values are available: tcp-fin, tcp-syn, tcp-rst, tcp-push, tcp-ack, tcp-urg.
You can combine primitives by using:
Negation has highest precedence. Alternation and concatenation have equal precedence and associate from left to right. Note that explicit and tokens, not juxtaposition, are now required for concatenation.
If an identifier is given without a keyword, the most recent keyword is assumed. For example:
not host vs and ace
is short for:
not host vs and host ace
which shouldn't be confused with:
not ( host vs or ace )
You can pass expression arguments to tcpdump as either a single argument or as multiple arguments, whichever is more convenient. Generally, if the expression contains shell metacharacters, it's easier to pass it as a single, quoted argument. Multiple arguments are concatenated with spaces before being parsed.
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 you specify the -e option, the link-level header is printed out. On Ethernets, the source and destination addresses, protocol, and packet length are printed.
On FDDI networks, the -e option causes tcpdump to print the frame-control field, the source and destination addresses, and the packet length. The frame-control field governs the interpretation of the rest of the packet. Normal packets (such as those containing IP datagrams) are “async” packets, with a priority value between 0 and 7; for example, async4. Such packets are assumed to contain an 802.2 Logical Link Control (LLC) packet; the LLC header is printed if it is not an ISO datagram or a so-called SNAP packet.
On Token Ring networks, the -e option causes tcpdump to print the access-control and frame-control fields, the source and destination addresses, and the packet length. As on FDDI networks, packets are assumed to contain an LLC packet. Regardless of whether the -e option is specified or not, the source routing information is printed for source-routed packets.
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.
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 isn't a special case, zero or more changes are printed. A change is indicated by U (urgent pointer), W (window), A (ack), S (sequence number), and I (packet ID), followed by a delta (+n or -n), or a new value (=n). Finally, the amount of data in the packet and compressed header length are printed.
For example, the following line shows an outbound compressed TCP packet, with an implicit connection identifier; the ack has changed by 6, the sequence number by 49, and the packet ID by 6; there are 3 bytes of data and 6 bytes of compressed header:
O ctcp * A+6 S+49 I+6 3 (6)
ARP/RARP packets
ARP/RARP output shows the type of request and its arguments. The format is intended to be self-explanatory. Here is a short sample taken from the start of an rlogin from host rtsg to host csam:
arp who-has csam tell rtsg arp reply csam is-at CSAM
The first line says that rtsg sent an arp packet asking for the Ethernet address of Internet host csam. Csam replies with its Ethernet address (in this example, Ethernet addresses are in uppercase, and Internet addresses are 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 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 hexadecimal 0806 (type ETHER_ARP) and the total length was 64 bytes.
TCP Packets
The general format of a tcp protocol line is:
src > dst: flags data-seqno ack window urgent options
Src and dst are the source and destination IP addresses and ports. Flags are some combination of S (SYN), F (FIN), P (PUSH), R (RST), W (ECN CWR) or E (ECN-Echo), or a single . (no flags). Data-seqno describes the portion of sequence space covered by the data in this packet (see example below). Ack is sequence number of the next data expected the other direction on this connection. Window is the number of bytes of receive buffer space available the other direction on this connection. Urg indicates there is urgent data in the packet. Options are tcp options enclosed in angle brackets (e.g., <mss 1024>).
The 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's the opening portion of an rlogin from host rtsg to host csam:
rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024> csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024> rtsg.1023 > csam.login: . ack 1 win 4096 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096 csam.login > rtsg.1023: . ack 2 win 4096 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1
The first line says that tcp port 1023 on rtsg sent a packet to port login on csam. The S indicates that the SYN flag was set. The packet sequence number was 768512 and it contained no data. (The notation is first:last(nbytes), which means “sequence numbers first up to but not including last which is nbytes bytes of user data.”) There was no piggy-backed ack, the available receive window was 4096 bytes and there was a max-segment-size option requesting an mss of 1024 bytes.
Csam replies with a similar packet except it includes a piggy-backed ack for rtsg's SYN. Rtsg then acks csam's SYN. The . means no flags were set. The packet contained no data so there is no data sequence number. Note that the ack sequence number is a small integer (1). The first time tcpdump sees a TCP “conversation”, it prints the sequence number from the packet. On subsequent packets of the conversation, the difference between the current packet's sequence number and this initial sequence number is printed. This means that sequence numbers after the first can be interpreted as relative byte positions in the conversation's data stream (with the first data byte each direction being 1). The -S option overrides this feature, causing the original sequence numbers to be output.
On the sixth 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 seventh 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 eighth and ninth lines, csam sends two bytes of urgent, pushed data to rtsg.
If the snapshot was small enough that tcpdump didn't capture the full TCP header, it interprets as much of the header as it can and then reports “[|tcp]” to indicate the remainder couldn't 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 doesn't 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 isn't 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:
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 the thirteenth octet:
| |
|---------------|
|C|E|U|A|P|R|S|F|
|---------------|
|7 5 3 0|
These are the TCP control bits we're 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 thirteenth 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 thirteenth octet of a TCP datagram have the decimal value 2”, which is exactly what we want.
Now, let's assume that we need to capture SYN packets, but we don't care if ACK or any other TCP control bit is set at the same time. Let's see what happens to octet 13 when a TCP datagram with SYN-ACK set arrives:
|C|E|U|A|P|R|S|F|
|---------------|
|0 0 0 1 0 0 1 0|
|---------------|
|7 6 5 4 3 2 1 0|
Now bits 1 and 4 are set in the thirteenth 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 thirteenth octet with the binary value of a SYN:
00010010 SYN-ACK 00000010 SYN
AND 00000010 (we want SYN) AND 00000010 (we want SYN)
-------- --------
= 00000010 = 00000010
We see that this AND operation delivers the same result regardless whether ACK or another TCP control bit is set. The decimal representation of the AND value as well as the result of this operation is 2 (binary 00000010), so we know that for packets with SYN set the following relation must hold true:
( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )
This points us to the tcpdump filter expression
tcpdump -i xl0 'tcp[13] & 2 == 2'
Note that you should use single quotes or a backslash in the expression to hide the AND ('&') special character from the shell.
UDP Packets
UDP format is illustrated by this rwho packet:
actinide.who > broadcast.who: udp 84
This says that port who on host actinide sent a udp datagram to port who on host broadcast, the Internet broadcast address. The packet contained 84 bytes of user data.
Some UDP services are recognized (from the source or destination port number) and the higher level protocol information printed. In particular, Domain Name service requests (RFCs 1034 and 1035) and Sun RPC calls (RFC 1050) to NFS.
UDP Name Server Requests
Name server requests are formatted as:
src > dst: id op? flags qtype qclass name (len) h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)
Host h2opolo asked the domain server on helios for an address record (qtype=A) associated with the name ucbvax.berkeley.edu. The query ID was 3. The + indicates the recursion desired flag was set. The query length was 37 bytes, not including the UDP and IP protocol headers. The query operation was the normal one, Query, so the op field was omitted. If the op had been anything else, it would have been printed between the 3 and the +. Similarly, the qclass was the normal one, C_IN, and omitted. Any other qclass would have been printed immediately after the A.
A few anomalies are checked and may result in extra fields enclosed in square brackets: If a query contains an answer, authority records or additional records section, ancount, nscount, or arcount are printed as [na], [nn], or [nau], where n is the appropriate count. If any of the response bits are set (AA, RA or rcode) or any of the “must be zero” bits are set in bytes two and three, [b2&3=x] is printed, where x is the hexadecimal value of header bytes two and three.
UDP Name Server Responses
Name server responses are formatted as
src > dst: id op rcode flags a/n/au type class data (len) helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (273) helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
In the first example, helios responds to query ID 3 from h2opolo with 3 answer records, 3 name server records and 7 additional records. The first answer record is type A (address) and its data is Internet address 128.32.137.3. The total size of the response was 273 bytes, excluding UDP and IP headers. The op (Query) and response code (NoError) were omitted, as was the class (C_IN) of the A record.
In the second example, helios responds to query 2 with a response code of non-existent domain (NXDomain) with no answers, one name server and no authority records. The * indicates that the authoritative answer bit was set. Since there were no answers, no type, class or data were printed.
Other flag characters that might appear are - (recursion available, RA, not set) and | (truncated message, TC, set). If the question section doesn't contain exactly one entry, [nq] is printed.
Note that name server requests and responses tend to be large and the default snaplen of 68 bytes may not capture enough of the packet to print. Use the -s flag to increase the snaplen if you need to seriously investigate name server traffic. For example, -s 128.
SMB/CIFS decoding
tcpdump now includes fairly extensive SMB/CIFS/NBT decoding for data on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and NetBEUI SMB data is also done.
By default, a fairly minimal decode is done, with a much more detailed decode done if -v is used. Be warned that with -v a single SMB packet may take up a page or more, so only use -v if you really want all the details.
NFS Requests and Replies
Sun NFS (Network File System) requests and replies are printed as:
src.xid > dst.nfs: len op args src.nfs > dst.xid: reply stat len op results
For example:
sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165 wrl.nfs > sushi.6709: reply ok 40 readlink “../var” sushi.201b > wrl.nfs: 144 lookup fh 9,74/4096.6878 “xcolors” wrl.nfs > sushi.201b: reply ok 128 lookup fh 9,74/4134.3150
In the first line, host sushi sends a transaction with ID 6709 to wrl (note that the number following the source host is a transaction ID, not the source port). The request was 112 bytes, excluding the UDP and IP headers. The operation was a readlink (read symbolic link) on file handle (fh) 21,24/10.731657119. (If you're 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.) The wrl host replies ok with 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. Note that the data printed depends on the operation type. The format is intended to be self-explanatory if read in conjunction with an NFS protocol spec.
If you specify the -v (verbose) option, 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
(The -v option also prints the IP header TTL, ID, length, and fragmentation fields, which have been omitted from this example.) In the first line, sushi asks wrl to read 8192 bytes from file 21,11/12.195, at byte offset 24576. The wrl host 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 don't 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 (expressed as a 6-digit octal number; see the st_mode member in the stat structure), the uid and gid, and the file size.
If you specify more than one -v option, 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 don't 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 doesn't closely follow the corresponding request, it might not be parsable.
AFS Requests and Replies
Transarc AFS (Andrew File System) requests and replies are printed as:
src.sport > dst.dport: rx packet-type
src.sport > dst.dport: rx packet-type service call call-name args
src.sport > dst.dport: rx packet-type service reply call-name args
elvis.7001 > pike.afsfs:
rx data fs call rename old fid 536876964/1/1 “.newsrc.new”
new fid 536876964/1/1 “.newsrc”
pike.afsfs > elvis.7001: rx data fs reply rename
In the first line, host elvis sends a RX packet to pike. This was a RX data packet to the fs (fileserver) service, and is the start of an RPC call. The RPC call was a rename, with the old directory file ID of 536876964/1/1 and an old filename of .newsrc.new, and a new directory file ID of 536876964/1/1 and a new filename of .newsrc. The host pike responds with a RPC reply to the rename call (which was successful, because it was a data packet and not an abort packet).
In general, all AFS RPCs are decoded at least by RPC call name. Most AFS RPCs have at least some of the arguments decoded (generally only the “interesting” arguments, for some definition of interesting).
The format is intended to be self-describing, but it will probably not be useful to people who aren't familiar with the workings of AFS and RX.
If the -v (verbose) flag is given twice, acknowledgment 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 negotiation information is also printed from RX ack packets.
If you specify the -v option 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 don't explicitly identify the RPC operation. Instead, tcpdump keeps track of “recent” requests, and matches them to the replies using the call number and service ID. If a reply doesn't closely follow the corresponding request, it might not be parsable.
KIP AppleTalk (DDP in UDP)
AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulated and dumped as DDP packets (i.e., all the UDP header information is discarded). The file /etc/atalk.names is used to translate AppleTalk net and node numbers to names. Lines in this file have the form:
number name 1.254 ether 16.1 icsd-net 1.254.110 ace
The first two lines give the names of AppleTalk networks. The third line gives the name of a particular host. (A host is distinguished from a net by the third octet in the number; a net number must have two octets and a host number must have three octets.) The number and name should be separated by whitespace (blanks or tabs). The /etc/atalk.names file may contain blank lines or comment lines (lines starting with a #).
AppleTalk addresses are printed in the form:
net.host.port 144.1.209.2 > icsd-net.112.220 office.2 > icsd-net.112.220 jssmag.149.235 > icsd-net.2
(If the /etc/atalk.names doesn't exist or doesn't contain an entry for some AppleTalk host/net number, addresses are printed in numeric form.) In the first example, NBP (DDP port 2) on net 144.1 node 209 is sending to whatever is listening on port 220 of net icsd node 112. The second line is the same except the full name of the source node is known (office). The third line is a send from port 235 on net jssmag node 149 to broadcast on the icsd-net NBP port (note that the broadcast address (255) is indicated by a net name with no host number; for this reason it's a good idea to keep node names and net names distinct in /etc/atalk.names).
NBP (name binding protocol) and ATP (AppleTalk transaction protocol) packets have their contents interpreted. Other protocols just dump the protocol name (or number if no name is registered for the protocol) and packet size.
NBP packets are formatted like the following examples:
icsd-net.112.220 > jssmag.2: nbp-lkup 190: “=:LaserWriter@*” jssmag.209.2 > icsd-net.112.220: nbp-reply 190: “RM1140:LaserWriter@*” 250 techpit.2 > icsd-net.112.220: nbp-reply 190: “techpit:LaserWriter@*” 186
The first line is a name-lookup request for laserwriters sent by net icsd host 112 and broadcast on net jssmag. The nbp ID for the lookup is 190. The second line shows a reply for this request (note that it has the same id) from host jssmag.209 saying that it has a laserwriter resource named RM1140 registered on port 250. The third line is another reply to the same request saying host techpit has laserwriter techpit registered on port 186.
ATP packet formatting is demonstrated by the following example:
jssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
The jssmag.209 host initiates transaction ID 12266 with host helios by requesting up to 8 packets (the <0-7>). The hexadecimal number at the end of the line is the value of the userdata field in the request.
The helios host responds with 8 512-byte packets. The :digit following the transaction ID gives the packet sequence number in the transaction and the number in parentheses is the amount of data in the packet, excluding the ATP header. The * on packet 7 indicates that the EOM bit was set.
The jssmag.209 host then requests that packets 3 and 5 be retransmitted; helios resends them, and then jssmag.209 releases the transaction. Finally, jssmag.209 initiates the next request. The * on the request indicates that XO (“exactly once”) wasn't set.
IP Fragmentation
Fragmented Internet datagrams are printed as:
(frag id:size@offset+) (frag id:size@offset)
The first form indicates there are more fragments. The second indicates this is the last fragment.
Id is the fragment id. Size is the fragment size (in bytes) excluding the IP header. Offset is this fragment's offset (in bytes) in the original datagram.
The fragment information is output for each fragment. The first fragment contains the higher level protocol header and the frag info is printed after the protocol info. Fragments after the first contain no higher level protocol header and the frag info is printed after the source and destination addresses. For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa over a CSNET connection that doesn't appear to handle 576 byte datagrams:
arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+) arizona > rtsg: (frag 595a:204@328) rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560
There are a couple of things to note here: First, addresses in the second 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 io-pkt 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 io-pkt serviced the “new packet” interrupt.
Print all packets arriving at or departing from sundown:
tcpdump host sundown
Print traffic between helios and either hot or ace:
tcpdump host helios and \( hot or ace \)
Print all IP packets between ace and any host except helios:
tcpdump ip host ace and not helios
Print all traffic between local hosts and hosts at Berkeley:
tcpdump net ucb-ether
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)'
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
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'
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)'
Print IP packets longer than 576 bytes sent through gateway snup:
tcpdump 'gateway snup and ip[2:2] > 576'
Print IP broadcast or multicast packets that weren't sent via Ethernet broadcast or multicast:
tcpdump 'ether[0] & 1 = 0 and ip[16] >= 224'
Print all ICMP packets that aren't echo requests/replies (i.e., not ping packets):
tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
The original authors are Van Jacobson, Craig Leres, and Steven McCanne, all of the Lawrence Berkeley National Laboratory, University of California, Berkeley, CA. The tcpdump utility is currently maintained by tcpdump.org.
IPv6/IPsec support is added by WIDE/KAME project. This program uses Eric Young's SSLeay library, under specific configuration.