If you enable it and these don't appear, it's likely that your UTM is having issues loading them into the kernel and should contact Sophos Technical Support for assistance. Lsmod grep -i sip The output should look similar to: ipnatsip 6784 0 ipconntracksip 16924 1 ipnatsip Viewing active SIP registrations using tcpdump. Tcpdump less 32: tcpdump greater 128 You can use the symbols for less than, greater than, and less than or equal / greater than or equal signs as well. // filtering for size using symbols: tcpdump 32: tcpdump tcpdump‘s flag field output.
Updated: 21 December 2020IndexReturn to Main Contents
NAME
tcpdump - dump traffic on a networkSYNOPSIS
tcpdump[-AbdDefhHIJKlLnNOpqStuUvxX#] [-Bbuffer_size][-ccount][--count][-Cfile_size]
[-Espi@ipaddr algo:secret,..]
[-Ffile][-Grotate_seconds][-iinterface]
[
--immediate-mode][-jtstamp_type][-mmodule][-Msecret][--number][--print][-Qin|out|inout]
[-rfile][-ssnaplen][-Ttype][--version]
[-Vfile][-wfile][-Wfilecount][-ydatalinktype]
[-zpostrotate-command][-Zuser]
[--time-stamp-precision=tstamp_precision]
[--micro][--nano]
[expression]
DESCRIPTION
Tcpdump prints out a description of the contents of packets on anetwork interface that match the Boolean expression; thedescription is preceded by a time stamp, printed, by default, as hours,minutes, seconds, and fractions of a second since midnight. It can alsobe run with the-wflag, which causes it to save the packet data to a file for lateranalysis, and/or with the-rflag, which causes it to read from a saved packet file rather than toread packets from a network interface. It can also be run with the-Vflag, which causes it to read a list of saved packet files. In all cases,only packets that matchexpressionwill be processed bytcpdump.
Tcpdumpwill, if not run with the-cflag, continue capturing packets until it is interrupted by a SIGINTsignal (generated, for example, by typing your interrupt character,typically control-C) or a SIGTERM signal (typically generated with thekill(1)command); if run with the-cflag, it will capture packets until it is interrupted by a SIGINT orSIGTERM signal or the specified number of packets have been processed.
Whentcpdumpfinishes capturing packets, it will report counts of:
On platforms that support the SIGINFO signal, such as most BSDs(including macOS) and Digital/Tru64 UNIX, it will report those countswhen it receives a SIGINFO signal (generated, for example, by typingyour ``status' character, typically control-T, although on someplatforms, such as macOS, the ``status' character is not set bydefault, so you must set it withstty(1)in order to use it) and will continue capturing packets. On platforms thatdo not support the SIGINFO signal, the same can be achieved by using theSIGUSR1 signal.
Using the SIGUSR2 signal along with the-wflag will forcibly flush the packet buffer into the output file.
Reading packets from a network interface may require that you havespecial privileges; see thepcap(3PCAP)man page for details. Reading a saved packet file doesn't requirespecial privileges.
OPTIONS
For the expression syntax, seepcap-filter(7).
The expression argument can be passed to tcpdump as either a singleShell argument, or as multiple Shell arguments, whichever is more convenient.Generally, if the expression contains Shell metacharacters, such asbackslashes used to escape protocol names, it is easier to pass it asa single, quoted argument rather than to escape the Shellmetacharacters.Multiple arguments are concatenated with spaces before being parsed.
EXAMPLES
To print all packets arriving at or departing from sundown:
To print traffic between helios and either hot or ace:
To print all IP packets between ace and any host except helios:
To print all traffic between local hosts and hosts at Berkeley:
To print all ftp traffic through internet gateway snup:(note that the expression is quoted to prevent the shell from(mis-)interpreting the parentheses):
To print traffic neither sourced from nor destined for local hosts(if you gateway to one other net, this stuff should never make itonto your local net).
To print the start and end packets (the SYN and FIN packets) of eachTCP conversation that involves a non-local host.
To print the TCP packets with flags RST and ACK both set.(i.e. select only the RST and ACK flags in the flags field, and if the resultis 'RST and ACK both set', match)
To print all IPv4 HTTP packets to and from port 80, i.e. print onlypackets that contain data, not, for example, SYN and FIN packets andACK-only packets. (IPv6 is left as an exercise for the reader.)
To print IP packets longer than 576 bytes sent through gateway snup:
To print IP broadcast or multicast packets that werenotsent via Ethernet broadcast or multicast:
To print all ICMP packets that are not echo requests/replies (i.e., notping packets):
OUTPUT FORMAT
The output of tcpdump is protocol dependent.The followinggives a brief description and examples of most of the formats.
Timestamps
By default, all output lines are preceded by a timestamp.The timestampis the current clock time in the form
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 networkinterface finished receiving the packet from the network and when thekernel applied a time stamp to the packet; that time lag could include adelay between the time when the network interface finished receiving apacket from the network and the time when an interrupt was delivered tothe kernel to get it to read the packet and a delay between the timewhen the kernel serviced the `new packet' interrupt and the time when itapplied 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 printthe `frame control' field, the source and destination addresses,and the packet length.(The `frame control' field governs theinterpretation of the rest of the packet.Normal packets (suchas those containing IP datagrams) are `async' packets, with a priorityvalue between 0 and 7; for example, `async4'.Such packetsare assumed to contain an 802.2 Logical Link Control (LLC) packet;the LLC header is printed if it is not an ISO datagram or aso-called SNAP packet.
On Token Ring networks, the '-e' option causes tcpdump to printthe `access control' and `frame control' fields, the source anddestination addresses, and the packet length.As on FDDI networks,packets are assumed to contain an LLC packet.Regardless of whetherthe '-e' option is specified or not, the source routing information isprinted for source-routed packets.
On 802.11 networks, the '-e' option causes tcpdump to printthe `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 withthe SLIP compression algorithm described in RFC-1144.)
On SLIP links, a direction indicator (``I' for inbound, ``O' for outbound),packet type, and compression information are printed out.The packet type is printed first.The three types are ip, utcp, and ctcp.No further link information is printed for ip packets.For TCP packets, the connection identifier is printed following the type.If the packet is compressed, its encoded header is printed out.The special cases are printed out as*S+n and *SA+n, where n is the amount by whichthe sequence number (or sequence number and ack) has changed.If it is not a special case,zero or more changes are printed.A change is indicated by U (urgent pointer), W (window), A (ack),S (sequence number), and I (packet ID), followed by a delta (+n or -n),or a new value (=n).Finally, the amount of data in the packet and compressed header lengthare 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 ofdata and 6 bytes of compressed header:
ARP/RARP Packets
ARP/RARP output shows the type of request and its arguments.Theformat is intended to be self explanatory.Here is a short sample taken from the start of an `rlogin' fromhost rtsg to host csam:
The first line says that rtsg sent an ARP packet askingfor the Ethernet address of internet host csam.Csamreplies with its Ethernet address (in this example, Ethernet addressesare in caps and internet addresses in lower case).This would look less redundant if we had done tcpdump -n:
If we had done tcpdump -e, the fact that the first packet isbroadcast and the second is point-to-point would be visible:
For the first packet this says the Ethernet source address is RTSG, thedestination is the Ethernet broadcast address, the type fieldcontained 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-vflag is specified, information from the IPv4 header is shown inparentheses after the IP or the link-layer header.The general format of this information is:
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 ispart of a fragmented datagram or not.flags are the MF and DF flags; + is reported if MF is set,and DF is reported if F is set. If neither are set, . isreported.proto is the protocol ID field.length is the total length field.options are the IP options, if any.Next, for TCP and UDP packets, the source and destination IP addressesand TCP or UDP ports, with a dot between each IP address and itscorresponding port, will be printed, with a > separating the source anddestination. For other protocols, the addresses will be printed, witha > separating the source and destination. Higher level protocolinformation, if any, will be printed after that.
For fragmented IP datagrams, the first fragment contains the higherlevel protocol header; fragments after the first contain no higher levelprotocol header. Fragmentation information will be printed only withthe-vflag, in the IP header information, as described above.
TCP Packets
(N.B.:The following description assumes familiarity withthe TCP protocol described in RFC-793.If you are not familiarwith the protocol, this description will notbe of much use to you.)
The general format of a TCP protocol line is:
Src and dst are the source and destination IPaddresses 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 coveredby the data in this packet (see example below).Ackno is sequence number of the next data expected the otherdirection on this connection.Window is the number of bytes of receive buffer space availablethe 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 fieldsdepend on the contents of the packet's TCP protocol header andare output only if appropriate.
Here is the opening portion of an rlogin from host rtsg tohost csam.
The first line says that TCP port 1023 on rtsg sent a packetto port loginon 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 `sequencenumbers firstup to but not including last'.)There was no piggy-backed ACK, the available receive window was 4096bytes and there was a max-segment-size option requesting an MSS of1024 bytes.Csam replies with a similar packet except it includes a piggy-backedACK 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 sequence number or length.Note that the ACK sequencenumber is a small integer (1).The first time tcpdump sees aTCP `conversation', it prints the sequence number from the packet.On subsequent packets of the conversation, the difference betweenthe current packet's sequence number and this initial sequence numberis printed.This means that sequence numbers after thefirst can be interpretedas relative byte positions in the conversation's data stream (with thefirst data byte each direction being `1').`-S' will override thisfeature, causing the original sequence numbers to be output.
On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20in 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 tobut not including byte 21.Most of this data is apparently sitting in thesocket 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 capturethe full TCP header, it interprets as much of the header as it canand then reports ``[|tcp]' to indicate the remainder could notbe interpreted.If the header contains a bogus option (one with a lengththat's either too small or beyond the end of the header), tcpdumpreports it as ``[bad opt]' and does not interpret any furtheroptions (since it's impossible to tell where they start).If the headerlength indicates options are present but the IP datagram length is notlong enough for the options to actually be there, tcpdump reportsit 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:
- 2) Recipient responds with SYN, ACK
Recall the structure of a TCP header without options:
A TCP header usually holds 20 octets of data, unless options arepresent.The first line of the graph contains octets 0 - 3, thesecond line shows octets 4 - 7 etc.
Starting to count with 0, the relevant TCP control bits are containedin octet 13:
Let's have a closer look at octet no. 13:
These are the TCP control bits we are interestedin.We have numbered the bits in this octet from 0 to 7, right toleft, 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 arriveswith the SYN bit set in its header:
Looking at thecontrol bits section we see that only bit number 1 (SYN) is set.
Assuming that octet number 13 is an 8-bit unsigned integer innetwork byte order, the binary value of this octet is
This relationship can be expressed as
- tcpdump -i xl0 tcp[13] 2
The expression says 'let the 13th octet of a TCP datagram havethe decimal value 2', which is exactly what we want.
Now, let's assume that we need to capture SYN packets, but wedon't care if ACK or any other TCP control bit is set at thesame time.Let's see what happens to octet 13 when a TCP datagramwith SYN-ACK set arrives:
Now bits 1 and 4 are set in the 13th octet.The binary value ofoctet 13 is
In order to achieve our goal, we need to logically AND thebinary value of octet 13 with some other value to preservethe 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 withthe binary value of a SYN:
We see that this AND operation delivers the same resultregardless whether ACK or another TCP control bit is set.The decimal representation of the AND value as well asthe result of this operation is 2 (binary 00000010),so we know that for packets with SYN set the followingrelation must hold true:
tcpdump -i xl0 'tcp[13] & 2 2'
Some offsets and field values may be expressed as namesrather than as numeric values. For example tcp[13] maybe replaced with tcp[tcpflags]. The following TCP flagfield values are also available: tcp-fin, tcp-syn, tcp-rst,tcp-push, tcp-ack, tcp-urg.
This can be demonstrated as:
Note that you should use single quotes or a backslashin the expression to hide the AND ('&') special characterfrom the shell.
UDP Packets
UDP format is illustrated by this rwho packet:
This says that port who on host actinide sent a UDPdatagram to port who on host broadcast, the Internetbroadcast address.The packet contained 84 bytes of user data.Some UDP services are recognized (from the source or destinationport number) and the higher level protocol information printed.In particular, Domain Name service requests (RFC-1034/1035) and SunRPC calls (RFC-1050) to NFS.
UDP Name Server Requests
(N.B.:The following description assumes familiarity withthe Domain Service protocol described in RFC-1035.If you are not familiarwith the protocol, the following description will appear to be writtenin Greek.)
Name server requests are formatted as
Host h2opolo asked the domain server on helios for anaddress record (qtype=A) associated with the name ucbvax.berkeley.edu.The query id was `3'.The `+' indicates the recursion desired flagwas set.The query length was 37 bytes, not including the UDP andIP protocol headers.The query operation was the normal one, Query,so the op field was omitted.If the op had been anything else, it wouldhave been printed between the `3' and the `+'.Similarly, the qclass was the normal one,C_IN, and omitted.Any other qclass would have been printedimmediately after the `A'.A few anomalies are checked and may result in extra fields enclosed insquare brackets: If a query contains an answer, authority records oradditional records section,ancount,nscount,orarcountare printed as `[na]', `[nn]' or `[nau]' where nis 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
In the first example, helios responds to query id 3 from
h2opolowith 3 answer records, 3 name server records and 7 additional records.The first answer record is type A (address) and its data is internetaddress 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 aresponse code of non-existent domain (NXDomain) with no answers,one name server and no authority records.The `*' indicates thatthe authoritative answer bit was set.Since there were noanswers, 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 dataon UDP/137, UDP/138 and TCP/139.Some primitive decoding of IPX andNetBEUI SMB data is also done.
By default a fairly minimal decode is done, with a much more detaileddecode done if -v is used.Be warned that with -v a single SMB packetmay take up a page or more, so only use -v if you really want all thegory details.
For information on SMB packet formats and what all the fields mean seehttps://download.samba.org/pub/samba/specs/ and other online resources.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:
In the first line, host sushi sends a transaction with id 26377to wrl.The request was 112 bytes,excluding the UDP and IP headers.The operation was a readlink(read symbolic link) on file handle (fh) 21,24/10.731657119.(If one is lucky, as in this case, the file handle can be interpretedas a major,minor device number pair, followed by the inode number andgeneration number.) In the second line, wrl replies `ok' withthe same transaction id and the contents of the link.In the third line, sushi asks (using a new transaction id) wrlto lookup the name `xcolors' in directory file 9,74/4096.6878. Inthe fourth line, wrl sends a reply with the respective transaction id.
Note that the data printeddepends on the operation type.The format is intended to be selfexplanatory if read in conjunction withan NFS protocol spec.Also note that older versions of tcpdump printed NFS packets in aslightly different format: the transaction id (xid) would be printedinstead of the non-NFS port number of the packet.
If the -v (verbose) flag is given, additional information is printed.For example:
(-v also prints the IP header TTL, ID, length, and fragmentation fields,which have been omitted from this example.) In the first line,sushi asks wrl to read 8192 bytes from file 21,11/12.195,at byte offset 24576.Wrl replies `ok'; the packet shown on thesecond line is the first fragment of the reply, and hence is only 1472bytes long (the other bytes will follow in subsequent fragments, butthese fragments do not have NFS or even UDP headers and so might not beprinted, depending on the filter expression used).Because the -v flagis given, some of the file attributes (which are returned in additionto the file data) are printed: the file type (``REG', for regular file),the file mode (in octal), the UID and GID, and the file size.If the -v flag is given more than once, even more details are printed.
NFS reply packets do not explicitly identify the RPC operation.Instead,tcpdump keeps track of ``recent' requests, and matches them to thereplies using the transaction ID.If a reply does not closely follow thecorresponding request, it might not be parsable.
AFS Requests and Replies
Transarc AFS (Andrew File System) requests and replies are printedas:
In the first line, host elvis sends a RX packet to pike.This wasa RX data packet to the fs (fileserver) service, and is the start ofan RPC call.The RPC call was a rename, with the old directory file idof 536876964/1/1 and an old filename of `.newsrc.new', and a new directoryfile id of 536876964/1/1 and a new filename of `.newsrc'.The host pikeresponds with a RPC reply to the rename call (which was successful, becauseit was a data packet and not an abort packet).In general, all AFS RPCs are decoded at least by RPC call name.MostAFS RPCs have at least some of the arguments decoded (generally onlythe `interesting' arguments, for some definition of interesting).
The format is intended to be self-describing, but it will probablynot be useful to people who are not familiar with the workings ofAFS and RX.
If the -v (verbose) flag is given twice, acknowledgement packets andadditional 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 the -v flag is given three times, the security index and service idare printed.
Error codes are printed for abort packets, with the exception of Ubikbeacon packets (because abort packets are used to signify a yes votefor the Ubik protocol).
AFS reply packets do not explicitly identify the RPC operation.Instead,tcpdump keeps track of ``recent' requests, and matches them to thereplies using the call number and service ID.If a reply does not closelyfollow thecorresponding request, it might not be parsable.
KIP AppleTalk (DDP in UDP)
AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulatedand dumped as DDP packets (i.e., all the UDP header information isdiscarded).The file/etc/atalk.namesis used to translate AppleTalk net and node numbers to names.Lines in this file have the form
The first two lines give the names of AppleTalk networks.The thirdline gives the name of a particular host (a host is distinguishedfrom a net by the 3rd octet in the number -a net number must have two octets and a host number musthave three octets.) The number and name should be separated bywhitespace (blanks or tabs).The/etc/atalk.namesfile may contain blank lines or comment lines (lines starting witha `#').AppleTalk addresses are printed in the form
(If the/etc/atalk.namesdoesn't exist or doesn't contain an entry for some AppleTalkhost/net number, addresses are printed in numeric form.)In the first example, NBP (DDP port 2) on net 144.1 node 209is 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 nodeis known (`office').The third line is a send from port 235 onnet jssmag node 149 to broadcast on the icsd-net NBP port (note thatthe broadcast address (255) is indicated by a net name with no hostnumber - for this reason it's a good idea to keep node names andnet names distinct in /etc/atalk.names).NBP (name binding protocol) and ATP (AppleTalk transaction protocol)packets have their contents interpreted.Other protocols just dumpthe protocol name (or number if no name is registered for theprotocol) and packet size.
NBP packets are formatted like the following examples:
The first line is a name lookup request for laserwriters sent by net icsd host112 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 thesame id) from host jssmag.209 saying that it has a laserwriterresource named 'RM1140' registered on port 250.The third line isanother 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 initiates transaction id 12266 with host helios by requestingup to 8 packets (the `<0-7>').The hex number at the end of the lineis the value of the `userdata' field in the request.Helios responds with 8 512-byte packets.The `:digit' following thetransaction id gives the packet sequence number in the transactionand the number in parens is the amount of data in the packet,excluding the ATP header.The `*' on packet 7 indicates that theEOM bit was set.
Jssmag.209 then requests that packets 3 & 5 be retransmitted.Heliosresends them then jssmag.209 releases the transaction.Finally,jssmag.209 initiates the next request.The `*' on the requestindicates that XO (`exactly once') was not set.
SEE ALSO
stty(1),pcap(3PCAP),bpf(4),nit(4P),pcap-savefile(5),pcap-filter(7),pcap-tstamp(7)- https://www.tcpdump.org/
The original distribution is available via anonymous ftp:
On Linux systems with 2.0[.x] kernels:
We recommend that you upgrade to a 2.2 or later kernel.
Some attempt should be made to reassemble IP fragments or, at leastto compute the right length for the higher level protocol.
Name server inverse queries are not dumped correctly: the (empty)question section is printed rather than real query in the answersection.Some believe that inverse queries are themselves a bug andprefer to fix the program generating them rather than tcpdump.
A packet trace that crosses a daylight savings time change will giveskewed time stamps (the time change is ignored).
Filter expressions on fields other than those in Token Ring headers willnot correctly handle source-routed Token Ring packets.
Filter expressions on fields other than those in 802.11 headers will notcorrectly handle 802.11 data packets with both To DS and From DS set.
ip6 protoshould chase header chain, but at this moment it does not.ip6 protochainis 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.
Index
- NAME
- SYNOPSIS
- DESCRIPTION
- OPTIONS
- EXAMPLES
- OUTPUT FORMAT
- SEE ALSO
- AUTHORS
- BUGS
Time: 01:01:40 GMT, January 29, 2021[Valid HTML 4.01][Valid CSS]Section: User Commands (1)
Updated: 21 December 2020
IndexReturn to Main Contents
NAME
tcpdump - dump traffic on a networkSYNOPSIS
tcpdump[-AbdDefhHIJKlLnNOpqStuUvxX#] [-Bbuffer_size][-ccount][
--count][-Cfile_size][-Espi@ipaddr algo:secret,..]
[-Ffile][-Grotate_seconds][-iinterface]
[--immediate-mode][-jtstamp_type][-mmodule]
[-Msecret][--number][--print][-Qin|out|inout]
[-rfile][-ssnaplen][-Ttype][--version]
[-Vfile][-wfile][-Wfilecount][-ydatalinktype]
[-zpostrotate-command][-Zuser]
[--time-stamp-precision=tstamp_precision]
[--micro][--nano]
[expression]
DESCRIPTION
Tcpdump prints out a description of the contents of packets on anetwork interface that match the Boolean expression; thedescription is preceded by a time stamp, printed, by default, as hours,minutes, seconds, and fractions of a second since midnight. It can alsobe run with the-wflag, which causes it to save the packet data to a file for lateranalysis, and/or with the-rflag, which causes it to read from a saved packet file rather than toread packets from a network interface. It can also be run with the-Vflag, which causes it to read a list of saved packet files. In all cases,only packets that matchexpressionwill be processed bytcpdump.
Tcpdumpwill, if not run with the-cflag, continue capturing packets until it is interrupted by a SIGINTsignal (generated, for example, by typing your interrupt character,typically control-C) or a SIGTERM signal (typically generated with thekill(1)command); if run with the-cflag, it will capture packets until it is interrupted by a SIGINT orSIGTERM signal or the specified number of packets have been processed.
Whentcpdumpfinishes capturing packets, it will report counts of:
On platforms that support the SIGINFO signal, such as most BSDs(including macOS) and Digital/Tru64 UNIX, it will report those countswhen it receives a SIGINFO signal (generated, for example, by typingyour ``status' character, typically control-T, although on someplatforms, such as macOS, the ``status' character is not set bydefault, so you must set it withstty(1)in order to use it) and will continue capturing packets. On platforms thatdo not support the SIGINFO signal, the same can be achieved by using theSIGUSR1 signal.
Using the SIGUSR2 signal along with the-wflag will forcibly flush the packet buffer into the output file.
Reading packets from a network interface may require that you havespecial privileges; see thepcap(3PCAP)man page for details. Reading a saved packet file doesn't requirespecial privileges.
OPTIONS
For the expression syntax, seepcap-filter(7).
The expression argument can be passed to tcpdump as either a singleShell argument, or as multiple Shell arguments, whichever is more convenient.Generally, if the expression contains Shell metacharacters, such asbackslashes used to escape protocol names, it is easier to pass it asa single, quoted argument rather than to escape the Shellmetacharacters.Multiple arguments are concatenated with spaces before being parsed.
EXAMPLES
To print all packets arriving at or departing from sundown:
To print traffic between helios and either hot or ace:
To print all IP packets between ace and any host except helios:
To print all traffic between local hosts and hosts at Berkeley:
To print all ftp traffic through internet gateway snup:(note that the expression is quoted to prevent the shell from(mis-)interpreting the parentheses):
To print traffic neither sourced from nor destined for local hosts(if you gateway to one other net, this stuff should never make itonto your local net).
To print the start and end packets (the SYN and FIN packets) of eachTCP conversation that involves a non-local host.
To print the TCP packets with flags RST and ACK both set.(i.e. select only the RST and ACK flags in the flags field, and if the resultis 'RST and ACK both set', match)
To print all IPv4 HTTP packets to and from port 80, i.e. print onlypackets that contain data, not, for example, SYN and FIN packets andACK-only packets. (IPv6 is left as an exercise for the reader.)
To print IP packets longer than 576 bytes sent through gateway snup:
To print IP broadcast or multicast packets that werenotsent via Ethernet broadcast or multicast:
To print all ICMP packets that are not echo requests/replies (i.e., notping packets):
OUTPUT FORMAT
The output of tcpdump is protocol dependent.The followinggives a brief description and examples of most of the formats.
Timestamps
By default, all output lines are preceded by a timestamp.The timestampis the current clock time in the form
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 networkinterface finished receiving the packet from the network and when thekernel applied a time stamp to the packet; that time lag could include adelay between the time when the network interface finished receiving apacket from the network and the time when an interrupt was delivered tothe kernel to get it to read the packet and a delay between the timewhen the kernel serviced the `new packet' interrupt and the time when itapplied 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 printthe `frame control' field, the source and destination addresses,and the packet length.(The `frame control' field governs theinterpretation of the rest of the packet.Normal packets (suchas those containing IP datagrams) are `async' packets, with a priorityvalue between 0 and 7; for example, `async4'.Such packetsare assumed to contain an 802.2 Logical Link Control (LLC) packet;the LLC header is printed if it is not an ISO datagram or aso-called SNAP packet.
On Token Ring networks, the '-e' option causes tcpdump to printthe `access control' and `frame control' fields, the source anddestination addresses, and the packet length.As on FDDI networks,packets are assumed to contain an LLC packet.Regardless of whetherthe '-e' option is specified or not, the source routing information isprinted for source-routed packets.
On 802.11 networks, the '-e' option causes tcpdump to printthe `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 withthe SLIP compression algorithm described in RFC-1144.)
On SLIP links, a direction indicator (``I' for inbound, ``O' for outbound),packet type, and compression information are printed out.The packet type is printed first.The three types are ip, utcp, and ctcp.No further link information is printed for ip packets.For TCP packets, the connection identifier is printed following the type.If the packet is compressed, its encoded header is printed out.The special cases are printed out as*S+n and *SA+n, where n is the amount by whichthe sequence number (or sequence number and ack) has changed.If it is not a special case,zero or more changes are printed.A change is indicated by U (urgent pointer), W (window), A (ack),S (sequence number), and I (packet ID), followed by a delta (+n or -n),or a new value (=n).Finally, the amount of data in the packet and compressed header lengthare 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 ofdata and 6 bytes of compressed header:
ARP/RARP Packets
ARP/RARP output shows the type of request and its arguments.Theformat is intended to be self explanatory.Here is a short sample taken from the start of an `rlogin' fromhost rtsg to host csam:
The first line says that rtsg sent an ARP packet askingfor the Ethernet address of internet host csam.Csamreplies with its Ethernet address (in this example, Ethernet addressesare in caps and internet addresses in lower case).This would look less redundant if we had done tcpdump -n:
If we had done tcpdump -e, the fact that the first packet isbroadcast and the second is point-to-point would be visible:
For the first packet this says the Ethernet source address is RTSG, thedestination is the Ethernet broadcast address, the type fieldcontained 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-vflag is specified, information from the IPv4 header is shown inparentheses after the IP or the link-layer header.The general format of this information is:
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 ispart of a fragmented datagram or not.flags are the MF and DF flags; + is reported if MF is set,and DF is reported if F is set. If neither are set, . isreported.proto is the protocol ID field.length is the total length field.options are the IP options, if any.Next, for TCP and UDP packets, the source and destination IP addressesand TCP or UDP ports, with a dot between each IP address and itscorresponding port, will be printed, with a > separating the source anddestination. For other protocols, the addresses will be printed, witha > separating the source and destination. Higher level protocolinformation, if any, will be printed after that.
For fragmented IP datagrams, the first fragment contains the higherlevel protocol header; fragments after the first contain no higher levelprotocol header. Fragmentation information will be printed only withthe-vflag, in the IP header information, as described above.
TCP Packets
(N.B.:The following description assumes familiarity withthe TCP protocol described in RFC-793.If you are not familiarwith the protocol, this description will notbe of much use to you.)
The general format of a TCP protocol line is:
Src and dst are the source and destination IPaddresses 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 coveredby the data in this packet (see example below).Ackno is sequence number of the next data expected the otherdirection on this connection.Window is the number of bytes of receive buffer space availablethe 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 fieldsdepend on the contents of the packet's TCP protocol header andare output only if appropriate.
Here is the opening portion of an rlogin from host rtsg tohost csam.
The first line says that TCP port 1023 on rtsg sent a packetto port loginon 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 `sequencenumbers firstup to but not including last'.)There was no piggy-backed ACK, the available receive window was 4096bytes and there was a max-segment-size option requesting an MSS of1024 bytes.Csam replies with a similar packet except it includes a piggy-backedACK 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 sequence number or length.Note that the ACK sequencenumber is a small integer (1).The first time tcpdump sees aTCP `conversation', it prints the sequence number from the packet.On subsequent packets of the conversation, the difference betweenthe current packet's sequence number and this initial sequence numberis printed.This means that sequence numbers after thefirst can be interpretedas relative byte positions in the conversation's data stream (with thefirst data byte each direction being `1').`-S' will override thisfeature, causing the original sequence numbers to be output.
On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20in 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 tobut not including byte 21.Most of this data is apparently sitting in thesocket 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 capturethe full TCP header, it interprets as much of the header as it canand then reports ``[|tcp]' to indicate the remainder could notbe interpreted.If the header contains a bogus option (one with a lengththat's either too small or beyond the end of the header), tcpdumpreports it as ``[bad opt]' and does not interpret any furtheroptions (since it's impossible to tell where they start).If the headerlength indicates options are present but the IP datagram length is notlong enough for the options to actually be there, tcpdump reportsit as ``[bad hdr length]'.
Sophos Appliance TCPDUMP – Sophos Ideas
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:
- 2) Recipient responds with SYN, ACK
Recall the structure of a TCP header without options:
A TCP header usually holds 20 octets of data, unless options arepresent.The first line of the graph contains octets 0 - 3, thesecond line shows octets 4 - 7 etc.
Starting to count with 0, the relevant TCP control bits are containedin octet 13:
Let's have a closer look at octet no. 13:
These are the TCP control bits we are interestedin.We have numbered the bits in this octet from 0 to 7, right toleft, 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 arriveswith the SYN bit set in its header:
Looking at thecontrol bits section we see that only bit number 1 (SYN) is set.
Assuming that octet number 13 is an 8-bit unsigned integer innetwork byte order, the binary value of this octet is
This relationship can be expressed as
- tcpdump -i xl0 tcp[13] 2
The expression says 'let the 13th octet of a TCP datagram havethe decimal value 2', which is exactly what we want.
Now, let's assume that we need to capture SYN packets, but wedon't care if ACK or any other TCP control bit is set at thesame time.Let's see what happens to octet 13 when a TCP datagramwith SYN-ACK set arrives:
Now bits 1 and 4 are set in the 13th octet.The binary value ofoctet 13 is
In order to achieve our goal, we need to logically AND thebinary value of octet 13 with some other value to preservethe 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 withthe binary value of a SYN:
We see that this AND operation delivers the same resultregardless whether ACK or another TCP control bit is set.The decimal representation of the AND value as well asthe result of this operation is 2 (binary 00000010),so we know that for packets with SYN set the followingrelation must hold true:
tcpdump -i xl0 'tcp[13] & 2 2'
Some offsets and field values may be expressed as namesrather than as numeric values. For example tcp[13] maybe replaced with tcp[tcpflags]. The following TCP flagfield values are also available: tcp-fin, tcp-syn, tcp-rst,tcp-push, tcp-ack, tcp-urg.
This can be demonstrated as:
Note that you should use single quotes or a backslashin the expression to hide the AND ('&') special characterfrom the shell.
Software engineering for mac. UDP Packets
UDP format is illustrated by this rwho packet:
This says that port who on host actinide sent a UDPdatagram to port who on host broadcast, the Internetbroadcast address.The packet contained 84 bytes of user data.Some UDP services are recognized (from the source or destinationport number) and the higher level protocol information printed.In particular, Domain Name service requests (RFC-1034/1035) and SunRPC calls (RFC-1050) to NFS.
UDP Name Server Requests
(N.B.:The following description assumes familiarity withthe Domain Service protocol described in RFC-1035.If you are not familiarwith the protocol, the following description will appear to be writtenin Greek.)
Name server requests are formatted as
Sophos Xg Tcpdump To File
Host h2opolo asked the domain server on helios for anaddress record (qtype=A) associated with the name ucbvax.berkeley.edu.The query id was `3'.The `+' indicates the recursion desired flagwas set.The query length was 37 bytes, not including the UDP andIP protocol headers.The query operation was the normal one, Query,so the op field was omitted.If the op had been anything else, it wouldhave been printed between the `3' and the `+'.Similarly, the qclass was the normal one,C_IN, and omitted.Any other qclass would have been printedimmediately after the `A'.A few anomalies are checked and may result in extra fields enclosed insquare brackets: If a query contains an answer, authority records oradditional records section,ancount,nscount,orarcountare printed as `[na]', `[nn]' or `[nau]' where nis 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
In the first example, helios responds to query id 3 from h2opolowith 3 answer records, 3 name server records and 7 additional records.The first answer record is type A (address) and its data is internetaddress 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 aresponse code of non-existent domain (NXDomain) with no answers,one name server and no authority records.The `*' indicates thatthe authoritative answer bit was set.Since there were noanswers, 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 dataon UDP/137, UDP/138 and TCP/139.Some primitive decoding of IPX andNetBEUI SMB data is also done.
By default a fairly minimal decode is done, with a much more detaileddecode done if -v is used.Be warned that with -v a single SMB packetmay take up a page or more, so only use -v if you really want all thegory details.
For information on SMB packet formats and what all the fields mean seehttps://download.samba.org/pub/samba/specs/ and other online resources.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:
In the first line, host sushi sends a transaction with id 26377to wrl.The request was 112 bytes,excluding the UDP and IP headers.The operation was a readlink(read symbolic link) on file handle (fh) 21,24/10.731657119.(If one is lucky, as in this case, the file handle can be interpretedas a major,minor device number pair, followed by the inode number andgeneration number.) In the second line, wrl replies `ok' withthe same transaction id and the contents of the link.In the third line, sushi asks (using a new transaction id) wrlto lookup the name `xcolors' in directory file 9,74/4096.6878. Inthe fourth line, wrl sends a reply with the respective transaction id.
Note that the data printeddepends on the operation type.The format is intended to be selfexplanatory if read in conjunction withan NFS protocol spec.Also note that older versions of tcpdump printed NFS packets in aslightly different format: the transaction id (xid) would be printedinstead of the non-NFS port number of the packet.
If the -v (verbose) flag is given, additional information is printed.For example:
(-v also prints the IP header TTL, ID, length, and fragmentation fields,which have been omitted from this example.) In the first line,sushi asks wrl to read 8192 bytes from file 21,11/12.195,at byte offset 24576.Wrl replies `ok'; the packet shown on thesecond line is the first fragment of the reply, and hence is only 1472bytes long (the other bytes will follow in subsequent fragments, butthese fragments do not have NFS or even UDP headers and so might not beprinted, depending on the filter expression used).Because the -v flagis given, some of the file attributes (which are returned in additionto the file data) are printed: the file type (``REG', for regular file),the file mode (in octal), the UID and GID, and the file size.If the -v flag is given more than once, even more details are printed.
NFS reply packets do not explicitly identify the RPC operation.Instead,tcpdump keeps track of ``recent' requests, and matches them to thereplies using the transaction ID.If a reply does not closely follow thecorresponding request, it might not be parsable.
AFS Requests and Replies
Transarc AFS (Andrew File System) requests and replies are printedas:
In the first line, host elvis sends a RX packet to pike.This wasa RX data packet to the fs (fileserver) service, and is the start ofan RPC call.The RPC call was a rename, with the old directory file idof 536876964/1/1 and an old filename of `.newsrc.new', and a new directoryfile id of 536876964/1/1 and a new filename of `.newsrc'.The host pikeresponds with a RPC reply to the rename call (which was successful, becauseit was a data packet and not an abort packet).In general, all AFS RPCs are decoded at least by RPC call name.MostAFS RPCs have at least some of the arguments decoded (generally onlythe `interesting' arguments, for some definition of interesting).
The format is intended to be self-describing, but it will probablynot be useful to people who are not familiar with the workings ofAFS and RX.
If the -v (verbose) flag is given twice, acknowledgement packets andadditional 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 the -v flag is given three times, the security index and service idare printed.
Error codes are printed for abort packets, with the exception of Ubikbeacon packets (because abort packets are used to signify a yes votefor the Ubik protocol).
AFS reply packets do not explicitly identify the RPC operation.Instead,tcpdump keeps track of ``recent' requests, and matches them to thereplies using the call number and service ID.If a reply does not closelyfollow thecorresponding request, it might not be parsable.
KIP AppleTalk (DDP in UDP)
AppleTalk DDP packets encapsulated in UDP datagrams are de-encapsulatedand dumped as DDP packets (i.e., all the UDP header information isdiscarded).The file/etc/atalk.namesis used to translate AppleTalk net and node numbers to names.Lines in this file have the form
The first two lines give the names of AppleTalk networks.The thirdline gives the name of a particular host (a host is distinguishedfrom a net by the 3rd octet in the number -a net number must have two octets and a host number musthave three octets.) The number and name should be separated bywhitespace (blanks or tabs).The/etc/atalk.namesfile may contain blank lines or comment lines (lines starting witha `#').AppleTalk addresses are printed in the form
(If the/etc/atalk.namesdoesn't exist or doesn't contain an entry for some AppleTalkhost/net number, addresses are printed in numeric form.)In the first example, NBP (DDP port 2) on net 144.1 node 209is 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 nodeis known (`office').The third line is a send from port 235 onnet jssmag node 149 to broadcast on the icsd-net NBP port (note thatthe broadcast address (255) is indicated by a net name with no hostnumber - for this reason it's a good idea to keep node names andnet names distinct in /etc/atalk.names).NBP (name binding protocol) and ATP (AppleTalk transaction protocol)packets have their contents interpreted.Other protocols just dumpthe protocol name (or number if no name is registered for theprotocol) and packet size.
NBP packets are formatted like the following examples:
The first line is a name lookup request for laserwriters sent by net icsd host112 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 thesame id) from host jssmag.209 saying that it has a laserwriterresource named 'RM1140' registered on port 250.The third line isanother 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 initiates transaction id 12266 with host helios by requestingup to 8 packets (the `<0-7>').The hex number at the end of the lineis the value of the `userdata' field in the request.Helios responds with 8 512-byte packets.The `:digit' following thetransaction id gives the packet sequence number in the transactionand the number in parens is the amount of data in the packet,excluding the ATP header.The `*' on packet 7 indicates that theEOM bit was set.
Jssmag.209 then requests that packets 3 & 5 be retransmitted.Heliosresends them then jssmag.209 releases the transaction.Finally,jssmag.209 initiates the next request.The `*' on the requestindicates that XO (`exactly once') was not set.
SEE ALSO
stty(1),pcap(3PCAP),bpf(4),nit(4P),pcap-savefile(5),pcap-filter(7),pcap-tstamp(7)- https://www.tcpdump.org/
The original distribution is available via anonymous ftp:
On Linux systems with 2.0[.x] kernels:
We recommend that you upgrade to a 2.2 or later kernel.
Some attempt should be made to reassemble IP fragments or, at leastto compute the right length for the higher level protocol.
Name server inverse queries are not dumped correctly: the (empty)question section is printed rather than real query in the answersection.Some believe that inverse queries are themselves a bug andprefer to fix the program generating them rather than tcpdump.
A packet trace that crosses a daylight savings time change will giveskewed time stamps (the time change is ignored).
Filter expressions on fields other than those in Token Ring headers willnot correctly handle source-routed Token Ring packets.
Filter expressions on fields other than those in 802.11 headers will notcorrectly handle 802.11 data packets with both To DS and From DS set.
ip6 protoshould chase header chain, but at this moment it does not.ip6 protochainis 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.
Index
- NAME
- SYNOPSIS
- DESCRIPTION
- OPTIONS
- EXAMPLES
- OUTPUT FORMAT
- SEE ALSO
- AUTHORS
- BUGS
Time: 01:01:40 GMT, January 29, 2021[Valid HTML 4.01][Valid CSS]