Tcpdump Sophos Xg

Section: User Commands (1)
Updated: 21 December 2020

How to: Dump on XG. I want to share my experience in dumping on XG with you. Most of the time, i have to write down, how it is done, so i will summarize it here. First of all, get a SSH Session to your XG. You have to use the 'admin' to login. Switch to Advanced Shell (Option 5. We can now perform a tcpdump. When attempting to upload to Sophos XG it requires the key be in key format. I ran an openssl command to convert privkey.pem to privkey.key. I then went to Sophos XG to upload thinking all would be well. Unfortunately I received this error:' Certificate could not be uploaded due to invalid private key or passphrase. Choose a proper key '.

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tcpdump - dump traffic on a network


tcpdump[-AbdDefhHIJKlLnNOpqStuUvxX#] [-Bbuffer_size]
[-E[email protected] algo:secret,..]
[-Msecret][--number][--print][-Qin out inout]


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:

tcpdumphas received and processed);
packets ``received by filter' (the meaning of this depends on the OS onwhich you're runningtcpdump,and possibly on the way the OS was configured - if a filter wasspecified on the command line, on some OSes it counts packets regardlessof whether they were matched by the filter expression and, even if theywere matched by the filter expression, regardless of whethertcpdumphas read and processed them yet, on other OSes it counts only packets that werematched by the filter expression regardless of whethertcpdumphas read and processed them yet, and on other OSes it counts onlypackets that were matched by the filter expression and were processed bytcpdump);
packets ``dropped by kernel' (this is the number of packets that weredropped, due to a lack of buffer space, by the packet capture mechanismin the OS on whichtcpdumpis running, if the OS reports that information to applications; if not,it will be reported as 0).

On platforms that support the SIGINFO signal, such as most BSDs(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.


Print each packet (minus its link level header) in ASCII. Handy forcapturing web pages.
Print the AS number in BGP packets in ASDOT notation rather than ASPLAINnotation.
-B buffer_size
Set the operating system capture buffer size to buffer_size, inunits of KiB (1024 bytes).
-c count
Exit after receiving count packets.
Print only on stderr the packet count when reading capture file(s) insteadof parsing/printing the packets. If a filter is specified on the commandline, tcpdump counts only packets that were matched by the filterexpression.
-C file_size
Before writing a raw packet to a savefile, check whether the file iscurrently larger than file_size and, if so, close the currentsavefile and open a new one. Savefiles after the first savefile willhave the name specified with the-wflag, with a number after it, starting at 1 and continuing upward.The units of file_size are millions of bytes (1,000,000 bytes,not 1,048,576 bytes).
Dump the compiled packet-matching code in a human readable form tostandard output and stop.
Please mind that although code compilation is always DLT-specific,typically it is impossible (and unnecessary) to specify which DLT to usefor the dump because tcpdump uses either the DLT of the input pcapfile specified with-r,or the default DLT of the network interface specified with-i,or the particular DLT of the network interface specified with-yand-irespectively. In these cases the dump shows the same exact code thatwould filter the input file or the network interface without-d.
However, when neither-rnor-iis specified, specifying-dprevents tcpdump from guessing a suitable network interface (see-i).In this case the DLT defaults to EN10MB and can be set to another validvalue manually with-y.
Dump packet-matching code as aCprogram fragment.
Dump packet-matching code as decimal numbers (preceded with a count).
Print the list of the network interfaces available on the system and onwhichtcpdumpcan capture packets. For each network interface, a number and aninterface name, possibly followed by a text description of theinterface, are printed. The interface name or the number can be suppliedto the-iflag to specify an interface on which to capture.
This can be useful on systems that don't have a command to list them(e.g., Windows systems, or UNIX systems lackingifconfig -a);the number can be useful on Windows 2000 and later systems, where theinterface name is a somewhat complex string.
The-Dflag will not be supported iftcpdumpwas built with an older version oflibpcapthat lacks thepcap_findalldevs(3PCAP)function.
Print the link-level header on each dump line. This can be used, forexample, to print MAC layer addresses for protocols such as Ethernet andIEEE 802.11.
Use [email protected] algo:secret for decrypting IPsec ESP packets thatare addressed to addr and contain Security Parameter Index valuespi. This combination may be repeated with comma or newline separation.
Note that setting the secret for IPv4 ESP packets is supported at this time.
Algorithms may bedes-cbc,3des-cbc,blowfish-cbc,rc3-cbc,cast128-cbc, ornone.The default is des-cbc.The ability to decrypt packets is only present if tcpdump was compiledwith cryptography enabled.
secret is the ASCII text for ESP secret key.If preceded by 0x, then a hex value will be read.
The option assumes RFC2406 ESP, not RFC1827 ESP.The option is only for debugging purposes, andthe use of this option with a true `secret' key is discouraged.By presenting IPsec secret key onto command lineyou make it visible to others, viaps(1)and other occasions.
In addition to the above syntax, the syntax file name may be usedto have tcpdump read the provided file in. The file is opened uponreceiving the first ESP packet, so any special permissions that tcpdumpmay have been given should already have been given up.
Print `foreign' IPv4 addresses numerically rather than symbolically(this option is intended to get around serious brain damage inSun's NIS server --- usually it hangs forever translating non-localinternet numbers).
The test for `foreign' IPv4 addresses is done using the IPv4 address andnetmask of the interface on which capture is being done. If thataddress or netmask are not available, available, either because theinterface on which capture is being done has no address or netmask orbecause the capture is being done on the Linux 'any' interface, whichcan capture on more than one interface, this option will not workcorrectly.
-F file
Use file as input for the filter expression.An additional expression given on the command line is ignored.
-G rotate_seconds
If specified, rotates the dump file specified with the-woption every rotate_seconds seconds.Savefiles will have the name specified by-wwhich should include a time format as defined bystrftime(3).If no time format is specified, each new file will overwrite the previous.Whenever a generated filename is not unique, tcpdump will overwrite thepre-existing data; providing a time specification that is coarser than thecapture period is therefore not advised.
If used in conjunction with the-Coption, filenames will take the form of `file<count>'.
Print the tcpdump and libpcap version strings, print a usage message,and exit.
Print the tcpdump and libpcap version strings and exit.
Attempt to detect 802.11s draft mesh headers.
-i interface
Listen, report the list of link-layer types, report the list of timestamp types, or report the results of compiling a filter expression oninterface. If unspecified and if the-dflag is not given, tcpdump searches the systeminterface list for the lowest numbered, configured up interface(excluding loopback), which may turn out to be, for example, ``eth0'.
On Linux systems with 2.2 or later kernels, aninterfaceargument of ``any' can be used to capture packets from all interfaces.Note that captures on the ``any' device will not be done in promiscuousmode.
If the-Dflag is supported, an interface number as printed by that flag can beused as theinterfaceargument, if no interface on the system has that number as a name.
Put the interface in 'monitor mode'; this is supported only on IEEE802.11 Wi-Fi interfaces, and supported only on some operating systems.
Note that in monitor mode the adapter might disassociate from thenetwork with which it's associated, so that you will not be able to useany wireless networks with that adapter. This could prevent accessingfiles on a network server, or resolving host names or network addresses,if you are capturing in monitor mode and are not connected to anothernetwork with another adapter.
This flag will affect the output of the-Lflag. If-Iisn't specified, only those link-layer types available when not inmonitor mode will be shown; if-Iis specified, only those link-layer types available when in monitor modewill be shown.
Capture in 'immediate mode'. In this mode, packets are delivered totcpdump as soon as they arrive, rather than being buffered forefficiency. This is the default when printing packets rather thansaving packets to a ``savefile' if the packets are being printed to aterminal rather than to a file or pipe.
-j tstamp_type
Set the time stamp type for the capture to tstamp_type. The namesto use for the time stamp types are given inpcap-tstamp(7);not all the types listed there will necessarily be valid for any giveninterface.
List the supported time stamp types for the interface and exit. If thetime stamp type cannot be set for the interface, no time stamp types arelisted.
When capturing, set the time stamp precision for the capture totstamp_precision. Note that availability of high precision timestamps (nanoseconds) and their actual accuracy is platform and hardwaredependent. Also note that when writing captures made with nanosecondaccuracy to a savefile, the time stamps are written with nanosecondresolution, and the file is written with a different magic number, toindicate that the time stamps are in seconds and nanoseconds; not allprograms that read pcap savefiles will be able to read those captures.
When reading a savefile, convert time stamps to the precision specifiedby timestamp_precision, and display them with that resolution. Ifthe precision specified is less than the precision of time stamps in thefile, the conversion will lose precision.
The supported values for timestamp_precision are micro formicrosecond resolution and nano for nanosecond resolution. Thedefault is microsecond resolution.
Shorthands for --time-stamp-precision=micro or--time-stamp-precision=nano, adjusting the time stampprecision accordingly. When reading packets from a savefile, using--micro truncates time stamps if the savefile was created withnanosecond precision. In contrast, a savefile created with microsecondprecision will have trailing zeroes added to the time stamp when--nano is used.
Don't attempt to verify IP, TCP, or UDP checksums. This is useful forinterfaces that perform some or all of those checksum calculation inhardware; otherwise, all outgoing TCP checksums will be flagged as bad.
Make stdout line buffered.Useful if you want to see the datawhile capturing it.E.g.,
Note that on Windows,``line buffered' means ``unbuffered', so thatWinDump will write each character individually if-lis specified.
-Uis similar to-lin its behavior, but it will cause output to be ``packet-buffered', sothat the output is written to stdout at the end of each packet ratherthan at the end of each line; this is buffered on all platforms,including Windows.
List the known data link types for the interface, in the specified mode,and exit. The list of known data link types may be dependent on thespecified mode; for example, on some platforms, a Wi-Fi interface mightsupport one set of data link types when not in monitor mode (forexample, it might support only fake Ethernet headers, or might support802.11 headers but not support 802.11 headers with radio information)and another set of data link types when in monitor mode (for example, itmight support 802.11 headers, or 802.11 headers with radio information,only in monitor mode).
-m module
Load SMI MIB module definitions from file module.This optioncan be used several times to load several MIB modules into tcpdump.
-M secret
Use secret as a shared secret for validating the digests found inTCP segments with the TCP-MD5 option (RFC 2385), if present.
Don't convert addresses (i.e., host addresses, port numbers, etc.) to names.
Don't print domain name qualification of host names.E.g.,if you give this flag then tcpdump will print ``nic'instead of ``'.
Print an optional packet number at the beginning of the line.
Do not run the packet-matching code optimizer.This is useful onlyif you suspect a bug in the optimizer.
Don't put the interfaceinto promiscuous mode.Note that the interface might be in promiscuousmode for some other reason; hence, `-p' cannot be used as an abbreviation for`ether host {local-hw-addr} or ether broadcast'.
Print parsed packet output, even if the raw packets are being saved to afile with the-wflag.
-Q direction
Choose send/receive direction direction for which packets should becaptured. Possible values are `in', `out' and `inout'. Not availableon all platforms.
Quick (quiet?) output.Print less protocol information so outputlines are shorter.
-r file
Read packets from file (which was created with the-woption or by other tools that write pcap or pcapng files).Standard input is used if file is ``-'.
Print absolute, rather than relative, TCP sequence numbers.
-s snaplen
Snarf snaplen bytes of data from each packet rather than thedefault of 262144 bytes.Packets truncated because of a limited snapshotare indicated in the output with ``[ proto]', where protois the name of the protocol level at which the truncation has occurred.
Note that taking larger snapshots both increasesthe amount of time it takes to process packets and, effectively,decreases the amount of packet buffering.This may cause packets to belost.Note also that taking smaller snapshots will discard data from protocolsabove the transport layer, which loses information that may beimportant. NFS and AFS requests and replies, for example, are verylarge, and much of the detail won't be available if a too-short snapshotlength is selected.
If you need to reduce the snapshot size below the default, you shouldlimit snaplen to the smallest number that will capture theprotocol information you're interested in. Settingsnaplen to 0 sets it to the default of 262144,for backwards compatibility with recent older versions oftcpdump.
-T type
Force packets selected by 'expression' to be interpreted thespecified type.Currently known types areaodv (Ad-hoc On-demand Distance Vector protocol),carp (Common Address Redundancy Protocol),cnfp (Cisco NetFlow protocol),domain (Domain Name System),lmp (Link Management Protocol),pgm (Pragmatic General Multicast),pgm_zmtp1 (ZMTP/1.0 inside PGM/EPGM),ptp (Precision Time Protocol),radius (RADIUS),resp (REdis Serialization Protocol),rpc (Remote Procedure Call),rtcp (Real-Time Applications control protocol),rtp (Real-Time Applications protocol),snmp (Simple Network Management Protocol),someip (SOME/IP),tftp (Trivial File Transfer Protocol),vat (Visual Audio Tool),vxlan (Virtual eXtensible Local Area Network),wb (distributed White Board)andzmtp1 (ZeroMQ Message Transport Protocol 1.0).
Note that the pgm type above affects UDP interpretation only, the nativePGM is always recognised as IP protocol 113 regardless. UDP-encapsulated PGM isoften called 'EPGM' or 'PGM/UDP'.
Note that the pgm_zmtp1 type above affects interpretation of both nativePGM and UDP at once. During the native PGM decoding the application data of anODATA/RDATA packet would be decoded as a ZeroMQ datagram with ZMTP/1.0 frames.During the UDP decoding in addition to that any UDP packet would be treated asan encapsulated PGM packet.
Don't print a timestamp on each dump line.
Print the timestamp, as seconds since January 1, 1970, 00:00:00, UTC, andfractions of a second since that time, on each dump line.
Print a delta (microsecond or nanosecond resolution depending on the--time-stamp-precisionoption) between current and previous line on each dump line.The default is microsecond resolution.
Print a timestamp, as hours, minutes, seconds, and fractions of a secondsince midnight, preceded by the date, on each dump line.
Print a delta (microsecond or nanosecond resolution depending on the--time-stamp-precisionoption) between current and first line on each dump line.The default is microsecond resolution.
Print undecoded NFS handles.
If the-woption is not specified, or if it is specified but the--printflag is also specified, make the printed packet output``packet-buffered'; i.e., as the description of the contents of eachpacket is printed, it will be written to the standard output, ratherthan, when not writing to a terminal, being written only when the outputbuffer fills.
If the-woption is specified, make the saved raw packet output``packet-buffered'; i.e., as each packet is saved, it will be writtento the output file, rather than being written only when the outputbuffer fills.
The-Uflag will not be supported iftcpdumpwas built with an older version oflibpcapthat lacks thepcap_dump_flush(3PCAP)function.
When parsing and printing, produce (slightly more) verbose output.For example, the time to live,identification, total length and options in an IP packet are printed.Also enables additional packet integrity checks such as verifying theIP and ICMP header checksum.
When writing to a file with the-woption and at the same time not reading from a file with the-roption, report to stderr, once per second, the number of packets captured. InSolaris, FreeBSD and possibly other operating systems this periodic updatecurrently can cause loss of captured packets on their way from the kernel totcpdump.
Even more verbose output.For example, additional fields areprinted from NFS reply packets, and SMB packets are fully decoded.
Even more verbose output.For example,telnet SB .. SE optionsare printed in full.With-XTelnet options are printed in hex as well.
-V file
Read a list of filenames from file. Standard input is usedif file is ``-'.
-w file
Write the raw packets to file rather than parsing and printingthem out.They can later be printed with the -r option.Standard output is used if file is ``-'.
This output will be buffered if written to a file or pipe, so a programreading from the file or pipe may not see packets for an arbitraryamount of time after they are received. Use the-Uflag to cause packets to be written as soon as they are received.
The MIME type application/vnd.tcpdump.pcap has been registeredwith IANA for pcap files. The filename extension .pcapappears to be the most commonly used along with .cap and.dmp. Tcpdump itself doesn't check the extension whenreading capture files and doesn't add an extension when writing them(it uses magic numbers in the file header instead). However, manyoperating systems and applications will use the extension if it ispresent and adding one (e.g. .pcap) is recommended.
Seepcap-savefile(5)for a description of the file format.
-W filecount
Used in conjunction with the-Coption, this will limit the numberof files created to the specified number, and begin overwriting filesfrom the beginning, thus creating a 'rotating' buffer.In addition, it will namethe files with enough leading 0s to support the maximum number offiles, allowing them to sort correctly.
Used in conjunction with the-Goption, this will limit the number of rotated dump files that getcreated, exiting with status 0 when reaching the limit.
If used in conjunction with both-Cand-G,the-Woption will currently be ignored, and will only affect the file name.
When parsing and printing,in addition to printing the headers of each packet, print the data ofeach packet (minus its link level header) in hex.The smaller of the entire packet orsnaplenbytes will be printed. Note that this is the entire link-layerpacket, so for link layers that pad (e.g. Ethernet), the padding byteswill also be printed when the higher layer packet is shorter than therequired padding.In the current implementation this flag may have the same effect as-xxif the packet is truncated.
When parsing and printing,in addition to printing the headers of each packet, print the data ofeach packet,includingits link level header, in hex.
When parsing and printing,in addition to printing the headers of each packet, print the data ofeach packet (minus its link level header) in hex and ASCII.This is very handy for analysing new protocols.In the current implementation this flag may have the same effect as-XXif the packet is truncated.
When parsing and printing,in addition to printing the headers of each packet, print the data ofeach packet,includingits link level header, in hex and ASCII.
-y datalinktype
Set the data link type to use while capturing packets (see-L)or just compiling and dumping packet-matching code (see-d)to datalinktype.
-z postrotate-command
Used in conjunction with the-Cor-Goptions, this will maketcpdumprun 'postrotate-command file' wherefileis the savefile being closed after each rotation. For example, specifying-z gzipor-z bzip2will compress each savefile using gzip or bzip2.
Note that tcpdump will run the command in parallel to the capture, usingthe lowest priority so that this doesn't disturb the capture process.
And in case you would like to use a command that itself takes flags ordifferent arguments, you can always write a shell script that will take thesavefile name as the only argument, make the flags & arguments arrangementsand execute the command that you want.
-Z user
Iftcpdumpis running as root, after opening the capture device or input savefile,but before opening any savefiles for output, change the user ID touserand the group ID to the primary group ofuser.
This behavior can also be enabled by default at compile time.
selects which packets will be dumped.If no expressionis given, all packets on the net will be dumped.Otherwise,only packets for which expression is `true' will be dumped.

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.


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):


The output of tcpdump is protocol dependent.The followinggives a brief description and examples of most of the formats.


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.

Google home app for mac laptop free. 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 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

tcpdump filterexpression, because that would select only those packets that haveSYN-ACK set, but not those with only SYN set.Remember that we don't careif 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 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 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 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 see and other online resources.The SMB patches were written by Andrew Tridgell([email protected]).

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 `', 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.



The original distribution is available via anonymous ftp:

On Linux systems with 2.0[.x] kernels:

capturing on some PPP devices won't work correctly.

We recommend that you upgrade to a 2.2 or later kernel.

Some attempt should be made to reassemble IP fragments or, at leastto compute the right length for the higher level protocol.

Sophos Xg Tcpdump Mac Address

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.

Tcpdump Sophos Xg

Arithmetic expression against transport layer headers, like tcp[0],does not work against IPv6 packets.It only looks at IPv4 packets.

Tcpdump Sophos Xg Update


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