May the source be with you,
but remember the KISS principle ;-)
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Like almost all open source sniffing applications tcpdump the libpcap library. People with Windows distributions are best to check the Windows PCAP page for references to WinDUMP. Ethereal (wireshark) is often used for analyzing the raw dumps.
In the simplest setup tcpdump dumps all traffic to the screen in a text format, which you can brose via more ot other pager.
tcpdump can also be used to see if a given service may be unresponsive because your packets are simply not reaching the remote machine. Since tcpdump is a commandline tool, you'll very probably need to add filters - especially when you're firing tcpdump up on a remote machine, where you're logged in via SSH. Otherwise you'll get lots of packet dumps of SSH packets that are telling you of packets dumped that belong to ssh telling you of packets dumped...
You can limit tcpdump "scope" in several way that can be combined
tcpdump -s0 -l -i eth0 port 25
This will dump all packets aimed at, or originating from, a TCP or UDP port 25.
The '-l' is to do line buffering, so we'll actually see each packet as it crosses
tcpdump -s0 -l not port 22
tcpdump -s0 -l src or dst dp.remote.net
The tcpdump filter syntax is actually powerful enough to enable you to create a filter that provides only set of packets you want. Although sometimes you look at wrong direction and too strict filter can make determination of the problem more difficult. So it is safer to limit communication by host name if you are studying a single affected host or a couple of them that by port.
Tcpdump prints verbose information about the sniffed traffic with the -v option. It can print hex dump of the packet with option -x. For printing full packet you need to use option -s. One important feature is the ability to write the "raw packets" to a file using -s0 -w name_of_file.
You can later analyze the capture using any program that understand pcap raw dump format, for example tcpdump, snort or Ethereal. I think almost all useful network programs now understand this format and can reuse raw packet tcpdump-generated files. If you forgot the option -s 0 packets will truncated and you will not be able to analyze them for example with snort. To print them from raw tcpdump file you can use option -r. For example tcpdump -r file_with_capture. You can use option r with -v (versose) and -vv to get more information.
Two very useful options are (see also Most useful options)
TCPDUMP will only process packets that match the filter expression. Such a filter expression can be passed on the command line, or read from a file using the -F filename parameter.
# tcpdump -F path_to_filter
Tcpdump filter language is similar to snort (actually this language originated in tcpdump and later was adopted and extended by snort). For more detailed info on TCPDUMP and filter expressions, please consult the TCPDUMP man page, either via the man command or online at tcpdump.org.
Here are several examples that might serve as a basic of you own collection (See also Filters to Detect, Filters to Protect The Mechanics of Writing TCPdump Filters.)
tcpdump dst host 10.10.10.10 or scr host 10.10.10.10Here we try to get TCP packets with source port 80
tcpdump src port 80 and tcp
Write the syntax of a tcpdump command that captures packets containing IP datagrams with a source or destination IP address equal to 10.0.1.12.tcpdump host 10.0.1.12
Write the syntax of a tcpdump command that captures packets containing ICMP messages with a source or destination IP address equal to 10.0.1.12.tcpdump icmp and host 10.0.1.12
Write the syntax of a tcpdump command that captures packets containing IP datagrams between two hosts with IP addresses 10.0.1.11 and 10.0.1.12, both on interface eth1.tcpdump -i eth1 host 10.0.1.11 and host 10.0.1.12
Write a tcpdump filter expression that captures packets containing TCP segments with a source or destination IP address equal to 10.0.1.12.tcp and host 10.0.1.12
Write a tcpdump filter expression that, in addition to the constraints in Question 5, only captures packets using port number 23.tcp port 23 and host 10.0.1.12
tcpdump '(host 220.127.116.11 and net 192.168.1) and ((tcp port 80 or port 443))'
Ethereal/Wireshark can display tcpdump logs via GUI interface and you can filter raw logs via Etherreal/Wireshark too. It can also act as data collection tool of its own. It will display all the connections it traced during the capture. There are a couple ways to look for bandwidth hogs.
The "Statistics" menu has a couple of useful options. The "Protocol Hierarchy" shows what % of packets in the trace is from each type of protocol. In the case of a bandwith hog, at least what protocol is the culprit should be easy to spot here.
The "Conversations" screen is also helpful for looking for bandwidth hogs. Since you can sort the "conversations" by number of packets, the culprit is likely to hop to the top. This isn't always the case, as it could easily be many small connections killing the bandwidth, not one big heavy connection.
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17 August 1999 (slac.stanford.edu) This report details an investigation of the TCPDUMP utility.Tcpdump is a powerful tool that allows us to sniff network packets and make some statistical analysis out of those dumps. One major drawback to tcpdump is the size of the flat file containing the text output. But tcpdump allows us to precisely see all the traffic and enables us to create statistical monitoring scripts.
In our case, looking at an ethernet segment, tcpdump operates by putting the network card into promiscuous mode in order to capture all the packets going through the wire. Tcpdump runs using BSD Packet Filter (BPF) which is the method of collecting data from this network interface running into promiscuous mode. BPF receives copies from the driver of sent packets and received packets. Before traveling through the kernel all the way up to the user process the user can set a filter so only interesting packets go through the Kernel. SUN OS uses Network Interface Tap (NIT) which only allows to capture packets received from the interface but no packets sent by the host. Still the SUN OS tcpdump does the trick but it performs its own filtering at the user process level which means that more data goes through the kernel.
We use tcpdump to measure the response time and the packet loss percentages. It can also tell us about lack of reachability for some distant server.
Using tcpdump we have a view on any TCP/UDP connection Establishment and Termination. TCP uses a special mechanism to set and close connections (we will discuss this later on); we measure the time lapse between the packets involved with this mechanism in order to know how fast some connections operate.
The syntax I used to copy all of the packets being sent and received by the exploit is as follows:
tcpdump -nXvSs 0 ip and host 192.168.1.101 -w sploit_log
The above noted tcpdump filter will copy all packets with a valid IP header in them, and write them to the binary log called "sploit_log". Lastly the IP address of the lab machine being tested is 192.168.1.101.
Now that I have the binary log file and the exploit has been executed I can look at the packets themselves, and see if there is something telling in the packets themselves. I use the following tcpdump syntax to look at the file that was generated:
tcpdump -r sploit_log -nXvSs 0 |more
Last week we took a deep dive into decoding TCP headers, so that we would know what the heck we were looking at when firing up tcpdump. Today we'll dig into specific tcpdump commands for looking at traffic from specific protocols, hosts, or ports, and how to save tcpdump's output to various file formats for analysis with other utilities.
Exporting tcpdumps To File
tcpdump merely captures packet headers; it does not have reporting or analysis capabilities. If you are going to do any kind of extended packet capture and analysis, it is best to capture it to a file, then use something like Ethereal or Snort to sort it out. As with all things Linux, there are several ways to do this. Use the -l switch to create re-directable output, or as man tcpdump says "Make stdout line buffered." The following command stores output in webdump.txt, and displays it on the screen at the same time:
In these here spyware-infested times, one of the most useful things you can do with a packet sniffer is monitor the traffic coming off Windows PCs.
# tcpdump port 80 -l > webdump.txt & tail -f webdump.txt
And so does this:
# tcpdump -l | tee webdump.txt
This creates a human-readable ASCII text file. Another way to do this is to capture the raw data in a binary file. This is a good thing to do when you want to capture a huge glob of unfiltered traffic and analyze it later with Ethereal or Snort:
# tcpdump -w rawdump
You can also replay it in tcpdump by naming the file to be read with the -r switch, then re-directing the output to another file:
# tcpdump -r rawdump > rawdump.txt
tcpdump runs until you stop it with Ctrl+c, so you might want to set a limit on the number of packets to capture, so that it will stop itself:
# tcpdump -c1000 -w rawdump
tcpdump defaults to the lowest-numbered NIC, which is usually eth0. Use the -i flag to specify a different NIC:
# tcpdump -i eth1 -c1000 -w rawdump
These two commands are the tcpdump workhorses. The first one captures traffic on a specific port. The second one captures traffic on a specific host on the LAN. (See Part 1, "Hubs Are Blabbermouths" and "Foiled By Switches" for more on how to do this.)
# tcpdump port nn
# tcpdump host 18.104.22.168
You can select several hosts on your LAN, and capture the traffic that passes between them:
# tcpdump host workstation4 and workstation11 and workstation13
Or capture all the LAN traffic between workstation4 and the LAN, except for workstation11:
# tcpdump workstation4 except workstation11
It can be useful to show the MAC addresses, especially when you're debugging network problems, because the MAC address is the definitive identification for a host. Use the -e flag:
# tcpdump -e host workstation4 and workstation11 and workstation13
Yes, astute reader, you read it right- you can spy on the traffic exchanged between certain computers on your LAN.
Excluding ThingsThe default is to capture all packets. You can capture all packets except those for certain ports, like this:
# tcpdump not port 110 and not port 25 and not port 53 and not port 22
A useful option is to declutter the display with the -t flag, which suppresses the timestamps:
# tcpdump -t not port 110 and not port 25
You can speed up performance and decrease clutter further by turning off DNS lookups with the -n flag:
# tcpdump -tn not port 110 and not port 25
Or you can go nuts and increase the amount of data shown with the -v and -vv flags. -vv is the most verbose:
# tcpdump -vv
Here is a sample of what you'll see with -vv:
192.168.1.5.35401 > 22.214.171.124.995: R [tcp sum ok] 394522529:394522529(0) win 0 (DF) (ttl 64, id 684, len 40)
126.96.36.199.1985 > 188.8.131.52.1985: [udp sum ok] udp 20 [tos 0xc0] (ttl 2, id 0, len 48)
Some of the additional fields displayed are:
checksum. A checksum is computed for each segment; damaged segments are discarded.
ttl is time to live. The standard ttl is two minutes; if a packet is still floating around undelivered after two minutes, it is discarded. In this example, the limit is 64 seconds.
id is a segment identification number, to assist in re-assembling datagram fragments.
tos, type of service, contains the values for precedence, delay, throughput, and reliability. Applications can theoretically set these values; for example, streaming video wants low latency, which is the delay value. In practice, this field is mostly useless in IPv4, because routers can be configured to ignore the TOS field, and there is no way to enforce the sensible use of it. IPv6 promises to make it meaningful, but again the problem lies in the implementation, not the standard.
len is the length, in bytes, of the segment.
Filtering On Protocols
A very useful tcpdump filter is the ability to filter on different protocols. Suppose you want to see only udp traffic:
# tcpdump udp
tcpdump recognizes the keywords ip, igmp, tcp, udp, and icmp. You can also specify any protocol named in /etc/protocols by using the proto qualifier. This example captures ospf (Open Shortest Path First) traffic, which is useful for eavesdropping on routers:
# tcpdump ip proto OSPFIGP
To capture traffic on a specific host and restrict by protocol, do this:
# tcpdump host server02 and ip
# tcpdump host server03 and not udp
# tcpdump host server03 and ip and igmp and not udp
When you look at what goes over the wires after the simple act of clicking on a URL, or checking email, it is amazing that it works at all. In these here spyware-infested times, one of the most useful things you can do with a packet sniffer is monitor the traffic coming off Windows PCs. Nothing can be hidden, it's all there for your keen probing eye to find.
- See Scripting Clinic: Have a bash with this Linux Shell for more information on the various bash-isms used here, such as the pipe, re-direct, and control operators.
- See Mind Your Packets with Ethereal to learn more about using ethereal to parse and analyze packets.
- Unlike my columns, RFCs are less-than-riveting reading. but they contain complete information.
- RFC 793 describes the transmission control protocol (tcp) in exhaustive detail.
- RFC 1180 is an excellent tcp tutorial.
- tcpdump home page
[Dec 28, 2004] tcpdump -- A collection of tcpdump filters.
# A collection of tcpdump filters. # [[shells might require escaping of special characters]] # == # This document: http://www.rdrs.net/document/ # Related: http://www.rdrs.net/snippets/src/pcap_example.c # Last update: Tue Dec 28, 2004 # == # If you have tips, suggestions or additional filters # that haven't been listed here, drop me a short note. # Address info can be found at http://www.rdrs.net/about.html # # Thnkx.. # # ####### # TCP # # filter ssh tcp[(tcp>>2):4] = 0x5353482D && (tcp[((tcp>>2)+4):2] = 0x312E || \ tcp[((tcp>>2)+4):2] = 0x322E) # filter "combine" rlogin (tcp[(ip[2:2]-((ip&0x0f)<<2))-1]=0) && \ ((ip[2:2]-((ip&0x0f)<<2) - (tcp>>2)) != 0) && \ ((ip[2:2]-((ip&0x0f)<<2) - (tcp>>2)) <= 128) # filter ftp tcp[(tcp>>2):4] = 0x3232302d || tcp[(tcp>>2):4] = 0x32323020 # URG set and ACK not set tcp & 0x30 = 0x20 # IMAP service exploit tcp && (tcp & 2 != 0) && (dst port 143) # filter root backdoor tcp[(tcp>>2):2] = 0x2320 && \ (ip[2:2] - ((ip&0x0f)<<2) - (tcp>>2)) == 2 # RST set and FIN set tcp & 0x05 = 5 # filter out napster ((ip[2:2] - ((ip&0x0f)<<2) - (tcp>>2)) = 4 && \ tcp[(tcp>>2):4] = 0x53454e44) || \ ((ip[2:2] - ((ip&0x0f)<<2) - (tcp>>2)) = 3 && \ tcp[(tcp>>2):2] = 0x4745 && tcp[(tcp>>2)+2]=0x54) # telnet tcp[2:2] = 23 # again telnet but beter... (tcp[(tcp>>2):2] > 0xfffa) && (tcp[(tcp>>2):2] < 0xffff) # attempted ftp connection to other hosts on the network than the ftp server dst net 184.108.40.206/22 && dst port 21 \ && (tcp & 0x3f = 2) && !(dst host ftp.bla.org) # attempts to include data on the initial SYN. tcp & 0xff = 2 && \ (ip[2:2] - ((ip & 0x0f) * 4) - ((tcp & 0xf0) / 4)) != 0 # active open (syn set without ack) (tcp & 0x12 < 16) # winnuke DOS attack (tcp[2:2] = 139) && (tcp & 0x20 != 0) && (tcp & 0x01 = 1) # destination port less than 1024 tcp[2:2] < 1024 # SYN set and FIN set tcp & 0x03 = 3 # one of the reserved bits of tcp is set tcp & 0xc0 != 0 # DNS zone transfer tcp && dst port 53 # active open connection, syn is set, ack is not tcp & 0x12 = 2 # X11 ports (tcp[2:2] >= 6000) && (tcp[2:2] < 7000) # TCP port 6667 with ACK flag set and payload starting at byte 12 # that does not include the asciiwords "PING", "PONG", "JOIN", or "QUIT". (tcp & 0x10 = 1) && (tcp[0:2]=6667 || tcp[2:2]=6667) \ && (not ip[32:4] = 1346981447 || not ip[32:4] = 1347374663 \ || not ip[32:4] = 1246710094 || not ip[32:4] = 1364543828) # except ack push (tcp & 0xe7) != 0 # all packets with the PUSH flag set tcp & 8 != 0 # all packets with the RST flag set tcp & 4 != 0 # filter out gnutella tcp[(tcp>>2):4] = 0x474e5554 && \ tcp[(4+(tcp>>2)):4] = 0x454c4c41 && tcp[8+(tcp>>2)] = 0x20 # catch default hping 2 pings tcp  = 0 && tcp = 0 # FIN set and ACK not set tcp & 0x11 = 1 # null scan filter with no flags set tcp = 0 # could also be written as tcp & 0xff = 0 # no flags set, null packet tcp & 0x3f = 0 # syn-fyn tcp = 3 # syn-fyn both flags set (tcp & 0x03) = 3 # only syn.. tcp & 0x02) != 0 # reserved bits set tcp >= 64 # incomming http requests (tcp[13:1]&18 = 2) && (port 80) && (ip dst 192.168.1.40) # broadcasts x.x.x.255 ip = 0xff # broadcasts x.x.x.0 ip = 0x00 # Incomming SYN packets tcp && (tcp & 0x02 != 0) && \ (tcp & 0x10 = 0) && (not dst port 53) && \ (not dst port 80) && (not dst port 25) && (not dst port 21) # SMB dst port 139 && tcp[13:1] & 18 = 2 # ACK flag set, ack value is ZERO. Not normal for three-way handshake. # Possible capture of NMAP(1) os fingerprinting. tcp & 0xff = 0x10 && tcp[8:4] = 0 # high-order reserved bits should be ZERO. NMAP(1) sometimes sets the # bit that is in the 64 position for os fingerprinting. tcp >= 64 # SYN set and RST set tcp & 0x06 = 6 # PSH set and ACK not set tcp & 0x18 = 8 # Some filters combined for a general [catch [[bad]] events filter] (tcp && (tcp & 3 != 0) && ((dst port 143) || \ (dst port 111) || (tcp & 3 != 0 && tcp & 0x10 = 0 && \ dst net 172.16 && dst port 1080) || \ (dst port 512 || dst port 513 || dst port 514) || \ ((ip = 0xff) && not (net 172.16/16 || net 192.168/16)) || \ (ip[12:4] = ip[16:4]))) || (not tcp && igrp && not dst port 520 && \ ((dst port 111) || (udp port 2049) || ((ip = 0xff) && \ not (net 172.16/16 || net 192.168/16)) || (ip[12:4] = ip[16:4]))) # RIP info -s 1024 port routed # in/out going fragmentation attack tcp && ip[6:2]&16383 != 0 ####### # IP # # all packets with more than 20 bytes of payload (ip[2:2] - ((ip&0x0f)<<2) - (tcp>>2)) <= 20 # ping of death attack ((ip & 0x20 = 0) && (ip[6:2] & 0x1fff != 0)) && \ ((65535 < (ip[2:2] + 8 * (ip[6:2] & 0x1fff)) # more fragments bit is not set [but] the fragment offset is not zero ((ip[6:1] & 0x20 = 0) && (ip[6:2] & 0x1fff != 0)) # any packet with a header more than 20 bytes. ip & 0x0f > 5 # any packet with more fragments set ip & 0x20 !=0 # packets with TTL's less than 5 ip < 5 # source ip equal to destination ip [classic land attack] ip[12:4] = ip[16:4] # another, land attack (tcp[0:2] = tcp[2:2]) && (ip[12:4] = ip[16:4]) # IP options (ip & 0x0f) != 5 # broadcasts to xxx.xxx.xxx.255 || xxx.xxx.xxx.0 (ip=0xff) || (ip=0x00) # fragmented packet with zero offset ip[6:2] & 0x1fff = 0 # and more fragments [terminal] (ip & 0x20 = 0) && (ip[6:2] & 0x1fff != 0) # and even more fragments [intervening] (ip & 0x20 != 0) && (ip[6:2] & 0x1fff != 0) # my head was fragmented [initially] (ip & 0x20 != 0) && (ip[6:2] & 0x1fff = 0) # fragmented packets with more coming ip[6:1] & 0x20 != 0 # more fragments bit is not set, [but] the fragment offset is not zero (ip[6:1] & 0x20 = 0) && (ip[6:2] & 0x1fff != 0)) # unroutable addresses not ((ip < 3) || net 5 || net 10 || net 127 || net 172.16 \ || net 192.168 || (ip > 239)) # IP options ip[0:1] & 0x0f > 5 # loose source routing, [(ip[0:1] & 0x0f > 5)] # ip opts: # 7,0x44,0x83,0x89 # record route,timestamp,loose source routing,strict source routing # loose source routing ip[20:1] & 0xff = 131 # other IP versions than ipv4 ip && (ip & 0xf0 != 0x40) ####### # ICMP # # fragmentation needed but DF flag set (icmp = 3) && (icmp = 4) # fragmented ICMP icmp && (ip[6:1] & 0x20 != 0) # in/out going smurf attack icmp && (ip[19:1] = 255) # in/out going fragmentation attack icmp && ip[6:2] & 16383 != 0 # Loki Filter ((icmp = 0) || (icmp = 8)) && ((icmp[6:2] = 0xf001) || (icmp[6:2] = 0x01f0) # ICMP address mask requests icmp = 17 # Frag required but DF set* ((icmp = 3) && (icmp = 4)) # source route failed (icmp = 3) && (icmp = 5) # all ICMP except ping icmp && icmp != 8 && icmp != 0 # source quench : icmp = 4 # redirect : icmp = 5 # router advertisement : icmp = 9 # router solicitation : icmp = 10 # parameter problem : icmp = 12 # timestamp request : icmp = 13 # timestamp reply : icmp = 14 # information request : icmp = 15 # information reply : icmp = 16 # address mask request : icmp = 17 # address mask reply : icmp = 18 ####### # UDP # # teardrop attack udp && (ip[6:1] & 0x20 != 0) # catch anything udp to port 500 udp -n -vv udp && dst port 500 # catch udp packets with impossible udp lengths (udp[4:2] < 0) || (udp[4:2] > 1500) # back Orifice -n -vv udp && dst port 31337 # UNIX traceroute destports between 33000 and 33999 (udp[2:2] >= 33000) && (udp[2:2] <= 33999) # or alternatively.. udp[2:2] >= 33000 && udp[2:2] < 34000 && ip = 1 # UDP port scan udp && src port = dst port
Tcpdump AWK filter
I wrote this filter for a friend that told me that I couldn't do this in AWK. I also learned AWK doing this. This is NOT the proper way to do this, Perl or C is definatly better. But, due to how simple it is, it can quickly be changed to display the data in any way you want and support more prootcols too. Right now it only supports IP, TCP, UDP, and ICMP. Drop me a line if you have any comments or questions.
There is also a few things that can be configured, such as the margins, the byte order of your machine, and also the starting position of the ip header in the output. The last option is due to a bug konwn in tcpdump when dumping on FDDI interfaces. See the script for more information.
Script is at end of examples. Beware, my AWK code looks like C without semicolons.
Usage: tcpdump -l -s65536 -x -i interface | fil
Setting snaplen to 0 means use the required length
to catch whole packets.
-r Used to print of view previous capture. For example tcpdump -r file_with_capture. There are several useful options that should be used with it.
TCPDUMP(8) man page
Tcpdump prints out the headers of packets on a network interface that match the boolean expression. It can also be run with the -w flag, which causes it to save the packet data to a file for later analysis, and/or with the -b flag, which causes it to read from a saved packet file rather than to read packets from a network interface. In all cases, only packets that match expression will be processed by tcpdump.
Tcpdump will, if not run with the -c flag, continue cap turing packets until it is interrupted by a SIGINT signal (generated, for example, by typing your interrupt charac ter, typically control-C) or a SIGTERM signal (typically generated with the kill(1) command); if run with the -c flag, it will capture packets until it is interrupted by a SIGINT or SIGTERM signal or the specified number of packets have been processed.
When tcpdump finishes capturing packets, it will report counts of:
packets ``received by filter'' (the meaning of this depends on the OS on which you're running tcpdump, and possibly on the way the OS was configured - if a filter was specified on the command line, on some OSes it counts packets regardless of whether they were matched by the filter expression, and on other OSes it counts only packets that were matched by the filter expression and were processed by tcpdump);
packets ``dropped by kernel'' (this is the number of packets that were dropped, due to a lack of buffer space, by the packet capture mechanism in the OS on which tcpdump is running, if the OS reports that information to applications; if not, it will be reported as 0).
On platforms that support the SIGINFO signal, such as most BSDs, it will report those counts when it receives a SIG INFO signal (generated, for example, by typing your ``sta tus'' character, typically control-T) and will continue you have special privileges:
Under SunOS 3.x or 4.x with NIT or BPF: You must have read access to /dev/nit or /dev/bpf*.
Under Solaris with DLPI: You must have read/write access to the network pseudo device, e.g. /dev/le. On at least some versions of Solaris, however, this is not suffi cient to allow tcpdump to capture in promiscuous mode; on those versions of Solaris, you must be root, or tcpdump must be installed setuid to root, in order to capture in promiscuous mode.
Under HP-UX with DLPI: You must be root or tcpdump must be installed setuid to root.
Under IRIX with snoop: You must be root or tcpdump must be installed setuid to root.
Under Linux: You must be root or tcpdump must be installed setuid to root.
Under Ultrix and Digital UNIX: Once the super-user has enabled promiscuous-mode operation using pfconfig(8), any user may capture network traffic with tcpdump.
Under BSD: You must have read access to /dev/bpf*.
Reading a saved packet file doesn't require special privileges.
On Linux systems with 2.2 or later kernels, an interface argument
of ``any'' can be used to cap ture packets from all interfaces. Note that cap
tures on the ``any'' device will not be done in promiscuous mode.
Filter expression selects which packets will be dumped. If no expression is given, all packets on the net will be dumped. Otherwise, only packets for which expres sion is `true' will be dumped. ber) preceded by one or more qualifiers. There are three different kinds of qualifier:
[`fddi' is actually an alias for `ether'; the parser treats them identically as meaning ``the data link level used on the specified network interface.'' FDDI headers contain Ethernet-like source and destination addresses, and often contain Ethernet-like packet types, so you can filter on these FDDI fields just as with the analogous Ethernet fields. FDDI headers also contain other fields, but you cannot name them explicitly in a filter expression.
Similarly, `tr' is an alias for `ether'; the previous paragraph's statements about FDDI headers also apply to Token Ring headers.]
In addition to the above, there are some special `primitive' keywords that don't follow the pattern: gateway, broadcast, less, greater and arithmetic expressions. All of these are described below.
E.g., `host foo and not port ftp and not port ftp-data'. To save typing, identical qualifier lists can be omitted. E.g., `tcp dst port ftp or ftp-data or domain' is exactly the same as `tcp dst port ftp or tcp dst port ftp-data or tcp dst port domain'.
[In the case of FDDI (e.g., `fddi protocol arp') and Token Ring (e.g., `tr protocol arp'), for most of those protocols, the pro tocol identification comes from the 802.2 Logical Link Control (LLC) header, which is usually layered on top of the FDDI or Token Ring header.
When filtering for most protocol identifiers on FDDI or Token Ring, tcpdump checks only the protocol ID field of an LLC header in so-called SNAP format with an Organizational Unit Identifier (OUI) of 0x000000, for encapsulated Ethernet; it doesn't check whether the packet is in SNAP format with an OUI of 0x000000.
The exceptions are iso, for which it checks the DSAP (Destination Service Access Point) and SSAP (Source Service Access Point) fields of the LLC header, stp and netbeui, packet with an OUI of 0x080007 and the Appletalk etype.
In the case of Ethernet, tcpdump checks the Ethernet type field for most of those proto cols; the exceptions are iso, sap, and net beui, for which it checks for an 802.3 frame and then checks the LLC header as it does for FDDI and Token Ring, atalk, where it checks both for the Appletalk etype in an Ethernet frame and for a SNAP-format packet as it does for FDDI and Token Ring, aarp, where it checks for the Appletalk ARP etype in either an Ethernet frame or an 802.2 SNAP frame with an OUI of 0x000000, and ipx, where it checks for the IPX etype in an Eth ernet frame, the IPX DSAP in the LLC header, the 802.3 with no LLC header encapsulation of IPX, and the IPX etype in a SNAP frame.]
For example, `ether & 1 != 0' catches all multicast traffic. The expression `ip & 0xf != 5' catches all IP packets with options. The expression `ip[6:2] & 0x1fff = 0' catches only unfragmented datagrams and frag zero of fragmented datagrams. This always means the first byte of the TCP header, and never means the first byte of an intervening fragment.
Some offsets and field values may be expressed as names rather than as numeric values. The following protocol header field offsets are available: icmptype (ICMP type field), icmpcode (ICMP code field), and tcpflags (TCP flags field).
The following ICMP type field values are available: icmp-echoreply, icmp-unreach, icmp-sourcequench, icmp-redirect, icmp-echo, icmp-routeradvert, icmp-routersolicit, icmp- timxceed, icmp-paramprob, icmp-tstamp, icmp- tstampreply, icmp-ireq, icmp-ireqreply, icmp-maskreq, icmp-maskreply.
The following TCP flags field values are available: tcp-fin, tcp-syn, tcp-rst, tcp- push, tcp-push, tcp-ack, tcp-urg.
A parenthesized group of primitives and operators (parentheses are special to the Shell and must be escaped).
Negation (`!' or `not').
Concatenation (`&&' or `and').
Alternation (`||' or `or').
Negation has highest precedence. Alternation and concatenation have equal precedence and associate left to right. Note that explicit and tokens, not juxtaposition, are now required for concatenation.
If an identifier is given without a keyword, the most recent keyword is assumed. For example,
not host vs and ace
is short for not host vs and host ace
which should not be confused with not ( host vs or ace )
Expression arguments can be passed to tcpdump as either a single argument or as multiple arguments, whichever is more convenient. Generally, if the expression contains Shell metacharacters, it is before being parsed.
To print all packets arriving at or departing from sun down:
tcpdump host sundownTo print traffic between helios and either hot or ace:
tcpdump host helios and \( hot or ace \)To print all IP packets between ace and any host except helios:
tcpdump ip host ace and not heliosTo print all traffic between local hosts and hosts at Berkeley:
tcpdump net ucb-etherTo print all ftp traffic through internet gateway snup: (note that the expression is quoted to prevent the shell from (mis-)interpreting the parentheses):
tcpdump 'gateway snup and (port ftp or ftp-data)'To print traffic neither sourced from nor destined for local hosts (if you gateway to one other net, this stuff should never make it onto your local net).
tcpdump ip and not net localnetTo print the start and end packets (the SYN and FIN pack ets) of each TCP conversation that involves a non-local host.
tcpdump 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0 and not src and dst net localnet'To print IP packets longer than 576 bytes sent through gateway snup:
tcpdump 'gateway snup and ip[2:2] > 576'To print IP broadcast or multicast packets that were not sent via ethernet broadcast or multicast:
tcpdump 'ether & 1 = 0 and ip >= 224'To print all ICMP packets that are not echo requests/replies (i.e., not ping packets):
tcpdump 'icmp[icmptype] != icmp-echo and icmp[icmptype] != icmp-echoreply'
The output of tcpdump is protocol dependent. The following gives a brief description and examples of most of the formats.
Link Level Headersaddresses, protocol, and packet length are printed.
On FDDI networks, the '-e' option causes tcpdump to print the `frame control' field, the source and destination addresses, and the packet length. (The `frame control' field governs the interpretation of the rest of the packet. Normal packets (such as those containing IP data grams) are `async' packets, with a priority value between 0 and 7; for example, `async4'. Such packets are assumed to contain an 802.2 Logical Link Control (LLC) packet; the LLC header is printed if it is not an ISO datagram or a so-called SNAP packet.
Arp/rarp output shows the type of request and its argu ments. The format is intended to be self explanatory. arp who-has csam tell rtsg arp reply csam is-at CSAM The first line says that rtsg sent an arp packet asking for the ethernet address of internet host csam. Csam replies with its ethernet address (in this example, ether net addresses are in caps and internet addresses in lower case).
This would look less redundant if we had done tcpdump -n: arp who-has 220.127.116.11 tell 18.104.22.168 arp reply 22.214.171.124 is-at 02:07:01:00:01:c4
If we had done tcpdump -e, the fact that the first packet is broadcast and the second is point-to-point would be visible: RTSG Broadcast 0806 64: arp who-has csam tell rtsg CSAM RTSG 0806 64: arp reply csam is-at CSAM For the first packet this says the ethernet source address is RTSG, the destination is the ethernet broadcast address, the type field contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes.
(N.B.:The following description assumes familiarity with the TCP protocol described in RFC-793. If you are not familiar with the protocol, neither this description nor tcpdump will be of much use to you.)
The general format of a tcp protocol line is: src > dst: flags data-seqno ack window urgent options Src and dst are the source and destination IP addresses and ports. Flags are some combination of S (SYN), F (FIN), P (PUSH) or R (RST) or a single `.' (no flags). Data-seqno describes the portion of sequence space covered by the data in this packet (see example below). Ack is sequence number of the next data expected the other direc tion on this connection. Window is the number of bytes of receive buffer space available the other direction on this connection. Urg indicates there is `urgent' data in the packet. Options are tcp options enclosed in angle brack ets (e.g., <mss 1024>).
Src, dst and flags are always present. The other fields depend on the contents of the packet's tcp protocol header and are output only if appropriate.
Here is the opening portion of an rlogin from host rtsg to host csam. rtsg.1023 > csam.login: S 768512:768512(0) win 4096 <mss 1024> csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024> rtsg.1023 > csam.login: . ack 1 win 4096 rtsg.1023 > csam.login: P 2:21(19) ack 1 win 4096 csam.login > rtsg.1023: P 1:2(1) ack 21 win 4077 csam.login > rtsg.1023: P 2:3(1) ack 21 win 4077 urg 1 csam.login > rtsg.1023: P 3:4(1) ack 21 win 4077 urg 1 The first line says that tcp port 1023 on rtsg sent a packet to port login on csam. The S indicates that the SYN flag was set. The packet sequence number was 768512 and it contained no data. (The notation is `first:last(nbytes)' which means `sequence numbers first up to but not including last which is nbytes bytes of user data'.) There was no piggy-backed ack, the available receive window was 4096 bytes and there was a max-segment- size option requesting an mss of 1024 bytes.
Csam replies with a similar packet except it includes a piggy-backed ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no flags were set. The packet con tained no data so there is no data sequence number. Note that the ack sequence number is a small integer (1). The first time tcpdump sees a tcp `conversation', it prints the sequence number from the packet. On subsequent pack ets of the conversation, the difference between the cur rent packet's sequence number and this initial sequence number is printed. This means that sequence numbers after the first can be interpreted as relative byte positions in the conversation's data stream (with the first data byte each direction being `1'). `-S' will override this fea ture, causing the original sequence numbers to be output.
On the 6th line, rtsg sends csam 19 bytes of data (bytes 2 through 20 in the rtsg -> csam side of the conversation). The PUSH flag is set in the packet. On the 7th line, csam says it's received data sent by rtsg up to but not includ ing byte 21. Most of this data is apparently sitting in the socket buffer since csam's receive window has gotten 19 bytes smaller. Csam also sends one byte of data to rtsg in this packet. On the 8th and 9th lines, csam sends two bytes of urgent, pushed data to rtsg.
If the snapshot was small enough that tcpdump didn't cap ture the full TCP header, it interprets as much of the header as it can and then reports ``[|tcp]'' to indicate the remainder could not be interpreted. If the header contains a bogus option (one with a length that's either too small or beyond the end of the header), tcpdump reports it as ``[bad opt]'' and does not interpret any further options (since it's impossible to tell where they start). If the header length indicates options are pre sent but the IP datagram length is not long enough for the options to actually be there, tcpdump reports it as ``[bad hdr length]''.
There are 8 bits in the control bits section of the TCP header:
CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
Let's assume that we want to watch packets used in estab lishing a TCP connection. Recall that TCP uses a 3-way handshake protocol when it initializes a new connection; the connection sequence with regard to the TCP control bits is
1) Caller sends SYN 2) Recipient responds with SYN, ACK 3) Caller sends ACK
Now we're interested in capturing packets that have only the SYN bit set (Step 1). Note that we don't want packets from step 2 (SYN-ACK), just a plain initial SYN. What we need is a correct filter expression for tcpdump.
Recall the structure of a TCP header without options:
015 31 ----------------------------------------------------------------- | source port | destination port | ----------------------------------------------------------------- | sequence number | ----------------------------------------------------------------- | acknowledgment number | ----------------------------------------------------------------- | HL | rsvd |C|E|U|A|P|R|S|F| window size | ----------------------------------------------------------------- | TCP checksum | urgent pointer | -----------------------------------------------------------------A TCP header usually holds 20 octets of data, unless options are present. The first line of the graph contains octets 0 - 3, the second line shows octets 4 - 7 etc. Starting to count with 0, the relevant TCP control bits are contained in octet 13:
0 7| 15| 23| 31 ----------------|---------------|---------------|---------------- | HL | rsvd |C|E|U|A|P|R|S|F| window size | ----------------|---------------|---------------|---------------- | | 13th octet | | |Let's have a closer look at octet no. 13:
| | |---------------| |7 5 3 0|These are the TCP control bits we are interested in. We have numbered the bits in this octet from 0 to 7, right to left, so the PSH bit is bit number 3, while the URG bit is number 5.
Recall that we want to capture packets with only SYN set. Let's see what happens to octet 13 if a TCP datagram arrives with the SYN bit set in its header:
|C|E|U|A|P|R|S|F| |---------------| |0 0 0 0 0 0 1 0| |---------------| |7 6 5 4 3 2 1 0|Looking at the control bits section we see that only bit number 1 (SYN) is set. Assuming that octet number 13 is an 8-bit unsigned integer in network byte order, the binary value of this octet is
00000010and its decimal representation is
7 6 5 4 3 2 1 0 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2We're almost done, because now we know that if only SYN is set, the value of the 13th octet in the TCP header, when interpreted as a 8-bit unsigned integer in network byte order, must be exactly 2.
This relationship can be expressed as tcp == 2
We can use this expression as the filter for tcpdump in order to watch packets which have only SYN set: tcpdump -i xl0 tcp == 2
The expression says "let the 13th octet of a TCP datagram have the decimal value 2", which is exactly what we want.
Now, let's assume that we need to capture SYN packets, but we don't care if ACK or any other TCP control bit is set at the same time. Let's see what happens to octet 13 when a TCP datagram with SYN-ACK set arrives:
|C|E|U|A|P|R|S|F| |---------------| |7 6 5 4 3 2 1 0|
Now bits 1 and 4 are set in the 13th octet. The binary value of octet 13 is
00010010which translates to decimal
7 6 5 4 3 2 1 0 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18Now we can't just use 'tcp == 18' in the tcpdump fil ter expression, because that would select only those pack ets that have SYN-ACK set, but not those with only SYN set. Remember that we don't care if ACK or any other con trol bit is set as long as SYN is set. In order to achieve our goal, we need to logically AND the binary value of octet 13 with some other value to preserve the SYN bit. We know that we want SYN to be set in any case, so we'll logically AND the value in the 13th octet with the binary value of a SYN:
00010010 SYN-ACK00000010 SYN AND 00000010 (we want SYN) AND 00000010 (we want SYN) -------- -------- = 00000010 = 00000010
We see that this AND operation delivers the same result regardless whether ACK or another TCP control bit is set. The decimal representation of the AND value as well as the result of this operation is 2 (binary 00000010), so we know that for packets with SYN set the following relation must hold true:
( ( value of octet 13 ) AND ( 2 ) ) == ( 2 )This points us to the tcpdump filter expression
tcpdump -i xl0 'tcp & 2 == 2'Note that you should use single quotes or a backslash in the expression to hide the AND ('&') special character from the shell.
UDP format is illustrated by this rwho packet:
actinide.who > broadcast.who: udp 84This says that port who on host actinide sent a udp data
Some UDP services are recognized (from the source or destination port number) and the higher level protocol infor mation printed. In particular, Domain Name service requests (RFC-1034/1035) and Sun RPC calls (RFC-1050) to NFS.
UDP Name Server Requests
(N.B.:The following description assumes familiarity with the Domain Service protocol described in RFC-1035. If you are not familiar with the protocol, the following descrip tion will appear to be written in greek.)
Name server requests are formatted as src > dst: id op? flags qtype qclass name (len) h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37) Host h2opolo asked the domain server on helios for an address record (qtype=A) associated with the name ucb vax.berkeley.edu. The query id was `3'. The `+' indi cates the recursion desired flag was set. The query length was 37 bytes, not including the UDP and IP protocol headers. The query operation was the normal one, Query, so the op field was omitted. If the op had been anything else, it would have been printed between the `3' and the `+'. Similarly, the qclass was the normal one, C_IN, and omitted. Any other qclass would have been printed immedi ately after the `A'.
A few anomalies are checked and may result in extra fields enclosed in square brackets: If a query contains an answer, authority records or additional records section, ancount, nscount, or arcount are printed as `[na]', `[nn]' or `[nau]' where n is the appropriate count. If any of the response bits are set (AA, RA or rcode) or any of the `must be zero' bits are set in bytes two and three, `[b2&3=x]' is printed, where x is the hex value of header bytes two and three.
UDP Name Server Responses
Name server responses are formatted as src > dst: id op rcode flags a/n/au type class data (len) helios.domain > h2opolo.1538: 3 3/3/7 A 126.96.36.199 (273) helios.domain > h2opolo.1537: 2 NXDomain* 0/1/0 (97)
In the first example, helios responds to query id 3 from h2opolo with 3 answer records, 3 name server records and 7 additional records. The first answer record is type A (address) and its data is internet address 188.8.131.52. 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 response code of non-existent domain (NXDomain) with no answers, one name server and no authority records. The `*' indicates that the authoritative answer bit was set. Since there were no answers, no type, class or data were printed.
Other flag characters that might appear are `-' (recursion available, RA, not set) and `|' (truncated message, TC, set). If the `question' section doesn't contain exactly one entry, `[nq]' is printed.
Note that name server requests and responses tend to be large and the default snaplen of 68 bytes may not capture enough of the packet to print. Use the -s flag to increase the snaplen if you need to seriously investigate name server traffic. `-s 128' has worked well for me.
tcpdump now includes fairly extensive SMB/CIFS/NBT decod ing for data on UDP/137, UDP/138 and TCP/139. Some primi tive decoding of IPX and NetBEUI SMB data is also done.
By default a fairly minimal decode is done, with a much more detailed decode done if -v is used. Be warned that with -v a single SMB packet may take up a page or more, so only use -v if you really want all the gory details.
If you are decoding SMB sessions containing unicode strings then you may wish to set the environment variable USE_UNICODE to 1. A patch to auto-detect unicode srings would be welcome.
For information on SMB packet formats and what all te fields mean see www.cifs.org or the pub/samba/specs/ directory on your favourite samba.org mirror site. The SMB patches were written by Andrew Tridgell (email@example.com).
NFS Requests and Replies
Sun NFS (Network File System) requests and replies are printed as: src.xid > dst.nfs: len op args src.nfs > dst.xid: reply stat len op results
sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165 wrl.nfs > sushi.6709: reply ok 40 readlink "../var" sushi.201b > wrl.nfs: 144 lookup fh 9,74/4096.6878 "xcolors"
In the first line, host sushi sends a transaction with id 6709 to wrl (note that the number following the src host is a transaction id, not the source port). The request was 112 bytes, excluding the UDP and IP headers. The operation was a readlink (read symbolic link) on file han dle (fh) 21,24/10.731657119. (If one is lucky, as in this case, the file handle can be interpreted as a major,minor device number pair, followed by the inode number and gen eration number.) Wrl replies `ok' with the contents of the link.
In the third line, sushi asks wrl to lookup the name `xcolors' in directory file 9,74/4096.6878. Note that the data printed depends on the operation type. The format is intended to be self explanatory if read in conjunction with an NFS protocol spec.
If the -v (verbose) flag is given, additional information is printed. For example:
sushi.1372a > wrl.nfs: 148 read fh 21,11/12.195 8192 bytes @ 24576 wrl.nfs > sushi.1372a: reply ok 1472 read REG 100664 ids 417/0 sz 29388
(-v also prints the IP header TTL, ID, length, and frag mentation fields, which have been omitted from this exam ple.) 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 the second line is the first fragment of the reply, and hence is only 1472 bytes long (the other bytes will follow in subsequent fragments, but these fragments do not have NFS or even UDP headers and so might not be printed, depending on the filter expression used). Because the -v flag is given, some of the file attributes (which are returned in addition to the file data) are printed: the file type (``REG'', for regu lar file), the file mode (in octal), the uid and gid, and the file size.
If the -v flag is given more than once, even more details are printed.
Note that NFS requests are very large and much of the detail won't be printed unless snaplen is increased. Try using `-s 192' to watch NFS traffic.
NFS reply packets do not explicitly identify the RPC oper ation. Instead, tcpdump keeps track of ``recent'' requests, and matches them to the replies using the trans action ID. If a reply does not closely follow the corre
Transarc AFS (Andrew File System) requests and replies are printed as:
src.sport > dst.dport: rx packet-type src.sport > dst.dport: rx packet-type service call call-name args src.sport > dst.dport: rx packet-type service reply call-name args
elvis.7001 > pike.afsfs: rx data fs call rename old fid 536876964/1/1 ".newsrc.new" new fid 536876964/1/1 ".newsrc" pike.afsfs > elvis.7001: rx data fs reply rename
In the first line, host elvis sends a RX packet to pike. This was a RX data packet to the fs (fileserver) service, and is the start of an RPC call. The RPC call was a rename, with the old directory file id of 536876964/1/1 and an old filename of `.newsrc.new', and a new directory file id of 536876964/1/1 and a new filename of `.newsrc'. The host pike responds with a RPC reply to the rename call (which was successful, because it was a data packet and not an abort packet).
In general, all AFS RPCs are decoded at least by RPC call name. Most AFS RPCs have at least some of the arguments decoded (generally only the `interesting' arguments, for some definition of interesting).
The format is intended to be self-describing, but it will probably not be useful to people who are not familiar with the workings of AFS and RX.
If the -v (verbose) flag is given twice, acknowledgement packets and additional header information is printed, such as the 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 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 id are printed.
Error codes are printed for abort packets, with the excep tion of Ubik beacon packets (because abort packets are used to signify a yes vote for the Ubik protocol).
Note that AFS requests are very large and many of the arguments won't be printed unless snaplen is increased. Try using `-s 256' to watch AFS traffic. ation. Instead, tcpdump keeps track of ``recent'' requests, and matches them to the replies using the call number and service ID. If a reply does not closely follow the corresponding request, it might not be parsable.
KIP Appletalk (DDP in UDP)
Appletalk DDP packets encapsulated in UDP datagrams are de-encapsulated and dumped as DDP packets (i.e., all the UDP header information is discarded). The file /etc/atalk.names is used to translate appletalk net and node numbers to names. Lines in this file have the form number name
1.254 ether 16.1 icsd-net 1.254.110 ace
The first two lines give the names of appletalk networks. The third line gives the name of a particular host (a host is distinguished from a net by the 3rd octet in the number - a net number must have two octets and a host number must have three octets.) The number and name should be sepa rated by whitespace (blanks or tabs). The /etc/atalk.names file may contain blank lines or comment lines (lines starting with a `#').
Appletalk addresses are printed in the form net.host.port 184.108.40.206 > icsd-net.112.220 office.2 > icsd-net.112.220 jssmag.149.235 > icsd-net.2 (If the /etc/atalk.names doesn't exist or doesn't contain an entry for some appletalk host/net number, addresses are printed in numeric form.) In the first example, NBP (DDP port 2) on net 144.1 node 209 is sending to whatever is listening on port 220 of net icsd node 112. The second line is the same except the full name of the source node is known (`office'). The third line is a send from port 235 on net jssmag node 149 to broadcast on the icsd-net NBP port (note that the broadcast address (255) is indi cated by a net name with no host number - for this reason it's a good idea to keep node names and net names distinct in /etc/atalk.names).
NBP (name binding protocol) and ATP (Appletalk transaction protocol) packets have their contents interpreted. Other protocols just dump the protocol name (or number if no name is registered for the protocol) and packet size.
NBP packets are formatted like the following examples: techpit.2 > icsd-net.112.220: nbp-reply 190: "techpit:LaserWriter@*" 186 The first line is a name lookup request for laserwriters sent by net icsd host 112 and broadcast on net jssmag. The nbp id for the lookup is 190. The second line shows a reply for this request (note that it has the same id) from host jssmag.209 saying that it has a laserwriter resource named "RM1140" registered on port 250. The third line is another reply to the same request saying host techpit has laserwriter "techpit" registered on port 186.
ATP packet formatting is demonstrated by the following example: jssmag.209.165 > helios.132: atp-req 12266<0-7> 0xae030001 helios.132 > jssmag.209.165: atp-resp 12266:0 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:1 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:2 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:4 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:6 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp*12266:7 (512) 0xae040000 jssmag.209.165 > helios.132: atp-req 12266<3,5> 0xae030001 helios.132 > jssmag.209.165: atp-resp 12266:3 (512) 0xae040000 helios.132 > jssmag.209.165: atp-resp 12266:5 (512) 0xae040000 jssmag.209.165 > helios.132: atp-rel 12266<0-7> 0xae030001 jssmag.209.133 > helios.132: atp-req* 12267<0-7> 0xae030002
Jssmag.209 initiates transaction id 12266 with host helios by requesting up to 8 packets (the `<0-7>'). The hex num ber at the end of the line is the value of the `userdata' field in the request.
Helios responds with 8 512-byte packets. The `:digit' following the transaction id gives the packet sequence number in the transaction and the number in parens is the amount of data in the packet, excluding the atp header. The `*' on packet 7 indicates that the EOM bit was set. Jssmag.209 then requests that packets 3 & 5 be retransmit ted. Helios resends them then jssmag.209 releases the transaction. Finally, jssmag.209 initiates the next request. The `*' on the request indicates that XO (`exactly once') was not set.
Fragmented Internet datagrams are printed as (frag id:size@offset+) (frag id:size@offset) (The first form indicates there are more fragments. The second indicates this is the last fragment.)
offset (in bytes) in the original datagram.
The fragment information is output for each fragment. The first fragment contains the higher level protocol header and the frag info is printed after the protocol info. Fragments after the first contain no higher level protocol header and the frag info is printed after the source and destination addresses. For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa over a CSNET connec tion that doesn't appear to handle 576 byte datagrams: arizona.ftp-data > rtsg.1170: . 1024:1332(308) ack 1 win 4096 (frag 595a:328@0+) arizona > rtsg: (frag 595a:204@328) rtsg.1170 > arizona.ftp-data: . ack 1536 win 2560
There are a couple of things to note here: First, addresses in the 2nd line don't include port numbers. This is because the TCP protocol information is all in the first fragment and we have no idea what the port or sequence numbers are when we print the later fragments. Second, the tcp sequence information in the first line is printed as if there were 308 bytes of user data when, in fact, there are 512 bytes (308 in the first frag and 204 in the second). If you are looking for holes in the sequence space or trying to match up acks with packets, this can fool you. A packet with the IP don't fragment flag is marked with a trailing (DF).
By default, all output lines are preceded by a timestamp. The timestamp is the current clock time in the form hh:mm:ss.frac and is as accurate as the kernel's clock. The timestamp reflects the time the kernel first saw the packet. No attempt is made to account for the time lag between when the ethernet interface removed the packet from the wire and when the kernel serviced the `new packet' interrupt.
traffic(1C), nit(4P), bpf(4), pcap(3)
The original authors are: Van Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley National Laboratory, University of Cali fornia, Berkeley, CA. It is currently being maintained by tcpdump.org. The current version is available via http: The original distribution is available via anonymous ftp: ftp://ftp.ee.lbl.gov/tcpdump.tar.Z IPv6/IPsec support is added by WIDE/KAME project. This program uses Eric Young's SSLeay library, under specific configuration.
Please send problems, bugs, questions, desirable enhance ments, etc. to: firstname.lastname@example.org Please send source code contributions, etc. to: email@example.com NIT doesn't let you watch your own outbound traffic, BPF will. We recommend that you use the latter.
Name server inverse queries are not dumped correctly: the (empty) question section is printed rather than real query in the answer section. Some believe that inverse queries are themselves a bug and prefer to fix the program gener ating them rather than tcpdump.
A packet trace that crosses a daylight savings time change will give skewed time stamps (the time change is ignored). headers assume that all FDDI and Token Ring packets are SNAP-encapsulated Ethernet packets. This is true for IP, ARP, and DECNET Phase IV, but is not true for protocols such as ISO CLNS. Therefore, the filter may inadvertently accept certain packets that do not properly match the fil ter expression.
Filter expressions on fields other than those that manipu late Token Ring headers will not correctly handle source- routed Token Ring packets.
ip6 proto should chase header chain, but at this moment it does not. ip6 protochain is supplied for this behavior.
Arithmetic expression against transport layer headers, like tcp, does not work against IPv6 packets. It only looks at IPv4 packets.
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Last modified: September 24, 2013