Group8_VINTAGE BOYS: Tcpdump presentation

Group8_Vintage Boys: Reading tcpdump’s output

By default tcpdump produces one line of text per every packet it intercepts. It 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.

Each line starts with a time stamp. It tells you very precise time when packet arrived.

Next comes protocol name. Unfortunately, tcpdump understands very limited number of protocols. It won’t tell you the difference between packets belonging to HTTP and for instance FTP stream. Instead, it will mark such packets as IP packets. It does have some limited understanding of TCP. For instance it identifies TCP synchronization packets such as SYN, ACK, FIN and others. This information printed after source and destination IP addresses (if it IP packet).Source and destination addresses follow protocol name. For IP packets, these are IP addresses. For other protocols, tcpdump does not print any identifiers unless explicitly asked to do so (using -e command line switch).

Finally, tcpdump prints some information about the packet. For instance, it prints TCP sequence numbers, flags, ARP/ICMP commands, etc.

Here’s an example of typical tcpdump output.

17:50:03.089893 IP 69.61.72.101.www > 212.150.66.73.48777: P 1366488174:1366488582

(408) ack 2337505545 win 7240 <nop,nop,timestamp 1491222906 477679143>

This packet is the part of HTTP data stream. We can understand the meaning of each and every field in the packet description in later part of our discussion.

Here’s an other example.

17:50:00.718266 arp who-has 69.61.72.185 tell 69.61.72.1

This is ARP packet. It’s slightly more self explanatory than TCP packets. Again, to see exact meaning of each field in the packet description we will discuss it later on.

Here are some more examples for understanding tcpdump with their explanation.

To  print  all  packets arriving at or departing from game:

# tcpdump host game

To print traffic between helios and either hot or ace:

# tcpdump host helios and \( hot or ace \)

To print all IP packets between ace and  any  host  except helios:

# tcpdump ip host ace and not helios

To print all traffic between local hosts and hosts at IIT:

# tcpdump net iit-ether

To print all ftp traffic through  internet  gateway  snup:

# tcpdump ‘gateway snup and (port ftp or ftp-data)’

To print traffic neither sourced  from  nor  destined  for local  hosts  (if you gateway to one other net, this stuff should never make it onto your local net).

# tcpdump ip and not net localnet

GROUP8_Vintage Boys: TCPDUMP Syntax

The TCPDUMP Syntax and there explanations are as following:

tcpdump

[ -AdDeflLnNOpqRStuUvxX ] [ -c count ] [ -C file_size ] [ -F file ] [ -i interface ] [ -m module ] [ -M secret ] [ -r file ] [ -s snaplen ] [ -T type ] [ -w file ] [ -W filecount ] [ -E spi@ipaddr algo:secret,... ] [ -y datalinktype ] [ -Z user ] [ expression ]

-A	Print each packet (minus its link level header) in ASCII. Handy for capturing web pages.
-c	Exit after receiving count packets.
-C	Before writing a raw packet to a savefile, check whether the file is currently larger than file_size and, if so, close the current savefile and open a new one. Savefiles after the first savefile will have the name specified with the -w flag, with a number after it, starting at 1 and continuing upward. The units of file_size are millions of bytes (1,000,000 bytes, not 1,048,576 bytes).
-d	Dump the compiled packet-matching code in a human readable form to standard output and stop.
-dd	Dump packet-matching code as a C program fragment.
-ddd	Dump packet-matching code as decimal numbers (preceded with a count).
-D	Print the list of the network interfaces available on the system and on which tcpdump can capture packets. For each network interface, a number and an interface name, possibly followed by a text description of the interface, is printed. The interface name or the number can be supplied to the -i flag to specify an interface on which to capture. This can be useful on systems that don’t have a command to list them (e.g., Windows systems, or UNIX systems lacking ifconfig -a); the number can be useful on Windows 2000 and later systems, where the interface name is a somewhat complex string. The -D flag will not be supported if tcpdump was built with an older version of libpcap that lacks the pcap_findalldevs() function.
-e	Print the link-level header on each dump line.
-E	Use spi@ipaddr algo:secret for decrypting IPsec ESP packets that are addressed to addr and contain Security Parameter Index value spi. This combination may be repeated with comma or newline separation. Note that setting the secret for IPv4 ESP packets is supported at this time. Algorithms may be des-cbc, 3des-cbc, blowfish-cbc, rc3-cbc, cast128-cbc, or none. The default is des-cbc. The ability to decrypt packets is only present if tcpdump was compiled with cryptography enabled. secret is the ASCII text for ESP secret key. If preceded by 0x, then a hex value will be read. The option assumes RFC2406 ESP, not RFC1827 ESP. The option is only for debugging purposes, and the use of this option with a true `secret’ key is discouraged. By presenting IPsec secret key onto command line you make it visible to others, via ps and other occasions. In addition to the above syntax, the syntax file name may be used to have tcpdump read the provided file in. The file is opened upon receiving the first ESP packet, so any special permissions that tcpdump may have been given should already have been given up.
-f	Print `foreign’ IPv4 addresses numerically rather than symbolically (this option is intended to get around serious brain damage in Sun’s NIS server — usually it hangs forever translating non-local Internet numbers). The test for `foreign’ IPv4 addresses is done using the IPv4 address and netmask of the interface on which capture is being done. If that address or netmask are not available, available, either because the interface on which capture is being done has no address or netmask or because the capture is being done on the Linux “any” interface, which can capture on more than one interface, this option will not work correctly.
-F	Use file as input for the filter expression. An additional expression given on the command line is ignored.
-i	Listen on interface. If unspecified, tcpdump searches the system interface list for the lowest numbered, configured up interface (excluding loopback). Ties are broken by choosing the earliest match. On Linux systems with 2.2 or later kernels, an interface argument of “any” can be used to capture packets from all interfaces. Note that captures on the “any” device will not be done in promiscuous mode. If the -D flag is supported, an interface number as printed by that flag can be used as the interface argument.
-l	Make stdout line buffered. Useful if you want to see the data while capturing it. E.g., “tcpdump -l | tee dat” or “tcpdump -l > dat & tail -f dat”.
-L	List the known data link types for the interface and exit.
-m	Load SMI MIB module definitions from file module. This option can be used several times to load several MIB modules into tcpdump.
-M	Use secret as a shared secret for validating the digests found in TCP segments with the TCP-MD5 option (RFC 2385), if present.
-n	Don’t convert addresses (i.e., host addresses, port numbers, etc.) to names.
-N	Don’t print domain name qualification of host names. E.g., if you give this flag then tcpdump will print “nic” instead of “nic.ddn.mil”.
-O	Do not run the packet-matching code optimizer. This is useful only if you suspect a bug in the optimizer.
-p	Don’t put the interface into promiscuous mode. Note that the interface might be in promiscuous mode for some other reason; hence, `-p’ cannot be used as an abbreviation for `ether host {local-hw-addr} or ether broadcast’.
-q	Quick (quiet?) output. Print less protocol information so output lines are shorter.
-R	Assume ESP/AH packets to be based on old specification (RFC1825 to RFC1829). If specified, tcpdump will not print replay prevention field. Since there is no protocol version field in ESP/AH specification, tcpdump cannot deduce the version of ESP/AH protocol.
-r	Read packets from file (which was created with the -w option). Standard input is used if file is “-”.
-S	Print absolute, rather than relative, TCP sequence numbers.
-s	Snarf snaplen bytes of data from each packet rather than the default of 68 (with SunOS’s NIT, the minimum is actually 96). 68 bytes is adequate for IP, ICMP, TCP and UDP but may truncate protocol information from name server and NFS packets (see below). Packets truncated because of a limited snapshot are indicated in the output with “[|proto]”, where proto is the name of the protocol level at which the truncation has occurred. Note that taking larger snapshots both increases the amount of time it takes to process packets and, effectively, decreases the amount of packet buffering. This may cause packets to be lost. You should limit snaplen to the smallest number that will capture the protocol information you’re interested in. Setting snaplen to 0 means use the required length to catch whole packets.
	-T Force packets selected by “expression” to be interpreted the specified type. Currently known types are aodv (Ad-hoc On-demand Distance Vector protocol), cnfp (Cisco NetFlow protocol), rpc (Remote Procedure Call), rtp (Real-Time Applications protocol), rtcp (Real-Time Applications control protocol), snmp (Simple Network Management Protocol), tftp (Trivial File Transfer Protocol), vat (Visual Audio Tool), and wb (distributed White Board).
-t	Don’t print a timestamp on each dump line.
-tt	Print an unformatted timestamp on each dump line.
-ttt	Print a delta (in micro-seconds) between current and previous line on each dump line.
-tttt	Print a timestamp in default format proceeded by date on each dump line.
-u	Print undecoded NFS handles.
-U	Make output saved via the -w option “packet-buffered”; i.e., as each packet is saved, it will be written to the output file, rather than being written only when the output buffer fills. The -U flag will not be supported if tcpdump was built with an older version of libpcap that lacks the pcap_dump_flush() function.
-v	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 the IP and ICMP header checksum. When writing to a file with the -w option, report, every 10 seconds, the number of packets captured.
-vv	Even more verbose output. For example, additional fields are printed from NFS reply packets, and SMB packets are fully decoded.
-vvv	Even more verbose output. For example, telnet SB … SE options are printed in full. With -X Telnet options are printed in hex as well.
-w	Write the raw packets to file rather than parsing and printing them out. They can later be printed with the -r option. Standard output is used if file is “-”.
-W	Used in conjunction with the -C option, this will limit the number of files created to the specified number, and begin overwriting files from the beginning, thus creating a ‘rotating’ buffer. In addition, it will name the files with enough leading 0s to support the maximum number of files, allowing them to sort correctly.
-x	When parsing and printing, in addition to printing the headers of each packet, print the data of each packet (minus its link level header) in hex. The smaller of the entire packet or snaplen bytes will be printed. Note that this is the entire link-layer packet, so for link layers that pad (e.g. Ethernet), the padding bytes will also be printed when the higher layer packet is shorter than the required padding.
-xx	When parsing and printing, in addition to printing the headers of each packet, print the data of each packet, including its link level header, in hex.
-X	When parsing and printing, in addition to printing the headers of each packet, print the data of each packet (minus its link level header) in hex and ASCII. This is very handy for analyzing new protocols.
-XX	When parsing and printing, in addition to printing the headers of each packet, print the data of each packet, including its link level header, in hex and ASCII.
-y	Set the data link type to use while capturing packets to datalinktype.
-Z	Drops privileges (if root) and changes user ID to user and the group ID to the primary group of user. This behavior can also be enabled by default at compile time.