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TCPDUMP !!!TCPDUMP NAME SYNOPSIS DESCRIPTION OPTIONS EXAMPLES OUTPUT FORMAT SEE ALSO AUTHORS BUGS ---- !!NAME tcpdump - dump traffic on a network !!SYNOPSIS __tcpdump__ [[ __-adeflnNOpqRStvxX__ ] [[ __-c__ ''count'' ] [[ __-F__ ''file'' ] [[ __-i__ ''interface'' ] [[ __-m__ ''module'' ] [[ __-r__ ''file'' ] [[ __-s__ ''snaplen'' ] [[ __-T__ ''type'' ] [[ __-w__ ''file'' ] [[ __-E__ ''algo:secret'' ] [[ ''expression'' ] !!DESCRIPTION ''Tcpdump'' prints out the headers of packets on a network interface that match the boolean ''expression''. __Under SunOS with nit or bpf:__ To run ''tcpdump'' 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''. __Under HP-UX with dlpi:__ You must be root or it must be installed setuid to root. __Under IRIX with snoop:__ You must be root or it must be installed setuid to root. __Under Linux:__ You must be root or it 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 run __tcpdump__. __Under BSD:__ You must have read access to ''/dev/bpf*''. !!OPTIONS __-a__ Attempt to convert network and broadcast addresses to names. __-c__ Exit after receiving ''count'' packets. __-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). __-e__ Print the link-level header on each dump line. __-E__ Use ''algo:secret'' for decrypting IPsec ESP packets. 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'' the ascii text for ESP secret key. We cannot take arbitrary binary value at this moment. The option assumes RFC2406 ESP, not RFC1827 ESP. The option is only for debugging purposes, and the use of this option with truly `secret' key is discouraged. By presenting IPsec secret key onto command line you make it visible to others, via ps(1) and other occasions. __-f__ Print `foreign' internet addresses numerically rather than symbolically (this option is intended to get around serious brain damage in Sun's yp server -- usually it hangs forever translating non-local internet numbers). __-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. __-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 __-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''. __-m__ Load SMI MIB module definitions from file ''module''. This option can be used several times to load several MIB modules into ''tcpdump''. __-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__ Read packets from ''file'' (which was created with the -w option). Standard input is used if ''file'' is ``-''. __-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'' ''type''. Currently known types are __cnfp__ (Cisco !NetFlow protocol), __rpc__ (Remote Procedure Call), __rtp__ (Real-Time Applications protocol), __rtcp__ (Real-Time Applications control protocol), __snmp__ (Simple Network Management Protocol), __vat__ (Visual Audio Tool), and __wb__ (distributed White Board). __-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. __-S__ Print absolute, rather than relative, TCP sequence numbers. __-t__ ''Don't'' print a timestamp on each dump line. __-tt__ Print an unformatted timestamp on each dump line. __-v__ (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. __-vv__ Even more verbose output. For example, additional fields are printed from NFS reply packets. __-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 ``-''. __-x__ Print each packet (minus its link level header) in hex. The smaller of the entire packet or ''snaplen'' bytes will be printed. __-X__ When printing hex, print ascii too. Thus if __-x__ is also set, the packet is printed in hex/ascii. This is very handy for analysing new protocols. Even if __-x__ is not also set, some parts of some packets may be printed in hex/ascii. ''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 ''expression'' is `true' will be dumped. The ''expression'' consists of one or more ''primitives.'' Primitives usually consist of an ''id'' (name or number) preceded by one or more qualifiers. There are three different kinds of qualifier: ''type'' qualifiers say what kind of thing the id name or number refers to. Possible types are __host__, __net__ and __port__. E.g., `host foo', `net 128.3', `port 20'. If there is no type qualifier, __host__ is assumed. ''dir'' qualifiers specify a particular transfer direction to and/or from ''id''. Possible directions are __src__, __dst__, __src or dst__ and __src and dst__. E.g., `src foo', `dst net 128.3', `src or dst port ftp-data'. If there is no dir qualifier, __src or dst__ is assumed. For `null' link layers (i.e. point to point protocols such as slip) the __inbound__ and __outbound__ qualifiers can be used to specify a desired direction. ''proto'' qualifiers restrict the match to a particular protocol. Possible protos are: __ether__, __fddi__, __tr__, __ip__, __ip6__, __arp__, __rarp__, __decnet__, __tcp__ and __udp__. E.g., `ether src foo', `arp net 128.3', `tcp port 21'. If there is no proto qualifier, all protocols consistent with the type are assumed. E.g., `src foo' means `(ip or arp or rarp) src foo' (except the latter is not legal syntax), `net bar' means `(ip or arp or rarp) net bar' and `port 53' means `(tcp or udp) port 53'. [[`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. More complex filter expressions are built up by using the words __and__, __or__ and __not__ to combine primitives. 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'. Allowable primitives are: __dst host__ ''host'' True if the IPv4/v6 destination field of the packet is ''host'', which may be either an address or a name. __src host__ ''host'' True if the IPv4/v6 source field of the packet is ''host''. __host__ ''host'' True if either the IPv4/v6 source or destination of the packet is ''host''. Any of the above host expressions can be prepended with the keywords, __ip__, __arp__, __rarp__, or __ip6__ as in: __ip host__ ''host '' which is equivalent to: __ether proto__ ''ip'' __and host__ ''host '' If ''host'' is a name with multiple IP addresses, each address will be checked for a match. __ether dst__ ''ehost'' True if the ethernet destination address is ''ehost''. ''Ehost'' may be either a name from /etc/ethers or a number (see ''ethers''(3N) for numeric format). __ether src__ ''ehost'' True if the ethernet source address is ''ehost''. __ether host__ ''ehost'' True if either the ethernet source or destination address is ''ehost''. __gateway__ ''host'' True if the packet used ''host'' as a gateway. I.e., the ethernet source or destination address was ''host'' but neither the IP source nor the IP destination was ''host''. ''Host'' must be a name and must be found in both /etc/hosts and /etc/ethers. (An equivalent expression is __ether host__ ''ehost'' __and not host__ ''host '' which can be used with either names or numbers for ''host / ehost''.) This syntax does not work in IPv6-enabled configuration at this moment. __dst net__ ''net'' True if the IPv4/v6 destination address of the packet has a network number of ''net''. ''Net'' may be either a name from /etc/networks or a network number (see ''networks(4)'' for details). __src net__ ''net'' True if the IPv4/v6 source address of the packet has a network number of ''net''. __net__ ''net'' True if either the IPv4/v6 source or destination address of the packet has a network number of ''net''. __net__ ''net'' __mask__ ''mask'' True if the IP address matches ''net'' with the specific netmask. May be qualified with __src__ or __dst__. Note that this syntax is not valid for IPv6 ''net''. __net__ ''net''/''len'' True if the IPv4/v6 address matches ''net'' a netmask ''len'' bits wide. May be qualified with __src__ or __dst__. __dst port__ ''port'' True if the packet is ip/tcp, ip/udp, ip6/tcp or ip6/udp and has a destination port value of ''port''. The ''port'' can be a number or a name used in /etc/services (see ''tcp''(4P) and ''udp''(4P)). If a name is used, both the port number and protocol are checked. If a number or ambiguous name is used, only the port number is checked (e.g., __dst port 513__ will print both tcp/login traffic and udp/who traffic, and __port domain__ will print both tcp/domain and udp/domain traffic). __src port__ ''port'' True if the packet has a source port value of ''port''. __port__ ''port'' True if either the source or destination port of the packet is ''port''. Any of the above port expressions can be prepended with the keywords, __tcp__ or __udp__, as in: __tcp src port__ ''port '' which matches only tcp packets whose source port is ''port''. __less__ ''length'' True if the packet has a length less than or equal to ''length''. This is equivalent to: __len __ ''length''__. __ __greater__ ''length'' True if the packet has a length greater than or equal to ''length''. This is equivalent to: __len __ ''length''__. __ __ip proto__ ''protocol'' True if the packet is an IP packet (see ''ip''(4P)) of protocol type ''protocol''. ''Protocol'' can be a number or one of the names ''icmp'', ''icmp6'', ''igmp'', ''igrp'', ''pim'', ''ah'', ''esp'', ''udp'', or ''tcp''. Note that the identifiers ''tcp'', ''udp'', and ''icmp'' are also keywords and must be escaped via backslash (), which is \ in the C-shell. Note that this primitive does not chase protocol header chain. __ip6 proto__ ''protocol'' True if the packet is an IPv6 packet of protocol type ''protocol''. Note that this primitive does not chase protocol header chain. __ip6 protochain__ ''protocol'' True if the packet is IPv6 packet, and contains protocol header with type ''protocol'' in its protocol header chain. For example, __ip6 protochain 6 __ matches any IPv6 packet with TCP protocol header in the protocol header chain. The packet may contain, for example, authentication header, routing header, or hop-by-hop option header, between IPv6 header and TCP header. The BPF code emitted by this primitive is complex and cannot be optimized by BPF optimizer code in ''tcpdump'', so this can be somewhat slow. __ip protochain__ ''protocol'' Equivalent to __ip6 protochain__ ''protocol'', but this is for IPv4. __ether broadcast__ True if the packet is an ethernet broadcast packet. The ''ether'' keyword is optional. __ip broadcast__ True if the packet is an IP broadcast packet. It checks for both the all-zeroes and all-ones broadcast conventions, and looks up the local subnet mask. __ether multicast__ True if the packet is an ethernet multicast packet. The ''ether'' keyword is optional. This is shorthand for `__ether[[0] __'. __ip multicast__ True if the packet is an IP multicast packet. __ip6 multicast__ True if the packet is an IPv6 multicast packet. __ether proto__ ''protocol'' True if the packet is of ether type ''protocol''. ''Protocol'' can be a number or one of the names ''ip'', ''ip6'', ''arp'', ''rarp'', ''atalk'', ''aarp'', ''decnet'', ''sca'', ''lat'', ''mopdl'', ''moprc'', or ''iso''. Note these identifiers are also keywords and must be escaped via backslash (). [[In the case of FDDI (e.g., `__fddi protocol arp__'), the protocol identification comes from the 802.2 Logical Link Control (LLC) header, which is usually layered on top of the FDDI header. ''Tcpdump'' assumes, when filtering on the protocol identifier, that all FDDI packets include an LLC header, and that the LLC header is in so-called SNAP format. The same applies to Token Ring.] __decnet src__ ''host'' True if the DECNET source address is ''host'', which may be an address of the form ``10.123'', or a DECNET host name. [[DECNET host name support is only available on Ultrix systems that are configured to run DECNET.] __decnet dst__ ''host'' True if the DECNET destination address is ''host''. __decnet host__ ''host'' True if either the DECNET source or destination address is ''host''. __ip__, __ip6__, __arp__, __rarp__, __atalk__, __aarp__, __decnet__, __iso__ Abbreviations for: __ether proto__ ''p '' where ''p'' is one of the above protocols. __lat__, __moprc__, __mopdl__ Abbreviations for: __ether proto__ ''p '' where ''p'' is one of the above protocols. Note that ''tcpdump'' does not currently know how to parse these protocols. __vlan__ ''[[vlan_id]'' True if the packet is an IEEE 802.1Q VLAN packet. If ''[[vlan_id]'' is specified, only true is the packet has the specified ''vlan_id''. Note that the first __vlan__ keyword encountered in ''expression'' changes the decoding offsets for the remainder of ''expression'' on the assumption that the packet is a VLAN packet. __tcp__, __udp__, __icmp__ Abbreviations for: __ip proto__ ''p'' __or ip6 proto__ ''p '' where ''p'' is one of the above protocols. __iso proto__ ''protocol'' True if the packet is an OSI packet of protocol type ''protocol''. ''Protocol'' can be a number or one of the names ''clnp'', ''esis'', or ''isis''. __clnp__, __esis__, __isis__ Abbreviations for: __iso proto__ ''p '' where ''p'' is one of the above protocols. Note that ''tcpdump'' does an incomplete job of parsing these protocols. ''expr relop expr'' True if the relation holds, where ''relop'' is one of ''expr'' is an arithmetic expression composed of integer constants (expressed in standard C syntax), the normal binary operators [[+, -, *, /, '' ''proto'' __[[__ ''expr'' __:__ ''size'' __] __ ''Proto'' is one of __ether, fddi, tr, ip, arp, rarp, tcp, udp, icmp__ or __ip6__, and indicates the protocol layer for the index operation. Note that ''tcp, udp'' and other upper-layer protocol types only apply to IPv4, not IPv6 (this will be fixed in the future). The byte offset, relative to the indicated protocol layer, is given by ''expr''. ''Size'' is optional and indicates the number of bytes in the field of interest; it can be either one, two, or four, and defaults to one. The length operator, indicated by the keyword __len__, gives the length of the packet. For example, `__ether[[0] __' catches all multicast traffic. The expression `__ip[[0] __' catches all IP packets with options. The expression `__ip[[6:2] __' catches only unfragmented datagrams and frag zero of fragmented datagrams. This check is implicitly applied to the __tcp__ and __udp__ index operations. For instance, __tcp[[0]__ always means the first byte of the TCP ''header'', and never means the first byte of an intervening fragment. Primitives may be combined using: 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 easier to pass it as a single, quoted argument. Multiple arguments are concatenated with spaces before being parsed. !!EXAMPLES To print all packets arriving at or departing from ''sundown'': __tcpdump host sundown __ 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 Berkeley: __tcpdump net ucb-ether __ To 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__ ''localnet '' To print the start and end packets (the SYN and FIN packets) of each TCP conversation that involves a non-local host. __tcpdump 'tcp[[13] __ ''localnet''__' __ To print IP packets longer than 576 bytes sent through gateway ''snup'': __tcpdump 'gateway snup and ip[[2:2] __ To print IP broadcast or multicast packets that were ''not'' sent via ethernet broadcast or multicast: __tcpdump 'ether[[0] __ To print all ICMP packets that are not echo requests/replies (i.e., not ping packets): __tcpdump 'icmp[[0] != 8 and icmp[[0] != 0' __ !!OUTPUT FORMAT The output of ''tcpdump'' is protocol dependent. The following gives a brief description and examples of most of the formats. __Link Level Headers__ If 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 print the `frame control' field, the source and destination addresses, and the packet length. (The `frame control' field governs the interpretation of the rest of the packet. Normal packets (such as those containing IP datagrams) are `async' packets, with a priority value between 0 and 7; for example, `__async4__'. Such packets are assumed to contain an 802.2 Logical Link Control (LLC) packet; the LLC header is printed if it is ''not'' an ISO datagram or a so-called SNAP packet. On Token Ring networks, the '-e' option causes ''tcpdump'' to print the `access control' and `frame control' fields, the source and destination addresses, and the packet length. As on FDDI networks, packets are assumed to contain an LLC packet. Regardless of whether the '-e' option is specified or not, the source routing information is printed for source-routed packets. ''(N.B.: The following description assumes familiarity with the 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 which the 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 length are printed. For example, the following line shows an outbound compressed TCP packet, with an implicit connection identifier; the ack has changed by 6, the sequence number by 49, and the packet ID by 6; there are 3 bytes of data and 6 bytes of compressed header: __O ctcp * A+6 S+49 I+6 3 (6) __ __ARP/RARP Packets__ Arp/rarp output shows the type of request and its arguments. The format is intended to be self explanatory. Here is a short sample taken from the start of an `rlogin' from host ''rtsg'' to host ''csam'': arp who-has csam tell rtsg arp reply csam is-at CSAM The first line says that rtsg sent an arp packet asking for the ethernet address of internet host csam. Csam replies with its ethernet address (in this example, ethernet addresses are in caps and internet addresses in lower case). This would look less redundant if we had done __tcpdump -n__: arp who-has 128.3.254.6 tell 128.3.254.68 arp reply 128.3.254.6 is-at 02:07:01:00:01:c4 If we had done __tcpdump -e__, the fact that the first packet is broadcast and the second is point-to-point would be visible: RTSG Broadcast 0806 64: arp who-has csam tell rtsg CSAM RTSG 0806 64: arp reply csam is-at CSAM For the first packet this says the ethernet source address is RTSG, the destination is the ethernet broadcast address, the type field contained hex 0806 (type ETHER_ARP) and the total length was 64 bytes. __TCP Packets__ ''(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 '' ''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 direction on this connection. ''Window'' is the number of bytes of receive buffer space available the other direction on this connection. ''Urg'' indicates there is `urgent' data in the packet. ''Options'' are tcp options enclosed in angle brackets (e.g., '' ''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 The first line says that tcp port 1023 on rtsg sent a packet to port ''login'' on csam. The __S__ indicates that the ''SYN'' flag was set. The packet sequence number was 768512 and it contained no data. (The notation is `first:last(nbytes)' which means `sequence numbers ''first'' up to but not including ''last'' which is ''nbytes'' bytes of user data'.) There was no piggy-backed ack, the available receive window was 4096 bytes and there was a max-segment-size option requesting an mss of 1024 bytes. Csam replies with a similar packet except it includes a piggy-backed ack for rtsg's SYN. Rtsg then acks csam's SYN. The `.' means no flags were set. The packet contained no data so there is no data sequence number. Note that the ack sequence number is a small integer (1). The first time ''tcpdump'' sees a tcp `conversation', it prints the sequence number from the packet. On subsequent packets of the conversation, the difference between the current packet's sequence number and this initial sequence number is printed. This means that sequence numbers after the first can be interpreted as relative byte positions in the conversation's data stream (with the first data byte each direction being `1'). `-S' will override this feature, 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 including byte 21. Most of this data is apparently sitting in the socket buffer since csam's receive window has gotten 19 bytes smaller. Csam also sends one byte of data to rtsg in this packet. On the 8th and 9th lines, csam sends two bytes of urgent, pushed data to rtsg. If the snapshot was small enough that ''tcpdump'' didn't capture the full TCP header, it interprets as much of the header as it can and then reports ``[[|''tcp'']'' to indicate the remainder 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 present but the IP datagram length is not long enough for the options to actually be there, ''tcpdump'' reports it as ``[[''bad hdr length'']''. __Capturing TCP packets with particular flag combinations (SYN-ACK, URG-ACK, etc.)__ There are 6 bits in the control bits section of the TCP header: ''URG | ACK | PSH | RST | SYN | FIN'' Let's assume that we want to watch packets used in establishing a TCP connection. Recall that TCP uses a 3-way handshake protocol when it initializes a new connection; the connection sequence with regard to the TCP control bits is 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: 0 15 31 ----------------------------------------------------------------- | source port | destination port | ----------------------------------------------------------------- | sequence number | ----------------------------------------------------------------- | acknowledgment number | ----------------------------------------------------------------- | HL | reserved |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 fist 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 | reserved |U|A|P|R|S|F| window size | ----------------|---------------|---------------|---------------- | | 13th octet | | | Let's have a closer look at octet no. 13: | | |---------------| | |U|A|P|R|S|F| |---------------| |7 5 3 0| We see that this octet contains 2 bytes from the reserved field. According to RFC 793 this field is reserved for future use and must be 0. The remaining 6 bits 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: | |U|A|P|R|S|F| |---------------| |0 0 0 0 0 0 1 0| |---------------| |7 6 5 4 3 2 1 0| We already mentioned that bits number 7 and 6 belong to the reserved field, so they must must be 0. Looking at the control bits section we see that only bit number 1 (SYN) is set. Assuming that octet number 13 is an 8-bit unsigned integer in network byte order, the binary value of this octet is 00000010 and its decimal representation is 7 6 5 4 3 2 1 0 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 0*2 + 1*2 + 0*2 = 2 We're almost done, because now we know that if only SYN is set, the value of the 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[[13] == 2__ We can use this expression as the filter for ''tcpdump'' in order to watch packets which have only SYN set: __tcpdump -i xl0 tcp[[13] == 2__ The expression says 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: | |U|A|P|R|S|F| |---------------| |0 0 0 1 0 0 1 0| |---------------| |7 6 5 4 3 2 1 0| Now bits 1 and 4 are set in the 13th octet. The binary value of octet 13 is 00010010 which translates to decimal 7 6 5 4 3 2 1 0 0*2 + 0*2 + 0*2 + 1*2 + 0*2 + 0*2 + 1*2 + 0*2 = 18 Now we can't just use 'tcp[[13] == 18' in the ''tcpdump'' filter expression, because that would select only those packets that have SYN-ACK set, but not those with only SYN set. Remember that we don't care if ACK or any other control bit is set as long as SYN is set. In order to achieve our goal, we need to logically AND the binary value of octet 13 with some other value to preserve the SYN bit. We know that we want SYN to be set in any case, so we'll logically AND the value in the 13th octet with the binary value of a SYN: 00010010 SYN-ACK 00000010 SYN AND 00000010 (we want SYN) AND 00000010 (we want SYN) -------- -------- = 00000010 = 00000010 We see that this AND operation delivers the same result regardless whether ACK or another TCP control bit is set. The decimal representation of the AND value as well as the result of this operation is 2 (binary 00000010), so we know that for packets with SYN set the following relation must hold true: ( ( value of octet 13 ) AND ( 2 ) ) == ( 2 ) This points us to the ''tcpdump'' filter expression __tcpdump -i xl0 'tcp[[13] __ Note that you should use single quotes or a backslash in the expression to hide the AND (' __UDP Packets__ UDP format is illustrated by this rwho packet: actinide.who This says that port ''who'' on host ''actinide'' sent a udp datagram to port ''who'' on host ''broadcast'', the Internet broadcast address. The packet contained 84 bytes of user data. Some UDP services are recognized (from the source or destination port number) and the higher level protocol information printed. In particular, Domain Name service requests (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 description will appear to be written in greek.)'' Name server requests are formatted as ''src ''h2opolo.1538 '' Host ''h2opolo'' asked the domain server on ''helios'' for an address record (qtype=A) associated with the name ''ucbvax.berkeley.edu.'' The query id was `3'. The `+' indicates the ''recursion desired'' flag was set. The query length was 37 bytes, not including the UDP and IP protocol headers. The query operation was the normal one, ''Query'', so the op field was omitted. If the op had been anything else, it would have been printed between the `3' and the `+'. Similarly, the qclass was the normal one, ''C_IN'', and omitted. Any other qclass would have been printed immediately after the `A'. A few anomalies are checked and may result in extra fields enclosed in square brackets: If a query contains an answer, name server or authority section, ''ancount'', ''nscount'', or ''arcount'' are printed as `[[''n''a]', `[[''n''n]' or `[[''n''au]' 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''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 ''helios.domain '' In the first example, ''helios'' responds to query id 3 from ''h2opolo'' with 3 answer records, 3 name server records and 7 authority records. The first answer record is type A (address) and its data is internet address 128.32.137.3. The total size of the response was 273 bytes, excluding UDP and IP headers. The op (Query) and response code (!NoError) were omitted, as was the class (C_IN) of the A record. In the second example, ''helios'' responds to query 2 with a response code of non-existent domain (NXDomain) with no answers, one name server and no authority records. The `*' indicates that the ''authoritative answer'' bit was set. Since there were no answers, no type, class or data were printed. Other flag characters that might appear are `-' (recursion available, RA, ''not'' set) and `|' (truncated message, TC, set). If the `question' section doesn't contain exactly one entry, `[[''n''q]' 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. __SMB/CIFS decoding__ ''tcpdump'' now includes fairly extensive SMB/CIFS/NBT decoding for data on UDP/137, UDP/138 and TCP/139. Some primitive decoding of IPX and NetBEUI SMB data is also done. By default a fairly minimal decode is done, with a much more detailed decode done if -v is used. Be warned that with -v a single SMB packet may take up a page or more, so only use -v if you really want all the 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 (tridge@samba.org). __NFS Requests and Replies__ Sun NFS (Network File System) requests and replies are printed as: ''src.xid ''sushi.6709 '' 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 handle (''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 generation 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 (-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 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 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. 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 operation. Instead, ''tcpdump'' keeps track of ``recent'' requests, and matches them to the replies using the transaction ID. If a reply does not closely follow the corresponding request, it might not be parsable. __AFS Requests and Replies__ Transarc AFS (Andrew File System) requests and replies are printed as: ''src.sport ''elvis.7001 '' 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 exception of Ubik beacon packets (because abort packets are used to signify a yes vote for the Ubik protocol). Note that AFS requests are very large and many of the arguments won't be printed unless ''snaplen'' is increased. Try using `__-s 256__' to watch AFS traffic. AFS reply packets do not explicitly identify the RPC operation. Instead, ''tcpdump'' keeps track of ``recent'' requests, and matches them to the replies using the call number and service ID. If a reply 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 separated by whitespace (blanks or tabs). The ''/etc/atalk.names'' file may contain blank lines or comment lines (lines starting with a `#'). Appletalk addresses are printed in the form ''net.host.port ''144.1.209.2 '' (If the ''/etc/atalk.names'' doesn't exist or doesn't contain an entry for some appletalk host/net number, addresses are printed in numeric form.) In the first example, NBP (DDP port 2) on net 144.1 node 209 is sending to whatever is listening on port 220 of net icsd node 112. The second line is the same except the full name of the source node is known (`office'). The third line is a send from port 235 on net jssmag node 149 to broadcast on the icsd-net NBP port (note that the broadcast address (255) is indicated by a net name with no host number - for this reason it's a good idea to keep node names and net names distinct in /etc/atalk.names). NBP (name binding protocol) and ATP (Appletalk transaction protocol) packets have their contents interpreted. Other protocols just dump the protocol name (or number if no name is registered for the protocol) and packet size. __NBP packets__ are formatted like the following examples: icsd-net.112.220 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 __ATP packet__ formatting is demonstrated by the following example: jssmag.209.165 Jssmag.209 initiates transaction id 12266 with host helios by requesting up to 8 packets (the ` 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 not'' set. __IP Fragmentation__ Fragmented Internet datagrams are printed as __(frag__ ''id''__:__''size''__@__''offset''__+) (frag__ ''id''__:__''size''__@__''offset''__) __ (The first form indicates there are more fragments. The second indicates this is the last fragment.) ''Id'' is the fragment id. ''Size'' is the fragment size (in bytes) excluding the IP header. ''Offset'' is this fragment's offset (in bytes) in the original datagram. The fragment information is output for each fragment. The first fragment contains the higher level protocol header and the frag info is printed after the protocol info. Fragments after the first contain no higher level protocol header and the frag info is printed after the source and destination addresses. For example, here is part of an ftp from arizona.edu to lbl-rtsg.arpa over a CSNET connection that doesn't appear to handle 576 byte datagrams: arizona.ftp-data 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)__. __Timestamps__ By default, all output lines are preceded by a timestamp. The timestamp is the current clock time in the form ''hh:mm:ss.frac '' and is as accurate as the kernel's clock. The timestamp reflects the time 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. !!SEE ALSO traffic(1C), nit(4P), bpf(4), pcap(3) !!AUTHORS The original authors are: Van Jacobson, Craig Leres and Steven !McCanne, all of the Lawrence Berkeley National Laboratory, University of California, Berkeley, CA. It is currently being maintained by tcpdump.org. The current version is available via http: ''http://www.tcpdump.org/'' 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. !!BUGS Please send problems, bugs, questions, desirable enhancements, etc. to: tcpdump-workers@tcpdump.org Please send source code contributions, etc. to: patches@tcpdump.org NIT doesn't let you watch your own outbound traffic, BPF will. We recommend that you use the latter. On Linux systems with 2.0[[.x] kernels: packets on the loopback device will be seen twice; packet filtering cannot be done in the kernel, so that all packets must be copied from the kernel in order to be filtered in user mode; all of a packet, not just the part that's within the snapshot length, will be copied from the kernel (the 2.0[[.x] packet capture mechanism, if asked to copy only part of a packet to userland, will not report the true length of the packet; this would cause most IP packets to get an error from __tcpdump__). We recommend that you upgrade to a 2.2 or later kernel. Some attempt should be made to reassemble IP fragments or, at least to compute the right length for the higher level protocol. Name server inverse queries are not dumped correctly: the (empty) question section is printed rather than real query in the answer section. Some believe that inverse queries are themselves a bug and prefer to fix the program generating them rather than ''tcpdump''. A packet trace that crosses a daylight savings time change will give skewed time stamps (the time change is ignored). Filter expressions that manipulate FDDI or Token Ring 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 filter expression. Filter expressions on fields other than those that manipulate 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[[0]__, does not work against IPv6 packets. It only looks at IPv4 packets. ----
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FirewallNotes
tcpdump(1)
IPW2x00WirelessChipset
Man8t
PacketSniffer
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TcpDump
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