TCPDUMP(8) AerieBSD 1.0 Refernce Manual TCPDUMP(8)

NAME

tcpdump — dump traffic on a network

SYNOPSIS

tcpdump .Bk -words [-adefILlNnOopqStvXx] [-c count] [-D direction] .Oo Fl E Oo Ar espalg : Oc Ns espkeyOc [-F file] [-i interface] [-r file] [-s snaplen] [-T type] [-w file] [-y datalinktype] [expression] .Ek

DESCRIPTION

tcpdump prints out the headers of packets on a network interface that match the boolean expression. You must have read access to /dev/bpf*.

The options are as follows:
-a
Attempt to convert network and broadcast addresses to names.
-c count
Exit after receiving count packets.
-D direction
Select packets flowing in the specified direction. Valid directions are: -in and -out. The default is to accept packets flowing in any direction.
-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 Oo espalg: Oc espkey Try to decrypt RFC 2406 ESP (Encapsulating Security Payload) traffic using the specified hex key espkey. Supported algorithms for espalg are: -aes128, -aes128-hmac96, -blowfish, -blowfish-hmac96, -cast, -cast-hmac96, -des3, -des3-hmac96, -des and -des-hmac96. The algorithm defaults to -aes128-hmac96. This option should be used for debugging only, since the key will show up in ps(1) output.
-e
Print the link-level header on each dump line.
-F file
Use file as input for the filter expression. Any additional expressions given on the command line are ignored.
-f
Print “foreign” internet addresses numerically rather than symbolically. This option is intended to get around serious brain damage in Sun's yp server \(em usually it hangs forever translating non-local internet numbers.
-I
Print the interface on each dump line.
-i interface
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.
-L
List the supported data link types for the interface and exit.
-l
Make stdout line buffered. Useful if you want to see the data while capturing it. For example:

     # tcpdump -l | tee dat or      # tcpdump -l > dat & tail -f dat
-N
Do not print domain name qualification of host names. For example, if you specify this flag then tcpdump will print “nic” instead of “nic.ddn.mil”.
-n
Do not convert addresses (host addresses, port numbers, etc.) to names.
-O
Do not run the packet-matching code optimizer. This is useful only if you suspect a bug in the optimizer.
-o
Print a guess of the possible operating system(s) of hosts that sent TCP SYN packets. See pf.os(5) for a description of the passive operating system fingerprints.
-p
Do not put the interface into promiscuous mode. 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 file
Read packets from a 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 snaplen
Analyze at most the first snaplen bytes of data from each packet rather than the default of 96. 96 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 snaplen are indicated in the output with “[\*(Ba ]”, where proto is the name of the protocol level at which the truncation has occurred. 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.
-T type
Force packets selected by expression to be interpreted as the specified type. Currently known types are -vrrp (Virtual Router Redundancy protocol), -cnfp (Cisco NetFlow protocol), -rpc (Remote Procedure Call), -rtp (Real-Time Applications protocol), -rtcp (Real-Time Applications control protocol), -sack (RFC 2018 TCP Selective Acknowledgements Options), -tcp (Transmission Control Protocol), -vat (Visual Audio Tool), and -wb (distributed White Board).
-t
Do not print a timestamp on each dump line.
-tt
Print an unformatted timestamp on each dump line.
-ttt
Print day and month in timestamp.
-tttt
Print timestamp difference between packets.
-ttttt
Print timestamp difference since the first packet.
-v
(Slightly more) verbose output. For example, the time to live (TTL) and type of service (ToS) information in an IP packet are printed.
-vv
Even more verbose output. For example, additional fields are printed from NFS reply packets.
-w file
Write the raw packets to file rather than parsing and printing them out. They can be analyzed later with the -r option. Standard output is used if file is "-".
-X
Print each packet (minus its link-level header) in hex and ASCII. The smaller of the entire packet or snaplen bytes will be printed.
-x
Print each packet (minus its link-level header) in hex. The smaller of the entire packet or snaplen bytes will be printed.
-y datalinktype
Set the data link type to use while capturing to datalinktype. Commonly used types include -EN10MB, -IEEE802_11, and -IEEE802_11_RADIO. The choices applicable to a particular device can be listed using -L.

expression selects which packets will be dumped. If no expression is given, all packets on the net will be dumped. Otherwise, only packets satisfying expression 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 qualifiers:
type
Specify which kind of address component 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
Specify a particular transfer direction to and/or from id. Possible directions are -src, -dst, -srcor dst, -srcand dst, -addr1, -addr2, -addr3, and -addr4. E.g., “src foo”, “dst net 128.3”, “src or dst port ftp-data”. If there is no dir qualifier, -srcor dst is assumed. The -addr1, -addr2, -addr3, and -addr4 qualifiers are only valid for IEEE 802.11 Wireless LAN link layers. For null link layers (i.e., point-to-point protocols such as SLIP (Serial Line Internet Protocol) or the pflog(4) header), the -inbound and -outbound qualifiers can be used to specify a desired direction.
proto
Restrict the match to a particular protocol. Possible protocols are: -ah, -arp, -atalk, -decnet, -esp, -ether, -fddi, -icmp, -icmp6, -igmp, -igrp, -ip, -ip6, -lat, -mopdl, -moprc, -pim, -rarp, -sca, -stp, -tcp, -udp, and -wlan. E.g., “ether src foo”, “arp net 128.3”, “tcp port 21”, “wlan addr1 0:2:3:4:5:6”. If there is no protocol 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 (Fiber Distributed Data Interface) 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.

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:
-dsthost host
True if the IP destination field of the packet is host, which may be either an address or a name.
-srchost host
True if the IP source field of the packet is host.
-hosthost
True if either the IP source or destination of the packet is host.

Any of the above host expressions can be prepended with the keywords, -ip, -arp, or -rarp as in:

     Cm ip host Ar host

which is equivalent to: -etherproto ip -andhost host

If host is a name with multiple IP addresses, each address will be checked for a match.
-etherdst ehost
True if the Ethernet destination address is ehost. ehost may be either a name from /etc/ethers or a number (see ethers(3) for a numeric format).
-ethersrc ehost
True if the Ethernet source address is ehost.
-etherhost ehost
True if either the Ethernet source or destination address is ehost.
-gatewayhost
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 -etherhost ehost -andnot host host

which can be used with either names or numbers for host.
-dstnet net
True if the IP 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(5) for details).
-srcnet net
True if the IP source address of the packet has a network number of net.
-netnet
True if either the IP source or destination address of the packet has a network number of net.
-dstport port
True if the packet is IP/TCP or IP/UDP and has a destination port value of port. The port can be a number or name from services(5) (see tcp(4) and udp(4/)). 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., “-dstport 513” will print both TCP/login traffic and UDP/who traffic, and “-dstport domain” will print both TCP/domain and UDP/domain traffic.
-srcport port
True if the packet has a source port value of port.
-portport
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:

     Cm tcp src port Ar port

which matches only TCP packets whose source port is port.
-lesslength
True if the packet has a length less than or equal to length. This is equivalent to:

     Cm len <= Ar length

-greaterlength
True if the packet has a length greater than or equal to length. This is equivalent to:

     Cm len >= Ar length

-ipproto proto
True if the packet is an IP packet (see ip(4)) of protocol type proto. proto can be a number or name from protocols(5), such as -icmp, -udp, or -tcp. These identifiers are also keywords and must be escaped using a backslash character (‘\e’).
-etherbroadcast
True if the packet is an Ethernet broadcast packet. The -ether keyword is optional.
-ipbroadcast
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.
-ethermulticast
True if the packet is an Ethernet multicast packet. The -ether keyword is optional. This is shorthand for “ -ether ”.
-ipmulticast
True if the packet is an IP multicast packet.
-etherproto proto
True if the packet is of ether type proto. proto can be a number or one of the names -ip, -ip6, -arp, -rarp, -atalk, -atalkarp, -decnet, -decdts, -decdns, -lanbridge, -lat, -mopdl, -moprc, -pup, -sca, -sprite, -stp, -vexp, -vprod, or -xns. These identifiers are also keywords and must be escaped using a backslash character (‘\e’). In the case of FDDI (e.g., “-fddiprotocol 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.
-decnetsrc 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 systems that are configured to run DECNET.
-decnetdst host
True if the DECNET destination address is host.
-decnethost host
True if either the DECNET source or destination address is host.
-ifnameinterface
True if the packet was logged as coming from the specified interface (applies only to packets logged by pf(4/)).
-oninterface
Synonymous with the ifname modifier.
-rnrnum
True if the packet was logged as matching the specified PF rule number in the main ruleset (applies only to packets logged by pf(4/)).
-rulenumnum
Synonymous with the rnr modifier.
-reasoncode
True if the packet was logged with the specified PF reason code. The known codes are: match, bad-offset, fragment, bad-timestamp, short, normalize, and memory (applies only to packets logged by pf(4/)).
-rsetname
True if the packet was logged as matching the specified PF ruleset name of an anchored ruleset (applies only to packets logged by pf(4/)).
-rulesetname
Synonymous with the rset modifier.
-srnrnum
True if the packet was logged as matching the specified PF rule number of an anchored ruleset (applies only to packets logged by pf(4/)).
-subrulenumnum
Synonymous with the srnr modifier.
-actionact
True if PF took the specified action when the packet was logged. Valid actions are: pass, block, nat, rdr, binat and scrub (applies only to packets logged by pf(4/)).
-wlanaddr1 ehost
True if the first IEEE 802.11 address is ehost.
-wlanaddr2 ehost
True if the second IEEE 802.11 address is ehost.
-wlanaddr3 ehost
True if the third IEEE 802.11 address is ehost.
-wlanaddr4 ehost
True if the fourth IEEE 802.11 address is ehost. The fourth address field is only used for WDS (Wireless Distribution System) frames.
-wlanhost ehost
True if either the first, second, third, or fourth IEEE 802.11 address is ehost.
-typetype
True if the IEEE 802.11 frame type matches the specified type. Valid types are: data, mgt, ctl, or a numeric value.
-subtypesubtype
True if the IEEE 802.11 frame subtype matches the specified subtype. Valid subtypes are: assocreq, assocresp, reassocreq, reassocresp, probereq, proberesp, beacon, atim, disassoc, auth, deauth, data, or a numeric value.
-dirdir
True if the IEEE 802.11 frame direction matches the specified dir. Valid directions are: nods, tods, fromds, dstods, or a numeric value.
-atalk, -ip, -ip6, -arp, -decnet, -lat, -moprc, -mopdl, -rarp, -sca Abbreviations for: -etherproto p where p is one of the above protocols. tcpdump does not currently know how to parse -lat, -moprc, or -mopdl.
-ah, -esp, -icmp, -icmp6, -igmp, -igrp, -pim, -tcp, -udp Abbreviations for: -ipproto p where p is one of the above protocols.
exprrelop expr
True if the relation holds, where relop is one of ">", "<", ">=", "<=", "=", "!=", and expr is an arithmetic expression composed of integer constants (expressed in standard C syntax), the normal binary operators .Pf ( Ns Ql + , "-", "*", "/", "&", "|")), a length operator, and special packet data accessors. To access data inside the packet, use the following syntax: .Sm off proto[expr: size] .Sm on

proto is one of -ether, -fddi, -ip, -arp, -rarp, -tcp, -udp, or -icmp, and indicates the protocol layer for the index operation. 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” catches all multicast traffic. The expression “-ip” catches all IP packets with options. The expression “-ip” 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” 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. Allowable primitives and operators are: Negation ( “-\&!” or “-not” ) Concatenation ( “-&&” or “-and” ) Alternation ( “-||” or “-or” )

Negation has highest precedence. Alternation and concatenation have equal precedence and associate left to right. 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, -nothost vs -and ace

is short for -nothost vs -andhost ace

which should not be confused with -not (-hostvs -orace)

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 (the expression is quoted to prevent the shell from mis-interpreting the parentheses):

     # 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:

     # 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 connection that involves a non-local host: 

# tcpdump 'tcp[13] & 3 != 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[0] & 1 = 0 and ip[16] >= 224'

To print all ICMP packets that are not echo requests/replies (i.e., not ping packets):

     # tcpdump 'icmp[0] != 8 and icmp[0] != 0'

To print and decrypt all ESP packets with SPI 0x00001234:

     # tcpdump -E des3-hmac96:ab...def 'ip[20:4] = 0x00001234'

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 the packet filter logging interface pflog(4), logging reason (rule match, bad-offset, fragment, bad-timestamp, short, normalize, memory), action taken (pass/block), direction (in/out) and interface information are printed out for each packet.

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.

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+ and -*SA+, 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

In this example, Ethernet addresses are in caps and internet addresses in lower case. 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 CSAM.

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

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: flagssrc-os data-seqno ack window urgent options

src and dst are the source and destination IP addresses and ports. flags is some combination of ‘S’ (SYN), ‘F’ (FIN), ‘P’ (PUSH), or ‘R’ (RST), ‘W’ (congestion Window reduced), ‘E’ (ecn ECHO) or a single "\&." (no flags). src-os will list a guess of the source host's operating system if the -o command line flag was passed to tcpdump. data-seqno describes the portion of sequence space covered by the data in this packet (see example below). ack is the sequence number of the next data expected by the other end of this connection. window is the number of bytes of receive buffer space available at the other end of this connection. urg indicates there is urgent data in the packet. options are TCP options enclosed in angle brackets 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 csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 rtsg.1023 > csam.login: . ack 1 win 4096 rtsg.1023 > csam.login: P 1:2(1) ack 1 win 4096 csam.login > rtsg.1023: . ack 2 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 host csam. The "S" indicates that the SYN flag was set. The packet sequence number was 768512 and it contained no data. The notation is .Sm off ‘ first: last (nbytes ) ’ .Sm on 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. The ack sequence number is a 32-bit integer. The first time tcpdump sees a TCP connection, it prints the sequence number from the packet. On subsequent packets of the connection, 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 connection'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 connection ). 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.

UDP Packets

UDP format is illustrated by this rwho packet:

     actinide.who > broadcast.who: udp 84

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

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 > dst: idop flagsqtype qclass name (len)

For example:

     h2opolo.1538 > helios.domain: 3+ A? ucbvax.berkeley.edu. (37)

Host h2opolo asked the domain server on helios for an address record (qtype) 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 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 was 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]”, “[n]”, or “[n]” 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= ]” 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: idop rcode flags a / n / au typeclass data (len)

For example: helios.domain > h2opolo.1538: 3 3/3/7 A 128.32.137.3 (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 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 rcode (NoError) were omitted, as was the class (C_IN) of the A record.

In the second example, helios responds to query op 2 with an rcode 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 ‘\*(Ba’ (truncated message, TC, set). If the question section doesn't contain exactly one entry, “[n]” is printed.

Name server requests and responses tend to be large and the default snaplen of 96 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.

NFS Requests and Replies

Sun NFS (Network File System) requests and replies are printed as: src. > dst. : src. > dst. : 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" wrl.nfs > sushi.201b: reply ok 128 lookup fh 9,74/4134.3150

In the first line, host sushi sends a transaction with ID 6709 to wrl. 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 op was a readlink (read symbolic link) on fh (“file handle”) 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 with a stat of ok and the contents of the link.

In the third line, sushi asks wrl to look up the name “xcolors” in directory file 9,74/4096.6878. 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, 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 with a statof 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.

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 xid (transaction 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. .

For example: 144.1.209.2 > icsd-net.112.220 office.2 > icsd-net.112.220 jssmag.149.235 > icsd-net.2

If /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-net 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. The broadcast address (255) is indicated by a net name with no host number; for this reason it is 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 > jssmag.2: nbp-lkup 190: "=:LaserWriter@*" jssmag.209.2 > icsd-net.112.220: nbp-reply 190: "RM1140:LaserWriter@*" 250 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 icsdi-net 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 .Sm off (the “‹0\-7›”). .Sm on The hex number at the end of the line is the value of the userdata field in the request.

Helios responds with 8 512-byte packets. The “: ” following the transaction ID gives the packet sequence number in the transaction and the number in parentheses 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 retransmitted. 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.

IP Fragmentation

Fragmented Internet datagrams are printed as ( -fragid : size @ offset [+] )

A "+" indicates there are more fragments. The last fragment will have no "+".

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 fragment info is printed after the protocol info. Fragments after the first contain no higher level protocol header and the fragment 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 > 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)”.

Timestamps

By default, all output lines are preceded by a timestamp. The timestamp is the current clock time in the form .Sm off hh: mm : ss . frac .Sm on 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.

IP Checksum Offload

Some network cards support IP checksum offload. Packet headers for such interfaces erroneously indicate a bad checksum, since the checksum is not calculated until after tcpdump sees the packet.

SEE ALSO

ethers(3), pcap(3), bpf(4), ip(4), pf(4), pflog(4), tcp(4), udp(4), networks(5), pf.os(5), protocols(5), services(5) .Rs .%R RFC 793 .%T Transmission Control Protocol .%D September 1981 .Re .Rs .%R RFC 1034 .%T Domain Names \- Concepts and Facilities .%D November 1987 .Re .Rs .%R RFC 1035 .%T Domain Names \- Implementation and Specification .%D November 1987 .Re .Rs .%R RFC 1050 .%T RPC: Remote Procedure Call .%D April 1988 .Re .Rs .%R RFC 1144 .%T Compressing TCP/IP Headers for Low-Speed Serial Links .%D February 1990 .Re .Rs .%R RFC 2018 .%T TCP Selective Acknowledgement Options .%D October 1996 .Re .Rs .%R RFC 2406 .%T IP Encapsulating Security Payload (ESP) .%D November 1998 .Re

AUTHORS

-nosplit Van Jacobson ‹van@ee.lbl.gov›, Craig Leres ‹leres@ee.lbl.gov›, and Steven McCanne ‹mccanne@ee.lbl.gov›, all of the Lawrence Berkeley Laboratory, University of California, Berkeley, CA.

BUGS

Please send bug reports to ‹tcpdump@ee.lbl.gov› or ‹libpcap@ee.lbl.gov›.

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 the 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.

Apple Ethertalk DDP packets could be dumped as easily as KIP DDP packets but aren't. Even if we were inclined to do anything to promote the use of Ethertalk (we aren't, LBL doesn't allow Ethertalk on any of its networks so we'd have no way of testing this code).

A packet trace that crosses a daylight saving time change will give skewed time stamps (the time change is ignored).

Filter expressions that manipulate FDDI headers assume that all FDDI packets are 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.

AerieBSD 1.0 Reference Manual August 26 2008 TCPDUMP(8)