TCPDUMP(8)

NAME
tcpdump - dump traffic on a network

SYNOPSIS
tcpdump [-AadefILlNnOopqStvXx] [-B fildrop] [-c count] [-D direction] [-E [espalg:]espkey] [-F file] [-i interface] [-r file] [-s snaplen] [-T type] [-w file] [-y datalinktype] [expression]

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  Print each packet in ASCII. If the -e option is also specified, the link-level header will be included. The smaller of the entire packet or snaplen bytes will be printed.

-a  Attempt to convert network and broadcast addresses to names.

-B fildrop
    Configure the drop action specified by fildrop to be used when the filter expression matches a packet. The actions are:

    pass
        Matching packets are accepted and captured.
    capture
        Matching packets are dropped and captured.
    drop
        Matching packets are dropped but not captured.

    The default action is pass.

-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 [espalg:]espkey
    Try to decrypt RFC 4835 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 — 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. interface may be either a network interface or a USB interface, for example usb0.

-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 116. 116 bytes is adequate for IPv6, 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 “[|proto]”, 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:

    cnfp
        Cisco NetFlow protocol
    erspan
        Cisco Encapsulated Remote Switch Port Analyzer (ERSPAN) over GRE
    geneve
        Generic Network Virtualization Encapsulation
    gre Generic Routing Encapsulation over UDP
    mpls
        Multiprocol Label Switching over UDP
    rpc Remote Procedure Call
    rtcp
        Real-Time Applications control protocol
    rtp Real-Time Applications protocol
    sack
        RFC 2018 TCP Selective Acknowledgements Options
    tcp Transmission Control Protocol
    tftp
        Trivial File Transfer Protocol
    vat Visual Audio Tool
    vrrp
        Virtual Router Redundancy protocol
    vxlan
        Virtual eXtensible Local Area Network
    wb  distributed White Board
    wg  WireGuard tunnel

-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 in hex and ASCII. If the -e option is also specified, the link-level header will be included. The smaller of the entire packet or snaplen bytes will be printed.

-x  Print each packet in hex. If the -e option is also specified, the link-level header will be included. 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, src or dst, src and 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, src or 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:

dst host host
    True if the IP destination field of the packet is host, which may be either an address or a name.

src host host
    True if the IP source field of the packet is host.

host host
    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:

        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 ether_aton(3) for a 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.

dst net net
    True if the IP destination address of the packet has a network number of net. net may be either a name from /etc/hosts or a network number (see hosts(5) for details).

src net net
    True if the IP source address of the packet has a network number of net.

net net
    True if either the IP source or destination address of the packet has a network number of net.

dst port 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., “dst port 513” will print both TCP/login traffic and UDP/who traffic, and “dst 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 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 (‘\’).

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] & 1 != 0 ”.

ip multicast
    True if the packet is an IP multicast packet.

ether proto 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 (‘\’). 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.

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 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.

ifname interface
    True if the packet was logged as coming from the specified interface (applies only to packets logged by pf(4)).

on interface
    Synonymous with the ifname modifier.

rnr num
    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)).

rulenum num
    Synonymous with the rnr modifier.

reason code
    True if the packet was logged with the specified PF reason code. The known codes are: match, bad-offset, fragment, short, normalize, memory, bad-timestamp, congestion, ip-option, proto-cksum, state-mismatch, state-insert, state-limit, src-limit, and synproxy (applies only to packets logged by pf(4)).

rset name
    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)).

ruleset name
    Synonymous with the rset modifier.

srnr num
    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)).

subrulenum num
    Synonymous with the srnr modifier.

action act
    True if PF took the specified action when the packet was logged. Valid actions are: pass, block, and match (applies only to packets logged by pf(4)).

wlan addr1 ehost
    True if the first IEEE 802.11 address is ehost.

wlan addr2 ehost
    True if the second IEEE 802.11 address is ehost.

wlan addr3 ehost
    True if the third IEEE 802.11 address is ehost.

wlan addr4 ehost
    True if the fourth IEEE 802.11 address is ehost. The fourth address field is only used for WDS (Wireless Distribution System) frames.

wlan host ehost
    True if either the first, second, third, or fourth IEEE 802.11 address is ehost.

type type
    True if the IEEE 802.11 frame type matches the specified type. Valid types are: data, mgt, ctl, or a numeric value.

subtype subtype
    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.

dir dir
    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: ether proto 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: ip proto p where p is one of the above protocols.

expr relop 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 ‘(+’, ‘-’, ‘*’, ‘/’, ‘&’, ‘|’), a length operator, and special packet data accessors. To access data inside the packet, use the following syntax:

    proto[expr:size]

    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[0] & 1 != 0” catches all multicast traffic. The expression “ip[0] & 0xf != 5” catches all IP packets with options. The expression “ip[6:2] & 0x1fff = 0” catches only unfragmented datagrams and frag zero of fragmented datagrams. This 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. 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,

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 (the expression is quoted to prevent the shell from misinterpreting 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 network 192.168.7.0/24 (if you gateway to one other net, this stuff should never make it onto your local network):

    # tcpdump ip and not net 192.168.7.0/24

To print the start and end packets (the SYN and FIN packets) of each TCP connection that involves a host that is not in local network 192.168.7.0/24:

    # tcpdump 'tcp[13] & 3 != 0 and not src and dst net 192.168.7.0/24'

To print only the SYN packets of HTTP connections:

    # tcpdump 'tcp[tcpflags] = tcp-syn and port http'

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 only echo request ICMP packets:

    # tcpdump 'icmp[icmptype] = icmp-echo'

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

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

To print raw wireless frames passing the iwn0 interface:
    # tcpdump -i iwn0 -y IEEE802_11_RADIO -v

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+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

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: flags src-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. urgent 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 <mss 1024>
    csam.login > rtsg.1023: S 947648:947648(0) ack 768513 win 4096 <mss 1024>
    rtsg.1023 > csam.login: . ack 1 win 4096
    rtsg.1023 > csam.login: P 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 ‘ 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. 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: id op? flags qtype 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=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 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 “[na]”, “[nn]”, or “[nau]” where n is the appropriate count. If any of the response bits are set ( AA, RA or rcode) or any of the “must be zero” bits are set in bytes two and three, “[b2&3=x]” is printed, where x is the hex value of header bytes two and three.

  UDP Name Server Responses
Name server responses are formatted as

src > dst: id op rcode flags a / n / au type class data (len)

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 ‘|’ (truncated message, TC, set). If the question section doesn't contain exactly one entry, “[nq]” 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.xid > dst.nfs: len op args

src.nfs > dst.xid: reply stat len op results

    sushi.6709 > wrl.nfs: 112 readlink fh 21,24/10.73165
    wrl.nfs > sushi.6709: reply ok 40 readlink "../var"
    sushi.201b > wrl.nfs:
    	144 lookup fh 9,74/4096.6878 "xcolors"
    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 stat of 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.

  IP Fragmentation
Fragmented Internet datagrams are printed as

( frag id : 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 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.

  IP and Protocol Checksum Offload
Some network cards support IP and/or protocol 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
ether_aton(3), pcap_open_live(3), bpf(4), ip(4), pf(4), pflog(4), tcp(4), udp(4), hosts(5), pcap-filter(5), pf.os(5), protocols(5), services(5)

STANDARDS
September 1981, RFC 793, Transmission Control Protocol.

P. Mockapetris, November 1987, RFC 1034, Domain Names – Concepts and Facilities.

P. Mockapetris, November 1987, RFC 1035, Domain Names – Implementation and Specification.

April 1988, RFC 1050, RPC: Remote Procedure Call Protocol Specification.

V. Jacobson, February 1990, RFC 1144, Compressing TCP/IP Headers for Low-Speed Serial Links.

M. Mathis, J. Mahdavi, S. Floyd, A. Romanow, October 1996, RFC 2018, TCP Selective Acknowledgement Options.

V. Manral, April 2007, RFC 4835, Cryptographic Algorithm Implementation Requirements for Encapsulating Security Payload (ESP) and Authentication Header (AH).

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
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.

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.