I need to record all outgoing
A records on a RedHat PC. I tried using tcpdump :
It makes an output file like:
So I need to process that to get the
yahoo.com :
Is there any better solution to gather all the outgoing
A record requests?
p.s.: collecting DNS A records is only needed to have an up-to-date list of websites that are reachable via HTTPS. So I can generate xml files for HTTPSEverywhere Firefox Add-on. So this is just a part of a script.
LanceBaynes
LanceBaynesLanceBaynes
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2 Answers
grawitygrawity
If you don't have wireshark installed then
should work for you. As you wanted to limit the output to the second to last value then I would parse your log file with:
If you want it live then:
should do it, (here sed and awk are interchangeable; and I would pick awk.)
Anthon
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Alexx RocheAlexx Roche
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1. To monitor HTTP traffic including request and response headers and message body:
tcpdump -A -s 0 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
2. To monitor HTTP traffic including request and response headers and message body from a particular source:
tcpdump -A -s 0 'src example.com and tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'
3. To monitor HTTP traffic including request and response headers and message body from local host to local host:
tcpdump -A -s 0 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)' -i lo
Tcpdump Multiple Ports
4. To only include HTTP requests, modify “tcp port 80” to “tcp dst port 80” in above commands
5. Capture TCP packets from local host to local host
tcpdump -i lo
Tcpdump is a command used on various Linux operating systems (OSs) that gathers TCP/IP packets that pass through a network adapter. Much like a packet sniffer tool, tcpdump can not only analyze the network traffic but also save it to a file.
Unlike some commands that are provided by the operating system by default, you might find that you can't use tcpdump because it isn't installed. To install tcpdump, execute apt-get installtcpdump or yum install tcpdump, depending on your OS.
How Tcpdump Works
Tcpdump prints out the headers of packets on a network interface that match the Boolean expression. It can also be run with the -w flag, which causes it to save the packet data to a file for later analysis, and/or with the -r flag, which causes it to read from a saved packet file rather than to read packets from a network interface. In all cases, only packets that match expression will be processed by tcpdump.
Tcpdump will, if not run with the -c flag, continue capturing packets until it's interrupted by a SIGINT signal (generated, for example, by typing your interrupt character, typically Ctrl+C) or a SIGTERM signal (typically generated with the kill(1) command); if run with the -c flag, it will capture packets until it is interrupted by a SIGINT or SIGTERM signal or the specified number of packets have been processed.
The switches mentioned above are explained in detail later in this article.
When tcpdump finishes capturing packets, it will report counts of:
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On platforms that support the SIGINFO signal, such as most BSDs (Berkeley Software Distributions), it will report those counts when it receives a SIGINFO signal (generated, for example, by typing your 'status' character, typically Ctrl+T) and will continue capturing packets.
Tcpdump Compatibility
Reading packets from a network interface with the tcpdump command might require that you have special privileges (reading a saved packet file doesn't require such privileges):
Tcpdump Command Syntax
Like all computer commands, the tcpdump command works properly only if the syntax is correct:
tcpdump [ -adeflnNOpqRStuvxX ] [ -ccount ]
[ -Cfile_size ] [ -Ffile ]
[ -iinterface ] [ -mmodule ] [ -rfile ]
[ -ssnaplen ] [ -Ttype ] [ -Uuser ] [ -wfile ]
[ -Ealgo:secret ] [ expression ]
Tcpdump Command Options
These are all the options you can use with the tcpdump command:
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—for example, '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'.)
These are the primitives allowed with the tcpdump command:
Proto is one of ether, fddi, tr, ppp, slip, link, ip, arp, rarp, tcp, udp, icmp, or ip6, and indicates the protocol layer for the index operation (ether, fddi, tr, ppp, slip, and link all refer to the link layer). Note that tcp, udp, and other upper-layer protocol types only apply to IPv4, not IPv6. 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.
Some offsets and field values may be expressed as names rather than as numeric values. The following protocol header field offsets are available: icmptype (ICMP type field), icmpcode (ICMP code field), and tcpflags (TCP flags field).
The following ICMP type field values are available: icmp-echoreply, icmp-unreach, icmp-sourcequench, icmp-redirect, icmp-echo, icmp-routeradvert, icmp-routersolicit, icmp-timxceed, icmp-paramprob, icmp-tstamp, icmp-tstampreply, icmp-ireq, icmp-ireqreply, icmp-maskreq, icmp-maskreply.
The following TCP flags field values are available: tcp-fin, tcp-syn, tcp-rst, tcp-push, tcp-push, tcp-ack, tcp-urg.
Primitives may be combined using any of the following:
Negation has highest precedence. Alternation and concatenation have equal precedence and associate left to right. Note that explicit and tokens, not juxtaposition, are 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. However, this 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.
Tcpdump Examples
The above tcpdump command is used to print all packets arriving at or departing from sundown.
This tcpdump example prints traffic between helios and either hot or ace.
You can use this tcpdump command to print all IP packets between ace and any host except helios.
In the above example, tcpdump prints all traffic between local hosts and hosts at Berkeley.
This next tcpdump command example is used to print all FTP traffic through internet gateway snup. Note that the expression is quoted to prevent the shell from misinterpreting the parentheses.
In the above tcpdump example, the command prints traffic neither sourced from nor destined for local hosts.
For the above example of tcpdump, the command is used to print the start and end packets (the SYN and FIN packets) of each TCP conversation that involves a nonlocal host.
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The above command will print IP packets longer than 576 bytes sent through gateway snup.
The tcpdump command shown above prints IP broadcast or multicast packets that were not sent via Ethernet broadcast or multicast.
In this last example of tcpdump, the command prints all ICMP packets that are not echo requests or replies (i.e., not ping packets).
Tcpdump 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 Ethernet networks, 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 areip, 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:
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:
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:
If we had done tcpdump -e, the fact that the first packet is broadcast and the second is point-to-point would be visible:
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 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). Ackno is the sequence number of the next data expected in the other direction on this connection. Window is the number of bytes of receive buffer space available in 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., <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.
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 piggybacked 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 sixth 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 seventh 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 eighth and ninth 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]'.
Capture packets with particular flag combinations. There are eight bits in the control bits section of the TCP header:
CWR | ECE | URG | ACK | PSH | RST | SYN | FIN
Let's assume that we want to watch packets used in establishing a TCP connection. Recall that TCP uses a three-way handshake protocol when it initializes a new connection; the connection sequence with regard to the TCP control bits is:
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:
A TCP header usually holds 20 octets of data, unless options are present. The first line of the graph contains octets 0–3, the second line shows octets 4–7, etc.
Starting to count with 0, the relevant TCP control bits are contained in octet 13:
Let's have a closer look at octet no. 13:
These are the TCP control bits in which we are interested. 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:
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
Its decimal representation is:
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 that have only SYN set:
tcpdump -i xl0 tcp[13] 2
The expression says 'let the 13th octet of a TCP datagram have the decimal value 2,' which is exactly what we want.
Now, let's assume that we need to capture SYN packets, but we don't care if ACK or any other TCP control bit is set at the same time. Look at what happens to octet 13 when a TCP datagram with SYN-ACK set arrives:
Bits 1 and 4 are now set in the 13th octet. The binary value of octet 13 is:
00010010
which translates to decimal:
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:
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] & 2 2'
Note that you should use single quotes or a backslash in the expression to hide the AND ('&') special character from the shell.
UDP packets. UDP format is illustrated by this rwho packet:
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 make little sense.)
Name server requests are formatted as:
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, authority records or additional records section, ancount, nscount, orarcount 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:
In the first example, helios responds to query id 3 from h2opolo with three answer records, three name server records, and seven additional 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 nonexistent domain (NXDomain) with no answers, one name server, and no authority records. The '*' indicates that the authoritative answer bit was set. Since there were no answers, no type, class, or data were printed.
Other flag characters that might appear are '-' (recursion available, RA, not set) and '|' (truncated message, TC, set). If the 'question' section doesn't contain exactly one entry, '[nq]' is printed.
Note that name server requests and responses tend to be large, and the default snaplen of 68 bytes may not capture enough of the packet to print. Use the -s flag to increase the snaplen if you need to seriously investigate name server traffic. '-s 128' has worked well for me.
SMB/CIFS decoding. tcpdump 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 are 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 strings would be welcome.
For information on SMB packet formats and what all the fields mean see www.cifs.org or the pub/samba/specs/ directory on your favorite samba.org mirror site. The SMB patches were written by Andrew Tridgell ([email protected]).
NFS requests and replies. Sun NFS (Network File System) requests and replies are printed as:
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 depend 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:
(-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 snaplenis 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.
Transarc AFS (Andrew File System) requests and replies.
In the first line, host elvis sends an RX packet to pike. This was an 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, acknowledgment 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 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 this form:
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 third 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:
(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:
The first line is a name lookup request for laserwriters sent by net icsd host 112 and broadcast on net jssmag. The nbp id for the lookup is 190. The second line shows a reply for this request (note that it has the same id) from host jssmag.209 saying that it has a laserwriter resource named 'RM1140' registered on port 250. The third line is another reply to the same request saying host techpit has laserwriter 'techpit' registered on port 186.
ATP packet formatting is demonstrated by the following example:
Jssmag.209 initiates transaction id 12266 with host helios by requesting up to eight packets (the `<0-7>'). The hex number at the end of the line is the value of the 'userdata' field in the request.
Helios responds with eight 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 and 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 like this:
(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:
There are a couple of things to note here: First, addresses in the second 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 above form 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.
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