From cfda4eb127247f00333a1d588e9a0a41ac3a4db2 Mon Sep 17 00:00:00 2001 From: Gerald Combs Date: Tue, 30 May 2006 19:45:12 +0000 Subject: EUG -> WSUG svn path=/trunk/; revision=18256 --- docbook/wsug_src/WSUG_chapter_capture.xml | 1033 +++++++++++++++++++++++++++++ 1 file changed, 1033 insertions(+) create mode 100644 docbook/wsug_src/WSUG_chapter_capture.xml (limited to 'docbook/wsug_src/WSUG_chapter_capture.xml') diff --git a/docbook/wsug_src/WSUG_chapter_capture.xml b/docbook/wsug_src/WSUG_chapter_capture.xml new file mode 100644 index 0000000000..23a00c13a7 --- /dev/null +++ b/docbook/wsug_src/WSUG_chapter_capture.xml @@ -0,0 +1,1033 @@ + + + + + Capturing Live Network Data + +
+ Introduction + + Capturing live network data is one of the major features of Ethereal. + + + The Wireshark capture engine provides the following features: + + + + + Capture from different kinds of network hardware (Ethernet, Token Ring, + ATM, ...). + + + Stop the capture on different triggers like: amount of captured data, + captured time, captured number of packets. + + + Simultaneously show decoded packets while keep on capturing. + + + Filter packets, reducing the amount of data to be captured, see . + + + Capturing into multiple files while doing a long term capture, and in + addition the option to form a ringbuffer of these files, keeping only + the last x files, useful for a "very long term" capture, see . + + + The capture engine still lacks the following features: + + + Simultaneous capturing from multiple network interfaces (however, you + can start multiple instances of Ethereal and merge capture files later). + + + Stop capturing (or doing some other action), depending on the captured + data. + + + +
+ +
Prerequisites + + Setting up Ethereal to capture packets for the first time can be tricky. + + Tip! + A comprehensive guide "How To setup a Capture" is available at: + http://wiki.ethereal.com/CaptureSetup. + + + Here are some common pitfalls: + + + + You need to have root / Administrator privileges to start a live + capture. + + + + + You need to choose the right network interface to capture packet data + from. + + + + + You need to capture at the right place in the network to see the + traffic you want to see. + + + + + ... and a lot more!. + + + + + + If you have any problems setting up your capture environment, you should + have a look at the guide mentioned above. + +
+ +
Start Capturing + + One of the following methods can be used to start capturing packets with + Ethereal: + + + + You can get an overview of the available local interfaces using the + " + Capture Interfaces" dialog box, see + . You can start a + capture from this dialog box, using (one of) the "Capture" button(s). + + + + + You can start capturing using the + " + Capture Options" dialog box, see + . + + + + + If you have selected the right capture options before, you can + immediately start a capture using the + " + Capture Start" menu / toolbar item. The capture + process will start immediately. + + + + + If you already know the name of the capture interface, you can start + Ethereal from the command line and use the following: + +ethereal -i eth0 -k + + This will start Ethereal capturing on interface eth0, more details + can be found at: . + + + + +
+ +
+ The "Capture Interfaces" dialog box + + When you select "Interfaces..." from the Capture menu, Ethereal pops + up the "Capture Interfaces" dialog box as shown in + . + Warning! + + As the "Capture Interfaces" dialog is showing live captured data, it is + consuming a lot of system ressources. Close this dialog as soon as + possible to prevent excessive system load. + + + Note! + + This dialog box will only show the local interfaces Ethereal knows + of. As Ethereal might not be able to detect all local interfaces, and it + cannot detect the remote interfaces available, there could be more capture + interfaces available than listed. + + +
+ The "Capture Interfaces" dialog box + +
+ + Description + + + The interface description provided by the operating system. + + + + IP + + + The first IP address Ethereal could resolve from this interface. + If no address could be resolved (e.g. no DHCP server available), + "unknown" will be displayed. If more than one IP address could be + resolved, only the first is shown (unpredictable which one in that + case). + + + + Packets + + + The number of packets captured from this interface, since this + dialog was opened. Will be greyed out, if no packet was captured + in the last second. + + + + Packets/s + + + Number of packets captured in the last second. Will be greyed out, + if no packet was captured in the last second. + + + + Stop + + + Stop a currently running capture. + + + + Capture + + + Start a capture on this interface immediately, using the settings + from the last capture. + + + + Prepare + + + Open the Capture Options dialog with this interface selected, see + . + + + + Close + + + Close this dialog box. + + + + + +
+ +
+ The "Capture Options" dialog box + + When you select Start... from the Capture menu (or use the corresponding + item in the "Main" toolbar), Ethereal pops + up the "Capture Options" dialog box as shown in + . + +
+ The "Capture Options" dialog box + +
+ Tip! + + If you are unsure which options to choose in this dialog box, just try + keeping the defaults as this should work well in many cases. + + + + You can set the following fields in this dialog box: + +
Capture frame + + Interface + + + This field specifies the interface you want to capture on. + You can only capture on one interface, and you can only + capture on interfaces that Ethereal has found on the + system. It is a drop-down list, so simply click on the + button on the right hand side and select the interface you + want. It defaults to the first non-loopback interface that + supports capturing, and if there are none, the first + loopback interface. On some systems, loopback interfaces + cannot be used for capturing (loopback interfaces are not available + on Windows platforms). + + + This field performs the same function as the + -i <interface> command line option. + + + + IP address + + + The IP address(es) of the selected interface. If no address could + be resolved from the system, "unknown" will be shown. + + + + Link-layer header type + + + Unless you are in the rare situation that you need this, just keep + the default. For a detailed description, see + + + + + Buffer size: n megabyte(s) + + + Enter the buffer size to be used while capturing. This is the size + of the kernel buffer which will keep the captured packets, until + they are written to disk. If you encounter packet drops, try + increasing this value. + + + Note + This option is only available on Windows platforms. + + + + + + Capture packets in promiscuous mode + + + + This checkbox allows you to specify that Ethereal + should put the interface in promiscuous mode when capturing. + If you do not specify this, Ethereal will only capture the + packets going to or from your computer (not + all packets on your LAN segment). + + + Note + + If some other process has put the interface in + promiscuous mode you may be capturing in promiscuous + mode even if you turn off this option + + + + Note + + Even in promiscuous mode you still won't necessarily see all packets + on your LAN segment, see for + some more explanations. + + + + + Limit each packet to n bytes + + + This field allows you to specify the maximum amount of + data that will be captured for each packet, and is + sometimes referred to as the snaplen. If disabled, + the default is 65535, which will be sufficient for most + protocols. Some rules of thumb: + + + + + If you are unsure, just keep the default value. + + + + + If you don't need all of the data in a packet - for example, if you + only need the link-layer, IP, and TCP headers - you might want to + choose a small snapshot length, as less CPU time is required for + copying packets, less buffer space is required for packets, and thus + perhaps fewer packets will be dropped if traffic is very heavy. + + + + + If you don't capture all of the data in a packet, you might find that + the packet data you want is in the part that's dropped, or that + reassembly isn't possible as the data required for reassembly is + missing. + + + + + + Capture Filter + + + This field allows you to specify a capture filter. + Capture filters are discussed in more details in + . It defaults to empty, or + no filter. + + + You can also click on the button labelled Capture Filter, and Ethereal + will bring up the Capture Filters dialog box and allow you to create + and/or select a filter. Please see + + + + + +
+
Capture File(s) frame + + An explanation about capture file usage can be found in . + + + File + + + This field allows you to specify the file name that will be + used for the capture file. This field is left blank by default. + If the field is left blank, the capture data will be stored in a + temporary file, see for + details. + + + You can also click on the button to the right of this field to + browse through the filesystem. + + + + Use multiple files + + + Instead of using a single file, Ethereal will automatically switch + to a new one, if a specific trigger condition is reached. + + + + Next file every n megabyte(s) + + + Multiple files only: Switch to the next file after the given + number of byte(s)/kilobyte(s)/megabyte(s)/gigabyte(s) have been + captured. + + + + Next file every n minute(s) + + + Multiple files only: Switch to the next file after the given + number of second(s)/minutes(s)/hours(s)/days(s) have elapsed. + + + + Ring buffer with n files + + + Multiple files only: Form a ring buffer of the capture files, with + the given number of files. + + + + Stop capture after n file(s) + + + Multiple files only: Stop capturing after switching to the next + file the given number of times. + + + + +
+
Stop Capture... frame + + ... after n packet(s) + + + Stop capturing after the given number of packets have been + captured. + + + + ... after n megabytes(s) + + + Stop capturing after the given number of + byte(s)/kilobyte(s)/megabyte(s)/gigabyte(s) have been captured. + This option is greyed out, if "Use multiple files" is selected. + + + + ... after n minute(s) + + + Stop capturing after the given number of + second(s)/minutes(s)/hours(s)/days(s) have elapsed. + + + + +
+
Display Options frame + + + + Update list of packets in real time + + + + This option allows you to specify that Ethereal + should update the packet list pane in real time. If you + do not specify this, Ethereal does not display any + packets until you stop the capture. When you check this, + Ethereal captures in a separate process + and feeds the captures to the display process. + + + + + + Automatic scrolling in live capture + + + + This option allows you to specify that Ethereal + should scroll the packet list pane as new packets come + in, so you are always looking at the last packet. If you + do not specify this, Ethereal simply adds new packets onto + the end of the list, but does not scroll the packet list + pane. This option is greyed out if + "Update list of packets in real time" is disabled. + + + + + + Hide capture info dialog + + + + If this option is checked, the following capture info dialog will be + hidden. + + + + +
+
Name Resolution frame + + + Enable MAC name resolution + + + This option allows you to control whether or not + Ethereal translates MAC addresses into names, see + . + + + + + Enable network name resolution + + + This option allows you to control whether or not + Ethereal translates network addresses into names, see + . + + + + + Enable transport name resolution + + + This option allows you to control whether or not + Ethereal translates transport addresses into protocols, see + . + + + + +
+
Buttons + + Once you have set the values you desire and have selected the + options you need, simply click on OK to commence the + capture, or Cancel to cancel the capture. + + + If you start a capture, Ethereal allows you to stop capturing when + you have enough packets captured, for details see + . + +
+
+ +
Capture files and file modes + + While capturing, the underlying libpcap capturing engine will grab the + packets from the network card and keep the packet data in a (relatively) + small kernel buffer. This data is read by Wireshark and saved into + the capture file(s) the user specified. + + + + Different modes of operation are available when saving this packet data to + the capture file(s). + + + Tip! + + Working with large files (several 100 MB's) can be quite slow. If you plan + to do a long term capture or capturing from a high traffic network, think + about using one of the "Multiple files" options. This will spread the + captured packets over several smaller files which can be much more + pleasant to work with. + + + Note! + + Using Multiple files may cut context related information. + Ethereal keeps context information of the loaded packet data, so it can + report context related problems (like a stream error) and keeps information + about context related protocols (e.g. where data is exchanged at the + establishing phase and only referred to in later packets). + As it keeps this information only for the loaded file, using one of + the multiple file modes may cut these contexts. If the establishing phase + is saved in one file and the things you would like to see is in another, + you might not see some of the valuable context related information. + + + Tip! + + Information about the folders used for the capture file(s), can be found + in . + + + + Capture file mode selected by capture options + + + + + + + + "File" option + "Use multiple files" option + "Ring buffer with n files" option + Mode + Resulting filename(s) used + + + + + - + - + - + Single temporary file + etherXXXXXX (where XXXXXX is a unique number) + + + foo.cap + - + - + Single named file + foo.cap + + + foo.cap + x + - + Multiple files, continuous + foo_00001_20040205110102.cap, foo_00002_20040205110102.cap, ... + + + foo.cap + x + x + Multiple files, ring buffer + foo_00001_20040205110102.cap, foo_00002_20040205110102.cap, ... + + + +
+ + + Single temporary file + + + A temporary file will be created and used (this is the default). After the + capturing is stopped, this file can be saved later under a user specified + name. + + + + + Single named file + + + A single capture file will be used. If you want to place the new capture + file to a specific folder, choose this mode. + + + + + Multiple files, continuous + + + Like the "Single named file" mode, but a new file is created and used, + after reaching one of the multiple file switch conditions (one of the + "Next file every ..." values). + + + + + Multiple files, ring buffer + + + Much like "Multiple files continuous", reaching one of the multiple files + switch conditions (one of the "Next file every ..." values) will switch + to the next file. This will be a newly created file if value of "Ring + buffer with n files" is not reached, otherwise it will replace the oldest + of the formerly used files (thus forming a "ring"). + + + This mode will limit the maximum disk usage, even for an unlimited amount of + capture input data, keeping the latest captured data. + + + + +
+ +
Link-layer header type + + In the usual case, you won't have to choose this link-layer header type. + The following paragraphs describe the exceptional cases, where + selecting this type is possible, so you will have a guide what to do: + + + If you are capturing on an 802.11 device on some versions of BSD, this + might offer a choice of "Ethernet" or "802.11". "Ethernet" will cause + the captured packets to have fake Ethernet headers; "802.11" will cause + them to have IEEE 802.11 headers. Unless the capture needs to be read by + an application that doesn't support 802.11 headers, you should select + "802.11". + + + If you are capturing on an Endace DAG card connected to a synchronous + serial line, this might offer a choice of "PPP over serial" or + "Cisco HDLC"; if the protocol on the serial line is PPP, select + "PPP over serial", and if the protocol on the serial line is Cisco HDLC, + select "Cisco HDLC". + + + If you are capturing on an Endace DAG card connected to an ATM network, + this might offer a choice of "RFC 1483 IP-over-ATM" or "Sun raw ATM". + If the only traffic being captured is RFC 1483 LLC-encapsulated IP, or if + the capture needs to be read by an application that doesn't support SunATM + headers, select "RFC 1483 IP-over-ATM", otherwise select "Sun raw ATM". + + + If you are capturing on an Ethernet device, this might offer a choice of + "Ethernet" or "DOCSIS". If you are capturing traffic from a Cisco Cable + Modem Termination System that is putting DOCSIS traffic onto the Ethernet + to be captured, select "DOCSIS", otherwise select "Ethernet". + +
+ +
Filtering while capturing + + Ethereal uses the libpcap filter language for capture filters. + This is explained in the tcpdump man page, which can be hard to + understand, so it's explained here to some extent. + + + Tip! + + You will find a lot of Capture Filter examples at &EtherealWikiCaptureFiltersPage;. + + + + You enter the capture filter into the Filter field of the Wireshark + Capture Options dialog box, as shown in + . The following is an outline + of the syntax of the tcpdump capture filter language. + See the expression option at the tcpdump manual page for details: + . + + + A capture filter takes the form of a series of primitive expressions + connected by conjunctions (and/or) and optionally + preceded by not: + +[not] primitive [and|or [not] primitive ...] + + An example is shown in . + + + + A capture filter for telnet than captures traffic to and from a + particular host + + +tcp port 23 and host 10.0.0.5 + + + This example captures telnet traffic to and from the host + 10.0.0.5, and shows how to use two primitives and the + and conjunction. Another example is shown in + , and shows how to capture all + telnet traffic except that from 10.0.0.5. + + + Capturing all telnet traffic not from 10.0.0.5 + +tcp port 23 and not host 10.0.0.5 + + + + + + XXX - add examples to the following list. + + + A primitive is simply one of the following: + + + [src|dst] host <host> + + + This primitive allows you to filter on a host IP + address or name. You can optionally precede the + primitive with the keyword src|dst + to specify that you are only interested in source or + destination addresses. If these are not present, + packets where the specified address appears as either + the source or the destination address will be selected. + + + + + + ether [src|dst] host <ehost> + + + + This primitive allows you to filter on Ethernet host + addresses. You can optionally include the keyword + src|dst between the keywords + ether and host + to specify that you are only interested in source + or destination addresses. If these are not present, + packets where the specified address appears in either + the source or destination address will be selected. + + + + + gateway host <host> + + + This primitive allows you to filter on packets that + used host as a gateway. That is, where + the Ethernet source or destination was + host but neither the source nor + destination IP address was host. + + + + + + + [src|dst] net <net> [{mask <mask>}|{len <len>}] + + + + + This primitive allows you to filter on network numbers. + You can optionally precede this primitive with the + keyword src|dst to specify that you + are only interested in a source or destination network. + If neither of these are present, packets will be + selected that have the specified network in either the + source or destination address. In addition, you can + specify either the netmask or the CIDR prefix for the + network if they are different from your own. + + + + + + [tcp|udp] [src|dst] port <port> + + + + This primitive allows you to filter on TCP and UDP port + numbers. You can optionally precede this primitive with + the keywords src|dst and + tcp|udp which allow you to specify + that you are only interested in source or destination + ports and TCP or UDP packets respectively. The + keywords tcp|udp must appear before + src|dst. + + + If these are not specified, packets will be selected + for both the TCP and UDP protocols and when the + specified address appears in either the source or + destination port field. + + + + + less|greater <length> + + + This primitive allows you to filter on packets whose + length was less than or equal to the specified length, + or greater than or equal to the specified length, + respectively. + + + + + ip|ether proto <protocol> + + + This primitive allows you to filter on the specified + protocol at either the Ethernet layer or the IP layer. + + + + + ether|ip broadcast|multicast + + + This primitive allows you to filter on either + Ethernet or IP broadcasts or multicasts. + + + + + <expr> relop <expr> + + + This primitive allows you to create complex filter + expressions that select bytes or ranges of bytes in + packets. Please see the tcpdump man page at + for more details. + + + + + +
+ +
While a Capture is running ... + + While a capture is running, the following dialog box is shown: +
+ The "Capture Info" dialog box + +
+ This dialog box will inform you about the number of captured packets and + the time since the capture was started. The selection which protocols + are counted cannot be changed. + + Tip! + + This Capture Info dialog box can be hidden, using the "Hide capture info + dialog" option in the Capture Options dialog box. + + +
Stop the running capture + + A running capture session will be stopped in one of the following ways: + + + Using the + " + Stop" button from the Capture Info dialog box + . + + Note! + + The Capture Info dialog box might be hidden, if the option "Hide capture + info dialog" is used. + + + + + Using the menu item + "Capture/ + Stop". + + + + Using the toolbar item + " + Stop". + + + + Pressing the accelerator keys: Ctrl+E. + + + + The capture will be automatically stopped, if one of the + Stop Conditions is exceeded, e.g. the maximum amount + of data was captured. + + + + +
+
Restart a running capture + + A running capture session can be restarted with the same capture options + than the last time, this will remove all packets previously captured. + This can be useful, if some uninteresting packets are captured and + there's no need to keep them. + + + Restart is a convenience function and + equivalent to a capture stop following by an immediate capture start. + A restart can be triggered in one of the following ways: + + + Using the menu item + "Capture/ + Restart". + + + + Using the toolbar item + " + Restart". + + + + +
+
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