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Capturing and Decoding Packet Data

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Capturing and Decoding Packet Data

Table Of Contents

Capturing and Decoding Packet Data

Configuring Capture Settings

Working with Alarm Capture Triggers

Capturing Using an Address Filter

Capturing Using a Protocol Filter

Capturing Using a Custom Filter

Viewing Packet Decode Information

Viewing Packets in the Packet Browser

Filtering Packets Displayed in the Capture Decode Window

Viewing Protocol Decode Information

Downloading Capture Buffer to a File

Setting Up Custom Capture Filters

Creating Custom Capture Filters

Tips for Creating Custom Capture Filter Expressions

Editing Custom Capture Filters

Deleting Custom Capture Filters

Setting Up Custom Decode Filters

Creating Custom Decode Filters

Tips for Creating Custom Decode Filter Expressions

Editing Custom Decode Filters

Deleting Custom Decode Filters


Capturing and Decoding Packet Data


The Capture tab has features for setting up, controlling, and displaying packet capture data.

The overall procedure for working with capture settings is:

1. Use the Settings option to configure capture settings and filters. (See the "Configuring Capture Settings" section.)

2. Use the Custom Filters option to create and save specialized filters when capturing data. (See the "Setting Up Custom Capture Filters" section.)

3. After packets are captured, use the Decode option to decode and view captured packets. See:

The "Viewing Packet Decode Information" section.

The "Viewing Protocol Decode Information" section.

4. Use the Download option to save and download the capture buffer. (See the "Downloading Capture Buffer to a File" section.)

Configuring Capture Settings

You must set up the capture settings and filters before starting the actual capture. After packets are captured, you can use decode filters to further narrow down the packets displayed in the Packet Decoder window.


Step 1 Click the Capture tab.

Step 2 Click Settings.

The Capture Settings Dialog Box (Figure 6-1) is displayed.

Figure 6-1 Capture Settings Dialog Box

In the top of the dialog box, there are four status indicators.

Status Indicator
Description

Capture Status

The current status of the capture:

Running—Packet capture is in progress.

Paused—Packet capture is paused. Captured packets remain in buffer, but no new packets are captured.

Stopped—Capture is stopped (by user) and capture buffer is cleared.

Locked—Capture is locked because the buffer is full. This is displayed only when Lock When Full is selected for the Buffer Mode.

Packets Captured

The number of packets captured and stored in the capture buffer.

Note When the capture buffer is full and capture is in wrap-when-full mode, the number of packets captured may fluctuate as new packets arrive and old packets are discarded from the buffer.

First Started

Shows when the current capture started. You can pause and restart the capture as many times as necessary. If you stop the capture and start a new capture, this field shows the start time of the new capture.

Buffer

Current buffer state—Empty, Space Available, Full (Wrap), or Full (Locked).

Capture Trigger Set

Shows if a capture trigger was set.

Time Last Event Triggered

Time the last capture was started or stopped based on an alarm trigger.


Step 3 Enter information in the Capture Settings Dialog Box (Table 6-1) as appropriate.

Table 6-1 Capture Settings Dialog Box 

Field
Description
Usage Notes

Capture Packets from

The data source from which to capture packets.

Buffer Mode

Determines what action to take when the capture buffer is full.

Select Lock when full to stop the capture when the buffer is full.

Select Wrap when full to continue the capture when the buffer is full. The oldest packets are discarded as new packets arrive.

Buffer Size

The size of the capture buffer in KB.

Enter a number from 64 up to your platform maximum. If system memory is low, the actual buffer size allocated might be less than the number specified here. After starting the capture, this field shows the actual buffer size allocated.

Note The WS-SVC-NAM-1 device allows up to 96,000 KB, and the WS-SVC-NAM-2 device allows up to 128,000 KB. The NM-NAM device allows up to 50,000 KB.

Packet Slice Size

The slice size in bytes, used to limit the size of the captured packets. If the packet size is larger than the specified slice size, the packet is "sliced" before it is saved in the capture buffer.

For example, if the packet is 1000 bytes and slice size is 200 bytes, only the first 200 bytes of the packet is stored in the capture buffer.

Enter a number from 12 to 1984.

If you have a small buffer but want to capture as many packets as possible, use a small slice size.


Step 4 In the Capture Filter pane, select inclusive or exclusive filter mode. (Inclusive filters capture only packets that match the filter conditions; exclusive captures exclude those packets that match the filter conditions.)

Step 5 Select one of the following check boxes to enable the applicable filter types:

Address to filter traffic based on a type of IP, IPIP4, IPv6, GRE.IP, or MAC address. (See the "Capturing Using an Address Filter" section.)

Protocol to filter traffic based on specific protocols. (See the "Capturing Using a Protocol Filter" section.)

Custom to use a customized filter. (See the "Capturing Using a Custom Filter" section.)


Note To create a custom capture filter, see the "Setting Up Custom Capture Filters" section.


Step 6 Do one of the following:

To start the capture, click Start.

To temporarily pause a running capture, click Pause.

To stop a running capture and clear the capture buffer, click Stop.

To discard and reset all of your selections and start over, click Reset.


Working with Alarm Capture Triggers

You can create two types of capture triggers from the Setup > Alarms section—start or stop. Only one capture trigger may be set at a time. For the capture triggers to work, you must set the capture buffer settings in advance. For more information on setting capture triggers, see the "Setting Alarm Thresholds" section.

For start capture triggers, the capture buffer should be in a paused state. If the buffer is stopped or running, a start capture trigger does not work. For optimal performance, we recommend that you set the buffer mode to Lock when full.

For stop capture triggers, the capture buffer should be running. If the buffer is paused or stopped, a stop trigger does not work. For optimal performance, we recommend that you set the buffer mode to Wrap when full.

Capturing Using an Address Filter

If you selected the Address check box, enter information in the Capture Settings Address Filter Dialog Box (Table 6-2) as appropriate.


Note When filtering on tunnel addresses such as IPIP4 or GRE.IP, the filters will match the addresses on the inner and outer IP header.


Table 6-2 Capture Settings Address Filter Dialog Box 

Field
Description
Usage Notes

Address

Indicated what address to filter by.

Select MAC to use the source/ destination MAC address of the packets.

Select IP to use the source/destination IP addresses of the packets.

Select IPIP4 for IP addresses including those tunneled over IP protocol 4.

Select GRE.IP for IP addresses including those tunneled over GRE.

Select IPv6 for addresses using IP version 6.

Both directions.

Indicates whether the filter is applied to traffic in both directions.

If the source is host A and the destination is host B, enabling both directions filters packets from A to B and B to A.

If the source is host A and the destination is not specified, enabling both directions filters packets both to and from host A.

Source

Source address of the packets.

For IP, IPIP4, and GRE.IP address, enter a valid IPv4 address in dotted-quad format n.n.n.n, where n is 0 to 255.

For IPv6 address, enter a valid IPv6 address in any allowed IPv6 address format. For example:

1080::8:800:200C:417A

::FFF:129.144.52.38

Note See RFC 2373 for valid text representations.

For MAC address, enter hh hh hh hh hh hh, where hh is a hexadecimal number from 0 to 9 or a to f.

Source Mask

The mask applied to the source address.

If a bit in the Source Mask is set to 1, the corresponding bit in the address is relevant.

If a bit in the Source Mask is set to 0, the corresponding bit in the address is ignored.

For IP, IPIP4, and GRE.IP address, enter a valid IPv4 address in dotted-quad format n.n.n.n, where n is 0 to 255. The default (if blank) is 255.255.255.255.

For IPv6 address, enter a valid IPv6 address in any allowed IPv6 address format. The default mask (if blank) for IPv6 addresses is ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff

Note See RFC 2373 for valid text representations.

For MAC address, enter hh hh hh hh hh hh, where hh is a hexadecimal number from 0 to 9 or a to f. The default is
ff ff ff ff ff ff.

Destination

Destination address of the packets.

For IP, IPIP4, and GRE.IP address, enter a valid IPv4 address in dotted-quad format n.n.n.n, where n is 0 to 255. The default (if blank) is 255.255.255.255.

For IPv6 address, enter a valid IPv6 address in any allowed IPv6 address format. For example:

1080::8:800:200C:417A

::FFF:129.144.52.38

Note See RFC 2373 for valid text representations.

For MAC address, enter hh hh hh hh hh hh, where hh is a hexadecimal number from 0 to 9 or a to f. The default is
ff ff ff ff ff ff.

Dest. Mask

The mask applied to the destination address.

If a bit in the Dest. Mask is set to 1, the corresponding bit in the address is relevant.

If a bit in the Dest. Mask is set to 0, the corresponding bit in the address is ignored.

For IP, IPIP4, and GRE.IP address, enter a valid IPv4 address in dotted-quad format n.n.n.n, where n is 0 to 255. The default (if blank) is 255.255.255.255.

For IPv6 address, enter a valid IPv6 address in any allowed IPv6 address format. The default mask (if blank) for IPv6 addresses is ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff

Note See RFC 2373 for valid text representations.

For MAC address, enter hh hh hh hh hh hh, where hh is a hexadecimal number from 0 to 9 or a to f. The default is
ff ff ff ff ff ff.


Capturing Using a Protocol Filter

If you selected the Protocol check box, select one or more protocols to capture from the list.

Capturing Using a Custom Filter


Step 1 Select the Custom check box.


Note The Address Filter and Protocol Filter check boxes are disabled if you select the Custom Filter check box and vice versa.


Step 2 Select a custom capture filter from the list.


Note If the list is empty, see the "Creating Custom Capture Filters" section for instructions on creating custom capture filters.


Step 3 (Optional.) To view or edit the selected custom capture filter, select
Custom Filters > Capture Filters.


For example, to capture only HTTP and HTTPS packets in the 111.122 Class B network, do the following:


Step 1 Select the Inclusive check box.

Step 2 Select the Address check box.

Step 3 Select the IP button.

Step 4 Select the Both Directions check box.

Step 5 In the Source, enter 111.122.0.0.

Step 6 In the Source Mask, enter 255.255.0.0.

Step 7 Select the Protocol check box.

Step 8 Press Shift-Click to select HTTP and HTTPS from the list.


Viewing Packet Decode Information

After some packets have been captured in the buffer, you can use the Packet Decoder to view the packet contents.


Note If several people use the Packet Decoder simultaneously, they see the same capture buffer.


The Packet Decoder window has three parts:

Packet browser pane (top of window).

Protocol decode (See the "Viewing Protocol Decode Information" section).

Packet hexadecimal dump.


Step 1 Click the Capture tab.

Step 2 Click Decode.

The Packet Decoder dialog box is displayed. Packet Browser (Table 6-3) describes the information displayed in the packet browser pane.


Table 6-3 Packet Browser 

Field
Description

Pkt

Packet numbers, listed numerically in capture sequence. If the decode (display) filter is active, the packet numbers might not be consecutive.

Time

Time the packet was captured relative to the first packet displayed (not the first packet in the buffer).

Note To see the absolute time, see the Detail window.

Size

Size of the packet, in bytes.

Source

Packet source, which might be displayed as hostname, IP, IPX, or MAC address.

Note To turn hostname resolution on and off for IP addresses, click the Setup tab and change this setting under Preferences.

Destination

Packet destination, which might be displayed as hostname, IP, IPX, or MAC address.

Protocol

Top-level protocol of the packet.

Info

Brief text information about the packet contents.


Viewing Packets in the Packet Browser

Use the packet browser to browse the list of captured packets. You can:

Filter by protocol, IP address, MAC address, text, and custom decode filter.

Use the Next, Previous, and Go To buttons to load packets from the capture buffer.


Note The capture must be paused or stopped for you to use these features.


Filtering Packets Displayed in the Capture Decode Window


Step 1 Select one of the following filter types from the list:

Protocol—To filter by protocol.

IP Addr—To filter by IP address.

MAC Addr—To filter by MAC address.

Text—To filter by matching text in the packet summary.

Custom—To use a custom decode filter.

Step 2 Specify the protocol name, IP address, MAC address, matching text, or custom decode filter.


Note Do not use hyphens (-) in the text filter.


Step 3 Click Filter.


Viewing Protocol Decode Information


Step 1 Highlight the packet number about which you want more information.

Detailed information about that packet is displayed in the Protocol Decode and hexadecimal dump panes at the bottom of the window.


Note If you highlight the details in the Protocol Decode pane, the corresponding bytes are highlighted in the hexadecimal dump pane
below it.


Step 2 To review the information, use the scrolling bar in the lower panes.



Tip Protocols are color coded both in the Packet Browser and the Protocol Decode pane.

Click the protocol name in the Protocol Decode pane to collapse and expand protocol information.

To adjust the size of any of the panes, click and drag the pane frame up or down.


Downloading Capture Buffer to a File

Use this option to download the capture buffer to a file in Sniffer format.


Step 1 Click the Capture tab.

Step 2 Click Download.

The Capture Download Window (Figure 6-2) is displayed.

Figure 6-2 Capture Download Window

The Capture Status line displays the status of the current capture.

A display area in the main window shows whether the capture buffer has already been saved to a file.

If the capture buffer has not been saved, the following message is displayed:

Capture buffer has not been saved to file. 

If the capture buffer has been saved, a message similar to the following is displayed:

Capture buffer saved to file on Thu, 9 Aug 2001 00:00:00 +0000.
File Size: bytes Packets
Saved in File: 246085
Packets in Buffer Now: 69511 

Step 3 Do one of the following:


Note You must save the capture buffer to a file before you can download it.


To save the buffer to a file, click Save Buffer to File. The Capture Download Window—Saving Capture Buffer Window (Figure 6-3) is displayed:

Figure 6-3 Capture Download Window—Saving Capture Buffer Window


Tip To refresh the information after a few moments, click Update Status.


To download the file to your desktop, click Download File, then go to Step 4.

The Capture Download Window shows the status after the capture buffer has been successfully saved.

Step 4 If you clicked Download File, the File Download Dialog Box (Figure 6-4) is displayed. Do one of the following:

To open the file on your desktop, select Open this file from its current location.

To save the file to your hard drive, select Save this file to disk, then enter a destination file location and name in the File Download Dialog Box (Figure 6-4).

Figure 6-4 File Download Dialog Box

In both cases. the File Transfer Dialog Box (Figure 6-5) shows:

The estimated amount of time before the file opens on your desktop or is written to your hard drive.

The location to which the file is being transferred (a temporary folder to open on your desktop or the output filename you specified).

The file transfer rate in KB/sec.

Figure 6-5 File Transfer Dialog Box


Setting Up Custom Capture Filters

You can use custom capture filters to create and save specialized filters to disregard everything except the information you are interested in when you capture data.

For more information about using custom filters when capturing data, see the "Capturing Using a Custom Filter" section.

See these topics for help setting up and managing custom capture filters:

Creating Custom Capture Filters.

Editing Custom Capture Filters.

Deleting Custom Capture Filters.

Creating Custom Capture Filters


Step 1 Click the Capture tab.

Step 2 Click Custom Filters.

The Custom Capture Filters dialog box is displayed.

Step 3 Click Create.

The Custom Capture Filter Dialog Box (Table 6-4) is displayed.

Step 4 Enter information in each of the fields as appropriate.

Table 6-4 Custom Capture Filter Dialog Box 

Field
Description
Usage Notes

Filter Name

Name of the new filter.

Enter a name.

Description

Brief description of the filter.

Enter a description from 1 to 35 characters.

Protocol

The protocol to match with the packet.

Select the encapsulation from the drop-down list, then select the protocol.

Data

The data pattern to be matched with the packet.

If the packet is too short and does not have enough data to match, the packet match fails.

Enter hh hh hh ..., where hh are hexadecimal numbers from 0 to 9 or a to f.

Leave blank if not applicable.

Data Mask

The mask applied to the data matching.

If a bit in the Data Mask is set to 1, the corresponding bit in the packet is relevant in the matching algorithm.

If a bit in the Data Mask is set to 0, the corresponding bit in the packet is ignored.

If you do not specify the Data Mask, or if it is shorter than the Data field, the Data Mask is padded with "1" bits up to the length of the Data field.

Enter hh hh hh ..., where hh are hexadecimal numbers from 0 to 9 or a to f.

Leave blank if all data bits are relevant.

Data Not Mask

The mask applied to reverse data matching.

For those bits in the Data Not Mask that are set to 0 (or not specified), the relevant bits in the packet must match the corresponding bit in the Data field.

For those bits in the Data Not Mask that are set to 1, at least one relevant bit in the packet must be different than the corresponding bit in the Data field.

If you do not specify the Data Not Mask, or if it is shorter than the Data field, the Data Not Mask is padded with "0" bits up to the length of the Data field.

Enter hh hh hh ..., where hh are hexadecimal numbers from 0 to 9 or a to f.

Leave blank for no reverse data matching.

Offset

The offset (in bytes, from the Base) where packet data-matching begins. This offset applies to the Data, Data Mask, and Data Not Mask fields.

Enter a decimal number.

Base

The base from which the offset is calculated.

If you select absolute, the offset is calculated from the absolute beginning of the packet (the beginning of the Ethernet frame). You must account for an 802.1q header when calculating an offset for WS-SVC-NAM-1 and NAM-2 devices.

If you select protocol, the offset is calculated from the beginning of the protocol portion of the packet. If the packet does not contain the protocol, the packet fails this match.

Select absolute or a protocol.

Status

The status to match with the packet.

For Ethernet packet captures, the status bits are:

Bit 0—Packet is longer than 1518 octets.

Bit 1—Packet is shorter than 64 octets.

Bit 2—CRC or alignment error.

For example, an Ethernet fragment has a status value of 6 (bits 1 and 2 set).

Enter a number from 0 to 65535.

Leave blank if not applicable.

Status Mask

The mask applied to the status matching.

If a Status Mask bit is set to 1, the corresponding bit in the packet status is relevant in the matching algorithm.

If a Status Mask bit is set to 0, the corresponding bit in the packet status is ignored.

If you do not specify a Status Mask, or if it is shorter than the Status field, the Status Mask is padded with "1" bits up to the length of the Status field.

Enter a number from 0 to 65535.

Leave blank if all status bits are relevant.

Status Not Mask

The mask applied to reverse status matching.

For those bits in the Status Not Mask that are set to 0 (or not specified), the relevant status bits of the packet must match the corresponding bit in the Status field.

For those bits in the Status Not Mask that are set to 1, at least one relevant bit of the status packet must be different than the corresponding bit in the Status field.

If you do not specify a Status Not Mask, it is padded with "0" bits.

Enter a number from 0 to 65535.

Leave blank for no reverse status matching.


Step 5 Do one of the following:

To create the filter, click Apply.

To cancel the changes, click Reset.


Tips for Creating Custom Capture Filter Expressions

The TOS value is stored in byte 1 (the second byte) in the IP header. To match the IP packet with a TOS value of 16 (0x10), enter:

Data—10
Offset—1
Base—IP

The source address of an IP packet is stored in bytes 12 to 15 in the IP header. To match IP packets with a source address of 15.16.17.18, enter:

Data—0f 10 11 12
Offset—12
Base—IP

To match IP packets with a source address of 15.*.*.18 (where * is any number from 0 to 255), enter:

Data—0f 00 00 12
Data Mask—ff 00 00 ff
Offset—12
Base—IP

To match IP packets with a source address of 15.16.17.18 and a destination address different than 15.16.17.19, enter:

Data—f0 10 12 12 0f 10 11 13
Data Mask—ff ff ff ff ff ff ff ff
Data Not Mask—00 00 00 00 00 00 00 00
Offset—12
Base—IP

Editing Custom Capture Filters


Step 1 Click the Capture tab.

Step 2 Click Custom Filters.

The Custom Capture Filters dialog box is displayed.

Step 3 Select the filter to edit, then click Edit.

The Custom Capture Filter dialog box (see Table 6-4) is displayed.

Step 4 Enter information in each of the fields as appropriate.

Step 5 Do one of the following:

To apply the changes, click Apply.

To cancel the changes, click Reset.


Deleting Custom Capture Filters


Step 1 Click the Capture tab.

Step 2 Click Custom Filters.

The Custom Capture Filters dialog box is displayed.

Step 3 Select the filter to delete, then click Delete.

Step 4 In the confirmation dialog box, do one of the following:

To delete the filter, click OK.

To cancel, click Cancel.


Setting Up Custom Decode Filters

Use custom decode filters to create and save customized filters to use in the Decode window to limit which packets are to be displayed.

See these topics for help setting up and managing custom decode filters:

Creating Custom Decode Filters.

Editing Custom Decode Filters.

Deleting Custom Decode Filters.

Creating Custom Decode Filters


Step 1 Click the Capture tab.

Step 2 Click Custom Filters.

Step 3 In the contents, click Decode Filters.

The Custom Decode Filters dialog box is displayed.

Step 4 Click Create.

The Custom Decode Filter Dialog Box (Table 6-5) is displayed.

Step 5 Enter information in each of the fields as appropriate.

Table 6-5 Custom Decode Filter Dialog Box 

Field
Description
Usage Notes

Filter Name

The name of the capture filter.

Enter the name of the filter to be created.

Description

The description of the capture filter.

Enter a description of the filter.

Protocol

The protocol to match with the packet.

Select a protocol from the list. (Select All to match all packets regardless of protocol.)

Address
(MAC or IP)

Indicates whether to filter by MAC or IP address.

Select MAC to filter using the source/destination MAC address of the packets.

Select IP to filter using the source/destination addresses of the packets.

Both Directions

Indicates whether the filter is applied to traffic in both directions.

If the source is host A and the destination is host B, enabling both directions filters packets from A to B and B to A.

If the source is host A and the destination is not specified, enabling both directions filters packets both to and from host A.

Source

Source address of the packets.

For IP address, enter n.n.n.n, where n is 0 to 255 or n.n.n.n/s where s is the subnet mask
(0 to 32).

For MAC address, enter hh hh hh ..., where hh are hexadecimal numbers from 0 to 9 or a to f.

Destination

Destination address of the packets.

For IP address, enter n.n.n.n, where n is 0 to 255 or n.n.n.n/s where s is the subnet mask
(0 to 32).

For MAC address, enter
hh hh hh hh hh hh, where hh are hexadecimal numbers from
0-9 or a-f.

Offset

The offset (in bytes) from the Base where packet data-matching begins.

Enter a decimal number.

Base

The base from which the offset is calculated.

If you select absolute, the offset is calculated from the absolute beginning of the packet (for example, the beginning of the Ethernet frame).

If you select protocol, the offset is calculated from the beginning of the protocol portion of the packet. If the packet does not contain the protocol, the packet fails this match.

Select absolute or a protocol.

Data Pattern

The data to be matched with the packet.

Enter hh hh hh ..., where hh are hexadecimal numbers from 0-9 or a-f.

Leave blank if not applicable.

Filter Expression

An advanced feature to set up complex filter conditions.

The simplest filter allows you to check for the existence of a protocol or field. For example, to see all packets that contain the IPX protocol, you can use the simple filter expression ipx.

See the "Tips for Creating Custom Decode Filter Expressions" section.


Step 6 Do one of the following:

To create the filter, click Apply.

To cancel the changes, click Reset.


Tips for Creating Custom Decode Filter Expressions

You can construct custom decode filter expressions using the following logical and comparison operators:

and

Logical AND

or

Logical OR

xor

Logical XOR

not

Logical NOT

==

Equal

!=

Not equal

>

Greater than

<

Less than

>=

Greater than or equal to

<=

Less than or equal to


You can also group subexpressions within parentheses.

You can use the following fields in filter expressions:

Field
Filter By
Format

eth.addr
eth.src
eth.dst

MAC address

hh:hh:hh:hh:hh:hh, where h is a hexadecimal number from 0 to 9 or a to f.

ip.addr
ip.src
ip.dst

IP address

n.n.n.n or n.n.n.n/s , where n is a number from 0 to 255 and s is a 0-32 hostname that does not contain a hyphen.

tcp.port
tcp.srcport
tcp.dstport

TCP port number

A decimal number from 0 to 65535.

udp.port
udp.srcport
udp.dstport

UDP port number

A decimal number from 0 to 65535.

protocol

Protocol

Click the Protocol list in the Custom Decode Filter dialog box to see the list of protocols on which you can filter.

protocol [offset:length]

Protocol data pattern

hh:hh:hh:hh..., where hh is a hexadecimal number fro 0 to 9 or a to f.

offset and length are decimal numbers.

offset starts at 0 and is relative to the beginning of the protocol portion of the packet.

frame.pkt_len

Packet length

A decimal number that represents the packet length, not the truncated capture packet length.


Examples of Custom Decode Filter Expressions

To match SNMP packets from 111.122.133.144, enter:

snmp and (ip.src == 111.122.133.144) 

To match IP packets from the 111.122 Class B network, enter:

ip.addr == 111.122.0.0/16 

To match TCP packets to and from port 80, enter:

tcp.port == 80 

The TOS value is stored in byte 1 (the second byte) in the IP header. To match the IP packet with the TOS value 16 (0x10), enter:

ip[1:1] == 10 

The TCP acknowledgement number is stored in bytes 8 through 11 in the TCP header. To match the TCP packet with acknowledgement number 12345678 (0xBC614E), enter:

tcp[8:4] == 00:BC:61:4E

Note You can use a filter expression with other fields in the Custom Decode Filter dialog box. In this case, the filter expression is ANDed with other conditions.
Invalid or conflicting filter expressions result in no packet match.


Editing Custom Decode Filters


Step 1 Click the Capture tab.

Step 2 Click Custom Filters.

Step 3 In the contents, click Decode Filters.

The Custom Decode Filters dialog box is displayed.

Step 4 Select the filter to edit, then click Edit.

Step 5 Change the information in each of the fields as appropriate.

Step 6 Do one of the following:

To apply the changes, click Apply.

To cancel the changes, click Reset.


Deleting Custom Decode Filters


Step 1 Click the Capture tab.

Step 2 Click Custom Filters.

Step 3 In the contents, click Decode Filters.

The Custom Decode Filters dialog box is displayed.

Step 4 Select the filter to delete, then click Delete.

Step 5 In the confirmation dialog box, do one of the following:

To delete the filter, click OK.

To cancel, click Cancel.