6.4. Building display filter expressions

Wireshark 2.1

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6.4. Building display filter expressions

Wireshark provides a simple but powerful display filter language that allows you to build quite complex filter expressions. You can compare values in packets as well as combine expressions into more specific expressions. The following sections provide more information on doing this.

[Tip] Tip

You will find a lot of Display Filter examples at the Wireshark Wiki Display Filter page at: https://wiki.wireshark.org/DisplayFilters.

6.4.1. Display filter fields

Every field in the packet details pane can be used as a filter string, this will result in showing only the packets where this field exists. For example: the filter string: tcp will show all packets containing the tcp protocol.

There is a complete list of all filter fields available through the menu item Help → Supported Protocols in the page “Display Filter Fields” of the “Supported Protocols” dialog.

6.4.2. Comparing values

You can build display filters that compare values using a number of different comparison operators. They are shown in Table 6.4, “Display Filter comparison operators”.

[Tip] Tip

You can use English and C-like terms in the same way, they can even be mixed in a filter string.

Table 6.4. Display Filter comparison operators

English C-like Description and example

eq

==

Equal.

ne

!=

Not equal.

gt

>

Greater than.

lt

<

Less than.

ge

>=

Greater than or equal to.

le

<=

Less than or equal to.

contains

Protocol, field or slice contains a value.

matches

~

Protocol or text field match Perl regualar expression.

bitwise_and

&

Compare bit field value.

In addition, all protocol fields have a type. Display Filter Field Types provides a list of the types and example of how to express them.

Display Filter Field Types

Unsigned integer

Can be 8, 16, 24, 32, or 64 bits. You can express integers in decimal, octal, or hexadecimal. The following display filters are equivalent: 

ip.len le 1500
ip.len le 02734
ip.len le 0x436
Signed integer
Can be 8, 16, 24, 32, or 64 bits. As with unsigned integers you can use decimal, octal, or hexadecimal.
Boolean

A boolean field is present in the protocol decode only if its value is true. For example, is present, and thus true, only if the SYN flag is present in a TCP segment header. 

The filter expression  +tcp.flags.syn+ will select only  those packets for which
this flag exists, that is,  TCP segments where the segment header contains the
SYN flag. Similarly, to find source-routed token ring packets, use a filter
expression of  +tr.sr+.
Ethernet address

6 bytes separated by a colon (:), dot (.) or dash (-) with one or two bytes between separators: 

eth.dst == ff:ff:ff:ff:ff:ff
eth.dst == ff-ff-ff-ff-ff-ff
eth.dst == ffff.ffff.ffff
IPv4 address

ip.addr == 192.168.0.1 

Classless InterDomain Routing (CIDR) notation can be used to test if
an IPv4 address is in a certain subnet. For example, this display
filter will find all packets in the 129.111 Class-B network:
ip.addr == 129.111.0.0/16
IPv6 address 

As with IPv4 addresses, IPv6 addresses can match a subnet.
Text string 
udp contains 81:60:03

The example above match packets that contains the 3-byte sequence 0x81, 0x60, 0x03 anywhere in the UDP header or payload.

sip.To contains "a1762"

Above example match packets where SIP To-header contains the string "a1762" anywhere in the header.

http.host matches "acme\.(org|com|net)"

The example above match HTTP packets where the HOST header contains acme.org or acme.com or acme.net. Note: Wireshark needs to be built with libpcre in order to be able to use the resp. operator.

tcp.flags & 0x02

That expression will match all packets that contain a "tcp.flags" field with the 0x02 bit, i.e. the SYN bit, set.

6.4.3. Combining expressions

You can combine filter expressions in Wireshark using the logical operators shown in Table 6.5, “Display Filter Logical Operations”

Table 6.5. Display Filter Logical Operations

English C-like Description and example

and

&&

Logical AND.

or

||

Logical OR.

xor

^^

Logical XOR.

not

!

Logical NOT.

[…]

See “Substring Operator” below.

in

See “Membership Operator” below.

6.4.4. Substring Operator

Wireshark allows you to select subsequences of a sequence in rather elaborate ways. After a label you can place a pair of brackets [] containing a comma separated list of range specifiers.

eth.src[0:3] == 00:00:83

The example above uses the n:m format to specify a single range. In this case n is the beginning offset and m is the length of the range being specified.

eth.src[1-2] == 00:83

The example above uses the n-m format to specify a single range. In this case n is the beginning offset and m is the ending offset.

eth.src[:4] == 00:00:83:00

The example above uses the :m format, which takes everything from the beginning of a sequence to offset m. It is equivalent to 0:m

eth.src[4:] == 20:20

The example above uses the n: format, which takes everything from offset n to the end of the sequence.

eth.src[2] == 83

The example above uses the n format to specify a single range. In this case the element in the sequence at offset n is selected. This is equivalent to n:1.

eth.src[0:3,1-2,:4,4:,2] ==
00:00:83:00:83:00:00:83:00:20:20:83

Wireshark allows you to string together single ranges in a comma separated list to form compound ranges as shown above.

6.4.5. Membership Operator.

Wireshark allows you to test a field for membership in a set of values or fields. After the field name, use the in operator followed by the set items surrounded by braces {}.

tcp.port in {80 443 8080}

This can be considered a shortcut operator, as the previous expression could have been expressed as:

tcp.port == 80 || tcp.port == 443 || tcp.port == 8080

6.4.6. A Common Mistake

Using the != operator on combined expressions like eth.addr, ip.addr, tcp.port, and udp.port will probably not work as expected.

Often people use a filter string to display something like which will display all packets containing the IP address 1.2.3.4.

Then they use to see all packets not containing the IP address 1.2.3.4 in it. Unfortunately, this does not do the expected.

Instead, that expression will even be true for packets where either source or destination IP address equals 1.2.3.4. The reason for this, is that the expression must be read as “the packet contains a field named ip.addr with a value different from 1.2.3.4”. As an IP datagram contains both a source and a destination address, the expression will evaluate to true whenever at least one of the two addresses differs from 1.2.3.4.

If you want to filter out all packets containing IP datagrams to or from IP address 1.2.3.4, then the correct filter is as it reads “show me all the packets for which it is not true that a field named ip.addr exists with a value of 1.2.3.4”, or in other words, “filter out all packets for which there are no occurrences of a field named ip.addr with the value 1.2.3.4”.

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