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11. More classifiers

Classifiers are the way by which the kernel decides which queue a packet should be placed into. There are various different classifiers, each of which can be used for different purposes.


Bases the decision on how the firewall has marked the packet.


Bases the decision on fields within the packet (i.e. source IP address, etc)


Bases the decision on which route the packet will be routed by.

rsvp, rsvp6

Bases the decision on the target (destination,protocol) and optionally the source as well. (I think)


FIXME: Fill me in

Note that in general there are many ways in which you can classify packet and that it generally comes down to preference as to which system you wish to use.

Classifiers in general accept a few arguments in common. They are listed here for convenience:


The protocol this classifier will accept. Generally you will only be accepting only IP traffic. Required.


The handle this classifier is to be attached to. This handle must be an already existing class. Required.


The priority of this classifier. Higher numbers get tested first.


This handle means different things to different filters.

FIXME: Add more

All the following sections will assume you are trying to shape the traffic going to HostA. They will assume that the root class has been configured on 1: and that the class you want to send the selected traffic to is 1:1.

11.1 The "fw" classifier

The "fw" classifier relies on the firewall tagging the packets to be shaped. So, first we will setup the firewall to tag them:

# iptables -I PREROUTING -t mangle -p tcp -d HostA \
 -j MARK --set-mark 1

Now all packets to that machine are tagged with the mark 1. Now we build the packet shaping rules to actually shape the packets. Now we just need to indicate that we want the packets that are tagged with the mark 1 to go to class 1:1. This is accomplished with the command:

# tc filter add dev eth1 protocol ip parent 1:0 prio 1 handle 1 fw classid 1:1

This should be fairly self-explanatory. Attach to the 1:0 class a filter with priority 1 to filter all packet marked with 1 in the firewall to class 1:1. Note how the handle here is used to indicate what the mark should be.

That's all there is to it! This is the (IMHO) easy way, the other ways are I think harder to understand. Note that you can apply the full power of the firewalling code with this classifier, including matching MAC addresses, user IDs and anything else the firewall can match.

11.2 The "u32" classifier

The U32 filter is the most advanced filter available in the current implementation. It entirely based on hashing tables, which make it robust when there are many filter rules.

In its simplest form the U32 filter is a list of records, each consisting of two fields: a selector and an action. The selectors, described below, are compared with the currently processed IP packet until the first match and the associated action is performed. The simplest type of action would be directing the packet into defined CBQ class.

The commandline of tc filter program, used to configure the filter, consists of three parts: filter specification, a selector and an action. The filter specification can be defined as:

tc filter add dev IF [ protocol PROTO ]
                     [ (preference|priority) PRIO ]
                     [ parent CBQ ]

The protocol field describes protocol that the filter will be applied to. We will only discuss case of ip protocol. The preference field (priority can be used alternatively) sets the priority of currently defined filter. This is important, since you can have several filters (lists of rules) with different priorities. Each list will be passed in the order the rules were added, then list with lower priority (higher preference number) will be processed. The parent field defines the CBQ tree top (e.g. 1:0), the filter should be attached to.

The options decribed apply to all filters, not only U32.

U32 selector

The U32 selector contains definition of the pattern, that will be matched to the currently processed packet. Precisely, it defines which bits are to be matched in the packet header and nothing more, but this simple method is very powerful. Let's take a look at the following examples, taken directly from a pretty complex, real-world filter:

# filter parent 1: protocol ip pref 10 u32 fh 800::800 order 2048 key ht 800 bkt 0 flowid 1:3 \
  match 00100000/00ff0000 at 0

For now, leave the first line alone - all these parameters describe the filter's hash tables. Focus on the selector line, containing match keyword. This selector will match to IP headers, whose second byte will be 0x10 (0010). As you can guess, the 00ff number is the match mask, telling the filter exactly which bits to match. Here it's 0xff, so the byte will match if it's exactly 0x10. The at keyword means that the match is to be started at specified offset (in bytes) -- in this case it's beginning of the packet. Translating all that to human language, the packet will match if its Type of Service field will have `low delay' bits set. Let's analyze another rule:

# filter parent 1: protocol ip pref 10 u32 fh 800::803 order 2051 key ht 800 bkt 0 flowid 1:3 \
  match 00000016/0000ffff at nexthdr+0

The nexthdr option means next header encapsulated in the IP packet, i.e. header of upper-layer protocol. The match will also start here at the beginning of the next header. The match should occur in the second, 32-bit word of the header. In TCP and UDP protocols this field contains packet's destination port. The number is given in big-endian format, i.e. older bits first, so we simply read 0x0016 as 22 decimal, which stands for SSH service if this was TCP. As you guess, this match is ambigous without a context, and we will discuss this later.

Having understood all the above, we will find the following selector quite easy to read: match c0a80100/ffffff00 at 16. What we got here is a three byte match at 17-th byte, counting from the IP header start. This will match for packets with destination address anywhere in 192.168.1/24 network. After analyzing the examples, we can summarize what we have learnt.

General selectors

General selectors define the pattern, mask and offset the pattern will be matched to the packet contents. Using the general selectors you can match virtually any single bit in the IP (or upper layer) header. They are more difficult to write and read, though, than specific selectors that described below. The general selector syntax is:

match [ u32 | u16 | u8 ] PATTERN MASK [ at OFFSET | nexthdr+OFFSET]

One of the keywords u32, u16 or u8 specifies length of the pattern in bits. PATTERN and MASK should follow, of length defined by the previous keyword. The OFFSET parameter is the offset, in bytes, to start matching. If nexthdr+ keyword is given, the offset is relative to start of the upper layer header.

Some examples:

# tc filter add dev ppp14 parent 1:0 prio 10 u32 \
     match u8 64 0xff at 8 \
     flowid 1:4

Packet will match to this rule, if its time to live (TTL) is 64. TTL is the field starting just after 8-th byte of the IP header.

# tc filter add dev ppp14 parent 1:0 prio 10 u32 \
     match u8 0x10 0xff at nexthdr+13 \
     protocol tcp \
     flowid 1:3 \

This rule will only match TCP packets with ACK bit set. Here we can see an example of using two selectors, the final result will be logical AND of their results. If we take a look at TCP header diagram, we can see that the ACK bit is second older bit (0x10) in the 14-th byte of the TCP header (at nexthdr+13). As for the second selector, if we'd like to make our life harder, we could write match u8 0x06 0xff at 9 instead if using the specific selector protocol tcp, because 6 is the number of TCP protocol, present in 10-th byte of the IP header. On the other hand, in this example we couldn't use any specific selector for the first match - simply because there's no specific selector to match TCP ACK bits.

Specific selectors

The following table contains a list of all specific selectors the author of this section has found in the tc program source code. They simply make your life easier and increase readability of your filter's configuration.

FIXME: table placeholder - the table is in separate file ,,selector.html''

FIXME: it's also still in Polish :-(

FIXME: must be sgml'ized

Some examples:

# tc filter add dev ppp0 parent 1:0 prio 10 u32 \
     match ip tos 0x10 0xff \
     flowid 1:4

The above rule will match packets, which have the TOS field set to 0x10. The TOS field starts at second byte of the packet and is one byte big, so we coul write an equivalent general selector: match u8 0x10 0xff at 1. This gives us hint to the internals of U32 filter -- the specific rules are always translated to general ones, and in this form they are stored in the kernel memory. This leads to another conclusion -- the tcp and udp selectors are exactly the same and this is why you can't use single match tcp dst 53 0xffff selector to match TCP packets sent to given port -- they will also match UDP packets sent to this port. You must remember to also specify the protocol and end up with the following rule:

# tc filter add dev ppp0 parent 1:0 prio 10 u32 \
        match tcp dst 53 0xffff \
        match ip protocol 0x6 0xff \
        flowid 1:2

11.3 The "route" classifier

This classifier filters based on the results of the routing tables. When a packet that is traversing through the classes reaches one that is marked with the "route" filter, it splits the packets up based on information in the routing table.

# tc filter add dev eth1 parent 1:0 protocol ip prio 100 route

Here we add a route classifier onto the parent node 1:0 with priority 100. When a packet reaches this node (which, since it is the root, will happen immediately) it will consult the routing table and if one matches will send it to the given class and give it a priority of 100. Then, to finally kick it into action, you add the appropriate routing entry:

The trick here is to define 'realm' based on either destination or source. The way to do it is like this:

# ip route add Host/Network via Gateway dev Device realm RealmNumber

For instance, we can define our destination network with a realm number 10:

# ip route add via dev eth1 realm 10

When adding route filters, we can use realm numbers to represent the networks or hosts and specify how the routes match the filters.

# tc filter add dev eth1 parent 1:0 protocol ip prio 100 \
  route to 10 classid 1:10

The above rule says packets going to the network match class id 1:10.

Route filter can also be used to match source routes. For example, there is a subnetwork attached to the Linux router on eth2.

# ip route add dev eth2 realm 2
# tc filter add dev eth1 parent 1:0 protocol ip prio 100 \
  route from 2 classid 1:2

Here the filter specifies that packets from the subnetwork (realm 2) will match class id 1:2.

11.4 The "rsvp" classifier

FIXME: Fill me in

11.5 The "tcindex" classifier

FIXME: Fill me in

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