| Universal 32bit classifier in tc(8) | Linux | Universal 32bit classifier in tc(8) |
u32 - universal 32bit traffic control filter
tc filter ... [ handle HANDLE ] u32 OPTION_LIST [ offset OFFSET ] [ hashkey HASHKEY ] [ classid CLASSID ] [ divisor uint_value ] [ order u32_value ] [ ht HANDLE ] [ sample SELECTOR [ divisor uint_value ] ] [ link HANDLE ] [ indev ifname ] [ skip_hw | skip_sw ] [ help ]
HANDLE := { u12_hex_htid:[u8_hex_hash:[u12_hex_nodeid] | 0xu32_hex_value }
OPTION_LIST := [ OPTION_LIST ] OPTION
HASHKEY := [ mask u32_hex_value ] [ at 4*int_value ]
CLASSID := { root | none | [u16_major]:u16_minor | u32_hex_value }
OFFSET := [ plus int_value ] [ at 2*int_value ] [ mask u16_hex_value ] [ shift int_value ] [ eat ]
OPTION := { match SELECTOR | action ACTION }
SELECTOR := { u32 VAL_MASK_32 | u16 VAL_MASK_16 | u8 VAL_MASK_8 | ip IP | ip6 IP6 | { tcp | udp } TCPUDP | icmp ICMP | mark VAL_MASK_32 | ether ETHER }
IP := { { src | dst } { default | any | all | ip_address [ / { prefixlen | netmask } ] } AT | { dsfield | ihl | protocol | precedence | icmp_type | icmp_code } VAL_MASK_8 | { sport | dport } VAL_MASK_16 | nofrag | firstfrag | df | mf }
IP6 := { { src | dst } { default | any | all | ip6_address [/prefixlen ] } AT | priority VAL_MASK_8 | { protocol | icmp_type | icmp_code } VAL_MASK_8 | flowlabel VAL_MASK_32 | { sport | dport } VAL_MASK_16 }
TCPUDP := { src | dst } VAL_MASK_16
ICMP := { type VAL_MASK_8 | code VAL_MASK_8 }
ETHER := { src | dst } ether_address AT
VAL_MASK_32 := u32_value u32_hex_mask [ AT ]
VAL_MASK_16 := u16_value u16_hex_mask [ AT ]
VAL_MASK_8 := u8_value u8_hex_mask [ AT ]
AT := [ at [ nexthdr+ ] int_value ]
The Universal/Ugly 32bit filter allows one to match arbitrary bitfields in the packet. Due to breaking everything down to values, masks and offsets, It is equally powerful and hard to use. Luckily many abstracting directives are present which allow defining rules on a higher level and therefore free the user from having to fiddle with bits and masks in many cases.
There are two general modes of invocation: The first mode creates a new filter to delegate packets to different destinations. Apart from the obvious ones, namely classifying the packet by specifying a CLASSID or calling an action, one may link one filter to another one (or even a list of them), effectively organizing filters into a tree-like hierarchy.
Typically filter delegation is done by means of a hash table, which leads to the second mode of invocation: it merely serves to set up these hash tables. Filters can select a hash table and provide a key selector from which a hash is to be computed and used as key to lookup the table's bucket which contains filters for further processing. This is useful if a high number of filters is in use, as the overhead of performing the hash operation and table lookup becomes negligible in that case. Using hashtables with u32 basically involves the following pattern:
Options and selectors require values to be specified in a specific format, which is often non-intuitive. Therefore the terminals in SYNOPSIS have been given descriptive names to indicate the required format and/or maximum allowed numeric value: Prefixes u32, u16 and u8 indicate four, two and single byte unsigned values. E.g. u16 indicates a two byte-sized value in range between 0 and 65535 (0xFFFF) inclusive. A prefix of int indicates a four byte signed value. A middle part of _hex_ indicates that the value is parsed in hexadecimal format. Otherwise, the value's base is automatically detected, i.e. values prefixed with 0x are considered hexadecimal, a leading 0 indicates octal format and decimal format otherwise. There are some values with special formatting as well: ip_address and netmask are in dotted-quad formatting as usual for IPv4 addresses. An ip6_address is specified in common, colon-separated hexadecimal format. Finally, prefixlen is an unsigned, decimal integer value in range from 0 to the address width in bits (32 for IPv4 and 128 for IPv6).
Sometimes values need to be dividable by a certain number. In that case a name of the form N*val was chosen, indicating that val must be dividable by N. Or the other way around: the resulting value must be a multiple of N.
U32 recognizes the following options:
Basically the only real selector is u32 . All others merely provide a higher level syntax and are internally translated into u32 .
tc filter add dev eth0 parent 999:0 prio 99 protocol ip u32 \
match ip src 192.168.8.0/24 classid 1:1
This attaches a filter to the qdisc identified by 999:0. It's priority is 99, which affects in which order multiple filters attached to the same parent are consulted (the lower the earlier). The filter handles packets of protocol type ip, and matches if the IP header's source address is within the 192.168.8.0/24 subnet. Matching packets are classified into class 1.1. The effect of this command might be surprising at first glance:
filter parent 1: protocol ip pref 99 u32
filter parent 1: protocol ip pref 99 u32 \
fh 800: ht divisor 1
filter parent 1: protocol ip pref 99 u32 \
fh 800::800 order 2048 key ht 800 bkt 0 flowid 1:1 \
match c0a80800/ffffff00 at 12
So parent 1: is assigned a new u32 filter, which contains a hash table of size 1 (as the divisor indicates). The table ID is 800. The third line then shows the actual filter which was added above: it sits in table 800 and bucket 0, classifies packets into class ID 1:1 and matches the upper three bytes of the four byte value at offset 12 to be 0xc0a808, which is 192, 168 and 8.
Now for something more complicated, namely creating a custom hash table:
tc filter add dev eth0 prio 99 handle 1: u32 divisor 256
This creates a table of size 256 with handle 1: in priority 99. The effect is as follows:
filter parent 1: protocol all pref 99 u32 filter parent 1: protocol all pref 99 u32 fh 1: ht divisor 256 filter parent 1: protocol all pref 99 u32 fh 800: ht divisor 1
So along with the requested hash table (handle 1:), the kernel has created his own table of size 1 to hold other filters of the same priority.
The next step is to create a filter which links to the created hash table:
tc filter add dev eth0 parent 1: prio 1 u32 \
link 1: hashkey mask 0x0000ff00 at 12 \
match ip src 192.168.0.0/16
The filter is given a lower priority than the hash table itself so u32 consults it before manually traversing the hash table. The options link and hashkey determine which table and bucket to redirect to. In this case the hash key should be constructed out of the second byte at offset 12, which corresponds to an IP packet's third byte of the source address field. Along with the match statement, this effectively maps all class C networks below 192.168.0.0/16 to different buckets of the hash table.
Filters for certain subnets can be created like so:
tc filter add dev eth0 parent 1: prio 99 u32 \
ht 1: sample u32 0x00000800 0x0000ff00 at 12 \
match ip src 192.168.8.0/24 classid 1:1
The bucket is defined using the sample option: In this case, the second byte at offset 12 must be 0x08, exactly. In this case, the resulting bucket ID is obviously 8, but as soon as sample selects an amount of data which could exceed the divisor, one would have to know the kernel-internal algorithm to deduce the destination bucket. This filter's match statement is redundant in this case, as the entropy for the hash key does not exceed the table size and therefore no collisions can occur. Otherwise it's necessary to prevent matching unwanted packets.
Matching upper layer fields is problematic since IPv4 header length is variable and IPv6 supports extension headers which affect upper layer header offset. To overcome this, there is the possibility to specify nexthdr+ when giving an offset, and to make things easier there are the tcp and udp matches which use nexthdr+ implicitly. This offset has to be calculated in beforehand though, and the only way to achieve that is by doing it in a separate filter which then links to the filter which wants to use it. Here is an example of doing so:
tc filter add dev eth0 parent 1:0 protocol ip handle 1: \
u32 divisor 1
tc filter add dev eth0 parent 1:0 protocol ip \
u32 ht 1: \
match tcp src 22 FFFF \
classid 1:2
tc filter add dev eth0 parent 1:0 protocol ip \
u32 ht 800: \
match ip protocol 6 FF \
match u16 0 1fff at 6 \
offset at 0 mask 0f00 shift 6 \
link 1:
This is what is being done: In the first call, a single element sized hash table is created so there is a place to hold the linked to filter and a known handle (1:) to reference to it. The second call then adds the actual filter, which pushes packets with TCP source port 22 into class 1:2. Using ht, it is moved into the hash table created by the first call. The third call then does the actual magic: It matches IPv4 packets with next layer protocol 6 (TCP), only if it's the first fragment (usually TCP sets DF bit, but if it doesn't and the packet is fragmented, only the first one contains the TCP header), and then sets the offset based on the IP header's IHL field (right-shifting by 6 eliminates the offset of the field and at the same time converts the value into byte unit). Finally, using link, the hash table from first call is referenced which holds the filter from second call.
tc(8),
cls_u32.txt at http://linux-tc-notes.sourceforge.net/
| 25 Sep 2015 | iproute2 |