# phpman > man > tc-flow(8) Flow filter in [tc(8)](https://www.chedong.com/phpMan.php/man/tc/8/markdown) Linux Flow filter in [tc(8)](https://www.chedong.com/phpMan.php/man/tc/8/markdown) ## NAME flow - flow based traffic control filter ## SYNOPSIS Mapping mode: **tc** **filter** ... **flow** **map** **key** _KEY_ [ _OPS_ ] [ _OPTIONS_ ] Hashing mode: **tc** **filter** ... **flow** **hash** **keys** _KEY_LIST_ [ **perturb** _secs_ ] [ _OPTIONS_ ] _OPS_ := [ _OPS_ ] _OP_ _OPTIONS_ := [ **divisor** _NUM_ ] [ **baseclass** _ID_ ] [ **match** _EMATCH_TREE_ ] [ **action** _ACTION_SPEC_ ] _KEY_LIST_ := [ _KEY_LIST_ ] _KEY_ _OP_ := { **or** | **and** | **xor** | **rshift** | **addend** } _NUM_ _ID_ := _X_:_Y_ _KEY_ := { **src** | **dst** | **proto** | **proto-src** | **proto-dst** | **iif** | **priority** | **mark** | **nfct** | **nfct-src** | **nfct-dst** | **nfct-proto-src** | **nfct-proto-dst** | **rt-classid** | **sk-uid** | **sk-gid** | **vlan-** **tag** | **rxhash** } ## DESCRIPTION The **flow** classifier is meant to extend the **SFQ** hashing capabilities without hard-coding new hash functions. It also allows deterministic mappings of keys to classes. ## OPTIONS **action** _ACTION_SPEC_ Apply an action from the generic actions framework on matching packets. **baseclass** _ID_ An offset for the resulting class ID. _ID_ may be **root**, **none** or a hexadecimal class ID in the form [_X_**:**]_Y_. _X_ must match qdisc's/class's major handle (if omitted, the correct value is chosen automatically). If the whole **baseclass** is omitted, _Y_ defaults to 1. **divisor** _NUM_ Number of buckets to use for sorting into. Keys are calculated modulo _NUM_. **hash** **keys** _KEY-LIST_ Perform a **jhash2** operation over the keys in _KEY-LIST_, the result (modulo the **divisor** if given) is taken as class ID, optionally offset by the value of **baseclass**. It is possible to specify an interval (in seconds) after which **jhash2**'s entropy source is recreated using the **perturb** parameter. **map** **key** _KEY_ Packet data identified by _KEY_ is translated into class IDs to push the packet into. The value may be mangled by _OPS_ before using it for the mapping. They are applied in the order listed here: **and** _NUM_ Perform bitwise **AND** operation with numeric value _NUM_. **or** _NUM_ Perform bitwise **OR** operation with numeric value _NUM_. **xor** _NUM_ Perform bitwise **XOR** operation with numeric value _NUM_. **rshift** _NUM_ Shift the value of _KEY_ to the right by _NUM_ bits. **addend** _NUM_ Add _NUM_ to the value of _KEY_. For the **or**, **and**, **xor** and **rshift** operations, _NUM_ is assumed to be an unsigned, 32bit integer value. For the **addend** operation, _NUM_ may be much more complex: It may be pre‐ fixed by a minus ('-') sign to cause subtraction instead of addition and for keys of **src**, **dst**, **nfct-src** and **nfct-dst** it may be given in IP address notation. See below for an illustrating example. **match** _EMATCH_TREE_ Match packets using the extended match infrastructure. See [**tc-ematch**(8)](https://www.chedong.com/phpMan.php/man/tc-ematch/8/markdown) for a detailed description of the allowed syntax in _EMATCH_TREE_. ## KEYS In mapping mode, a single key is used (after optional permutation) to build a class ID. The resulting ID is deducible in most cases. In hashing more, a number of keys may be specified which are then hashed and the output used as class ID. This ID is not deducible in before‐ hand, and may even change over time for a given flow if a **perturb** interval has been given. The range of class IDs can be limited by the **divisor** option, which is used for a modulus. **src**, **dst** Use source or destination address as key. In case of IPv4 and TIPC, this is the actual address value. For IPv6, the 128bit address is folded into a 32bit value by XOR'ing the four 32bit words. In all other cases, the kernel-internal socket address is used (after folding into 32bits on 64bit systems). **proto** Use the layer four protocol number as key. ### proto-src Use the layer four source port as key. If not available, the kernel-internal socket address is used instead. ### proto-dst Use the layer four destination port as key. If not available, the associated kernel- internal dst_entry address is used after XOR'ing with the packet's layer three proto‐ col number. **iif** Use the incoming interface index as key. ### priority Use the packet's priority as key. Usually this is the IP header's DSCP/ECN value. **mark** Use the netfilter **fwmark** as key. **nfct** Use the associated conntrack entry address as key. **nfct-src**, **nfct-dst**, **nfct-proto-src**, **nfct-proto-dst** These are conntrack-aware variants of **src**, **dst**, **proto-src** and **proto-dst**. In case of NAT, these are basically the packet header's values before NAT was applied. ### rt-classid Use the packet's destination routing table entry's realm as key. ### sk-uid **sk-gid** For locally generated packets, use the user or group ID the originating socket belongs to as key. ### vlan-tag Use the packet's vlan ID as key. **rxhash** Use the flow hash as key. ## EXAMPLES Classic SFQ hash: tc filter add ... flow hash \ keys src,dst,proto,proto-src,proto-dst divisor 1024 Classic SFQ hash, but using information from conntrack to work properly in combination with NAT: tc filter add ... flow hash \ keys nfct-src,nfct-dst,proto,nfct-proto-src,nfct-proto-dst \ divisor 1024 Map destination IPs of 192.168.0.0/24 to classids 1-256: tc filter add ... flow map \ key dst addend -192.168.0.0 divisor 256 Alternative to the above: tc filter add ... flow map \ key dst and 0xff The same, but in reverse order: tc filter add ... flow map \ key dst and 0xff xor 0xff ## SEE ALSO [**tc**(8)](https://www.chedong.com/phpMan.php/man/tc/8/markdown), [**tc-ematch**(8)](https://www.chedong.com/phpMan.php/man/tc-ematch/8/markdown), [**tc-sfq**(8)](https://www.chedong.com/phpMan.php/man/tc-sfq/8/markdown) iproute2 20 Oct 2015 Flow filter in [tc(8)](https://www.chedong.com/phpMan.php/man/tc/8/markdown)