Manual:IP/Firewall/Mangle

Applies to RouterOS: v3, v4

Summary

Sub-menu: /ip firewall mangle

Mangle is a kind of 'marker' that marks packets for future processing with special marks. Many other facilities in RouterOS make use of these marks, e.g. queue trees, NAT, routing. They identify a packet based on its mark and process it accordingly. The mangle marks exist only within the router, they are not transmitted across the network.

Additionally, the mangle facility is used to modify some fields in the IP header, like TOS (DSCP) and TTL fields.

Properties

Property	Description
action (action name; Default: accept)	Action to take if packet is matched by the rule: accept - accept the packet. Packet is not passed to next firewall rule. add-dst-to-address-list - add destination address to Address list specified by address-list parameter add-src-to-address-list - add source address to Address list specified by address-list parameter change-dscp - change Differentiated Services Code Point (DSCP) field value specified by the new-dscp parameter change-mss - change Maximum Segment Size field value of the packet to a value specified by the new-mss parameter change-ttl - change Time to Live field value of the packet to a value specified by the new-ttl parameter jump - jump to the user defined chain specified by the value of jump-target parameter log - add a message to the system log containing following data: in-interface, out-interface, src-mac, protocol, src-ip:port->dst-ip:port and length of the packet. After packet is matched it is passed to next rule in the list, similar as passthrough mark-connection - place a mark specified by the new-connection-mark parameter on the entire connection that matches the rule mark-packet - place a mark specified by the new-packet-mark parameter on a packet that matches the rule mark-routing - place a mark specified by the new-routing-mark parameter on a packet. This kind of marks is used for policy routing purposes only passthrough - ignore this rule and go to next one (useful for statistics). return - pass control back to the chain from where the jump took place set-priority - set priority speciefied by the new-priority parameter on the packets sent out through a link that is capable of transporting priority (VLAN or WMM-enabled wireless interface). Read more> strip-ipv4-options - strip IPv4 option fields from IP header.
address-list (string; Default: )	Name of the address list to be used. Applicable if action is add-dst-to-address-list or add-src-to-address-list
address-list-timeout (time; Default: 00:00:00)	Time interval after which the address will be removed from the address list specified by address-list parameter. Used in conjunction with add-dst-to-address-list or add-src-to-address-list actions Value of 00:00:00 will leave the address in the address list forever
chain (name; Default: )	Specifies to which chain rule will be added. If the input does not match the name of an already defined chain, a new chain will be created.
comment (string; Default: )	Descriptive comment for the rule.
connection-bytes (integer-integer; Default: )	Matches packets only if a given amount of bytes has been transfered through the particular connection. 0 - means infinity, for example connection-bytes=2000000-0 means that the rule matches if more than 2MB has been transfered through the relevant connection
connection-limit (integer,netmaks; Default: )	Restrict connection limit per address or address block/td>
connection-mark (no-mark | string; Default: )	Matches packets marked via mangle facility with particular connection mark. If no-mark is set, rule will match any unmarked connection.
connection-rate (Integer 0..4294967295; Default: )	Connection Rate is a firewall matcher that allow to capture traffic based on present speed of the connection. Read more >>
connection-state (estabilished | invalid | new | related; Default: )	Interprets the connection tracking analysis data for a particular packet: established - a packet which belongs to an existing connection invalid - a packet which could not be identified for some reason new - a packet which begins a new connection related - a packet which is related to, but not part of an existing connection, such as ICMP errors or a packet which begins FTP data connection
connection-type (ftp | h323 | irc | pptp | quake3 | sip | tftp; Default: )	Matches packets from related connections based on information from their connection tracking helpers. A relevant connection helper must be enabled under /ip firewall service-port
content (string; Default: )	Match packets that contain specified text
dscp (integer: 0..63; Default: )	Matches DSCP IP header field.
dst-address (IP/netmask | IP range; Default: )	Matches packets which destination is equal to specified IP or falls into specified IP range.
dst-address-list (name; Default: )	Matches destination address of a packet against user-defined address list
dst-address-type (unicast | local | broadcast | multicast; Default: )	Matches destination address type: unicast - IP address used for point to point transmission local - if dst-address is assigned to one of router's interfaces broadcast - packet is sent to all devices in subnet multicast - packet is forwarded to defined group of devices
dst-limit (integer,time,integer,dst-address | dst-port | src-address, time; Default: )	Matches packets if given pps limit is exceeded. As opposed to the limit matcher, every destination IP address / destination port has it's own limit. Parameters are written in following format: count,time,burst,mode,expire. count - maximum average packet rate measured in packets per time interval time - specifies the time interval in which the packet rate is measured burst - number of packets which are not counted by packet rate mode - the classifier for packet rate limiting expire - specifies interval after which recored ip address /port will be deleted
dst-port (integer[-integer]: 0..65535; Default: )	List of destination port numbers or port number ranges
fragment (yes|no; Default: )	Matches fragmented packets. First (starting) fragment does not count. If connection tracking is enabled there will be no fragments as system automatically assembles every packet
hotspot (auth | from-client | http | local-dst | to-client; Default: )
icmp-options (integer:integer; Default: )	Matches ICMP type:code fileds
in-bridge-port (name; Default: )	Actual interface the packet has entered the router, if incoming interface is bridge
in-interface (name; Default: )	Interface the packet has entered the router
ingress-priority (integer: 0..63; Default: )	Matches ingress priority of the packet. Priority may be derived from VLAN, WMM or MPLS EXP bit. Read more >>
ipv4-options (any | loose-source-routing | no-record-route | no-router-alert | no-source-routing | no-timestamp | none | record-route | router-alert | strict-source-routing | timestamp; Default: )	Matches IPv4 header options. any - match packet with at least one of the ipv4 options loose-source-routing - match packets with loose source routing option. This option is used to route the internet datagram based on information supplied by the source no-record-route - match packets with no record route option. This option is used to route the internet datagram based on information supplied by the source no-router-alert - match packets with no router alter option no-source-routing - match packets with no source routing option no-timestamp - match packets with no timestamp option record-route - match packets with record route option router-alert - match packets with router alter option strict-source-routing - match packets with strict source routing option timestamp - match packets with timestamp
jump-target (name; Default: )	Name of the target chain to jump to. Applicable only if action=jump
layer7-protocol (name; Default: )	Layer7 filter name defined in layer7 protocol menu.
limit (integer,time,integer; Default: )	Matches packets if given pps limit is exceeded. Parameters are written in following format: count,time,burst. count - maximum average packet rate measured in packets per time interval time - specifies the time interval in which the packet rate is measured burst - number of packets which are not counted by packet rate
log-prefix (string; Default: )	Adds specified text at the beginning of every log message. Applicable if action=log
new-connection-mark (string; Default: )
new-dscp (integer: 0..63; Default: )
new-mss (integer; Default: )
new-packet-mark (string; Default: )
new-priority (integer; Default: )
new-routing-mark (string; Default: )
new-ttl (decrement | increment | set:integer; Default: )
nth (integer,integer; Default: )	Matches every nth packet. Read more >>
out-bridge-port (name; Default: )	Actual interface the packet is leaving the router, if outgoing interface is bridge
out-interface (; Default: )	Interface the packet is leaving the router
p2p (all-p2p | bit-torrent | blubster | direct-connect | edonkey | fasttrack | gnutella | soulseek | warez | winmx; Default: )	Matches packets from various peer-to-peer (P2P) protocols. Does not work on encrypted p2p packets.
packet-mark (no-mark | string; Default: )	Matches packets marked via mangle facility with particular packet mark. If no-mark is set, rule will match any unmarked packet.
packet-size (integer[-integer]:0..65535; Default: )	Matches packets of specified size or size range in bytes.
per-connection-classifier (ValuesToHash:Denominator/Remainder; Default: )	PCC matcher allows to divide traffic into equal streams with ability to keep packets with specific set of options in one particular stream. Read more >>
port (integer[-integer]: 0..65535; Default: )	Matches if any (source or destination) port matches the specified list of ports or port ranges. Applicable only if protocol is TCP or UDP
protocol (name or protocol ID; Default: tcp)	Matches particular IP protocol specified by protocol name or number
psd (integer,time,integer,integer; Default: )	Attempts to detect TCP and UDP scans. Parameters are in following format WeightThreshold, DelayThreshold, LopPortWeight, HighPortWeight WeightThreshold - total weight of the latest TCP/UDP packets with different destination ports coming from the same host to be treated as port scan sequence DelayThreshold - delay for the packets with different destination ports coming from the same host to be treated as possible port scan subsequence LowPortWeight - weight of the packets with privileged (<=1024) destination port HighPortWeight - weight of the packet with non-priviliged destination port
random (integer: 1..99; Default: )	Matches packets randomly with given probability.
routing-mark (string; Default: )	Matches packets marked by mangle facility with particular routing mark
src-address (Ip/Netmaks, Ip range; Default: )	Matches packets which source is equal to specified IP or falls into specified IP range.
src-address-list (name; Default: )	Matches source address of a packet against user-defined address list
src-address-type (unicast | local | broadcast | multicast; Default: )	Matches source address type: unicast - IP address used for point to point transmission local - if address is assigned to one of router's interfaces broadcast - packet is sent to all devices in subnet multicast - packet is forwarded to defined group of devices
src-port (integer[-integer]: 0..65535; Default: )	List of source ports and ranges of source ports. Applicable only if protocol is TCP or UDP.
src-mac-address (MAC address; Default: )	Matches source MAC address of the packet
tcp-flags (ack | cwr | ece | fin | psh | rst | syn | urg; Default: )	Matches specified TCP flags ack - acknowledging data cwr - congestion window reduced ece - ECN-echo flag (explicit congestion notification) fin - close connection psh - push function rst - drop connection syn - new connection urg - urgent data
tcp-mss (integer: 0..65535; Default: )	Matches TCP MSS value of an IP packet
time (time-time,sat | fri | thu | wed | tue | mon | sun; Default: )	Allows to create filter based on the packets' arrival time and date or, for locally generated packets, departure time and date
ttl (equal | greater-than | less-than | not-equal : integer(0..255); Default: )	Matches packets TTL value.

Stats

/ip firewall filter print stats will show additional read-only properties

Property	Description
bytes (integer)	Total amount of bytes matched by the rule
packets (integer)	Total amount of packets matched by the rule

By default print is equivalent to print static and shows only static rules.

[admin@dzeltenais_burkaans] /ip firewall mangle> print stats
Flags: X - disabled, I - invalid, D - dynamic 
 #   CHAIN              ACTION                  BYTES           PACKETS        
 0   prerouting         mark-routing            17478158        127631         
 1   prerouting         mark-routing            782505          4506           

To print also dynamic rules use print all.

[admin@dzeltenais_burkaans] /ip firewall mangle> print all stats
Flags: X - disabled, I - invalid, D - dynamic 
 #   CHAIN              ACTION                  BYTES           PACKETS        
 0   prerouting         mark-routing            17478158        127631         
 1   prerouting         mark-routing            782505          4506           
 2 D forward            change-mss              0               0              
 3 D forward            change-mss              0               0              
 4 D forward            change-mss              0               0              
 5 D forward            change-mss              129372          2031  

Or to print only dynamic rules use print dynamic

[admin@dzeltenais_burkaans] /ip firewall mangle> print stats dynamic 
Flags: X - disabled, I - invalid, D - dynamic 
 #   CHAIN              ACTION                  BYTES           PACKETS        
 0 D forward            change-mss              0               0              
 1 D forward            change-mss              0               0              
 2 D forward            change-mss              0               0              
 3 D forward            change-mss              132444          2079 

Menu specific commands

Property	Description
reset-counters (id)	Reset statistics counters for specified firewall rules.
reset-counters-all ()	Reset statistics counters for all firewall rules.

Basic examples

Change MSS

It is a well known fact that VPN links have smaller packet size due to incapsulation overhead. A large packet with MSS that exceeds the MSS of the VPN link should be fragmented prior to sending it via that kind of connection. However, if the packet has DF flag set, it cannot be fragmented and should be discarded. On links that have broken path MTU discovery (PMTUD) it may lead to a number of problems, including problems with FTP and HTTP data transfer and e-mail services.

In case of link with broken PMTUD, a decrease of the MSS of the packets coming through the VPN link solves the problem. The following example demonstrates how to decrease the MSS value via mangle:

/ip firewall mangle 
add out-interface=pppoe-out protocol=tcp tcp-flags=syn action=change-mss new-mss=1300 chain=forward

Marking packets

Marking each packet is quite resource expensive especially if rule has to match against many parameters from IP header or address list containing hundreds of entries.

Lets say we want to

• mark all tcp packets except tcp/80 and match these packets against first address list
• mark all udp packets and match them against second address list.

/ip firewall mangle
  add chain=forward protocol=tcp port=!80 dst-address-list=first action=mark-packet new-packet-mark=first
  add chain=forward protocol=udp dst-address-list=second action=mark-packet new-packet-mark=second

Setup looks quite simple and probably will work without problems in small networks. Now multiply count of rules by 10, add few hundred entries in address list, run 100Mbit of traffic over this router and you will see how rapidly CPU usage is increasing. The reason for such behavior is that each rule reads IP header of every packet and tries to match collected data against parameters specified in firewall rule.

Fortunately if connection tracking is enabled, we can use connection marks to optimize our setup.

/ip firewall mangle
  add chain=forward protocol=tcp port=!80 dst-address-list=first connection-state=new action=mark-connection \ 
new-connection-mark=first
  add chain=forward connection-mark=first action=mark-packet new-packet-mark=first passthrough=no

  add chain=forward protocol=udp dst-address-list=second connection-state=new action=mark-connection \ 
new-connection-mark=second
  add chain=forward connection-mark=second action=mark-packet new-packet-mark=second passthrough=no

Now first rule will try to match data from IP header only from first packet of new connection and add connection mark. Next rule will no longer check IP header for each packet, it will just compare connection marks resulting in lower CPU consumption. Additionally passthrough=no was added that helps to reduce CPU consumption even more.

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