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3. New netfilter matches

In this section, we will attempt to explain the usage of new netfilter matches. The patches will appear in alphabetical order. Additionally, we will not explain patches that break other patches. But this might come later.

Generally speaking, for matches, you can get the help hints from a particular module by typing :

# iptables -m the_match_you_want --help

This would display the normal iptables help message, plus the specific ``the_match_you_want'' match help message at the end.

3.1 ah-esp patch

This patch by Yon Uriarte <yon@astaro.de> adds 2 new matches :

• ``ah'' : lets you match an AH packet based on its Security Parameter Index (SPI).
• ``esp'' : lets you match an ESP packet based on its SPI.

This patch can be quite useful for people using IPSEC who are willing to discriminate connections based on their SPI.

For example, we will drop all the AH packets that have a SPI equal to 500 :

# iptables -A INPUT -p 51 -m ah --ahspi 500 -j DROP

# iptables --list
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
DROP       ipv6-auth--  anywhere             anywhere           ah spi:500

Supported options for the ah match are :

--ahspi [!] spi[:spi]

-> match spi (range)

The esp match works exactly the same :

# iptables -A INPUT -p 50 -m esp --espspi 500 -j DROP

# iptables --list
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
DROP       ipv6-crypt--  anywhere             anywhere           esp spi:500 

Supported options for the esp match are :

--espspi [!] spi[:spi]

-> match spi (range)

Do not forget to specify the proper protocol through ``-p 50'' or ``-p 51'' (for esp & ah respectively) when you use the ah or esp matches, or else the rule insertion will simply abort for obvious reasons.

3.2 conntrack patch

This patch by Marc Boucher <marc+nf@mbsi.ca> adds a new general conntrack match module (a superset of the state match) that allows you to match on additional conntrack information.

For example, if you want to allow all the RELATED connections for TCP protocols only, then you can proceed as follows :

# iptables -A FORWARD -m conntrack --ctstate RELATED --ctproto tcp -j ACCEPT

# iptables --list
Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  anywhere             anywhere           ctstate RELATED

Supported options for the conntrack match are :

[!] --ctstate [INVALID|ESTABLISHED|NEW|RELATED|SNAT|DNAT][,...]

-> State(s) to match. The "new" `SNAT' and `DNAT' states are virtual ones, matching if the original source address differs from the reply destination, or if the original destination differs from the reply source.

[!] --ctproto proto

-> Protocol to match; by number or name, eg. `tcp'.

--ctorigsrc [!] address[/mask]

-> Original source specification.

--ctorigdst [!] address[/mask]

-> Original destination specification.

--ctreplsrc [!] address[/mask]

-> Reply source specification.

--ctrepldst [!] address[/mask]

-> Reply destination specification.

[!] --ctstatus [NONE|EXPECTED|SEEN_REPLY|ASSURED][,...]

-> Status(es) to match.

[!] --ctexpire time[:time]

-> Match remaining lifetime in seconds against value or range of values (inclusive).

3.3 iplimit patch

This patch by Gerd Knorr <kraxel@bytesex.org> adds a new match that will allow you to restrict the number of parallel TCP connections from a particular host or network.

For example, let's limit the number of parallel HTTP connections made by a single IP address to 4 :

# iptables -A INPUT -p tcp --syn --dport http -m iplimit --iplimit-above 4 -j REJECT

# iptables --list
Chain INPUT (policy ACCEPT)
target   prot opt source    destination         
REJECT   tcp  --  anywhere  anywhere     tcp dpt:http flags:SYN,RST,ACK/SYN #conn/32 > 4 reject-with icmp-port-unreachable

Or you might want to limit the number of parallel connections made by a whole class A for example :

# iptables -A INPUT -p tcp --syn --dport http -m iplimit --iplimit-mask 8 --iplimit-above 4 -j REJECT

# iptables --list
Chain INPUT (policy ACCEPT)
target   prot opt source    destination         
REJECT   tcp  --  anywhere  anywhere     tcp dpt:http flags:SYN,RST,ACK/SYN #conn/8 > 4 reject-with icmp-port-unreachable

Supported options for the iplimit patch are :

[!] --iplimit-above n

-> match if the number of existing tcp connections is (not) above n

--iplimit-mask n

-> group hosts using mask

3.4 ipv4options patch

This patch by Fabrice MARIE <fabrice@celestix.com> adds a news match that allows you to match packets based on the IP options they have set.

For example, let's drop all packets that have the record-route or the timestamp IP option set :

# iptables -A INPUT -m ipv4options --rr -j DROP
# iptables -A INPUT -m ipv4options --ts -j DROP

# iptables --list
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
DROP       all  --  anywhere             anywhere            IPV4OPTS RR
DROP       all  --  anywhere             anywhere            IPV4OPTS TS

Supported options for the ipv4options match are :

--ssrr

-> match strict source routing flag.

--lsrr

-> match loose source routing flag.

--no-srr

-> match packets with no source routing.

[!] --rr

-> match record route flag.

[!] --ts

-> match timestamp flag.

[!] --ra

-> match router-alert option.

[!] --any-opt

-> Match a packet that has at least one IP option (or that has no IP option at all if ! is chosen).

3.5 length patch

This patch by James Morris <jmorris@intercode.com.au> adds a new match that allows you to match a packet based on its length.

For example, let's drop all the pings with a packet size greater than 85 bytes :

# iptables -A INPUT -p icmp --icmp-type echo-request -m length --length 85:0xffff -j DROP

# iptables --list
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
DROP       icmp --  anywhere             anywhere           icmp echo-request length 85:65535

Supported options for the length match are :

[!] --length length[:length]

-> Match packet length against value or range of values (inclusive)

Values of the range not present will be implied. The implied value for minimum is 0, and for maximum is 65535.

3.6 mport patch

This patch by Andreas Ferber <af@devcon.net> adds a new match that allows you to specify ports with a mix of port-ranges and single ports for UDP and TCP protocols.

For example, if you want to block ftp, ssh, telnet and http in one line, you can :

# iptables -A INPUT -p tcp -m mport --ports 20:23,80 -j DROP

# iptables --list
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
DROP       tcp  --  anywhere             anywhere           mport ports ftp-data:telnet,http

Supported options for the mport match are :

--source-ports port[,port:port,port...]

-> match source port(s)

--sports port[,port:port,port...]

-> match source port(s)

--destination-ports port[,port:port,port...]

-> match destination port(s)

--dports port[,port:port,port...]

-> match destination port(s)

--ports port[,port:port,port]

-> match both source and destination port(s)

3.7 nth patch

This patch by Fabrice MARIE <fabrice@celestix.com> adds a new match that allows you to match a particular Nth packet received by the rule.

For example, if you want to drop every 2 ping packets, you can do as follows :

# iptables -A INPUT -p icmp --icmp-type echo-request -m nth --every 2 -j DROP

# iptables --list
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
DROP       icmp --  anywhere             anywhere           icmp echo-request every 2th 

Extensions by Richard Wagner <rwagner@cloudnet.com> allows you to create an easy and quick method to produce load-balancing for both inbound and outbound connections.

For example, if you want to balance the load to the 3 addresses 10.0.0.5, 10.0.0.6 and 10.0.0.7, then you can do as follows :

# iptables -t nat -A POSTROUTING -o eth0 -m nth --counter 7 --every 3 --packet 0 -j SNAT --to-source 10.0.0.5
# iptables -t nat -A POSTROUTING -o eth0 -m nth --counter 7 --every 3 --packet 1 -j SNAT --to-source 10.0.0.6
# iptables -t nat -A POSTROUTING -o eth0 -m nth --counter 7 --every 3 --packet 2 -j SNAT --to-source 10.0.0.7

# iptables -t nat --list
Chain POSTROUTING (policy ACCEPT)
target     prot opt source               destination         
SNAT       all  --  anywhere             anywhere           every 3th packet #0 to:10.0.0.5 
SNAT       all  --  anywhere             anywhere           every 3th packet #1 to:10.0.0.6 
SNAT       all  --  anywhere             anywhere           every 3th packet #2 to:10.0.0.7 

Supported options for the nth match are :

--every Nth

-> Match every Nth packet.

[--counter] num

-> Use counter 0-15 (default:0).

[--start] num

-> Initialize the counter at the number `num' instead of 0. Must be between 0 and (Nth-1).

[--packet] num

-> Match on the `num' packet. Must be between 0 and Nth-1. If `--packet' is used for a counter, then there must be Nth number of --packet rules, covering all values between 0 and (Nth-1) inclusively.

3.8 pkttype patch

This patch by Michal Ludvig <michal@logix.cz> adds a new match that allows you to match a packet based on its type : host/broadcast/multicast.

If For example you want to silently drop all the broadcasted packets :

# iptables -A INPUT -m pkttype --pkt-type broadcast -j DROP

# iptables --list
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
DROP       all  --  anywhere             anywhere           PKTTYPE = broadcast 

Supported options for this match are :

--pkt-type [!] packettype

-> match packet type where packet type is one of

host

-> to us

broadcast

-> to all

multicast

-> to group

3.9 pool patch

Patch by Patrick Schaaf <bof@bof.de>. Joakim Axelsson and Patrick are in the process of re-writing it, therefore they will replace this section with the actual explanations once its written.

3.10 psd patch

This patch by Dennis Koslowski <dkoslowski@astaro.de> adds a new match that will attempt to detect port scans.

In its simplest form, psd match can be used as follows :

# iptables -A INPUT -m psd -j DROP

# iptables --list
Chain INPUT (policy ACCEPT)
target  prot opt source    destination         
DROP    all  --  anywhere  anywhere    psd weight-threshold: 21 delay-threshold: 300 lo-ports-weight: 3 hi-ports-weight: 1

Supported options for psd match are :

[--psd-weight-threshold threshold]

-> Portscan detection weight threshold

[--psd-delay-threshold delay]

-> Portscan detection delay threshold

[--psd-lo-ports-weight lo]

-> Privileged ports weight

[--psd-hi-ports-weight hi]

-> High ports weight

3.11 quota patch

This patch by Sam Johnston <samj@samj.net> adds a new match that allows you to set quotas. When the quota is reached, the rule doesn't match any more.

For example, if you want to limit put a quota of 50Megs on incoming http data you can do as follows :

# iptables -A INPUT -p tcp --dport 80 -m quota --quota 52428800 -j ACCEPT
# iptables -A INPUT -p tcp --dport 80 -j DROP

# iptables --list
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
ACCEPT     tcp  --  anywhere             anywhere           tcp dpt:http quota: 52428800 bytes
DROP       tcp  --  anywhere             anywhere           tcp dpt:http 

Supported options for quota match are :

--quota quota

-> The quota you want to set.

3.12 random patch

This patch by Fabrice MARIE <fabrice@celestix.com> adds a new match that allows you to math a packet randomly based on given probability.

For example, if you want to drop 50% of the pings randomly, you can do as follows :

# iptables -A INPUT -p icmp --icmp-type echo-request -m random --average 50 -j DROP

# iptables --list
Chain INPUT (policy ACCEPT)
target     prot opt source       destination         
DROP       icmp --  anywhere     anywhere        icmp echo-request  random 50% 

Supported options for random match are :

[--average percent]

-> The probability in percentage of the match. If omitted, a probability of 50% percent is set. Percentage must be within : 1 <= percent <= 99.

3.13 realm patch

This patch by Sampsa Ranta <sampsa@netsonic.fi> adds a new match that allows you to use realm key from routing as match criteria similar to the one found in the packet classifier.

For example, to log all the outgoing packet with a realm of 10, you can do the following :

# iptables -A OUTPUT -m realm --realm 10 -j LOG

# iptables --list
Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination         
LOG        all  --  anywhere             anywhere           REALM match 0xa LOG level warning

Supported options for the realm match are :

--realm [!] value[/mask]

-> Match realm

3.14 recent patch

This patch by Stephen Frost <sfrost@snowman.net> adds a new match that allows you to dynamically create a list of IP addresses and then match against that list in a few different ways.

For example, you can create a `badguy' list out of people attempting to connect to port 139 on your firewall and then DROP all future packets from them without considering them.

# iptables -A FORWARD -m recent --name badguy --rcheck --seconds 60 -j DROP
# iptables -A FORWARD -p tcp -i eth0 --dport 139 -m recent --name badguy --set -j DROP

# iptables --list
Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
DROP       all  --  anywhere             anywhere           recent: CHECK seconds: 60
DROP       tcp  --  anywhere             anywhere           tcp dpt:netbios-ssn recent: SET

Supported options for the recent match are :

--name name

-> Specify the list to use for the commands. If no name is given then 'DEFAULT' will be used.

[!] --set

-> This will add the source address of the packet to the list. If the source address is already in the list, this will update the existing entry. This will always return success or failure if `!' is passed in.

[!] --rcheck

-> This will check if the source address of the packet is currently in the list and return true if it is, and false otherwise. Opposite is returned if `!' is passed in.

[!] --update

-> This will check if the source address of the packet is currently in the list. If it is then that entry will be updated and the rule will return true. If the source address is not in the list then the rule will return false. Opposite is returned if `!' is passed in.

[!] --remove

-> This will check if the source address of the packet is currently in the list and if so that address will be removed from the list and the rule will return true. If the address is not found, false is returned. Opposite is returned if `!' is passed in.

[!] --seconds seconds

-> This option must be used in conjunction with one of `rcheck' or `update'. When used, this will narrow the match to only happen when the address is in the list and was seen within the last given number of seconds. Opposite is returned if `!' is passed in.

[!] --hitcount hits

-> This option must be used in conjunction with one of `rcheck' or `update'. When used, this will narrow the match to only happen when the address is in the list and packets had been received greater than or equal to the given value. This option may be used along with `seconds' to create an even narrower match requiring a certain number of hits within a specific time frame. Opposite returned if `!' passed in.

--rttl

-> This option must be used in conjunction with one of `rcheck' or `update'. When used, this will narrow the match to only happen when the address is in the list and the TTL of the current packet matches that of the packet which hit the --set rule. This may be useful if you have problems with people faking their source address in order to DoS you via this module by disallowing others access to your site by sending bogus packets to you.

3.15 record-rpc patch

This patch by Marcelo Barbosa Lima <marcelo.lima@dcc.unicamp.br> adds a new match that allows you to match if the source of the packet has requested that port through the portmapper before, or it is a new GET request to the portmapper, allowing effective RPC filtering.

To match RPC connection tracking information, simply do the following :

# iptables -A INPUT -m record_rpc -j ACCEPT

# iptables --list
Chain INPUT (policy ACCEPT)
target     prot opt source               destination 
ACCEPT     all  --  anywhere             anywhere

The record_rpc match does not take any option.

Do not worry for the match information not printed, it's simply because the print() function of this match is empty :

/* Prints out the union ipt_matchinfo. */
static void
print(const struct ipt_ip *ip,
      const struct ipt_entry_match *match,
      int numeric)
{
}

3.16 string patch

This patch by Emmanuel Roger <winfield@freegates.be> adds a new match that allows you to match a string anywhere in the packet.

For example, to match packets containing the string ``cmd.exe'' anywhere in the packet and queue them to a userland IDS, you could use :

# iptables -A INPUT -m string --string 'cmd.exe' -j QUEUE

# iptables --list
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         
QUEUE      all  --  anywhere             anywhere           STRING match cmd.exe 

Please do use this match with caution. A lot of people want to use this match to stop worms, along with the DROP target. This is a major mistake. It would be defeated by any IDS evasion method.

In a similar fashion, a lot of people have been using this match as a mean to stop particular functions in HTTP like POST or GET by dropping any HTTP packet containing the string POST. Please understand that this job is better done by a filtering proxy. Additionally, any HTML content with the word POST would get dropped with the former method. This match has been designed to be able to queue to userland interesting packets for better analysis, that's all. Dropping packet based on this would be defeated by any IDS evasion method.

Supported options for the string match are :

--string [!] string

-> Match a string in a packet

3.17 time patch

This patch by Fabrice MARIE <fabrice@celestix.com> adds a new match that allows you to match a packet based on its arrival or departure (for locally generated packets) timestamp.

for example, to accept packets that have an arrival time from 8:00H to 18:00H from Monday to Friday you can do as follows :

# iptables -A INPUT -m time --timestart 8:00 --timestop 18:00 --days Mon,Tue,Wed,Thu,Fri -j ACCEPT

# iptables --list 
Chain INPUT (policy ACCEPT)
target     prot opt source           destination
ACCEPT     all  --  anywhere         anywhere        TIME from 8:0 to 18:0 on Mon,Tue,Wed,Thu,Fri 

Supported options for the time match are :

--timestart value

-> minimum HH:MM

--timestop value

-> maximum HH:MM

--days listofdays

-> a list of days to apply, from (case sensitive)

• Mon
• Tue
• Wed
• Thu
• Fri
• Sat
• Sun

3.18 ttl patch

This patch by Harald Welte <laforge@gnumonks.org> adds a new match that allows you to match a packet based on its TTL.

For example if you want to log any packet that have a TTL less than 5, you can do as follows :

# iptables -A INPUT -m ttl --ttl-lt 5 -j LOG

# iptables --list
Chain INPUT (policy ACCEPT)
target     prot opt source               destination
LOG        all  --  anywhere             anywhere           TTL match TTL < 5 LOG level warning

Options supported by the ttl match are :

--ttl-eq value

-> Match time to live value

--ttl-lt value

-> Match TTL < value

--ttl-gt value

-> Match TTL > value