Linux Packet Filtering and iptables

Explicit matches are those that have to be specifically loaded with the -m or --match option. State matches, for example, demand the directive -m state prior to entering the actual match that you want to use. Some of these matches may be protocol specific . Some may be unconnected with any specific protocol - for example connection states. These might be NEW (the first packet of an as yet unestablished connection), ESTABLISHED (a connection that is already registered in the kernel), RELATED (a new connection that was created by an older, established one) etc. A few may just have been evolved for testing or experimental purposes, or just to illustrate what iptables is capable of. This in turn means that not all of these matches may at first sight be of any use. Nevertheless, it may well be that you personally will find a use for specific explicit matches. And there are new ones coming along all the time, with each new iptables release. Whether you find a use for them or not depends on your imagination and your needs. The difference between implicitly loaded matches and explicitly loaded ones, is that the implicitly loaded matches will automatically be loaded when, for example, you match on the properties of TCP packets, while explicitly loaded matches will never be loaded automatically - it is up to you to discover and activate explicit matches.

These matches are used for the IPSEC AH and ESP protocols. IPSEC is used to create secure tunnels over an insecure Internet connection. The AH and ESP protocols are used by IPSEC to create these secure connections. The AH and ESP matches are really two separate matches, but are both described here since they look very much alike, and both are used in the same function.

I will not go into detail to describe IPSEC here, instead look at the following pages and documents for more information:

• RFC 2401 - Security Architecture for the Internet Protocol

There is also a ton more documentation on the Internet on this, but you are free to look it up as needed.

To use the AH/ESP matches, you need to use -m ah to load the AH matches, and -m esp to load the ESP matches.

In 2.2 and 2.4 kernels, Linux used something called FreeS/WAN for the IPSEC implementation, but as of Linux kernel 2.5.47 and up, Linux kernels have a direct implementation of IPSEC that requires no patching of the kernel. This is a total rewrite of the IPSEC implementation on Linux.

Table 10-5. AH match options

Match	--ahspi
Kernel	2.5 and 2.6
Example	iptables -A INPUT -p 51 -m ah --ahspi 500
Explanation	This matches the AH Security Parameter Index (SPI) number of the AH packets. Please note that you must specify the protocol as well, since AH runs on a different protocol than the standard TCP, UDP or ICMP protocols. The SPI number is used in conjunction with the source and destination address and the secret keys to create a security association (SA). The SA uniquely identifies each and every one of the IPSEC tunnels to all hosts. The SPI is used to uniquely distinguish each IPSEC tunnel connected between the same two peers. Using the --ahspi match, we can match a packet based on the SPI of the packets. This match can match a whole range of SPI values by using a : sign, such as 500:520, which will match the whole range of SPI's.

Table 10-6. ESP match options

Match	--espspi
Kernel	2.5 and 2.6
Example	iptables -A INPUT -p 50 -m esp --espspi 500
Explanation	The ESP counterpart Security Parameter Index (SPI) is used exactly the same way as the AH variant. The match looks exactly the same, with the esp/ah difference. Of course, this match can match a whole range of SPI numbers as well as the AH variant of the SPI match, such as --espspi 200:250 which matches the whole range of SPI's.

The conntrack match is an extended version of the state match, which makes it possible to match packets in a much more granular way. It let's you look at information directly available in the connection tracking system, without any "frontend" systems, such as in the state match. For more information about the connection tracking system, take a look at the chapter.

There are a number of different matches put together in the conntrack match, for several different fields in the connection tracking system. These are compiled together into the list below. To load these matches, you need to specify -m conntrack.

Table 10-7. Conntrack match options

Match	--ctstate
Kernel	2.5 and 2.6
Example	iptables -A INPUT -p tcp -m conntrack --ctstate RELATED
Explanation	This match is used to match the state of a packet, according to the conntrack state. It is used to match pretty much the same states as in the original state match. The valid entries for this match are: INVALID ESTABLISED NEW RELATED SNAT DNAT The entries can be used together with each other separated by a comma. For example, -m conntrack --ctstate ESTABLISHED,RELATED. It can also be inverted by putting a ! in front of --ctstate. For example: -m conntrack ! --ctstate ESTABLISHED,RELATED, which matches all but the ESTABLISHED and RELATED states.
Match	--ctproto
Kernel	2.5 and 2.6
Example	iptables -A INPUT -p tcp -m conntrack --ctproto TCP
Explanation	This matches the protocol, the same as the --protocol does. It can take the same types of values, and is inverted using the ! sign. For example, -m conntrack ! --ctproto TCP matches all protocols but the TCP protocol.
Match	--ctorigsrc
Kernel	2.5 and 2.6
Example	iptables -A INPUT -p tcp -m conntrack --ctorigsrc 192.168.0.0/24
Explanation	--ctorigsrc matches based on the original source IP specification of the conntrack entry that the packet is related to. The match can be inverted by using a ! between the --ctorigsrc and IP specification, such as --ctorigsrc ! 192.168.0.1. It can also take a netmask of the CIDR form, such as --ctorigsrc 192.168.0.0/24.
Match	--ctorigdst
Kernel	2.5 and 2.6
Example	iptables -A INPUT -p tcp -m conntrack --ctorigdst 192.168.0.0/24
Explanation	This match is used exactly as the --ctorigsrc, except that it matches on the destination field of the conntrack entry. It has the same syntax in all other respects.
Match	--ctreplsrc
Kernel	2.5 and 2.6
Example	iptables -A INPUT -p tcp -m conntrack --ctreplysrc 192.168.0.0/24
Explanation	The --ctreplysrc match is used to match based on the original conntrack reply source of the packet. Basically, this is the same as the --ctorigsrc, but instead we match the reply source expected of the upcoming packets. This target can, of course, be inverted and address a whole range of addresses, just the same as the the previous targets in this class.
Match	--ctrepldst
Kernel	2.5 and 2.6
Example	iptables -A INPUT -p tcp -m conntrack --ctreplydst 192.168.0.0/24
Explanation	The --ctreplydst match is the same as the --ctreplysrc match, with the exception that it matches the reply destination of the conntrack entry that matched the packet. It too can be inverted, and accept ranges, just as the --ctreplysrc match.
Match	--ctstatus
Kernel	2.5 and 2.6
Example	iptables -A INPUT -p tcp -m conntrack --ctstatus RELATED
Explanation	This matches the status of the connection, as described in the chapter. It can match the following statuses. NONE - The connection has no status at all. EXPECTED - This connection is expected and was added by one of the expectation handlers. SEEN_REPLY - This connection has seen a reply but isn't assured yet. ASSURED - The connection is assured and will not be removed until it times out or the connection is closed by either end. This can also be inverted by using the ! sign. For example -m conntrack ! --ctstatus ASSURED which will match all but the ASSURED status.
Match	--ctexpire
Kernel	2.5 and 2.6
Example	iptables -A INPUT -p tcp -m conntrack --ctexpire 100:150
Explanation	This match is used to match on packets based on how long is left on the expiration timer of the conntrack entry, measured in seconds. It can either take a single value and match against, or a range such as in the example above. It can also be inverted by using the ! sign, such as this -m conntrack ! --ctexpire 100. This will match every expiration time, which does not have exactly 100 seconds left to it.

This match is used to match on packets based on their DSCP (Differentiated Services Code Point) field. This is documented in the RFC. The match is explicitly loaded by specifying -m dscp. The match can take two mutually exclusive options, described below.

Table 10-8. DSCP match options

Match	--dscp
Kernel	2.5 and 2.6
Example	iptables -A INPUT -p tcp -m dscp --dscp 32
Explanation	This option takes a DSCP value in either decimal or in hex. If the option value is in decimal, it would be written like 32 or 16, et cetera. If written in hex, it should be prefixed with 0x, like this: 0x20. It can also be inverted by using the ! character, like this: -m dscp ! --dscp 32.
Match	--dscp-class
Kernel	2.5 and 2.6
Example	iptables -A INPUT -p tcp -m dscp --dscp-class BE
Explanation	The --dscp-class match is used to match on the DiffServ class of a packet. The values can be any of the BE, EF, AFxx or CSx classes as specified in the various RFC's. This match can be inverted just the same way as the --dscp option.

Please note that the --dscp and --dscp-class options are mutually exclusive and can not be used in conjunction with each other.

The ECN match is used to match on the different ECN fields in the TCP and IPv4 headers. ECN is described in detail in the RFC. The match is explicitly loaded by using -m ecn in the command line. The ECN match takes three different options as described below.

Table 10-9. ECN match options

Match	--ecn
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m ecn --ecn-tcp-cwr
Explanation	This match is used to match the CWR (Congestion Window Received) bit, if it has been set. The CWR flag is set to notify the other endpoint of the connection that they have received an ECE, and that they have reacted to it. Per default this matches if the CWR bit is set, but the match may also be inversed using an exclamation point.
Match	--ecn-tcp-ece
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m ecn --ecn-tcp-ece
Explanation	This match can be used to match the ECE (ECN-Echo) bit. The ECE is set once one of the endpoints has received a packet with the CE bit set by a router. The endpoint then sets the ECE in the returning ACK packet, to notify the other endpoint that it needs to slow down. The other endpoint then sends a CWR packet as described in the --ecn-tcp-cwr explanation. This matches per default if the ECE bit is set, but may be inversed by using an exclamation point.
Match	--ecn-ip-ect
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m ecn --ecn-ip-ect 1
Explanation	The --ecn-ip-ect match is used to match the ECT (ECN Capable Transport) codepoints. The ECT codepoints has several types of usage. Mainly, they are used to negotiate if the connection is ECN capable by setting one of the two bits to 1. The ECT is also used by routers to indicate that they are experiencing congestion, by setting both ECT codepoints to 1. The ECT values are all available in the in the table below. The match can be inversed using an exclamation point, for example ! --ecn-ip-ect 2 which will match all ECN values but the ECT(0) codepoint. The valid value range is 0-3 in iptables. See the above table for their values.

Table 10-10. ECN Field in IP

Iptables value	ECT	CE	[Obsolete] RFC 2481 names for the ECN bits.
0	0	0	Not-ECT, ie. non-ECN capable connection.
1	0	1	ECT(1), New naming convention of ECT codepoints in RFC 3168.
2	1	0	ECT(0), New naming convention of ECT codepoints in RFC 3168.
3	1	1	CE (Congestion Experienced), Used to notify endpoints of congestion

This is a rather unorthodox match in comparison to the other matches, in the sense that it uses a little bit specific syntax. The match is used to match packets, based on which conntrack helper that the packet is related to. For example, let's look at the FTP session. The Control session is opened up, and the ports/connection is negotiated for the Data session within the Control session. The ip_conntrack_ftp helper module will find this information, and create a related entry in the conntrack table. Now, when a packet enters, we can see which protocol it was related to, and we can match the packet in our ruleset based on which helper was used.

Table 10-11. Helper match options

Match	--helper
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m helper --helper ftp-21
Explanation	The --helper option is used to specify a string value, telling the match which conntrack helper to match. In the basic form, it may look like --helper irc. This is where the syntax starts to change from the normal syntax. We can also choose to only match packets based on which port that the original expectation was caught on. For example, the FTP Control session is normally transferred over port 21, but it may as well be port 954 or any other port. We may then specify upon which port the expectation should be caught on, like --helper ftp-954.

The IP range match is used to match IP ranges, just as the --source and --destination matches are able to do as well. However, this match adds a different kind of matching in the sense that it is able to match in the manner of from IP - to IP, which the --source and --destination matches are unable to. This may be needed in some specific network setups, and it is rather a bit more flexible.

Table 10-12. IP range match options

Match	--src-range
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m iprange --src-range 192.168.1.13-192.168.2.19
Explanation	This matches a range of source IP addresses. The range includes every single IP address from the first to the last, so the example above includes everything from 192.168.1.13 to 192.168.2.19. The match may also be inverted by adding an !. The above example would then look like -m iprange ! --src-range 192.168.1.13-192.168.2.19, which would match every single IP address, except the ones specified.
Match	--dst-range
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m iprange --dst-range 192.168.1.13-192.168.2.19
Explanation	The --dst-range works exactly the same as the --src-range match, except that it matches destination IP's instead of source IP's.

The length match is used to match packets based on their length. It is very simple. If you want to limit packet length for some strange reason, or want to block ping-of-death-like behaviour, use the length match.

Table 10-13. Length match options

Match	--length
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m length --length 1400:1500
Explanation	The example --length will match all packets with a length between 1400 and 1500 bytes. The match may also be inversed using the ! sign, like this: -m length ! --length 1400:1500 . It may also be used to match only a specific length, removing the : sign and onwards, like this: -m length --length 1400. The range matching is, of course, inclusive, which means that it includes all packet lengths in between the values you specify.

The limit match extension must be loaded explicitly with the -m limit option. This match can, for example, be used to advantage to give limited logging of specific rules etc. For example, you could use this to match all packets that do not exceed a given value, and after this value has been exceeded, limit logging of the event in question. Think of a time limit: You could limit how many times a certain rule may be matched in a certain time frame, for example to lessen the effects of DoS syn flood attacks. This is its main usage, but there are more usages, of course. The limit match may also be inverted by adding a ! flag in front of the limit match. It would then be expressed as -m limit ! --limit 5/s.This means that all packets will be matched after they have broken the limit.

To further explain the limit match, it is basically a token bucket filter. Consider having a leaky bucket where the bucket leaks X packets per time-unit. X is defined depending on how many matching packets we get, so if we get 3 packets, the bucket leaks 3 packets per that time-unit. The --limit option tells us how many packets to refill the bucket with per time-unit, while the --limit-burst option tells us how big the bucket is in the first place. So, setting --limit 3/minute --limit-burst 5, and then receiving 5 matches will empty the bucket. After 20 seconds, the bucket is refilled with another token, and so on until the --limit-burst is reached again or until they get used.

Consider the example below for further explanation of how this may look.

1. We set a rule with -m limit --limit 5/second --limit-burst 10/second. The limit-burst token bucket is set to 10 initially. Each packet that matches the rule uses a token.

2. We get packet that matches, 1-2-3-4-5-6-7-8-9-10, all within a 1/1000 of a second.

3. The token bucket is now empty. Once the token bucket is empty, the packets that qualify for the rule otherwise no longer match the rule and proceed to the next rule if any, or hit the chain policy.

4. For each 1/5 s without a matching packet, the token count goes up by 1, upto a maximum of 10. 1 second after receiving the 10 packets, we will once again have 5 tokens left.

5. And of course, the bucket will be emptied by 1 token for each packet it receives.

Table 10-14. Limit match options

Match	--limit
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -m limit --limit 3/hour
Explanation	This sets the maximum average match rate for the limit match. You specify it with a number and an optional time unit. The following time units are currently recognized: /second /minute /hour /day. The default value here is 3 per hour, or 3/hour. This tells the limit match how many times to allow the match to occur per time unit (e.g. per minute).
Match	--limit-burst
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -m limit --limit-burst 5
Explanation	This is the setting for the burst limit of the limit match. It tells iptables the maximum number of packets to match within the given time unit. This number gets decremented by one for every time unit (specified by the --limit option) in which the event does not occur, back down to the lowest possible value, 1. If the event is repeated, the counter is again incremented, until the count reaches the burst limit. And so on. The default --limit-burst value is 5. For a simple way of checking out how this works, you can use the example one-rule-script. Using this script, you can see for yourself how the limit rule works, by simply sending ping packets at different intervals and in different burst numbers. All echo replies will be blocked until the threshold for the burst limit has again been reached.

The MAC (Ethernet Media Access Control) match can be used to match packets based on their MAC source address. As of writing this documentation, this match is a little bit limited, however, in the future this may be more evolved and may be more useful. This match can be used to match packets on the source MAC address only as previously said.

Do note that to use this module we explicitly load it with the -m mac option. The reason that I am saying this is that a lot of people wonder if it should not be -m mac-source, which it should not.

Table 10-15. MAC match options

Match	--mac-source
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -m mac --mac-source 00:00:00:00:00:01
Explanation	This match is used to match packets based on their MAC source address. The MAC address specified must be in the form XX:XX:XX:XX:XX:XX, else it will not be legal. The match may be reversed with an ! sign and would look like --mac-source ! 00:00:00:00:00:01. This would in other words reverse the meaning of the match, so that all packets except packets from this MAC address would be matched. Note that since MAC addresses are only used on Ethernet type networks, this match will only be possible to use for Ethernet interfaces. The MAC match is only valid in the PREROUTING, FORWARD and INPUT chains and nowhere else.

The mark match extension is used to match packets based on the marks they have set. A mark is a special field, only maintained within the kernel, that is associated with the packets as they travel through the computer. Marks may be used by different kernel routines for such tasks as traffic shaping and filtering. As of today, there is only one way of setting a mark in Linux, namely the MARK target in iptables. This was previously done with the FWMARK target in ipchains, and this is why people still refer to FWMARK in advanced routing areas. The mark field is currently set to an unsigned integer, or 4294967296 possible values on a 32 bit system. In other words, you are probably not going to run into this limit for quite some time.

Table 10-16. Mark match options

Match	--mark
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -t mangle -A INPUT -m mark --mark 1
Explanation	This match is used to match packets that have previously been marked. Marks can be set with the MARK target which we will discuss in the next section. All packets traveling through Netfilter get a special mark field associated with them. Note that this mark field is not in any way propagated, within or outside the packet. It stays inside the computer that made it. If the mark field matches the mark, it is a match. The mark field is an unsigned integer, hence there can be a maximum of 4294967296 different marks. You may also use a mask with the mark. The mark specification would then look like, for example, --mark 1/1. If a mask is specified, it is logically AND ed with the mark specified before the actual comparison.

The multiport match extension can be used to specify multiple destination ports and port ranges. Without the possibility this match gives, you would have to use multiple rules of the same type, just to match different ports.

You can not use both standard port matching and multiport matching at the same time, for example you can't write: --sport 1024:63353 -m multiport --dport 21,23,80. This will simply not work. What in fact happens, if you do, is that iptables honors the first element in the rule, and ignores the multiport instruction.

Table 10-17. Multiport match options

Match	--source-port
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m multiport --source-port 22,53,80,110
Explanation	This match matches multiple source ports. A maximum of 15 separate ports may be specified. The ports must be comma delimited, as in the above example. The match may only be used in conjunction with the -p tcp or -p udp matches. It is mainly an enhanced version of the normal --source-port match.
Match	--destination-port
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m multiport --destination-port 22,53,80,110
Explanation	This match is used to match multiple destination ports. It works exactly the same way as the above mentioned source port match, except that it matches destination ports. It too has a limit of 15 ports and may only be used in conjunction with -p tcp and -p udp.
Match	--port
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m multiport --port 22,53,80,110
Explanation	This match extension can be used to match packets based both on their destination port and their source port. It works the same way as the --source-port and --destination-port matches above. It can take a maximum of 15 ports and can only be used in conjunction with -p tcp and -p udp. Note that the --port match will only match packets coming in from and going to the same port, for example, port 80 to port 80, port 110 to port 110 and so on.

The owner match extension is used to match packets based on the identity of the process that created them. The owner can be specified as the process ID either of the user who issued the command in question, that of the group, the process, the session, or that of the command itself. This extension was originally written as an example of what iptables could be used for. The owner match only works within the OUTPUT chain, for obvious reasons: It is pretty much impossible to find out any information about the identity of the instance that sent a packet from the other end, or where there is an intermediate hop to the real destination. Even within the OUTPUT chain it is not very reliable, since certain packets may not have an owner. Notorious packets of that sort are (among other things) the different ICMP responses. ICMP responses will never match.

Table 10-18. Owner match options

Match	--uid-owner
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m owner --uid-owner 500
Explanation	This packet match will match if the packet was created by the given User ID (UID). This could be used to match outgoing packets based on who created them. One possible use would be to block any other user than root from opening new connections outside your firewall. Another possible use could be to block everyone but the http user from sending packets from the HTTP port.
Match	--gid-owner
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m owner --gid-owner 0
Explanation	This match is used to match all packets based on their Group ID (GID). This means that we match all packets based on what group the user creating the packets is in. This could be used to block all but the users in the network group from getting out onto the Internet or, as described above, only to allow members of the http group to create packets going out from the HTTP port.
Match	--pid-owner
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m owner --pid-owner 78
Explanation	This match is used to match packets based on the Process ID (PID) that was responsible for them. This match is a bit harder to use, but one example would be only to allow PID 94 to send packets from the HTTP port (if the HTTP process is not threaded, of course). Alternatively we could write a small script that grabs the PID from a ps output for a specific daemon and then adds a rule for it. For an example, you could have a rule as shown in the example.
Match	--sid-owner
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m owner --sid-owner 100
Explanation	This match is used to match packets based on the Session ID used by the program in question. The value of the SID, or Session ID of a process, is that of the process itself and all processes resulting from the originating process. These latter could be threads, or a child of the original process. So, for example, all of our HTTPD processes should have the same SID as their parent process (the originating HTTPD process), if our HTTPD is threaded (most HTTPDs are, Apache and Roxen for instance). To show this in example, we have created a small script called . This script could possibly be run every hour or so together with some extra code to check if the HTTPD is actually running and start it again if necessary, then flush and re-enter our OUTPUT chain if needed.

The packet type match is used to match packets based on their type. I.e., are they destined to a specific person, to everyone or to a specific group of machines or users. These three groups are generally called unicast, broadcast and multicast, as discussed in the chapter. The match is loaded by using -m pkttype.

Table 10-19. Packet type match options

Match	--pkttype
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m owner --pkttype unicast
Explanation	The --pkttype match is used to tell the packet type match which packet type to match. It can either take unicast , broadcast or multicast as an argument, as in the example. It can also be inverted by using a ! like this: -m pkttype --pkttype ! broadcast, which will match all other packet types.

The recent match is a rather large and complex matching system, which allows us to match packets based on recent events that we have previously matched. For example, if we would see an outgoing IRC connection, we could set the IP addresses into a list of hosts, and have another rule that allows identd requests back from the IRC server within 15 seconds of seeing the original packet.

Before we can take a closer look at this match, let's try and explain a little bit how it works. First of all, we use several different rules to accomplish the use of the recent match. The recent match uses several different lists of recent events. The default list being used is the DEFAULT list. We create a new entry in a list with the set option, so once a rule is entirely matched (the set option is always a match), we also add an entry in the recent list specified. The list entry contains a timestamp, and the source IP address used in the packet that triggered the set option. Once this has happened, we can use a series of different recent options to match on this information, as well as update the entries timestamp, et cetera.

Finally, if we would for some reason want to remove a list entry, we would do this using the remove match from the recent module. All rules using the recent match, must load the recent module (-m recent) as usual. Before we go on with an example of the recent match, let's take a look at all the options.

Table 10-20. Recent match options

Match	--name
Kernel	2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m recent --name examplelist
Explanation	The name option gives the name of the list to use. Per default the DEFAULT list is used, which is probably not what we want if we are using more than one list.
Match	--set
Kernel	2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m recent --set
Explanation	This creates a new list entry in the named recent list, which contains a timestamp and the source IP address of the host that triggered the rule. This match will always return success, unless it is preceded by a ! sign, in which case it will return failure.
Match	--rcheck
Kernel	2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m recent --name examplelist --rcheck
Explanation	The --rcheck option will check if the source IP address of the packet is in the named list. If it is, the match will return true, otherwise it returns false. The option may be inverted by using the ! sign. In the later case, it will return true if the source IP address is not in the list, and false if it is in the list.
Match	--update
Kernel	2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m recent --name examplelist --update
Explanation	This match is true if the source combination is available in the specified list and it also updates the last-seen time in the list. This match may also be reversed by setting the ! mark in front of the match. For example, ! --update.
Match	--remove
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -m recent --name example --remove
Explanation	This match will try to find the source address of the packet in the list, and returns true if the packet is there. It will also remove the corresponding list entry from the list. The command is also possible to inverse with the ! sign.
Match	--seconds
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -m recent --name example --check --seconds 60
Explanation	This match is only valid together with the --check and --update matches. The --seconds match is used to specify how long since the "last seen" column was updated in the recent list. If the last seen column was older than this amount in seconds, the match returns false. Other than this the recent match works as normal, so the source address must still be in the list for a true return of the match.
Match	--hitcount
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -m recent --name example --check --hitcount 20
Explanation	The --hitcount match must be used together with the --check or --update matches and it will limit the match to only include packets that have seen at least the hitcount amount of packets. If this match is used together with the --seconds match, it will require the specified hitcount packets to be seen in the specific timeframe. This match may also be reversed by adding a ! sign in front of the match. Together with the --seconds match, this means that a maximum of this amount of packets may have been seen during the specified timeframe. If both of the matches are inversed, then a maximum of this amount of packets may have been seen during the last minumum of seconds.
Match	--rttl
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -m recent --name example --check --rttl
Explanation	The --rttl match is used to verify that the TTL value of the current packet is the same as the original packet that was used to set the original entry in the recent list. This can be used to verify that people are not spoofing their source address to deny others access to your servers by making use of the recent match.
Match	--rsource
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -m recent --name example --rsource
Explanation	The --rsource match is used to tell the recent match to save the source address and port in the recent list. This is the default behavior of the recent match.
Match	--rdest
Kernel	2.4, 2.5 and 2.6
Example	iptables -A INPUT -m recent --name example --rdest
Explanation	The --rdest match is the opposite of the --rsource match in that it tells the recent match to save the destination address and port to the recent list.

I have created a small sample script of how the recent match can be used, which you can find in the section.

Briefly, this is a poor replacement for the state engine available in netfilter. This version was created with a http server in mind, but will work with any TCP connection. First we have created two chains named http-recent and http-recent-final. The http-recent chain is used in the starting stages of the connection, and for the actual data transmission, while the http-recent-final chain is used for the last and final FIN, FIN/ACK handshake.

This is a very bad replacement for the built in state engine and can not handle all of the possibilities that the state engine can handle. However, it is a good example of what can be done with the recent match without being too specific. Do not use this example in a real world environment. It is slow, handles special cases badly, and should generally never be used more than as an example. For example, it does not handle closed ports on connection, asyncronuous FIN handshake (where one of the connected parties closes down, while the other continues to send data), etc.

Let's follow a packet through the example ruleset. First a packet enters the INPUT chain, and we send it to the http-recent chain.

1. The first packet should be a SYN packet, and should not have the ACK,FIN or RST bits set. Hence it is matched using the --tcp-flags SYN,ACK,FIN,RST SYN line. At this point we add the connection to the httplist using -m recent --name httplist --set line. Finally we accept the packet.

2. After the first packet we should receive a SYN/ACK packet to acknowledge that the SYN packet was received. This can be matched using the --tcp-flags SYN,ACK,FIN,RST SYN,ACK line. FIN and RST should be illegal at this point as well. At this point we update the entry in the httplist using -m recent --name httplist --update and finally we ACCEPT the packet.

3. By now we should get a final ACK packet, from the original creater of the connection, to acknowledge the SYN/ACK sent by the server. SYN, FIN and RST are illegal at this point of the connection, so the line should look like --tcp-flags SYN,ACK,FIN,RST ACK. We update the list in exactly the same way as in the previous step, and ACCEPT it.

4. At this point the data tranmission can start. The connection should never contain any SYN packet now, but it will contain ACK packets to acknowledge the data packets that are sent. Each time we see any packet like this, we update the list and ACCEPT the packets.

5. The transmission can be ended in two ways, the simplest is the RST packet. RST will simply reset the connection and it will die. With FIN, the other endpoint answers with a FIN,ACK, and this closes down the connection so that the original source of the FIN can no longer send any data. The receiver of the FIN, will still be able to send data, hence we send the connection to a "final" stage chain to handle the rest.

6. In the http-recent-final chain we check if the packet is still in the httplist, and if so, we send it to the http-recent-final1 chain. In that chain we remove the connection from the httplist and add it to the http-recent-final list instead. If the connection has already been removed and moved over to the http-recent-final list, we send te packet to the http-recent-final2 chain.

7. In the final http-recent-final2 chain, we wait for the non-closed side to finish sending its data, and to close the connection from their side as well. Once this is done, the connection is completely removed.

As you can see the recent list can become quite complex, but it will give you a huge set of possibilities if need be. Still, try and remember not to reinvent the wheel. If the ability you need is already implemented, try and use it instead of trying to create your own solution.

The state match extension is used in conjunction with the connection tracking code in the kernel. The state match accesses the connection tracking state of the packets from the conntracking machine. This allows us to know in what state the connection is, and works for pretty much all protocols, including stateless protocols such as ICMP and UDP. In all cases, there will be a default timeout for the connection and it will then be dropped from the connection tracking database. This match needs to be loaded explicitly by adding a -m state statement to the rule. You will then have access to one new match called state. The concept of state matching is covered more fully in the chapter, since it is such a large topic.

Table 10-21. State matches

Match	--state
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -m state --state RELATED,ESTABLISHED
Explanation	This match option tells the state match what states the packets must be in to be matched. There are currently 4 states that can be used. INVALID, ESTABLISHED, NEW and RELATED. INVALID means that the packet is associated with no known stream or connection and that it may contain faulty data or headers. ESTABLISHED means that the packet is part of an already established connection that has seen packets in both directions and is fully valid. NEW means that the packet has or will start a new connection, or that it is associated with a connection that has not seen packets in both directions. Finally, RELATED means that the packet is starting a new connection and is associated with an already established connection. This could for example mean an FTP data transfer, or an ICMP error associated with a TCP or UDP connection. Note that the NEW state does not look for SYN bits in TCP packets trying to start a new connection and should, hence, not be used unmodified in cases where we have only one firewall and no load balancing between different firewalls. However, there may be times where this could be useful. For more information on how this could be used, read the chapter.

The tcpmss match is used to match a packet based on the Maximum Segment Size in TCP. This match is only valid for SYN and SYN/ACK packets. For a more complete explanation of the MSS value, see the appendix, the and the documents. This match is loaded using -m tcpmss and takes only one option.

Table 10-22. TCPMSS match options

Match	--mss
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp --tcp-flags SYN,ACK,RST SYN -m tcpmss --mss 2000:2500
Explanation	The --mss option tells the tcpmss match which Maximum Segment Sizes to match. This can either be a single specific MSS value, or a range of MSS values separated by a :. The value may also be inverted as usual using the ! sign, as in the following example: -m tcpmss ! --mss 2000:2500 . This example will match all MSS values, except for values in the range 2000 through 2500.

The TOS match can be used to match packets based on their TOS field. TOS stands for Type Of Service, consists of 8 bits, and is located in the IP header. This match is loaded explicitly by adding -m tos to the rule. TOS is normally used to inform intermediate hosts of the precedence of the stream and its content (it doesn't really, but it informs of any specific requirements for the stream, such as it having to be sent as fast as possible, or it needing to be able to send as much payload as possible). How different routers and administrators deal with these values depends. Most do not care at all, while others try their best to do something good with the packets in question and the data they provide.

Table 10-23. TOS matches

Match	--tos
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A INPUT -p tcp -m tos --tos 0x16
Explanation	This match is used as described above. It can match packets based on their TOS field and their value. This could be used, among other things together with the iproute2 and advanced routing functions in Linux, to mark packets for later usage. The match takes a hex or numeric value as an option, or possibly one of the names resulting from 'iptables -m tos -h'. At the time of writing it contained the following named values: Minimize-Delay 16 (0x10), Maximize-Throughput 8 (0x08), Maximize-Reliability 4 (0x04), Minimize-Cost 2 (0x02), and Normal-Service 0 (0x00). Minimize-Delay means to minimize the delay in putting the packets through - example of standard services that would require this include telnet, SSH and FTP-control. Maximize-Throughput means to find a path that allows as big a throughput as possible - a standard protocol would be FTP-data. Maximize-Reliability means to maximize the reliability of the connection and to use lines that are as reliable as possible - a couple of typical examples are BOOTP and TFTP. Minimize-Cost means minimizing the cost of packets getting through each link to the client or server; for example finding the route that costs the least to travel along. Examples of normal protocols that would use this would be RTSP (Real Time Stream Control Protocol) and other streaming video/radio protocols. Finally, Normal-Service would mean any normal protocol that has no special needs.

The TTL match is used to match packets based on their TTL (Time To Live) field residing in the IP headers. The TTL field contains 8 bits of data and is decremented once every time it is processed by an intermediate host between the client and recipient host. If the TTL reaches 0, an ICMP type 11 code 0 (TTL equals 0 during transit) or code 1 (TTL equals 0 during reassembly) is transmitted to the party sending the packet and informing it of the problem. This match is only used to match packets based on their TTL, and not to change anything. The latter, incidentally, applies to all kinds of matches. To load this match, you need to add an -m ttl to the rule.

Table 10-24. TTL matches

Match	--ttl
Kernel	2.3, 2.4, 2.5 and 2.6
Example	iptables -A OUTPUT -m ttl --ttl 60
Explanation	This match option is used to specify the TTL value to match. It takes a numeric value and matches this value within the packet. There is no inversion and there are no other specifics to match. It could, for example, be used for debugging your local network - e.g. LAN hosts that seem to have problems connecting to hosts on the Internet - or to find possible ingress by Trojans etc. The usage is relatively limited, however; its usefulness really depends on your imagination. One example would be to find hosts with bad default TTL values (could be due to a badly implemented TCP/IP stack, or simply to misconfiguration).

The unclean match takes no options and requires no more than explicitly loading it when you want to use it. Note that this option is regarded as experimental and may not work at all times, nor will it take care of all unclean packages or problems. The unclean match tries to match packets that seem malformed or unusual, such as packets with bad headers or checksums and so on. This could be used to DROP connections and to check for bad streams, for example; however you should be aware that it could possibly break legal connections.