Netfilter Extensions HOWTO: New netfilter targets

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## 4. New netfilter targets

In this section, we will attempt to explain the usage of new netfilter targets. 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 targets, you can get the help hints from a particular module by typing :

> `

>     # iptables -j THE_TARGET_YOU_WANT --help

> `

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

## 4.1 ftos patch

This patch by Matthew G. Marsh <mgm@paktronix.com> adds a new target that allows you to set the TOS of packets to an arbitrary value.

For example, if you want to set the TOS of all the outgoing packets to be 15, you can do as follows :

> `

>     # iptables -t mangle -A OUTPUT -j FTOS --set-ftos 15

>     # iptables -t mangle --list
>     Chain OUTPUT (policy ACCEPT)
>     target     prot opt source               destination
>     FTOS       all  --  anywhere             anywhere           TOS set 0x0f

> `

Supported options for the FTOS target are :

--set-ftos value

  -> Set TOS field in packet header to value. This value can be in decimal (ex: `32`) or in hex (ex: `0x20`)

## 4.2 IPV4OPTSSTRIP patch

This patch by Fabrice MARIE <fabrice@netfilter.org> adds a new target that allows you to strip all the IP options from an IPv4 packet.

It's simpled loaded as follows :

> `

>     # iptables -t mangle -A PREROUTING -j IPV4OPTSSTRIP

>     # iptables -t mangle --list
>     Chain PREROUTING (policy ACCEPT)
>     target     prot opt source               destination
>     IPV4OPTSSTRIP  all  --  anywhere             anywhere

> `

This target doesn't support any option.

## 4.3 NETLINK patch

This patch by Gianni Tedesco <gianni@ecsc.co.uk> adds a new target that allows you to send dropped packets to userspace via a netlink socket.

For example, if you want to drop all pings and send them to a userland netlink socket instead, you can do as follows :

> `

>     # iptables -A INPUT -p icmp --icmp-type echo-request -j NETLINK --nldrop

>     # iptables --list
>     Chain INPUT (policy ACCEPT)
>     target     prot opt source               destination
>     NETLINK    icmp --  anywhere             anywhere           icmp echo-request nldrop

> `

Supported options for the NETLINK target are :

--nldrop

  -> Drop the packet too

--nlmark <number>

  -> Mark the packet

--nlsize <bytes>

  -> Limit packet size

For more information on netlink sockets, you can refer to the Netlink Sockets Tour.

## 4.4 NETMAP patch

This patch by Svenning Soerensen <svenning@post5.tele.dk> adds a new target that allows you create a static 1:1 mapping of the network address, while keeping host addresses intact.

For example, if you want to alter the destination of incoming connections from 1.2.3.0/24 to 5.6.7.0/24, you can do as follows :

> `

>     # iptables -t nat -A PREROUTING -d 1.2.3.0/24 -j NETMAP --to 5.6.7.0/24

>     # iptables -t nat --list
>     Chain PREROUTING (policy ACCEPT)
>     target     prot opt source               destination
>     NETMAP     all  --  anywhere             1.2.3.0/24         5.6.7.0/24

> `

Supported options for NETMAP target are :

--to address[/mask]

  -> Network address to map to.

## 4.5 ROUTE patch

This patch by Cédric de Launois <delaunois@info.ucl.ac.be> adds a new target which allows you to setup unusual routes not supported by the standard kernel routing table. The ROUTE target lets you route a received packet through an interface or towards a host, even if the regular destination of the packet is the router itself. The ROUTE target is also able to change the incoming interface of a packet. Packets are directly put on the wire and do not traverse any other table.

This target does not modify the packets and is a final target. It has to be used inside the mangle table.

Whenever possible, you should use the MARK target together with iproute2 instead of this ROUTE target. However, this target is useful to force the use of an interface or a next hop and to change the incoming interface of a packet. People also use it for easiness and to simplify their rules (one rule to route a packet is easier that one MARK rule + one iproute2 rule).

Options supported by the ROUTE target are :

--oif ifname

  Send the packet out using `ifname' network interface. The destination host must be on the same link or the interface must be a tunnel. Otherwise, arp resolution cannot be performed and the packet is dropped.

--iif ifname

  Change the packet's incoming interface to `ifname'.

--gw ip

  Route the packet via this gateway. The packet is routed as if its destination IP address was this ip.

For example, assume that you want to redirect ssh packets towards a server inside your network, without modifying those packets in any way (this excludes the use of the standard port forwarding mechanism). A solution is to use an ipip tunnel and the ROUTE target to reroute ssh packets to the real ssh server, which has the same IP address as the router. It is not possible to reroute those packets using the standard routing mechanisms, because the kernel locally delivers a packet having a destination address belonging to the router itself.

Time for ASCII art :

                  eth0  +------+ 192.168.0.1        192.168.0.2 +----+
        ----------------|router|--------------------------------|host|
        IP: 150.150.0.1 +------+                                +----+
                           | | tunl1              IP: 150.150.0.1 | |
                           | +------------------------------------+ |
                           +----------------------------------------+
                                          IPIP tunnel

For the example above, you can do as follows :

> `

>     # iptables -A PREROUTING -t mangle -i eth0 -p tcp --dport 22 -j ROUTE --oif tunl1
>     # iptables -A PREROUTING -t mangle -i tunl1 -j ROUTE --oif eth0

>     # iptables -L PREROUTING -t mangle
>     Chain PREROUTING (policy ACCEPT)
>     target     prot opt source               destination
>     ROUTE      tcp  --  anywhere             anywhere           tcp dpt:ssh ROUTE oif tunl1
>     ROUTE      all  --  anywhere             anywhere           ROUTE oif eth0

> `

Another example : if you want to quickly and easily balance the load between two gateways 10.0.0.1 and 10.0.0.2, then you can do as follows :

> `

>     # iptables -A PREROUTING -t mangle -m random --average 50 -j ROUTE --gw 10.0.0.1
>     # iptables -A PREROUTING -t mangle -j ROUTE --gw 10.0.0.2

>     # iptables -L PREROUTING -t mangle
>     Chain PREROUTING (policy ACCEPT)
>     target     prot opt source               destination
>     ROUTE      all  --  anywhere             anywhere           random 50% ROUTE gw 10.0.0.1
>     ROUTE      all  --  anywhere             anywhere           ROUTE gw 10.0.0.2

> `

## 4.6 SAME patch

This patch by Martin Josefsson <gandalf@wlug.westbo.se> adds a new target which is similar to SNAT and will gives a client the same address for each connection.

For example, if you want to modify the source address of the connections to be 1.2.3.4-1.2.3.7 you can do as follows :

> `

>     # iptables -t nat -A POSTROUTING -j SAME --to 1.2.3.4-1.2.3.7

>     # iptables -t nat --list
>     Chain POSTROUTING (policy ACCEPT)
>     target     prot opt source               destination
>     SAME       all  --  anywhere             anywhere           same:1.2.3.4-1.2.3.7

> `

Options supported by the SAME target are :

--to <ipaddr>-<ipaddr>

  -> Addresses to map source to. May be specified more than once for multiple ranges.

--nodst

  -> Don't use destination-ip in source selection

## 4.7 tcp-MSS patch

This patch by Marc Boucher <marc+nf@mbsi.ca> adds a new target that allows you to examine and alter the MSS value of TCP SYN packets, to control the maximum size for that connection.

As explained by Marc himself, THIS IS A HACK, used to overcome criminally brain-dead ISPs or servers which block ICMP Fragmentation Needed packets.

Typical usage would be :

> `

>     # iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu

>     # iptables --list
>     Chain FORWARD (policy ACCEPT)
>     target     prot opt source               destination
>     TCPMSS     tcp  --  anywhere             anywhere           tcp flags:SYN,RST/SYN TCPMSS clamp to PMTU

> `

Options supported by the tcp-MSS target are (mutually-exclusive) :

--set-mss value

  explicitly set MSS option to specified value

--clamp-mss-to-pmtu

  automatically clamp MSS value to (path_MTU - 40)

## 4.8 TTL patch

This patch by Harald Welte <laforge@gnumonks.org> adds a new target that enables the user to set the TTL value of an IP packet or to increment/decrement it by a given value.

For example, if you want to set the TTL of all outgoing connections to 126, you can do as follows :

> `

>     # iptables -t mangle -A OUTPUT -j TTL --ttl-set 126

>     # iptables -t mangle --list
>     Chain OUTPUT (policy ACCEPT)
>     target     prot opt source               destination
>     TTL        all  --  anywhere             anywhere           TTL set to 126

> `

Supported options for the TTL target are :

--ttl-set value

  -> Set TTL to <value>

--ttl-dec value

  -> Decrement TTL by <value>

--ttl-inc value

  -> Increment TTL by <value>

## 4.9 ulog patch

This patch by Harald Welte <laforge@gnumonks.org> adds a new target which supplies a more advanced packet logging mechanism than the standard LOG target. The `libipulog/' contains a library for receiving the ULOG messages.

Harald maintains a web page containing the proper documentation for ULOG, so there is no point for me to explain this here..

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