Oskar Andreasson - Iptables Tutorial 1.2.2

Table 11-18. TCPMSS target options

Works under Linux kernel 2.5 and 2.6.

The TOS target is used to set the Type of Service field within the IP header. The TOS field consists of 8 bits which are used to help in routing packets. This is one of the fields that can be used directly within iproute2 and its subsystem for routing policies. Worth noting, is that if you handle several separate firewalls and routers, this is the only way to propagate routing information within the actual packet between these routers and firewalls. As previously noted, the MARK target - which sets a MARK associated with a specific packet - is only available within the kernel, and can't be propagated with the packet. If you feel a need to propagate routing information for a specific packet or stream, you should therefore set the TOS field, which was developed for this.

There are currently a lot of routers on the Internet which do a pretty bad job at this, so as of now it may prove to be a bit useless to attempt TOS mangling before sending the packets on to the Internet. At best the routers will not pay any attention to the TOS field. At worst, they will look at the TOS field and do the wrong thing. However, as stated above, the TOS field can most definitely be put to good use if you have a large WAN or LAN with multiple routers. You then in fact have the possibility of giving packets different routes and preferences, based on their TOS value - even though this might be confined to your own network.

The TOS target is only capable of setting specific values, or named values on packets. These predefined TOS values can be found in the kernel include files, or more precisely, the Linux/ip.h file. The reasons are many, and you should actually never need to set any other values; however, there are ways around this limitation. To get around the limitation of only being able to set the named values on packets, you can use the FTOS patch available at the Paksecured Linux Kernel patches site maintained by Matthew G. Marsh. However, be cautious with this patch! You should not need to use any other than the default values, except in extreme cases.

Note that this target is only valid within the mangle table and can't be used outside it.

Also note that some old versions (1.2.2 or below) of iptables provided a broken implementation of this target which did not fix the packet checksum upon mangling, hence rendering the packets bad and in need of retransmission. That in turn would most probably lead to further mangling and the connection never working.

The TOS target only takes one option as described below.

Table 11-19. TOS target options

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.

The TTL target is used to modify the Time To Live field in the IP header. One useful application of this is to change all Time To Live values to the same value on all outgoing packets. One reason for doing this is if you have a bully ISP which don't allow you to have more than one machine connected to the same Internet connection, and who actively pursues this. Setting all TTL values to the same value, will effectively make it a little bit harder for them to notice that you are doing this. We may then reset the TTL value for all outgoing packets to a standardized value, such as 64 as specified in the Linux kernel.

For more information on how to set the default value used in Linux, read the ip-sysctl.txt , which you may find within the Other resources and links appendix.

The TTL target is only valid within the mangle table, and nowhere else. It takes 3 options as of writing this, all of them described below in the table.

Table 11-20. TTL target options

Works under Linux kernel 2.3, 2.4, 2.5 and 2.6.