Advanced Linux Routing and Traffic Control: NetFilter and Iproute

So far we have learned how Iproute works, and have mentioned netfilter many times. Here, you can just take the opportunity to see Rusty's notoriously unreliable guide. NetFilter itself can be found here.

NetFilter allows us to filter packets or tamper with the packet header. A special feature is that we can put a digital tag on the packet. You can use the--set-mark mechanism.

For example, this command marks all packets sent to 25/TCP (outgoing mail):

# iptables-a prerouting-i eth0-t mangle-p TCP--dport 25 \
J MARK--set-mark 1

For example, 11545.html "> We have multiple connections, one is based on flowmeter fees, more expensive but soon, the other is a monthly, but relatively slow. Of course we'd like to send email packets to the cheaper route.

We have labeled "1" for those packets, and we now command the routing strategy database to implement this feature:

# echo 201 Mail.out >>/etc/iproute2/rt_tables
# IP rule Add fwmark 1 table mail.out
# IP Rule ls
0:from All lookup Local
32764:from all Fwmark 1 lookup mail.out
32766:from All lookup Main
32767:from All Lookup Default

Now we build a route to that cheap link to generate the Mail.out routing table:

#/SBIN/IP route add default via 195.96.98.253 dev ppp0 table mail.out

This is done. We may need some exceptions, and there are many ways to achieve them. We can modify the NetFilter command to exclude some hosts, or we can insert some lower priority rules to send packets to the main routing table for the excluded host.

We can also identify different service types of packets by identifying their TOS bits, and then establish rules for them separately. You can even use this method to support interactive services such as ISDN lines.

Needless to say, this, of course, can also be used on machines that are doing NAT ("camouflage").

Important Reminder: We received reports that the MASQ and SNAT functions are in conflict with the packet markings. Rusty Russell explained in this post. Turn off filtering in reverse direction to work properly.

Note: If you want to mark a packet, you need some configuration within and between:

ip:advanced Router (config_ip_advanced_router) [y/n/?]
Ip:policy Routing (config_ip_multiple_tables) [y/n/?]
Ip:use netfilter MARK http://www.aliyun.com/zixun/aggregation/9541.html ">value as routing key (Config_ip_route_fwmark ) [y/n/?]