2 Very funky action. I do plan to add to a few more things to it
3 This is the basic stuff. Idea borrowed from the way ethernet switches
4 mirror and redirect packets. The main difference with say a vannila
5 ethernet switch is that you can use u32 classifier to select a
6 flow to be mirrored. High end switches typically can select based
7 on more than just a port (eg a 5 tuple classifier). They may also be
8 capable of redirecting.
12 mirred <DIRECTION> <ACTION> [index INDEX] <dev DEVICENAME>
14 DIRECTION := <ingress | egress>
15 ACTION := <mirror | redirect>
16 INDEX is the specific policy instance id
17 DEVICENAME is the devicename
20 - Ingress is not supported at the moment. It will be in the
21 future as well as mirror/redirecting to a socket.
24 - Mirror takes a copy of the packet and sends it to specified
25 dev ("port" in ethernet switch/bridging terminology)
27 steals the packet and redirects to specified destination dev.
29 What NOT to do if you dont want your machine to crash:
30 ------------------------------------------------------
33 Loops are not hard to create in the egress qdiscs.
35 Here are simple rules to follow if you dont want to get
37 A) Do not have the same packet go to same netdevice twice
38 in a single graph of policies. Your machine will just hang!
39 This is design intent _not a bug_ to teach you some lessons.
41 In the future if there are easy ways to do this in the kernel
42 without affecting other packets not interested in this feature
43 I will add them. At the moment that is not clear.
45 Some examples of bad things NOT to do:
46 1) redirecting eth0 to eth0
48 3) eth0->lo-> eth1-> eth0
50 B) Do not redirect from one IFB device to another.
51 Remember that IFB is a very specialized case of packet redirecting
52 device. Instead of redirecting it puts packets at the exact spot
53 on the stack it found them from.
54 Redirecting from ifbX->ifbY will actually not crash your machine but your
55 packets will all be dropped (this is much simpler to detect
56 and resolve and is only affecting users of ifb as opposed to the
59 In the case of A) the problem has to do with a recursive contention
60 for the devices queue lock and in the second case for the transmit lock.
65 1) Mirror all packets arriving on eth0 to be sent out on eth1.
66 You may have a sniffer or some accounting box hooked up on eth1.
69 tc qdisc add dev eth0 ingress
70 tc filter add dev eth0 parent ffff: protocol ip prio 10 u32 \
71 match u32 0 0 flowid 1:2 action mirred egress mirror dev eth1
74 If you replace "mirror" with "redirect" then not a copy but rather
75 the original packet is sent to eth1.
77 2) Host A is hooked up to us on eth0
79 # redirect all packets arriving on ingress of lo to eth0
81 tc qdisc add dev lo ingress
82 tc filter add dev lo parent ffff: protocol ip prio 10 u32 \
83 match u32 0 0 flowid 1:2 action mirred egress redirect dev eth0
86 On host A start a tcpdump on interface connecting to us.
88 on our host ping -c 2 127.0.0.1
90 Ping would fail since all packets are heading out eth0
91 tcpudmp on host A would show them
93 if you substitute the redirect with mirror above as in:
94 tc filter add dev lo parent ffff: protocol ip prio 10 u32 \
95 match u32 0 0 flowid 1:2 action mirred egress mirror dev eth0
97 Then you should see the packets on both host A and the local
98 stack (i.e ping would work).
100 3) Even more funky example:
103 #allow 1 out 10 packets on ingress of lo to randomly make it to the
104 # host A (Randomness uses the netrand generator)
107 tc filter add dev lo parent ffff: protocol ip prio 10 u32 \
108 match u32 0 0 flowid 1:2 \
109 action drop random determ ok 10\
110 action mirred egress mirror dev eth0
114 # for packets from 10.0.0.9 going out on eth0 (could be local
115 # IP or something # we are forwarding) -
116 # if exceeding a 100Kbps rate, then redirect to eth1
120 tc qdisc add dev eth0 handle 1:0 root prio
121 tc filter add dev eth0 parent 1:0 protocol ip prio 6 u32 \
122 match ip src 10.0.0.9/32 flowid 1:16 \
123 action police rate 100kbit burst 90k ok \
124 action mirred egress mirror dev eth1
127 A more interesting example is when you mirror flows to a dummy device
128 so you could tcpdump them (dummy by defaults drops all packets it sees).
129 This is a very useful debug feature.
131 Lets say you are policing packets from alias 192.168.200.200/32
132 you dont want those to exceed 100kbps going out.
135 tc qdisc add dev eth0 handle 1:0 root prio
136 tc filter add dev eth0 parent 1: protocol ip prio 10 u32 \
137 match ip src 192.168.200.200/32 flowid 1:2 \
138 action police rate 100kbit burst 90k drop
141 If you run tcpdump on eth0 you will see all packets going out
142 with src 192.168.200.200/32 dropped or not (since tcpdump shows
143 all packets being egressed).
144 Extend the rule a little to see only the packets making it out.
147 tc qdisc add dev eth0 handle 1:0 root prio
148 tc filter add dev eth0 parent 1: protocol ip prio 10 u32 \
149 match ip src 192.168.200.200/32 flowid 1:2 \
150 action police rate 10kbit burst 90k drop \
151 action mirred egress mirror dev dummy0
154 Now fire tcpdump on dummy0 to see only those packets ..
155 tcpdump -n -i dummy0 -x -e -t
157 Essentially a good debugging/logging interface (sort of like
158 BSDs speacialized log device does without needing one).
160 If you replace mirror with redirect, those packets will be
161 blackholed and will never make it out.