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Dummynet was originally invented to simulate poor or lossy links, so
the author could test network protocols in bad conditions. However, it also allows you to control bandwidth usage. One example the developer uses
on his web page is using
dummynet to simulate an ADSL link to the moon! We don't want to
throttle our client that badly, but it's nice to know it's possible.
Articles about IPFW
One nice thing about
dummynet is that it works on arbitrary ports, IP
addresses, and protocols. If you want to restrict the bandwidth usage
of IPSec tunnels, Sendmail, or anything, you can do it.
Dummynet is part of IPFW. You can control all sorts of packet flows with
dummynet. One of the most
basic controls is the amount of traffic through a given IP address.
The IPFW integration was my problem. I'm loathe to muck with the kernel of a working production system. It's just not a good idea. Things have only gone wrong a couple of times, but those failures have been pretty spectacular. New kernel problems might only take a couple of hours to fix, but ISPs don't get downtime. Customers don't like outage explanations like "rebuilding the kernel." Their eyes glaze over, then and they find another ISP that doesn't use those "kernel" thingies.
You must have IPFW in your kernel to use
dummynet. I added the
following configuration to the system kernel.
options IPFIREWALL options IPFIREWALL_VERBOSE options DUMMYNET options IPFIREWALL_DEFAULT_TO_ACCEPT
Technically I didn't need the
IPFIREWALL_VERBOSE statement, but I like
logs. They give me a warm fuzzy feeling. I've been known to keep
logs for 30 months at a time (generally when I thought I was
rotating them daily, but was actually rotating them once a month).
Because we're not using IPFW for packet filtering, but for bandwidth control, I'm using the default accept mode. If you're doing packet filtering, you probably don't want this.
We can use two different types of
dummynet filters -- pipes and queues.
For this example, we'll use pipes. We'll look at queues some other
Dummynet expects to have some outside agency forward packets to it.
IPFW includes this functionality. For one
dummynet operation, you
need at least two IPFW configuration statements. If you aren't
familiar with IPFW commands, check out the FreeBSD Basics IPFW
dummynet, like all IPFW subsystems, with the
command. First, redirect the packets from the IP address of the site
(192.168.1.88) to a
ipfw add 100 pipe 1 ip from 192.168.1.88 to any
The syntax is pretty straightforward. We're adding a rule, number
100, and giving it a pipe. Packets that match
ip from 192.168.1.88
to any -- packets from this web site -- will be sent to pipe
Our pipe configuration is really very basic, too.
ipfw pipe 1 config bw 128Kbit/s
The pipe is created and labeled. The
config statement tells IPFW
that this is a configuration statement. The
bw means that this is a
bandwidth control and 128 Kbps is the bandwidth allowed.
The most disconcerting thing was when I checked the IPFW rule list.
# ipfw list 00100 pipe 1 ip from 192.168.1.88 to any 65535 allow ip from any to any #
Where's my pipe?
It turns out that the pipes are stored in a separate list. To view
your pipes, do
ipfw pipe list.
# ipfw pipe list
00001: 128.000 Kbit/s 0 ms 50 sl. 1 queues (1 buckets) droptail
mask: 0x00 0x00000000/0x0000 -> 0x00000000/0x0000
BKT Prot ___Source IP/port____ ____Dest. IP/port____ Tot_pkt/bytes Pkt/Byte Drp
0 tcp 192.168.1.88/80 22.214.171.124/2415 128050681 35518324182 0 0 50486587 #
Wow, impressive! We can see where traffic is coming from, where it's going to, and how many packets have been passed and dropped.
Our rules covered the straightforward
dummynet configuration. There's
many more options that we'll look at another time. If you're
interested in experimenting with network congestion,
dummynet is your
I put this rule in and the bandwidth instantly dropped. My friend went away happy, his client didn't have to pay for a T1 for a month, and I got back to changing the rodent cages, watering the fish, and cleaning the garage.
One of the nice things about FreeBSD is that you get your software for free. You can use terribly inexpensive hardware. The one irremediable expense is bandwidth. FreeBSD lets you control your bandwidth usage as well, keeping your costs in check.
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