Because IP accounting is closely related to IP firewall, the same tool
was designated to configure it, so ipfwadm,
ipchains or iptables are used to configure IP accounting. The command syntax is very similar to
that of the firewall rules, so we won't focus on it, but we will discuss
what you can discover about the nature of your network traffic using this
feature.
The general syntax for IP accounting with ipfwadm is:
# ipfwadm -A [direction] [command] [parameters] |
The direction argument is new. This is simply coded as
in,
out, or
both.
These directions are from the perspective of the linux machine itself, so
in means data coming into the machine from a network
connection and out means data that is being transmitted by
this host on a network connection. The both direction is the
sum of both the incoming and outgoing directions.
The general command syntax for ipchains
and iptables is:
# ipchains -A chain rule-specification
# iptables -A chain rule-specification |
The ipchains and iptables
commands allow you to specify direction in a manner more consistent
with the firewall rules. IP Firewall Chains doesn't allow you to
configure a rule that aggregates both directions, but it does allow you
to configure rules in the forward chain that the
older implementation did not. We'll see the difference that makes in
some examples a little later.
The commands are much the same as firewall rules, except that the
policy rules do not apply here. We can add, insert, delete, and list
accounting rules. In the case of ipchains and
iptables, all valid rules are accounting rules, and
any command that doesn't specify the -j option
performs accounting only.
The rule specification parameters for IP accounting are the same as
those used for IP firewall. These are what we use to define precisely
what network traffic we wish to count and total.
Let's work with an example to illustrate how we'd use IP accounting.
Imagine we have a Linux-based router that serves two departments
at the Virtual Brewery. The router has two Ethernet devices,
eth0 and eth1, each of which
services a department; and a PPP device, ppp0, that
connects us via a high-speed serial link to the main campus of the
Groucho Marx University.
Let's also imagine that for billing purposes we want to know the total
traffic generated by each of the departments across the serial link, and
for management purposes we want to know the total traffic generated
between the two departments.
The following table shows the interface addresses we will use in our
example:
To answer the question, “How much data does each department
generate on the PPP link?”, we could use a rule that looks like
this:
# ipfwadm -A both -a -W ppp0 -S 172.16.3.0/24 -b
# ipfwadm -A both -a -W ppp0 -S 172.16.4.0/24 -b |
or:
# ipchains -A input -i ppp0 -d 172.16.3.0/24
# ipchains -A output -i ppp0 -s 172.16.3.0/24
# ipchains -A input -i ppp0 -d 172.16.4.0/24
# ipchains -A output -i ppp0 -s 172.16.4.0/24 |
and with
iptables:
# iptables -A FORWARD -i ppp0 -d 172.16.3.0/24
# iptables -A FORWARD -o ppp0 -s 172.16.3.0/24
# iptables -A FORWARD -i ppp0 -d 172.16.4.0/24
# iptables -A FORWARD -o ppp0 -s 172.16.4.0/24 |
The first half of each of these set of rules say, “Count all data
traveling in either direction across the interface named ppp0 with a source
or destination (remember the function of the -b flag in
ipfwadm and iptables) address of
172.16.3.0/24.” The second half of each ruleset is
the same, but for the second Ethernet network at our site.
To answer the second question, “How much data travels between the
two departments?”, we need a rule that looks like this:
# ipfwadm -A both -a -S 172.16.3.0/24 -D 172.16.4.0/24 -b |
or:
# ipchains -A forward -s 172.16.3.0/24 -d 172.16.4.0/24 -b |
or:
# iptables -A FORWARD -s 172.16.3.0/24 -d 172.16.4.0/24
# iptables -A FORWARD -s 172.16.4.0/24 -d 172.16.3.0/24 |
These rules will count all datagrams with a source address belonging
to one of the department networks and a destination address belonging
to the other.
Okay, let's suppose we also want a better idea of exactly what sort of traffic
is being carried across our PPP link. We might, for example, want to know
how much of the link the FTP, smtp, and World Wide Web services are consuming.
A script of rules to enable us to collect this information might look like:
#!/bin/sh
# Collect FTP, smtp and www volume statistics for data carried on our
# PPP link using ipfwadm
#
ipfwadm -A both -a -W ppp0 -P tcp -S 0/0 ftp ftp-data
ipfwadm -A both -a -W ppp0 -P tcp -S 0/0 smtp
ipfwadm -A both -a -W ppp0 -P tcp -S 0/0 www |
or:
#!/bin/sh
# Collect ftp, smtp and www volume statistics for data carried on our
# PPP link using ipchains
#
ipchains -A input -i ppp0 -p tcp -s 0/0 ftp-data:ftp
ipchains -A output -i ppp0 -p tcp -d 0/0 ftp-data:ftp
ipchains -A input -i ppp0 -p tcp -s 0/0 smtp
ipchains -A output -i ppp0 -p tcp -d 0/0 smtp
ipchains -A input -i ppp0 -p tcp -s 0/0 www
ipchains -A output -i ppp0 -p tcp -d 0/0 www |
or:
#!/bin/sh
# Collect ftp, smtp and www volume statistics for data carried on our
# PPP link using iptables.
#
iptables -A FORWARD -i ppp0 -m tcp -p tcp --sport ftp-data:ftp
iptables -A FORWARD -o ppp0 -m tcp -p tcp --dport ftp-data:ftp
iptables -A FORWARD -i ppp0 -m tcp -p tcp --sport smtp
iptables -A FORWARD -o ppp0 -m tcp -p tcp --dport smtp
iptables -A FORWARD -i ppp0 -m tcp -p tcp --sport www
iptables -A FORWARD -o ppp0 -m tcp -p tcp --dport www |
There are a couple of interesting features to this
configuration. Firstly, we've specified the protocol. When we specify
ports in our rules, we must also specify a protocol because TCP and UDP
provide separate sets of ports. Since all of these services are
TCB-based, we've specified it as the protocol. Secondly, we've
specified the two services ftp and
ftp-data in one command. ipfwadm
allows you to specify single ports, ranges of ports, or arbitrary lists
of ports. The ipchains command allows either single
ports or ranges of ports, which is what we've used here. The syntax
"ftp-data:ftp" means "ports ftp-data (20) through
ftp (21)," and is how we encode ranges of ports in both
ipchains and iptables. When you
have a list of ports in an accounting rule, it means that any data
received for any of the ports in the list will cause the data to be
added to that entry's totals. Remembering that the FTP service uses
two ports, the command port and the data transfer port, we've added
them together to total the FTP traffic. Lastly, we've specified the
source address as “0/0,” which is
special notation that matches all addresses and is required by both
the ipfwadm and ipchains
commands in order to specify ports.
We can expand on the second point a little to give us a different view
of the data on our link. Let's now imagine that we class FTP, SMTP,
and World Wide Web traffic as essential traffic, and all other traffic
as nonessential. If we were interested in seeing the ratio of
essential traffic to nonessential traffic, we could do something like:
# ipfwadm -A both -a -W ppp0 -P tcp -S 0/0 ftp ftp-data smtp www
# ipfwadm -A both -a -W ppp0 -P tcp -S 0/0 1:19 22:24 26:79 81:32767 |
If you have already examined your /etc/services file, you
will see that the second rule covers all ports except (ftp, ftp-data, smtp,
and www).
How do we do this with the ipchains or
iptables commands, since they allow only one argument in
their port specification? We can exploit user-defined chains in accounting
just as easily as in firewall rules. Consider the following approach:
# ipchains -N a-essent
# ipchains -N a-noness
# ipchains -A a-essent -j ACCEPT
# ipchains -A a-noness -j ACCEPT
# ipchains -A forward -i ppp0 -p tcp -s 0/0 ftp-data:ftp -j a-essent
# ipchains -A forward -i ppp0 -p tcp -s 0/0 smtp -j a-essent
# ipchains -A forward -i ppp0 -p tcp -s 0/0 www -j a-essent
# ipchains -A forward -j a-noness |
Here we create two user-defined chains, one called
a-essent, where we capture accounting data for
essential services and another called
a-noness, where we
capture accounting data for nonessential services. We then add rules to
our forward chain that match our essential services and jump to the
a-essent chain, where we have just one rule that accepts
all datagrams and counts them. The last rule in our forward chain is a rule
that jumps to our
a-noness chain, where again we have just
one rule that accepts all datagrams and counts them. The rule that jumps to the
a-noness chain will not be reached by any of our essential
services, as they will have been accepted in their own chain. Our tallies for
essential and nonessential services will therefore be available in the
rules within those chains. This is just one approach you could take; there
are others. Our
iptables implementation of the same
approach would look like:
# iptables -N a-essent
# iptables -N a-noness
# iptables -A a-essent -j ACCEPT
# iptables -A a-noness -j ACCEPT
# iptables -A FORWARD -i ppp0 -m tcp -p tcp --sport ftp-data:ftp -j a-essent
# iptables -A FORWARD -i ppp0 -m tcp -p tcp --sport smtp -j a-essent
# iptables -A FORWARD -i ppp0 -m tcp -p tcp --sport www -j a-essent
# iptables -A FORWARD -j a-noness |
This looks simple enough. Unfortunately, there is a small but unavoidable
problem when trying to do accounting by service type. You will remember that
we discussed the role the MTU plays in TCP/IP networking in an earlier
chapter. The MTU defines the largest datagram that will be transmitted on
a network device. When a datagram is received by a router that is larger than
the MTU of the interface that needs to retransmit it, the router performs a
trick called fragmentation. The router breaks the large
datagram into small pieces no longer than the MTU of the interface and then
transmits these pieces. The router builds new headers to put in front of each
of these pieces, and these are what the remote machine uses to reconstruct the
original data. Unfortunately, during the fragmentation process the port
is lost for all but the first fragment. This means that the IP accounting
can't properly count fragmented datagrams. It can reliably count only
the first fragment, or unfragmented datagrams. There is a small trick permitted
by ipfwadm that ensures that while we won't be able to know
exactly what port the second and later fragments were from, we can still
count them. An early version of Linux accounting software assigned
the fragments a fake port number, 0xFFFF, that we could count. To ensure that
we capture the second and later fragments, we could use a rule like:
# ipfwadm -A both -a -W ppp0 -P tcp -S 0/0 0xFFFF |
The IP chains implementation has a slightly more sophisticated solution, but
the result is much the same. If using the ipchains command
we'd instead use:
# ipchains -A forward -i ppp0 -p tcp -f |
and with
iptables we'd use:
# iptables -A FORWARD -i ppp0 -m tcp -p tcp -f |
These won't tell us what the original port for this data was, but at least we are able to see how much of our data is fragments, and be able to account
for the volume of traffic they consume.
In 2.2 kernels you can select a kernel compile-time option that
negates this whole issue if your Linux machine is acting as the single
access point for a network. If you enable the IP:
always defragment option when you compile your
kernel, all received datagrams will be reassembled by the Linux router
before routing and retransmission. This operation is performed before
the firewall and accounting software sees the datagram, and thus you
will have no fragments to deal with. In 2.4 kernels you compile and
load the netfilter
forward-fragment module.
The ICMP protocol does not use service port numbers and is therefore a little
bit more difficult to collect details on. ICMP uses a number of different
types of datagrams. Many of these are harmless and normal, while others
should only be seen under special circumstances. Sometimes people with too
much time on their hands attempt to maliciously disrupt the network
access of a user by generating large numbers of ICMP messages. This is
commonly called ping flooding. While IP accounting
cannot do anything to prevent this problem (IP firewalling can help, though!)
we can at least put accounting rules in place that will show us if anybody
has been trying.
ICMP doesn't use ports as TCP and UDP do. Instead ICMP has ICMP message
types. We can build rules to account for each ICMP message type. To do this,
we place the ICMP message and type number in place of the port field in the
ipfwadm accounting commands. We listed the ICMP message
types in Section 9.6.3.5,” so refer to it
if you need to remember what they are.
An IP accounting rule to collect information about the volume of ping data
that is being sent to you or that you are generating might look like:
# ipfwadm -A both -a -P icmp -S 0/0 8
# ipfwadm -A both -a -P icmp -S 0/0 0
# ipfwadm -A both -a -P icmp -S 0/0 0xff |
or, with
ipchains:
# ipchains -A forward -p icmp -s 0/0 8
# ipchains -A forward -p icmp -s 0/0 0
# ipchains -A forward -p icmp -s 0/0 -f |
or, with
iptables:
# iptables -A FORWARD -m icmp -p icmp --sports echo-request
# iptables -A FORWARD -m icmp -p icmp --sports echo-reply
# iptables -A FORWARD -m icmp -p icmp -f |
The first rule collects information about the
“ICMP Echo Request” datagrams (ping requests), and the
second rule collects information about the “ICMP Echo Reply”
datagrams (ping replies). The third rule collects information about ICMP
datagram fragments. This is a trick similar to that described for fragmented
TCP and UDP datagrams.
If you specify source and/or destination addresses in your rules, you can
keep track of where the pings are coming from, such as whether they originate
inside or outside your network. Once you've determined where the rogue
datagrams are coming from, you can decide whether you want to put firewall
rules in place to prevent them or take some other action, such as
contacting the owner of the offending network to advise them of the problem,
or perhaps even legal action if the problem is a malicious act.
Let's now imagine that we are interested in knowing how much of the
traffic on our link is TCP, UDP, and ICMP. We would use rules like
the following:
# ipfwadm -A both -a -W ppp0 -P tcp -D 0/0
# ipfwadm -A both -a -W ppp0 -P udp -D 0/0
# ipfwadm -A both -a -W ppp0 -P icmp -D 0/0 |
or:
# ipchains -A forward -i ppp0 -p tcp -d 0/0
# ipchains -A forward -i ppp0 -p udp -d 0/0
# ipchains -A forward -i ppp0 -p icmp -d 0/0 |
or:
# iptables -A FORWARD -i ppp0 -m tcp -p tcp
# iptables -A FORWARD -o ppp0 -m tcp -p tcp
# iptables -A FORWARD -i ppp0 -m udp -p udp
# iptables -A FORWARD -o ppp0 -m udp -p udp
# iptables -A FORWARD -i ppp0 -m icmp -p icmp
# iptables -A FORWARD -o ppp0 -m icmp -p icmp |
With these rules in place, all of the traffic flowing across the
ppp0 interface will be analyzed to determine
whether it is TCP, UDP, or IMCP traffic, and the appropriate counters will
be updated for each. The
iptables example splits incoming
flow from outgoing flow as its syntax demands it.
