Linux Network Administratoramp;#039;s Guide (3rd Edition) [Electronic resources]

8.2. Configuring IP Accounting

Because IP accounting is closely related to IP firewall, the same tool was designated to configure it, so the iptables command is 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 command syntax is:

# iptables -A  chain rule-specification 

The iptables command allows you to specify direction in a manner consistent with the firewall rules.

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 firewalls. These are what we use to define precisely what network traffic we wish to count and total.

8.2.1. Accounting by Address

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 that 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.

Table 8-1 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:

# 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 two rules say, "Count all data traveling in either direction across the interface named ppp0 with a source or destination address of 172.16.3.0/24." The second set of two rules do the same thing, 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:

# 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 packets with a source address belonging to one of the department networks and a destination address belonging to the other.

8.2.2. Accounting by Service Port

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 (HTTP) services are consuming.

A script of rules to enable us to collect this information might look like this:

#!/bin/sh
# Collect ftp, smtp and www volume statistics for data carried on our
# PPP link using iptables.
#
iptables -A FORWARD -i ppp0 -p tcp --sport 20:21
iptables -A FORWARD -o ppp0 -p tcp --dport 20:21
iptables -A FORWARD -i ppp0 -p tcp --sport smtp
iptables -A FORWARD -o ppp0 -p tcp --dport smtp
iptables -A FORWARD -i ppp0 -p tcp --sport www
iptables -A FORWARD -o ppp0 -p tcp --dport www

There are a couple of interesting features to this configuration. First, 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 TCP based, we've specified it as the protocol. Second, we've specified the two services ftp and ftp-data in one command. The iptables command allows either single ports or ranges of ports, which is what we've used here. The syntax "20:21" means "ports 20 (ftp-data) through 21 (ftp)," and is how we encode ranges of ports in iptables (the tcp match extension allow you to use port names in range specifications, but the multiport match extension does notand you are better off using numbers for ranges anyway so you don't accidentally include more ports than you intend). 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.

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 (HTTP) 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 this:

# iptables -A FORWARD -i ppp0 -p tcp -m multiport \
    --sports ftp-data,ftp,smtp,www -j ACCEPT
# iptables -A FORWARD -j ACCEPT

The first rule would count our essential traffic while the second one would count everything else.

Alternatively, we can use user-defined chains (this would be useful if the rules for determining essential traffic were more complex):

# 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 -p tcp -m multiport \
    --sports ftp-data,ftp,smtp,www -j a-essent
# iptables -A FORWARD -j a-noness

Here we create two user-defined chainsone 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 packets 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 packets 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.

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 packet that will be transmitted on a network device. When a packet 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 packet 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 host 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 packets. It can reliably count only the first fragment or unfragmented packets. To ensure that we capture the second and later fragments, we could use a rule like this:

# 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 account for the volume of traffic they consume.

8.2.3. Accounting of ICMP Packets

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 packets. 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 (the generic term for this type of denial of service attack is packet flooding, but ping flooding is a common one). 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. We place the ICMP message and type number in place of the port field in the accounting commands.

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 this:

# 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" packets (ping requests), and the second rule collects information about the "ICMP Echo Reply" packets (ping replies). The third rule collects information about ICMP packet fragments. This is a trick similar to that described for fragmented TCP and UDP packets.

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 packets 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 taking legal action if the problem is a malicious act.

8.2.4. Accounting by Protocol

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:

# 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 ICMP traffic, and the appropriate counters will be updated for each.