Linux Network Administrator Guide, Second Edition

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.

Accounting of ICMP Datagrams

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 'ICMP datagram types', 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.

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:

# 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.

Using IP Accounting Results

It is all very well to be collecting this information, but how do we actually get to see it? To view the collected accounting data and the configured accounting rules, we use our firewall configuration commands, asking them to list our rules. The packet and byte counters for each of our rules are listed in the output.

The ipfwadm, ipchains, and iptables commands differ in how accounting data is handled, so we will treat them independently.

Listing Accounting Data with ipfwadm

The most basic means of listing our accounting data with the ipfwadm command is to use it like this:

# ipfwadm -A -l
IP accounting rules
 pkts bytes dir prot source               destination          ports
 9833 2345K i/o all  172.16.3.0/24      anywhere             n/a
56527   33M i/o all  172.16.4.0/24      anywhere             n/a

This will tell us the number of packets sent in each direction. If we use the extended output format with the -e option (not shown here because the output is too wide for the page), we are also supplied the options and applicable interface names. Most of the fields in the output will be self-explanatory, but the following may not:

dir

The direction in which the rule applies. Expected values here are in, out, or i/o, meaning both ways.

prot

The protocols to which the rule applies.

opt