My post about repositories wasn't just a little attempt to stave off work, it was part of a larger scheme.

I share the ADSL line in my digs with 3 other people. We do split-routing to save money, but we still have to divide the phone bill at the end of the month. Rather than buy a fixed cap, and have a fight over who's fault it was when we get capped, we are running a pay-per-use system (with local use free, subsidised by me). It means you don't have to restrain yourself for the common cap, but it also means I need to calculate who owes what.

For the first month, I used my old standby, bandwidthd. It uses pcap to count traffic, and gives you totals and graphs. For simplicity of logging, I gave each person a /28 for their machines and configured static DHCP leases. Then bandwidthd totalled up the internet use for each /28.

This was sub-optimal. bandwidthd either sees the local network, in which case it can't see which packets went out over which link. Or it can watch the international link, but then not know which user is responsible.

I could have installed some netflow utilities at this point, but I wanted to roll my own with the correct Linux approach (ulog) rather than any pcapping. ulogd is the easy ulog solution.

Ulogd can pick up packets that you "-j ULOG" from iptables. It receives them over a netlink interface. You can tell iptables how many bytes of each packet to send, and how many to queue up before sending them. E.g.

will log the first 48 bytes of any incoming packet to netlink-group 1. It will tag the packet as being "input", and send them in batches of 50. 48 bytes is usually enough to catch any data you could want from the headers. If you were only need size, 4 bytes will do, and for source and destination as well, 20.

Now, we tell ulogd to listen for this stuff and log it. Ulogd has a pluggable architecture. IPv4 decoding is a plugin, and there are various logging plugins for "-j LOG" emulation, Text files, pcap-files, MySQL, PostgreSQL, and SQLite. For my purposes, I used MySQL as the router in question already had MySQL on it (for Cacti). Otherwise, I would have opted for SQLite. Be warned that the etch version of ulogd doesn't automatically reconnect to the MySQL server should the connection break for any reason. I backported the lenny version to etch to get around that. (You also need to provide the reconnect and connect_timeout options.)

Besides the reconnection issue, the SQL implementations are quite nice. They have a set schema, and you just need to create a table with the columns in it that you are interested in. No other configuration (beyond connection details) is necessary.

My MySQL table:

My ulogd.conf:

The relevant parts of my firewall rules:

So, traffic for my /28 (sr) will be counted as sr-f or sr-p so I can tally up proxy & forwarded traffic separately. (Yes, I can count traffic with squid too, but doing it all in one place is simpler.) fb is random housemate Foo Bar, and gu guest (unreserved IP addresses).

You can query the usage this month with for example:

Your table will fill up fast. We are averaging around 200 000 rows per day. So obviously some aggregation is in order:

And every night, run something like:

Finally, I have a simple little PHP script that provides reporting and calculates dues. Done.