One of the things that I really like about pf, the OpenBSD firewall, is how it lets you define dynamic packet filtering rules — rules that filter based on your network interfaces’ current addresses at the time of filtering. For instance, if I want to allow SSH connections to my laptop only from my local network:

pass in on xl0 inet proto tcp from (xl0:network) to any \
    port ssh flags S/SFRA

(xl0:network) is not resolved to a specific address block at configuration load time; if you switch networks — say, if you go from home to work — the rule’s behavior will change accordingly.

Unless I have overlooked some recent change in Linux, this cannot be achieved in a direct fashion with iptables. You can insert a rule to reject non-LAN source addresses, but such a rule is static. When you change network addresses, the rule must be explicitly updated.

In lieu of rewriting all of netfilter to accommodate this use case (*cough*), I just wrote a shell script to help mitigate the pain of manually updating my laptop’s firewall rules — merely a shortcut to cut down on the amount of typing I do on any given day, but if you tend to move around as much as I do, all those keystrokes can add up :) So with this script you can, in one fell swoop, start and open up global access to an SSH server:

# ssh-serve any

Or only allow local access from the networks you’re connected to:

# ssh-serve lan

Or only local network access on a specific interface:

# ssh-serve lan eth0

Or only access from a given set of IP addresses and/or CIDR blocks:

# ssh-serve addr 192.168.0.104 10.18.0.0/16

Better yet, you can make the whole process automagical by hooking into your Linux distribution’s DHCP client. For instance, in Ubuntu Hardy Heron you can automate ssh-serve by creating a file /etc/dhcp3/dhclient-exit-hooks.d/ssh-serve:

# Allow SSH access from your local network only, and keep these filter
# rules up-to-date as you move from one network to another.
case $reason in
    BOUND|REBIND|REBOOT)
        ssh-serve lan
        ;;
esac

This script was written (and named) with Secure Shell in mind, but it could just as easily govern over any other service controlled by a standard SysV init script. See below the jump for the code…

This evening I had the chance to download Google’s newly-released (and by “released” I mean “beta”… hey, it’s Google) web browser, Chrome, and give it a try. They weren’t kidding when they said V8, the new JavaScript virtual machine in Chrome, should raise the bar for next-generation JavaScript implementations: it’s fast. How fast?

The above results are from Mozilla’s Dromaeo JavaScript performance test suite, so there’s little worry of this test being intentionally biased in Chrome’s favor. The scores above are the averages of five test executions on each web browser, running in the same Windows XP virtual machine on the same computer. Some notes:

• Each run of the test was performed in a fresh browser instance.
• IE 7 was unable to complete the test suite without crashing, although I am using a special, standalone version of IE 7 so this may be particular to my installation.
• In order to prevent IE 8 from complaining about the long JavaScript execution time, I set set the registry value MaxScriptStatements = (DWORD) 0xffffffff in the key \HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Styles.

So yes, Chrome does in fact have a much faster JavaScript engine than any other current web browser: in this test, more than eight times as fast as Firefox 3.0.1’s engine, and more than thirty times as fast as the latest beta of Internet Explorer 8. But how does the rest of the user experience stack up?