David Madore's WebLog: More geekiness: trying to make some sense of WiFi

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Entry #1336 [older|newer] / Entrée #1336 [précédente|suivante]:

(Sunday)

More geekiness: trying to make some sense of WiFi

My new laptop has a WiFi (aka 802.11) receptor (and emitter, of course), and it turns out that it works (to some extent, at least) under Linux, so I couldn't resist the temptation of playing with it. I'm a computer geek in general but one thing which particularly fascinates me among all computer things is network protocols (Ethernet, IPv6[#] and TCP being among my favorites), so certainly WiFi made me curious.

An 802.11 node can operate in at least four different modes: ad-hoc, managed, master and monitor. From what I understand, the ad-hoc mode is a kind of peer-to-peer mode, where nodes agree on a channel and SSID and talk directly to each other; the master and managed (i.e., slave) modes are used in more complex setups in which, if I get it right, the master asks somewhat like an Ethernet switch (so even if the clients can't talk to each other directly, they can talk through it), and even more complex networks can have secondary masters or repeaters; and the monitor mode is used to silently capture all 802.11 frames without emitting anything. (This is, of course, very badly explained everywhere I looked; even more unclear are questions like whether an ad-hoc node can talk to a master node or whether only managed nodes can associate with the master; the whole process of association is entirely unclear to me.) I managed to get my laptop's receiver to function at least in ad-hoc mode (to peer with another laptop, also in ad-hoc mode), in managed mode (I went to a random Golden Arches restaurant and checked that I could connect to their wireless network[#2]) and monitor mode. I was unable to set up a functional master node, however (or perhaps the other laptop I'm trying to test it with doesn't properly work as a managed node, but that would be surprising).

What is certainly fun is seeing what WiFi networks are detectable from a given point: I never thought there'd be so many of them. Just from my apartment (which is normally quite impervious to radio waves!) I get a scan which looks something like this:

          Cell 01 - Address: 00:11:F5:25:F8:F2
                    ESSID:"BODEAUXXAV"
                    Mode:Master
                    Frequency:2.462 GHz (Channel 11)
                    Quality=21/94  Signal level=-74 dBm  Noise level=-95 dBm
                    Encryption key:on
                    Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 18 Mb/s
                              24 Mb/s; 36 Mb/s; 54 Mb/s; 6 Mb/s; 9 Mb/s
                              12 Mb/s; 48 Mb/s
                    Extra:bcn_int=100
          Cell 02 - Address: 00:16:38:2C:47:2C
                    ESSID:"ALICE-2C4721"
                    Mode:Master
                    Frequency:2.462 GHz (Channel 11)
                    Quality=13/94  Signal level=-82 dBm  Noise level=-95 dBm
                    Encryption key:on
                    Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 18 Mb/s
                              24 Mb/s; 36 Mb/s; 54 Mb/s; 6 Mb/s; 9 Mb/s
                              12 Mb/s; 48 Mb/s
                    Extra:bcn_int=100
          Cell 03 - Address: 00:03:C9:87:83:BB
                    ESSID:"TECOM-AH4021-931340"
                    Mode:Master
                    Frequency:2.472 GHz (Channel 13)
                    Quality=13/94  Signal level=-82 dBm  Noise level=-95 dBm
                    Encryption key:on
                    Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 6 Mb/s
                              9 Mb/s; 12 Mb/s; 18 Mb/s; 24 Mb/s; 36 Mb/s
                              48 Mb/s; 54 Mb/s
                    Extra:bcn_int=100
          Cell 04 - Address: 00:03:C9:71:EC:7E
                    ESSID:"Wanadoo_a088"
                    Mode:Master
                    Frequency:2.457 GHz (Channel 10)
                    Quality=6/94  Signal level=-89 dBm  Noise level=-95 dBm
                    Encryption key:on
                    Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 6 Mb/s
                              9 Mb/s; 12 Mb/s; 18 Mb/s; 24 Mb/s; 36 Mb/s
                              48 Mb/s; 54 Mb/s
                    Extra:bcn_int=100

—and that's nothing compared with scans I get in the street where, on top of such private networks I see things like Eurospot and Ozone. No free WiFi anywhere I looked, however.

If I can set up a master node, maybe I'll offer free WiFi to anyone who can sense it, but… IPv6 only! and then see whether anyone uses it (I doubt it, but if the idea spread and if there were enough geeks of the kind, it might be an incentive). Or perhaps there are much funnier ideas one can play with.

[#] One of the wonderful things about IPv6, which often comes in handy when experimenting with various networks, is the autoconfiguration of link-local addresses. If I just plug in my laptop in an IPv6-aware computer, even if the latter has no globally defined address, then, without any configuration of any kind I can access it from that computer as fe80::216:36ff:fe2e:867f%eth0 (assuming eth0 is the name of the network device where it is plugged: this varies from one operating system to another); and if I didn't want to type in the hexadecimal address, I could register it whatever DNS domain I have handy. This may not sound impressive to people who aren't used to fooling around with network configurations, or to those who have a DHCP waiting for them wherever they go, but, believe me, it's really a saver compared to the clumsy manual setting up of local IPv4 addresses. So if you're going to link, say, two laptops together (whether by Ethernet cable or by WiFi), these link-local addresses are probably the simplest way to use the link once it's set up. And to detect whether there's an (IPv6-aware) node somewhere along a link eth0, all one needs to do is ping ff02::1%eth0, it isn't necessary to know anything about the global addresses along that link! This alone is sufficient reason to enable IPv6 wherever possible (even in the absence of a globally-visible IPv6 address; but in fact, since any IPv4 address automatically gives you one septillion IPv6 addresses to play with, one can nearly always conjure such global addresses… anyway).

[#2] I would have written a blog entry from there, except that the only kind of Internet connection they give you is a Web access, and I didn't have the proper tools to get around this, so I couldn't do much. On the other hand, I could have had some fun spying on what the woman behind me (also using a laptop) was browsing, but that would have been against my code of ethics.

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