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 msjPremium
join:2004-05-21
Fort Collins, CO
kudos:1 | reply to AthlGrond
Re: DHCP Spoofing / Half Bridging Well, I would just note that "DHCP Spoofing" and "Half Bridging" are marketing terms, and aren't very accurate in describing how this is done.
I don't have one of these modems (I have the Actiontec GT701WG) but I was interested in finding out how these modems do what they do (and possibly adding the feature to the Actiontec in the future).
So, from reading as much as I can find, I believe I understand how this works. The "DHCP Spoofing" part describes the process in which you get the external IP address on your machine. This process is the same as what has been done in the past with dumb (non routing) cable modems. When you first connect to the modem (or after power is cycled on the modem) the modem first delivers a local IP address to your machine, however the lease is very short). This allows you to connect to the modem and possibly do configuration, etc. Meanwhile the modem is establishing a connection via PPPoA to your ISP. Once it gets the external IP address to use it waits for the lease to expire. When your machines lease expires it makes a new request for an IP address via DHCP, and this time the modem gives you the external address with a normal lease. I assume that if the connection fails or doesn't complete before the first short lease expires the modem will just reissue the internal IP with another short lease.
Anyway, there is nothing new about that part of the feature, in fact it was the norm for older cable modems that didn't have a router built into the modem. I don't think it was called "DHCP Spoofing" in the past. I have a hard time with describing this as spoofing.
The second part of making this work is "Half Bridging". I don't like this name either. There is nothing "half" about it. It is just a different type of bridging. Normally when you use bridging you are bridging the ethernet (local LAN) device with the ATM device. The bridging support in Linux just passes the ethernet packets between the drivers. The ATM driver handles encapsulating/splitting the ethernet packets within ATM packets (Ethernet over ATM).
When you are not bridging the modem starts a PPPoA connection over the ATM device. It creates a virtual lan device (ppp0 on Linux) that is the local endpoint for the connection. It is this ppp0 device that gets the external IP address. The modem then routes packets between the local lan device (eth0 on Linux) and the ppp device.
In the "Half Bridging" scenario eth0 is not bridged to the ATM device. Instead the ppp daemon starts a normal PPPoA connection as before, creating the ppp0 device. However the external IP address is not assigned to that device. Instead the ppp0 device is bridged with eth0. Note that on Linux the bridging control command, brctl is used in both bridging cases. In one case it is used to bridge the ethernet device with the ATM device. In the other case it is used to bridge the ethernet device with the ppp device. If I had to choose a name for this feature I would probably call it "PPP bridging"
So, you might ask whether you could log into the Actiontec modem and use brctl to bridge eth0 with ppp0 after the connection is established. I wondered that also. However, the standard 2.4 kernel doesn't have support in the ppp lan device to support bridging. However, there is a patch available for that support. I'm currently playing around with OpenWRT on the Actiontec modem, and I may try to add that feature at some point (which of course would kind of defeat the purpose of having OpenWRT running on the modem  ).
Anyway, hopefully this has been informative and you will forgive me for the small rant! | | |
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 AthlGrondPremium,MVM
join:2002-04-25
Aurora, CO
 ·Comcast
| said by msj:In the other case it is used to bridge the ethernet device with the ppp device. If I had to choose a name for this feature I would probably call it "PPP bridging" I've seen "PPP Half Bridge" on some modem spec sheets.
I think part of the problem with this technology is that is isn't very well described (lack of standard terminology) or supported by the hardware manufacturers.
I imagine that its usefulness will eventually lead to it being a standard feature/option at some point.
--
You are now free to paint your hair wild colors and run around naked. -dg2 | |
NormanSPremium,MVM
join:2001-02-14
San Jose, CA
kudos:4 Reviews:
 ·SONIC.NET
·Pacific Bell - SBC
3 edits | reply to msj
said by msj:I don't have one of these modems (I have the Actiontec GT701WG) but I was interested in finding out how these modems do what they do (and possibly adding the feature to the Actiontec in the future). So, from reading as much as I can find, I believe I understand how this works. The "DHCP Spoofing" part describes the process in which you get the external IP address on your machine. This process is the same as what has been done in the past with dumb (non routing) cable modems. When you first connect to the modem (or after power is cycled on the modem) the modem first delivers a local IP address to your machine, however the lease is very short). This allows you to connect to the modem and possibly do configuration, etc. Meanwhile the modem is establishing a connection via PPPoA to your ISP. Once it gets the external IP address to use it waits for the lease to expire. When your machines lease expires it makes a new request for an IP address via DHCP, and this time the modem gives you the external address with a normal lease. I assume that if the connection fails or doesn't complete before the first short lease expires the modem will just reissue the internal IP with another short lease. Something like this?
2005/08/25 11:05:21 GMT E |System |=============== SYSTEM UP ===============
2005/08/25 11:05:21 GMT E |System |Current Mode: PPP on the modem (Public IP for LAN device)
2005/08/25 11:05:22 GMT E |DSL |DataPump Version 01.01.00.00
2005/08/25 11:05:22 GMT E |DSL |State: WAITING
2005/08/25 11:05:24 GMT E |Ethernet |Link 1 Up - 100Base-TX Full Duplex
2005/08/25 11:05:26 GMT E |DSL |State: INITIALIZING
2005/08/25 11:05:34 GMT E |DSL |HYBRID 1
2005/08/25 11:05:34 GMT E |DSL |Link up 1 US 384 DS 1536 (FAST:G.dmt)
2005/08/25 11:06:02 GMT E |DHCP Server |Smarttimeout invoked, lease timeout
2005/08/25 11:06:31 GMT E |DHCP Server |Address 192.168.1.64 given out to 00:09:5b:2c: f2:3b
2005/08/25 11:06:31 GMT E |PPPoE |tx PADI, id: 0000, ac: (NULL), sn: (NULL)
2005/08/25 11:06:31 GMT E |PPPoE |rx AC Name: 62011030047564-rback2.sntc01
2005/08/25 11:06:31 GMT E |PPPoE |tx PADR, id: 0000, ac: (NULL), sn: (NULL)
2005/08/25 11:06:31 GMT E |PPPoE |rx PADS id: C644
2005/08/25 11:06:31 GMT E |PPP |LCP neg PAP
2005/08/25 11:06:31 GMT E |PPP |LCP up
2005/08/25 11:06:33 GMT E |PPP |IPCP nak option: 3
2005/08/25 11:06:33 GMT E |PPP |IPCP nak option: 129
2005/08/25 11:06:33 GMT E |PPP |IPCP nak option: 131
2005/08/25 11:06:33 GMT E |PPP |IPCP up ip: 64.161.28.158,
gw: 64.161.31.254,
dns: 206.13.31.12, 20 6.13.28.12
2005/08/25 11:06:46 GMT E |DHCP Server |Address 64.161.28.158 given out to 00:09:5b:2c :f2:3b
2005/08/25 11:07:10 GMT E |SNTP Client |Updated time from Primary server 132.163.4.102
2005/08/25 11:26:36 GMT E |Ethernet |Link 1 Down
2005/08/25 11:26:37 GMT E |Ethernet |Link 1 Up - 100Base-TX Full Duplex
2005/08/25 12:13:09 GMT E |Ethernet |Link 1 Down
2005/08/25 12:13:11 GMT E |Ethernet |Link 1 Up - 100Base-TX Full Duplex
--
Norman
~A deam, dream, no dream
~Voices of the night go across the forest
~A dream, dream, no dream
~Good night my good child | |
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