Q I have broadband with Optus in Australia. I was unable to get Linux to connect to the modem (a Siemens SpeedStream 4200) until I got Gentoo 2006.1. Once it worked I found the difference was that it used dhcpcd and the others used pump or dhclient. Mandriva 2007 offered all three, but only dhcpcd worked. When I checked, I found that dhcpcd uses the -h hostname option. I haven't been able to get the other programs to work, but I have been able to get other distros (DSL-N, Knoppix and Ubuntu) by mounting the Gentoo partition and running dhcpcd, which brings up the net immediately. The others get what looks like a valid IP address but don't connect or drop the connection before I can use it. I think Optus has a special version of the modem, but it does use other brands of modem. I originally used the Windows setup disk on XP until I found Gentoo worked. What is the difference between the programs?
A It seems that this is a fairly common problem with this modem. When used with some DHCP programs, it does exactly what you describe: it gives out an IP address and then drops the connection. There are two possible solutions, the first of which you have already discovered. By using dhcpcd, you can pass a hostname to the modem with the -h option. There is an equivalent option with pump, -u or --hostname, but not with dhclient. It would appear that either the modem or your ISP is very picky about the format of any DHCP requests you send. Given the apparently flaky nature of the DHCP support in this modem, the second solution would be more reliable: to use static addressing. You need to find the IP address of the modem, which you can do after a successful dhcpcd negotiation, by connecting through Windows or by trial and error.
The default address varies according to the ISP it is intended to be used with, but the default for OptusNet should be 10.1.1.1. Once you know the modem's IP address, it is easy to configure your computer's Ethernet interface to use a static address. Pick an address on the same subnet as the modem, say 10.1.1.2 and set the gateway and DNS server addresses to that of the modem (10.1.1.1). The netmask needs to be set to 255.255.255.0. With a setup like this, you should have no more problems. DHCP is a great time saver when working with larger networks, or when moving from one network to another with a laptop. For a small home network, it is usually simpler to just give each device its own static address.