An ADSL connection is not just Ethernet; it's a bit more than that.
While it does carry Ethernet traffic, the frames aren't sent directly over the wire, but inside an ATM virtual circuit, using ATM Adaptation Layer 5. (That's what the VCI/VPI settings are for.)
That said, Linux could certainly handle ATM traffic by itself. (Many DSL modems do in fact run Linux.) Even Windows had direct ATM and PPPoA support a while ago.
But more importantly, the electrical signalling is completely different. In other words…
But what I'm saying is that each wire of the RJ45 have a specific function when connected, the same is the RJ11.
…the problem is that none of the wires of the RJ45 have functions that exactly correspond to the wires on the RJ11. (Not that the jacks themselves have any meaning, and I've heard RJ45 being used for phone lines too – it's "Ethernet" vs "phone" that's important.)
Most phone lines have only one pair, and send & receive over both wires at once. The whole idea of DSL is to run over a two-wire phone line alongside analog calls; there's a DSLAM on the other end that separates them again.
Ethernet uses at least four wires at once – one pair to send data, one pair to receive. Gigabit Ethernet requires all four pairs (eight wires); the other two are bidirectional.
ADSL uses QAM and OFDM to transmit bits over the wire. (The ADSL signalling was defined in ANSI T1.413, later ITU G.dmt, G.lite, ADSL2, ADSL2+…)
Ethernet uses PAM coding for 1 Gbit networks, MLT-3 for 100 Mbit. Old 10 Mbit networks use Manchester code. (The most relevant standards are 802.3ab for 1G, 802.3u for 100M, and 802.3i for 10M. See the variants table.)
(I hope I got the above right?)
Therefore, attempting to wire one to the other is similar to connecting Video Out to Microphone In and expecting to see things on your speakers.
That said, ADSL modem cards for PCI do exist (much like ISDN cards and modem cards used to exist), and you could get one of those for your server and connect the ADSL line directly to it. There was once a ServerFault thread about it.
Alternatively, you could use a regular "ADSL modem" but configure it in pure bridge mode, where it would not do any routing, only forward Ethernet traffic directly between the regular ports and the ATM/ADSL line. That way, your server could directly talk to your ISP and act as the LAN router while having only Ethernet.
(The latter used to be very common practice. I had to deal with several really clunky PPPoE clients on Win98/Win2000, until finally Windows XP gained native PPPoE support. On Linux, there is rp-pppoe
. Some ISPs just use direct DHCP.)