You cannot configure network address translation (NAT) in Mac OS X Server when the primary Internet connection uses PPPoE, because the PPPoE network interface will not appear in the “External network interface�? pop-up menu in Server Admin (located at Computers & Services > NAT > Settings).
To work around this issue, you need a DSL modem that negotiates the PPPoE authentication itself, rather than configuring the server to do so. After making the PPPoE connection, the modem passes the IP address to the server via DHCP.
From the server's perspective, it will then operate as a standard Ethernet connection, and the network interface to which the modem-negotiated IP address is assigned will be available as the external network interface when configuring NAT.
The ability of a modem to operate in this manner is sometimes known as "half-bridge mode" or "DHCP spoofing."
Note: You must work with your Internet Service Provider to ensure that the server's DSL line has a static IP address. (Mac OS X Server requires a static IP address regardless of the issue discussed in this document.)