A very common use of FreeBSD is virtual site hosting, where one server appears to the
network as many servers. This is achieved by assigning multiple network addresses to a
single interface.
A given network interface has one “real” address, and may have any number
of “alias” addresses. These aliases are normally added by placing alias
entries in /etc/rc.conf.
An alias entry for the interface fxp0 looks like:
ifconfig_fxp0_alias0="inet xxx.xxx.xxx.xxx netmask xxx.xxx.xxx.xxx"
Note that alias entries must start with alias0 and proceed
upwards in order, (for example, _alias1, _alias2, and so on). The configuration process will stop at the
first missing number.
The calculation of alias netmasks is important, but fortunately quite simple. For a
given interface, there must be one address which correctly represents the network's
netmask. Any other addresses which fall within this network must have a netmask of all
1s (expressed as either 255.255.255.255
or 0xffffffff).
For example, consider the case where the fxp0 interface is
connected to two networks, the 10.1.1.0 network with a netmask of
255.255.255.0 and the 202.0.75.16 network
with a netmask of 255.255.255.240. We want the system to appear
at 10.1.1.1 through 10.1.1.5 and at 202.0.75.17 through 202.0.75.20. As noted
above, only the first address in a given network range (in this case, 10.0.1.1 and 202.0.75.17) should have a real
netmask; all the rest (10.1.1.2 through 10.1.1.5 and 202.0.75.18 through 202.0.75.20) must be configured with a netmask of 255.255.255.255.
The following /etc/rc.conf entries configure the adapter
correctly for this arrangement:
ifconfig_fxp0="inet 10.1.1.1 netmask 255.255.255.0"
ifconfig_fxp0_alias0="inet 10.1.1.2 netmask 255.255.255.255"
ifconfig_fxp0_alias1="inet 10.1.1.3 netmask 255.255.255.255"
ifconfig_fxp0_alias2="inet 10.1.1.4 netmask 255.255.255.255"
ifconfig_fxp0_alias3="inet 10.1.1.5 netmask 255.255.255.255"
ifconfig_fxp0_alias4="inet 202.0.75.17 netmask 255.255.255.240"
ifconfig_fxp0_alias5="inet 202.0.75.18 netmask 255.255.255.255"
ifconfig_fxp0_alias6="inet 202.0.75.19 netmask 255.255.255.255"
ifconfig_fxp0_alias7="inet 202.0.75.20 netmask 255.255.255.255"
