InternetSharing(8) BSD System Manager's Manual InternetSharing(8)
NAME
InternetSharing -- simple NAT/router configuration daemon
SYNOPSIS
InternetSharing -d
DESCRIPTION
InternetSharing is the back-end for the Internet Sharing feature. It is responsible for configuring
the network interfaces, the DHCP server bootpd(8), the network address translation daemon natd(8), and
the Internet domain name server named(8). named(8) is run in caching-only mode and allows the DHCP
server to always offer the same DNS server address to the DHCP clients, regardless of the value of the
actual DNS server addresses.
The single command line option -d places additional debugging information to stdout/stderr.
InternetSharing is launched by launchd(8) both at start-up and when the user turns Internet Sharing on
in the Sharing preferences pane.
By default, InternetSharing configures the IP addresses for non-AirPort interfaces starting at
192.168.2.1, walking up by one class C network (subnet mask 255.255.255.0) for each subsequent inter-face interface
face i.e. 192.168.3.1, 192.168.4.1, 192.168.5.1, and so on. The AirPort interface by default is
assigned 10.0.2.1.
CONFIGURATION
InternetSharing reads the property list com.apple.nat.plist stored in the /Library/Preferences/System-Configuration. /Library/Preferences/SystemConfiguration.
Configuration.
Details of the com.apple.nat.plist are subject to change and are not completely documented here. The
plist is a contract between the Sharing preferences pane and InternetSharing. Any details provided here
are for informational purposes only.
The plist is a dictionary with a single sub-dictionary called NAT containing properties to control
which interfaces to use and other settings. It may also have a sub-dictionary called AirPort that is
used to configure the AirPort interface when it is put into access point mode.
[B] One property worth mentioning is SharingNetworkNumberStart. This property controls the behavior of
InternetSharing when it configures IP addresses for the local interfaces. The property is encoded as a
string containing the dotted decimal network IP address, assumed to be a class C network. For example:
<key>SharingNetworkNumberStart</key>
<string>192.168.100.0</string>
If the SharingNetworkNumberStart appears directly in the NAT dictionary, it controls the starting IP[/B]
address chosen for the non-AirPort interfaces. If the property appears within the AirPort sub-dictio-nary, sub-dictionary,
nary, it controls the IP address assigned to the AirPort interface.
The purpose of the property is to allow the user to avoid address collisions with existing NAT'd net-works. networks.
works.
