Apple - Internet sharing without DHCP

From the bootpd man page:

For each of the properties dhcp_enabled, bootp_enabled, old_netboot_enabled, netboot_enabled, and relay_enabled, the corresponding service can be enabled or disabled for all interfaces, or enabled for just a specific set of interfaces. To enable or disable globally, use a boolean value true or false respectively. To enable just for a specific set of interfaces, use either a string, for a single interface, or an array of strings, one element for each interface.

 For example, to enable DHCP on interfaces en0 and en1, disable BOOTP on all
 interfaces, enable NetBoot on en1, and enable relay agent on interface en1,
 /etc/bootpd.plist could contain:
 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
 <plist version="1.0">
 <dict>
         <key>bootp_enabled</key>
         <false/>
         <key>dhcp_enabled</key>
         <array>
                 <string>en0</string>
                 <string>en1</string>
         </array>
         <key>netboot_enabled</key>
         <string>en1</string>
         <key>relay_enabled</key>
         <array>
                 <string>en1</string>
         </array>
 </dict>
 </plist>

To disable dhcp globally, use:

    <key>dhcp_enabled</key>
    <false/>

Don't forget to send a SIGHUP (kill -1) to bootpd after making changes to the configuration file.