Creating a Bridged Network Connection in Terminal
Here is an interesting solution to share internet with a layer-2 bridge but without using NAT/DHCP (not required)/Internet Sharing Pref Panel at all:

Create a shell script ethernet-bridge.sh in /usr/local/sbin with the following content:

#! /bin/sh
# ######################################
#  coded by Nat!
#  2013 Mulle kybernetiK
#  GPL

command=${1:-start}
shift
proxyarp=${1:-no}
shift

start()
{
        sysctl -w net.inet.ip.forwarding=1
        sysctl -w net.inet.ip.fw.enable=1
        if [ "$proxyarp" != "no" ]
        then
                sysctl -w net.link.ether.inet.proxyall=1
        fi

        ifconfig bridge0 create
        ifconfig bridge0 addm en0
        ifconfig bridge0 addm en1
        ifconfig bridge0 up
        if [ $? -eq 0 ]
        then
                syslog -s "Ethernet Bridge is up"
        else
                syslog -s "Ethernet Bridge failure"
        fi
}


stop()
{
        ifconfig bridge0 destroy

        sysctl -w net.inet.ip.forwarding=0
        sysctl -w net.inet.ip.fw.enable=0
        sysctl -w net.link.ether.inet.proxyall=0

        syslog -s "Ethernet Bridge is down"
}



case "$command" in
        start*) start
                ;;

        stop*)  stop
                ;;
esac


and make it executable:

chmod 755 /usr/local/sbin/ethernet-bridge.sh


Then put the following XML into /Library/LaunchDaemons as com.ethernet-bridge.plist

&lt;?xml version="1.0" encoding="UTF-8"?&gt;
&lt;!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd"&gt;

&lt;plist version="1.0"&gt;
    &lt;dict&gt;
        &lt;key&gt;Label&lt;/key&gt;
        &lt;string&gt;com.ethernet-bridge.plist&lt;/string&gt;
        &lt;key&gt;ProgramArguments&lt;/key&gt;
        &lt;array&gt;
            &lt;string&gt;/usr/local/sbin/ethernet-bridge.sh&lt;/string&gt;
        &lt;/array&gt;
        &lt;key&gt;UserName&lt;/key&gt;
        &lt;string&gt;root&lt;/string&gt;
        &lt;key&gt;GroupName&lt;/key&gt;
        &lt;string&gt;wheel&lt;/string&gt;
        &lt;key&gt;RunAtLoad&lt;/key&gt;
        &lt;true/&gt;
        &lt;key&gt;LaunchOnlyOnce&lt;/key&gt;
        &lt;true/&gt;   
    &lt;/dict&gt;
&lt;/plist&gt;


Then modify the file and load it:

sudo chmod 644 /Library/LaunchDaemons/com.ethernet-bridge.plist
sudo chown root:wheel /Library/LaunchDaemons/com.ethernet-bridge.plist
sudo launchctl load /Library/LaunchDaemons/com.ethernet-bridge.plist


The plist will enable the bridge mode while booting.

Check with ifconfig -a if the bridge is visible.

To stop the bridge mode just enter:

sudo /usr/local/sbin/ethernet-bridge.sh stop


To restart the bridge mode just enter:

sudo /usr/local/sbin/ethernet-bridge.sh start


Now set up the clients with a fixed IP/netmask/Gateway (the internal IP-address of the briding machine)/DNS or properly set up a DHCP server on the bridging machine.

Additionally you have to set up a static route on your router to the network  you provide Internet Sharing to (in your example (en1)).

Until now the syslog messages don't work - I'll try to improve the script pertinently. 



A similar approach is outlined here, but I didn't get it to work in Yosemite