macOS Network – How to Set Up a Transparent Firewall Using pf

ethernetmacosNetwork

I'm trying to setup a transparent firewall with a Mac mini running macOS Sierra.
Basically I'm trying to do this (just with macOS): A Transparent Firewall Using OpenBSD

This is my network setup:

Internet <-ethernet-> Router <-ethernet->en0 Mac mini en4<-ethernet-> AppleTV

The Router also connects to several other devices via Wifi.

Now here's what I did on the Mac mini to make it a transparent firewall:

  1. I setup the bridge via these commands:

    sudo ifconfig bridge1 create
sudo ifconfig bridge1 addm en0 addm en4
sudo ifconfig bridge1 up

From the AppleTV's perspective it's now like the Mac mini doesn't exist.

  1. I setup pf by adding this to /etc/pf.conf:

    anchor "org.user"
load anchor "org.user" from "/etc/pf.anchors/org.user"

    and created /etc/pf.anchors/org.user with this content:

    # skip lo0
set skip on lo0

# options
set block-policy drop

# rules
block on en4 all

    Then I "restarted" pf with the new rules:

    sudo pfctl -f /etc/pf.conf
sudo pfctl -e

Now I would expect no traffic from AppleTV to go out into the network. But AppleTV still works like I never setup those pf rules.

What am I missing?

PS: Here's the ifconfig output:

MacMini:~ Daniel$ ifconfig
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> mtu 16384
    options=1203<RXCSUM,TXCSUM,TXSTATUS,SW_TIMESTAMP>
    inet 127.0.0.1 netmask 0xff000000 
    inet6 ::1 prefixlen 128 
    inet6 fe80::1%lo0 prefixlen 64 scopeid 0x1 
    nd6 options=201<PERFORMNUD,DAD>
gif0: flags=8010<POINTOPOINT,MULTICAST> mtu 1280
stf0: flags=0<> mtu 1280
en0: flags=8963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
    options=10b<RXCSUM,TXCSUM,VLAN_HWTAGGING,AV>
    ether a8:20:66:1e:9a:62 
    inet6 fe80::1cca:5c4b:64a7:7350%en0 prefixlen 64 secured scopeid 0x4 
    inet6 2a02:8070:a89:8f00:181c:12bc:a98c:7229 prefixlen 64 autoconf secured 
    inet6 2a02:8070:a89:8f00:d489:89da:9083:17d8 prefixlen 64 autoconf temporary 
    inet6 2a02:8070:a89:8f00::9 prefixlen 64 dynamic 
    inet 192.168.0.137 netmask 0xffffff00 broadcast 192.168.0.255
    nd6 options=201<PERFORMNUD,DAD>
    media: autoselect (1000baseT <full-duplex,flow-control,energy-efficient-ethernet>)
    status: active
en4: flags=8963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMPLEX,MULTICAST> mtu 1500
    options=b<RXCSUM,TXCSUM,VLAN_HWTAGGING>
    ether 00:23:56:1c:82:26 
    inet6 fe80::223:56ff:fe1c:8226%en4 prefixlen 64 scopeid 0x5 
    nd6 options=201<PERFORMNUD,DAD>
    media: autoselect (100baseTX <full-duplex>)
    status: active
en1: flags=8823<UP,BROADCAST,SMART,SIMPLEX,MULTICAST> mtu 1500
    ether 20:c9:d0:93:5e:f1 
    nd6 options=201<PERFORMNUD,DAD>
    media: autoselect (<unknown type>)
    status: inactive
fw0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 4078
    lladdr 00:3e:e1:ff:fe:b3:04:08 
    nd6 options=201<PERFORMNUD,DAD>
    media: autoselect <full-duplex>
    status: inactive
en3: flags=963<UP,BROADCAST,SMART,RUNNING,PROMISC,SIMPLEX> mtu 1500
    options=60<TSO4,TSO6>
    ether 32:00:1b:30:40:80 
    media: autoselect <full-duplex>
    status: inactive
bridge0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    options=63<RXCSUM,TXCSUM,TSO4,TSO6>
    ether 32:00:1b:30:40:80 
    Configuration:
        id 0:0:0:0:0:0 priority 0 hellotime 0 fwddelay 0
        maxage 0 holdcnt 0 proto stp maxaddr 100 timeout 1200
        root id 0:0:0:0:0:0 priority 0 ifcost 0 port 0
        ipfilter disabled flags 0x2
    member: en3 flags=3<LEARNING,DISCOVER>
            ifmaxaddr 0 port 8 priority 0 path cost 0
    nd6 options=201<PERFORMNUD,DAD>
    media: <unknown type>
    status: inactive
p2p0: flags=8802<BROADCAST,SIMPLEX,MULTICAST> mtu 2304
    ether 02:c9:d0:93:5e:f1 
    media: autoselect
    status: inactive
awdl0: flags=8902<BROADCAST,PROMISC,SIMPLEX,MULTICAST> mtu 1484
    ether 8e:36:65:e0:9a:1e 
    nd6 options=201<PERFORMNUD,DAD>
    media: autoselect
    status: inactive
utun0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 2000
    inet6 fe80::9a58:4bd5:9f77:f77a%utun0 prefixlen 64 scopeid 0xc 
    nd6 options=201<PERFORMNUD,DAD>
utun1: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1380
    inet6 fe80::1e34:cc36:613b:1ee6%utun1 prefixlen 64 scopeid 0xf 
    nd6 options=201<PERFORMNUD,DAD>
ppp0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1280
    inet 10.0.1.16 --> 1.0.0.1 netmask 0xffffff00 
bridge1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    options=3<RXCSUM,TXCSUM>
    ether aa:20:66:e1:cb:01 
    Configuration:
        id 0:0:0:0:0:0 priority 0 hellotime 0 fwddelay 0
        maxage 0 holdcnt 0 proto stp maxaddr 100 timeout 1200
        root id 0:0:0:0:0:0 priority 0 ifcost 0 port 0
        ipfilter disabled flags 0x2
    member: en0 flags=3<LEARNING,DISCOVER>
            ifmaxaddr 0 port 4 priority 0 path cost 0
    member: en4 flags=3<LEARNING,DISCOVER>
            ifmaxaddr 0 port 5 priority 0 path cost 0
    media: autoselect
    status: active
MacMini:~ Daniel$

Best Answer

I really doubt that you will get this to work with system tools. And though I'm not familiar with OpenBSD I also doubt that the linked example works without further mods with one exception: filtering by MAC addresses after enabling bridge packet tagging.

In OS X bridge packet tagging (i.e. ifconfig bridge1 rule pass in on en4 src 00:17:f2:f8:1a:3d tag APPLETV) isn't possible though.

pf in OS X is a layer-3 packet filter. It relies on IP-addresses and doesn't really work with interface names or MAC-addresses. So using an interface name in a rule (e.g. block on en4 all) is internally translated to block on <IP of en4> all and in your case block nothing because en4 has no IP-address.

Even after assigning an IP to en4 you wouldn't be able to block AppleTV traffic to the outside world because these IP-packets neither contain <IP of en4> as source nor as destination address.

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