Using Internet of Things devices like Wi-Fi-connected light bulbs, switches, motion detectors and such poses a challenge to secure your LAN. Devices’ firmware usually is of a pretty bad quality with administrator password hard-coded and exploits never patched. At the same time these devices heavily rely on zero-configuration protocols for ease of discovery, and these protocols were never developed with security in mind.

I’m trying to reconcile these inherently insecure devices working with inherently insecure protocols by using a specially configured Linux-based firewall. I’ll guide you through my thought process to show how to make everything work seamlessly, but (more) secure at the same time.

               Internet
                  |
(ETH00) ====== Router (192.168.0.1) ====== (192.168.0.X)
  |               |                         Secured Wi-Fi/Eth Network
Firewall          |
| | |          (172.16.0.0)
| | |          Guest Wi-Fi Network
| | (ETH01)
| | (192.168.0.X)
| | DeviceType0
| |
| (WLAN01)
| (192.168.0.X)
| DeviceType1
|
(WLAN00)
(192.168.0.X)
DeviceType2

Overview

  • All trusted or Secured devices are directly connected to the main Router connected to the Internet and are not affected by the setup. (Example: owner’s laptops and phones that have complete implicit trust are on the Secured network)
  • All completely untrusted or Guest devices devices live on a separate subnetwork and don’t pass any traffic to the Secured network. This is how Apple AirPort Guest Network works out of the box. (Example: A friend’s laptop is on the Guest network does not see any other devices)
  • All partially trusted or DeviceTypeX devices are connected through the Firewall and can only respond to requests from devices from Secured network. Secured network devices see DeviceTypeX devices, but not in reverse. Devices of the same Type can interact with each other. (Example: iPhone on the Secured network can see Belkin WeMo switch on DeviceType1 newtork, but not in reverse; or Belkin WeMo switch can talk to detector on the same DeviceType1 network, but can not talk to the HP Printer on DeviceType2 network).

Firewall Network Interfaces

Firewall has these network interfaces:

  • ETH00: wired, connects to Router to the Secured network
  • ETH01, ETH02, ETH03: wired, each connect to a specific DeviceType network
  • WLAN00, WLAN01, WLAN02, WLAN03: 4 different Wi-Fi networks sharing the same channel and the same network card
  • BRIDGE00: Bridge across all network interfaces. This is the interface all FORWARD rules would be applied against

Hardware Used

PC: Soekris net6501

  • Intel Atom CPU 600MHz-1.6GHz
  • 512MB-2048MB RAM
  • 4x Intel 82574L Gigabit Ethernet ports
  • RS-232 Serial port for BIOS and OS boot control
  • PCIe slot for extensions
  • Size: 11.4”W x 6.11”D x 1.41”H
  • Passive cooling

US-made, small box, passive cooling, good networking and relatively cheap.

If you’re using macOS, I’d recommend to buy any FTDI RS-232 USB cable and Serial App to simplify configuration over a serial port.

  • Supports Wi-Fi 802.11n, 2.4GHz and 5GHz
  • 3x3 antenna array
  • Access point: 4 different SSID on the same channel

Really good Linux support, relatively fast, cheap.

Software Used

ZeroShell

  • Small, simple Linux-based firewall/router
  • Web- and SSH-based interfaces, no X11
  • Firewall: full iptables support
  • Wi-Fi: access point w/ many different SSID on the same channel

Simple, ideal for network appliance like Soekris.

Configuration instructions:

  • Create all Wi-Fi networks from RS-232 | Wi-Fi manager, you can have up to 4 on the same channel
  • Create Setup | Network | New BRIDGE with all interfaces: BRIDGE00

Another distribution I evaluated was pfSense. pfSense 2.3 is built on top of FreeBSD 10 and does not support my Wi-Fi card of choice: TL-WDN4800. pfSense 2.4 built on top of FreeBSD 11 is in beta test as of April 2017 and only supports amd64, which requires kernel recompilation to work on Soekris net6501.

pfSense 2.3 installation instructions:

  • Text on the console would be garbled, press 3 to get into a command line interface of FreeBSD loader
  • Type set console=comconsole to fix console. You should see OK
  • Type set hint.acpi.0.disabled=1 to disable ACPI. Otherwise boot will hang without any log messages
  • Type boot -v to boot the loader and install as usual

IPv6

ZeroShell allows all IPv6 traffic by default and there is no configuration. Simplest way to deal with it is to drop it all.

Add a new script: Setup | Scripts/Cron | NAT And Virtual Servers

## Disable IPv6 support: routing and all, ZeroShell does not do it right by default
echo -n "[Disable IPv6]: "
ip6tables -P FORWARD DROP \
&& sysctl -w net.ipv6.conf.all.disable_ipv6=1 \
&& sysctl -w net.ipv6.conf.default.disable_ipv6=1 \
&& sysctl -w net.ipv6.conf.lo.disable_ipv6=1 \
&& echo SUCCESS || echo FAILED

ZeroShell Firewall Rules

All these rules are configured using ZeroShell Firewall. Firewall strings represent how the rule looks like in ZeroShell Web UI. I also provide relevant iptables rules to make sure you can re-implement them in any other Linux distribution.

All rules apply to FORWARD chain and bridged packets only.

1. Drop invalid packets

Technically speaking this rule is not required, but why not?

  • Connection State: INVALID
  • ACTION: DROP

Firewall: DROP all opt -- in * out * 0.0.0.0/0 -> 0.0.0.0/0 PHYSDEV match --physdev-is-bridged state INVALID /* Drop invalid packets */

iptables: -A FORWARD -m physdev --physdev-is-bridged -m state --state INVALID -j DROP

Broadcast Protocols

2. Allow DHCP requests to Secure network

There is no good way to filter out DHCP requests as they are broadcasts

  • Output: ETH00
  • Protocol Matching: UDP
  • Source Port: 68
  • Dest. Port: 67
  • IPTABLES: -m pkttype --pkt-type broadcast
  • ACTION: ACCEPT
  • LOG

Firewall: ACCEPT udp opt -- in * out * 0.0.0.0/0 -> 0.0.0.0/0 PHYSDEV match --physdev-out ETH00 --physdev-is-bridged udp spt:68 dpt:67 PKTTYPE = broadcast /* Allow DHCP requests to Secure network */

iptables: -A FORWARD -p udp -m physdev --physdev-out ETH00 --physdev-is-bridged -m udp --sport 68 --dport 67 -m pkttype --pkt-type broadcast -m limit --limit 10/min --limit-burst 15 -j LOG --log-prefix "FORWARD/002"

3. Drop the rest of the Broadcast

I’m not sure how ARP is handled, but it looks like it just works

  • IPTABLES: -m pkttype --pkt-type broadcast
  • ACTION: DROP

Firewall: DROP 4 opt -- in * out * 0.0.0.0/0 -> 0.0.0.0/0 PHYSDEV match --physdev-is-bridged PKTTYPE = broadcast /* Drop the rest of broadcast (IP w/o ARP) */

iptables: -A FORWARD -p ip -m physdev --physdev-is-bridged -m pkttype --pkt-type broadcast -j DROP

Multicast ZeroConf Protocols

ZeroConf protocols rely on Multicast IP messages to find devices available nearby. Two specific protocols I support are mDNS and SSDP.

These protocols are very different from a normal TCP/UDP Unicast as iptables does not know if that’s ESTABLISHED or NEW connection. And we only allow requests from Secured network to DeviceTypeX networks, and only responses from DeviceTypeX to Secured network.

4. Allow mDNS requests from Secured network

mDNS search request may contain several requests in the same packet. Any device may potentially respond if it knows something:

  • UDP Multicast to 224.0.0.251:5353
  • Binary UDP data
  • UDP data 17th bit is 0 (3rd byte highest bit)
  • Input: ETH00
  • Fragments: Not match second and further
  • IPTABLES: -m u32 --u32 0>>22&[email protected]>>15&1=0
  • nDPI: mdns
  • ACTION: ACCEPT

Firewall: ACCEPT all opt !f in * out * 0.0.0.0/0 -> 0.0.0.0/0 PHYSDEV match --physdev-in ETH00 --physdev-is-bridged ndpi protocol mdns u32 "0x0>>0x16&[email protected]>>0xf&0x1=0x0" /* Allow mDNS requests from Secured network (UDP payload, 3rd byte, first bit is 0) */

iptables: -A FORWARD ! -f -m physdev --physdev-in ETH00 --physdev-is-bridged -m ndpi --mdns -m u32 --u32 "0x0>>0x16&[email protected]>>0xf&0x1=0x0" -m limit --limit 10/min --limit-burst 15 -j LOG --log-prefix "FORWARD/004"

5. Allow mDNS responses to Secured network

mDNS search response may contain several responses in the same packet. Any device may potentially respond if it knows something:

  • UDP Multicast to 224.0.0.251:5353
  • Binary UDP data
  • UDP data 17th bit is 0 (3rd byte highest bit)
  • Output: ETH00
  • Fragments: Not match second and further
  • IPTABLES: -m u32 --u32 0>>22&[email protected]>>15&1=1
  • nDPI: mdns
  • ACTION: ACCEPT
  • LOG

Firewall: ACCEPT all opt !f in * out * 0.0.0.0/0 -> 0.0.0.0/0 PHYSDEV match --physdev-out ETH00 --physdev-is-bridged ndpi protocol mdns u32 "0x0>>0x16&[email protected]>>0xf&0x1=0x1" /* Allow mDNS responses to Secured network (UDP payload, 3rd byte, first bit is 1) */

iptables: -A FORWARD ! -f -m physdev --physdev-in ETH00 --physdev-is-bridged -m ndpi --mdns -m u32 --u32 "0x0>>0x16&[email protected]>>0xf&0x1=0x0" -j ACCEPT

6. Allow SSDP requests from Secured network

SSDP search request uses HTTPU as a transport layer:

  • UDP Multicast to 239.255.255.250:1900
  • From A.B.C.D:N
  • Text UDP data
  • Starts with M-SEARCH *
  • Output: ETH00
  • Fragments: Not match second and further
  • IPTABLES: -m string --algo bm --to 0x40 --string M-SEARCH
  • nDPI: ssdp
  • ACTION: ACCEPT
  • LOG

Firewall: ACCEPT all opt !f in * out * 0.0.0.0/0 -> 0.0.0.0/0 PHYSDEV match --physdev-out ETH00 --physdev-is-bridgedprotocol SSDP STRING match "M-SEARCH" ALGO name bm TO 64 /* Allow SSDP requests from Secured network (has M-SEARCH string) */

iptables: -A FORWARD ! -f -m physdev --physdev-in ETH00 --physdev-is-bridged -m ndpi --ssdp -m string --string "M-SEARCH" --algo bm --to 64 -m limit --limit 10/min --limit-burst 15 -j LOG --log-prefix "FORWARD/006"

7. Allow SSDP responses to Secured network

SSDP search response uses HTTPU as a transport layer:

  • UDP Unicast to A.B.C.D:N
  • Text UDP data
  • Starts with HTTP/1.1 200 OK

SSDP responses uses Unicast and not Multicast. And destination port depends on the original SSDP request port number

We ignore SSDP notifications (essentially broadcast messages):

  • UDP Multicast to 239.255.255.250:1900
  • Text UDP data
  • Starts with NOTIFY *
  • Output: ETH00
  • Fragments: Not match second and further
  • Protocol Matching: UDP
  • IPTABLES: -m string --algo bm --to 0x40 --string HTTP/1.1
  • nDPI: ssdp
  • ACTION: ACCEPT
  • LOG

Firewall: ACCEPT udp opt !f in * out * 0.0.0.0/0 -> 0.0.0.0/0 PHYSDEV match --physdev-out ETH00 --physdev-is-bridged STRING match "HTTP/1.1" ALGO name bm TO 64 /* Allow SSDP responses to Secured network (UDP unicast) */

iptables: -A FORWARD -p udp ! -f -m physdev --physdev-out ETH00 --physdev-is-bridged -m string --string "HTTP/1.1" --algo bm --to 64 -m limit --limit 10/min --limit-burst 15 -j LOG --log-prefix "FORWARD/007"

8. Drop the rest of the Multicast

  • IPTABLES: -m pkttype --pkt-type multicast
  • ACTION: DROP

Firewall: DROP all opt -- in * out * 0.0.0.0/0 -> 0.0.0.0/0 PHYSDEV match --physdev-is-bridged PKTTYPE = multicast /* Drop the rest of multicast */

iptables: -A FORWARD -m physdev --physdev-is-bridged -m pkttype --pkt-type multicast -j DROP

Unicast Network Protocols

There are many Unicast protocols that we should take into account. Rules of thumb:

  • Connections originating from Secured network are allowed
  • Responses to a connection from Secured network are allowed
  • Connections to Router are usually allowed
  • Connections to Internet are always allowed

9. Allow all traffic from Secured network

  • Input: ETH00
  • ACTION: ACCEPT

Firewall: ACCEPT all opt -- in * out * 0.0.0.0/0 -> 0.0.0.0/0 PHYSDEV match --physdev-in ETH00 --physdev-is-bridged /* Allow all traffic from Secured network */

iptables: -A FORWARD -m physdev --physdev-in ETH00 --physdev-is-bridged -j ACCEPT

  • Output: ETH00
  • Connection state: ESTABLISHED, RELATED
  • ACITON: ACCEPT

Firewall: ACCEPT all opt -- in * out * 0.0.0.0/0 -> 0.0.0.0/0 PHYSDEV match --physdev-out ETH00 --physdev-is-bridged state RELATED,ESTABLISHED /* Allow all established and related packets */

iptables: -A FORWARD -m physdev --physdev-out ETH00 --physdev-is-bridged -m state --state RELATED,ESTABLISHED -j ACCEPT

11. Allow an ICMP ping traffic to Router

Some devices rely on ping to the gateway to verify a connection status

Router address is hardcoded here to 192.168.0.1

  • Output: ETH00
  • Destination IP: 192.168.0.1
  • Protocol Matching: ICMP
  • ICMP Type: echo-request (ping)
  • ACTION: ACCEPT
  • LOG

Firewall: ACCEPT icmp opt -- in * out * 0.0.0.0/0 -> 192.168.0.1 PHYSDEV match --physdev-out ETH00 --physdev-is-bridged icmptype 8 /* Allow an ICMP ping traffic to AirPort */

iptables: -A FORWARD -d 192.168.0.1/32 -p icmp -m physdev --physdev-out ETH00 --physdev-is-bridged -m icmp --icmp-type 8 -m limit --limit 10/min --limit-burst 15 -j LOG --log-prefix "FORWARD/011"

12. Allow any UDP traffic to Router (DHCP, DNS etc.)

Router address is hardcoded here to 192.168.0.1

  • Output: ETH00
  • Destination IP: 192.168.0.1
  • Protocol Matching: UDP
  • ACTION: ACCEPT
  • LOG

Firewall: ACCEPT udp opt -- in * out * 0.0.0.0/0 -> 192.168.0.1 PHYSDEV match --physdev-out ETH00 --physdev-is-bridged /* Allow any UDP traffic to AirPort (DHCP, DNS etc.) */

iptables: -A FORWARD -d 192.168.0.1/32 -p udp -m physdev --physdev-out ETH00 --physdev-is-bridged -m limit --limit 10/min --limit-burst 15 -j LOG --log-prefix "FORWARD/012"

13. Allow any traffic outside of LAN network

We rely on our LAN to be limited to 192.168.0.0/16 network

  • Output: ETH00
  • Destination IP: NOT 192.168.0.0/16
  • ACTION: ACCEPT

Firewall: ACCEPT all opt -- in * out * 0.0.0.0/0 !-> 192.168.0.0/16 PHYSDEV match --physdev-out ETH00 --physdev-is-bridged /* Allow any traffic outside of LAN network */

iptables: -A FORWARD ! -d 192.168.0.0/16 -m physdev --physdev-out ETH00 --physdev-is-bridged -j ACCEPT

14. DROP by default

  • Policy: DROP

iptables: -P FORWARD DROP

End result: iptables rules

Here is an end result in form of iptables rules which you can use directly if desired.

iptables -S

-P INPUT ACCEPT
-P FORWARD DROP
-P OUTPUT ACCEPT
-N NetBalancer
-N SYS_DNS
-N SYS_GUI
-N SYS_HTTPS
-N SYS_INPUT
-N SYS_OUTPUT
-N SYS_SSH
-A INPUT -j SYS_GUI
-A INPUT -j SYS_INPUT
-A INPUT -p tcp -m tcp --dport 80 -j SYS_HTTPS
-A INPUT -p tcp -m tcp --dport 443 -j SYS_HTTPS
-A INPUT -p tcp -m tcp --dport 22 -j SYS_SSH
-A FORWARD -m physdev --physdev-is-bridged -m state --state INVALID -j DROP
-A FORWARD -p udp -m physdev --physdev-out ETH00 --physdev-is-bridged -m udp --sport 68 --dport 67 -m pkttype --pkt-type broadcast -m limit --limit 10/min --limit-burst 15 -j LOG --log-prefix "FORWARD/002"
-A FORWARD -p udp -m physdev --physdev-out ETH00 --physdev-is-bridged -m udp --sport 68 --dport 67 -m pkttype --pkt-type broadcast -j ACCEPT
-A FORWARD -p ip -m physdev --physdev-is-bridged -m pkttype --pkt-type broadcast -j DROP
-A FORWARD ! -f -m physdev --physdev-in ETH00 --physdev-is-bridged -m ndpi  --mdns  -m u32 --u32 "0x0>>0x16&[email protected]>>0xf&0x1=0x0" -m limit --limit 10/min --limit-burst 15 -j LOG --log-prefix "FORWARD/004"
-A FORWARD ! -f -m physdev --physdev-in ETH00 --physdev-is-bridged -m ndpi  --mdns  -m u32 --u32 "0x0>>0x16&[email protected]>>0xf&0x1=0x0" -j ACCEPT
-A FORWARD ! -f -m physdev --physdev-out ETH00 --physdev-is-bridged -m ndpi  --mdns  -m u32 --u32 "0x0>>0x16&[email protected]>>0xf&0x1=0x1" -m limit --limit 10/min --limit-burst 15 -j LOG --log-prefix "FORWARD/005"
-A FORWARD ! -f -m physdev --physdev-out ETH00 --physdev-is-bridged -m ndpi  --mdns  -m u32 --u32 "0x0>>0x16&[email protected]>>0xf&0x1=0x1" -j ACCEPT
-A FORWARD ! -f -m physdev --physdev-in ETH00 --physdev-is-bridged -m ndpi  --ssdp  -m string --string "M-SEARCH" --algo bm --to 64 -m limit --limit 10/min --limit-burst 15 -j LOG --log-prefix "FORWARD/006"
-A FORWARD ! -f -m physdev --physdev-in ETH00 --physdev-is-bridged -m ndpi  --ssdp  -m string --string "M-SEARCH" --algo bm --to 64 -j ACCEPT
-A FORWARD -p udp ! -f -m physdev --physdev-out ETH00 --physdev-is-bridged -m string --string "HTTP/1.1" --algo bm --to 64 -m limit --limit 10/min --limit-burst 15 -j LOG --log-prefix "FORWARD/007"
-A FORWARD -p udp ! -f -m physdev --physdev-out ETH00 --physdev-is-bridged -m string --string "HTTP/1.1" --algo bm --to 64 -j ACCEPT
-A FORWARD -m physdev --physdev-is-bridged -m pkttype --pkt-type multicast -j DROP
-A FORWARD -m physdev --physdev-in ETH00 --physdev-is-bridged -j ACCEPT
-A FORWARD -m physdev --physdev-out ETH00 --physdev-is-bridged -m state --state RELATED,ESTABLISHED -j ACCEPT
-A FORWARD -d 192.168.0.1/32 -p icmp -m physdev --physdev-out ETH00 --physdev-is-bridged -m icmp --icmp-type 8 -m limit --limit 10/min --limit-burst 15 -j LOG --log-prefix "FORWARD/011"
-A FORWARD -d 192.168.0.1/32 -p icmp -m physdev --physdev-out ETH00 --physdev-is-bridged -m icmp --icmp-type 8 -j ACCEPT
-A FORWARD -d 192.168.0.1/32 -p udp -m physdev --physdev-out ETH00 --physdev-is-bridged -m limit --limit 10/min --limit-burst 15 -j LOG --log-prefix "FORWARD/012"
-A FORWARD -d 192.168.0.1/32 -p udp -m physdev --physdev-out ETH00 --physdev-is-bridged -j ACCEPT
-A FORWARD ! -d 192.168.0.0/16 -m physdev --physdev-out ETH00 --physdev-is-bridged -j ACCEPT
-A OUTPUT -j SYS_OUTPUT
-A SYS_DNS -s 10.0.0.0/8 -j ACCEPT
-A SYS_DNS -s 172.16.0.0/12 -j ACCEPT
-A SYS_DNS -s 192.168.0.0/16 -j ACCEPT
-A SYS_DNS -s 192.168.0.0/24 -j ACCEPT
-A SYS_DNS -s 192.168.0.0/24 -j ACCEPT
-A SYS_DNS -s 192.168.250.0/24 -j ACCEPT
-A SYS_DNS -j DROP
-A SYS_GUI -s 192.168.0.2/32 -p tcp -m tcp --dport 12081 -j ACCEPT
-A SYS_HTTPS -i lo -j ACCEPT
-A SYS_HTTPS -m physdev --physdev-in ETH00 -j ACCEPT
-A SYS_HTTPS -j DROP
-A SYS_INPUT -i lo -j ACCEPT
-A SYS_INPUT -p udp -m udp --sport 53 -m state --state ESTABLISHED -j ACCEPT
-A SYS_INPUT -p tcp -m tcp --sport 53 -m state --state ESTABLISHED -j ACCEPT
-A SYS_INPUT -p udp -m udp --dport 53 -j SYS_DNS
-A SYS_INPUT -p tcp -m tcp --dport 53 -j SYS_DNS
-A SYS_INPUT -p tcp -m tcp --sport 80 -m state --state ESTABLISHED -j ACCEPT
-A SYS_INPUT -p tcp -m tcp --sport 8245 -m state --state ESTABLISHED -j ACCEPT
-A SYS_INPUT -p udp -m udp --sport 123 -m state --state ESTABLISHED -j ACCEPT
-A SYS_INPUT -j RETURN
-A SYS_OUTPUT -o lo -j ACCEPT
-A SYS_OUTPUT -p udp -m udp --dport 53 -j ACCEPT
-A SYS_OUTPUT -p tcp -m tcp --dport 80 -j ACCEPT
-A SYS_OUTPUT -p tcp -m tcp --dport 8245 -j ACCEPT
-A SYS_OUTPUT -p udp -m udp --dport 123 -j ACCEPT
-A SYS_OUTPUT -j RETURN
-A SYS_SSH -i lo -j ACCEPT
-A SYS_SSH -m physdev --physdev-in ETH00 -j ACCEPT
-A SYS_SSH -j DROP

ip6tables -S

-P INPUT ACCEPT
-P FORWARD DROP
-P OUTPUT ACCEPT