SlimShim – IP/MAC spoofing of a connected device without interrupting traffic

SlimShim is a script that performs IP and MAC address spoofing of a directly-connected device without interrupting traffic to and from that device. A SlimShim has 2 ethernet ports that is plugged in between the victim device and the switch/router. The SlimShim acts as a switch and injects spoofed packets below the radar (source ports).


  1. The device you shim will remain up and connected. Nothing on the victim device will change.
  2. This allows you to get past NAC (as long as you shim a trusted device).
  3. This allows you to MITM the device connecting through the SlimShim.
  4. This allows you to spoof as any device connected to the SlimShim.
  5. If you attack the victim, you will appear as the router.
  6. If your SlimShim has wifi, you can route through it and appear as the victim.
  7. Everything is transparent to the network.
  8. A network scanner cannot detect SlimShim
  9. Code is available at
  10. Only IPv4 is supported at the moment. IPv6 is coming soon.
  11. 802.1q vlans can be shimmed.
  12. Slimshim mimics the TTL of the victim device.
  13. You will need expert-level skills of Linux and networking to understand how this works.
  14. You need to plug the victim into eth1.  The network guess function sniffs for input packets on that interface.

TODO/Upcoming features:

  1. IPv6 support

UPDATE: After all the work of testing and scripting, it turns out I just re-invented the wheel. The idea was already conceived by Alva Lease ‘Skip’ Duckwall IV and he did a presentation at Defcon 19.  His script is called 8021xbridge.  I like my script better as it is better at network guessing and supports 802.1q.

SlimShim is a bash script that runs iptables and ebtables commands to mimic the IP and MAC of the victim. It will use an ephemeral port (source ports) range below the range used by Windows and Linux devices. SlimShim will forward a lower range of ephemeral ports to itself, such as ports 27000-32000.

SlimShim can run on any device that has 2 ethernet ports. I use it on a Raspberry Pi(with a usb ethernet) running Kali and a Nexx WT3020 running OpenWRT. To shim a PC, simply unplug it’s ethernet and plug the cable into the SlimShim (ETH1) and plug the other SlimShim ethernet(ETH0) into the PC. You may need a cross-over cable if your device can’t configure it automatically. The two interfaces on the SlimShim are bridged, so it acts as a switch. The victim device does not know it has been shimmed and neither does the router/switch.

I have wifi AP setup on my SlimShims so that I can connect to victim’s network with a laptop and still appear as the victim IP/MAC.

The hardest part of shimming a victim is to get the IP/MAC of the victim and router. On my SlimShims, I have the start on bootup via rc.local (/root/ >/tmp/slimrun.log 2>&1). The slimshim script will run 3 tcpdumps to attempt to guess the IP/MAC of the victim and router.  The script should also work if you are shimming a server that is receiving mostly inbound packets, such as a networked security camera (which is a funny story for another time).

The method I use to guess the network is as follows:

  1. Start a loop to sniff for packets.  If anything doesn’t look right, start over.
  2. Sniff for inbound packets on eth1, where the victim device is plugged in.  This gets the victim IP, MAC, TTL, and 802.1q vlan tag(if used).
  3. Get the route MAC by sniffing for packets that are sent to the victim IP, look for non-standard TTLs (because we want packets that were hopped), and ignore any local network packets (assuming we’re on a /24).
  4. Now for the hard part, which is getting the router IP.  We have to watch for an arp request for who has the router MAC we got from the previous sniff. This may take a while for the victim to re-query the router IP, so to speed it up I re-plumb eth1 and also use scapy(if installed) to flood the assumed /24 with arp requests.


The script will also watch for DNS queries and update /etc/resolv.conf with the nameserver that the victim is using.

The script will also forward all connections to to the SlimShim. This allows the victim PC to connect to the SlimShim, which is useful to me when I’m pentesting. The script will also forward port 2501 to it’s ssh service so I can connect to SlimShim from elsewhere on the network.

The redirectIngressPort and redirectIngressPorts functions can redirect traffic that is destined to the victim. This is useful for opening ports to be used for reverse-bind attacks when I attack the network.

The redirectEgressPort function can redirect traffic from the victim so that I can redirect DNS, or anything, for MITM attacks. I once redirected a victim running Splunk Forwarder to a malicious Splunk server that I setup and deployed an app that opened a reverse shell.

The source is available at It is GPLv3.

The slimshim script will create an environment log file in /root that you can source and use the variables for your own scripts.

So how can you protect your network against SlimShims? You can’t block it, but you can detect it by configuring your firewalls to log all connections that use a source port range of 27000-32000. Unless the attacker changed to a different set of source ports.

How to setup SlimShim on a Raspberry Pi running Kali:
First get a usb ethernet adapter because SlimShim requires 2 ethernets.
Setup your /etc/network/interfaces like so:

auto lo
iface lo inet loopback

auto eth0
iface eth0 inet manual
up ifconfig $IFACE up

auto eth1
iface eth1 inet manual
up ifconfig $IFACE up

auto wlan0
iface wlan0 inet static

Setup wifi. Add this to /etc/hostapd/hostapd.conf. Ssid is slimshim and password is Gaddafi’d! (anyone get the reference?)


Setup dhcp for the wifi interface. Add this to /etc/dhcp/dhcpd.conf

ddns-update-style none;
option domain-name “shim.lan”;
option domain-name-servers;
subnet netmask {
default-lease-time 600;
max-lease-time 7200;
log-facility local7;

The script will create a br-lan bridge and assign it with MAC of 00:01:01:01:01:01. The wifi interface will be (an RFC 5736 private network). You can ssh to it once you are connected to wifi. Add it as your default route if you want to route through the SlimShim.

Copy the code from Github and put in /root/. Now run

I have the start on bootup via rc.local (/root/ >/tmp/slimrun.log 2>&1).

If that ran without errors, try to telnet to a website to see if everything is working. You can sniff the interface to verify your IP/MAC spoofs. The source port range should be within the range specified in the slimshim script.

That should do it. Let me know if something doesn’t work for you.


How to setup SlimShim on a Nexx WT3020. These things are only $15-$20. They are powered by usb. They don’t have much storage or ram, so don’t expect much beyond shimming a victim and maybe run a small nmap scan.  I have also used GL-Inet MT300A devices.  They are slightly larger, but have more ram.

First install OpenWRT or LEDE. I have mine modified to use a usb thumbdrive for storage. The Nexx has very little storage and you won’t be able to do much without a thumbdrive extension. There is documentation on OpenWRT’s site on how to use a thumbdrive as an overlay filesystem. I use a sandisk ultrafit 16gb. Here is how to setup a root overlay:

  1. Install the following packages: blkid block-mount kmod-crypto-hash kmod-fs-ext4 kmod-lib-crc16 kmod-scsi-core kmod-usb-ohci kmod-usb-storage-extras kmod-usb-storage kmod-usb-uhci kmod-usb2 libblkid libpthread librt libuuid
  2. Partition and format your thumbdrive.  I use 2 partitions, first is ext4 and the second is swap.  Don’t forget to run mkswap on the swap partition.
  3. Plug the thumbdrive into the device and run: mount /dev/sda1 /mnt; tar -C /overlay -cvf – . |tar -C /mnt/-xf -; umount /mnt
  4. Add the following to /etc/config/fstab
    config 'mount'
        option target '/overlay'
        option device '/dev/sda1'
        option enabled '1'
        option options 'rw,sync'
        option enabled_fsck '1'
    config 'swap'
       option device '/dev/sda2'
       option enabled '1'
  5. Reboot and check to make sure the overlay fs is working with ‘df /root’

Run these uci commands:

uci delete wireless.radio0
uci set wireless.radio0=wifi-device
uci set wireless.radio0.type=’mac80211′
uci set’11’
uci set wireless.radio0.hwmode=’11g’
uci set wireless.radio0.path=’10180000.wmac’
uci set wireless.radio0.htmode=’HT20′
uci set wireless.radio0.txpower=’20’
uci set’00’

uci delete wireless.@wifi-iface[0]
uci add wireless wifi-iface
uci set wireless.@wifi-iface[0]=wifi-iface
uci set wireless.@wifi-iface[0].device=’radio0′
uci set wireless.@wifi-iface[0].mode=’ap’
uci set wireless.@wifi-iface[0].ssid=’slimshim’
uci set wireless.@wifi-iface[0].hidden=’1′
uci set wireless.@wifi-iface[0].encryption=’psk2′
uci set wireless.@wifi-iface[0].key=’Gaddafi’\”d!’
uci set wireless.@wifi-iface[0].network=’wifi’

uci delete dhcp.lan
uci delete dhcp.wan
uci delete dhcp.odhcpd
uci delete dhcp.wifi
uci set dhcp.lan=dhcp
uci set dhcp.lan.interface=’lan’
uci set dhcp.lan.ignore=’1′
uci set dhcp.wan=dhcp
uci set dhcp.wan.interface=’wan’
uci set dhcp.wan.ignore=’1′
uci set dhcp.odhcpd=odhcpd
uci set dhcp.odhcpd.maindhcp=’0′
uci set dhcp.odhcpd.leasefile=’/tmp/hosts/odhcpd’
uci set dhcp.odhcpd.leasetrigger=’/usr/sbin/odhcpd-update’
uci set dhcp.wifi=dhcp
uci set dhcp.wifi.leasetime=’12h’
uci set dhcp.wifi.limit=’150′
uci set dhcp.wifi.interface=’wifi’
uci set dhcp.wifi.start=’10’

uci set dropbear.@dropbear[0].GatewayPorts=’on’

while uci show firewall.@defaults[0] >/dev/null 2>&1 ; do
uci delete firewall.@defaults[0] >/dev/null 2>&1
while uci show firewall.@zone[0] >/dev/null 2>&1 ; do
uci delete firewall.@zone[0] >/dev/null 2>&1
while uci show firewall.@forwarding[0] >/dev/null 2>&1 ; do
uci delete firewall.@forwarding[0] >/dev/null 2>&1
while uci show firewall.@rule[0] >/dev/null 2>&1 ; do
uci delete firewall.@rule[0] >/dev/null 2>&1
while uci show firewall.@include[0] >/dev/null 2>&1 ; do
uci delete firewall.@include[0] >/dev/null 2>&1

uci add firewall defaults
uci set firewall.@defaults[0]=defaults
uci set firewall.@defaults[0].input=’ACCEPT’
uci set firewall.@defaults[0].output=’ACCEPT’
uci set firewall.@defaults[0].forward=’ACCEPT’
uci add firewall zone
uci set firewall.@zone[0]=zone
uci set firewall.@zone[0].name=’lan’
uci set firewall.@zone[0].input=’ACCEPT’
uci set firewall.@zone[0].output=’ACCEPT’
uci set firewall.@zone[0].forward=’ACCEPT’
uci set firewall.@zone[0].network=’ ‘
uci add firewall zone
uci set firewall.@zone[1]=zone
uci set firewall.@zone[1].name=’wan’
uci set firewall.@zone[1].output=’ACCEPT’
uci set firewall.@zone[1].network=’wan wan6′
uci set firewall.@zone[1].input=’ACCEPT’
uci set firewall.@zone[1].forward=’ACCEPT’
uci add firewall forwarding
uci set firewall.@forwarding[0]=forwarding
uci set firewall.@forwarding[0].src=’lan’
uci set firewall.@forwarding[0].dest=’wan’
uci add firewall include
uci set firewall.@include[0]=include
uci set firewall.@include[0].path=’/etc/firewall.user’

uci set network.lan._orig_ifname=’eth0.1′
uci set network.lan._orig_bridge=’true’
uci set network.lan.ifname=’eth0.1 eth0.2′
uci set network.lan.ipaddr=’′
uci set network.lan.netmask=’′
uci set network.lan.force_link=’1′
uci set network.lan.delegate=0
uci delete network.lan.ip6assign

uci set network.@switch_vlan[0].ports=’0 6t’
uci set network.@switch_vlan[0].vid=’1′
uci set network.@switch_vlan[1].ports=’1 2 3 4 6t’
uci set network.@switch_vlan[1].vid=’2′

uci set network.wifi=interface
uci set network.wifi._orig_ifname=’wlan0′
uci set network.wifi._orig_bridge=’false’
uci set network.wifi.proto=’static’
uci set network.wifi.ipaddr=’′
uci set network.wifi.netmask=’′
uci set network.wifi.delegate=’0′

uci delete network.wan
uci delete network.wan6

uci set system.@system[0].hostname=slimshim

uci commit

Next steps are to install the bash and scapy packages and copy the code from Github and put in /root/.  Now run and watch the output.

I have the start on bootup via rc.local (/root/ >/tmp/slimrun.log 2>&1).

If that ran without errors, try to telnet to a website to see if everything is working. You can sniff the interface to verify your IP/MAC spoofs. The source port range should be within the range specified in the slimshim script. The slimshim script will blink the power led at a slow pace when it wants you to unplug and replug the vicim ethernet to get the router’s MAC.