arpfox

arpfox is an arpspoof alternative written in Go that injects specially crafted ARP packets into a LAN.

A security researcher may run arpfox against any machine on the LAN (even the router) to alter its ARP cache table and divert network packets to another host, this is an ancient technique known as ARP spoofing.

Installing arpfox

You can install arpfox to /usr/local/bin with the following command (requires admin privileges):

curl -sL 'https://raw.githubusercontent.com/malfunkt/arpfox/master/install.sh' | sh

You can also grab the latest release from our releases page and install it on a different location.

Building arpfox from source

In order to build arpfox from source you'll need Go, a C compiler and libpcap's development files:

# Fedora
sudo dnf install -y libpcap-devel

# Arch Linux
sudo yay -S arpfox

# Debian/Ubuntu
sudo apt-get install -y libpcap-dev

# OSX
brew install libpcap

# FreeBSD
sudo pkg install libpcap

# Windows
# Install https://www.winpcap.org/

After installing libpcap, use go install to build and install arpfox:

go install github.com/malfunkt/arpfox
arpfox -h

Running arpfox

arpfox -i [interface] -t [target] [host]

Interface (-i)

Interface name (e.g.: eth0, en0, wlan0, etc).

Target specification (-t)

arpfox takes targets in the same format as nmap does. The following are all valid target specifications:

• 10.0.0.1
• 10.0.0.0/24
• 10.0.0.*
• 10.0.0.1-10
• 10.0.0.1, 10.0.0.5-10, 192.168.1.*, 192.168.10.0/24

Host

The host parameter defines the host you want to pose as, for instance, if you use the LAN router's IP address, the targeted machine will stop sending network packets to the router and will send them to you instead.

Root privileges

Depending on your OS, you may require root privileges to run arpfox

arpfox -i wlan0 -t 10.0.0.25 10.0.0.1
2016/09/05 20:06:12 wlan0: You don't have permission to capture on that device ((cannot open device) /dev/bpf: Permission denied)

sudo arpfox -i wlan0 -t 10.0.0.25 10.0.0.1
...

A practical example

Alice is a security researcher, and she wants to intercept and record all traffic between her own phone and the LAN router.

Her machine is already on the same LAN as the phone, and she knows the IP addresses of both the phone and of the router.

Phone:  10.0.0.101
Router: 10.0.0.1

Alice will attempt to make her laptop pose as the router in order for the phone to send all its traffic to the laptop.

If she succeeds, the phone will start sending traffic marked for the router (10.0.0.1) to Alice's machine, which (by default) will ignore the packets because they have a different destination, in order to instruct her machine to forward the packets to the legitimate destination instrad of dropping them Alice does something like:

# OSX
sudo sysctl net.inet.ip.forwarding=1

# FreeBSD
sudo sysctl -w net.inet.ip.forwarding=1

# Linux
sudo sysctl -w net.ipv4.ip_forward=1

# Windows (on PowerShell)
Get-NetIPInterface | select ifIndex,InterfaceAlias,AddressFamily,ConnectionState,Forwarding | Sort-Object -Property IfIndex | Format-Table
# (get the interface index)
Set-NetIPInterface -ifindex [interface index] -Forwarding Enabled

Besides just forwarding packets between the target and the router, Alice also wants to eavesdrop the traffic between the target and the router:

tcpdump -i en0 -A -n "src host 10.0.0.101 and (dst port 80 or dst port 443)"

At this point Alice hasn't started arpfox yet and the phone's ARP table still looks like this:

# 10.0.0.1's legitimate MAC address on the phone.
? (10.0.0.1) at 11:22:33:44:55:66 on wlan0 expires in 857 seconds [ethernet]

Now she's ready to use arpfox:

# arpfox -i [network interface] -t [target] [host]
arpfox -i en0 -t 10.0.0.101 10.0.0.1

-i en0 tells arpfox to inject packets via the en0 network interface, -t 10.0.0.101 10.0.0.1 tells arpfox to send unsolicited ARP replies to the phone (10.0.0.101) posing as the router (10.0.0.1).

After a few seconds, the phone's ARP table will get altered and the phone will think Alice's machine is the router:

# 10.0.0.1's MAC address was changed on the phone.
? (10.0.0.1) at 11:22:de:ad:be:ef on wlan0 expires in 1193 seconds [ethernet]

Finally, she takes the phone and goes to example.com, tcpdump starts showing traffic:

...
GET / HTTP/1.1
Host: example.org
Accept-Encoding: gzip, deflate
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
User-Agent: Mozilla/5.0 (iPhone; CPU iPhone OS 9_3_5 like Mac OS X) AppleWebKit/601.1 (KHTML, like Gecko) CriOS/52.0.2743.84 Mobile/13G36 Safari/601.1.46
Accept-Language: en-us
Cache-Control: max-age=0
Connection: keep-alive
...

Why does this happen?

ARP is the protocol most computers use to translate a MAC address into an IP address . The original proposal is described in RFC826.

The part that is flawed is described in the "Packet Reception" section:

Notice that the <protocol type, sender protocol address, sender hardware address> triplet is merged into the table before the opcode is looked at. This is on the assumption that communcation is bidirectional; if A has some reason to talk to B, then B will probably have some reason to talk to A. Notice also that if an entry already exists for the <protocol type, sender protocol address> pair, then the new hardware address supersedes the old one. Related Issues gives some motivation for this.

The opcode they're talking about is what tells the receiver if the packet is a request or a reply, except that nothing verifies that a reply is associated with a request nor that a request was made in the first place.

It only takes an unsolicited ARP packet of type reply to make a machine change that entry into its internal ARP table, this includes adding new addreses and replacing old ones.

How can I protect against this?

There are some programs that can help you against ARP spoofing. Sometimes programs like these may be inconvenient because we usually roam over different networks all the time and these programs require us to hack stuff before actually starting being productive. Keeping an static ARP table is simply not practical enough for most users.

One thing you can do, though, is assuming things like these could happen and try to encrypt your communications, specially on untrusted networks where any node, even the router, can be actively recording traffic without your consent. While this doesn't prevent attackers from keeping records on your communications, it does prevent them from knowing the actual contents of it.

You don't have to use a VPN to stay secure, you can build a proxy with SSH and configure your programs to use it:

ssh -D 9999 user@myownserver.org

A more advanced example could be found here.

A glimpse of our future

You know what is probably not going to help solving this problem in the coming years? Millions of already deployed IoT devices that cannot update themselves.