netkat

netcat version using raw sockets to avoid iptables and/or other OS filtering mechanisms.

Requirements

It only works on Linux with a Kernel version > 5.2 It requires CAP_NET_RAW and CAP_SYS_RESOURCE capabilities ... or just run it with sudo ;)

Install

make build

Usage

It requires root privileges:

sudo ./bin/netkat
Usage: nk [options] [hostname] [port]

  -debug
        Debug
  -interface string
        Specify interface to use. Default interface with default route
  -listen
        Bind and listen for incoming connections
  -source-port int
        Specify source port to use
  -udp
        Use UDP instead of default TCP

Docker image

It can be used as a container image, based in alpine:

docker run -it --privileged aojea/netkat:latest 192.168.68.1 80
2021/05/31 21:29:42 Using source address 172.17.0.2/16
2021/05/31 21:29:42 Creating raw socket
2021/05/31 21:29:42 Adding ebpf ingress filter on interface eth0
2021/05/31 21:29:42 filter {LinkIndex: 99, Handle: 0:1, Parent: ffff:fff2, Priority: 0, Protocol: 3}
2021/05/31 21:29:42 Creating user TCP/IP stack
2021/05/31 21:29:42 Dialing ...
2021/05/31 21:29:42 Connection established

Development

netkat uses the gvisor userspace TCP/IP stack "netstack"

It creates a RAW socket attached to the interface with the route to the destination IP, attaching a BPF filter for the traffic mathing the connection parameters specified as arguments.

It also creates an ingress queue and attaches an eBPF filter, to drop the traffic of the connection directed to the RAW socket, preventing that the host stack drops the connection.