Remote debugging the kernel

FreeBSD supports remote debugging using a serial interface. Since most virtual solutions support serial interfaces it can be quite convenient to deploy a kernel and start a debug session on another machine.

The setup assumes two (virtual) machines with a serial (rs232) connection in between.

Configure and build a kernel

Use our toolchain described in detail here and add the following options to your SMP file in /usr/tools/config/[VERSION]/SMP

#nomakeoptions DEBUG
options        GDB                     # Support remote GDB.
makeoptions    DEBUG=-g

Next clean and build a kernel

make clean-obj,kernel kernel

Install the kernel on test vm

With the build finished, we should have a kernel package on the build machine available at the following location:


Copy this package to the test vm and install it using our opnsense-update command:

opnsense-update -ikfr [version] -l /location/from/

Where [version] is the version part of the kernel package, such as 19.1.2

Configure the test vm

To be able to connect to the test machine using kgdb, you need to make sure some settings are set.

Edit /boot/device.hints and change or add hint.uart.0.flags to the following:


Also /boot/loader.conf.local should have a baud-rate configured for the serial device:


Test your setup

Login to the test machine and force a debug session in gdb, using the following commands:

# sysctl debug.kdb.current=gdb
# sysctl debug.kdb.enter=1
db> gdb
Step to enter the remote GDB backend.
db> c (continue)

Then go to the build machine, make sure gdb is installed (pkg install gdb) and go to the directory where the debug symbols are and start a session, ask a backtrace bt and continue normal operation c: