KGDB

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KGDB is a debugger for the Linux kernel and for the kernels of NetBSD and FreeBSD. It lets you debug a running or crashed kernel from another machine.

How it works
- You need two machines connected either by a serial link (RS-232 with a null modem cable) or over a network (KGDB over Ethernet, KGDBoE).
- The machine being debugged (the target) runs a patched kernel. The other machine (the host) runs GDB. They talk using the GDB remote protocol.

Origins and how it evolved
- KGDB began in NetBSD in 1997 and later appeared in FreeBSD (version 2.2).
- The concept and a remote GDB protocol patch were adapted for Linux, where a lighter version called “kgdb light” was merged into the 2.6.26 kernel.
- KGDB light is simplified: it uses RS-232 only and includes a driver to separate debugger input from console input, so only one serial port is needed.
- Linux KGDB supports several architectures: IA-32, x86-64, PowerPC, ARM, MIPS, and ESA/390. NetBSD and FreeBSD use RS-232 connectivity for KGDB on their supported architectures.

Maintainers and history
- Linux KGDB was maintained by Amit Kale (2000–2004), then Linsyssoft Technologies (2004–2006).
- Since 2006, Jason Wessel of Wind River Systems has been the official maintainer.
- The slimmed-down Linux version, kgdb light, was created by Ingo Molnar and Jason Wessel and merged into Linux 2.6.26.

Other uses
- FreeBSD also has a gdb-based program named kgdb for debugging kernel core dumps. It can be used for remote “live” kernel debugging over serial or FireWire, similar to the Linux KGDB setup.