.\"-
.\" Copyright (c) 2019 Conrad Meyer <cem@FreeBSD.org>
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" $FreeBSD$
.\"
.Dd November 10, 2022
.Dt NETGDB 4
.Os
.Sh NAME
.Nm netgdb
.Nd protocol for debugging the kernel with GDB over the network
.Sh SYNOPSIS
To compile NetGDB support into the kernel, place the following lines in your
kernel configuration file:
.Bd -ragged -offset indent
.Cd "options DDB"
.Cd "options GDB"
.Cd "options INET"
.Cd "options DEBUGNET"
.Cd "options NETGDB"
.Ed
.Sh DESCRIPTION
.Nm
is a UDP-based protocol for communicating with a remote GDB client via an
intermediary proxy.
.Pp
A
.Nm
session is started by using the
.Ic netgdb Fl s Ar server Oo Fl g Ar gateway Fl c Ar client Fl i Ar iface Oc
command in
.Xr ddb 4
to connect to a proxy server.
When the connection is made, the proxy server logs a message that a
.Nm
client has connected.
It subsequently establishes a TCP listening socket and logs a message
specifying which port it is listening on.
Then it waits for a GDB client to connect.
The GDB command to connect is:
.Bd -ragged -offset indent
.Ic target remote Aq Ar proxyip:proxyport
.Ed
.Pp
At this point, the server proxies traffic back and forth between
.Nm
and the ordinary GDB client, speaking the ordinary GDB remote protocol.
The
.Nm
session is identical to any other kernel GDB sesssion from the perspective
of the GDB debugger.
.Sh IMPLEMENTATION NOTES
The UDP protocol is based on the same packet structure and a subset of the
exact same message types as
.Xr netdump 4 .
It uses the
.Dv HERALD ,
.Dv DATA ( née VMCORE ) ,
and
.Dv FINISHED
message types.
Like
.Xr netdump 4 ,
the client's initial
.Dv HERALD
message is acknowledged from a random source port, and the client sends
subsequent communication to that port.
.Pp
Unlike
.Xr netdump 4 ,
the initial
.Dv HERALD
port is 20025.
Additionally,
the proxy server sends responses to the source port of the client's initial
.Dv HERALD ,
rather than a separate reserved port.
.Nm
message and acknowledgements are bidirectional.
The sequence number and acknowledgement protocol is otherwise identical to
the unidirectional version used by netdump; it just runs in both directions.
Acknowledgements are sent to and from the same addresses and ports as
regular messages.
.Pp
The first version of the
.Nm
protocol uses the protocol number
.Dv Sq 0x2515f095
in the 32-bit
.Va aux2
parameter of the initial
.Dv HERALD
message.
.Pp
The list of supported network drivers and protocol families is identical to
that of
.Xr netdump 4 .
.Sh DIAGNOSTICS
The following variable is available via both
.Xr sysctl 8
and
.Xr loader 8
(as a tunable):
.Bl -tag -width "indent"
.It Va debug.gdb.netgdb.debug
Control debug message verbosity.
Debug messages are disabled by default.
They may be enabled by setting the variable to a non-zero value.
.El
.Sh SEE ALSO
.Xr ddb 4 ,
.Xr gdb 4 ,
.Xr netdump 4
.Sh HISTORY
.Nm
first appeared in
.Fx 13.0 .
.Sh BUGS
.Nm
may only be used after the kernel has panicked, due to limitations in the
treatment of locking primitives under
.Xr ddb 4 .
.Sh SECURITY CONSIDERATIONS
Version 1 of the
.Nm
protocol has no security properties whatsoever.
All messages are sent and acknowledged in cleartext, and no message
authentication codes are used to prevent attackers from forging messages.
It is absolutely inappropriate for use across the public internet.