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.\" Copyright (c) 2019 Conrad Meyer <cem@FreeBSD.org>
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.Dd October 17, 2019
.Dt NETGDB 4
.Os
.Sh NAME
.Nm netgdb
.Nd protocol for debugging the kernel with GDB over the network
.Sh SYNOPSIS
NetGDB support is compiled by default, if DDB, GDB, and INET are enabled.
To build a kernel without it, add the following line to your kernel
configuration file:
.Bd -ragged -offset indent
.Cd "nooptions 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.