sbin/dumpon/dumpon.8

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.\"     From: @(#)swapon.8	8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
.Dd October 24, 2017
.Dt DUMPON 8
.Os
.Sh NAME
.Nm dumpon
.Nd "specify a device for crash dumps"
.Sh SYNOPSIS
.Nm
.Op Fl v
.Op Fl k Ar public_key_file
.Op Fl z
.Ar special_file
.Nm
.Op Fl v
.Cm off
.Nm
.Op Fl v
.Fl l
.Sh DESCRIPTION
The
.Nm
utility is used to specify a device where the kernel can save a crash
dump in the case of a panic.
.Pp
Calls to
.Nm
normally occur from the system multi-user initialization file
.Pa /etc/rc ,
controlled by the
.Dq dumpdev
and
.Dq dumppubkey
variables in the boot time configuration file
.Pa /etc/rc.conf .
.Pp
The default type of kernel crash dump is the mini crash dump.
Mini crash dumps hold only memory pages in use by the kernel.
Alternatively, full memory dumps can be enabled by setting the
.Va debug.minidump
.Xr sysctl 8
variable to 0.
.Pp
For systems using full memory dumps, the size of the specified dump
device must be at
least the size of physical memory.
Even though an additional 64 kB header is added to the dump, the BIOS for a
platform typically holds back some memory, so it is not usually
necessary to size the dump device larger than the actual amount of RAM
available in the machine.
Also, when using full memory dumps, the
.Nm
utility will refuse to enable a dump device which is smaller than the
total amount of physical memory as reported by the
.Va hw.physmem
.Xr sysctl 8
variable.
.Pp
The
.Op Fl k Ar public_key_file
flag causes
.Nm
to generate a one-time key for kernel crash dump encryption.
The key will be replaced by a new one when the
.Nm
utility is run again.
The key is encrypted using
.Ar public_key_file .
This process is sandboxed using
.Xr capsicum 4 .
Both plain and encrypted keys are sent to the kernel using
.Dv DIOCSKERNELDUMP
.Xr ioctl 2 .
A user can specify the
.Ar public_key_file
in the
.Dq dumppubkey
variable defined in
.Pa /etc/rc.conf
for use with the
.Pa /etc/rc.d/dumpon
.Xr rc 8
script.
This flag requires a kernel compiled with the
.Dv EKCD
kernel option.
.Pp
The
.Fl z
option configures the kernel to compress the dump in gzip format before writing
it to the dump device.
This reduces the amount of space required for the dump and accelerates
recovery with
.Xr savecore 8
since less data needs to be copied from the dump device.
When compression is enabled, the
.Nm
utility will not verify that the dump device is sufficiently large for a full
dump.
This flag requires a kernel compiled with the
.Dv GZIO
kernel option.
.Pp
The
.Fl l
flag causes
.Nm
to print the current dump device or _PATH_DEVNULL ("/dev/null") if no device is
configured.
.Pp
The
.Fl v
flag causes
.Nm
to be verbose about its activity.
.Sh IMPLEMENTATION NOTES
Since a
.Xr panic 9
condition may occur in a situation
where the kernel cannot trust its internal representation
of the state of any given file system,
one of the system swap devices,
and
.Em not
a device containing a file system,
should be used as the dump device.
.Pp
The
.Nm
utility operates by opening
.Ar special_file
and making a
.Dv DIOCSKERNELDUMP
.Xr ioctl 2
request on it to save kernel crash dumps.
If
.Ar special_file
is the text string:
.Dq Li off ,
.Nm
performs a
.Dv DIOCSKERNELDUMP
.Xr ioctl 2
on
.Pa /dev/null
and thus instructs the kernel not to save crash dumps.
.Pp
Since
.Nm
cannot be used during kernel initialization, the
.Va dumpdev
variable of
.Xr loader 8
must be used to enable dumps for system panics which occur
during kernel initialization.
.Sh FILES
.Bl -tag -width "/dev/{ada,da}?s?b" -compact
.It Pa /dev/{ada,da}?s?b
standard swap areas
.It Pa /etc/rc.conf
boot-time system configuration
.El
.Sh EXAMPLES
In order to generate an RSA private key a user can use the
.Xr genrsa 1
tool:
.Pp
.Dl # openssl genrsa -out private.pem 4096
.Pp
A public key can be extracted from the private key using the
.Xr rsa 1
tool:
.Pp
.Dl # openssl rsa -in private.pem -out public.pem -pubout
.Pp
Once the RSA keys are created the private key should be moved to a safe place.
Now
.Pa public.pem
can be used by
.Nm
to configure encrypted kernel crash dumps:
.Pp
.Dl # dumpon -k public.pem /dev/ada0s1b
.Pp
It is recommended to test if the kernel saves encrypted crash dumps using the
current configuration.
The easiest way to do that is to cause a kernel panic using the
.Xr ddb 4
debugger:
.Pp
.Dl # sysctl debug.kdb.panic=1
.Pp
In the debugger the following commands should be typed to write a core dump and
reboot:
.Pp
.Dl db> call doadump(0)
.Dl db> reset
.Pp
After reboot
.Xr savecore 8
should be able to save the core dump in the core directory which is
.Pa /var/crash
by default:
.Pp
.Dl # savecore /var/crash /dev/ada0s1b
.Pp
Three files should be created in the core directory:
.Pa info.# ,
.Pa key.#
and
.Pa vmcore_encrypted.#
where
.Dq #
is the number of the last core dump saved by
.Xr savecore 8 .
The
.Pa vmcore_encrypted.#
can be decrypted using the
.Xr decryptcore 8
utility:
.Pp
.Dl # decryptcore -p private.pem -k key.# -e vmcore_encrypted.# -c vmcore.#
.Pp
or shorter:
.Pp
.Dl # decryptcore -p private.pem -n #
.Pp
The
.Pa vmcore.#
can be now examined using
.Xr kgdb 1 :
.Pp
.Dl # kgdb /usr/obj/sys/GENERIC/kernel.debug vmcore.#
.Pp
or shorter:
.Pp
.Dl # kgdb -n # /usr/obj/sys/GENERIC/kernel.debug
.Pp
The core was decrypted properly if
.Xr kgdb 1
does not print any errors.
.Sh SEE ALSO
.Xr kgdb 1 ,
.Xr ddb 4 ,
.Xr fstab 5 ,
.Xr rc.conf 5 ,
.Xr config 8 ,
.Xr init 8 ,
.Xr loader 8 ,
.Xr rc 8 ,
.Xr decryptcore 8 ,
.Xr savecore 8 ,
.Xr swapon 8 ,
.Xr panic 9
.Sh HISTORY
The
.Nm
utility appeared in
.Fx 2.0.5 .
.Sh BUGS
Because the file system layer is already dead by the time a crash dump
is taken, it is not possible to send crash dumps directly to a file.
.Pp
It is currently not possible to configure both compression and encryption.
The encrypted dump format assumes that the kernel dump size is a multiple
of the cipher block size, which may not be true when the dump is compressed.