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.\"     From: @(#)swapon.8	8.1 (Berkeley) 6/5/93
.\" $FreeBSD$
.\"
.Dd June 13, 2018
.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 pubkey
.Op Fl Z
.Op Fl z
.Ar device
.Nm
.Op Fl v
.Op Fl k Ar pubkey
.Op Fl Z
.Op Fl z
.Op Fl g Ar gateway | Li default
.Fl s Ar server
.Fl c Ar client
.Ar iface
.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 dumpon_flags
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
.Nm
is used to configure a local storage device as the dump device.
With additional parameters, the kernel can instead be configured to
transmit a dump to a remote server using
.Xr netdump 4 .
This eliminates the need to reserve space for saving crash dumps and
is especially useful in diskless environments.
The
.Xr netdump 4
server address is specified with
.Fl s Ar server ,
and the local address is specified with
.Fl c Ar client .
The
.Fl g Ar gateway
parameter may be used to specify a first-hop router to the server,
or to specify that the currently configured default gateway is to
be used.
Note that the
.Xr netdump 4
configuration is not automatically updated if any network configuration
(e.g., the default route) changes after the
.Nm
invocation.
The name of the interface to be used must be specified as
.Ar iface .
The interface must be up in order to configure
.Xr netdump 4 .
.Pp
The
.Fl k Ar pubkey
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 pubkey .
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 pubkey
in the
.Dq dumpon_flags
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
and
.Fl Z
options configure the kernel to compress the dump 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.
The
.Fl z
and
.Fl Z
options cause the dump to be written in
.Xr gzip 1
and
.Xr zstd 1
format, respectively.
These flags require a kernel compiled with the
.Dv GZIO
or
.Dv ZSTDIO
kernel options.
.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 device
and making a
.Dv DIOCSKERNELDUMP
.Xr ioctl 2
request on it to save kernel crash dumps.
If
.Ar device
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 /boot/kernel/kernel vmcore.#
.Pp
or shorter:
.Pp
.Dl # kgdb -n #
.Pp
The core was decrypted properly if
.Xr kgdb 1
does not print any errors.
Note that the live kernel might be at a different path
which can be examined by looking at the kern.bootfile sysctl.
.Sh SEE ALSO
.Xr gzip 1 ,
.Xr kgdb 1 ,
.Xr zstd 1 ,
.Xr ddb 4 ,
.Xr fstab 5 ,
.Xr rc.conf 5 ,
.Xr config 8 ,
.Xr decryptcore 8 ,
.Xr init 8 ,
.Xr loader 8 ,
.Xr rc 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.