DUMPON(8) FreeBSD System Manager's Manual DUMPON(8)
NAME
dumpon - specify a device for crash dumps
SYNOPSIS
dumpon [-i index] [-r] [-v] [-C cipher] [-k pubkey] [-Z] [-z] device
dumpon [-i index] [-r] [-v] [-C cipher] [-k pubkey] [-Z] [-z]
[-g gateway] -s server -c client iface
dumpon [-v] off
dumpon [-v] -l
DESCRIPTION
The dumpon utility is used to configure where the kernel can save a crash
dump in the case of a panic.
System administrators should typically configure dumpon in a persistent
fashion using the rc.conf(5) variables dumpdev and dumpon_flags. For
more information on this usage, see rc.conf(5).
Starting in FreeBSD 13.0, dumpon can configure a series of fallback dump
devices. For example, an administrator may prefer netdump(4) by default,
but if the netdump(4) service cannot be reached or some other failure
occurs, they might choose a local disk dump as a second choice option.
General options
-i index Insert the specified dump configuration into the prioritized
fallback dump device list at the specified index, starting at
zero.
If -i is not specified, the configured dump device is appended
to the prioritized list.
-r Remove the specified dump device configuration or
configurations from the fallback dump device list rather than
inserting or appending it. In contrast, "dumpon off" removes
all configured devices. Conflicts with -i.
-k pubkey Configure encrypted kernel dumps.
A random, one-time symmetric key is automatically generated
for bulk kernel dump encryption every time dumpon is used.
The provided pubkey is used to encrypt a copy of the symmetric
key. The encrypted dump contents consist of a standard dump
header, the pubkey-encrypted symmetric key contents, and the
symmetric key encrypted core dump contents.
As a result, only someone with the corresponding private key
can decrypt the symmetric key. The symmetric key is necessary
to decrypt the kernel core. The goal of the mechanism is to
provide confidentiality.
The pubkey file should be a PEM-formatted RSA key of at least
2048 bits.
-C cipher Select the symmetric algorithm used for encrypted kernel crash
dump. The default is "chacha20" but "aes256-cbc" is also
available. (AES256-CBC mode does not work in conjunction with
compression.)
-l List the currently configured dump device(s), or /dev/null if
no devices are configured.
-v Enable verbose mode.
-Z Enable compression (Zstandard).
-z Enable compression (gzip). Only one compression method may be
enabled at a time, so -z is incompatible with -Z.
Zstandard provides superior compression ratio and performance.
Netdump
dumpon may also configure the kernel to dump to a remote netdumpd(8)
server. (The netdumpd(8) server is available in ports.) netdump(4)
eliminates the need to reserve space for crash dumps. It is especially
useful in diskless environments. When dumpon is used to configure
netdump, the device (or iface) parameter should specify a network
interface (e.g., igb1). The specified NIC must be up (online) to
configure netdump.
netdump(4) specific options include:
-c client The local IP address of the netdump(4) client.
-g gateway The first-hop router between client and server. If the -g
option is not specified and the system has a default route,
the default router is used as the netdump(4) gateway. If the
-g option is not specified and the system does not have a
default route, server is assumed to be on the same link as
client.
-s server The IP address of the netdumpd(8) server.
All of these options can be specified in the rc.conf(5) variable
dumpon_flags.
Minidumps
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 debug.minidump sysctl(8)
variable to 0.
Full dumps
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 dumpon utility will
refuse to enable a dump device which is smaller than the total amount of
physical memory as reported by the hw.physmem sysctl(8) variable.
IMPLEMENTATION NOTES
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.
The loader(8) variable dumpdev may be used to enable early kernel core
dumps for system panics which occur before userspace starts.
EXAMPLES
In order to generate an RSA private key, a user can use the genrsa(1)
tool:
# openssl genrsa -out private.pem 4096
A public key can be extracted from the private key using the rsa(1) tool:
# openssl rsa -in private.pem -out public.pem -pubout
Once the RSA keys are created in a safe place, the public key may be
moved to the untrusted netdump client machine. Now public.pem can be
used by dumpon to configure encrypted kernel crash dumps:
# dumpon -k public.pem /dev/ada0s1b
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 ddb(4) debugger:
# sysctl debug.kdb.panic=1
In the debugger the following commands should be typed to write a core
dump and reboot:
db> dump
db> reset
After reboot savecore(8) should be able to save the core dump in the
"dumpdir" directory, which is /var/crash by default:
# savecore /dev/ada0s1b
Three files should be created in the core directory: info.#, key.# and
vmcore_encrypted.# (where "#" is the number of the last core dump saved
by savecore(8)). The vmcore_encrypted.# can be decrypted using the
decryptcore(8) utility:
# decryptcore -p private.pem -k key.# -e vmcore_encrypted.# -c
vmcore.#
or shorter:
# decryptcore -p private.pem -n #
The vmcore.# can be now examined using kgdb(1):
# kgdb /boot/kernel/kernel vmcore.#
or shorter:
# kgdb -n #
The core was decrypted properly if 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(8).
The dumpon rc(8) script runs early during boot, typically before
networking is configured. This makes it unsuitable for configuring
netdump(4) when the client address is dynamic. To configure netdump(4)
when dhclient(8) binds to a server, dhclient-script(8) can be used to run
dumpon(8). For example, to automatically configure netdump(4) on the
vtnet0 interface, add the following to /etc/dhclient-exit-hooks.
case $reason in
BOUND|REBIND|REBOOT|RENEW)
if [ "$interface" != vtnet0 ] || [ -n "$old_ip_address" -a \
"$old_ip_address" = "$new_ip_address" ]; then
break
fi
if [ -n "$new_routers" ]; then
# Take the first router in the list.
gateway_flag="-g ${new_routers%% *}"
fi
# Configure as the highest-priority dump device.
dumpon -i 0 -c $new_ip_address -s $server $gateway_flag vtnet0
;;
esac
Be sure to fill in the server IP address and change the interface name if
needed.
SEE ALSO
gzip(1), kgdb(1), zstd(1), ddb(4), netdump(4), fstab(5), rc.conf(5),
config(8), decryptcore(8), init(8), loader(8), rc(8), savecore(8),
swapon(8), panic(9)
HISTORY
The dumpon utility appeared in FreeBSD 2.0.5.
Support for encrypted kernel core dumps and netdump was added in
FreeBSD 12.0.
AUTHORS
The dumpon manual page was written by Mark Johnston <markj@FreeBSD.org>,
Conrad Meyer <cem@FreeBSD.org>,
Konrad Witaszczyk <def@FreeBSD.org>, and countless others.
CAVEATS
To configure encrypted kernel core dumps, the running kernel must have
been compiled with the EKCD option.
Netdump does not automatically update the configured gateway if routing
topology changes.
The size of a compressed dump or a minidump is not a fixed function of
RAM size. Therefore, when at least one of these options is enabled, the
dumpon utility cannot verify that the device has sufficient space for a
dump. dumpon is also unable to verify that a configured netdumpd(8)
server has sufficient space for a dump.
-Z requires a kernel compiled with the ZSTDIO kernel option. Similarly,
-z requires the GZIO option.
BUGS
Netdump only supports IPv4 at this time.
SECURITY CONSIDERATIONS
The current encrypted kernel core dump scheme does not provide integrity
nor authentication. That is, the recipient of an encrypted kernel core
dump cannot know if they received an intact core dump, nor can they
verify the provenance of the dump.
RSA keys smaller than 1024 bits are practical to factor and therefore
weak. Even 1024 bit keys may not be large enough to ensure privacy for
many years, so NIST recommends a minimum of 2048 bit RSA keys. As a
seatbelt, dumpon prevents users from configuring encrypted kernel dumps
with extremely weak RSA keys. If you do not care for cryptographic
privacy guarantees, just use dumpon without specifying a -k pubkey
option.
This process is sandboxed using capsicum(4).
FreeBSD 13.1-RELEASE-p6 April 23, 2020 FreeBSD 13.1-RELEASE-p6
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