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28 .\" From: @(#)swapon.8 8.1 (Berkeley) 6/5/93
36 .Nd "specify a device for crash dumps"
68 utility is used to configure where the kernel can save a crash dump in the case
71 System administrators should typically configure
73 in a persistent fashion using the
79 For more information on this usage, see
85 can configure a series of fallback dump devices.
86 For example, an administrator may prefer
88 by default, but if the
90 service cannot be reached or some other failure occurs, they might choose a
91 local disk dump as a second choice option.
93 .Bl -tag -width _k_pubkey
95 Insert the specified dump configuration into the prioritized fallback dump
96 device list at the specified index, starting at zero.
100 is not specified, the configured dump device is appended to the prioritized
103 Remove the specified dump device configuration or configurations from the
104 fallback dump device list rather than inserting or appending it.
110 removes all configured devices.
114 Configure encrypted kernel dumps.
116 A random, one-time symmetric key is automatically generated for bulk kernel
117 dump encryption every time
122 is used to encrypt a copy of the symmetric key.
123 The encrypted dump contents consist of a standard dump header, the
124 pubkey-encrypted symmetric key contents, and the symmetric key encrypted core
127 As a result, only someone with the corresponding private key can decrypt the symmetric key.
128 The symmetric key is necessary to decrypt the kernel core.
129 The goal of the mechanism is to provide confidentiality.
133 file should be a PEM-formatted RSA key of at least 2048 bits.
135 Select the symmetric algorithm used for encrypted kernel crash dump.
141 (AES256-CBC mode does not work in conjunction with compression.)
143 List the currently configured dump device(s), or /dev/null if no devices are
148 Enable compression (Zstandard).
150 Enable compression (gzip).
151 Only one compression method may be enabled at a time, so
156 Zstandard provides superior compression ratio and performance.
160 may also configure the kernel to dump to a remote
165 server is available in ports.)
167 eliminates the need to reserve space for crash dumps.
168 It is especially useful in diskless environments.
171 is used to configure netdump, the
175 parameter should specify a network interface (e.g.,
177 The specified NIC must be up (online) to configure netdump.
180 specific options include:
181 .Bl -tag -width _g_gateway
183 The local IP address of the
187 The first-hop router between
193 option is not specified and the system has a default route, the default
194 router is used as the
199 option is not specified and the system does not have a default route,
201 is assumed to be on the same link as
204 The IP address of the
209 All of these options can be specified in the
214 The default type of kernel crash dump is the mini crash dump.
215 Mini crash dumps hold only memory pages in use by the kernel.
216 Alternatively, full memory dumps can be enabled by setting the
221 For systems using full memory dumps, the size of the specified dump
222 device must be at least the size of physical memory.
223 Even though an additional 64 kB header is added to the dump, the BIOS for a
224 platform typically holds back some memory, so it is not usually
225 necessary to size the dump device larger than the actual amount of RAM
226 available in the machine.
227 Also, when using full memory dumps, the
229 utility will refuse to enable a dump device which is smaller than the
230 total amount of physical memory as reported by the
234 .Sh IMPLEMENTATION NOTES
235 Because the file system layer is already dead by the time a crash dump
236 is taken, it is not possible to send crash dumps directly to a file.
242 may be used to enable early kernel core dumps for system panics which occur
243 before userspace starts.
245 In order to generate an RSA private key, a user can use the
249 .Dl # openssl genrsa -out private.pem 4096
251 A public key can be extracted from the private key using the
255 .Dl # openssl rsa -in private.pem -out public.pem -pubout
257 Once the RSA keys are created in a safe place, the public key may be moved to
258 the untrusted netdump client machine.
263 to configure encrypted kernel crash dumps:
265 .Dl # dumpon -k public.pem /dev/ada0s1b
267 It is recommended to test if the kernel saves encrypted crash dumps using the
268 current configuration.
269 The easiest way to do that is to cause a kernel panic using the
273 .Dl # sysctl debug.kdb.panic=1
275 In the debugger the following commands should be typed to write a core dump and
283 should be able to save the core dump in the
289 .Dl # savecore /dev/ada0s1b
291 Three files should be created in the core directory:
295 .Pa vmcore_encrypted.#
298 is the number of the last core dump saved by
301 .Pa vmcore_encrypted.#
302 can be decrypted using the
306 .Dl # decryptcore -p private.pem -k key.# -e vmcore_encrypted.# -c vmcore.#
310 .Dl # decryptcore -p private.pem -n #
314 can be now examined using
317 .Dl # kgdb /boot/kernel/kernel vmcore.#
323 The core was decrypted properly if
325 does not print any errors.
326 Note that the live kernel might be at a different path
327 which can be examined by looking at the
334 script runs early during boot, typically before networking is configured.
335 This makes it unsuitable for configuring
337 when the client address is dynamic.
343 .Xr dhclient-script 8
346 For example, to automatically configure
348 on the vtnet0 interface, add the following to
349 .Pa /etc/dhclient-exit-hooks .
352 BOUND|REBIND|REBOOT|RENEW)
353 if [ "$interface" != vtnet0 ] || [ -n "$old_ip_address" -a \\
354 "$old_ip_address" = "$new_ip_address" ]; then
357 if [ -n "$new_routers" ]; then
358 # Take the first router in the list.
359 gateway_flag="-g ${new_routers%% *}"
361 # Configure as the highest-priority dump device.
362 dumpon -i 0 -c $new_ip_address -s $server $gateway_flag vtnet0
367 Be sure to fill in the server IP address and change the interface name if
391 Support for encrypted kernel core dumps and netdump was added in
396 manual page was written by
397 .An Mark Johnston Aq Mt markj@FreeBSD.org ,
398 .An Conrad Meyer Aq Mt cem@FreeBSD.org ,
399 .An Konrad Witaszczyk Aq Mt def@FreeBSD.org ,
400 and countless others.
402 To configure encrypted kernel core dumps, the running kernel must have been
407 Netdump does not automatically update the configured
409 if routing topology changes.
411 The size of a compressed dump or a minidump is not a fixed function of RAM
413 Therefore, when at least one of these options is enabled, the
415 utility cannot verify that the
417 has sufficient space for a dump.
419 is also unable to verify that a configured
421 server has sufficient space for a dump.
424 requires a kernel compiled with the
433 Netdump only supports IPv4 at this time.
434 .Sh SECURITY CONSIDERATIONS
435 The current encrypted kernel core dump scheme does not provide integrity nor
437 That is, the recipient of an encrypted kernel core dump cannot know if they
438 received an intact core dump, nor can they verify the provenance of the dump.
440 RSA keys smaller than 1024 bits are practical to factor and therefore weak.
441 Even 1024 bit keys may not be large enough to ensure privacy for many
442 years, so NIST recommends a minimum of 2048 bit RSA keys.
445 prevents users from configuring encrypted kernel dumps with extremely weak RSA
447 If you do not care for cryptographic privacy guarantees, just use
453 This process is sandboxed using