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28 .\" From: @(#)swapon.8 8.1 (Berkeley) 6/5/93
36 .Nd "specify a device for crash dumps"
66 utility is used to configure where the kernel can save a crash dump in the case
69 System administrators should typically configure
71 in a persistent fashion using the
77 For more information on this usage, see
83 can configure a series of fallback dump devices.
84 For example, an administrator may prefer
86 by default, but if the
88 service cannot be reached or some other failure occurs, they might choose a
89 local disk dump as a second choice option.
91 .Bl -tag -width _k_pubkey
93 Insert the specified dump configuration into the prioritized fallback dump
94 device list at the specified index, starting at zero.
98 is not specified, the configured dump device is appended to the prioritized
101 Remove the specified dump device configuration or configurations from the
102 fallback dump device list rather than inserting or appending it.
108 removes all configured devices.
112 Configure encrypted kernel dumps.
114 A random, one-time symmetric key is automatically generated for bulk kernel
115 dump encryption every time
120 is used to encrypt a copy of the symmetric key.
121 The encrypted dump contents consist of a standard dump header, the
122 pubkey-encrypted symmetric key contents, and the symmetric key encrypted core
125 As a result, only someone with the corresponding private key can decrypt the symmetric key.
126 The symmetric key is necessary to decrypt the kernel core.
127 The goal of the mechanism is to provide confidentiality.
131 file should be a PEM-formatted RSA key of at least 1024 bits.
133 List the currently configured dump device(s), or /dev/null if no devices are
138 Enable compression (Zstandard).
140 Enable compression (gzip).
141 Only one compression method may be enabled at a time, so
146 Zstandard provides superior compression ratio and performance.
150 may also configure the kernel to dump to a remote
155 server is available in ports.)
157 eliminates the need to reserve space for crash dumps.
158 It is especially useful in diskless environments.
161 is used to configure netdump, the
165 parameter should specify a network interface (e.g.,
167 The specified NIC must be up (online) to configure netdump.
170 specific options include:
171 .Bl -tag -width _g_gateway
173 The local IP address of the
177 The first-hop router between
183 option is not specified and the system has a default route, the default
184 router is used as the
189 option is not specified and the system does not have a default route,
191 is assumed to be on the same link as
194 The IP address of the
199 All of these options can be specified in the
204 The default type of kernel crash dump is the mini crash dump.
205 Mini crash dumps hold only memory pages in use by the kernel.
206 Alternatively, full memory dumps can be enabled by setting the
211 For systems using full memory dumps, the size of the specified dump
212 device must be at least the size of physical memory.
213 Even though an additional 64 kB header is added to the dump, the BIOS for a
214 platform typically holds back some memory, so it is not usually
215 necessary to size the dump device larger than the actual amount of RAM
216 available in the machine.
217 Also, when using full memory dumps, the
219 utility will refuse to enable a dump device which is smaller than the
220 total amount of physical memory as reported by the
224 .Sh IMPLEMENTATION NOTES
225 Because the file system layer is already dead by the time a crash dump
226 is taken, it is not possible to send crash dumps directly to a file.
232 may be used to enable early kernel core dumps for system panics which occur
233 before userspace starts.
235 In order to generate an RSA private key, a user can use the
239 .Dl # openssl genrsa -out private.pem 4096
241 A public key can be extracted from the private key using the
245 .Dl # openssl rsa -in private.pem -out public.pem -pubout
247 Once the RSA keys are created in a safe place, the public key may be moved to
248 the untrusted netdump client machine.
253 to configure encrypted kernel crash dumps:
255 .Dl # dumpon -k public.pem /dev/ada0s1b
257 It is recommended to test if the kernel saves encrypted crash dumps using the
258 current configuration.
259 The easiest way to do that is to cause a kernel panic using the
263 .Dl # sysctl debug.kdb.panic=1
265 In the debugger the following commands should be typed to write a core dump and
268 .Dl db> call doadump(0)
273 should be able to save the core dump in the
279 .Dl # savecore /dev/ada0s1b
281 Three files should be created in the core directory:
285 .Pa vmcore_encrypted.#
288 is the number of the last core dump saved by
291 .Pa vmcore_encrypted.#
292 can be decrypted using the
296 .Dl # decryptcore -p private.pem -k key.# -e vmcore_encrypted.# -c vmcore.#
300 .Dl # decryptcore -p private.pem -n #
304 can be now examined using
307 .Dl # kgdb /boot/kernel/kernel vmcore.#
313 The core was decrypted properly if
315 does not print any errors.
316 Note that the live kernel might be at a different path
317 which can be examined by looking at the
342 Support for encrypted kernel core dumps and netdump was added in
347 manual page was written by
348 .An Mark Johnston Aq Mt markj@FreeBSD.org ,
349 .An Conrad Meyer Aq Mt cem@FreeBSD.org ,
350 .An Konrad Witaszczyk Aq Mt def@FreeBSD.org ,
351 and countless others.
353 To configure encrypted kernel core dumps, the running kernel must have been
358 Netdump does not automatically update the configured
360 if routing topology changes.
362 The size of a compressed dump or a minidump is not a fixed function of RAM
364 Therefore, when at least one of these options is enabled, the
366 utility cannot verify that the
368 has sufficient space for a dump.
370 is also unable to verify that a configured
372 server has sufficient space for a dump.
375 requires a kernel compiled with the
384 It is currently not possible to configure both compression and encryption.
385 The encrypted dump format assumes that the kernel dump size is a multiple
386 of the cipher block size, which may not be true when the dump is compressed.
388 Netdump only supports IPv4 at this time.
389 .Sh SECURITY CONSIDERATIONS
390 The current encrypted kernel core dump scheme does not provide integrity nor
392 That is, the recipient of an encrypted kernel core dump cannot know if they
393 received an intact core dump, nor can they verify the provenance of the dump.
395 RSA keys smaller than 1024 bits are practical to factor and therefore weak.
396 Even 1024 bit keys may not be large enough to ensure privacy for many
397 years, so NIST recommends a minimum of 2048 bit RSA keys.
400 prevents users from configuring encrypted kernel dumps with extremely weak RSA
402 If you do not care for cryptographic privacy guarantees, just use
408 This process is sandboxed using