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33 .Nd "specify a device for crash dumps"
65 utility is used to configure where the kernel can save a crash dump in the case
68 System administrators should typically configure
70 in a persistent fashion using the
76 For more information on this usage, see
82 can configure a series of fallback dump devices.
83 For example, an administrator may prefer
85 by default, but if the
87 service cannot be reached or some other failure occurs, they might choose a
88 local disk dump as a second choice option.
90 .Bl -tag -width _k_pubkey
92 Insert the specified dump configuration into the prioritized fallback dump
93 device list at the specified index, starting at zero.
97 is not specified, the configured dump device is appended to the prioritized
100 Remove the specified dump device configuration or configurations from the
101 fallback dump device list rather than inserting or appending it.
107 removes all configured devices.
111 Configure encrypted kernel dumps.
113 A random, one-time symmetric key is automatically generated for bulk kernel
114 dump encryption every time
119 is used to encrypt a copy of the symmetric key.
120 The encrypted dump contents consist of a standard dump header, the
121 pubkey-encrypted symmetric key contents, and the symmetric key encrypted core
124 As a result, only someone with the corresponding private key can decrypt the symmetric key.
125 The symmetric key is necessary to decrypt the kernel core.
126 The goal of the mechanism is to provide confidentiality.
130 file should be a PEM-formatted RSA key of at least 2048 bits.
132 Select the symmetric algorithm used for encrypted kernel crash dump.
138 (AES256-CBC mode does not work in conjunction with compression.)
140 List the currently configured dump device(s), or /dev/null if no devices are
145 Enable compression (Zstandard).
147 Enable compression (gzip).
148 Only one compression method may be enabled at a time, so
153 Zstandard provides superior compression ratio and performance.
157 may also configure the kernel to dump to a remote
162 server is available in ports.)
164 eliminates the need to reserve space for crash dumps.
165 It is especially useful in diskless environments.
168 is used to configure netdump, the
172 parameter should specify a network interface (e.g.,
174 The specified NIC must be up (online) to configure netdump.
177 specific options include:
178 .Bl -tag -width _g_gateway
180 The local IP address of the
184 The first-hop router between
190 option is not specified and the system has a default route, the default
191 router is used as the
196 option is not specified and the system does not have a default route,
198 is assumed to be on the same link as
201 The IP address of the
206 All of these options can be specified in the
211 The default type of kernel crash dump is the mini crash dump.
212 Mini crash dumps hold only memory pages in use by the kernel.
213 Alternatively, full memory dumps can be enabled by setting the
218 For systems using full memory dumps, the size of the specified dump
219 device must be at least the size of physical memory.
220 Even though an additional 64 kB header is added to the dump, the BIOS for a
221 platform typically holds back some memory, so it is not usually
222 necessary to size the dump device larger than the actual amount of RAM
223 available in the machine.
224 Also, when using full memory dumps, the
226 utility will refuse to enable a dump device which is smaller than the
227 total amount of physical memory as reported by the
231 .Sh IMPLEMENTATION NOTES
232 Because the file system layer is already dead by the time a crash dump
233 is taken, it is not possible to send crash dumps directly to a file.
239 may be used to enable early kernel core dumps for system panics which occur
240 before userspace starts.
242 In order to generate an RSA private key, a user can use the
246 .Dl # openssl genrsa -out private.pem 4096
248 A public key can be extracted from the private key using the
252 .Dl # openssl rsa -in private.pem -out public.pem -pubout
254 Once the RSA keys are created in a safe place, the public key may be moved to
255 the untrusted netdump client machine.
260 to configure encrypted kernel crash dumps:
262 .Dl # dumpon -k public.pem /dev/ada0s1b
264 It is recommended to test if the kernel saves encrypted crash dumps using the
265 current configuration.
266 The easiest way to do that is to cause a kernel panic using the
270 .Dl # sysctl debug.kdb.panic=1
272 In the debugger the following commands should be typed to write a core dump and
280 should be able to save the core dump in the
286 .Dl # savecore /dev/ada0s1b
288 Three files should be created in the core directory:
292 .Pa vmcore_encrypted.#
295 is the number of the last core dump saved by
298 .Pa vmcore_encrypted.#
299 can be decrypted using the
303 .Dl # decryptcore -p private.pem -k key.# -e vmcore_encrypted.# -c vmcore.#
307 .Dl # decryptcore -p private.pem -n #
311 can be now examined using
312 .Xr kgdb 1 Pq Pa ports/devel/gdb :
314 .Dl # kgdb /boot/kernel/kernel vmcore.#
320 The core was decrypted properly if
321 .Xr kgdb 1 Pq Pa ports/devel/gdb
322 does not print any errors.
323 Note that the live kernel might be at a different path
324 which can be examined by looking at the
331 script runs early during boot, typically before networking is configured.
332 This makes it unsuitable for configuring
334 when the client address is dynamic.
340 .Xr dhclient-script 8
343 For example, to automatically configure
345 on the vtnet0 interface, add the following to
346 .Pa /etc/dhclient-exit-hooks .
349 BOUND|REBIND|REBOOT|RENEW)
350 if [ "$interface" != vtnet0 ] || [ -n "$old_ip_address" -a \\
351 "$old_ip_address" = "$new_ip_address" ]; then
354 if [ -n "$new_routers" ]; then
355 # Take the first router in the list.
356 gateway_flag="-g ${new_routers%% *}"
358 # Configure as the highest-priority dump device.
359 dumpon -i 0 -c $new_ip_address -s $server $gateway_flag vtnet0
364 Be sure to fill in the server IP address and change the interface name if
368 .Xr kgdb 1 Pq Pa ports/devel/gdb ,
388 Support for encrypted kernel core dumps and netdump was added in
393 manual page was written by
394 .An Mark Johnston Aq Mt markj@FreeBSD.org ,
395 .An Conrad Meyer Aq Mt cem@FreeBSD.org ,
396 .An Konrad Witaszczyk Aq Mt def@FreeBSD.org ,
397 and countless others.
399 To configure encrypted kernel core dumps, the running kernel must have been
404 Netdump does not automatically update the configured
406 if routing topology changes.
408 The size of a compressed dump or a minidump is not a fixed function of RAM
410 Therefore, when at least one of these options is enabled, the
412 utility cannot verify that the
414 has sufficient space for a dump.
416 is also unable to verify that a configured
418 server has sufficient space for a dump.
421 requires a kernel compiled with the
430 Netdump only supports IPv4 at this time.
431 .Sh SECURITY CONSIDERATIONS
432 The current encrypted kernel core dump scheme does not provide integrity nor
434 That is, the recipient of an encrypted kernel core dump cannot know if they
435 received an intact core dump, nor can they verify the provenance of the dump.
437 RSA keys smaller than 1024 bits are practical to factor and therefore weak.
438 Even 1024 bit keys may not be large enough to ensure privacy for many
439 years, so NIST recommends a minimum of 2048 bit RSA keys.
442 prevents users from configuring encrypted kernel dumps with extremely weak RSA
444 If you do not care for cryptographic privacy guarantees, just use
450 This process is sandboxed using