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machine-id: be nice and generate compliant v4 UUIDs

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Newly generated machine IDs now qualify as randomized v4 UUIds. This is
trivial to do and hopefully increases adoption of the ID for various
purposes.
This commit is contained in:
Lennart Poettering 2011-07-25 19:31:07 +02:00
parent fb922d4f82
commit 8d41a963d6
2 changed files with 67 additions and 14 deletions
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43
man/machine-id.xml
View File

@ -55,10 +55,12 @@
<title>Description</title>
<para>The <filename>/etc/machine-id</filename> file
configures the unique machine id of the local system
that is set during installation. It should contain a
single newline-terminated, hexadecimal, lowercase 16
character machine ID string.</para>
contains the unique machine id of the local system
that is set during installation. The machine ID is a
single newline-terminated, hexadecimal, lowercase 32
character machine ID string. (When decoded from
hexadecimal this corresponds with a 16 byte/128 bit
string.)</para>
<para>The machine ID is usually generated from a
random source during system installation and stays
@ -69,7 +71,7 @@
<para>The machine ID does not change based on user
configuration, or when hardware is replaced.</para>
<para>This machine id follows the same format and
<para>This machine ID adheres to the same format and
logic as the D-Bus machine ID.</para>
<para>Programs may use this ID to identify the host
@ -81,6 +83,35 @@
call POSIX specifies.</para>
</refsect1>
<refsect1>
<title>Relation to OSF UUIDs</title>
<para>Note that the machine ID historically is not an
OSF UUID as defined by <ulink
url="http://tools.ietf.org/html/rfc4122">RFC
4122</ulink>, nor a Microsoft GUID. Starting with
systemd v30 newly generated machine IDs however do
qualify as v4 UUIDs.</para>
<para>In order to maintain compatibility with existing
installations, an application requiring a UUID should
decode the machine ID, and then apply the following
operations to turn it into a valid OSF v4 UUID. With
<literal>id</literal> being an unsigned character
array:</para>
<programlisting>/* Set UUID version to 4 --- truly random generation */
id[6] = (id[6] &amp; 0x0F) | 0x40;
/* Set the UUID variant to DCE */
id[8] = (id[8] &amp; 0x3F) | 0x80;</programlisting>
<para>(This code is inspired by
<literal>generate_random_uuid()</literal> of
<filename>drivers/char/random.c</filename> from the
kernel sources.)</para>
</refsect1>
<refsect1>
<title>History</title>
@ -88,7 +119,7 @@
<filename>/etc/machine-id</filename> originates in the
<filename>/var/lib/dbus/machine-id</filename> file
introduced by D-Bus. In fact this latter file might be a
symlink to the
symlink to
<varname>/etc/machine-id</varname>.</para>
</refsect1>

38
src/machine-id-setup.c
View File

@ -32,16 +32,28 @@
#include "util.h"
#include "log.h"
static void make_v4_uuid(unsigned char *id) {
/* Stolen from generate_random_uuid() of drivers/char/random.c
* in the kernel sources */
/* Set UUID version to 4 --- truly random generation */
id[6] = (id[6] & 0x0F) | 0x40;
/* Set the UUID variant to DCE */
id[8] = (id[8] & 0x3F) | 0x80;
}
static int generate(char id[34]) {
int fd;
char buf[16];
char *p, *q;
unsigned char buf[16], *p;
char *q;
ssize_t k;
assert(id);
/* First, try reading the D-Bus machine id, unless it is a symlink */
if ((fd = open("/var/lib/dbus/machine-id", O_RDONLY|O_CLOEXEC|O_NOCTTY|O_NOFOLLOW)) >= 0) {
fd = open("/var/lib/dbus/machine-id", O_RDONLY|O_CLOEXEC|O_NOCTTY|O_NOFOLLOW);
if (fd >= 0) {
k = loop_read(fd, id, 33, false);
close_nointr_nofail(fd);
@ -56,7 +68,8 @@ static int generate(char id[34]) {
}
/* If that didn't work, generate a random machine id */
if ((fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY)) < 0) {
fd = open("/dev/urandom", O_RDONLY|O_CLOEXEC|O_NOCTTY);
if (fd < 0) {
log_error("Failed to open /dev/urandom: %m");
return -errno;
}
@ -69,6 +82,11 @@ static int generate(char id[34]) {
return k < 0 ? (int) k : -EIO;
}
/* Turn this into a valid v4 UUID, to be nice. Note that we
* only guarantee this for newly generated UUIDs, not for
* pre-existing ones.*/
make_v4_uuid(buf);
for (p = buf, q = id; p < buf + sizeof(buf); p++, q += 2) {
q[0] = hexchar(*p >> 4);
q[1] = hexchar(*p & 15);
@ -96,10 +114,12 @@ int machine_id_setup(void) {
* will be owned by root it doesn't matter much, but maybe
* people look. */
if ((fd = open("/etc/machine-id", O_RDWR|O_CREAT|O_CLOEXEC|O_NOCTTY, 0444)) >= 0)
fd = open("/etc/machine-id", O_RDWR|O_CREAT|O_CLOEXEC|O_NOCTTY, 0444);
if (fd >= 0)
writable = true;
else {
if ((fd = open("/etc/machine-id", O_RDONLY|O_CLOEXEC|O_NOCTTY)) < 0) {
fd = open("/etc/machine-id", O_RDONLY|O_CLOEXEC|O_NOCTTY);
if (fd < 0) {
umask(m);
log_error("Cannot open /etc/machine-id: %m");
return -errno;
@ -126,7 +146,8 @@ int machine_id_setup(void) {
/* Hmm, so, the id currently stored is not useful, then let's
* generate one */
if ((r = generate(id)) < 0)
r = generate(id);
if (r < 0)
goto finish;
if (S_ISREG(st.st_mode) && writable) {
@ -146,7 +167,8 @@ int machine_id_setup(void) {
mkdir_p("/run/systemd", 0755);
if ((r = write_one_line_file("/run/systemd/machine-id", id)) < 0) {
r = write_one_line_file("/run/systemd/machine-id", id);
if (r < 0) {
log_error("Cannot write /run/systemd/machine-id: %s", strerror(-r));
unlink("/run/systemd/machine-id");