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machine-id: be nice and generate compliant v4 UUIDs 

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");