16.12.2013 06:29, Denys Vlasenko пишет:
> On Monday 09 December 2013 18:56, Michael Tokarev wrote:
>> There's no reason to call gethostbyname() on the value returned
>> by uname() when asked just for a short name of a host. This may
>> also be wrong, when uname is set to one value, but in /etc/hosts
>> (or elsewhere) the "canonical" name is different. This is often
>> the case for localhost entry in /etc/hosts:
>>
>> 127.0.0.1 localhost myname
>>
>> With this content of /etc/hosts, and uname being set to myname,
>> busybox hostname -s will return localhost, while regular
>> hostname utility returns myname.
>
> Can you send strace of regular hostname's run?
> I would like to see whether it does any name resolution...
Well. That's in fact *exactly* what I did here to find out why they
differ.
Here we go:
$ strace hostname
...
brk(0) = 0x232b000
brk(0x234c000) = 0x234c000
uname({sys="Linux", node="gandalf", ...}) = 0
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 2), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
0x7fdd63c9a000
write(1, "gandalf\n", 8gandalf
) = 8
exit_group(0) = ?
(it isn't interesting to see anything before the call to uname(2),
since it is the only way to get system name which might be processed
further using dns).
$ strace hostname -s
...
brk(0) = 0x8ca000
brk(0x8eb000) = 0x8eb000
uname({sys="Linux", node="gandalf", ...}) = 0
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 2), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
0x7f4f34853000
write(1, "gandalf\n", 8gandalf
) = 8
exit_group(0) = ?
With a "long" hostname:
# hostname gandalf.local
$ strace hostname -s
...
brk(0) = 0xe71000
brk(0xe92000) = 0xe92000
uname({sys="Linux", node="gandalf.local", ...}) = 0
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 2), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
0x7f9364ec4000
write(1, "gandalf\n", 8gandalf
) = 8
exit_group(0) = ?
$ strace hostname
...
brk(0) = 0x25b1000
brk(0x25d2000) = 0x25d2000
uname({sys="Linux", node="gandalf.local", ...}) = 0
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 2), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) =
0x7f2f185b8000
write(1, "gandalf.local\n", 14gandalf.local
) = 14
exit_group(0) = ?
In other words, without -d (domain) or -f (fqdn), hostname just
does the string operations against the result of uname(2).
However, with -d or -f added (with or withot -s), it calls
gethostbyname(), and strace shows this too (not provided, it
is quite large).
Thanks,
/mjt
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