On 03/09/2015 08:54 AM, Stéphane Aulery wrote:
> Le lundi 09 mars 2015 à 08:24:38, Michael Kerrisk (man-pages) a écrit :
>> Hello Stéphane,
>>
>> On 03/08/2015 01:03 PM, Stéphane Aulery wrote:
>>> Hi Michael,
>>>
>>> I made another patch with all your remarks.
>>
>> Did you send it already. (No hurry--it's just that I wonder if I missed it.)
>
> I was waiting your answers to the question bellow before.
>
>>>>> +.B RES_BLAST
>>>>> +When resolvers want to even out the load on their nameservers, following
>>>>> +them.
>>>>
>>>> I do not understand that last sentence. Could you clarify please.
>>>
>>> I must admit that it's not clear to me either. I think this means that
>>> server alias are swapped between each request. I have wrong? The word
>>> "blast" leaves me skeptical.
>>>
>>> What I found in code source:
>>>
>>> >From resolv.h:
>>>
>>> #define RES_BLAST 0x00020000 /* blast all recursive servers */
>>>
>>> Form res_send.c
>>>
>>> /*
>>> * Some resolvers want to even out the load on their nameservers.
>>> * Note that RES_BLAST overrides RES_ROTATE.
>>> */
>>> if (__builtin_expect ((statp->options & RES_ROTATE) != 0, 0) &&
>>> (statp->options & RES_BLAST) == 0) {
>>> struct sockaddr_in6 *ina;
>>> unsigned int map;
>>>
>>> n = 0;
>>> while (n < MAXNS && EXT(statp).nsmap[n] == MAXNS)
>>> n++;
>>> if (n < MAXNS) {
>>> ina = EXT(statp).nsaddrs[n];
>>> map = EXT(statp).nsmap[n];
>>> for (;;) {
>>> ns = n + 1;
>>> while (ns < MAXNS
>>> && EXT(statp).nsmap[ns] == MAXNS)
>>> ns++;
>>> if (ns == MAXNS)
>>> break;
>>> EXT(statp).nsaddrs[n] = EXT(statp).nsaddrs[ns];
>>> EXT(statp).nsmap[n] = EXT(statp).nsmap[ns];
>>> n = ns;
>>> }
>>> EXT(statp).nsaddrs[n] = ina;
>>> EXT(statp).nsmap[n] = map;
>>> }
>>> }
>>
>> Okay -- I leave it to your discretion to choose what to do here.
>
> No please, I need your help.
Sorry -- the code is not clear to me, and I don't have the time
to investigate deeper.
Google turns up this detail from the O'Reilly DNS and BIND
book:
"Blast" all recursive servers by sending queries to
the simultaneously. Not implemented yet.
The first sentence makes sense. I'm not sure whether
the second is true for glibc or not. But, I just grepped
the entire C source code for Fedora 20, and found no users
of RES_BLAST, so indeed maybe it is not implemented.
Thanks,
Michael
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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