Re: [PATCH] libnfsidmap: Query DNS for the the NFSv4 domain

[Steve Dickson <SteveD@redhat.com>]

On 08/14/2016 02:59 PM, Chuck Lever wrote:
> Hey Steve-
> 
> Sorry for the delay, and thanks for putting this together!
> 
> 
>> On Aug 11, 2016, at 5:26 PM, Steve Dickson <SteveD@redhat.com> wrote:
>>
>> In domain_from_dns(), when at the hostname is a FQHN
>> query the DNS server for the _nfsv4idmapdomain TXT
>> record. If the record exists, use that as the
>> NFSv4 domain.
>>
>> Note, this query will only happen if the domain name
>> is not set in the /etc/idmapd.conf
> 
> Is there a man page update that goes with this? The order in
> which the library searches for the domain name should be
> documented. idmapd.conf(5), maybe.
Yes... I should have made this an RFC patch since I just
wanted to get core out there for for comments... 
The man page needs to updated as well as the configure.ac.

> 
> Also, some indication of when a change to the TXT record can
> be observed by users could be mentioned.
I'm not sure what you are asking here... There is the TTL time
for TXT that is returned in the query but that is completely
controlled by the admin... 
 
> 
> I recall that the server-side ID mapper is a little different
> than the client side. Does this TXT record affect server
> idmapper behavior too? 
Yes rpc.idmapd and nfsidmap use the same code to resolve
the nfsv4 domain

> If so, when does a TXT record change take effect?
When the system admin changes it?? ;-) 

> 
> Each time nfsidmap is invoked, it will now fire off a DNS
> lookup. That might have performance implications, but of
> course the fix is to ensure the domain is set locally.
Yeah it would be nice if the resolver API cached results
but I don't think that happens and I don't think we want
to build in a cache in the libnfsidmap code...

> 
> Can that DNS lookup ever happen before the network is up?
I don't think so with systemd but if the network is down
there will be a lot other systems hung waiting on DNS
queries... 

steved.

> 
> 
>> Signed-off-by: Steve Dickson <steved@redhat.com>
>> ---
>> libnfsidmap.c | 104 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>> 1 file changed, 103 insertions(+), 1 deletion(-)
>>
>> diff --git a/libnfsidmap.c b/libnfsidmap.c
>> index 2db4d13..7b8c0ed 100644
>> --- a/libnfsidmap.c
>> +++ b/libnfsidmap.c
>> @@ -53,6 +53,10 @@
>> #include <stdarg.h>
>> #include <dlfcn.h>
>> #include <ctype.h>
>> +#include <resolv.h>
>> +#include <arpa/nameser.h>
>> +#include <arpa/nameser_compat.h>
>> +
>> #include "nfsidmap.h"
>> #include "nfsidmap_internal.h"
>> #include "cfg.h"
>> @@ -79,6 +83,11 @@ gid_t nobody_gid = (gid_t)-1;
>> #define IDMAPD_DEFAULT_DOMAIN "localdomain"
>> #endif
>>
>> +#ifndef NFS4DNSTXTREC
>> +#define NFS4DNSTXTREC "_nfsv4idmapdomain"
>> +#endif
>> +
>> +
>> /* Default logging fuction */
>> static void default_logger(const char *fmt, ...)
>> {
>> @@ -114,6 +123,93 @@ static int id_as_chars(char *name, uid_t *id)
>> 	return 1;
>> }
>>
>> +static int dns_txt_query(char *domain, char **nfs4domain)
>> +{
>> +	char *txtname = NFS4DNSTXTREC;
>> +	char *msg, *answ, *eom, *mptr; 
>> +	int len, status = -1;
>> +	HEADER *hdr;
>> +	
>> +	msg = calloc(1, NS_MAXMSG);
>> +	if (msg == NULL)
>> +		return -1;
>> +
>> +	answ = calloc(1, NS_MAXMSG);
>> +	if (answ == NULL) {
>> +		free(msg);
>> +		return -1;
>> +	}
>> +
>> +	if (res_init() < 0) {
>> +		IDMAP_LOG(2, ("libnfsidmap: res_init() failed for %s.%s: %s\n",
>> +			txtname, domain, hstrerror(h_errno)));
>> +		goto freemem;
>> +	}
>> +	len = res_querydomain(txtname, domain, C_IN, T_TXT, msg, NS_MAXMSG);
>> +	if (len < 0) {
>> +		IDMAP_LOG(2, ("libnfsidmap: res_querydomain() failed for %s.%s: %s\n",
>> +			txtname, domain, hstrerror(h_errno)));
>> +		goto freemem;
>> +	}
>> +	hdr = (HEADER *)msg;
>> +
>> +	/* See if there is an answer */
>> +	if (ntohs(hdr->ancount) < 1) {
>> +		IDMAP_LOG(2, ("libnfsidmap: No TXT record for %s.%s\n",
>> +			txtname, domain));
>> +		goto freemem;
>> +	}
>> +	/* find the EndOfMessage */
>> +	eom = msg + len;
>> +
>> +	/* skip header */
>> +	mptr = &msg[HFIXEDSZ];
>> +
>> +	/* skip name field in question section */
>> +	mptr += dn_skipname(mptr, eom) + QFIXEDSZ;
>> +
>> +	/* read in the question */
>> +	len = dn_expand(msg, eom, mptr, answ, NS_MAXDNAME);
>> +	if (len < 0) { /* does this really matter?? */
>> +		IDMAP_LOG(2, ("libnfsidmap: No question section for %s.%s: %s\n",
>> +			txtname, domain, hstrerror(h_errno)));
>> +		goto freemem;
>> +	}
>> +
>> +	/*
>> +	 * Now, dissect the answer section, Note: if there
>> +	 * are more than one answer only the first
>> +	 * one will be used. 
>> +	 */
>> +
>> +	/* skip passed the name field  */
>> +	mptr += dn_skipname(mptr, eom);
>> +	/* skip pass the type class and ttl fields */
>> +	mptr += 2 + 2 + 4;
>> +
>> +	/* make sure there is some data */
>> +	GETSHORT(len, mptr);
>> +	if (len < 0) {
>> +		IDMAP_LOG(2, ("libnfsidmap: No data in answer for %s.%s\n",
>> +			txtname, domain));
>> +		goto freemem;
>> +	}
>> +	/* get the lenght field */
>> +	len = (int)*mptr++;
>> +	/* copy the data */
>> +	memcpy(answ, mptr, len);
>> +	answ[len] = '\0';
>> +	
>> +	*nfs4domain = strdup(answ);
>> +	status = 0;
>> +
>> +freemem:
>> +	free(msg);
>> +	free(answ);
>> +
>> +	return (status);
>> +}
>> +
>> static int domain_from_dns(char **domain)
>> {
>> 	struct hostent *he;
>> @@ -125,7 +221,13 @@ static int domain_from_dns(char **domain)
>> 		return -1;
>> 	if ((c = strchr(he->h_name, '.')) == NULL || *++c == '\0')
>> 		return -1;
>> -	*domain = strdup(c);
>> +	/* 
>> +	 * Query DNS to see if the _nfsv4idmapdomain TXT record exists
>> +	 * If so use it... 
>> +	 */
>> +	if (dns_txt_query(c, domain) < 0)
>> +		*domain = strdup(c);
>> +
>> 	return 0;
>> }
>>
>> -- 
>> 2.7.4
>>
>> --
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> 
> --
> Chuck Lever
> 
> 
>