Re: [CFR] reflect resolv.conf update to running application

From: Robert Watson (rwatson_at_FreeBSD.org)
Date: 08/21/05

  • Next message: Peter Jeremy: "Re: [CFR] reflect resolv.conf update to running application" 
    Date: Sun, 21 Aug 2005 00:37:56 +0100 (BST)
To: Hajimu UMEMOTO <ume@FreeBSD.org>

    On Sat, 20 Aug 2005, Hajimu UMEMOTO wrote:

    > Our resolver reads resolv.conf once, and never re-read it. Recent
    > OpenBSD changed to re-read resolv.conf when it is updated. I believe it
    > is useful specially for mobile environment. So, I made a patch for our
    > resolver. Please review it.

    Two concerns:

    (1) Has anyone characterized the significant of the cost of doing a stat()
         for every DNS lookup for a significant workload? Does it matter?
         Most performance-critical network paths don't do name lookups in order
         to prevent indefinite stalls in lookup, but doing, say, 1,000
         additional stats a second is not a small issue.

    (2) By reading the configuration file more frequently and more quickly
         after a change, we increase the chances of a race condition in which
         the resolve reads a partially written resolv.conf file during an
         update. Does this happen in practice? I've always been very leery of
         re-reading configuration files automatically based on a time-stamp, as
         updates to files are not atomic at all.

    Robert N M Watson

    >
    > Index: lib/libc/net/getaddrinfo.c
    > diff -u -p lib/libc/net/getaddrinfo.c.orig lib/libc/net/getaddrinfo.c
    > --- lib/libc/net/getaddrinfo.c.orig Sat May 21 22:46:37 2005
    > +++ lib/libc/net/getaddrinfo.c Sat May 21 23:20:27 2005
    > @@ -2443,7 +2443,7 @@ res_searchN(name, target)
    > int got_nodata = 0, got_servfail = 0, tried_as_is = 0;
    > char abuf[MAXDNAME];
    >
    > - if ((_res.options & RES_INIT) == 0 && res_init() == -1) {
    > + if (_res_init() == -1) {
    > h_errno = NETDB_INTERNAL;
    > return (-1);
    > }
    > Index: lib/libc/net/gethostbydns.c
    > diff -u -p lib/libc/net/gethostbydns.c.orig lib/libc/net/gethostbydns.c
    > --- lib/libc/net/gethostbydns.c.orig Sat May 21 22:46:37 2005
    > +++ lib/libc/net/gethostbydns.c Sat May 21 23:20:27 2005
    > @@ -538,10 +538,6 @@ _dns_gethostbyaddr(void *rval, void *cb_
    > he = va_arg(ap, struct hostent *);
    > hed = va_arg(ap, struct hostent_data *);
    >
    > - if ((_res.options & RES_INIT) == 0 && res_init() == -1) {
    > - h_errno = NETDB_INTERNAL;
    > - return NS_UNAVAIL;
    > - }
    > switch (af) {
    > case AF_INET:
    > (void) sprintf(qbuf, "%u.%u.%u.%u.in-addr.arpa",
    > Index: lib/libc/net/gethostnamadr.c
    > diff -u -p lib/libc/net/gethostnamadr.c.orig lib/libc/net/gethostnamadr.c
    > --- lib/libc/net/gethostnamadr.c.orig Sat May 21 22:46:37 2005
    > +++ lib/libc/net/gethostnamadr.c Sat May 21 23:20:27 2005
    > @@ -44,6 +44,7 @@ __FBSDID("$FreeBSD: src/lib/libc/net/get
    > #include <resolv.h> /* XXX hack for _res */
    > #include "un-namespace.h"
    > #include "netdb_private.h"
    > +#include "res_config.h"
    >
    > extern int _ht_gethostbyname(void *, void *, va_list);
    > extern int _dns_gethostbyname(void *, void *, va_list);
    > @@ -264,7 +265,7 @@ gethostbyaddr_r(const char *addr, int le
    > { 0 }
    > };
    >
    > - if ((_res.options & RES_INIT) == 0 && res_init() == -1) {
    > + if (_res_init() == -1) {
    > h_errno = NETDB_INTERNAL;
    > return -1;
    > }
    > Index: lib/libc/net/getnetbydns.c
    > diff -u -p lib/libc/net/getnetbydns.c.orig lib/libc/net/getnetbydns.c
    > --- lib/libc/net/getnetbydns.c.orig Sat May 21 22:46:37 2005
    > +++ lib/libc/net/getnetbydns.c Sat May 21 23:20:27 2005
    > @@ -304,6 +304,10 @@ _dns_getnetbyaddr(void *rval, void *cb_d
    > netbr[1], netbr[0]);
    > break;
    > }
    > + if (_res_init() == -1) {
    > + h_errno = NETDB_INTERNAL;
    > + return NS_UNAVAIL;
    > + }
    > if ((buf = malloc(sizeof(*buf))) == NULL) {
    > h_errno = NETDB_INTERNAL;
    > return NS_NOTFOUND;
    > Index: lib/libc/net/name6.c
    > diff -u -p lib/libc/net/name6.c.orig lib/libc/net/name6.c
    > --- lib/libc/net/name6.c.orig Sat May 21 22:46:38 2005
    > +++ lib/libc/net/name6.c Sat May 21 23:20:27 2005
    > @@ -121,6 +121,7 @@ __FBSDID("$FreeBSD: src/lib/libc/net/nam
    > #include <unistd.h>
    > #include "un-namespace.h"
    > #include "netdb_private.h"
    > +#include "res_config.h"
    >
    > #ifndef _PATH_HOSTS
    > #define _PATH_HOSTS "/etc/hosts"
    > @@ -1810,11 +1811,9 @@ _dns_ghbyaddr(void *rval, void *cb_data,
    > return NS_NOTFOUND;
    > }
    >
    > - if ((_res.options & RES_INIT) == 0) {
    > - if (res_init() < 0) {
    > - *errp = h_errno;
    > - return NS_UNAVAIL;
    > - }
    > + if (_res_init() < 0) {
    > + *errp = h_errno;
    > + return NS_UNAVAIL;
    > }
    > memset(&hbuf, 0, sizeof(hbuf));
    > hbuf.h_name = NULL;
    > Index: lib/libc/net/res_config.h
    > diff -u lib/libc/net/res_config.h.orig lib/libc/net/res_config.h
    > --- lib/libc/net/res_config.h.orig Sat Mar 23 08:41:54 2002
    > +++ lib/libc/net/res_config.h Sat May 21 23:20:27 2005
    > @@ -8,3 +8,5 @@
    > #define MULTI_PTRS_ARE_ALIASES 1 /* fold multiple PTR records into aliases */
    > #define CHECK_SRVR_ADDR 1 /* confirm that the server requested sent the reply */
    > #define BIND_UPDATE 1 /* update support */
    > +
    > +int _res_init(void);
    > Index: lib/libc/net/res_init.c
    > diff -u -p lib/libc/net/res_init.c.orig lib/libc/net/res_init.c
    > --- lib/libc/net/res_init.c.orig Thu Feb 26 06:03:45 2004
    > +++ lib/libc/net/res_init.c Sat May 21 23:20:27 2005
    > @@ -78,11 +78,13 @@ __FBSDID("$FreeBSD: src/lib/libc/net/res
    > #include <sys/types.h>
    > #include <sys/param.h>
    > #include <sys/socket.h>
    > +#include <sys/stat.h>
    > #include <sys/time.h>
    > #include <netinet/in.h>
    > #include <arpa/inet.h>
    > #include <arpa/nameser.h>
    > #include <ctype.h>
    > +#include <errno.h>
    > #include <limits.h>
    > #include <resolv.h>
    > #include <stdio.h>
    > @@ -100,6 +102,7 @@ __FBSDID("$FreeBSD: src/lib/libc/net/res
    > #undef h_errno
    > extern int h_errno;
    >
    > +static void res_readconf(void);
    > static void res_setoptions(char *, char *);
    >
    > #ifdef RESOLVSORT
    > @@ -112,6 +115,18 @@ static u_int32_t net_mask(struct in_addr
    > # define isascii(c) (!(c & 0200))
    > #endif
    >
    > +#define timespecclear(tvp) ((tvp)->tv_sec = (tvp)->tv_nsec = 0)
    > +#define timespeccmp(tvp, uvp, cmp) \
    > + (((tvp)->tv_sec == (uvp)->tv_sec) ? \
    > + ((tvp)->tv_nsec cmp (uvp)->tv_nsec) : \
    > + ((tvp)->tv_sec cmp (uvp)->tv_sec))
    > +
    > +struct __res_conf_private {
    > + struct timespec mtimespec;
    > +};
    > +
    > +static struct __res_conf_private *___res_conf_private(void);
    > +
    > /*
    > * Check structure for failed per-thread allocations.
    > */
    > @@ -119,6 +134,7 @@ static struct res_per_thread {
    > struct __res_state res_state;
    > struct __res_state_ext res_state_ext;
    > struct __res_send_private res_send_private;
    > + struct __res_conf_private res_conf_private;
    > int h_errno;
    > } _res_per_thread_bogus = { .res_send_private = { .s = -1 } }; /* socket */
    >
    > @@ -146,21 +162,8 @@ static struct res_per_thread {
    > int
    > res_init()
    > {
    > - FILE *fp;
    > struct __res_send_private *rsp;
    > - char *cp, **pp;
    > - int n;
    > - char buf[MAXDNAME];
    > - int nserv = 0; /* number of nameserver records read from file */
    > - int haveenv = 0;
    > - int havesearch = 0;
    > -#ifdef RESOLVSORT
    > - int nsort = 0;
    > - char *net;
    > -#endif
    > -#ifndef RFC1535
    > - int dots;
    > -#endif
    > + char *cp;
    >
    > /*
    > * If allocation of memory for this thread's resolver has failed,
    > @@ -208,6 +211,37 @@ res_init()
    > if (!_res.id)
    > _res.id = res_randomid();
    >
    > + _res.pfcode = 0;
    > + res_readconf();
    > +
    > + if (issetugid())
    > + _res.options |= RES_NOALIASES;
    > + else if ((cp = getenv("RES_OPTIONS")) != NULL)
    > + res_setoptions(cp, "env");
    > + _res.options |= RES_INIT;
    > + return (0);
    > +}
    > +
    > +static void
    > +res_readconf()
    > +{
    > + struct __res_conf_private *rcp;
    > + FILE *fp;
    > + char *cp, **pp;
    > + int n;
    > + char buf[MAXDNAME];
    > + int nserv = 0; /* number of nameserver records read from file */
    > + int haveenv = 0;
    > + int havesearch = 0;
    > +#ifdef RESOLVSORT
    > + int nsort = 0;
    > + char *net;
    > +#endif
    > +#ifndef RFC1535
    > + int dots;
    > +#endif
    > + struct stat sb;
    > +
    > #ifdef USELOOPBACK
    > _res.nsaddr.sin_addr = inet_makeaddr(IN_LOOPBACKNET, 1);
    > #else
    > @@ -220,7 +254,6 @@ res_init()
    > memcpy(&_res_ext.nsaddr, &_res.nsaddr, _res.nsaddr.sin_len);
    > _res.nscount = 1;
    > _res.ndots = 1;
    > - _res.pfcode = 0;
    >
    > /* Allow user to override the local domain definition */
    > if (issetugid() == 0 && (cp = getenv("LOCALDOMAIN")) != NULL) {
    > @@ -262,7 +295,10 @@ res_init()
    > (line[sizeof(name) - 1] == ' ' || \
    > line[sizeof(name) - 1] == '\t'))
    >
    > + rcp = ___res_conf_private();
    > if ((fp = fopen(_PATH_RESCONF, "r")) != NULL) {
    > + if (fstat(fileno(fp), &sb) == 0)
    > + rcp->mtimespec = sb.st_mtimespec;
    > /* read the config file */
    > while (fgets(buf, sizeof(buf), fp) != NULL) {
    > /* skip comments */
    > @@ -396,11 +432,11 @@ res_init()
    > if (inet_aton(net, &a)) {
    > _res.sort_list[nsort].mask = a.s_addr;
    > } else {
    > - _res.sort_list[nsort].mask =
    > + _res.sort_list[nsort].mask =
    > net_mask(_res.sort_list[nsort].addr);
    > }
    > } else {
    > - _res.sort_list[nsort].mask =
    > + _res.sort_list[nsort].mask =
    > net_mask(_res.sort_list[nsort].addr);
    > }
    > _res_ext.sort_list[nsort].af = AF_INET;
    > @@ -465,13 +501,14 @@ res_init()
    > continue;
    > }
    > }
    > - if (nserv > 1)
    > + if (nserv > 1)
    > _res.nscount = nserv;
    > #ifdef RESOLVSORT
    > _res.nsort = nsort;
    > #endif
    > (void) fclose(fp);
    > - }
    > + } else
    > + timespecclear(&rcp->mtimespec);
    > if (_res.defdname[0] == 0 &&
    > gethostname(buf, sizeof(_res.defdname) - 1) == 0 &&
    > (cp = strchr(buf, '.')) != NULL)
    > @@ -507,12 +544,27 @@ res_init()
    > #endif
    > #endif /* !RFC1535 */
    > }
    > +}
    >
    > - if (issetugid())
    > - _res.options |= RES_NOALIASES;
    > - else if ((cp = getenv("RES_OPTIONS")) != NULL)
    > - res_setoptions(cp, "env");
    > - _res.options |= RES_INIT;
    > +int
    > +_res_init(void)
    > +{
    > + struct __res_conf_private *rcp;
    > + struct stat sb;
    > +
    > + if ((_res.options & RES_INIT) == 0)
    > + return (res_init());
    > +
    > + if (stat(_PATH_RESCONF, &sb) == -1) {
    > + /*
    > + * Lost the file, in chroot?
    > + * Don' trash settings
    > + */
    > + return (0);
    > + }
    > + rcp = ___res_conf_private();
    > + if (timespeccmp(&sb.st_mtimespec, &rcp->mtimespec, !=))
    > + res_readconf();
    > return (0);
    > }
    >
    > @@ -629,6 +681,7 @@ struct __res_state _res;
    > #endif
    > struct __res_state_ext _res_ext;
    > static struct __res_send_private _res_send_private = { .s = -1 }; /* socket */
    > +static struct __res_conf_private _res_conf_private;
    >
    > static thread_key_t res_key;
    > static once_t res_init_once = ONCE_INITIALIZER;
    > @@ -697,6 +750,14 @@ ___res_send_private(void)
    > if (thr_main() != 0)
    > return (&_res_send_private);
    > return (&allocate_res()->res_send_private);
    > +}
    > +
    > +struct __res_conf_private *
    > +___res_conf_private(void)
    > +{
    > + if (thr_main() != 0)
    > + return (&_res_conf_private);
    > + return (&allocate_res()->res_conf_private);
    > }
    >
    > int *
    > Index: lib/libc/net/res_query.c
    > diff -u -p lib/libc/net/res_query.c.orig lib/libc/net/res_query.c
    > --- lib/libc/net/res_query.c.orig Sat May 21 22:46:39 2005
    > +++ lib/libc/net/res_query.c Sat May 21 23:20:27 2005
    > @@ -200,7 +200,7 @@ res_search(name, class, type, answer, an
    > int trailing_dot, ret, saved_herrno;
    > int got_nodata = 0, got_servfail = 0, tried_as_is = 0;
    >
    > - if ((_res.options & RES_INIT) == 0 && res_init() == -1) {
    > + if (_res_init() == -1) {
    > h_errno = NETDB_INTERNAL;
    > return (-1);
    > }
    > Index: lib/libc/net/res_update.c
    > diff -u -p lib/libc/net/res_update.c.orig lib/libc/net/res_update.c
    > --- lib/libc/net/res_update.c.orig Mon Sep 16 01:51:09 2002
    > +++ lib/libc/net/res_update.c Sat May 21 23:20:27 2005
    > @@ -36,6 +36,8 @@ __FBSDID("$FreeBSD: src/lib/libc/net/res
    > #include <stdlib.h>
    > #include <string.h>
    >
    > +#include "res_config.h"
    > +
    > /*
    > * Separate a linked list of records into groups so that all records
    > * in a group will belong to a single zone on the nameserver.
    > @@ -84,7 +86,7 @@ res_update(ns_updrec *rrecp_in) {
    > u_int16_t dlen, class, qclass, type, qtype;
    > u_int32_t ttl;
    >
    > - if ((_res.options & RES_INIT) == 0 && res_init() == -1) {
    > + if (_res_init() == -1) {
    > h_errno = NETDB_INTERNAL;
    > return (-1);
    > }
    >
    >
    > Sincerely,
    >
    > --
    > Hajimu UMEMOTO @ Internet Mutual Aid Society Yokohama, Japan
    > ume@mahoroba.org ume@{,jp.}FreeBSD.org
    > http://www.imasy.org/~ume/
    > _______________________________________________
    > freebsd-arch@freebsd.org mailing list
    > http://lists.freebsd.org/mailman/listinfo/freebsd-arch
    > To unsubscribe, send any mail to "freebsd-arch-unsubscribe@freebsd.org"
    >
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