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ITS#7027 Implement priority/weight for DNS SRV records

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From RFC 2782:

  A client MUST attempt to contact the target host with the
  lowest-numbered priority it can reach.

This patch sorts the DNS SRV records by their priority, and
additionally gives records with a larger weight a higher probability
of appearing earlier. This way, the DNS SRV records are tried in the
order of their priority.
This commit is contained in:
James M Leddy 2011-08-23 17:02:03 -04:00 committed by Howard Chu
parent 7e7ce79bd7
commit 5de85b922a
1 changed files with 75 additions and 17 deletions
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92
libraries/libldap/dnssrv.c
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@ -174,6 +174,46 @@ int ldap_domain2dn(
return LDAP_SUCCESS;
}
#ifdef HAVE_RES_QUERY
#define DNSBUFSIZ (64*1024)
typedef struct srv_record {
u_short priority;
u_short weight;
u_short port;
char hostname[DNSBUFSIZ];
} srv_record;
static int srv_cmp(const void *aa, const void *bb){
srv_record *a=(srv_record *)aa;
srv_record *b=(srv_record *)bb;
u_long total;
if(a->priority < b->priority) {
return -1;
}
if(a->priority > b->priority) {
return 1;
}
if(a->priority == b->priority){
/* targets with same priority are in psudeo random order */
if (a->weight == 0 && b->weight == 0) {
if (rand() % 2) {
return -1;
} else {
return 1;
}
}
total = a->weight + b->weight;
if (rand() % total < a->weight) {
return -1;
} else {
return 1;
}
}
}
#endif /* HAVE_RES_QUERY */
/*
* Lookup and return LDAP servers for domain (using the DNS
* SRV record _ldap._tcp.domain).
@ -183,15 +223,16 @@ int ldap_domain2hostlist(
char **list )
{
#ifdef HAVE_RES_QUERY
#define DNSBUFSIZ (64*1024)
char *request;
char *hostlist = NULL;
srv_record *hostent_head=NULL;
int i;
int rc, len, cur = 0;
unsigned char reply[DNSBUFSIZ];
int hostent_count=0;
assert( domain != NULL );
assert( list != NULL );
if( *domain == '\0' ) {
return LDAP_PARAM_ERROR;
}
@ -223,8 +264,7 @@ int ldap_domain2hostlist(
unsigned char *p;
char host[DNSBUFSIZ];
int status;
u_short port;
/* int priority, weight; */
u_short port, priority, weight;
/* Parse out query */
p = reply;
@ -263,36 +303,51 @@ int ldap_domain2hostlist(
size = (p[0] << 8) | p[1];
p += 2;
if (type == T_SRV) {
int buflen;
status = dn_expand(reply, reply + len, p + 6, host, sizeof(host));
if (status < 0) {
goto out;
}
/* ignore priority and weight for now */
/* priority = (p[0] << 8) | p[1]; */
/* weight = (p[2] << 8) | p[3]; */
/* Get priority weight and port */
priority = (p[0] << 8) | p[1];
weight = (p[2] << 8) | p[3];
port = (p[4] << 8) | p[5];
if ( port == 0 || host[ 0 ] == '\0' ) {
goto add_size;
}
buflen = strlen(host) + STRLENOF(":65355 ");
hostlist = (char *) LDAP_REALLOC(hostlist, cur + buflen + 1);
if (hostlist == NULL) {
rc = LDAP_NO_MEMORY;
hostent_head = (srv_record *) LDAP_REALLOC(hostent_head, (hostent_count+1)*(sizeof(srv_record)));
if(hostent_head==NULL){
rc=LDAP_NO_MEMORY;
goto out;
}
if (cur > 0) {
/* not first time around */
hostlist[cur++] = ' ';
}
cur += sprintf(&hostlist[cur], "%s:%hu", host, port);
hostent_head[hostent_count].priority=priority;
hostent_head[hostent_count].weight=priority;
hostent_head[hostent_count].port=port;
strncpy(hostent_head[hostent_count].hostname, host,255);
hostent_count=hostent_count+1;
}
add_size:;
p += size;
}
}
qsort(hostent_head, hostent_count, sizeof(srv_record), srv_cmp);
for(i=0; i<hostent_count; i++){
int buflen;
buflen = strlen(hostent_head[i].hostname) + STRLENOF(":65355" );
hostlist = (char *) LDAP_REALLOC(hostlist, cur+buflen+1);
if (hostlist == NULL) {
rc = LDAP_NO_MEMORY;
goto out;
}
if(cur>0){
hostlist[cur++]=' ';
}
cur += sprintf(&hostlist[cur], "%s:%hd", hostent_head[i].hostname, hostent_head[i].port);
}
if (hostlist == NULL) {
/* No LDAP servers found in DNS. */
rc = LDAP_UNAVAILABLE;
@ -308,6 +363,9 @@ add_size:;
if (request != NULL) {
LDAP_FREE(request);
}
if (hostent_head != NULL) {
LDAP_FREE(hostent_head);
}
if (rc != LDAP_SUCCESS && hostlist != NULL) {
LDAP_FREE(hostlist);
}