core/ngx_resolver.c源文件分析
本章我们主要介绍一下ngx_resolver.c源文件，其主要是实现对nginx中涉及到的域名的解析。
1. ngx_resolver_hdr_t数据结构
/*
* Copyright (C) Igor Sysoev
* Copyright (C) Nginx, Inc.
*/
#include <ngx_config.h>
#include <ngx_core.h>
#include <ngx_event.h>
#define NGX_RESOLVER_UDP_SIZE 4096
#define NGX_RESOLVER_TCP_RSIZE (2 + 65535)
#define NGX_RESOLVER_TCP_WSIZE 8192
typedef struct {
//identifier 字段
u_char ident_hi;
u_char ident_lo;
u_char flags_hi;
u_char flags_lo;
//查询问题的个数
u_char nqs_hi;
u_char nqs_lo;
//'answer section' 段的应答资源记录数
u_char nan_hi;
u_char nan_lo;
// 'authority records section' 段授权资源记录数
u_char nns_hi;
u_char nns_lo;
//'additional records section'段的附加资源记录数
u_char nar_hi;
u_char nar_lo;
} ngx_resolver_hdr_t;ngx_resolver_hdr_t定义了DNS报文结构的头部。关于各字段的含义，请参看DNS协议详解及报文格式分析
2. ngx_resolver_qs_t数据结构
typedef struct {
u_char type_hi;
u_char type_lo;
u_char class_hi;
u_char class_lo;
} ngx_resolver_qs_t;此结构定义了DNS查询报文中的Queries区域。其中type用于指明查询类型； class用于指明查询类。
3. ngx_resolver_an_t数据结构
typedef struct {
u_char type_hi;
u_char type_lo;
u_char class_hi;
u_char class_lo;
u_char ttl[4];
u_char len_hi;
u_char len_lo;
} ngx_resolver_an_t;此结构定义了DNS应答报文中的Answers区域
4. 相关静态函数声明
#define ngx_resolver_node(n) \
(ngx_resolver_node_t *) \
((u_char *) (n) - offsetof(ngx_resolver_node_t, node))
ngx_int_t ngx_udp_connect(ngx_resolver_connection_t *rec);
ngx_int_t ngx_tcp_connect(ngx_resolver_connection_t *rec);
static void ngx_resolver_cleanup(void *data);
static void ngx_resolver_cleanup_tree(ngx_resolver_t *r, ngx_rbtree_t *tree);
static ngx_int_t ngx_resolve_name_locked(ngx_resolver_t *r,
ngx_resolver_ctx_t *ctx, ngx_str_t *name);
static void ngx_resolver_expire(ngx_resolver_t *r, ngx_rbtree_t *tree,
ngx_queue_t *queue);
static ngx_int_t ngx_resolver_send_query(ngx_resolver_t *r,
ngx_resolver_node_t *rn);
static ngx_int_t ngx_resolver_send_udp_query(ngx_resolver_t *r,
ngx_resolver_connection_t *rec, u_char *query, u_short qlen);
static ngx_int_t ngx_resolver_send_tcp_query(ngx_resolver_t *r,
ngx_resolver_connection_t *rec, u_char *query, u_short qlen);
static ngx_int_t ngx_resolver_create_name_query(ngx_resolver_t *r,
ngx_resolver_node_t *rn, ngx_str_t *name);
//创建DNS服务查询报文
static ngx_int_t ngx_resolver_create_srv_query(ngx_resolver_t *r,
ngx_resolver_node_t *rn, ngx_str_t *name);
static ngx_int_t ngx_resolver_create_addr_query(ngx_resolver_t *r,
ngx_resolver_node_t *rn, ngx_resolver_addr_t *addr);
static void ngx_resolver_resend_handler(ngx_event_t *ev);
static time_t ngx_resolver_resend(ngx_resolver_t *r, ngx_rbtree_t *tree,
ngx_queue_t *queue);
static ngx_uint_t ngx_resolver_resend_empty(ngx_resolver_t *r);
static void ngx_resolver_udp_read(ngx_event_t *rev);
static void ngx_resolver_tcp_write(ngx_event_t *wev);
static void ngx_resolver_tcp_read(ngx_event_t *rev);
static void ngx_resolver_process_response(ngx_resolver_t *r, u_char *buf,
size_t n, ngx_uint_t tcp);
static void ngx_resolver_process_a(ngx_resolver_t *r, u_char *buf, size_t n,
ngx_uint_t ident, ngx_uint_t code, ngx_uint_t qtype,
ngx_uint_t nan, ngx_uint_t trunc, ngx_uint_t ans);
static void ngx_resolver_process_srv(ngx_resolver_t *r, u_char *buf, size_t n,
ngx_uint_t ident, ngx_uint_t code, ngx_uint_t nan,
ngx_uint_t trunc, ngx_uint_t ans);
static void ngx_resolver_process_ptr(ngx_resolver_t *r, u_char *buf, size_t n,
ngx_uint_t ident, ngx_uint_t code, ngx_uint_t nan);
static ngx_resolver_node_t *ngx_resolver_lookup_name(ngx_resolver_t *r,
ngx_str_t *name, uint32_t hash);
static ngx_resolver_node_t *ngx_resolver_lookup_srv(ngx_resolver_t *r,
ngx_str_t *name, uint32_t hash);
static ngx_resolver_node_t *ngx_resolver_lookup_addr(ngx_resolver_t *r,
in_addr_t addr);
static void ngx_resolver_rbtree_insert_value(ngx_rbtree_node_t *temp,
ngx_rbtree_node_t *node, ngx_rbtree_node_t *sentinel);
static ngx_int_t ngx_resolver_copy(ngx_resolver_t *r, ngx_str_t *name,
u_char *buf, u_char *src, u_char *last);
static void ngx_resolver_timeout_handler(ngx_event_t *ev);
static void ngx_resolver_free_node(ngx_resolver_t *r, ngx_resolver_node_t *rn);
static void *ngx_resolver_alloc(ngx_resolver_t *r, size_t size);
static void *ngx_resolver_calloc(ngx_resolver_t *r, size_t size);
static void ngx_resolver_free(ngx_resolver_t *r, void *p);
static void ngx_resolver_free_locked(ngx_resolver_t *r, void *p);
static void *ngx_resolver_dup(ngx_resolver_t *r, void *src, size_t size);
static ngx_resolver_addr_t *ngx_resolver_export(ngx_resolver_t *r,
ngx_resolver_node_t *rn, ngx_uint_t rotate);
static void ngx_resolver_report_srv(ngx_resolver_t *r, ngx_resolver_ctx_t *ctx);
static u_char *ngx_resolver_log_error(ngx_log_t *log, u_char *buf, size_t len);
static void ngx_resolver_resolve_srv_names(ngx_resolver_ctx_t *ctx,
ngx_resolver_node_t *rn);
static void ngx_resolver_srv_names_handler(ngx_resolver_ctx_t *ctx);
static ngx_int_t ngx_resolver_cmp_srvs(const void *one, const void *two);
#if (NGX_HAVE_INET6)
static void ngx_resolver_rbtree_insert_addr6_value(ngx_rbtree_node_t *temp,
ngx_rbtree_node_t *node, ngx_rbtree_node_t *sentinel);
static ngx_resolver_node_t *ngx_resolver_lookup_addr6(ngx_resolver_t *r,
struct in6_addr *addr, uint32_t hash);
#endif5. 函数ngx_resolver_create()
ngx_resolver_t *
ngx_resolver_create(ngx_conf_t *cf, ngx_str_t *names, ngx_uint_t n)
{
ngx_str_t s;
ngx_url_t u;
ngx_uint_t i, j;
ngx_resolver_t *r;
ngx_pool_cleanup_t *cln;
ngx_resolver_connection_t *rec;
cln = ngx_pool_cleanup_add(cf->pool, 0);
if (cln == NULL) {
return NULL;
}
cln->handler = ngx_resolver_cleanup;
r = ngx_calloc(sizeof(ngx_resolver_t), cf->log);
if (r == NULL) {
return NULL;
}
cln->data = r;
r->event = ngx_calloc(sizeof(ngx_event_t), cf->log);
if (r->event == NULL) {
return NULL;
}
ngx_rbtree_init(&r->name_rbtree, &r->name_sentinel,
ngx_resolver_rbtree_insert_value);
ngx_rbtree_init(&r->srv_rbtree, &r->srv_sentinel,
ngx_resolver_rbtree_insert_value);
ngx_rbtree_init(&r->addr_rbtree, &r->addr_sentinel,
ngx_rbtree_insert_value);
ngx_queue_init(&r->name_resend_queue);
ngx_queue_init(&r->srv_resend_queue);
ngx_queue_init(&r->addr_resend_queue);
ngx_queue_init(&r->name_expire_queue);
ngx_queue_init(&r->srv_expire_queue);
ngx_queue_init(&r->addr_expire_queue);
#if (NGX_HAVE_INET6)
r->ipv6 = 1;
ngx_rbtree_init(&r->addr6_rbtree, &r->addr6_sentinel,
ngx_resolver_rbtree_insert_addr6_value);
ngx_queue_init(&r->addr6_resend_queue);
ngx_queue_init(&r->addr6_expire_queue);
#endif
r->event->handler = ngx_resolver_resend_handler;
r->event->data = r;
r->event->log = &cf->cycle->new_log;
r->ident = -1;
r->resend_timeout = 5;
r->tcp_timeout = 5;
r->expire = 30;
r->valid = 0;
r->log = &cf->cycle->new_log;
r->log_level = NGX_LOG_ERR;
if (n) {
if (ngx_array_init(&r->connections, cf->pool, n,
sizeof(ngx_resolver_connection_t))
!= NGX_OK)
{
return NULL;
}
}
for (i = 0; i < n; i++) {
if (ngx_strncmp(names[i].data, "valid=", 6) == 0) {
s.len = names[i].len - 6;
s.data = names[i].data + 6;
r->valid = ngx_parse_time(&s, 1);
if (r->valid == (time_t) NGX_ERROR) {
ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
"invalid parameter: %V", &names[i]);
return NULL;
}
continue;
}
#if (NGX_HAVE_INET6)
if (ngx_strncmp(names[i].data, "ipv6=", 5) == 0) {
if (ngx_strcmp(&names[i].data[5], "on") == 0) {
r->ipv6 = 1;
} else if (ngx_strcmp(&names[i].data[5], "off") == 0) {
r->ipv6 = 0;
} else {
ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
"invalid parameter: %V", &names[i]);
return NULL;
}
continue;
}
#endif
ngx_memzero(&u, sizeof(ngx_url_t));
u.url = names[i];
u.default_port = 53;
if (ngx_parse_url(cf->pool, &u) != NGX_OK) {
if (u.err) {
ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
"%s in resolver \"%V\"",
u.err, &u.url);
}
return NULL;
}
rec = ngx_array_push_n(&r->connections, u.naddrs);
if (rec == NULL) {
return NULL;
}
ngx_memzero(rec, u.naddrs * sizeof(ngx_resolver_connection_t));
for (j = 0; j < u.naddrs; j++) {
rec[j].sockaddr = u.addrs[j].sockaddr;
rec[j].socklen = u.addrs[j].socklen;
rec[j].server = u.addrs[j].name;
rec[j].resolver = r;
}
}
return r;
}本函数用于创建一个ngx_resolver_t对象。在介绍本函数的具体实现之前，我们先来大体看一下resolver的配置：
resolver 223.5.5.5 223.6.6.6 1.2.4.8 114.114.114.114 valid=3600s;
因此，如果解析到此resolver命令时调用ngx_resolver_create()函数，那么names将为后面所有以空格分隔的字符串。
下面介绍一下本函数的大体实现流程：
ngx_resolver_t *
ngx_resolver_create(ngx_conf_t *cf, ngx_str_t *names, ngx_uint_t n)
{
//1) 为要创建的ngx_resolver_t对象创建一个pool cleanup，这样后续该对象就可以随池的销毁而自动释放相应的空间。
//2) 创建ngx_resolver_t对象，并初始化该对象的event、name_rbtree、srv_rbtree、addr_rbtree等方面的属性
//3) 设置ngx_resolver_t对象event的事件处理器ngx_resolver_resend_handler，相应的TCP超时时间tcp_timeout，
// 以及对DNS结果的缓存时间valid
//4) 若n>0，则创建对应的connections对象数组，数组中的每一个元素表示与DNS服务器的连接（注意，对于一个DNS域名
// 可能会对应多个不同的IP地址)
}5. 函数ngx_resolver_cleanup()
static void
ngx_resolver_cleanup(void *data)
{
ngx_resolver_t *r = data;
ngx_uint_t i;
ngx_resolver_connection_t *rec;
if (r) {
ngx_log_debug0(NGX_LOG_DEBUG_CORE, ngx_cycle->log, 0,
"cleanup resolver");
ngx_resolver_cleanup_tree(r, &r->name_rbtree);
ngx_resolver_cleanup_tree(r, &r->srv_rbtree);
ngx_resolver_cleanup_tree(r, &r->addr_rbtree);
#if (NGX_HAVE_INET6)
ngx_resolver_cleanup_tree(r, &r->addr6_rbtree);
#endif
if (r->event) {
ngx_free(r->event);
}
rec = r->connections.elts;
for (i = 0; i < r->connections.nelts; i++) {
if (rec[i].udp) {
ngx_close_connection(rec[i].udp);
}
if (rec[i].tcp) {
ngx_close_connection(rec[i].tcp);
}
if (rec[i].read_buf) {
ngx_resolver_free(r, rec[i].read_buf->start);
ngx_resolver_free(r, rec[i].read_buf);
}
if (rec[i].write_buf) {
ngx_resolver_free(r, rec[i].write_buf->start);
ngx_resolver_free(r, rec[i].write_buf);
}
}
ngx_free(r);
}
}此函数是ngx_resolver_t对象的pool被销毁时的回调函数，主要完成相关内存资源的回收以及关闭对应的连接：
static void
ngx_resolver_cleanup(void *data)
{
//1) 回收相应的红黑树结构: name_rbtree、srv_rbtree、addr_rbtree、addr6_rbtree
//2) 释放ngx_resolver_t的event对象
//3) 关闭与DNS的连接，并释放连接相关的内存占用
//4) 释放ngx_resolver_t对象本身
}6. 函数ngx_resolver_cleanup_tree()
static void
ngx_resolver_cleanup_tree(ngx_resolver_t *r, ngx_rbtree_t *tree)
{
ngx_resolver_ctx_t *ctx, *next;
ngx_resolver_node_t *rn;
while (tree->root != tree->sentinel) {
rn = ngx_resolver_node(ngx_rbtree_min(tree->root, tree->sentinel));
ngx_queue_remove(&rn->queue);
for (ctx = rn->waiting; ctx; ctx = next) {
next = ctx->next;
if (ctx->event) {
ngx_resolver_free(r, ctx->event);
}
ngx_resolver_free(r, ctx);
}
ngx_rbtree_delete(tree, &rn->node);
ngx_resolver_free_node(r, rn);
}
}本函数按从小到大的顺序依次删除ngx_resolver_t中相应红黑树的节点。并将该节点所关联的rn->waiting链表中的上下文对象进行删除。
7. 函数ngx_resolve_start()
ngx_resolver_ctx_t *
ngx_resolve_start(ngx_resolver_t *r, ngx_resolver_ctx_t *temp)
{
in_addr_t addr;
ngx_resolver_ctx_t *ctx;
if (temp) {
addr = ngx_inet_addr(temp->name.data, temp->name.len);
if (addr != INADDR_NONE) {
temp->resolver = r;
temp->state = NGX_OK;
temp->naddrs = 1;
temp->addrs = &temp->addr;
temp->addr.sockaddr = (struct sockaddr *) &temp->sin;
temp->addr.socklen = sizeof(struct sockaddr_in);
ngx_memzero(&temp->sin, sizeof(struct sockaddr_in));
temp->sin.sin_family = AF_INET;
temp->sin.sin_addr.s_addr = addr;
temp->quick = 1;
return temp;
}
}
if (r->connections.nelts == 0) {
return NGX_NO_RESOLVER;
}
ctx = ngx_resolver_calloc(r, sizeof(ngx_resolver_ctx_t));
if (ctx) {
ctx->resolver = r;
}
return ctx;
}本函数用于创建ngx_resolver_t的上下文。如果传入的参数temp不为NULL，且temp->name能够被解析为一个IPv4地址，则复用temp，将其作为r的上下文对象； 否则新建一个新的上下文对象。
这里注意，当temp->name能够成功解析为IP地址时，会将temp->quick置为1，这样后续就不再需要请求DNS来解析了。
8. 函数ngx_resolve_name()
ngx_int_t
ngx_resolve_name(ngx_resolver_ctx_t *ctx)
{
size_t slen;
ngx_int_t rc;
ngx_str_t name;
ngx_resolver_t *r;
r = ctx->resolver;
if (ctx->name.len > 0 && ctx->name.data[ctx->name.len - 1] == '.') {
ctx->name.len--;
}
ngx_log_debug1(NGX_LOG_DEBUG_CORE, r->log, 0,
"resolve: \"%V\"", &ctx->name);
if (ctx->quick) {
ctx->handler(ctx);
return NGX_OK;
}
if (ctx->service.len) {
slen = ctx->service.len;
if (ngx_strlchr(ctx->service.data,
ctx->service.data + ctx->service.len, '.')
== NULL)
{
slen += sizeof("_._tcp") - 1;
}
name.len = slen + 1 + ctx->name.len;
name.data = ngx_resolver_alloc(r, name.len);
if (name.data == NULL) {
return NGX_ERROR;
}
if (slen == ctx->service.len) {
ngx_sprintf(name.data, "%V.%V", &ctx->service, &ctx->name);
} else {
ngx_sprintf(name.data, "_%V._tcp.%V", &ctx->service, &ctx->name);
}
/* lock name mutex */
rc = ngx_resolve_name_locked(r, ctx, &name);
ngx_resolver_free(r, name.data);
} else {
/* lock name mutex */
rc = ngx_resolve_name_locked(r, ctx, &ctx->name);
}
if (rc == NGX_OK) {
return NGX_OK;
}
/* unlock name mutex */
if (rc == NGX_AGAIN) {
return NGX_OK;
}
/* NGX_ERROR */
if (ctx->event) {
ngx_resolver_free(r, ctx->event);
}
ngx_resolver_free(r, ctx);
return NGX_ERROR;
}在介绍本函数之前，我们先大体说明一下DNS的服务查询(SRV)的格式：
_ldap._tcp.example.com
下面我们再来简单介绍本函数的实现：
ngx_int_t
ngx_resolve_name(ngx_resolver_ctx_t *ctx)
{
//1) 如果ctx->quick为1，那么直接回调handler()即可
//2) 若指定了ctx->service，表示的是查询某一个服务，这时需要做特定的处理，然后调用ngx_resolve_name_locked()来完成'服务名到IP的映射'
//3) 否则，直接调用ngx_resolve_name_locked()来完成'域名到IP的映射'
}9. 函数ngx_resolve_name_done()
void
ngx_resolve_name_done(ngx_resolver_ctx_t *ctx)
{
ngx_uint_t i;
ngx_resolver_t *r;
ngx_resolver_ctx_t *w, **p;
ngx_resolver_node_t *rn;
r = ctx->resolver;
ngx_log_debug1(NGX_LOG_DEBUG_CORE, r->log, 0,
"resolve name done: %i", ctx->state);
if (ctx->quick) {
return;
}
if (ctx->event && ctx->event->timer_set) {
ngx_del_timer(ctx->event);
}
/* lock name mutex */
if (ctx->nsrvs) {
for (i = 0; i < ctx->nsrvs; i++) {
if (ctx->srvs[i].ctx) {
ngx_resolve_name_done(ctx->srvs[i].ctx);
}
if (ctx->srvs[i].addrs) {
ngx_resolver_free(r, ctx->srvs[i].addrs->sockaddr);
ngx_resolver_free(r, ctx->srvs[i].addrs);
}
ngx_resolver_free(r, ctx->srvs[i].name.data);
}
ngx_resolver_free(r, ctx->srvs);
}
if (ctx->state == NGX_AGAIN || ctx->state == NGX_RESOLVE_TIMEDOUT) {
rn = ctx->node;
if (rn) {
p = &rn->waiting;
w = rn->waiting;
while (w) {
if (w == ctx) {
*p = w->next;
goto done;
}
p = &w->next;
w = w->next;
}
ngx_log_error(NGX_LOG_ALERT, r->log, 0,
"could not cancel %V resolving", &ctx->name);
}
}
done:
if (ctx->service.len) {
ngx_resolver_expire(r, &r->srv_rbtree, &r->srv_expire_queue);
} else {
ngx_resolver_expire(r, &r->name_rbtree, &r->name_expire_queue);
}
/* unlock name mutex */
/* lock alloc mutex */
if (ctx->event) {
ngx_resolver_free_locked(r, ctx->event);
}
ngx_resolver_free_locked(r, ctx);
/* unlock alloc mutex */
if (r->event->timer_set && ngx_resolver_resend_empty(r)) {
ngx_del_timer(r->event);
}
}此函数用于处理当解析完成（可能成功，也可能失败），进行相应的收尾工作。下面我们先简要介绍一下通过服务名来查询IP返回报文的基本格式：
_Service._Proto.Name TTL Class SRV Priority Weight Port Target
下面是本函数的基本流程：
void
ngx_resolve_name_done(ngx_resolver_ctx_t *ctx)
{
//1) 如果ctx->quick为真，此种情况根本没有向DNS发出解析请求，因此这里可以直接返回
//2) 如果ctx->event上仍还有定时器在运行，那么清除相应定时器
//3) 若是 SRV查询，即ctx->nsrvs>0，那么释放服务所占用的空间
//4) 如果状态为NGX_AGAIN或者NGX_RESOLVE_TIMEDOUT，那么将该上下文从waiting链表中移除
//5) 从超时队列中移除1~2个超时节点（这里注意到，一次只会移除少数几个超时节点，从而把整个超时处理过程分摊到
// 整个程序运行过程中，此种处理方法在很多程序设计中值得借鉴）
//6) 删除ctx->event
//7) 删除ctx本身
//8) 若对应ngx_resolver_t的resend_queue并没有重发任务时，则移除ctx对应的resolver上的事件定时器
}10. 函数ngx_resolve_name_locked()
static ngx_int_t
ngx_resolve_name_locked(ngx_resolver_t *r, ngx_resolver_ctx_t *ctx,
ngx_str_t *name)
{
uint32_t hash;
ngx_int_t rc;
ngx_str_t cname;
ngx_uint_t i, naddrs;
ngx_queue_t *resend_queue, *expire_queue;
ngx_rbtree_t *tree;
ngx_resolver_ctx_t *next, *last;
ngx_resolver_addr_t *addrs;
ngx_resolver_node_t *rn;
ngx_strlow(name->data, name->data, name->len);
hash = ngx_crc32_short(name->data, name->len);
if (ctx->service.len) {
rn = ngx_resolver_lookup_srv(r, name, hash);
tree = &r->srv_rbtree;
resend_queue = &r->srv_resend_queue;
expire_queue = &r->srv_expire_queue;
} else {
rn = ngx_resolver_lookup_name(r, name, hash);
tree = &r->name_rbtree;
resend_queue = &r->name_resend_queue;
expire_queue = &r->name_expire_queue;
}
if (rn) {
/* ctx can be a list after NGX_RESOLVE_CNAME */
for (last = ctx; last->next; last = last->next);
if (rn->valid >= ngx_time()) {
ngx_log_debug0(NGX_LOG_DEBUG_CORE, r->log, 0, "resolve cached");
ngx_queue_remove(&rn->queue);
rn->expire = ngx_time() + r->expire;
ngx_queue_insert_head(expire_queue, &rn->queue);
naddrs = (rn->naddrs == (u_short) -1) ? 0 : rn->naddrs;
#if (NGX_HAVE_INET6)
naddrs += (rn->naddrs6 == (u_short) -1) ? 0 : rn->naddrs6;
#endif
if (naddrs) {
if (naddrs == 1 && rn->naddrs == 1) {
addrs = NULL;
} else {
addrs = ngx_resolver_export(r, rn, 1);
if (addrs == NULL) {
return NGX_ERROR;
}
}
last->next = rn->waiting;
rn->waiting = NULL;
/* unlock name mutex */
do {
ctx->state = NGX_OK;
ctx->valid = rn->valid;
ctx->naddrs = naddrs;
if (addrs == NULL) {
ctx->addrs = &ctx->addr;
ctx->addr.sockaddr = (struct sockaddr *) &ctx->sin;
ctx->addr.socklen = sizeof(struct sockaddr_in);
ngx_memzero(&ctx->sin, sizeof(struct sockaddr_in));
ctx->sin.sin_family = AF_INET;
ctx->sin.sin_addr.s_addr = rn->u.addr;
} else {
ctx->addrs = addrs;
}
next = ctx->next;
ctx->handler(ctx);
ctx = next;
} while (ctx);
if (addrs != NULL) {
ngx_resolver_free(r, addrs->sockaddr);
ngx_resolver_free(r, addrs);
}
return NGX_OK;
}
if (rn->nsrvs) {
last->next = rn->waiting;
rn->waiting = NULL;
/* unlock name mutex */
do {
next = ctx->next;
ngx_resolver_resolve_srv_names(ctx, rn);
ctx = next;
} while (ctx);
return NGX_OK;
}
/* NGX_RESOLVE_CNAME */
if (ctx->recursion++ < NGX_RESOLVER_MAX_RECURSION) {
cname.len = rn->cnlen;
cname.data = rn->u.cname;
return ngx_resolve_name_locked(r, ctx, &cname);
}
last->next = rn->waiting;
rn->waiting = NULL;
/* unlock name mutex */
do {
ctx->state = NGX_RESOLVE_NXDOMAIN;
ctx->valid = ngx_time() + (r->valid ? r->valid : 10);
next = ctx->next;
ctx->handler(ctx);
ctx = next;
} while (ctx);
return NGX_OK;
}
if (rn->waiting) {
if (ctx->event == NULL && ctx->timeout) {
ctx->event = ngx_resolver_calloc(r, sizeof(ngx_event_t));
if (ctx->event == NULL) {
return NGX_ERROR;
}
ctx->event->handler = ngx_resolver_timeout_handler;
ctx->event->data = ctx;
ctx->event->log = r->log;
ctx->ident = -1;
ngx_add_timer(ctx->event, ctx->timeout);
}
last->next = rn->waiting;
rn->waiting = ctx;
ctx->state = NGX_AGAIN;
do {
ctx->node = rn;
ctx = ctx->next;
} while (ctx);
return NGX_AGAIN;
}
ngx_queue_remove(&rn->queue);
/* lock alloc mutex */
if (rn->query) {
ngx_resolver_free_locked(r, rn->query);
rn->query = NULL;
#if (NGX_HAVE_INET6)
rn->query6 = NULL;
#endif
}
if (rn->cnlen) {
ngx_resolver_free_locked(r, rn->u.cname);
}
if (rn->naddrs > 1 && rn->naddrs != (u_short) -1) {
ngx_resolver_free_locked(r, rn->u.addrs);
}
#if (NGX_HAVE_INET6)
if (rn->naddrs6 > 1 && rn->naddrs6 != (u_short) -1) {
ngx_resolver_free_locked(r, rn->u6.addrs6);
}
#endif
if (rn->nsrvs) {
for (i = 0; i < rn->nsrvs; i++) {
if (rn->u.srvs[i].name.data) {
ngx_resolver_free_locked(r, rn->u.srvs[i].name.data);
}
}
ngx_resolver_free_locked(r, rn->u.srvs);
}
/* unlock alloc mutex */
} else {
rn = ngx_resolver_alloc(r, sizeof(ngx_resolver_node_t));
if (rn == NULL) {
return NGX_ERROR;
}
rn->name = ngx_resolver_dup(r, name->data, name->len);
if (rn->name == NULL) {
ngx_resolver_free(r, rn);
return NGX_ERROR;
}
rn->node.key = hash;
rn->nlen = (u_short) name->len;
rn->query = NULL;
#if (NGX_HAVE_INET6)
rn->query6 = NULL;
#endif
ngx_rbtree_insert(tree, &rn->node);
}
if (ctx->service.len) {
rc = ngx_resolver_create_srv_query(r, rn, name);
} else {
rc = ngx_resolver_create_name_query(r, rn, name);
}
if (rc == NGX_ERROR) {
goto failed;
}
if (rc == NGX_DECLINED) {
ngx_rbtree_delete(tree, &rn->node);
ngx_resolver_free(r, rn->query);
ngx_resolver_free(r, rn->name);
ngx_resolver_free(r, rn);
do {
ctx->state = NGX_RESOLVE_NXDOMAIN;
next = ctx->next;
ctx->handler(ctx);
ctx = next;
} while (ctx);
return NGX_OK;
}
rn->last_connection = r->last_connection++;
if (r->last_connection == r->connections.nelts) {
r->last_connection = 0;
}
rn->naddrs = (u_short) -1;
rn->tcp = 0;
#if (NGX_HAVE_INET6)
rn->naddrs6 = r->ipv6 ? (u_short) -1 : 0;
rn->tcp6 = 0;
#endif
rn->nsrvs = 0;
if (ngx_resolver_send_query(r, rn) != NGX_OK) {
goto failed;
}
if (ctx->event == NULL && ctx->timeout) {
ctx->event = ngx_resolver_calloc(r, sizeof(ngx_event_t));
if (ctx->event == NULL) {
goto failed;
}
ctx->event->handler = ngx_resolver_timeout_handler;
ctx->event->data = ctx;
ctx->event->log = r->log;
ctx->ident = -1;
ngx_add_timer(ctx->event, ctx->timeout);
}
if (ngx_queue_empty(resend_queue)) {
ngx_add_timer(r->event, (ngx_msec_t) (r->resend_timeout * 1000));
}
rn->expire = ngx_time() + r->resend_timeout;
ngx_queue_insert_head(resend_queue, &rn->queue);
rn->code = 0;
rn->cnlen = 0;
rn->valid = 0;
rn->ttl = NGX_MAX_UINT32_VALUE;
rn->waiting = ctx;
ctx->state = NGX_AGAIN;
do {
ctx->node = rn;
ctx = ctx->next;
} while (ctx);
return NGX_AGAIN;
failed:
ngx_rbtree_delete(tree, &rn->node);
if (rn->query) {
ngx_resolver_free(r, rn->query);
}
ngx_resolver_free(r, rn->name);
ngx_resolver_free(r, rn);
return NGX_ERROR;
}本函数实现的主要功能就是向DNS服务器发起域名查询或服务名查询。下面我们简要分析一下函数的实现步骤：
static ngx_int_t
ngx_resolve_name_locked(ngx_resolver_t *r, ngx_resolver_ctx_t *ctx,
ngx_str_t *name)
{
//1) 首先检查r->srv_rbtree或者r->name_rbtree，看是否有已经缓存有对应的信息
//2) 如果检查到有对应的缓存信息
// 2.1) 如果查找到的节点仍在有效期内
2.1.1） 如果当前已经有解析到的地址信息了，那么直接调用ctx所绑定的handler()回调函数即可
// 2.1.2） 否则，根据相应的情况重新发起 '域名查询' 或 '服务名查询', 或将状态设置为NGX_RESOLVE_NXDOMAIN,
// 然后直接回调ctx绑定的handler()
// 2.2) 否则，检查rn->waiting链表，如果不为NULL，表示当前仍有等待解析的任务，直接将ctx->state设置为NGX_AGAIN，然后返回
//3) 否则，构造相应的查询报文，然后发起查询请求
//4) 如果当前ctx->event为NULL，并且ctx又具有timeout属性，那么默认为此context绑定一个超时回调事件
}11. 函数ngx_resolve_addr()
ngx_int_t
ngx_resolve_addr(ngx_resolver_ctx_t *ctx)
{
u_char *name;
in_addr_t addr;
ngx_queue_t *resend_queue, *expire_queue;
ngx_rbtree_t *tree;
ngx_resolver_t *r;
struct sockaddr_in *sin;
ngx_resolver_node_t *rn;
#if (NGX_HAVE_INET6)
uint32_t hash;
struct sockaddr_in6 *sin6;
#endif
#if (NGX_SUPPRESS_WARN)
addr = 0;
#if (NGX_HAVE_INET6)
hash = 0;
sin6 = NULL;
#endif
#endif
r = ctx->resolver;
switch (ctx->addr.sockaddr->sa_family) {
#if (NGX_HAVE_INET6)
case AF_INET6:
sin6 = (struct sockaddr_in6 *) ctx->addr.sockaddr;
hash = ngx_crc32_short(sin6->sin6_addr.s6_addr, 16);
/* lock addr mutex */
rn = ngx_resolver_lookup_addr6(r, &sin6->sin6_addr, hash);
tree = &r->addr6_rbtree;
resend_queue = &r->addr6_resend_queue;
expire_queue = &r->addr6_expire_queue;
break;
#endif
default: /* AF_INET */
sin = (struct sockaddr_in *) ctx->addr.sockaddr;
addr = ntohl(sin->sin_addr.s_addr);
/* lock addr mutex */
rn = ngx_resolver_lookup_addr(r, addr);
tree = &r->addr_rbtree;
resend_queue = &r->addr_resend_queue;
expire_queue = &r->addr_expire_queue;
}
if (rn) {
if (rn->valid >= ngx_time()) {
ngx_log_debug0(NGX_LOG_DEBUG_CORE, r->log, 0, "resolve cached");
ngx_queue_remove(&rn->queue);
rn->expire = ngx_time() + r->expire;
ngx_queue_insert_head(expire_queue, &rn->queue);
name = ngx_resolver_dup(r, rn->name, rn->nlen);
if (name == NULL) {
goto failed;
}
ctx->name.len = rn->nlen;
ctx->name.data = name;
/* unlock addr mutex */
ctx->state = NGX_OK;
ctx->valid = rn->valid;
ctx->handler(ctx);
ngx_resolver_free(r, name);
return NGX_OK;
}
if (rn->waiting) {
if (ctx->event == NULL && ctx->timeout) {
ctx->event = ngx_resolver_calloc(r, sizeof(ngx_event_t));
if (ctx->event == NULL) {
return NGX_ERROR;
}
ctx->event->handler = ngx_resolver_timeout_handler;
ctx->event->data = ctx;
ctx->event->log = r->log;
ctx->ident = -1;
ngx_add_timer(ctx->event, ctx->timeout);
}
ctx->next = rn->waiting;
rn->waiting = ctx;
ctx->state = NGX_AGAIN;
ctx->node = rn;
/* unlock addr mutex */
return NGX_OK;
}
ngx_queue_remove(&rn->queue);
ngx_resolver_free(r, rn->query);
rn->query = NULL;
#if (NGX_HAVE_INET6)
rn->query6 = NULL;
#endif
} else {
rn = ngx_resolver_alloc(r, sizeof(ngx_resolver_node_t));
if (rn == NULL) {
goto failed;
}
switch (ctx->addr.sockaddr->sa_family) {
#if (NGX_HAVE_INET6)
case AF_INET6:
rn->addr6 = sin6->sin6_addr;
rn->node.key = hash;
break;
#endif
default: /* AF_INET */
rn->node.key = addr;
}
rn->query = NULL;
#if (NGX_HAVE_INET6)
rn->query6 = NULL;
#endif
ngx_rbtree_insert(tree, &rn->node);
}
if (ngx_resolver_create_addr_query(r, rn, &ctx->addr) != NGX_OK) {
goto failed;
}
rn->last_connection = r->last_connection++;
if (r->last_connection == r->connections.nelts) {
r->last_connection = 0;
}
rn->naddrs = (u_short) -1;
rn->tcp = 0;
#if (NGX_HAVE_INET6)
rn->naddrs6 = (u_short) -1;
rn->tcp6 = 0;
#endif
rn->nsrvs = 0;
if (ngx_resolver_send_query(r, rn) != NGX_OK) {
goto failed;
}
if (ctx->event == NULL && ctx->timeout) {
ctx->event = ngx_resolver_calloc(r, sizeof(ngx_event_t));
if (ctx->event == NULL) {
goto failed;
}
ctx->event->handler = ngx_resolver_timeout_handler;
ctx->event->data = ctx;
ctx->event->log = r->log;
ctx->ident = -1;
ngx_add_timer(ctx->event, ctx->timeout);
}
if (ngx_queue_empty(resend_queue)) {
ngx_add_timer(r->event, (ngx_msec_t) (r->resend_timeout * 1000));
}
rn->expire = ngx_time() + r->resend_timeout;
ngx_queue_insert_head(resend_queue, &rn->queue);
rn->code = 0;
rn->cnlen = 0;
rn->name = NULL;
rn->nlen = 0;
rn->valid = 0;
rn->ttl = NGX_MAX_UINT32_VALUE;
rn->waiting = ctx;
/* unlock addr mutex */
ctx->state = NGX_AGAIN;
ctx->node = rn;
return NGX_OK;
failed:
if (rn) {
ngx_rbtree_delete(tree, &rn->node);
if (rn->query) {
ngx_resolver_free(r, rn->query);
}
ngx_resolver_free(r, rn);
}
/* unlock addr mutex */
if (ctx->event) {
ngx_resolver_free(r, ctx->event);
}
ngx_resolver_free(r, ctx);
return NGX_ERROR;
}本函数进行DNS逆查询，即通过DNS，查询ctx->addr地址所对应的域名。下面简要介绍一下本函数的实现：
ngx_int_t
ngx_resolve_addr(ngx_resolver_ctx_t *ctx)
{
//1) 从红黑树中查询当前所缓存的IP地址到域名的映射
//2) 如果红黑树r->addr_rbtree中有对应的缓存信息
// 2.1) 如果缓存信息并未过期，则将该节点加入到addr_expire_queue或addr6_expire_queue中，同时调用对应的handler()回调函数
// 2.2) 如果对应节点的waiting链表不为NULL，则将当前上下文加入到waiting链表中， 并将ctx状态设置为NGX_AGAIN
//3) 如果在红黑树r->addr_rbtree中没有对应的缓存信息，创建一个ngx_resolver_node_t对象加入到ctx->resolver相应的红黑树中
//4) 创建DNS逆解析报文，并向对应的DNS服务器发送请求
//5) 将ctx->state设置为NGX_AGAIN，等待返回相应的处理结果
}12. 函数ngx_resolve_addr_done()
void
ngx_resolve_addr_done(ngx_resolver_ctx_t *ctx)
{
ngx_queue_t *expire_queue;
ngx_rbtree_t *tree;
ngx_resolver_t *r;
ngx_resolver_ctx_t *w, **p;
ngx_resolver_node_t *rn;
r = ctx->resolver;
switch (ctx->addr.sockaddr->sa_family) {
#if (NGX_HAVE_INET6)
case AF_INET6:
tree = &r->addr6_rbtree;
expire_queue = &r->addr6_expire_queue;
break;
#endif
default: /* AF_INET */
tree = &r->addr_rbtree;
expire_queue = &r->addr_expire_queue;
}
ngx_log_debug1(NGX_LOG_DEBUG_CORE, r->log, 0,
"resolve addr done: %i", ctx->state);
if (ctx->event && ctx->event->timer_set) {
ngx_del_timer(ctx->event);
}
/* lock addr mutex */
if (ctx->state == NGX_AGAIN || ctx->state == NGX_RESOLVE_TIMEDOUT) {
rn = ctx->node;
if (rn) {
p = &rn->waiting;
w = rn->waiting;
while (w) {
if (w == ctx) {
*p = w->next;
goto done;
}
p = &w->next;
w = w->next;
}
}
{
u_char text[NGX_SOCKADDR_STRLEN];
ngx_str_t addrtext;
addrtext.data = text;
addrtext.len = ngx_sock_ntop(ctx->addr.sockaddr, ctx->addr.socklen,
text, NGX_SOCKADDR_STRLEN, 0);
ngx_log_error(NGX_LOG_ALERT, r->log, 0,
"could not cancel %V resolving", &addrtext);
}
}
done:
ngx_resolver_expire(r, tree, expire_queue);
/* unlock addr mutex */
/* lock alloc mutex */
if (ctx->event) {
ngx_resolver_free_locked(r, ctx->event);
}
ngx_resolver_free_locked(r, ctx);
/* unlock alloc mutex */
if (r->event->timer_set && ngx_resolver_resend_empty(r)) {
ngx_del_timer(r->event);
}
}此函数用于处理当解析完成（可能成功，也可能失败），进行相应的收尾工作：
1) 如果状态为NGX_AGAIN或者NGX_RESOLVE_TIMEDOUT，那么将该上下文从waiting链表中移除
2) 从ctx->resolver的addr_expire_queue或addr6_expire_queue队列中移除相应的节点
3) 如果ctx->resolver上并没有相应的resend事件了，则将ctx->resolver上相应的定时器移除13. 函数ngx_resolver_expire()
static void
ngx_resolver_expire(ngx_resolver_t *r, ngx_rbtree_t *tree, ngx_queue_t *queue)
{
time_t now;
ngx_uint_t i;
ngx_queue_t *q;
ngx_resolver_node_t *rn;
ngx_log_debug0(NGX_LOG_DEBUG_CORE, r->log, 0, "resolver expire");
now = ngx_time();
for (i = 0; i < 2; i++) {
if (ngx_queue_empty(queue)) {
return;
}
q = ngx_queue_last(queue);
rn = ngx_queue_data(q, ngx_resolver_node_t, queue);
if (now <= rn->expire) {
return;
}
ngx_log_debug2(NGX_LOG_DEBUG_CORE, r->log, 0,
"resolver expire \"%*s\"", (size_t) rn->nlen, rn->name);
ngx_queue_remove(q);
ngx_rbtree_delete(tree, &rn->node);
ngx_resolver_free_node(r, rn);
}
}此函数用于淘汰相应超时队列上的1~2个超时节点。
14. 函数ngx_resolver_send_query()
static ngx_int_t
ngx_resolver_send_query(ngx_resolver_t *r, ngx_resolver_node_t *rn)
{
ngx_int_t rc;
ngx_resolver_connection_t *rec;
rec = r->connections.elts;
rec = &rec[rn->last_connection];
if (rec->log.handler == NULL) {
rec->log = *r->log;
rec->log.handler = ngx_resolver_log_error;
rec->log.data = rec;
rec->log.action = "resolving";
}
if (rn->naddrs == (u_short) -1) {
rc = rn->tcp ? ngx_resolver_send_tcp_query(r, rec, rn->query, rn->qlen)
: ngx_resolver_send_udp_query(r, rec, rn->query, rn->qlen);
if (rc != NGX_OK) {
return rc;
}
}
#if (NGX_HAVE_INET6)
if (rn->query6 && rn->naddrs6 == (u_short) -1) {
rc = rn->tcp6
? ngx_resolver_send_tcp_query(r, rec, rn->query6, rn->qlen)
: ngx_resolver_send_udp_query(r, rec, rn->query6, rn->qlen);
if (rc != NGX_OK) {
return rc;
}
}
#endif
return NGX_OK;
}此函数用于向DNS服务器发送相应的查询报文。
15. 函数ngx_resolver_send_udp_query()
static ngx_int_t
ngx_resolver_send_udp_query(ngx_resolver_t *r, ngx_resolver_connection_t *rec,
u_char *query, u_short qlen)
{
ssize_t n;
if (rec->udp == NULL) {
if (ngx_udp_connect(rec) != NGX_OK) {
return NGX_ERROR;
}
rec->udp->data = rec;
rec->udp->read->handler = ngx_resolver_udp_read;
rec->udp->read->resolver = 1;
}
n = ngx_send(rec->udp, query, qlen);
if (n == -1) {
return NGX_ERROR;
}
if ((size_t) n != (size_t) qlen) {
ngx_log_error(NGX_LOG_CRIT, &rec->log, 0, "send() incomplete");
return NGX_ERROR;
}
return NGX_OK;
}采用UDP方式向DNS服务器发送查询报文
16. 函数ngx_resolver_send_tcp_query()
static ngx_int_t
ngx_resolver_send_tcp_query(ngx_resolver_t *r, ngx_resolver_connection_t *rec,
u_char *query, u_short qlen)
{
ngx_buf_t *b;
ngx_int_t rc;
rc = NGX_OK;
if (rec->tcp == NULL) {
b = rec->read_buf;
if (b == NULL) {
b = ngx_resolver_calloc(r, sizeof(ngx_buf_t));
if (b == NULL) {
return NGX_ERROR;
}
b->start = ngx_resolver_alloc(r, NGX_RESOLVER_TCP_RSIZE);
if (b->start == NULL) {
ngx_resolver_free(r, b);
return NGX_ERROR;
}
b->end = b->start + NGX_RESOLVER_TCP_RSIZE;
rec->read_buf = b;
}
b->pos = b->start;
b->last = b->start;
b = rec->write_buf;
if (b == NULL) {
b = ngx_resolver_calloc(r, sizeof(ngx_buf_t));
if (b == NULL) {
return NGX_ERROR;
}
b->start = ngx_resolver_alloc(r, NGX_RESOLVER_TCP_WSIZE);
if (b->start == NULL) {
ngx_resolver_free(r, b);
return NGX_ERROR;
}
b->end = b->start + NGX_RESOLVER_TCP_WSIZE;
rec->write_buf = b;
}
b->pos = b->start;
b->last = b->start;
rc = ngx_tcp_connect(rec);
if (rc == NGX_ERROR) {
return NGX_ERROR;
}
rec->tcp->data = rec;
rec->tcp->write->handler = ngx_resolver_tcp_write;
rec->tcp->read->handler = ngx_resolver_tcp_read;
rec->tcp->read->resolver = 1;
ngx_add_timer(rec->tcp->write, (ngx_msec_t) (r->tcp_timeout * 1000));
}
b = rec->write_buf;
if (b->end - b->last < 2 + qlen) {
ngx_log_error(NGX_LOG_CRIT, &rec->log, 0, "buffer overflow");
return NGX_ERROR;
}
*b->last++ = (u_char) (qlen >> 8);
*b->last++ = (u_char) qlen;
b->last = ngx_cpymem(b->last, query, qlen);
if (rc == NGX_OK) {
ngx_resolver_tcp_write(rec->tcp->write);
}
return NGX_OK;
}以TCP方式向DNS服务器发送查询报文。如果连接没有建立，则创建对应的连接，并设置好读写缓冲。
[参看]
