Open vislee opened 7 years ago
域名在互联网中广泛应用，而http是建立在tcp/ip协议上的，tcp/ip协议只认识ip地址，所以就需要通过某个系统（DNS）把域名转换成ip地址供底层使用。 通常linux系统下，命令行使用dig查询，c语言使用gethostbyname或getaddrinfo函数查询。实际上都是发送一个网络请求到/etc/resolv.conf下的一个服务器查询。
nginx作为一个通用的服务器也会涉及到域名解析。例如，nginx 用作反向代理可以配置upstream是一个域名。proxy_pass http://www.baidu.com/$request_uri; 那么www.baidu.com这个域名是什么时候解析的呢？又是如何解析的呢？如果没有添加$request_uri这个变量解析会有区别么？
proxy_pass http://www.baidu.com/$request_uri;
nginx是单进程异步非阻塞服务，如果用getaddrinfo来解析势必会导致nginx阻塞。在启动阶段阻塞一小会儿不会有大的影响，如果在服务处理阶段阻塞，会导致该进程所服务的客户端延迟甚至出错。
proxy_pass 对应的域名解析分为有变量和没有变量，没有变量的是在启动阶段解析， 而有变量的是在每次请求解析的，每次请求解析在nginx是怎么做的？还是异步回调。
nginx会拼装DNS查询报文，所以现了解一下DNS报文格式。也可以略过直接看大框架逻辑。
DNS format +--+--+--+--+--+--+--+ | Header | 报文头 +--+--+--+--+--+--+--+ | Question | 查询的问题 +--+--+--+--+--+--+--+ | Answer | 应答 +--+--+--+--+--+--+--+ | Authority | 授权应答 +--+--+--+--+--+--+--+ | Additional | 附加信息 +--+--+--+--+--+--+--+
Header format 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ID | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |QR| opcode |AA|TC|RD|RA| Z | RCODE | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | QDCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ANCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | NSCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ARCOUNT | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
说明： ID: 2个字节(16bit)，标识字段，客户端会解析服务器返回的DNS应答报文，获取ID值与请求报文设置的ID值做比较，如果相同，则认为是同一个DNS会话。 FLAGS: 2个字节(16bit)的标志字段。包含以下属性: QR: 0表示查询报文，1表示响应报文; opcode: 通常值为0（标准查询），其他值为1（反向查询）和2（服务器状态请求）,[3,15]保留值; AA: 表示授权回答（authoritative answer）– 这个比特位在应答的时候才有意义，指出给出应答的服务器是查询域名的授权解析服务器; TC: 表示可截断的（truncated）–用来指出报文比允许的长度还要长，导致被截断; RD: 表示期望递归(Recursion Desired) – 这个比特位被请求设置，应答的时候使用的相同的值返回。如果设置了RD，就建议域名服务器进行递归解析，递归查询的支持是可选的; RA: 表示支持递归(Recursion Available) – 这个比特位在应答中设置或取消，用来代表服务器是否支持递归查询; Z : 保留值，暂未使用; RCODE: 应答码(Response code) - 这4个比特位在应答报文中设置，代表的含义如下: 0 : 没有错误。 QDCOUNT: 无符号16bit整数表示报文请求段中的问题记录数。 ANCOUNT: 无符号16bit整数表示报文回答段中的回答记录数。 ANCOUNT: 无符号16bit整数表示报文回答段中的回答记录数。 ARCOUNT: 无符号16bit整数表示报文附加段中的附加记录数。
Question format 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | ... | | QNAME | | ... | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | QTYPE | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | QCLASS | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
说明： QNAME 8bit为单位表示的查询名(广泛的说就是：域名). QTYPE 无符号16bit整数表示查询的协议类型. QCLASS 无符号16bit整数表示查询的类,比如，IN代表Internet.
Answer/Authority/Additional format 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | NAME | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | TYPE | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | CLASS | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | TTL | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | RDLENGTH | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | RDATA | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
说明： NAME 资源记录包含的域名. TYPE 表示DNS协议的类型. CLASS 表示RDATA的类. TTL 4字节无符号整数表示资源记录可以缓存的时间。0代表只能被传输，但是不能被缓存。 RDLENGTH 2个字节无符号整数表示RDATA的长度 RDATA 不定长字符串来表示记录，格式根TYPE和CLASS有关。比如，TYPE是A，CLASS 是 IN，那么RDATA就是一个4个字节的ARPA网络地址。
// 对应报文头的6个16bit字段。12个8bit typedef struct { // ID标识字段 u_char ident_hi; u_char ident_lo; // FLAG标识 u_char flags_hi; u_char flags_lo; // QDCOUNT 查询段中的问题记录数 u_char nqs_hi; u_char nqs_lo; // ANCOUNT 应答段中的问题记录数 u_char nan_hi; u_char nan_lo; // NSCOUNT 授权记录数 u_char nns_hi; u_char nns_lo; // ARCOUNT 附加记录数 u_char nar_hi; u_char nar_lo; } ngx_resolver_hdr_t; // 对应查询报文2个16bit字段。4个8bit typedef struct { // QTYPE 无符号16bit整数表示查询的协议类型. u_char type_hi; u_char type_lo; // 无符号16bit整数表示查询的类,比如，IN代表Internet. u_char class_hi; u_char class_lo; } ngx_resolver_qs_t; // 应答报文 4个字段 typedef struct { // 表示DNS协议的类型. u_char type_hi; u_char type_lo; // 表示RDATA的类. u_char class_hi; u_char class_lo; // 4字节无符号整数表示资源记录可以缓存的时间。0代表只能被传输，但是不能被缓存。 u_char ttl[4]; // 2个字节无符号整数表示RDATA的长度 u_char len_hi; u_char len_lo; } ngx_resolver_an_t;
ngx_inet_resolve_host函数。该函数实际上是封装了getaddrinfo函数，是同步阻塞解析。
异步回调解析，下面以proxy模块的代码来分析异步非阻塞回调的域名解析。
启动阶段，在proxy模块的ngx_http_proxy_eval函数中，url不是ip地址，则u->resolved->sockaddr不会赋值，u->resolved->host是域名。
请求处理阶段，在解析完header头，进入content阶段，调用proxy模块的ngx_http_proxy_handler回调函数，该函数通过调用ngx_http_read_client_request_body(r, ngx_http_upstream_init)，读取body并启动upstream。读取完body后（或者读到body）调用ngx_http_upstream_init函数，该函数又调用了ngx_http_upstream_init_request函数。
在ngx_http_upstream_init_request函数中会查找上游服务器。如果是域名则会调用ngx_resolve_start函数分配域名解析结构体，调用ngx_resolve_name函数解析，该函数会创建一个事件结构添加到epoll中，解析完毕后回调ctx->handler函数，即ngx_http_upstream_resolve_handler。
static void ngx_http_upstream_init_request(ngx_http_request_t *r) { ...... ctx = ngx_resolve_start(clcf->resolver, &temp); if (ctx == NULL) { ngx_http_upstream_finalize_request(r, u, NGX_HTTP_INTERNAL_SERVER_ERROR); return; } if (ctx == NGX_NO_RESOLVER) { ngx_log_error(NGX_LOG_ERR, r->connection->log, 0, "no resolver defined to resolve %V", host); ngx_http_upstream_finalize_request(r, u, NGX_HTTP_BAD_GATEWAY); return; } ctx->name = *host; ctx->handler = ngx_http_upstream_resolve_handler; ctx->data = r; ctx->timeout = clcf->resolver_timeout; u->resolved->ctx = ctx; if (ngx_resolve_name(ctx) != NGX_OK) { u->resolved->ctx = NULL; ngx_http_upstream_finalize_request(r, u, NGX_HTTP_INTERNAL_SERVER_ERROR); return; } ...... }
ngx_resolver.h|c
主要结构：
// resolver 信息，通过resolver指令制定的解析域名的服务器 typedef struct { ngx_connection_t *udp; ngx_connection_t *tcp; // tcp 连接的connection struct sockaddr *sockaddr; // resolver 地址 socklen_t socklen; ngx_str_t server; // resolver域名 ngx_log_t log; ngx_buf_t *read_buf; // tcp 读缓存区 ngx_buf_t *write_buf; // tcp 写缓存区 ngx_resolver_t *resolver; } ngx_resolver_connection_t; struct ngx_resolver_s { /* has to be pointer because of "incomplete type" */ ngx_event_t *event; void *dummy; ngx_log_t *log; /* event ident must be after 3 pointers as in ngx_connection_t */ ngx_int_t ident; /* simple round robin DNS peers balancer */ // ngx_resolver_connection_t 结构的数组，resolver服务地址。 ngx_array_t connections; ngx_uint_t last_connection; ngx_rbtree_t name_rbtree; ngx_rbtree_node_t name_sentinel; ngx_rbtree_t srv_rbtree; ngx_rbtree_node_t srv_sentinel; ngx_rbtree_t addr_rbtree; ngx_rbtree_node_t addr_sentinel; ngx_queue_t name_resend_queue; ngx_queue_t srv_resend_queue; ngx_queue_t addr_resend_queue; ngx_queue_t name_expire_queue; ngx_queue_t srv_expire_queue; ngx_queue_t addr_expire_queue; #if (NGX_HAVE_INET6) ngx_uint_t ipv6; /* unsigned ipv6:1; */ ngx_rbtree_t addr6_rbtree; ngx_rbtree_node_t addr6_sentinel; ngx_queue_t addr6_resend_queue; ngx_queue_t addr6_expire_queue; #endif time_t resend_timeout; time_t tcp_timeout; // tcp 连接超时时间 time_t expire; time_t valid; ngx_uint_t log_level; }; struct ngx_resolver_ctx_s { ngx_resolver_ctx_t *next; ngx_resolver_t *resolver; ngx_resolver_node_t *node; /* event ident must be after 3 pointers as in ngx_connection_t */ ngx_int_t ident; ngx_int_t state; ngx_str_t name; // 要解析的域名 ngx_str_t service; time_t valid; ngx_uint_t naddrs; ngx_resolver_addr_t *addrs; // 解析的IP // 如果域名本来就是ip，则把ip解析到该内存。不用向dns服务器发起请求 ngx_resolver_addr_t addr; struct sockaddr_in sin; ngx_uint_t count; ngx_uint_t nsrvs; ngx_resolver_srv_name_t *srvs; ngx_resolver_handler_pt handler; // 解析万后的回调函数 void *data; ngx_msec_t timeout; // 域名解析超时 unsigned quick:1; // 不用向dns服务器发起查询，也可以说是快速的同步的。 unsigned async:1; unsigned cancelable:1; ngx_uint_t recursion; ngx_event_t *event; // 只用做超时ngx_resolver_set_timeout函数初始化 }; typedef struct { ngx_rbtree_node_t node; ngx_queue_t queue; /* PTR: resolved name, A: name to resolve */ u_char *name; // 要解析的域名 #if (NGX_HAVE_INET6) /* PTR: IPv6 address to resolve (IPv4 address is in rbtree node key) */ struct in6_addr addr6; #endif u_short nlen; // 要解析的域名的长度 u_short qlen; // 发送dns解析命令的长度 u_char *query; // dns解析命令字符串 #if (NGX_HAVE_INET6) u_char *query6; #endif union { in_addr_t addr; in_addr_t *addrs; u_char *cname; ngx_resolver_srv_t *srvs; } u; u_char code; u_short naddrs; u_short nsrvs; u_short cnlen; #if (NGX_HAVE_INET6) union { struct in6_addr addr6; struct in6_addr *addrs6; } u6; u_short naddrs6; #endif time_t expire; time_t valid; uint32_t ttl; unsigned tcp:1; // 是否通过tcp方式方法查询请求 #if (NGX_HAVE_INET6) unsigned tcp6:1; #endif ngx_uint_t last_connection; // dns域名服务器地址下标 ngx_resolver_ctx_t *waiting; } ngx_resolver_node_t;
主要函数：
ngx_resolver_create // 创建域名解析结构体，用于后续的域名解析 ngx_resolve_start // 初始化域名解析上下文ngx_resolver_ctx_t ngx_resolve_name // 构建dns查询并发起查询请求 ngx_resolve_name_done // dns查询结束，清理资源 // 以下为实现的内部函数 ngx_resolver_create_name_query // 创建查询请求命令的字符串 ngx_resolver_send_query // 发送查询请求 ngx_resolver_send_udp_query // udp方式发送dns查询请求 ngx_resolver_send_tcp_query // tcp 方式发送，建立tcp连接会把连接可读可写添加到epoll ngx_resolver_tcp_write // tcp 方式可写回调函数 ngx_resolver_tcp_read // tcp 方式可读回调函数
主要函数实现
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; // 一般使用会定义一个局部变量ngx_resolver_ctx_t tmp; // 把tmp的指针传递进来，name赋值为要解析的域名 if (temp) { // 校验name是否就是ip地址 addr = ngx_inet_addr(temp->name.data, temp->name.len); if (addr != INADDR_NONE) { // name就是ip地址 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; // 直接返回tmp指针。 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; } 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) { // 不用发起dns查询，则直接调用业务赋值的回调函数。 // 例如：upstream模块的ngx_http_upstream_resolve_handler ctx->handler(ctx); return NGX_OK; } if (ctx->service.len) { // 目前开源的代码中没有用到 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) { goto failed; } 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 */ // 发起dns查询 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); } failed: ngx_resolver_free(r, ctx); return NGX_ERROR; } // 发起dns查询 // name 是域名 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 { // 先从缓存中查询，缓存没有命中则返回NULL 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 (ngx_resolver_set_timeout(r, ctx) != NGX_OK) { return NGX_ERROR; } last->next = rn->waiting; rn->waiting = ctx; ctx->state = NGX_AGAIN; ctx->async = 1; 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 < (ngx_uint_t) 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; } // 设置超时事件 if (ngx_resolver_set_timeout(r, ctx) != NGX_OK) { goto failed; } if (ngx_resolver_resend_empty(r)) { 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; ctx->async = 1; 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; } // 封装dns查询数据包 static ngx_int_t ngx_resolver_create_name_query(ngx_resolver_t *r, ngx_resolver_node_t *rn, ngx_str_t *name) { u_char *p, *s; size_t len, nlen; ngx_uint_t ident; ngx_resolver_qs_t *qs; ngx_resolver_hdr_t *query; nlen = name->len ? (1 + name->len + 1) : 1; len = sizeof(ngx_resolver_hdr_t) + nlen + sizeof(ngx_resolver_qs_t); #if (NGX_HAVE_INET6) p = ngx_resolver_alloc(r, r->ipv6 ? len * 2 : len); #else p = ngx_resolver_alloc(r, len); #endif if (p == NULL) { return NGX_ERROR; } rn->qlen = (u_short) len; rn->query = p; #if (NGX_HAVE_INET6) if (r->ipv6) { rn->query6 = p + len; } #endif query = (ngx_resolver_hdr_t *) p; ident = ngx_random(); ngx_log_debug2(NGX_LOG_DEBUG_CORE, r->log, 0, "resolve: \"%V\" A %i", name, ident & 0xffff); query->ident_hi = (u_char) ((ident >> 8) & 0xff); query->ident_lo = (u_char) (ident & 0xff); /* recursion query */ query->flags_hi = 1; query->flags_lo = 0; /* one question */ query->nqs_hi = 0; query->nqs_lo = 1; query->nan_hi = 0; query->nan_lo = 0; query->nns_hi = 0; query->nns_lo = 0; query->nar_hi = 0; query->nar_lo = 0; p += sizeof(ngx_resolver_hdr_t) + nlen; qs = (ngx_resolver_qs_t *) p; /* query type */ qs->type_hi = 0; qs->type_lo = NGX_RESOLVE_A; /* IN query class */ qs->class_hi = 0; qs->class_lo = 1; /* convert "www.example.com" to "\3www\7example\3com\0" */ len = 0; p--; *p-- = '\0'; if (name->len == 0) { return NGX_DECLINED; } for (s = name->data + name->len - 1; s >= name->data; s--) { if (*s != '.') { *p = *s; len++; } else { if (len == 0 || len > 255) { return NGX_DECLINED; } *p = (u_char) len; len = 0; } p--; } if (len == 0 || len > 255) { return NGX_DECLINED; } *p = (u_char) len; #if (NGX_HAVE_INET6) if (!r->ipv6) { return NGX_OK; } p = rn->query6; ngx_memcpy(p, rn->query, rn->qlen); query = (ngx_resolver_hdr_t *) p; ident = ngx_random(); ngx_log_debug2(NGX_LOG_DEBUG_CORE, r->log, 0, "resolve: \"%V\" AAAA %i", name, ident & 0xffff); query->ident_hi = (u_char) ((ident >> 8) & 0xff); query->ident_lo = (u_char) (ident & 0xff); p += sizeof(ngx_resolver_hdr_t) + nlen; qs = (ngx_resolver_qs_t *) p; qs->type_lo = NGX_RESOLVE_AAAA; #endif return NGX_OK; } // 发送dns查询 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) { // 建立连接并把可读事件添加到epoll if (ngx_udp_connect(rec) != NGX_OK) { return NGX_ERROR; } rec->udp->data = rec; // 可读回调 rec->udp->read->handler = ngx_resolver_udp_read; rec->udp->read->resolver = 1; } // 发送dns查询数据报 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; } // 设置超时事件 static ngx_int_t ngx_resolver_set_timeout(ngx_resolver_t *r, ngx_resolver_ctx_t *ctx) { if (ctx->event || ctx->timeout == 0) { return NGX_OK; } 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->event->cancelable = ctx->cancelable; ctx->ident = -1; ngx_add_timer(ctx->event, ctx->timeout); return NGX_OK; } // 超时回调 static void ngx_resolver_timeout_handler(ngx_event_t *ev) { ngx_resolver_ctx_t *ctx; ctx = ev->data; ctx->state = NGX_RESOLVE_TIMEDOUT; ctx->handler(ctx); } // 可读事件回调函数 // 解析DNS查询结果 static void ngx_resolver_udp_read(ngx_event_t *rev) { ssize_t n; ngx_connection_t *c; ngx_resolver_connection_t *rec; u_char buf[NGX_RESOLVER_UDP_SIZE]; c = rev->data; rec = c->data; do { n = ngx_udp_recv(c, buf, NGX_RESOLVER_UDP_SIZE); if (n < 0) { return; } ngx_resolver_process_response(rec->resolver, buf, n, 0); } while (rev->ready); } static void ngx_resolver_process_response(ngx_resolver_t *r, u_char *buf, size_t n, ngx_uint_t tcp) { char *err; ngx_uint_t i, times, ident, qident, flags, code, nqs, nan, trunc, qtype, qclass; #if (NGX_HAVE_INET6) ngx_uint_t qident6; #endif ngx_queue_t *q; ngx_resolver_qs_t *qs; ngx_resolver_hdr_t *response; ngx_resolver_node_t *rn; // n为接收数据的长度，是否小于dns报头大小 if (n < sizeof(ngx_resolver_hdr_t)) { goto short_response; } response = (ngx_resolver_hdr_t *) buf; ident = (response->ident_hi << 8) + response->ident_lo; flags = (response->flags_hi << 8) + response->flags_lo; nqs = (response->nqs_hi << 8) + response->nqs_lo; nan = (response->nan_hi << 8) + response->nan_lo; // 应答记录数 trunc = flags & 0x0200; // QR:0 opcode:0000 AA:0 TC:1 RD:0 RA:0 Z:000 RDCODE:0000 ngx_log_debug6(NGX_LOG_DEBUG_CORE, r->log, 0, "resolver DNS response %ui fl:%04Xi %ui/%ui/%ud/%ud", ident, flags, nqs, nan, (response->nns_hi << 8) + response->nns_lo, (response->nar_hi << 8) + response->nar_lo); /* response to a standard query */ 0xf870 : 1 1111 0 0 0 0 111 0000 0x8000 : 1 0000 0 0 0 0 000 0000 QR == 1 表示相应报文 if ((flags & 0xf870) != 0x8000 || (trunc && tcp)) { ngx_log_error(r->log_level, r->log, 0, "invalid %s DNS response %ui fl:%04Xi", tcp ? "TCP" : "UDP", ident, flags); return; } // Response code code = flags & 0xf; if (code == NGX_RESOLVE_FORMERR) { // 报文格式错误 times = 0; for (q = ngx_queue_head(&r->name_resend_queue); q != ngx_queue_sentinel(&r->name_resend_queue) && times++ < 100; q = ngx_queue_next(q)) { rn = ngx_queue_data(q, ngx_resolver_node_t, queue); qident = (rn->query[0] << 8) + rn->query[1]; if (qident == ident) { goto dns_error_name; } #if (NGX_HAVE_INET6) if (rn->query6) { qident6 = (rn->query6[0] << 8) + rn->query6[1]; if (qident6 == ident) { goto dns_error_name; } } #endif } goto dns_error; } if (code > NGX_RESOLVE_REFUSED) { goto dns_error; } // 查询记录数 if (nqs != 1) { err = "invalid number of questions in DNS response"; goto done; } i = sizeof(ngx_resolver_hdr_t); while (i < (ngx_uint_t) n) { if (buf[i] == '\0') { goto found; } i += 1 + buf[i]; } goto short_response; found: if (i++ == sizeof(ngx_resolver_hdr_t)) { err = "zero-length domain name in DNS response"; goto done; } if (i + sizeof(ngx_resolver_qs_t) + nan * (2 + sizeof(ngx_resolver_an_t)) > (ngx_uint_t) n) { goto short_response; } qs = (ngx_resolver_qs_t *) &buf[i]; qtype = (qs->type_hi << 8) + qs->type_lo; qclass = (qs->class_hi << 8) + qs->class_lo; ngx_log_debug2(NGX_LOG_DEBUG_CORE, r->log, 0, "resolver DNS response qt:%ui cl:%ui", qtype, qclass); if (qclass != 1) { ngx_log_error(r->log_level, r->log, 0, "unknown query class %ui in DNS response", qclass); return; } switch (qtype) { case NGX_RESOLVE_A: #if (NGX_HAVE_INET6) case NGX_RESOLVE_AAAA: #endif // 解析A记录，IP地址 // buf：DNS应答内容 n：内容长度 // ident：ID标识 code：应答码 qtype：协议类型 nan：应答记录数 trunc 是否截断 ngx_resolver_process_a(r, buf, n, ident, code, qtype, nan, trunc, i + sizeof(ngx_resolver_qs_t)); break; case NGX_RESOLVE_SRV: ngx_resolver_process_srv(r, buf, n, ident, code, nan, trunc, i + sizeof(ngx_resolver_qs_t)); break; case NGX_RESOLVE_PTR: ngx_resolver_process_ptr(r, buf, n, ident, code, nan); break; default: ngx_log_error(r->log_level, r->log, 0, "unknown query type %ui in DNS response", qtype); return; } return; short_response: err = "short DNS response"; done: ngx_log_error(r->log_level, r->log, 0, err); return; dns_error_name: ngx_log_error(r->log_level, r->log, 0, "DNS error (%ui: %s), query id:%ui, name:\"%*s\"", code, ngx_resolver_strerror(code), ident, (size_t) rn->nlen, rn->name); return; dns_error: ngx_log_error(r->log_level, r->log, 0, "DNS error (%ui: %s), query id:%ui", code, ngx_resolver_strerror(code), ident); return; } // 解析A记录 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) { char *err; u_char *cname; size_t len; int32_t ttl; uint32_t hash; in_addr_t *addr; ngx_str_t name; ngx_uint_t type, class, qident, naddrs, a, i, j, start; #if (NGX_HAVE_INET6) struct in6_addr *addr6; #endif ngx_resolver_an_t *an; ngx_resolver_ctx_t *ctx, *next; ngx_resolver_node_t *rn; ngx_resolver_addr_t *addrs; ngx_resolver_connection_t *rec; // 解析请求的域名 if (ngx_resolver_copy(r, &name, buf, buf + sizeof(ngx_resolver_hdr_t), buf + n) != NGX_OK) { return; } ngx_log_debug1(NGX_LOG_DEBUG_CORE, r->log, 0, "resolver qs:%V", &name); hash = ngx_crc32_short(name.data, name.len); /* lock name mutex */ rn = ngx_resolver_lookup_name(r, &name, hash); if (rn == NULL) { ngx_log_error(r->log_level, r->log, 0, "unexpected response for %V", &name); ngx_resolver_free(r, name.data); goto failed; } switch (qtype) { #if (NGX_HAVE_INET6) case NGX_RESOLVE_AAAA: if (rn->query6 == NULL || rn->naddrs6 != (u_short) -1) { ngx_log_error(r->log_level, r->log, 0, "unexpected response for %V", &name); ngx_resolver_free(r, name.data); goto failed; } if (trunc && rn->tcp6) { ngx_resolver_free(r, name.data); goto failed; } qident = (rn->query6[0] << 8) + rn->query6[1]; break; #endif default: /* NGX_RESOLVE_A */ if (rn->query == NULL || rn->naddrs != (u_short) -1) { ngx_log_error(r->log_level, r->log, 0, "unexpected response for %V", &name); ngx_resolver_free(r, name.data); goto failed; } if (trunc && rn->tcp) { ngx_resolver_free(r, name.data); goto failed; } // 请求ID标识 qident = (rn->query[0] << 8) + rn->query[1]; } if (ident != qident) { ngx_log_error(r->log_level, r->log, 0, "wrong ident %ui response for %V, expect %ui", ident, &name, qident); ngx_resolver_free(r, name.data); goto failed; } ngx_resolver_free(r, name.data); if (trunc) { ngx_queue_remove(&rn->queue); if (rn->waiting == NULL) { ngx_rbtree_delete(&r->name_rbtree, &rn->node); ngx_resolver_free_node(r, rn); goto next; } rec = r->connections.elts; rec = &rec[rn->last_connection]; switch (qtype) { #if (NGX_HAVE_INET6) case NGX_RESOLVE_AAAA: rn->tcp6 = 1; (void) ngx_resolver_send_tcp_query(r, rec, rn->query6, rn->qlen); break; #endif default: /* NGX_RESOLVE_A */ rn->tcp = 1; (void) ngx_resolver_send_tcp_query(r, rec, rn->query, rn->qlen); } rn->expire = ngx_time() + r->resend_timeout; ngx_queue_insert_head(&r->name_resend_queue, &rn->queue); goto next; } if (code == 0 && rn->code) { code = rn->code; } if (code == 0 && nan == 0) { #if (NGX_HAVE_INET6) switch (qtype) { case NGX_RESOLVE_AAAA: rn->naddrs6 = 0; if (rn->naddrs == (u_short) -1) { goto next; } if (rn->naddrs) { goto export; } break; default: /* NGX_RESOLVE_A */ rn->naddrs = 0; if (rn->naddrs6 == (u_short) -1) { goto next; } if (rn->naddrs6) { goto export; } } #endif code = NGX_RESOLVE_NXDOMAIN; } if (code) { #if (NGX_HAVE_INET6) switch (qtype) { case NGX_RESOLVE_AAAA: rn->naddrs6 = 0; if (rn->naddrs == (u_short) -1) { rn->code = (u_char) code; goto next; } break; default: /* NGX_RESOLVE_A */ rn->naddrs = 0; if (rn->naddrs6 == (u_short) -1) { rn->code = (u_char) code; goto next; } } #endif next = rn->waiting; rn->waiting = NULL; ngx_queue_remove(&rn->queue); ngx_rbtree_delete(&r->name_rbtree, &rn->node); /* unlock name mutex */ while (next) { ctx = next; ctx->state = code; ctx->valid = ngx_time() + (r->valid ? r->valid : 10); next = ctx->next; ctx->handler(ctx); } ngx_resolver_free_node(r, rn); return; } i = ans; naddrs = 0; cname = NULL; for (a = 0; a < nan; a++) { start = i; while (i < n) { if (buf[i] & 0xc0) { i += 2; goto found; } if (buf[i] == 0) { i++; goto test_length; } i += 1 + buf[i]; } goto short_response; test_length: if (i - start < 2) { err = "invalid name in DNS response"; goto invalid; } found: if (i + sizeof(ngx_resolver_an_t) >= n) { goto short_response; } an = (ngx_resolver_an_t *) &buf[i]; type = (an->type_hi << 8) + an->type_lo; class = (an->class_hi << 8) + an->class_lo; len = (an->len_hi << 8) + an->len_lo; ttl = (an->ttl[0] << 24) + (an->ttl[1] << 16) + (an->ttl[2] << 8) + (an->ttl[3]); if (class != 1) { ngx_log_error(r->log_level, r->log, 0, "unexpected RR class %ui", class); goto failed; } if (ttl < 0) { ttl = 0; } rn->ttl = ngx_min(rn->ttl, (uint32_t) ttl); i += sizeof(ngx_resolver_an_t); switch (type) { case NGX_RESOLVE_A: if (qtype != NGX_RESOLVE_A) { err = "unexpected A record in DNS response"; goto invalid; } if (len != 4) { err = "invalid A record in DNS response"; goto invalid; } if (i + 4 > n) { goto short_response; } naddrs++; break; #if (NGX_HAVE_INET6) case NGX_RESOLVE_AAAA: if (qtype != NGX_RESOLVE_AAAA) { err = "unexpected AAAA record in DNS response"; goto invalid; } if (len != 16) { err = "invalid AAAA record in DNS response"; goto invalid; } if (i + 16 > n) { goto short_response; } naddrs++; break; #endif case NGX_RESOLVE_CNAME: cname = &buf[i]; break; case NGX_RESOLVE_DNAME: break; default: ngx_log_error(r->log_level, r->log, 0, "unexpected RR type %ui", type); } i += len; } ngx_log_debug3(NGX_LOG_DEBUG_CORE, r->log, 0, "resolver naddrs:%ui cname:%p ttl:%uD", naddrs, cname, rn->ttl); if (naddrs) { switch (qtype) { #if (NGX_HAVE_INET6) case NGX_RESOLVE_AAAA: if (naddrs == 1) { addr6 = &rn->u6.addr6; rn->naddrs6 = 1; } else { addr6 = ngx_resolver_alloc(r, naddrs * sizeof(struct in6_addr)); if (addr6 == NULL) { goto failed; } rn->u6.addrs6 = addr6; rn->naddrs6 = (u_short) naddrs; } #if (NGX_SUPPRESS_WARN) addr = NULL; #endif break; #endif default: /* NGX_RESOLVE_A */ if (naddrs == 1) { addr = &rn->u.addr; rn->naddrs = 1; } else { addr = ngx_resolver_alloc(r, naddrs * sizeof(in_addr_t)); if (addr == NULL) { goto failed; } rn->u.addrs = addr; rn->naddrs = (u_short) naddrs; } #if (NGX_HAVE_INET6 && NGX_SUPPRESS_WARN) addr6 = NULL; #endif } j = 0; i = ans; for (a = 0; a < nan; a++) { for ( ;; ) { if (buf[i] & 0xc0) { i += 2; break; } if (buf[i] == 0) { i++; break; } i += 1 + buf[i]; } an = (ngx_resolver_an_t *) &buf[i]; type = (an->type_hi << 8) + an->type_lo; len = (an->len_hi << 8) + an->len_lo; i += sizeof(ngx_resolver_an_t); if (type == NGX_RESOLVE_A) { addr[j] = htonl((buf[i] << 24) + (buf[i + 1] << 16) + (buf[i + 2] << 8) + (buf[i + 3])); if (++j == naddrs) { #if (NGX_HAVE_INET6) if (rn->naddrs6 == (u_short) -1) { goto next; } #endif break; } } #if (NGX_HAVE_INET6) else if (type == NGX_RESOLVE_AAAA) { ngx_memcpy(addr6[j].s6_addr, &buf[i], 16); if (++j == naddrs) { if (rn->naddrs == (u_short) -1) { goto next; } break; } } #endif i += len; } } switch (qtype) { #if (NGX_HAVE_INET6) case NGX_RESOLVE_AAAA: if (rn->naddrs6 == (u_short) -1) { rn->naddrs6 = 0; } break; #endif default: /* NGX_RESOLVE_A */ if (rn->naddrs == (u_short) -1) { rn->naddrs = 0; } } if (rn->naddrs != (u_short) -1 #if (NGX_HAVE_INET6) && rn->naddrs6 != (u_short) -1 #endif && rn->naddrs #if (NGX_HAVE_INET6) + rn->naddrs6 #endif > 0) { #if (NGX_HAVE_INET6) export: #endif naddrs = rn->naddrs; #if (NGX_HAVE_INET6) naddrs += rn->naddrs6; #endif if (naddrs == 1 && rn->naddrs == 1) { addrs = NULL; } else { addrs = ngx_resolver_export(r, rn, 0); if (addrs == NULL) { goto failed; } } ngx_queue_remove(&rn->queue); rn->valid = ngx_time() + (r->valid ? r->valid : (time_t) rn->ttl); rn->expire = ngx_time() + r->expire; ngx_queue_insert_head(&r->name_expire_queue, &rn->queue); next = rn->waiting; rn->waiting = NULL; /* unlock name mutex */ while (next) { ctx = next; 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); } if (addrs != NULL) { ngx_resolver_free(r, addrs->sockaddr); ngx_resolver_free(r, addrs); } ngx_resolver_free(r, rn->query); rn->query = NULL; #if (NGX_HAVE_INET6) rn->query6 = NULL; #endif return; } if (cname) { /* CNAME only */ if (rn->naddrs == (u_short) -1 #if (NGX_HAVE_INET6) || rn->naddrs6 == (u_short) -1 #endif ) { goto next; } if (ngx_resolver_copy(r, &name, buf, cname, buf + n) != NGX_OK) { goto failed; } ngx_log_debug1(NGX_LOG_DEBUG_CORE, r->log, 0, "resolver cname:\"%V\"", &name); ngx_queue_remove(&rn->queue); rn->cnlen = (u_short) name.len; rn->u.cname = name.data; rn->valid = ngx_time() + (r->valid ? r->valid : (time_t) rn->ttl); rn->expire = ngx_time() + r->expire; ngx_queue_insert_head(&r->name_expire_queue, &rn->queue); ngx_resolver_free(r, rn->query); rn->query = NULL; #if (NGX_HAVE_INET6) rn->query6 = NULL; #endif ctx = rn->waiting; rn->waiting = NULL; if (ctx) { if (ctx->recursion++ >= NGX_RESOLVER_MAX_RECURSION) { /* unlock name mutex */ do { ctx->state = NGX_RESOLVE_NXDOMAIN; next = ctx->next; ctx->handler(ctx); ctx = next; } while (ctx); return; } for (next = ctx; next; next = next->next) { next->node = NULL; } (void) ngx_resolve_name_locked(r, ctx, &name); } /* unlock name mutex */ return; } ngx_log_error(r->log_level, r->log, 0, "no A or CNAME types in DNS response"); return; short_response: err = "short DNS response"; invalid: /* unlock name mutex */ ngx_log_error(r->log_level, r->log, 0, err); return; failed: next: /* unlock name mutex */ return; }
reading
selboo
commented
7 years ago
概述
域名在互联网中广泛应用，而http是建立在tcp/ip协议上的，tcp/ip协议只认识ip地址，所以就需要通过某个系统（DNS）把域名转换成ip地址供底层使用。 通常linux系统下，命令行使用dig查询，c语言使用gethostbyname或getaddrinfo函数查询。实际上都是发送一个网络请求到/etc/resolv.conf下的一个服务器查询。
nginx作为一个通用的服务器也会涉及到域名解析。例如，nginx 用作反向代理可以配置upstream是一个域名。
proxy_pass http://www.baidu.com/$request_uri;那么www.baidu.com这个域名是什么时候解析的呢？又是如何解析的呢？如果没有添加$request_uri这个变量解析会有区别么？nginx是单进程异步非阻塞服务，如果用getaddrinfo来解析势必会导致nginx阻塞。在启动阶段阻塞一小会儿不会有大的影响，如果在服务处理阶段阻塞，会导致该进程所服务的客户端延迟甚至出错。
proxy_pass 对应的域名解析分为有变量和没有变量，没有变量的是在启动阶段解析， 而有变量的是在每次请求解析的，每次请求解析在nginx是怎么做的？还是异步回调。
DNS报文
nginx会拼装DNS查询报文，所以现了解一下DNS报文格式。也可以略过直接看大框架逻辑。
报文格式
说明： ID: 2个字节(16bit)，标识字段，客户端会解析服务器返回的DNS应答报文，获取ID值与请求报文设置的ID值做比较，如果相同，则认为是同一个DNS会话。 FLAGS: 2个字节(16bit)的标志字段。包含以下属性: QR: 0表示查询报文，1表示响应报文; opcode: 通常值为0（标准查询），其他值为1（反向查询）和2（服务器状态请求）,[3,15]保留值; AA: 表示授权回答（authoritative answer）– 这个比特位在应答的时候才有意义，指出给出应答的服务器是查询域名的授权解析服务器; TC: 表示可截断的（truncated）–用来指出报文比允许的长度还要长，导致被截断; RD: 表示期望递归(Recursion Desired) – 这个比特位被请求设置，应答的时候使用的相同的值返回。如果设置了RD，就建议域名服务器进行递归解析，递归查询的支持是可选的; RA: 表示支持递归(Recursion Available) – 这个比特位在应答中设置或取消，用来代表服务器是否支持递归查询; Z : 保留值，暂未使用; RCODE: 应答码(Response code) - 这4个比特位在应答报文中设置，代表的含义如下: 0 : 没有错误。 QDCOUNT: 无符号16bit整数表示报文请求段中的问题记录数。 ANCOUNT: 无符号16bit整数表示报文回答段中的回答记录数。 ANCOUNT: 无符号16bit整数表示报文回答段中的回答记录数。 ARCOUNT: 无符号16bit整数表示报文附加段中的附加记录数。
说明： QNAME 8bit为单位表示的查询名(广泛的说就是：域名). QTYPE 无符号16bit整数表示查询的协议类型. QCLASS 无符号16bit整数表示查询的类,比如，IN代表Internet.
说明： NAME 资源记录包含的域名. TYPE 表示DNS协议的类型. CLASS 表示RDATA的类. TTL 4字节无符号整数表示资源记录可以缓存的时间。0代表只能被传输，但是不能被缓存。 RDLENGTH 2个字节无符号整数表示RDATA的长度 RDATA 不定长字符串来表示记录，格式根TYPE和CLASS有关。比如，TYPE是A，CLASS 是 IN，那么RDATA就是一个4个字节的ARPA网络地址。
ngx中报文结构体
nginx域名解析源码分析
ngx_inet_resolve_host函数。该函数实际上是封装了getaddrinfo函数，是同步阻塞解析。
异步回调解析，下面以proxy模块的代码来分析异步非阻塞回调的域名解析。
启动阶段，在proxy模块的ngx_http_proxy_eval函数中，url不是ip地址，则u->resolved->sockaddr不会赋值，u->resolved->host是域名。
请求处理阶段，在解析完header头，进入content阶段，调用proxy模块的ngx_http_proxy_handler回调函数，该函数通过调用ngx_http_read_client_request_body(r, ngx_http_upstream_init)，读取body并启动upstream。读取完body后（或者读到body）调用ngx_http_upstream_init函数，该函数又调用了ngx_http_upstream_init_request函数。
在ngx_http_upstream_init_request函数中会查找上游服务器。如果是域名则会调用ngx_resolve_start函数分配域名解析结构体，调用ngx_resolve_name函数解析，该函数会创建一个事件结构添加到epoll中，解析完毕后回调ctx->handler函数，即ngx_http_upstream_resolve_handler。
ngx_resolver.h|c
主要结构：
主要函数：
主要函数实现