🌐 evdns异步DNS - libevent源码深度解析系列（十九）
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🌐 evdns异步DNS - libevent源码深度解析系列（十九）
🌐 evdns异步DNS
一、概述
evdns 是libevent提供的异步DNS解析库，它允许应用程序在不阻塞的情况下进行DNS查询。evdns支持正向解析（域名到IP）、反向解析（IP到域名）、以及运行DNS服务器。
二、evdns_base结构体
struct evdns_base { struct event_base *event_base;
struct evdns_server_port **server_ports; int n_server_ports;
struct evdns_request **reqs; int n_reqs;
struct nameserver **servers; int n_servers;
struct timeval global_nameserver_probe_initial_timeout; struct timeval global_nameserver_probe_period; // ...};三、创建与销毁
3.1 创建evdns_base
/* 创建evdns_base * @param base event_base * @param init_nameservers 是否初始化系统DNS服务器 * 1: 从/etc/resolv.conf读取 * 0: 不初始化，需要手动添加 */struct evdns_base *evdns_base_new(struct event_base *base, int init_nameservers);
/* 释放evdns_base * @param fail_requests 是否让pending的请求失败 * 1: 请求回调会被调用并返回错误 * 0: 请求回调不会被调用 */void evdns_base_free(struct evdns_base *base, int fail_requests);3.2 基本示例
#include <stdio.h>#include <event2/event.h>#include <event2/dns.h>
int main() { struct event_base *base = event_base_new(); struct evdns_base *dns_base = evdns_base_new(base, 1);
if (!dns_base) { fprintf(stderr, "Failed to create dns base\n"); return 1; }
printf("evdns_base created successfully\n");
evdns_base_free(dns_base, 0); event_base_free(base); return 0;}四、DNS服务器配置
4.1 添加DNS服务器
/* 添加DNS服务器（IPv4地址） */int evdns_base_nameserver_add(struct evdns_base *base, unsigned long address);
/* 添加DNS服务器（支持IPv6） */int evdns_base_nameserver_ip_add(struct evdns_base *base, const char *ip_as_string);
/* 添加Unix域套接字DNS服务器 */int evdns_base_nameserver_sockaddr_add(struct evdns_base *base, const struct sockaddr *sa, ev_socklen_t len, unsigned flags);
/* 清除所有DNS服务器 */void evdns_base_clear_nameservers_and_suspend(struct evdns_base *base);
/* 恢复DNS解析 */int evdns_base_resume(struct evdns_base *base);
/* 设置DNS服务器探测超时 */void evdns_base_set_nameserver_probe_timeout(struct evdns_base *base, const struct timeval *timeout);4.2 配置示例
#include <stdio.h>#include <arpa/inet.h>#include <event2/event.h>#include <event2/dns.h>
int main() { struct event_base *base = event_base_new();
// 不自动初始化，手动添加DNS服务器 struct evdns_base *dns_base = evdns_base_new(base, 0);
// 添加Google DNS (8.8.8.8) evdns_base_nameserver_add(dns_base, inet_addr("8.8.8.8"));
// 添加Cloudflare DNS (1.1.1.1) evdns_base_nameserver_ip_add(dns_base, "1.1.1.1");
// 添加IPv6 DNS服务器 evdns_base_nameserver_ip_add(dns_base, "[2001:4860:4860::8888]:53");
printf("DNS servers configured\n");
evdns_base_free(dns_base, 0); event_base_free(base); return 0;}五、正向DNS解析
5.1 解析API
/* DNS回调函数类型 * @param result DNS结果码 * @param type 记录类型 (DNS_IPv4_A, DNS_IPv6_AAAA等) * @param count 返回的地址数量 * @param ttl 生存时间 * @param addresses 地址数组 * @param arg 用户参数 */typedef void (*evdns_callback_type)(int result, char type, int count, int ttl, void *addresses, void *arg);
/* 异步解析IPv4地址 * @param name 域名 * @param flags 解析标志 * @param callback 回调函数 * @param arg 用户参数 * @return evdns_request指针，可用于取消请求 */struct evdns_request *evdns_base_resolve_ipv4( struct evdns_base *base, const char *name, int flags, evdns_callback_type callback, void *arg);
/* 异步解析IPv6地址 */struct evdns_request *evdns_base_resolve_ipv6( struct evdns_base *base, const char *name, int flags, evdns_callback_type callback, void *arg);
/* 解析任意记录类型 */struct evdns_request *evdns_base_resolve( struct evdns_base *base, const char *name, int type, int flags, evdns_callback_type callback, void *arg);5.2 解析标志
#define DNS_QUERY_NO_SEARCH 0x01 // 不使用搜索列表#define DNS_IPv4_A 1 // A记录#define DNS_IPv6_AAAA 28 // AAAA记录#define DNS_PTR 12 // PTR记录#define DNS_CNAME 5 // CNAME记录#define DNS_MX 15 // MX记录#define DNS_TXT 16 // TXT记录5.3 DNS结果码
#define DNS_ERR_NONE 0 // 成功#define DNS_ERR_FORMAT 1 // 查询格式错误#define DNS_ERR_SERVERFAILED 2 // 服务器错误#define DNS_ERR_NOTEXIST 3 // 域名不存在#define DNS_ERR_NOTIMPL 4 // 未实现#define DNS_ERR_REFUSED 5 // 拒绝#define DNS_ERR_TRUNCATED 6 // 响应被截断#define DNS_ERR_UNKNOWN 7 // 未知错误#define DNS_ERR_TIMEOUT 8 // 超时#define DNS_ERR_SHUTDOWN 9 // DNS服务关闭#define DNS_ERR_CANCEL 10 // 请求被取消#define DNS_ERR_NODATA 11 // 无数据5.4 正向解析示例
#include <stdio.h>#include <stdlib.h>#include <arpa/inet.h>#include <event2/event.h>#include <event2/dns.h>
static void dns_callback(int result, char type, int count, int ttl, void *addresses, void *arg) { const char *hostname = (const char *)arg;
if (result == DNS_ERR_NONE) { printf("Resolved %s:\n", hostname);
if (type == DNS_IPv4_A) { struct in_addr *addrs = (struct in_addr *)addresses; for (int i = 0; i < count; i++) { printf(" IPv4: %s (TTL: %d)\n", inet_ntoa(addrs[i]), ttl); } } else if (type == DNS_IPv6_AAAA) { char addr_str[INET6_ADDRSTRLEN]; struct in6_addr *addrs = (struct in6_addr *)addresses; for (int i = 0; i < count; i++) { inet_ntop(AF_INET6, &addrs[i], addr_str, sizeof(addr_str)); printf(" IPv6: %s (TTL: %d)\n", addr_str, ttl); } } } else { printf("DNS lookup failed for %s: %s\n", hostname, evdns_err_to_string(result)); }
// 退出事件循环 event_base_loopbreak((struct event_base *)arg);}
int main(int argc, char *argv[]) { const char *hostname = argc > 1 ? argv[1] : "www.google.com";
struct event_base *base = event_base_new(); struct evdns_base *dns_base = evdns_base_new(base, 1);
printf("Resolving %s...\n", hostname);
// 解析IPv4 evdns_base_resolve_ipv4(dns_base, hostname, 0, dns_callback, (void *)hostname);
event_base_dispatch(base);
evdns_base_free(dns_base, 0); event_base_free(base);
return 0;}六、反向DNS解析
6.1 反向解析API
/* 反向解析IPv4地址 */struct evdns_request *evdns_base_resolve_reverse( struct evdns_base *base, const struct in_addr *in, int flags, evdns_callback_type callback, void *arg);
/* 反向解析IPv6地址 */struct evdns_request *evdns_base_resolve_reverse_ipv6( struct evdns_base *base, const struct in6_addr *in, int flags, evdns_callback_type callback, void *arg);6.2 反向解析示例
#include <stdio.h>#include <stdlib.h>#include <string.h>#include <arpa/inet.h>#include <event2/event.h>#include <event2/dns.h>
static void reverse_callback(int result, char type, int count, int ttl, void *addresses, void *arg) { if (result == DNS_ERR_NONE) { printf("PTR records:\n"); char **names = (char **)addresses; for (int i = 0; i < count; i++) { printf(" %s (TTL: %d)\n", names[i], ttl); } } else { printf("Reverse lookup failed: %s\n", evdns_err_to_string(result)); }
event_base_loopbreak((struct event_base *)arg);}
int main(int argc, char *argv[]) { const char *ip_str = argc > 1 ? argv[1] : "8.8.8.8";
struct event_base *base = event_base_new(); struct evdns_base *dns_base = evdns_base_new(base, 1);
struct in_addr addr; inet_pton(AF_INET, ip_str, &addr);
printf("Reverse lookup for %s...\n", ip_str); evdns_base_resolve_reverse(dns_base, &addr, 0, reverse_callback, base);
event_base_dispatch(base);
evdns_base_free(dns_base, 0); event_base_free(base);
return 0;}七、取消DNS请求
/* 取消DNS请求 */int evdns_cancel_request(struct evdns_base *base, struct evdns_request *req);取消示例
#include <stdio.h>#include <stdlib.h>#include <event2/event.h>#include <event2/dns.h>
static struct evdns_request *pending_req = NULL;
static void dns_callback(int result, char type, int count, int ttl, void *addresses, void *arg) { if (result == DNS_ERR_CANCEL) { printf("Request was cancelled\n"); } else if (result == DNS_ERR_NONE) { printf("DNS resolved successfully\n"); } else { printf("DNS error: %s\n", evdns_err_to_string(result)); }
event_base_loopbreak((struct event_base *)arg);}
static void timer_callback(evutil_socket_t fd, short events, void *arg) { struct event_base *base = arg; printf("Timeout, cancelling DNS request...\n");
if (pending_req) { evdns_cancel_request(NULL, pending_req); pending_req = NULL; }}
int main() { struct event_base *base = event_base_new(); struct evdns_base *dns_base = evdns_base_new(base, 1);
// 发起DNS请求 pending_req = evdns_base_resolve_ipv4(dns_base, "www.example.com", 0, dns_callback, base);
// 设置1秒超时 struct event *timer = event_new(base, -1, 0, timer_callback, base); struct timeval tv = {1, 0}; event_add(timer, &tv);
event_base_dispatch(base);
event_free(timer); evdns_base_free(dns_base, 0); event_base_free(base);
return 0;}八、DNS服务器
8.1 创建DNS服务器
/* DNS请求回调函数类型 */typedef void (*evdns_request_callback_fn_type)(struct evdns_server_request *, void *);
/* 创建DNS服务器端口 */struct evdns_server_port *evdns_add_server_port( struct event_base *base, evutil_socket_t socket, int flags, evdns_request_callback_fn_type cb, void *arg);
/* 创建DNS服务器端口（带地址） */struct evdns_server_port *evdns_add_server_port_with_listener( struct event_base *base, struct evconnlistener *listener, int flags, evdns_request_callback_fn_type cb, void *arg);
/* 释放DNS服务器端口 */void evdns_close_server_port(struct evdns_server_port *port);8.2 DNS服务器请求处理
/* 获取请求中的问题数量 */int evdns_server_request_get_requesting_addr( struct evdns_server_request *req, struct sockaddr *sa, socklen_t *len);
/* 获取问题 */int evdns_server_request_get_requesting_nameserver_addr( struct evdns_server_request *req, struct sockaddr *sa, socklen_t *len);
/* 回复A记录 */int evdns_server_request_add_a_reply(struct evdns_server_request *req, const char *name, int n, const void *addrs, int ttl);
/* 回复AAAA记录 */int evdns_server_request_add_aaaa_reply(struct evdns_server_request *req, const char *name, int n, const void *addrs, int ttl);
/* 回复PTR记录 */int evdns_server_request_add_ptr_reply(struct evdns_server_request *req, struct in_addr *in, const char *name, int ttl);
/* 回复CNAME记录 */int evdns_server_request_add_cname_reply(struct evdns_server_request *req, const char *name, const char *cname, int ttl);
/* 发送回复 */int evdns_server_request_respond(struct evdns_server_request *req, int err);
/* 丢弃请求 */int evdns_server_request_drop(struct evdns_server_request *req);8.3 DNS服务器示例
#include <stdio.h>#include <stdlib.h>#include <string.h>#include <arpa/inet.h>#include <event2/event.h>#include <event2/dns.h>#include <event2/dns_struct.h>
static void dns_server_callback(struct evdns_server_request *request, void *arg) { printf("DNS request received\n");
// 获取问题 int n_questions = request->n_questions;
for (int i = 0; i < n_questions; i++) { struct evdns_server_question *q = request->questions[i]; printf("Question %d: name=%s, type=%d, class=%d\n", i, q->name, q->type, q->dns_question_class);
// 简单处理：对于特定域名返回固定IP if (q->type == EVDNS_TYPE_A && strcmp(q->name, "test.local.") == 0) { struct in_addr addr; inet_pton(AF_INET, "192.168.1.100", &addr); evdns_server_request_add_a_reply(request, q->name, 1, &addr, 60); } else if (q->type == EVDNS_TYPE_AAAA && strcmp(q->name, "test.local.") == 0) { struct in6_addr addr; inet_pton(AF_INET6, "::1", &addr); evdns_server_request_add_aaaa_reply(request, q->name, 1, &addr, 60); } else { // 其他请求返回NXDOMAIN evdns_server_request_respond(request, DNS_ERR_NOTEXIST); return; } }
evdns_server_request_respond(request, DNS_ERR_NONE);}
int main() { struct event_base *base = event_base_new();
// 创建UDP socket int fd = socket(AF_INET, SOCK_DGRAM, 0); evutil_make_socket_nonblocking(fd);
int reuse = 1; setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &reuse, sizeof(reuse));
struct sockaddr_in sin; memset(&sin, 0, sizeof(sin)); sin.sin_family = AF_INET; sin.sin_port = htons(5353); // 使用5353端口 sin.sin_addr.s_addr = INADDR_ANY;
bind(fd, (struct sockaddr *)&sin, sizeof(sin));
// 创建DNS服务器 struct evdns_server_port *dns_port = evdns_add_server_port( base, fd, 0, dns_server_callback, NULL);
printf("DNS server listening on port 5353\n"); printf("Try: nslookup -port=5353 test.local 127.0.0.1\n");
event_base_dispatch(base);
evdns_close_server_port(dns_port); evutil_closesocket(fd); event_base_free(base);
return 0;}九、DNS配置选项
9.1 配置API
/* 设置搜索域名 */int evdns_base_search_add(struct evdns_base *base, const char *domain);
/* 清除搜索域名 */void evdns_base_search_clear(struct evdns_base *base);
/* 设置搜索域名数量限制 */void evdns_base_search_ndots_set(struct evdns_base *base, int ndots);
/* 设置是否使用TCP */int evdns_base_set_option(struct evdns_base *base, const char *option, const char *val);
/* 常用选项 */// "timeout" - 超时时间（秒）// "max-timeouts" - 最大超时次数// "attempts" - 重试次数// "randomize-case" - 随机化大小写（DNS安全）// "use-vc" - 使用TCP// "ndots" - 点的数量阈值9.2 配置示例
#include <stdio.h>#include <event2/event.h>#include <event2/dns.h>
int main() { struct event_base *base = event_base_new(); struct evdns_base *dns_base = evdns_base_new(base, 0);
// 添加DNS服务器 evdns_base_nameserver_add(dns_base, inet_addr("8.8.8.8"));
// 设置选项 evdns_base_set_option(dns_base, "timeout", "5"); // 5秒超时 evdns_base_set_option(dns_base, "attempts", "3"); // 重试3次 evdns_base_set_option(dns_base, "use-vc", NULL); // 使用TCP evdns_base_set_option(dns_base, "randomize-case", "1"); // 启用DNS安全
// 添加搜索域名 evdns_base_search_add(dns_base, "example.com"); evdns_base_search_add(dns_base, "local");
// 设置ndots evdns_base_search_ndots_set(dns_base, 2);
printf("DNS configuration complete\n");
evdns_base_free(dns_base, 0); event_base_free(base); return 0;}十、错误处理
10.1 错误字符串
/* 将DNS错误码转换为字符串 */const char *evdns_err_to_string(int err);10.2 错误处理示例
#include <stdio.h>#include <event2/dns.h>
void print_dns_error(int err) { switch (err) { case DNS_ERR_NONE: printf("Success\n"); break; case DNS_ERR_FORMAT: printf("Query format error\n"); break; case DNS_ERR_SERVERFAILED: printf("Server failed\n"); break; case DNS_ERR_NOTEXIST: printf("Domain name does not exist\n"); break; case DNS_ERR_TIMEOUT: printf("Query timeout\n"); break; case DNS_ERR_CANCEL: printf("Request cancelled\n"); break; default: printf("Unknown error: %s\n", evdns_err_to_string(err)); }}十一、完整示例：异步HTTP客户端DNS解析
#include <stdio.h>#include <stdlib.h>#include <string.h>#include <arpa/inet.h>#include <event2/event.h>#include <event2/dns.h>#include <event2/bufferevent.h>
typedef struct { struct event_base *base; struct evdns_base *dns_base; const char *hostname; int port;} http_context_t;
static void http_event_cb(struct bufferevent *bev, short events, void *arg) { http_context_t *ctx = arg;
if (events & BEV_EVENT_CONNECTED) { printf("Connected to %s:%d\n", ctx->hostname, ctx->port);
// 发送HTTP请求 char request[256]; snprintf(request, sizeof(request), "GET / HTTP/1.1\r\n" "Host: %s\r\n" "Connection: close\r\n" "\r\n", ctx->hostname);
bufferevent_write(bev, request, strlen(request)); } else if (events & (BEV_EVENT_EOF | BEV_EVENT_ERROR)) { if (events & BEV_EVENT_ERROR) { printf("Connection error\n"); } bufferevent_free(bev); event_base_loopexit(ctx->base, NULL); }}
static void http_read_cb(struct bufferevent *bev, void *arg) { char buf[4096]; size_t n;
while ((n = bufferevent_read(bev, buf, sizeof(buf) - 1)) > 0) { buf[n] = '\0'; printf("%s", buf); }}
static void dns_callback(int result, char type, int count, int ttl, void *addresses, void *arg) { http_context_t *ctx = arg;
if (result != DNS_ERR_NONE || count == 0) { printf("DNS lookup failed for %s: %s\n", ctx->hostname, evdns_err_to_string(result)); event_base_loopexit(ctx->base, NULL); return; }
struct in_addr *addrs = (struct in_addr *)addresses; printf("Resolved %s to %s\n", ctx->hostname, inet_ntoa(addrs[0]));
// 创建连接 struct bufferevent *bev = bufferevent_socket_new(ctx->base, -1, BEV_OPT_CLOSE_ON_FREE); bufferevent_setcb(bev, http_read_cb, NULL, http_event_cb, ctx); bufferevent_enable(bev, EV_READ | EV_WRITE);
struct sockaddr_in sin; memset(&sin, 0, sizeof(sin)); sin.sin_family = AF_INET; sin.sin_port = htons(ctx->port); sin.sin_addr = addrs[0];
bufferevent_socket_connect(bev, (struct sockaddr *)&sin, sizeof(sin));}
int main(int argc, char *argv[]) { const char *hostname = argc > 1 ? argv[1] : "www.example.com"; int port = argc > 2 ? atoi(argv[2]) : 80;
struct event_base *base = event_base_new(); struct evdns_base *dns_base = evdns_base_new(base, 1);
http_context_t ctx = { .base = base, .dns_base = dns_base, .hostname = hostname, .port = port };
printf("Resolving %s...\n", hostname); evdns_base_resolve_ipv4(dns_base, hostname, 0, dns_callback, &ctx);
event_base_dispatch(base);
evdns_base_free(dns_base, 0); event_base_free(base);
return 0;}十二、最佳实践
| 场景 | 建议 |
|---|---|
| 生产环境 | 配置多个DNS服务器，提高可用性 |
| 超时设置 | 根据网络环境调整超时时间 |
| 错误处理 | 实现重试机制和降级策略 |
| DNS缓存 | 应用层实现缓存，减少DNS查询 |
| IPv6支持 | 同时解析IPv4和IPv6地址 |
| DNS安全 | 启用randomize-case防止DNS欺骗 |
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🌐 evdns异步DNS - libevent源码深度解析系列（十九）
https://xutopia.top/blog/libevent-evdns/