这篇笔记分析了库函数getaddrinfo()的代码实现。

原型解读

int getaddrinfo(const char *hostname, const char *servname,    const struct addrinfo *hints, struct addrinfo **res);

hostname: 和gethostbyname()的入参hostname相同，要查询的域名；
servname：要查询的服务名，如果指定，则返回值中还会有服务名对应的端口号；也就是该函数还有getservbyname()的功能；
hints：表面意思为暗示，使用该参数可以对查询结果进行约束，只返回那些满足条件的查询结果；后面称该参数为过滤器；
res：该参数用于保存指向查询结果地址的指针，保存查询结果的内存有该函数内部分配，使用完毕后调用者需要调用freeaddrinfo()清除，调用者只需要提供保存struct addrinfo *的地址即可。

关于该函数的更详细介绍可以参考getaddrinfo(3)。

入口

int getaddrinfo(const char *hostname, const char *servname,    const struct addrinfo *hints, struct addrinfo **res){//增加了入参NETID_UNSET和MARK_UNSETreturn android_getaddrinfofornet(hostname, servname, hints, NETID_UNSET, MARK_UNSET, res);}int android_getaddrinfofornet(const char *hostname, const char *servname,    const struct addrinfo *hints, unsigned netid, unsigned mark, struct addrinfo **res){//入参中netid和mark值可以影响DNS请求包从哪个网卡出去，属于网络相关的配置，将这些信息组织成参数    //netcontext继续向下传递struct android_net_context netcontext = {.app_netid = netid,.app_mark = mark,.dns_netid = netid,.dns_mark = mark,.uid = NET_CONTEXT_INVALID_UID,        };return android_getaddrinfofornetcontext(hostname, servname, hints, &netcontext, res);}

这里有必要对这两个函数做个说明：

getaddrinfo()是标准的libc接口，系统中Framework以下的C/C++代码可以调用该接口实现DNS查询；
android_getaddrinfofornet()是Android对libc的扩展，当APP在Framework通过InetAddress类提供的接口进行DNS查询时，最终会通过JNI调用到该函数，也就是说该接口是给Framework使用的。

android_getaddrinfofornetcontext()

该函数的作用主要是对输入参数的合法性进行检查，以及处理hostname为空或者是纯数字格式的情形，如果需要真正发起DNS请求，调用explore_fqdn()完成。

/* 定义了faimly、socktype、protocol字段的有效组合 */static const struct explore explore[] = {#ifdef INET6{ PF_INET6, SOCK_DGRAM, IPPROTO_UDP, "udp", 0x07 },{ PF_INET6, SOCK_STREAM, IPPROTO_TCP, "tcp", 0x07 },{ PF_INET6, SOCK_RAW, ANY, NULL, 0x05 },#endif{ PF_INET, SOCK_DGRAM, IPPROTO_UDP, "udp", 0x07 },{ PF_INET, SOCK_STREAM, IPPROTO_TCP, "tcp", 0x07 },{ PF_INET, SOCK_RAW, ANY, NULL, 0x05 },{ PF_UNSPEC, SOCK_DGRAM, IPPROTO_UDP, "udp", 0x07 },{ PF_UNSPEC, SOCK_STREAM, IPPROTO_TCP, "tcp", 0x07 },{ PF_UNSPEC, SOCK_RAW, ANY, NULL, 0x05 },{ -1, 0, 0, NULL, 0 },};int android_getaddrinfofornetcontext(const char *hostname, const char *servname,    const struct addrinfo *hints, const struct android_net_context *netcontext,    struct addrinfo **res){//这里定义了3个addrinfo结构变量，后面对于这些变量的使用非常绕，要注意理解下面的注释说明struct addrinfo sentinel;struct addrinfo *cur;int error = 0;struct addrinfo ai;struct addrinfo ai0;struct addrinfo *pai;const struct explore *ex;/* hostname is allowed to be NULL *//* servname is allowed to be NULL *//* hints is allowed to be NULL */assert(res != NULL);assert(netcontext != NULL);    //sentinel清零，并且cur指向该结构memset(&sentinel, 0, sizeof(sentinel));cur = &sentinel;    //pai指向ai，并且对ai的成员进行初始化pai = &ai;pai->ai_flags = 0;pai->ai_family = PF_UNSPEC;pai->ai_socktype = ANY;pai->ai_protocol = ANY;pai->ai_addrlen = 0;pai->ai_canonname = NULL;pai->ai_addr = NULL;pai->ai_next = NULL;    //hostname和servname二者不能同时为空，至少要指定一个if (hostname == NULL && servname == NULL)return EAI_NONAME;//如果指定了过滤器，则对过滤器成员进行检查，因为作为过滤器，并非addrinfo中的每个成员都是可以指定的if (hints) {/* error check for hints */        //作为过滤器，addrinfo的这四个成员是不能指定的，必须保持为0才行if (hints->ai_addrlen || hints->ai_canonname ||    hints->ai_addr || hints->ai_next)ERR(EAI_BADHINTS); /* xxx */        //ai_flags中有没有指定非法的标记if (hints->ai_flags & ~AI_MASK)ERR(EAI_BADFLAGS);        //ai_faimly成员只可取下面这三个值，其它值均非法switch (hints->ai_family) {case PF_UNSPEC:case PF_INET:#ifdef INET6case PF_INET6:#endifbreak;default:ERR(EAI_FAMILY);}        //从这里可以看出，过滤器参数最终是被保存在了ai中，这就是ai的作用memcpy(pai, hints, sizeof(*pai));/* * if both socktype/protocol are specified, check if they * are meaningful combination. */        //如果同时指定了hints参数中的ai_socktype和ai_protocol字段（非0），那么继续检查这两个参数的组合是否合理        //因为并非所有的组合都是有效的，比如socktype指定为STREAM，但是protocol指定为DGRAM就不合理if (pai->ai_socktype != ANY && pai->ai_protocol != ANY) {        //expore数组定义了faimly、socktype、protocol的有效组合for (ex = explore; ex->e_af >= 0; ex++) {if (pai->ai_family != ex->e_af)continue;if (ex->e_socktype == ANY)continue;if (ex->e_protocol == ANY)continue;                //如果是非法组合返回BADHINTS错误if (pai->ai_socktype == ex->e_socktype && pai->ai_protocol != ex->e_protocol) {ERR(EAI_BADHINTS);}}}}//end of "if (hints)"/* * check for special cases.  (1) numeric servname is disallowed if * socktype/protocol are left unspecified. (2) servname is disallowed * for raw and other inet{,6} sockets. */    //这段逻辑用于检查服务名和hints参数是否存在冲突（见注释）if (MATCH_FAMILY(pai->ai_family, PF_INET, 1)#ifdef PF_INET6 || MATCH_FAMILY(pai->ai_family, PF_INET6, 1)#endif    ) {        //ai0的作用就是备份hints的内容ai0 = *pai;/* backup *pai */if (pai->ai_family == PF_UNSPEC) {#ifdef PF_INET6pai->ai_family = PF_INET6;#elsepai->ai_family = PF_INET;#endif}error = get_portmatch(pai, servname);if (error)ERR(error);//将备份的hints参数内容再赋值回去，保证这步检查不会修改hints本身的内容*pai = ai0;}ai0 = *pai;    //处理hostname为空，或者hostname为纯数字的情形/* NULL hostname, or numeric hostname */for (ex = explore; ex->e_af >= 0; ex++) {*pai = ai0;/* PF_UNSPEC entries are prepared for DNS queries only */if (ex->e_af == PF_UNSPEC)continue;if (!MATCH_FAMILY(pai->ai_family, ex->e_af, WILD_AF(ex)))continue;if (!MATCH(pai->ai_socktype, ex->e_socktype, WILD_SOCKTYPE(ex)))continue;if (!MATCH(pai->ai_protocol, ex->e_protocol, WILD_PROTOCOL(ex)))continue;if (pai->ai_family == PF_UNSPEC)pai->ai_family = ex->e_af;if (pai->ai_socktype == ANY && ex->e_socktype != ANY)pai->ai_socktype = ex->e_socktype;if (pai->ai_protocol == ANY && ex->e_protocol != ANY)pai->ai_protocol = ex->e_protocol;if (hostname == NULL)        //处理只请求服务名转换的情况，转黄结果保存到cur->ai_next中。            //从这里可以看出sentinel的作用来，它本身不保存查询结果，但是查询结果就保存在            //sentinel.ai_next后面的addrinfo链表中，而cur指针永远指向当前链表的最后一个成员，            //这样便于查询过程中不断添加查询结果，见下面的while循环error = explore_null(pai, servname, &cur->ai_next);else        //尝试按照数字格式解析hostname以及服务名装换error = explore_numeric_scope(pai, hostname, servname,    &cur->ai_next);if (error)goto free;//移动cur指针，使其指向当前查询结果链表的最后一个成员while (cur->ai_next)cur = cur->ai_next;}/* * XXX * If numeric representation of AF1 can be interpreted as FQDN * representation of AF2, we need to think again about the code below. */    //如果这里非空，说明上一个for循环中已经解析的结果，请求的hostname就是为空或者是纯数字格式，    //那么无需再发起DNS查询，返回结果即可if (sentinel.ai_next)goto good;//hostname为空的场景上面应该已经处理，不应该会执行到这里if (hostname == NULL)ERR(EAI_NODATA);    //hostname为数字格式的情形上面应该已经处理，不应该会执行到这里if (pai->ai_flags & AI_NUMERICHOST)ERR(EAI_NONAME);//这里是Android的变化，就是将DNS查询导向到netd中，然后由netd处理后再回到这里，    //这个过程和gethostbyname()中的处理流程一致，可见gethostbyname()实现中的分析#if defined(__ANDROID__)//非netd调用会阻塞到这里，等待netd查询完后返回int gai_error = android_getaddrinfo_proxy(hostname, servname, hints, res, netcontext->app_netid);//如果是netd调用，那么android_getaddrinfo_proxy()会返回EAI_SYSTEM错误，继续后续的逻辑    if (gai_error != EAI_SYSTEM) {return gai_error;}#endif/* * hostname as alphabetical name. * we would like to prefer AF_INET6 than AF_INET, so we'll make a * outer loop by AFs. */    //按照explore数组中指定的有效组合，调用explore_fqdn()进行DNS查询for (ex = explore; ex->e_af >= 0; ex++) {*pai = ai0;/* require exact match for family field */if (pai->ai_family != ex->e_af)continue;if (!MATCH(pai->ai_socktype, ex->e_socktype,WILD_SOCKTYPE(ex))) {continue;}if (!MATCH(pai->ai_protocol, ex->e_protocol,WILD_PROTOCOL(ex))) {continue;}//这步会执行，以为explore中，socktype字段都是固定的，并非通配if (pai->ai_socktype == ANY && ex->e_socktype != ANY)pai->ai_socktype = ex->e_socktype;if (pai->ai_protocol == ANY && ex->e_protocol != ANY)pai->ai_protocol = ex->e_protocol;//进行DNS查询error = explore_fqdn(pai, hostname, servname, &cur->ai_next, netcontext);//查询结果指针cur前移while (cur && cur->ai_next)cur = cur->ai_next;}//sentinel.ai_next不为空，说明有查询结果，所以设置error为0if (sentinel.ai_next)error = 0;//有错误发生，需要调用freeaddrinfo()释放可能已经分配的内存空间if (error)goto free;if (error == 0) {if (sentinel.ai_next) { good: //查询过程没有问题 && 查询到了结果，返回成功*res = sentinel.ai_next;return SUCCESS;} else        //查询过程没有问题(error=0)，但是没有查到结果(sentinel.ai_next=NULL)，也是失败error = EAI_FAIL;} free: bad: //查询失败，或者查询过程出错，释放内存后返回错误码if (sentinel.ai_next)freeaddrinfo(sentinel.ai_next);*res = NULL;return error;}

explore_fqdn()

看到该函数的代码，一定会非常熟悉，在gethostbyname()的实现中，已经见过这种查询表方式，这里不再赘述。

/* * FQDN hostname, DNS lookup */static int explore_fqdn(const struct addrinfo *pai, const char *hostname,    const char *servname, struct addrinfo **res,    const struct android_net_context *netcontext){struct addrinfo *result;struct addrinfo *cur;int error = 0;    //_files_getaddrinfo()完成基于文件的查询；_dns_getaddrinfo()完成基于DNS服务器的查询；    //_yp_getaddrinfo()不会被调用static const ns_dtab dtab[] = {NS_FILES_CB(_files_getaddrinfo, NULL){ NSSRC_DNS, _dns_getaddrinfo, NULL },/* force -DHESIOD */NS_NIS_CB(_yp_getaddrinfo, NULL){ 0, 0, 0 }};assert(pai != NULL);/* hostname may be NULL *//* servname may be NULL */assert(res != NULL);result = NULL;/* * if the servname does not match socktype/protocol, ignore it. */    //如果指定了servname，检查socktype和protocol是否有冲突，Android中不涉及if (get_portmatch(pai, servname) != 0)return 0;//调用nsdispatch()进行查询分发switch (nsdispatch(&result, dtab, NSDB_HOSTS, "getaddrinfo",default_dns_files, hostname, pai, netcontext)) {case NS_TRYAGAIN:error = EAI_AGAIN;goto free;case NS_UNAVAIL:error = EAI_FAIL;goto free;case NS_NOTFOUND:error = EAI_NODATA;goto free;case NS_SUCCESS:error = 0;        //如果hostname查询成功，如果指定了servname，将对应的端口名转换为端口号，Android不涉及for (cur = result; cur; cur = cur->ai_next) {GET_PORT(cur, servname);/* canonname should be filled already */}break;}*res = result;return 0;free:if (result)freeaddrinfo(result);return error;}

_dns_getaddrinfo()

我们不关注基于文件的查询_files_getaddrinfo()的实现，只看基于DNS服务器的查询实现。

static int _dns_getaddrinfo(void *rv, void *cb_data, va_list ap){struct addrinfo *ai;querybuf *buf, *buf2;const char *name;const struct addrinfo *pai;struct addrinfo sentinel, *cur;struct res_target q, q2;res_state res;const struct android_net_context *netcontext;//从explore_fqdn()传入的三个参数name = va_arg(ap, char *);pai = va_arg(ap, const struct addrinfo *);netcontext = va_arg(ap, const struct android_net_context *);memset(&q, 0, sizeof(q));memset(&q2, 0, sizeof(q2));memset(&sentinel, 0, sizeof(sentinel));cur = &sentinel;//buf和buf2用于保存查询结果，见下文的getanswer()buf = malloc(sizeof(*buf));if (buf == NULL) {h_errno = NETDB_INTERNAL;return NS_NOTFOUND;}buf2 = malloc(sizeof(*buf2));if (buf2 == NULL) {free(buf);h_errno = NETDB_INTERNAL;return NS_NOTFOUND;}//这段逻辑非常重要，决定了是否发起AAAA与A查询以及它们的顺序switch (pai->ai_family) {case AF_UNSPEC://当没有指定具体的failmy时，优先IPv6/* prefer IPv6 */q.name = name;q.qclass = C_IN;q.answer = buf->buf;q.anslen = sizeof(buf->buf);int query_ipv6 = 1, query_ipv4 = 1;        //如果设置了AI_ADDRCONFIG标记，那么表示要发起对应的请求时，本机必须至少有一个相应类型的IP地址，        //_have_ipv6()和_have_ipv4()就是分别用来检测本机是否有IPv4和IPv6地址的if (pai->ai_flags & AI_ADDRCONFIG) {query_ipv6 = _have_ipv6(netcontext->app_mark, netcontext->uid);query_ipv4 = _have_ipv4(netcontext->app_mark, netcontext->uid);}        //注意：在需要发起AAAA请求的条件中会判断是否需要发起A请求，这是为了在AAAA请求查询失败时可以继续发起A请求if (query_ipv6) {q.qtype = T_AAAA;if (query_ipv4) {            //将q2接入到q的后面，这样可以在AAAA请求查询失败时可以继续发起A请求q.next = &q2;q2.name = name;q2.qclass = C_IN;q2.qtype = T_A;q2.answer = buf2->buf;q2.anslen = sizeof(buf2->buf);}} else if (query_ipv4) {q.qtype = T_A;} else {free(buf);free(buf2);return NS_NOTFOUND;}break    //如果已经已经具体指定了faimy，那么按照指定的failmy发起A或者AAAA即可case AF_INET:q.name = name;q.qclass = C_IN;        //A查询q.qtype = T_A;q.answer = buf->buf;q.anslen = sizeof(buf->buf);break;case AF_INET6:q.name = name;q.qclass = C_IN;        //AAAA查询q.qtype = T_AAAA;q.answer = buf->buf;q.anslen = sizeof(buf->buf);break;default:    //其它faimy均为非法值free(buf);free(buf2);return NS_UNAVAIL;}//获取线程独立的resolver状态结构res = __res_get_state();if (res == NULL) {free(buf);free(buf2);return NS_NOTFOUND;}/* this just sets our netid val in the thread private data so we don't have to * modify the api's all the way down to res_send.c's res_nsend.  We could * fully populate the thread private data here, but if we get down there * and have a cache hit that would be wasted, so we do the rest there on miss */    //将netid、mark和qhook参数设置到res_stats中，这三个参数在res_nsend()中都有使用res_setnetcontext(res, netcontext);    //getaddrinfo()将res_search.c中的三个接口重新实现了一遍，这样做的原因见上面的注释(本人没有理解!!!)if (res_searchN(name, &q, res) < 0) {__res_put_state(res);free(buf);free(buf2);return NS_NOTFOUND;}    //解析查询结果报文ai = getanswer(buf, q.n, q.name, q.qtype, pai);if (ai) {cur->ai_next = ai;while (cur && cur->ai_next)cur = cur->ai_next;}    //如果有A请求查询结果，继续解析A请求查询结果报文if (q.next) {ai = getanswer(buf2, q2.n, q2.name, q2.qtype, pai);if (ai)cur->ai_next = ai;}free(buf);free(buf2);    //如果查询失败，返回对应的错误码if (sentinel.ai_next == NULL) {__res_put_state(res);switch (h_errno) {case HOST_NOT_FOUND:return NS_NOTFOUND;case TRY_AGAIN:return NS_TRYAGAIN;default:return NS_UNAVAIL;}}//按照RFC 6762中要求的顺序将查询结果排序后返回，关于这个排序有需要再研究吧_rfc6724_sort(&sentinel, netcontext->app_mark, netcontext->uid);__res_put_state(res);*((struct addrinfo **)rv) = sentinel.ai_next;return NS_SUCCESS;}

后面的代码不再继续分析，它和res_search()中的逻辑非常类似，感兴趣的可以自行分析。

其它

MATCH宏

#define MATCH(x, y, w) \((x) == (y) || (/*CONSTCOND*/(w) && ((x) == ANY || (y) == ANY)))

作用是检查x和y是否匹配，返回布尔值，匹配分两种情况：

x和y的值确实是相等的，那么匹配，对应表达式中的(x) == (y)
x和y不等，但是w参数非0，并且x和y中有任何一个是通配的，即值为ANY(0)，那么也认为是匹配的。实际上，这里的w含义是wildcard，即通配，如果w为0，即表示把通配考虑进去，否则按照x和y的真实值是否相等来判断是否匹配。

Android(安卓)DNS之getaddrinfo()的实现

原型解读

入口

android_getaddrinfofornetcontext()

explore_fqdn()

_dns_getaddrinfo()

其它

MATCH宏

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