bionic dns
#############


res_query.c 
==========================================================

`res_query.c <https://android.googlesource.com/platform/bionic/+/master/libc/dns/resolv/res_query.c>`_
的 res_nquery 是执行查询的主函数，查询结果为answer。

.. code-block:: c

    int
    res_nquery(res_state statp,
           const char *name,	/* domain name */
           int class, int type,	/* class and type of query */
           u_char *answer,	/* buffer to put answer */
           int anslen)		/* size of answer buffer */

res_mkquery.c
==========================================================

`res_mkquery.c <https://android.googlesource.com/platform/bionic/+/master/libc/dns/resolv/res_mkquery.c>`_
的 res_nmkquery 负责组查询包。

res_stat的statp里设置一些dns option，例如，是否recurse，是否开启dnssec。

data 在正向query场景下可以置NULL。

buf是最终组装好的查询包

.. code-block:: c

    int
    res_nmkquery(res_state statp,
             int op,			/* opcode of query */
             const char *dname,		/* domain name */
             int class, int type,	/* class and type of query */
             const u_char *data,	/* resource record data */
             int datalen,		/* length of data */
             const u_char *newrr_in,	/* new rr for modify or append */
             u_char *buf,		/* buffer to put query */
             int buflen)		/* size of buffer */

res_send.c
==========================================================

`res_send.c <https://android.googlesource.com/platform/bionic/+/master/libc/dns/resolv/res_send.c>`_
的res_nsend负责将查询包送到目标server的socket，并从server的socket获取应答包。

.. code-block:: c

    int
    res_nsend(res_state statp,
          const u_char *buf, int buflen, u_char *ans, int anssiz)  

res_nsend 从 statp->nsaddr_list[ns] 取server信息。

以UDP查询为例，调用send_dg发送查询包：

.. code-block:: c

    n = send_dg(statp, &params, buf, buflen, ans, anssiz, &terrno,
                        ns, &v_circuit, &gotsomewhere, &now, &rcode, &delay);

send_dg从ns里取socket，发buf查询包，取回应答包ans

params是一些应答参数，例如等待时间

.. code-block:: c

    static int
    send_dg(res_state statp, struct __res_params* params,
        const u_char *buf, int buflen, u_char *ans, int anssiz,
        int *terrno, int ns, int *v_circuit, int *gotsomewhere,
        time_t *at, int *rcode, int* delay)
    ...
        s = EXT(statp).nssocks[ns];
    ...
        if (send(s, (const char*)buf, (size_t)buflen, 0) != buflen) {
    ...
        if (sendto(s, (const char*)buf, buflen, 0, nsap, nsaplen) != buflen)
    ...
    resplen = recvfrom(s, (char*)ans, (size_t)anssiz,0,
                   (struct sockaddr *)(void *)&from, &fromlen);


res_init.c 
==========================================================

`res_init.c <https://android.googlesource.com/platform/bionic/+/master/libc/dns/resolv/res_init.c>`_

__res_vinit是初始化读取一些系统resolver列表，放到statp里。

如果#ifdef USELOOPBACK为true，会先把loopback的v4, v6地址添加到res_sockaddr_union u数组里，再调res_setservers放到statp里。

然后是读取环境变量，resolv.conf之类的操作，同样添到statp里。

.. code-block:: c

    int
    __res_vinit(res_state statp, int preinit)

    ...

    res_setservers(statp, u, nserv);

getaddrinfo.c 
==========================================================

`getaddrinfo.c <https://android.googlesource.com/platform/bionic/+/master/libc/dns/net/getaddrinfo.c>`_

querybuf的hdr即为包头，buf即为dns包内容，注意hdr与buf的起始地址相同，内存是共用的。

.. code-block:: c

    typedef union {
        HEADER hdr;
        u_char buf[MAXPACKET];
    } querybuf;

    static struct addrinfo *
    getanswer(const querybuf *answer, int anslen, const char *qname, int qtype, const struct addrinfo *pai)


参考资料
==========================================================

`AOSP DNS 解析器 <https://source.android.com/devices/architecture/modular-system/dns-resolver>`_

`C Programming with the Resolver Library Routines <http://web.deu.edu.tr/doc/oreily/networking/dnsbind/ch14_02.htm>`_