getaddrinfo默认使用UDP访问DNS SERVER，并依次使用/etc/resolv.conf中的search行的项：
...
socket(PF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 4
connect(4, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("172.17.255.10")}, 16) = 0
poll([{fd=4, events=POLLOUT}], 1, 0) = 1 ([{fd=4, revents=POLLOUT}])
sendto(4, "\332C\1\0\0\1\0\0\0\0\0\0\7nginx-1\5dbyin\3svc\7c"..., 49, MSG_NOSIGNAL, NULL, 0) = 49
poll([{fd=4, events=POLLIN}], 1, 1000) = 1 ([{fd=4, revents=POLLIN}])
ioctl(4, FIONREAD, [141]) = 0
recvfrom(4, "\332C\201\3\0\1\0\0\0\1\0\0\7nginx-1\5dbyin\3svc\7c"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("172.17.255.10")}, [16]) = 141
close(4) = 0
socket(PF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 4
connect(4, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("172.17.255.10")}, 16) = 0
poll([{fd=4, events=POLLOUT}], 1, 0) = 1 ([{fd=4, revents=POLLOUT}])
sendto(4, "\36^\1\0\0\1\0\0\0\0\0\0\7nginx-1\3svc\7cluster"..., 43, MSG_NOSIGNAL, NULL, 0) = 43
poll([{fd=4, events=POLLIN}], 1, 1000) = 1 ([{fd=4, revents=POLLIN}])
ioctl(4, FIONREAD, [135]) = 0
recvfrom(4, "\36^\201\3\0\1\0\0\0\1\0\0\7nginx-1\3svc\7cluster"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("172.17.255.10")}, [16]) = 135
close(4) = 0
socket(PF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 4
connect(4, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("172.17.255.10")}, 16) = 0
poll([{fd=4, events=POLLOUT}], 1, 0) = 1 ([{fd=4, revents=POLLOUT}])
sendto(4, "}\210\1\0\0\1\0\0\0\0\0\0\7nginx-1\7cluster\5loc"..., 39, MSG_NOSIGNAL, NULL, 0) = 39
poll([{fd=4, events=POLLIN}], 1, 1000) = 1 ([{fd=4, revents=POLLIN}])
ioctl(4, FIONREAD, [131]) = 0
recvfrom(4, "}\210\201\3\0\1\0\0\0\1\0\0\7nginx-1\7cluster\5loc"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("172.17.255.10")}, [16]) = 131
close(4) = 0
write(2, "ping: unknown host nginx-1\n", 27ping: unknown host nginx-1
) = 27
exit_group(2) = ?
__libc_res_nsearch -> __libc_res_nquerydomain -> __libc_res_nquery -> __libc_res_nsend.
int
__libc_res_nsearch(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 */
u_char **answerp,
u_char **answerp2,
int *nanswerp2,
int *resplen2)
{
///...
/*
* We do at least one level of search if
* - there is no dot and RES_DEFNAME is set, or
* - there is at least one dot, there is no trailing dot,
* and RES_DNSRCH is set.
*/
if ((!dots && (statp->options & RES_DEFNAMES) != 0) || /// ex: hostname
(dots && !trailing_dot && (statp->options & RES_DNSRCH) != 0)) {
int done = 0;
for (domain = (const char * const *)statp->dnsrch; ///try every entry if failed
*domain && !done;
domain++) {
searched = 1;
if (domain[0][0] == '\0' ||
(domain[0][0] == '.' && domain[0][1] == '\0'))
root_on_list++;
ret = __libc_res_nquerydomain(statp, name, *domain,
class, type,
answer, anslen, answerp,
answerp2, nanswerp2,
resplen2);
在/etc/resolv.conf中配置options:use-vc，让getaddrinfo使用TCP访问Server（glibc 2.14及以上.
int
__libc_res_nsend(res_state statp, const u_char *buf, int buflen,
const u_char *buf2, int buflen2,
u_char *ans, int anssiz, u_char **ansp, u_char **ansp2,
int *nansp2, int *resplen2)
{
//...
if (__builtin_expect (v_circuit, 0)) { /// use tcp
/* Use VC; at most one attempt per server. */
try = statp->retry;
n = send_vc(statp, buf, buflen, buf2, buflen2,
&ans, &anssiz, &terrno,
ns, ansp, ansp2, nansp2, resplen2);
if (n < 0)
return (-1);
if (n == 0 && (buf2 == NULL || *resplen2 == 0))
goto next_ns;
} else {
/* Use datagrams. */
n = send_dg(statp, buf, buflen, buf2, buflen2,
&ans, &anssiz, &terrno,
ns, &v_circuit, &gotsomewhere, ansp,
ansp2, nansp2, resplen2);
if (n < 0)
return (-1);
if (n == 0 && (buf2 == NULL || *resplen2 == 0))
goto next_ns;
if (v_circuit)
// XXX Check whether both requests failed or
// XXX whether one has been answered successfully
goto same_ns;
}
参考 getaddrinfo.