使用 wget（curl、httplib2）时，我总是会遇到 5 秒的解析超时。当我执行 wget -4 或 wget -6 时不会出现此问题。使用 dig 或 host 也可以解析。可以使用 /etc/resolv.conf 中的“options timeout:2”来降低延迟
其 strace 输出为：
15:19:09.908234 uname({sys="Linux", node="openkvk", ...}) = 0
15:19:09.908413 socket(PF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 3
15:19:09.908482 connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("212.83.206.202")}, 16) = 0
15:19:09.908569 poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
15:19:09.908637 sendto(3, "F\347\1\0\0\1\0\0\0\0\0\0\10slashdot\3org\0\0\1\0\1", 30, MSG_NOSIGNAL, NULL, 0) = 30
15:19:09.908735 poll([{fd=3, events=POLLIN|POLLOUT}], 1, 5000) = 1 ([{fd=3, revents=POLLOUT}])
15:19:09.908804 sendto(3, "F\331\1\0\0\1\0\0\0\0\0\0\10slashdot\3org\0\0\34\0\1", 30, MSG_NOSIGNAL, NULL, 0) = 30
15:19:09.908884 poll([{fd=3, events=POLLIN}], 1, 4999) = 1 ([{fd=3, revents=POLLIN}])
15:19:09.911230 ioctl(3, FIONREAD, [46]) = 0
15:19:09.911308 recvfrom(3, "F\347\201\200\0\1\0\1\0\0\0\0\10slashdot\3org\0\0\1\0\1\300\f"..., 2048, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("212.83.206.202")}, [16]) = 46
15:19:09.911445 poll([{fd=3, events=POLLIN}], 1, 4997) = 0 (Timeout)
15:19:14.909096 poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
15:19:14.909189 sendto(3, "F\347\1\0\0\1\0\0\0\0\0\0\10slashdot\3org\0\0\1\0\1", 30, MSG_NOSIGNAL, NULL, 0) = 30
15:19:14.909292 poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
15:19:14.911569 ioctl(3, FIONREAD, [46]) = 0
15:19:14.911644 recvfrom(3, "F\347\201\200\0\1\0\1\0\0\0\0\10slashdot\3org\0\0\1\0\1\300\f"..., 2048, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("212.83.206.202")}, [16]) = 46
15:19:14.911772 poll([{fd=3, events=POLLOUT}], 1, 4997) = 1 ([{fd=3, revents=POLLOUT}])
15:19:14.911837 sendto(3, "F\331\1\0\0\1\0\0\0\0\0\0\10slashdot\3org\0\0\34\0\1", 30, MSG_NOSIGNAL, NULL, 0) = 30
15:19:14.912051 poll([{fd=3, events=POLLIN}], 1, 4997) = 1 ([{fd=3, revents=POLLIN}])
15:19:14.914034 ioctl(3, FIONREAD, [106]) = 0
Tcpdump 输出：
tcpdump -vvv port 53
tcpdump: listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
15:19:09.908687 IP (tos 0x0, ttl 64, id 37830, offset 0, flags [DF], proto UDP (17), length 58)
93.157.136.242.38551 > 212.83.206.202.domain: [bad udp cksum 3576!] 18151+ A? slashdot.org. (30)
15:19:09.908852 IP (tos 0x0, ttl 64, id 37831, offset 0, flags [DF], proto UDP (17), length 58)
93.157.136.242.38551 > 212.83.206.202.domain: [bad udp cksum 2876!] 18137+ AAAA? slashdot.org. (30)
15:19:09.911081 IP (tos 0x0, ttl 59, id 0, offset 0, flags [DF], proto UDP (17), length 74)
212.83.206.202.domain > 93.157.136.242.38551: [udp sum ok] 18151 q: A? slashdot.org. 1/0/0 slashdot.org. [22m21s] A 216.34.181.45 (46)
15:19:14.909246 IP (tos 0x0, ttl 64, id 37832, offset 0, flags [DF], proto UDP (17), length 58)
93.157.136.242.38551 > 212.83.206.202.domain: [bad udp cksum 3576!] 18151+ A? slashdot.org. (30)
15:19:14.911442 IP (tos 0x0, ttl 59, id 0, offset 0, flags [DF], proto UDP (17), length 74)
212.83.206.202.domain > 93.157.136.242.38551: [udp sum ok] 18151 q: A? slashdot.org. 1/0/0 slashdot.org. [22m16s] A 216.34.181.45 (46)
15:19:14.911882 IP (tos 0x0, ttl 64, id 37833, offset 0, flags [DF], proto UDP (17), length 58)
93.157.136.242.38551 > 212.83.206.202.domain: [bad udp cksum 2876!] 18137+ AAAA? slashdot.org. (30)
15:19:14.913906 IP (tos 0x0, ttl 59, id 0, offset 0, flags [DF], proto UDP (17), length 134)
212.83.206.202.domain > 93.157.136.242.38551: [udp sum ok] 18137 q: AAAA? slashdot.org. 0/1/0 ns: slashdot.org. [3m39s] SOA ns-1.ch3.sourceforge.com. hostmaster.corp.sourceforge.com. 2011101300 14400 1800 604800 3600 (106)
dig/host 等工具的性能与使用 wget -4 或 wget -6 类似，即时，没有任何问题。
我已经分别尝试了下列每个服务器。
nameserver 2001:4018:3:12::202
nameserver 2001:4018:1:64::202
nameserver 212.83.206.202
nameserver 213.207.64.202
nameserver 8.8.8.8
nameserver 8.8.4.4
什么原因可能导致这种延迟？
请求的输出：wget -4（有效）
23:41:10.428217 IP (tos 0x0, ttl 64, id 11790, offset 0, flags [DF], proto UDP (17), length 55)
93.157.136.242.51379 > 212.83.206.202.domain: [bad udp cksum e070!] 65414+ A? vircon.nl. (27)
23:41:10.430442 IP (tos 0x0, ttl 59, id 0, offset 0, flags [DF], proto UDP (17), length 71)
212.83.206.202.domain > 93.157.136.242.51379: [udp sum ok] 65414 q: A? vircon.nl. 1/0/0 vircon.nl. [15h59m29s] A 31.193.104.52 (43)
23:41:10.427920 socket(PF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 3
23:41:10.427982 connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("212.83.206.202")}, 16) = 0
23:41:10.428095 poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
23:41:10.428166 sendto(3, "\377\206\1\0\0\1\0\0\0\0\0\0\6vircon\2nl\0\0\1\0\1", 27, MSG_NOSIGNAL, NULL, 0) = 27
23:41:10.428262 poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
23:41:10.430572 ioctl(3, FIONREAD, [43]) = 0
23:41:10.430651 recvfrom(3, "\377\206\201\200\0\1\0\1\0\0\0\0\6vircon\2nl\0\0\1\0\1\300\f\0\1\0"..., 1024, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("212.83.206.202")}, [16]) = 43
23:41:10.430774 close(3)
wget (损坏)
23:40:13.577621 IP (tos 0x0, ttl 64, id 20475, offset 0, flags [DF], proto UDP (17), length 55)
93.157.136.242.55276 > 212.83.206.202.domain: [bad udp cksum 16c!] 62764+ A? vircon.nl. (27)
23:40:13.577786 IP (tos 0x0, ttl 64, id 20476, offset 0, flags [DF], proto UDP (17), length 55)
93.157.136.242.55276 > 212.83.206.202.domain: [bad udp cksum 9654!] 61847+ AAAA? vircon.nl. (27)
23:40:13.579879 IP (tos 0x0, ttl 59, id 0, offset 0, flags [DF], proto UDP (17), length 71)
212.83.206.202.domain > 93.157.136.242.55276: [udp sum ok] 62764 q: A? vircon.nl. 1/0/0 vircon.nl. [22h20m8s] A 31.193.104.52 (43)
23:40:18.582490 IP (tos 0x0, ttl 64, id 20477, offset 0, flags [DF], proto UDP (17), length 55)
93.157.136.242.55276 > 212.83.206.202.domain: [bad udp cksum 16c!] 62764+ A? vircon.nl. (27)
23:40:18.584709 IP (tos 0x0, ttl 59, id 0, offset 0, flags [DF], proto UDP (17), length 71)
212.83.206.202.domain > 93.157.136.242.55276: [udp sum ok] 62764 q: A? vircon.nl. 1/0/0 vircon.nl. [16h21s] A 31.193.104.52 (43)
23:40:18.585163 IP (tos 0x0, ttl 64, id 20478, offset 0, flags [DF], proto UDP (17), length 55)
93.157.136.242.55276 > 212.83.206.202.domain: [bad udp cksum 9654!] 61847+ AAAA? vircon.nl. (27)
23:40:18.587212 IP (tos 0x0, ttl 59, id 0, offset 0, flags [DF], proto UDP (17), length 106)
212.83.206.202.domain > 93.157.136.242.55276: [udp sum ok] 61847 q: AAAA? vircon.nl. 0/1/0 ns: vircon.nl. [56m25s] SOA ns1.vircon.nl. hostmaster.vircon.nl. 2011120602 10800 3600 604800 86400 (78)
23:40:13.577353 socket(PF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 3
23:40:13.577417 connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("212.83.206.202")}, 16) = 0
23:40:13.577502 poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
23:40:13.577572 sendto(3, "\365,\1\0\0\1\0\0\0\0\0\0\6vircon\2nl\0\0\1\0\1", 27, MSG_NOSIGNAL, NULL, 0) = 27
23:40:13.577667 poll([{fd=3, events=POLLIN|POLLOUT}], 1, 5000) = 1 ([{fd=3, revents=POLLOUT}])
23:40:13.577737 sendto(3, "\361\227\1\0\0\1\0\0\0\0\0\0\6vircon\2nl\0\0\34\0\1", 27, MSG_NOSIGNAL, NULL, 0) = 27
23:40:13.577892 poll([{fd=3, events=POLLIN}], 1, 4999) = 1 ([{fd=3, revents=POLLIN}])
23:40:13.580007 ioctl(3, FIONREAD, [43]) = 0
23:40:13.580105 recvfrom(3, "\365,\201\200\0\1\0\1\0\0\0\0\6vircon\2nl\0\0\1\0\1\300\f\0\1\0"..., 2048, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("212.83.206.202")}, [16]) = 43
23:40:13.580229 poll([{fd=3, events=POLLIN}], 1, 4997) = 0 (Timeout)
23:40:18.582338 poll([{fd=3, events=POLLOUT}], 1, 0) = 1 ([{fd=3, revents=POLLOUT}])
23:40:18.582433 sendto(3, "\365,\1\0\0\1\0\0\0\0\0\0\6vircon\2nl\0\0\1\0\1", 27, MSG_NOSIGNAL, NULL, 0) = 27
23:40:18.582535 poll([{fd=3, events=POLLIN}], 1, 5000) = 1 ([{fd=3, revents=POLLIN}])
23:40:18.584835 ioctl(3, FIONREAD, [43]) = 0
23:40:18.584912 recvfrom(3, "\365,\201\200\0\1\0\1\0\0\0\0\6vircon\2nl\0\0\1\0\1\300\f\0\1\0"..., 2048, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("212.83.206.202")}, [16]) = 43
23:40:18.585048 poll([{fd=3, events=POLLOUT}], 1, 4997) = 1 ([{fd=3, revents=POLLOUT}])
23:40:18.585117 sendto(3, "\361\227\1\0\0\1\0\0\0\0\0\0\6vircon\2nl\0\0\34\0\1", 27, MSG_NOSIGNAL, NULL, 0) = 27
23:40:18.585300 poll([{fd=3, events=POLLIN}], 1, 4997) = 1 ([{fd=3, revents=POLLIN}])
23:40:18.587325 ioctl(3, FIONREAD, [78]) = 0
23:40:18.587394 recvfrom(3, "\361\227\201\200\0\1\0\0\0\1\0\0\6vircon\2nl\0\0\34\0\1\300\f\0\6\0"..., 2005, 0, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("212.83.206.202")}, [16]) = 78
23:40:18.587513 close(3)
答案1
您的问题描述于http://udrepper.livejournal.com/20948.html，这是由于 IPv4 和 IPv6 上的并行查找无法与某些 DNS 服务器和防火墙协同工作所致。
可以通过在 /etc/resolv.conf 中的选项中添加“single-request”来修复此问题，如上面的链接所述。
答案2
如果你更喜欢 NetworkManager 服务，那么使用nmtui/nmcli更好。（以下cmds在centos7上测试）
更改设置（针对遭受 5 秒 DNS 延迟问题的用户）
nmcli con modify YourConnName ipv4.dns-options "single-request-reopen timeout:2"
您可以通过执行以下命令找到 YourConnName：
nmcli con show
应用设置
nmcli con down YourConnName ; nmcli con up YourConnName
PS 将设置更改为默认设置，即空
nmcli c m YourConnName ipv4.dns-options ''