我希望有人能够帮助阐明ping6GNU/Linux 机器上所表现出的不同行为。
该机器是利诺德已分配 IPv4 和 IPv6 地址的主机。它以双栈配置运行。Linode 人员提供了两个 IPv4 解析器和两个 IPv6 解析器。（对于这个问题，我将只使用每种类型中的一个）。
当我配置我的/etc/resolv.conf文件以使用 IPv4 解析器时；
# /etc/resolv.conf
nameserver 72.14.179.5
ping6对每个回显回复执行反向查找，并相应地显示地址。（这是我所期望的。）
$ ping6 -c 6 google.com
PING google.com(dfw06s33-in-x05.1e100.net) 56 data bytes
64 bytes from dfw06s33-in-x05.1e100.net: icmp_seq=1 ttl=49 time=34.7 ms
64 bytes from dfw06s33-in-x05.1e100.net: icmp_seq=2 ttl=49 time=34.7 ms
64 bytes from dfw06s33-in-x05.1e100.net: icmp_seq=3 ttl=49 time=34.7 ms
64 bytes from dfw06s33-in-x05.1e100.net: icmp_seq=4 ttl=49 time=34.7 ms
64 bytes from dfw06s33-in-x05.1e100.net: icmp_seq=5 ttl=49 time=34.7 ms
64 bytes from dfw06s33-in-x05.1e100.net: icmp_seq=6 ttl=49 time=34.7 ms
如果我运行tcpdump，当我运行ping6命令时，我可以看到预期的 1 个正向查找和 6 个反向查找。
$ sudo tcpdump -n -i eth0 udp port 53
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
21:00:23.128883 IP 23.239.30.203.37353 > 72.14.179.5.53: 18114+ AAAA? google.com. (28)
21:00:23.129194 IP 72.14.179.5.53 > 23.239.30.203.37353: 18114 1/0/0 AAAA 2607:f8b0:4000:805::1003 (56)
21:00:23.129380 IP 23.239.30.203.58841 > 72.14.179.5.53: 32482+ PTR? 3.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.5.0.8.0.0.0.0.4.0.b.8.f.7.0.6.2.ip6.arpa. (90)
21:00:23.129661 IP 72.14.179.5.53 > 23.239.30.203.58841: 32482 1/0/0 PTR dfw06s33-in-x03.1e100.net. (129)
21:00:23.164677 IP 23.239.30.203.47867 > 72.14.179.5.53: 30527+ PTR? 3.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.5.0.8.0.0.0.0.4.0.b.8.f.7.0.6.2.ip6.arpa. (90)
21:00:23.165086 IP 72.14.179.5.53 > 23.239.30.203.47867: 30527 1/0/0 PTR dfw06s33-in-x03.1e100.net. (129)
21:00:24.166410 IP 23.239.30.203.55723 > 72.14.179.5.53: 37565+ PTR? 3.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.5.0.8.0.0.0.0.4.0.b.8.f.7.0.6.2.ip6.arpa. (90)
21:00:24.167221 IP 72.14.179.5.53 > 23.239.30.203.55723: 37565 1/0/0 PTR dfw06s33-in-x03.1e100.net. (129)
21:00:25.166735 IP 23.239.30.203.52095 > 72.14.179.5.53: 15353+ PTR? 3.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.5.0.8.0.0.0.0.4.0.b.8.f.7.0.6.2.ip6.arpa. (90)
21:00:25.167138 IP 72.14.179.5.53 > 23.239.30.203.52095: 15353 1/0/0 PTR dfw06s33-in-x03.1e100.net. (129)
21:00:26.167883 IP 23.239.30.203.60999 > 72.14.179.5.53: 48567+ PTR? 3.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.5.0.8.0.0.0.0.4.0.b.8.f.7.0.6.2.ip6.arpa. (90)
21:00:26.168327 IP 72.14.179.5.53 > 23.239.30.203.60999: 48567 1/0/0 PTR dfw06s33-in-x03.1e100.net. (129)
21:00:27.169385 IP 23.239.30.203.33681 > 72.14.179.5.53: 42354+ PTR? 3.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.5.0.8.0.0.0.0.4.0.b.8.f.7.0.6.2.ip6.arpa. (90)
21:00:27.169847 IP 72.14.179.5.53 > 23.239.30.203.33681: 42354 1/0/0 PTR dfw06s33-in-x03.1e100.net. (129)
21:00:28.170818 IP 23.239.30.203.36738 > 72.14.179.5.53: 15194+ PTR? 3.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.5.0.8.0.0.0.0.4.0.b.8.f.7.0.6.2.ip6.arpa. (90)
21:00:28.171221 IP 72.14.179.5.53 > 23.239.30.203.36738: 15194 1/0/0 PTR dfw06s33-in-x03.1e100.net. (129)
然而，如果我配置/etc/resolv.conf使用 IPv6 解析器；
# /etc/resolv.conf
nameserver 2600:3c00::3
ping6执行反向查找所有其他回显请求。（我感到很困惑）
$ ping6 -c 6 google.com
PING google.com(2607:f8b0:4000:803::1000) 56 data bytes
64 bytes from dfw06s27-in-x00.1e100.net: icmp_seq=1 ttl=49 time=33.1 ms
64 bytes from 2607:f8b0:4000:803::1000: icmp_seq=2 ttl=49 time=33.2 ms
64 bytes from dfw06s27-in-x00.1e100.net: icmp_seq=3 ttl=49 time=33.2 ms
64 bytes from 2607:f8b0:4000:803::1000: icmp_seq=4 ttl=49 time=33.2 ms
64 bytes from dfw06s27-in-x00.1e100.net: icmp_seq=5 ttl=49 time=33.1 ms
64 bytes from 2607:f8b0:4000:803::1000: icmp_seq=6 ttl=49 time=33.2 ms
tcpdump我通过运行时查看输出来确认这一点ping6。我看到一个正向查找，但只有 3 个反向查找。
$ sudo tcpdump -n -i eth0 udp port 53
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes
21:10:12.163444 IP6 2600:3c00::f03c:91ff:fe73:ee8f.35982 > 2600:3c00::3.53: 47249+ AAAA? google.com. (28)
21:10:12.164457 IP6 2600:3c00::3.53 > 2600:3c00::f03c:91ff:fe73:ee8f.35982: 47249 1/0/0 AAAA 2607:f8b0:4000:803::1000 (56)
21:10:12.198491 IP6 2600:3c00::f03c:91ff:fe73:ee8f.37631 > 2600:3c00::3.53: 18439+ PTR? 0.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.3.0.8.0.0.0.0.4.0.b.8.f.7.0.6.2.ip6.arpa. (90)
21:10:12.207233 IP6 2600:3c00::3.53 > 2600:3c00::f03c:91ff:fe73:ee8f.37631: 18439 1/0/0 PTR dfw06s27-in-x00.1e100.net. (129)
21:10:14.201780 IP6 2600:3c00::f03c:91ff:fe73:ee8f.55836 > 2600:3c00::3.53: 9503+ PTR? 0.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.3.0.8.0.0.0.0.4.0.b.8.f.7.0.6.2.ip6.arpa. (90)
21:10:14.202272 IP6 2600:3c00::3.53 > 2600:3c00::f03c:91ff:fe73:ee8f.55836: 9503 1/0/0 PTR dfw06s27-in-x00.1e100.net. (129)
21:10:16.204599 IP6 2600:3c00::f03c:91ff:fe73:ee8f.46736 > 2600:3c00::3.53: 38520+ PTR? 0.0.0.1.0.0.0.0.0.0.0.0.0.0.0.0.3.0.8.0.0.0.0.4.0.b.8.f.7.0.6.2.ip6.arpa. (90)
21:10:16.205006 IP6 2600:3c00::3.53 > 2600:3c00::f03c:91ff:fe73:ee8f.46736: 38520 1/0/0 PTR dfw06s27-in-x00.1e100.net. (129)
任何见解，都将不胜感激。
答案1
ping 和 ping6 的代码都使用一个共享函数char * pr_addr(void *sa, socklen_t salen)从ping.c格式化hostname (address)输出中的子字符串。pr_addr调用getnameinfo()，这是操作系统提供的典型库函数。因此查找行为没有区别