求解 Linux 对本地主机名的解析方式

[enlightment]

centos7 本机的 hostname 为 xxxa
吃时 ping xxxa 解析到了 eth1 对应的 ip，这个解析走的是哪里的 dns ？为什么会解析到 eth1 的 ip 而不是 eth2 eth3 的？

[D0n9]

先走 /etc/hosts

[enlightment]

/etc/hosts 并里没有单独对 xxxa 做任何解析

[3dwelcome]

主机名解析底层就是调用 gethostbyname，你说为什么不对应第二个 ip，理论上应该对应 0.0.0.0，绑定所有的本机 ip，但 linux 程序员就是写了获取回来的第一个 ip，你也没办法。

[enlightment]

@3dwelcome 这个是内核里写的逻辑吗？

[3dwelcome]

严格来说不算内核，就算是个提供给程序的调用库。

[julyclyde]

strace 一下 ping 看看？

[lenqu]

traceroute 一下，看看那走的节点

[huangmingyou]

man 5 nsswitch.conf

[huangmingyou]

# /etc/nsswitch.conf
#
# Example configuration of GNU Name Service Switch functionality.
# If you have the `glibc-doc-reference' and `info' packages installed, try:
# `info libc "Name Service Switch"' for information about this file.

passwd: compat
group: compat
shadow: compat
gshadow: files

hosts: files mdns4_minimal [NOTFOUND=return] dns
networks: files

protocols: db files
services: db files
ethers: db files
rpc: db files

netgroup: nis

[targetFree]

@3dwelcome 好像 gethostbyname 已被 getaddrinfo 替代

[baobao1270]

systemd resolved?

[enlightment]

@julyclyde
[root@cent7 ~]# strace -f -e open ping cent7.550gtmp
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libcap.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libidn.so.11", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libcrypto.so.10", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libresolv.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libm.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libattr.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libdl.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/lib64/libz.so.1", O_RDONLY|O_CLOEXEC) = 3
open("/etc/pki/tls/legacy-settings", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
open("/etc/resolv.conf", O_RDONLY|O_CLOEXEC) = 4
open("/etc/nsswitch.conf", O_RDONLY|O_CLOEXEC) = 4
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 4
open("/lib64/libnss_files.so.2", O_RDONLY|O_CLOEXEC) = 4
open("/etc/host.conf", O_RDONLY|O_CLOEXEC) = 4
open("/etc/hosts", O_RDONLY|O_CLOEXEC) = 4
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 4
open("/lib64/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = 4
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 4
open("/lib64/libnss_myhostname.so.2", O_RDONLY|O_CLOEXEC) = 4
open("/lib64/librt.so.1", O_RDONLY|O_CLOEXEC) = 4
open("/lib64/libdw.so.1", O_RDONLY|O_CLOEXEC) = 4
open("/lib64/libgcc_s.so.1", O_RDONLY|O_CLOEXEC) = 4
open("/lib64/libpthread.so.0", O_RDONLY|O_CLOEXEC) = 4
open("/usr/lib64/elfutils/tls/x86_64/libelf.so.1", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/usr/lib64/elfutils/tls/libelf.so.1", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/usr/lib64/elfutils/x86_64/libelf.so.1", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/usr/lib64/elfutils/libelf.so.1", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/lib64/libelf.so.1", O_RDONLY|O_CLOEXEC) = 4
open("/usr/lib64/elfutils/liblzma.so.5", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/lib64/liblzma.so.5", O_RDONLY|O_CLOEXEC) = 4
open("/usr/lib64/elfutils/libbz2.so.1", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
open("/lib64/libbz2.so.1", O_RDONLY|O_CLOEXEC) = 4
open("/etc/hosts", O_RDONLY|O_CLOEXEC) = 4
open("/etc/gai.conf", O_RDONLY|O_CLOEXEC) = -1 ENOENT (No such file or directory)
PING cent7.550gtmp (10.0.182.120) 56(84) bytes of data.
open("/etc/hosts", O_RDONLY|O_CLOEXEC) = 4
64 bytes from cent7.550gtmp (10.0.112.1): icmp_seq=1 ttl=64 time=0.050 ms
64 bytes from cent7.550gtmp (10.0.112.1): icmp_seq=2 ttl=64 time=0.104 ms
64 bytes from cent7.550gtmp (10.0.112.1): icmp_seq=3 ttl=64 time=0.049 ms
64 bytes from cent7.550gtmp (10.0.112.1): icmp_seq=4 ttl=64 time=0.047 ms
^C
strace: Process 56574 detached