从 ubuntu 12.04 升级后，我有一个奇怪的 ping 问题。到 14.04。我无法使用主机名 ping，但使用 ip 可以正常工作。我对其他服务或程序没有任何其他 dns 问题。我以普通用户身份工作，但不是 root 用户。作为 root 用户 host 和 dig 可以将 dnsname 解析为 ip。busybox ping 也适用于主机名。
这是错误：
root@myhost:~# ping ubuntu.com
ping: unknown host ubuntu.com
这有效：
root@myhost:~# host ubuntu.com
ubuntu.com has address 91.189.94.40
ubuntu.com mail is handled by 10 mx.canonical.com.
root@myhost:~# ping 91.189.94.40 -c 1
PING 91.189.94.40 (91.189.94.40) 56(84) bytes of data.
64 bytes from 91.189.94.40: icmp_seq=1 ttl=53 time=16.1 ms
root@myhost:~# busybox ping ubuntu.com -c 1
PING ubuntu.com (91.189.94.40): 56 data bytes
64 bytes from 91.189.94.40: seq=0 ttl=53 time=16.189 ms
user@myhost:~$ ping ubuntu.com -c 1
PING ubuntu.com (91.189.94.40) 56(84) bytes of data.
64 bytes from ovinnik.canonical.com (91.189.94.40): icmp_seq=1 ttl=53 time=16.1 ms
文件 /etc/nsswitch.conf
root@myhost:~# cat /etc/nsswitch.conf
# /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
hosts: files mdns4_minimal [NOTFOUND=return] dns mdns4
networks: files
protocols: db files
services: db files
ethers: db files
rpc: db files
netgroup: nis
一些额外的信息。感谢 strace 提示。appamor 未激活。
strace：打开 /etc/resolv.conf 似乎是个问题，但作为 root 的工作较少
root@myhost:~# strace -e open ping -c 1 ubuntu.com
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libcap.so.2", O_RDONLY|O_CLOEXEC) = 3
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
open("/etc/resolv.conf", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/etc/nsswitch.conf", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/lib/x86_64-linux-gnu/tls/x86_64/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/lib/x86_64-linux-gnu/tls/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/lib/x86_64-linux-gnu/x86_64/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/lib/x86_64-linux-gnu/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/usr/lib/x86_64-linux-gnu/tls/x86_64/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/usr/lib/x86_64-linux-gnu/tls/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/usr/lib/x86_64-linux-gnu/x86_64/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/usr/lib/x86_64-linux-gnu/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/lib/tls/x86_64/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/lib/tls/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/lib/x86_64/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/lib/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/usr/lib/tls/x86_64/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/usr/lib/tls/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/usr/lib/x86_64/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
open("/usr/lib/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = -1 EACCES (Permission denied)
ping: unknown host ubuntu.com
+++ exited with 2 +++
得到：
root@myhost:~# getent hosts ubuntu.com 91.189.94.40 ubuntu.com
获取帽：
root@myhost:~# getcap -rv /bin/ping
/bin/ping
权限：
root@myhost:/etc# ls -lha /etc/resolv.conf
lrwxrwxrwx 1 root root 29 Jan 9 11:11 /etc/resolv.conf -> ../run/resolvconf/resolv.conf
root@myhost:/etc# ls -lha /run/resolvconf/resolv.conf
-rw-r--r-- 1 root root 237 Jan 10 08:52 /run/resolvconf/resolv.conf
root@myhost:/etc# ls -lha /etc/hosts
-rw-r--r-- 1 root root 485 Jan 8 09:15 /etc/hosts
root@myhost:/etc# ls -lha /etc/nsswitch.conf
-rw-r--r-- 1 root root 513 Jan 8 09:08 /etc/nsswitch.conf
resolv.conf：（与另一台主机相同，是否有效）
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN
nameserver 213.133.98.98
nameserver 213.133.99.99
nameserver 213.133.100.100
