Ubuntu Desktop: Error resolving, name or service not known (but DNS works!)

Written by Claudio Kuenzler - 0 comments

Published on November 24th 2015 - Listed in Linux DNS

Yesterday I hit a very strange error, when I attempted to run ntpdate on a Ubuntu 14.04 Desktop:

root@ubuntudesktop:~# ntpdate ntp.example.local
Error resolving ntp.example.local: Name or service not known (-2)
24 Nov 09:17:16 ntpdate[21746]: Can't find host ntp.example.local: Name or service not known (-2)
24 Nov 09:17:16 ntpdate[21746]: no servers can be used, exiting

The domain (ntp.example.local) could not be resolved.

Looks like a typical DNS resolving issue - at first sight. However DNS is working and resolving correctly:

root@ubuntudesktop:~# dig -t A ntp.example.local +short
dc001.example.local.
10.10.110.20

root@ubuntudesktop:~# nslookup ntp.example.local
[...]
ntp.example.local    canonical name = dc001.example.local.
Name:    dc001.example.local
Address: 10.10.110.20

The same command was tested on a Ubuntu 14.04 Server in the same subnet and there it worked without problems. What's the difference between the Desktop and Server version?

Strace gives more details. I will spare you the full output and show only the relevant output:

root@ubuntudesktop:~# strace ntpdate ntp.example.local
[...]
open("/etc/hosts", O_RDONLY|O_CLOEXEC)  = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=298, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f01da876000
read(3, "127.0.0.1\tlocalhost\n194.40.217.2"..., 4096) = 298
read(3, "", 4096)                       = 0
close(3)                                = 0
munmap(0x7f01da876000, 4096)            = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=87882, ...}) = 0
mmap(NULL, 87882, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f01da861000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libnss_mdns4_minimal.so.2", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\20\f\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=10432, ...}) = 0
mmap(NULL, 2105560, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f01d968a000
mprotect(0x7f01d968c000, 2093056, PROT_NONE) = 0
mmap(0x7f01d988b000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1000) = 0x7f01d988b000
close(3)                                = 0
mprotect(0x7f01d988b000, 4096, PROT_READ) = 0
munmap(0x7f01da861000, 87882)           = 0
socket(PF_LOCAL, SOCK_STREAM, 0)        = 3
fcntl(3, F_GETFD)                       = 0
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
connect(3, {sa_family=AF_LOCAL, sun_path="/var/run/avahi-daemon/socket"}, 110) = 0
fcntl(3, F_GETFL)                       = 0x2 (flags O_RDWR)
fstat(3, {st_mode=S_IFSOCK|0777, st_size=0, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f01da876000
lseek(3, 0, SEEK_CUR)                   = -1 ESPIPE (Illegal seek)
write(3, "RESOLVE-HOSTNAME-IPV4 ntp.example."..., 38) = 38
read(3, "-15 Timeout reached\n", 4096)  = 20
close(3)                                = 0
munmap(0x7f01da876000, 4096)            = 0
write(2, "Error resolving ntp.example.local:"..., 64Error resolving ntp.example.local: Name or service not known (-2)
) = 64
[...]

On the Ubuntu Server, the strace output starts the same way, however there is a small difference:

root@ubuntuserver:~# strace ntpdate ntp.example.local
[...]
open("/etc/resolv.conf", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=242, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc831bc4000
read(3, "# Dynamic resolv.conf(5) file fo"..., 4096) = 242
read(3, "", 4096)                       = 0
close(3)                                = 0
munmap(0x7fc831bc4000, 4096)            = 0
open("/etc/hosts", O_RDONLY|O_CLOEXEC)  = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=225, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc831bc4000
read(3, "127.0.0.1\tlocalhost\n194.40.217.1"..., 4096) = 225
read(3, "", 4096)                       = 0
close(3)                                = 0
munmap(0x7fc831bc4000, 4096)            = 0
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=43388, ...}) = 0
mmap(NULL, 43388, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7fc831bba000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\0\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\0\21\0\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0644, st_size=22952, ...}) = 0
mmap(NULL, 2117896, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7fc8309d5000
mprotect(0x7fc8309da000, 2093056, PROT_NONE) = 0
mmap(0x7fc830bd9000, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x4000) = 0x7fc830bd9000
close(3)                                = 0
mprotect(0x7fc8309d1000, 4096, PROT_READ) = 0
mprotect(0x7fc830bd9000, 4096, PROT_READ) = 0
munmap(0x7fc831bba000, 43388)           = 0
stat("/etc/resolv.conf", {st_mode=S_IFREG|0644, st_size=242, ...}) = 0
open("/etc/resolv.conf", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=242, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fc831bc4000
read(3, "# Dynamic resolv.conf(5) file fo"..., 4096) = 242
read(3, "", 4096)                       = 0
close(3)                                = 0
munmap(0x7fc831bc4000, 4096)            = 0
socket(PF_INET, SOCK_DGRAM|SOCK_NONBLOCK, IPPROTO_IP) = 3
connect(3, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("10.10.110.20")}, 16) = 0
[...]

As you can see in the strace output of the Server, the DNS name could be resolved to 10.10.110.20.
But where does the difference start? It's this line:

Desktop: open("/lib/x86_64-linux-gnu/libnss_mdns4_minimal.so.2", O_RDONLY|O_CLOEXEC) = 3
Server: open("/lib/x86_64-linux-gnu/libnss_dns.so.2", O_RDONLY|O_CLOEXEC) = 3

Ubuntu Desktop uses the DNS library libnss_mdns4_minimal.so, but Server uses the "normal" DNS library of libnss.
This behaviour is configured in /etc/nsswitch.conf which tells the Desktop version to use mdns4:

root@ubuntudesktop:~# cat /etc/nsswitch.conf | grep hosts
hosts:          files mdns4_minimal [NOTFOUND=return] dns

Comparison with the Server:

root@ubuntuserver:~# cat /etc/nsswitch.conf | grep hosts
hosts:          files dns

After removing the "mdns4_minimal [NOTFOUND=return]" entry from the hosts line in nsswitch.conf, ntpdate finally worked:

root@ubuntudesktop:~# ntpdate ntp.example.local
24 Nov 09:28:12 ntpdate[22745]: adjust time server 10.162.214.20 offset -0.008526 sec

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