Can you configure a specific interface to reach the NTP server?

Question

I have to configure NTP on a CentOS server. I have 7 interfaces and I can reach the NTP server through one interface. Also the server doesn't have access to internet. When I try ping:

ping 10.0.10.12
connect: Network is unreachable

But when I ping with source interface, the ping is successful.

ping -I eth0 10.0.10.12
PING 10.0.0.131 (10.0.10.12) from 10.69.4.16 eth0: 56(84) bytes of data.
64 bytes from 10.0.10.12: icmp_seq=1 ttl=127 time=4.07 ms
64 bytes from 10.0.10.12: icmp_seq=2 ttl=127 time=0.309 ms
64 bytes from 10.0.10.12: icmp_seq=3 ttl=127 time=0.318 ms

ntpstat command output:

unsynchronised
 time server re-starting
  polling server every 8s

ntpq -p command output:

     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 10.0.10.12      .INIT.          16 -    -  512    0    0.000    0.000   0.000

I also tried ntpdate - d 10.0.10.12:

ntpdate -d 10.0.0.131
23 Jul 09:14:31 ntpdate[16715]: ntpdate 4.2.6p5@1.2349-o Sat Nov 23 18:21:48 UTC 2013 (1)
Looking for host 10.0.0.131 and service ntp
host found : 10.0.0.131
23 Jul 09:14:31 ntpdate[16715]: no servers can be used, exiting

The NTP server is configured in the ntp.conf file, I think that the problem is something with the network. I was wondering if I can make explicit source interface for reaching the NTP server?

Lenniey · Answer 1 · 2019-07-24T06:54:46.723

3

~~You can add a specific host route for your NTP server:~~

route add -host 10.0.10.12 dev eth0, for example.

As per Anton's completely correct comment, here is the command in iproute2 syntax:

ip route add 10.0.10.12 dev eth0

edited Jul 24 '19 at 06:54

answered Jul 23 '19 at 07:31

Lenniey

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Stop using of the deprecated commands! – Anton Danilov Jul 23 '19 at 07:55
Force of habit. I added the new command. – Lenniey Jul 23 '19 at 08:05
This is in the eth0 table: ip route show table eth0table, 10.69.0.0/16 dev eth0 scope link, default via 10.69.4.1 dev eth0. – ananetworking Jul 23 '19 at 08:34
@ananetworking unfortunately, the server `10.0.10.12` isn't in `10.69.0.0/16`, so that route won't help. I should try Lenniey's answer before complaining about it! – MadHatter Jul 24 '19 at 07:19

score -1 · Accepted Answer · answered Jul 23 '19 at 09:50

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I resolved this concluding that NTP requires default gateway for working properly. This was solution for me:

ip route add default via 127.0.0.1 dev lo

answered Jul 23 '19 at 09:50

ananetworking

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A default route via localhost? I'd assume this complicates things further, but YMMV... – Lenniey Jul 23 '19 at 09:58

Can you configure a specific interface to reach the NTP server?

2 Answers2