我有一个虚拟机，大约在同一时间连续三天断开自己的连接。我不知道为什么。
/var/log/messages:
Jan 25 20:43:50 server dhclient[576]: DHCPREQUEST on eth1 to 255.255.255.255 port 67 (xid=0x369f2988)
Jan 25 20:43:56 server named[2114]: no longer listening on 94.237.40.155#53
Jan 25 20:43:56 server named[2114]: no longer listening on 10.5.0.127#53
Jan 25 20:43:56 server named[2114]: no longer listening on 94.237.40.185#53
Jan 25 20:43:56 server avahi-daemon[514]: Withdrawing address record for 94.237.40.185 on eth3.
Jan 25 20:43:56 server avahi-daemon[514]: Leaving mDNS multicast group on interface eth3.IPv4 with address 94.237.40.185.
Jan 25 20:43:56 server avahi-daemon[514]: Interface eth3.IPv4 no longer relevant for mDNS.
Jan 25 20:43:56 server avahi-daemon[514]: Withdrawing address record for 10.5.0.127 on eth1.
Jan 25 20:43:56 server avahi-daemon[514]: Leaving mDNS multicast group on interface eth1.IPv4 with address 10.5.0.127.
Jan 25 20:43:56 server avahi-daemon[514]: Interface eth1.IPv4 no longer relevant for mDNS.
Jan 25 20:43:56 server avahi-daemon[514]: Withdrawing address record for 94.237.40.155 on eth0.
Jan 25 20:43:56 server avahi-daemon[514]: Leaving mDNS multicast group on interface eth0.IPv4 with address 94.237.40.155.
Jan 25 20:43:56 server avahi-daemon[514]: Interface eth0.IPv4 no longer relevant for mDNS.
Jan 25 20:43:56 server NetworkManager[509]: <info> [1516909436.0282] policy: set 'System eth0' (eth0) as default for IPv4 routing and DNS
Jan 25 20:43:57 server ntpd[540]: Deleting interface #6 eth3, 94.237.40.185#123, interface stats: received=591, sent=591, dropped=0, active_time=86399 secs
Jan 25 20:43:57 server ntpd[540]: 129.250.35.251 interface 94.237.40.185 -> (none)
Jan 25 20:43:57 server ntpd[540]: 78.46.53.8 interface 94.237.40.185 -> (none)
Jan 25 20:43:57 server ntpd[540]: 46.4.99.122 interface 94.237.40.185 -> (none)
Jan 25 20:43:57 server ntpd[540]: 88.198.12.78 interface 94.237.40.185 -> (none)
Jan 25 20:43:57 server ntpd[540]: Deleting interface #5 eth1, 10.5.0.127#123, interface stats: received=0, sent=0, dropped=0, active_time=86399 secs
Jan 25 20:43:57 server ntpd[540]: Deleting interface #4 eth0, 94.237.40.155#123, interface stats: received=0, sent=0, dropped=0, active_time=86399 secs
Jan 25 20:43:58 server dhclient[575]: DHCPREQUEST on eth0 to 255.255.255.255 port 67 (xid=0x4d5a4814)
Jan 25 20:43:58 server dhclient[577]: DHCPREQUEST on eth3 to 255.255.255.255 port 67 (xid=0x632e1122)
为什么服务器决定断开自己的网络连接？我如何防止它这样做？