6.6.1 on Ultra 6 Loses DNS lookup after a few days

I share your preference. I cannot offer a solution. This is just a suggestion.

My idea is: Currently the router is responsible for assigning the ip address to the nas. Now it would be interesting to see, whether the problem continues after you take away that responsibility from the router and leave the nas with a fix ip address.

If that helps, you a) would have a temporay fix and b) could continue to research for something which gets you closer to your preferred configuration.

This time the unit locked up so I had to reboot it. I set the IP back to DHCP. Running your other command I noticed that I had not updated my NTP address and that the 2nd ethernet connection had a router and DNS entry set that it cannot connect to. I have Eth1 directly connected to a PC with a cross over cable. I removed the router and DNS entries from that connection. I am wondering if that was the issue??? How are the network connections weighted? Which link will it try by default? I see they are both listed as favorite.

/net/connman/service/ethernet_001f33ea8125_cable
  Type = ethernet
  Security = [  ]
  State = ready
  Favorite = True
  Immutable = False
  AutoConnect = True
  Name = Wired
  Ethernet = [ Method=auto, Interface=eth0, Address=00:1F:33:EA:81:25, MTU=1500 ]
  IPv4 = [ Method=dhcp, Address=10.10.10.20, Netmask=255.255.255.0, Gateway=10.10.10.1 ]
  IPv4.Configuration = [ Method=dhcp ]
  IPv6 = [  ]
  IPv6.Configuration = [ Method=off ]
  Nameservers = [ 10.10.10.1 ]
  Nameservers.Configuration = [  ]
  Timeservers = [ 10.10.10.26, time-a.netgear.com, 10.10.10.1 ]
  Timeservers.Configuration = [ 10.10.10.26, time-a.netgear.com ]
  Routes = [  ]
  Routes.Configuration = [  ]
  Domains = [ NULEAF ]
  Domains.Configuration = [  ]
  Proxy = [ Method=direct ]
  Proxy.Configuration = [  ]
  Provider = [  ]
/net/connman/service/ethernet_001f33ea8124_cable
  Type = ethernet
  Security = [  ]
  State = ready
  Favorite = True
  Immutable = False
  AutoConnect = True
  Name = Wired
  Ethernet = [ Method=auto, Interface=eth1, Address=00:1F:33:EA:81:24, MTU=1500 ]
  IPv4 = [ Method=manual, Address=10.101.1.3, Netmask=255.255.255.0 ]
  IPv4.Configuration = [ Method=manual, Address=10.101.1.3, Netmask=255.255.255.0 ]
  IPv6 = [  ]
  IPv6.Configuration = [ Method=off ]
  Nameservers = [  ]
  Nameservers.Configuration = [  ]
  Timeservers = [ 10.10.10.26, time-a.netgear.com ]
  Timeservers.Configuration = [ 10.10.10.26, time-a.netgear.com ]
  Routes = [  ]
  Routes.Configuration = [  ]
  Domains = [  ]
  Domains.Configuration = [  ]
  Proxy = [ Method=direct ]
  Proxy.Configuration = [  ]
  Provider = [  ]

Once I removed that Router and DNS entry from Eth1 I could no longer resolve DNS So it really seems like it wants to use that link

root@NAS:~# ping google.com
ping: bad address 'google.com'
root@NAS:~# ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8): 56 data bytes
64 bytes from 8.8.8.8: seq=0 ttl=46 time=9.523 ms
64 bytes from 8.8.8.8: seq=1 ttl=46 time=9.473 ms

So next I disconencted Eth1 and then I could ping again. Any ideas on how to have the two connections used but favor Eth0 for internet and DNS?

Nothing has changed that I am aware of and my system has been up for almost 2 weeks now.