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In my company when I install an Ubuntu I cannot ping any local machine:
$ ping foobar.mycompany.local
ping foobar.mycompany.local: Name or service not known
But if I put dns in /etc/nsswitch.conf at the beginning of the list, then it works:
# hosts: files mdns4_minimal [NOTFOUND=return] dns
hosts: dns files mdns4_minimal [NOTFOUND=return]
I would like to understand why.
nowox
Posted 2019-03-25T09:41:27.807
Reputation: 1 779
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Your company uses a DNS domain ending with .local, which is actually a special-purpose suffix and is reserved by IETF for Multicast DNS. So because you have a mDNS client installed (mdns4_minimal), it gets configured for priority handling of all *.local names.
(It is unfortunately still common practice in corporate intranets to just make up a nonexistent domain name or IP address range and hope that it'll remain nonexistent forever...)
Go through your configured modules one by one:
hosts: files mdns4_minimal [NOTFOUND=return] dns
[NOTFOUND=return] indicates that processing should not continue after this error; i.e. "not found" should be immediately returned to the program.Why the extra "[NOTFOUND=return]"? According to various sources, it's there to speed up unsuccessful queries and to prevent information leakage, and to reduce load on public DNS servers.
Let's say someone's network actually used mDNS (which is common on Linux/macOS). If the user tried to resolve "MyLittleLaptop.local" and it wasn't found, the system would keep trying the next module ('dns'), and the query would be sent to the public DNS (e.g. to the school's DNS servers, or to the coffee shop's router).
But – according to the reservation by IETF – *.local names cannot possibly exist in public DNS, so such a query would be useless, all it does is reveal your personal information to the network admin. So the [NOTFOUND=return] tag is added to stop it from reaching DNS completely.
If your corporate network uses .local for internal DNS and you're fairly sure it'll never use mDNS, you can remove the whole module – resulting in:
hosts: files dns
If you want to prioritize DNS, but keep the possibility open for using mDNS, then move it to the end:
hosts: files dns mdns_minimal
user1686
Posted 2019-03-25T09:41:27.807
Reputation: 283 655
man nsswitch.conf it's unclear where you would add it and how that might look like. The current answer could be that (for whatever reason) you don't query the DNS server otherwise. – Seth – 2019-03-25T09:45:37.173
What other modules are specified in your nsswitch.conf
hosts:line? – user1686 – 2019-03-25T10:20:30.257@grawity I have edited my question and added this information – nowox – 2019-03-25T10:23:01.027
Do you have mdns setup? If not in the first example you would need to have that host on your hosts files or it would return a notfound. On the second example it would check your DNS. – Seth – 2019-03-25T10:49:05.407
I did not have configured anything it is a new fresh install Ubuntu 18.04 – nowox – 2019-03-25T10:50:39.883
So your company doesn't provide an MDNS (Multicast DNS) service, but it does provide a DNS service. Which is why you need to put
dnsinto this line (and I'm a bit surprised Ubuntu doesn't put it there by default), because otherwise it won't be used to look up the hostname. – dirkt – 2019-03-25T11:41:53.220