Set DNS server on OS X even when without internet connection

SEE UPDATE BELOW!

I also enjoy using Dnsmasq on my local machine, and I had this problem too. Here is the solution:

From man 5 resolver:

The configuration for a particular client may be read from a file
having the format described in this man page. These are at present
located by the system in the /etc/resolv.conf file and in the files
found in the /etc/resolver directory.

/etc/resolver/ is not present by default; you must create it yourself.

Also from the man page:

domain
  Domain name associated with this resolver configuration. This
  option is normally not required by the Mac OS X DNS search system
  when the resolver configuration is read from a file in the
  /etc/resolver directory. In that case the file name is used as the
  domain name.

So if you wanted all dns queries for the top level domain of dev to be routed to the local nameserver, you would:

# mkdir /etc/resolver
# echo 'nameserver 127.0.0.1' > /etc/resolver/dev

configd does not alter files in /etc/resolver/, so this setting will persist through network changes and reboots.

UPDATE 17 July 2012

Unfortunately, as of OS X Lion, the top resolver (as shown by scutil --dns) disappears when no interfaces are active:

# scutil --dns # Online
DNS configuration

resolver #1
  nameserver[0] : 127.0.0.1

...

resolver #8
  domain   : dev
  nameserver[0] : 127.0.0.1

# scutil --dns # Offline
DNS configuration

resolver #1

...

resolver #8
  domain   : dev
  nameserver[0] : 127.0.0.1

Notice that resolver #1 is empty, but that the /etc/resolver derived nameserver entry remains.

It turns out that since you can specify the resolver domain directly in the /etc/resolver/ file, specifying the special Internet root domain . causes the creation of a global resolver entry that looks like:

resolver #8
  nameserver[0] : 127.0.0.1

Now all DNS queries are routed to localhost, even when offline.

Of course, you will still have to resolve your chosen domains as 127.0.0.1 using something like dnsmasq's --address option:

# dnsmasq --address=/dev/127.0.0.1

In summary:

• Set all your network interface dns servers to 127.0.0.1:

    networksetup -setdnsservers Ethernet 127.0.0.1
    networksetup -setdnsservers Wi-Fi 127.0.0.1
    ...

• Create a file /etc/resolver/whatever:

    nameserver 127.0.0.1
    domain .

• Set up a local DNS server and be happy.

cf. http://opensource.apple.com/source/configd/configd-395.11/dnsinfo/dnsinfo_flatfile.c

Why not make the entries in /etc/hosts instead? I'm having trouble thinking of a situation where you'd need to actually be running a full blown DNS server. I use host file entries all the time to accomplish things like this on my Macs.

The resolver in OS X works differently than that in Linux or other Unixes. This is probably part of what's causing you grief. Like for instance it has a preferences for which method of resolution to use first and it caches the results of all queries for a period of time.

Have you added the DNS server to the interface in the Network preference pane? This should ensure that the resolver uses that server for it's queries should it decide to look for a DNS entry.

(answering b/c I can't comment yet...)

How are you going offline?

(best answer I got right now)

# man -S 5 resolver
 .
 .
 .
 Note that the /etc/resolv.conf file,
 which contains configuration for the default (or "primary") DNS resolver
 client, is maintained automatically by Mac OS X and should not be edited manu-
 ally.  Changes to the DNS configuration should be made by using the Network
 Preferences panel.

So, you should be able to enter something in the UI, and it should stick. I had done this a couple times when I had not like the DNS server my local DHCP server provides.