I am using OPENDNS to block 3 or 4 urls.
Router is pointed at opendns servers, opendns is configured to block specific DNS entries.
if you are using your router to block only URLs it won't work for HTTPS queries to those sites since HTTPS is end to end encrypted and the router won't know. You have to block the DNS queries so the IP address is never obtained.
When I get home if I remember I will share my list of blocked domains. Note I have been blocking since the day the browser blocking update was annoiced and I have not gotten a system update notice or browser update notice since. But it hasn't been that many days yet so YMMV until it gets more validation.
Note: eshop still works fine with the following blocked domains.
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Do this first to remove any current updates pending:
Completely power off your system.
Boot in recovery mode (press L+R+A+?+Power, hold the first four about 1 second, more won't hurt the system but only your fingers...
)
It will ask you as usual to update. However, it will take a little longer for the screen to come up compared to the speed it would have had if you didn't already have an update.
If you click yes, the update will be completely redownloaded then installed...
If you click no, it will power off and you'll go home free!
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--edit--
nus.cdn.c.shop.nintendowifi.net (http) (Where the update content is hosted)
nus.c.shop.nintendowifi.net (https) (Where updates are checked)
cbvc.cdn.nintendo.net (browserhax blocking)
--edit--
doing this on an N3DS XL USA 10.1 connected to the internet through homepass on router and through normal wifi.