The problem is that superuser.com, stackexchange.com and many other sites didn't work.
When I was debugging the problem, I tried nslookup:
user@host:~$ nslookup superuser.com 8.8.8.8
Server: 8.8.8.8
Address: 8.8.8.8#53
** server can't find superuser.com: SERVFAIL
user@host:~$ nslookup superuser.com 8.8.4.4
Server: 8.8.4.4
Address: 8.8.4.4#53
** server can't find superuser.com: SERVFAIL
But then, using this page: http://www.kloth.net/services/nslookup.php
... here is the nslookup result for superuser.com from server 8.8.8.8, querytype=A :
DNS server handling your query: 8.8.8.8
DNS server's address: 8.8.8.8#53
Non-authoritative answer:
Name: superuser.com
Address: 104.16.32.193
Name: superuser.com
Address: 104.16.125.192
Name: superuser.com
Address: 104.16.126.192
Name: superuser.com
Address: 104.16.127.192
Name: superuser.com
Address: 104.16.33.193
So there was clearly a problem trying to reach Google's DNS IP. I then made a telnet to see if port 53 was open in my router:
user@host:~$ telnet 8.8.8.8 53
Trying 8.8.8.8...
Connected to 8.8.8.8.
Escape character is '^]'.
Connection closed by foreign host.
Everything is working, so my last suspect is that the ISP is hijacking the IP somehow. How could I test it?
EDIT:
mtr trace:
Host Loss% Snt Last Avg Best Wrst StDev
1. 192.168.2.254 0.0% 120 0.5 0.4 0.2 0.7 0.0
2. 170.210.178.1 0.0% 120 1.1 1.0 0.7 1.3 0.0
3. riu-unpaz-100M.BUENOS-AIRES.riu.edu.ar 0.0% 120 1.8 4.4 1.7 182.0 18.3
4. rgda3-dc-sol.riu.edu.ar 0.0% 119 2.8 3.2 2.1 7.1 0.8
5. google-riu.riu.edu.ar 0.0% 119 2.3 2.5 2.0 16.1 1.5
6. 66.249.95.205 0.8% 119 2.6 2.8 2.4 3.6 0.0
7. google-public-dns-a.google.com 0.0% 119 2.5 2.5 2.2 3.2 0.0
