BIND and network unreachable messages...

Sometimes you'll see messages like cá cược thể thao bet365_cách nạp tiền vào bet365_ đăng ký bet365 following ones in your log file (messages are slightly obfuscated to protect innocent :)):

Jun 29 14:32:11 someserver named[1459]: error (network unreachable) resolving 'www.eolprocess.com/A/IN': 2001:503:a83e::2:30#53
Jun 29 14:32:11 someserver named[1459]: error (network unreachable) resolving 'www.eolprocess.com/A/IN': 2001:503:231d::2:30#53

What cá cược thể thao bet365_cách nạp tiền vào bet365_ đăng ký bet365se messages say is that network that contains address 2001:503:231d::2:30 is unreachable. So, what's happening?

The problem is that all modern operating systems support IPv6 out of cá cược thể thao bet365_cách nạp tiền vào bet365_ đăng ký bet365 box. The same is for growing number of software packages, among cá cược thể thao bet365_cách nạp tiền vào bet365_ đăng ký bet365m is BIND too. So, operating system configures IPv6 address on interface and application thinks that IPv6 works and configures it. But, IPv6 doesn't work outside of cá cược thể thao bet365_cách nạp tiền vào bet365_ đăng ký bet365 local network (cá cược thể thao bet365_cách nạp tiền vào bet365_ đăng ký bet365re is no IPv6 capable router) so, IPv6 addresses, unless in local networks, are unreachable.

So, you might ask now: but everything ocá cược thể thao bet365_cách nạp tiền vào bet365_ đăng ký bet365rwise works, why is this case special! Well, cá cược thể thao bet365_cách nạp tiền vào bet365_ đăng ký bet365 problem is that some DNS servers, anywhere in hierarchy, support IPv6, but not all. And when our resolver gets IPv6 address in response, it defaults to it and ignores IPv4. It obviously can not reach it so it logs a message and cá cược thể thao bet365_cách nạp tiền vào bet365_ đăng ký bet365n tries IPv4. Once again, note that this IPv6 address can pop up anywhere in hierarchy, it isn't necessary to be on cá cược thể thao bet365_cách nạp tiền vào bet365_ đăng ký bet365 last DNS server. In this concrete case name server for eolprocess.com doesn't support IPv6, but some name server for cá cược thể thao bet365_cách nạp tiền vào bet365_ đăng ký bet365 top level com domain do support it!

To prevent those messages from appearing add option -4 to bind during startup. On CentOS (Fedora/RHEL) add or modify cá cược thể thao bet365_cách nạp tiền vào bet365_ đăng ký bet365 line OPTIONS in /etc/sysconfig/named so that it includes option -4, i.e.

OPTIONS="-4"