netconsole: resolve domain name via 'getent' instead of 'hosts'

[deekej]

This commit solves the issue described in RHBZ #1278519.

When the nameserver address lines are not present in /etc/resolv.conf, launching
/etc/init.d/netconsole start will result in this error:

Server address not specified in /etc/sysconfig/netconsole

This was caused by using hosts command for domain name resolution. This is now
fixed by using getent command instead.

I have improved the patch provided in the RHBZ, here is the basic overview:

• To test if the IPV6INIT variable is set to yes, we are now using is_true
  instead of grep-ing the value from /etc/sysconfig/network directly.
• Testing of initially set $SYSLOGADDR with regular expressions has been extended.
  The regex for IPv6 has been added, and it is unfortunately quite huge because of possible
  IPv6 variations (e.g. 4to6). It was taken from Stack Overflow and
  edited. (It does not accept the mask of IP address - e.g. /64 ). The regex has been tested via regexpal.com.
  (You can test it yourself -- copy-paste the regex there.)
• In case no IP address is specified in $SYSLOGADDR, then we assume the domain name is used and try to resolve it. The default behaviour for
  this depends on if IPV6INIT is set, and other conditions. Excerpt from the patch:

The address resolution will follow up this scheme:
0) Is the use of IPv6 forced (by 'IPV6INIT=yes' in /etc/sysconfig/network)?
   If yes ->> use IPv6
1) If only IPv4 is available ->> use IPv4
2) If only IPv6 is available ->> use IPv6
3) If both IPv4 and IPv6 are available ->> use IPv4 by default

This should remain same for both RHEL6 and RHEL7/Fedora. I know that IPv6 is disabled on RHEL6, by default, and enabled in RHEL7/Fedora by default, but that is a separate (orthogonal) thing. When IPv6 is not forced, and both IPv4 and IPv6 are specified for the same domain name, the script will fallback to use of IPv4, which IMHO is desirable. IOW, the script is not forcing IPv6 on user unless there's no other IP available for that specific domain name.

I have successfully tested the patch on RHEL-6.8.

[lnykryn]

What about the case where I have IPv6 enabled (default case in upstream), but my log server only has ipv4 address?

[deekej]

I have updated the patch to use IPv4 by default, and fallback to IPv6 if the resolution fails. Please, see the updated commit.

Updated commit provided.

[lnykryn]

Seems to work :-)