Bug#941026: netcfg_gateway_reachable wrongly rejects IPv6 link-local addresses

[Andrew Kanaber <adk-debianbugs@jump.net.uk>]

Package: netcfg
Version: 1.160
Severity: normal

When doing static IP configuration, netcfg will reject a gateway address
outside the host's network as defined by the netmask. This is wrong for IPv6
because the gateway can legitimately be a link-local address in fe80::/64
instead of the host's network range. Ubuntu have fixed this bug in their
version, see LP#1382295
https://bugs.launchpad.net/ubuntu/+source/netcfg/+bug/1382295

The relevant function is netcfg_gateway_reachable in netcfg-common.c which
simply checks gateway_address & netmask == host_address. It should also allow
IPv6 addresses in the link local prefix fe80::/64.

Less importantly, the error message it triggers could be a bit clearer, "The
gateway address you entered is unreachable" sounds like it might be a network
error when it's purely a user-input parsing rejection - if the code had
actually tried the link-local address it would've worked. 

Thanks for your help,

Andrew Kanaber