[Bug 190824] New: [patch] [libc] getaddrinfo: addrconfig doesn't ignore loopback addresses

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=190824

            Bug ID: 190824
           Summary: [patch] [libc] getaddrinfo: addrconfig doesn't ignore
                    loopback addresses
           Product: Base System
           Version: 11.0-CURRENT
          Hardware: Any
                OS: Any
            Status: Needs Triage
          Severity: Affects Many People
          Priority: ---
         Component: kern
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: justin.mcomie at gmail.com

The AI_ADDRCONFIG option of getaddrinfo is intended to reduce latency by
excluding queries
for address families not configured on the system.  However loopback addresses
are currently
treated as "configured" addresses, effectively breaking this functionality.
This patch
addresses this by retooling addrconfig() to exclude addresses on loopback
interfaces when
looking for configured addresses.  The code is based on getifaddrs.c.

- This patch does not address link-local addresses with respect to
AI_ADDRCONFIG.
- This patch does not affect the ability to resolve numeric hosts such as
'127.0.0.1', '::1',
  and 'ffff::%eth0', since getaddrinfo bypasses the call to addrconfig when
such an address
  is requested.

Testing:

- Host info:
$ ifconfig
em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
    options=9b<RXCSUM,TXCSUM,VLAN_MTU,VLAN_HWTAGGING,VLAN_HWCSUM>
    ether 00:0c:29:0d:a3:78
    inet 172.16.69.133 netmask 0xffffff00 broadcast 172.16.69.255 
    nd6 options=29<PERFORMNUD,IFDISABLED,AUTO_LINKLOCAL>
    media: Ethernet autoselect (1000baseT <full-duplex>)
    status: active
lo0: flags=8049<UP,LOOPBACK,RUNNING,MULTICAST> metric 0 mtu 16384
    options=600003<RXCSUM,TXCSUM,RXCSUM_IPV6,TXCSUM_IPV6>
    inet6 ::1 prefixlen 128 
    inet6 fe80::1%lo0 prefixlen 64 scopeid 0x2 
    inet 127.0.0.1 netmask 0xff000000 
    nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>


- Using a test program like
http://www.logix.cz/michal/devel/various/getaddrinfo.c.xp
  and filtering to just show the address families:

- With AI_ADDRCONFIG NOT set:
$ ./gai google.com | egrep -o IPv. | uniq
IPv4
IPv6
$ ./gai  127.0.0.1 | egrep -o IPv. | uniq
IPv4
$ ./gai  fe80::1%lo0 | egrep -o IPv. | uniq
IPv6

- With AI_ADDRCONFIG set:
$ ./gai google.com | egrep -o IPv. | uniq
IPv4
$ ./gai 127.0.0.1 | egrep -o IPv. | uniq
IPv4
$ ./gai fe80::1%lo0 | egrep -o IPv. | uniq
IPv6

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