[PATCH] nss: Use gethostbyname4_r when requesting IPv6 addresses (bug 14413)

[Patrick Griffis]

If getaddrinfo() is called with family of AF_INET6 it would return IPv6
addresses lacking scope_id information.

This was a regression caused by bug 14505. That patch avoided calling
gethostbyname4_r unless AF_UNSPEC is passed. The reasoning was that
gethostbyname4_r has no ability to only ask for a specific family.
That is true however gethostbyname3_r returns a list of hostent structs
which lack scope_id information.

The solution is to always use gethostbyname4_r if IPv6 is requested.
This does have the downside of IPv4 addresses being looked up by
modules even when they are not needed.
---
 nss/getaddrinfo.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/nss/getaddrinfo.c b/nss/getaddrinfo.c
index 6726ace6fd..f448222821 100644
--- a/nss/getaddrinfo.c
+++ b/nss/getaddrinfo.c
@@ -644,9 +644,12 @@ get_nss_addresses (const char *name, const struct addrinfo *req,
       no_data = 0;
       nss_gethostbyname4_r *fct4 = NULL;
 
-      /* gethostbyname4_r sends out parallel A and AAAA queries and
-	 is thus only suitable for PF_UNSPEC.  */
-      if (req->ai_family == PF_UNSPEC)
+      /* The gethostbyname4_r API is always expected to return both address families
+       * which means we would prefer gethostbyname3_r when requesting only IPv4 addresses.
+       * Unfortunately when requesting IPv6 only we have to use this API for link-local
+       * information (scope_id is not in the hostent struct).
+       * This means we may collect unncessary IPv4 addresses. */
+      if (req->ai_family != PF_INET)
 	fct4 = __nss_lookup_function (nip, "gethostbyname4_r");
 
       if (fct4 != NULL)
-- 
2.51.0