connect: treat an interface bindlocal() problem as a non-fatal error

I am using curl_easy_setopt(CURLOPT_INTERFACE, "if!something") to force
transfers to use a particular interface but the transfer fails with
CURLE_INTERFACE_FAILED, "Failed binding local connection end" if the
interface I specify has no IPv6 address. The cause is as follows:

The remote hostname resolves successfully and has an IPv6 address and an
IPv4 address.

cURL attempts to connect to the IPv6 address first.

bindlocal (in lib/connect.c) fails because Curl_if2ip cannot find an
IPv6 address on the interface.

This is a fatal error in singleipconnect()

This change will make cURL try the next IP address in the list.

Also included are two changes related to IPv6 address scope:

- Filter the choice of address in Curl_if2ip to only consider addresses
with the same scope ID as the connection address (mismatched scope for
local and remote address does not result in a working connection).

- bindlocal was ignoring the scope ID of addresses returned by
Curl_if2ip . Now it uses them.

Bug: http://curl.haxx.se/bug/view.cgi?id=1189

1 files changed, 11 insertions, 6 deletions

diff --git a/lib/ftp.c b/lib/ftp.c
index dec4800b6..e490dd206 100644
--- a/lib/ftp.c
+++ b/lib/ftp.c
@@ -1066,12 +1066,17 @@ static CURLcode ftp_state_use_port(struct connectdata *conn,
 
     if(*addr != '\0') {
       /* attempt to get the address of the given interface name */
-      if(!Curl_if2ip(conn->ip_addr->ai_family, addr,
-                     hbuf, sizeof(hbuf)))
-        /* not an interface, use the given string as host name instead */
-        host = addr;
-      else
-        host = hbuf; /* use the hbuf for host name */
+      switch(Curl_if2ip(conn->ip_addr->ai_family, conn->scope, addr,
+                     hbuf, sizeof(hbuf))) {
+        case IF2IP_NOT_FOUND:
+          /* not an interface, use the given string as host name instead */
+          host = addr;
+          break;
+        case IF2IP_AF_NOT_SUPPORTED:
+          return CURLE_FTP_PORT_FAILED;
+        case IF2IP_FOUND:
+          host = hbuf; /* use the hbuf for host name */
+      }
     }
     else
       /* there was only a port(-range) given, default the host */