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, 9 insertions, 1 deletions

diff --git a/lib/if2ip.h b/lib/if2ip.h
index 858ec2f5b..ac5875237 100644
--- a/lib/if2ip.h
+++ b/lib/if2ip.h
@@ -24,7 +24,15 @@
 #include "curl_setup.h"
 
 bool Curl_if_is_interface_name(const char *interf);
-char *Curl_if2ip(int af, const char *interf, char *buf, int buf_size);
+
+typedef enum {
+  IF2IP_NOT_FOUND = 0, /* Interface not found */
+  IF2IP_AF_NOT_SUPPORTED = 1, /* Int. exists but has no address for this af */
+  IF2IP_FOUND = 2 /* The address has been stored in "buf" */
+} if2ip_result_t;
+
+if2ip_result_t Curl_if2ip(int af, unsigned int remote_scope,
+                          const char *interf, char *buf, int buf_size);
 
 #ifdef __INTERIX