CURLOPT_INTERFACE: avoid resolving interfaces names

Do not try to resolve interfaces names via DNS by recognizing interface
names in a few ways.  If the interface option argument has a prefix of
"if!" then treat the argument as only an interface.  Similarly, if the
interface argument is the name of an interface (even if it does not have
an IP address assigned), treat it as an interface name.  Finally, if the
interface argument is prefixed by "host!" treat it as a hostname that
must be resolved by /etc/hosts or DNS.

These changes allow a client using the multi interfaces to avoid
blocking on name resolution if the interface loses its IP address or
disappears.

1 files changed, 9 insertions, 0 deletions

diff --git a/docs/libcurl/curl_easy_setopt.3 b/docs/libcurl/curl_easy_setopt.3
index 7cf9a0408..5f1629d4a 100644
--- a/docs/libcurl/curl_easy_setopt.3
+++ b/docs/libcurl/curl_easy_setopt.3
@@ -843,6 +843,15 @@ negotiation. (Added in 7.19.4).
 Pass a char * as parameter. This sets the interface name to use as outgoing
 network interface. The name can be an interface name, an IP address, or a host
 name.
+
+Starting with 7.24.0: If the parameter starts with "if!" then it is treated as
+only as interface name and no attempt will ever be named to do treat it as an
+IP address or to do name resolution on it.  If the parameter starts with
+\&"host!" it is treated as either an IP address or a hostname.  Hostnames are
+resolved synchronously.  Using the if! format is highly recommended when using
+the multi interfaces to avoid allowing the code to block.  If "if!" is
+specified but the parameter does not match an existing interface,
+CURLE_INTERFACE_FAILED is returned.
 .IP CURLOPT_LOCALPORT
 Pass a long. This sets the local port number of the socket used for
 connection. This can be used in combination with \fICURLOPT_INTERFACE\fP and