-S: if not valid IP addr, treat it as a device name to bind to

src/dns_poller.c

@@ -153,7 +153,8 @@ static void set_bootstrap_source_addr(ares_channel channel,
     }
     ares_set_local_ip6(channel, (const unsigned char *)&addr_v6);
   } else {
-    WLOG("Bootstrap source address '%s' is not a valid IP literal", source_addr);
+    // otherwise treat it as a device/interface name
+    ares_set_local_dev(channel, source_addr);
     return;
   }
 }

src/dns_poller.h

@@ -37,7 +37,7 @@ typedef struct {
 // provided ev_loop. `bootstrap_dns` is a comma-separated list of DNS servers to
 // use for the lookup `hostname` every `interval_seconds`. For each successful
 // lookup, `cb` will be called with the resolved address.
-// `source_addr` optionally binds bootstrap DNS lookups to a specific IP.
+// `source_addr` optionally binds bootstrap DNS lookups to a specific IP/device.
 // `family` should be AF_INET for IPv4 or AF_UNSPEC for both IPv4 and IPv6.
 //
 // Note: hostname *not* copied. It should remain valid until

src/https_client.c

@@ -321,7 +321,7 @@ static void https_fetch_ctx_init(https_client_t *client,
     ASSERT_CURL_EASY_SETOPT(ctx, CURLOPT_PROXY, client->opt->curl_proxy);
   }
   if (client->opt->source_addr) {
-    DLOG_REQ("Using source address: %s", client->opt->source_addr);
+    DLOG_REQ("Using source address or device: %s", client->opt->source_addr);
     ASSERT_CURL_EASY_SETOPT(ctx, CURLOPT_INTERFACE, client->opt->source_addr);
   }
   if (client->opt->ca_info) {

src/options.c

@@ -260,7 +260,7 @@ void options_show_usage(int __attribute__((unused)) argc, char **argv) {
   printf("                         supports it (http, https, socks4a, socks5h), otherwise\n");
   printf("                         initial DNS resolution will still be done via the\n");
   printf("                         bootstrap DNS servers.\n");
-  printf("  -S source_addr         Source IPv4/v6 address for outbound HTTPS and bootstrap DNS.\n");
+  printf("  -S source_addr/dev     Source IPv4/v6 address or device for outbound HTTPS and bootstrap DNS.\n");
   printf("                         (Default: system default)\n");
   printf("  -x                     Use HTTP/1.1 instead of HTTP/2. Useful with broken\n"
          "                         or limited builds of libcurl.\n");

src/options.h

@@ -46,7 +46,7 @@ struct Options {
   // e.g. "socks5://127.0.0.1:1080"
   const char *curl_proxy;
 
-  // Source address for outbound HTTPS connections
+  // Source address or device for outbound HTTPS connections and ares
   const char *source_addr;
 
   // 1 = Use only HTTP/1.1 for limited OpenWRT libcurl (which is not built with HTTP/2 support)