--dns-servers flag and CURLOPT_DNS_SERVERS ignores port number

[Roguelazer]

I did this

Started a DNS server on localhost:5353 and ran curl --dns-servers 127.0.0.1:5353 www.example.com

The documentation indicated that port numbers are passed through to c-ares; however, I believe that this is not the case.

I expected the following

curl to make a DNS request to 127.0.0.1:5353

What happened instead?

curl made a DNS request to 127.0.0.1:53. I confirmed with strace that nothing ever tried to talk to :5353 and that starting my custom server as root on port 53 instead worked.

curl/libcurl version

curl 7.65.1 (x86_64-redhat-linux-gnu) libcurl/7.65.1 OpenSSL/1.0.2s zlib/1.2.3 c-ares/1.15.0 libidn2/2.0.5 libssh2/1.8.2 nghttp2/1.39.1
Release-Date: 2019-06-05
Protocols: dict file ftp ftps gopher http https imap imaps ldap ldaps pop3 pop3s rtsp scp sftp smb smbs smtp smtps telnet tftp 
Features: AsynchDNS GSS-API HTTP2 HTTPS-proxy IDN IPv6 Kerberos Largefile libz NTLM NTLM_WB SPNEGO SSL UnixSockets

operating system

CentOS 6

I believe the following patch will fix this issue

diff --git a/lib/asyn-ares.c b/lib/asyn-ares.c
index 8561a4724..198d3d6e6 100644
--- a/lib/asyn-ares.c
+++ b/lib/asyn-ares.c
@@ -734,7 +734,11 @@ CURLcode Curl_set_dns_servers(struct Curl_easy *data,
     return CURLE_OK;
 
 #if (ARES_VERSION >= 0x010704)
+#if (ARES_VERSION >= 0x011100)
+  ares_result = ares_set_servers_ports_csv(data->state.resolver, servers);
+#else
   ares_result = ares_set_servers_csv(data->state.resolver, servers);
+#endif
   switch(ares_result) {
   case ARES_SUCCESS:
     result = CURLE_OK;

I have not tested this patch and there do not appear to be any unit tests of this functionality.

[bagder]

Thanks! The report and fix seem correct.

Can you turn the fix into a PR or would you like me to do it for you?