localhost resolves to IPv4 then IPv6 in unexpected order

[Chilledheart]

Following the thread in #11458. I can confirm the issue is irrelative to proxy setups.

I did this

I pick one VM from mine at IPv6-enabled public network for example.

(IPv6 addresses come at first)

azureuser@east-asia-two:~$ curl -v -I http://www.google.com:12345
*   Trying 2404:6800:4005:810::2004:12345...
*   Trying 142.250.66.100:12345...

(IPv4 addresses come at first)

azureuser@east-asia-two:~$ curl -v -I http://localhost:12345
*   Trying 127.0.0.1:12345...
* connect to 127.0.0.1 port 12345 failed: Connection refused
*   Trying ::1:12345...
* connect to ::1 port 12345 failed: Connection refused

I expected the following

See

curl/lib/hostip.c

Lines 557 to 588 in f2aac0d

	static struct Curl_addrinfo *get_localhost(int port, const char *name)
	{
	struct Curl_addrinfo *ca;
	const size_t ss_size = sizeof(struct sockaddr_in);
	const size_t hostlen = strlen(name);
	struct sockaddr_in sa;
	unsigned int ipv4;
	unsigned short port16 = (unsigned short)(port & 0xffff);
	/* memset to clear the sa.sin_zero field */
	memset(&sa, 0, sizeof(sa));
	sa.sin_family = AF_INET;
	sa.sin_port = htons(port16);
	if(Curl_inet_pton(AF_INET, "127.0.0.1", (char *)&ipv4) < 1)
	return NULL;
	memcpy(&sa.sin_addr, &ipv4, sizeof(ipv4));
	ca = calloc(sizeof(struct Curl_addrinfo) + ss_size + hostlen + 1, 1);
	if(!ca)
	return NULL;
	ca->ai_flags = 0;
	ca->ai_family = AF_INET;
	ca->ai_socktype = SOCK_STREAM;
	ca->ai_protocol = IPPROTO_TCP;
	ca->ai_addrlen = (curl_socklen_t)ss_size;
	ca->ai_addr = (void *)((char *)ca + sizeof(struct Curl_addrinfo));
	memcpy(ca->ai_addr, &sa, ss_size);
	ca->ai_canonname = (char *)ca->ai_addr + ss_size;
	strcpy(ca->ai_canonname, name);
	ca->ai_next = get_localhost6(port, name);
	return ca;
	}

the DNS response for localhost is manually crafted, so it is neither done by getaddrinfo nor affected by /etc/hosts file.

To be clear, for some usage of IPv6 localhost, it could be disabled manually by removing the line inside /etc/hosts

(IPv6 addresses come at first)

azureuser@east-asia-two:~$ curl -v -I http://localhost:12345
*   Trying ::1:12345...
* connect to ::1 port 12345 failed: Connection refused
*   Trying 127.0.0.1:12345...
* connect to 127.0.0.1 port 12345 failed: Connection refused

curl/libcurl version

curl 7.81.0 (x86_64-pc-linux-gnu) libcurl/7.81.0 OpenSSL/3.0.2 zlib/1.2.11 brotli/1.0.9 zstd/1.4.8 libidn2/2.3.2 libpsl/0.21.0 (+libidn2/2.3.2) libssh/0.9.6/openssl/zlib nghttp2/1.43.0 librtmp/2.3 OpenLDAP/2.5.14
Release-Date: 2022-01-05
Protocols: dict file ftp ftps gopher gophers http https imap imaps ldap ldaps mqtt pop3 pop3s rtmp rtsp scp sftp smb smbs smtp smtps telnet tftp
Features: alt-svc AsynchDNS brotli GSS-API HSTS HTTP2 HTTPS-proxy IDN IPv6 Kerberos Largefile libz NTLM NTLM_WB PSL SPNEGO SSL TLS-SRP UnixSockets zstd

operating system

Ubuntu 22.04

Linux east-asia-two 5.15.0-1041-azure #48-Ubuntu SMP Tue Jun 20 20:34:08 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux

[Chilledheart]

I can also confirm without get_localhost call, curl can invoke getaddrinfo correctly for localhost:

(base) ➜  build git:(master) ✗ ./src/curl -v -I http://localhost:12345
* processing: http://localhost:12345
*   Trying [::1]:12345...
* connect to ::1 port 12345 failed: Connection refused
*   Trying 127.0.0.1:12345...
* connect to 127.0.0.1 port 12345 failed: Connection refused

with patch

diff --git a/lib/hostip.c b/lib/hostip.c
index 8b452a8e3..633301b1a 100644
--- a/lib/hostip.c
+++ b/lib/hostip.c
@@ -777,8 +777,8 @@ enum resolve_t Curl_resolv(struct Curl_easy *data,
       if(conn->ip_version == CURL_IPRESOLVE_V6 && !Curl_ipv6works(data))
         return CURLRESOLV_ERROR;

-      if(strcasecompare(hostname, "localhost") ||
-         tailmatch(hostname, ".localhost"))
+      if(strcasecompare(hostname, "localhost.fake") ||
+         tailmatch(hostname, ".localhost.fake"))
         addr = get_localhost(port, hostname);
 #ifndef CURL_DISABLE_DOH
       else if(allowDOH && data->set.doh && !ipnum)