Proxy DNS Errors: Unknown Proxy Server and curl (5)

Before your device can use a proxy, it has to resolve the proxy's host name, such as gate.proxyfield.io, to an IP address. When that lookup fails, there is nothing to connect to, and every app reports it in its own words:

Chrome's ERR_PROXY_CONNECTION_FAILED (net error -130) covers both a failed name lookup and a failed socket connection to the proxy (Chromium net_error_list.h); the error page text comes from Chromium's error page strings. cURL's exit code 5 is CURLE_COULDNT_RESOLVE_PROXY (libcurl error codes); the cURL fix is below.

1. Check the host name character by character. For ProxyField it is gate.proxyfield.io, and the port, 7000, goes in the port field. Common breakers in an app's or a system's proxy form:

   • a scheme in a host-only field (http://gate.proxyfield.io where only the host belongs; on the cURL command line a scheme is fine);
   • the port typed into the host field (gate.proxyfield.io:7000 in a host-only box);
   • a leading or trailing space from copy and paste;
   • the account's username typed where the host goes.

   Where each value goes on each device: proxy server address and proxy port.

2. Look the host up yourself. On Windows, macOS or Linux:

   ShellCopy Shell code

   nslookup gate.proxyfield.io

   On macOS and Linux, dig +short gate.proxyfield.io does the same in one line. An address back means DNS works and the problem is in your proxy settings. "Non-existent domain", "NXDOMAIN" or "can't find" means the lookup fails on this network.

3. Try another DNS resolver. If the lookup fails on your default resolver, test a public one, for example nslookup gate.proxyfield.io 1.1.1.1. If that works, your network's DNS is blocking or failing: change the DNS server in your network settings, or ask your network admin.

4. Flush the DNS cache after a change, so the old failed answer isn't reused: ipconfig /flushdns on Windows; sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder on macOS; resolvectl flush-caches on Linux with systemd-resolved.

5. Check for a hosts-file entry or a PAC script that overrides the proxy host. A stale entry in /etc/hosts (or C:\Windows\System32\drivers\etc\hosts) wins over DNS. If your system uses automatic proxy setup, the PAC file may name a proxy host that no longer exists.

6. Turn off the proxy if you didn't set one. An "unknown proxy server" error on a home computer often means old proxy settings from an app or extension; see how to turn off a proxy and proxy settings controlled by an extension.

When the lookup works but the connection still fails, the problem is the next step, the connection to the proxy: see cannot establish proxy connection.

Exit code 5 is CURLE_COULDNT_RESOLVE_PROXY: cURL's documentation says the address of the given proxy host could not be resolved. cURL has not contacted the website yet. The causes are a wrong proxy name, a misbehaving DNS server, or a misconfigured system DNS (curl exit codes). Work through it in this order:

1. Find which proxy cURL is using. cURL reads http_proxy, HTTPS_PROXY, ALL_PROXY and NO_PROXY from the environment (curl proxy environment variables), and -x / --proxy overrides them (curl man page):

   ShellCopy Shell code

   env | grep -i proxy

   An old host in one of these variables is the most common cause of error 5 when you did not type -x at all. A .curlrc file can also hold a proxy = line.

2. Watch the request. -v prints the proxy name cURL tries to resolve, so a typo or stray character shows up at once:

   ShellCopy Shell code

   curl -v -x http://gate.proxyfield.io:7000 -U "<login>-country-us-city-new_york:<password>" https://api.ipify.org

   -x takes host:port (gate.proxyfield.io:7000) or a URL with a scheme (http://gate.proxyfield.io:7000, socks5h://gate.proxyfield.io:7000); both work. A scheme breaks only a host-only field in a GUI, not -x.

3. Resolve the proxy host yourself.

   ShellCopy Shell code

   nslookup gate.proxyfield.io
   dig +short gate.proxyfield.io

   No address back means the problem is DNS on this machine or network (steps 3 and 4 in the section above). An address back, while cURL still says error 5, means cURL is resolving a different name: go back to step 1.

4. Prove the proxy settings are the cause. Send one request with every proxy setting ignored:

   ShellCopy Shell code

   curl --noproxy '*' https://example.com
   # or clear the variables for one command only:
   env -u http_proxy -u https_proxy -u HTTP_PROXY -u HTTPS_PROXY -u ALL_PROXY curl https://example.com

   If that works, your internet connection and the website are fine, and the proxy configuration is at fault: fix or remove the variable, the .curlrc line or the -x value.

5. Check NO_PROXY. Hosts listed in NO_PROXY / no_proxy go direct even when a proxy is set, so a request you expected to go through the proxy may not. More on --noproxy and the variables: cURL; setting them per shell: command-line proxy settings.

"Could not resolve host" (error 6) is different: the proxy is not in play, or the request went direct, and the lookup of the website's own name failed. With an HTTP proxy or socks5h://, the proxy resolves the website's name, so error 6 usually means the request did not go through the proxy at all.

"Proxy DNS" also means the second lookup: the website's host name.

When the proxy resolves the name, your own resolver does not see the lookup. With local resolution, your resolver sees every site you visit, and a site with region-specific DNS may send you to the wrong region. Firefox's setting is explained on Firefox proxy setup.

ProxyField runs HTTP, HTTPS and SOCKS5 on the same gate.proxyfield.io:7000, with the same username and password; with socks5h://gate.proxyfield.io:7000 in cURL or Python, the gateway resolves the website's name.