Sometimes it’s useful to be able to craft a request to one server, using a DNS name that’s either not defined or currently pointed to a different IP. (Migrating webservers, testing a new webserver config out, etc.)

Historically for HTTP calls this was easy, just set the Host header as you make the http request to the IP directly:

	curl -H "Host: caiustheory.com" http://10.200.0.1/

However, HTTPS throws a bit of a spanner in the works, if we just try to connect using an overridden Host header, we get an error back from the server if it’s not configured with a certificate for the IP address:

	$ curl -H "Host: caiustheory.com" https://10.200.0.1/
	curl: (51) SSL: no alternative certificate subject name matches target host name '10.200.0.1'

Usually at this point I’d just start editing /etc/hosts to add 10.200.0.1 caius.name to it and carry on testing. This is a pain when you’re testing more than one server, or you’re on a machine where you don’t have root access to edit /etc/hosts.

In later versions of curl there’s a solution for this built into the binary, in the form of the --resolve flag. You can tell it to override the DNS lookup for a specific hostname/port combination. This in turn means that the correct host is forwarded to the server for the correct SSL certificate to be chosen to serve the request based on host.

It takes the form --resolve HOST:PORT:IP where HOST is the human-friendly host, PORT is the webserver’s port (convention is 80 for HTTP, 443 for HTTPS) and IP being the destination IP you want to hit. (As opposed to the one in DNS currently.)

$ curl --silent --head --resolve caiustheory.com:443:10.200.0.1 https://caiustheory.com | head -n1
HTTP/2 200 

And voila, you don’t need to fiddle with editing /etc/hosts. Just use --resolve to hit a different IP for a given host.