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The --help output for curl lists a --resolve option, which states

--resolve <host:port:address> Force resolve of HOST:PORT to ADDRESS

I'm not having any luck getting it to work though. The basic command I'm trying to run is

curl --resolve foo.example.com:443:localhost https://foo.example.com:443/

and I keep getting the response Couldn't resolve host 'foo.example.com'. I want do do this because I'm testing a certificate for foo.example.com, but I'm not testing it on the actual server. Instead, I'm testing it on a dummy machine. I know that I can edit /etc/hosts so that foo.example.com resolves to localhost, but this curl approach seems like it would be the "correct" way to go, if I could make it work.

Does anybody see what I'm doing wrong here?

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up vote 18 down vote accepted

It appears that the address needs to be a numeric IP address, not a hostname.

Try this:

curl --resolve foo.example.com:443: https://foo.example.com:443/
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And that solves it. Thank you very much! – tsuraan Mar 1 '12 at 1:15

Some additional help in troubleshooting:

Ensure you're listening the right port for the protocol: http=80; https=443 & etc

Also curl -v will give the HEADER information for the response server which can be helpful.

curl -v --resolve foo.example.com:443: https://foo.example.com:443/
## The client request headers prepended by >
> OPTIONS /v1/..
> HOSTS foo.example.com
## The server response prepended by <
< HTTP/1.1 501 Not Implemented
## The html returned
<H1>Unsupported Request</H1>

Basically in this case the OPTIONS HTTP method was not implemented at the edge server for the CDN.

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(1) This problem was solved almost 3½ years ago.  (2) The problem was that curl couldn't figure out what host to (attempt to) connect to.  How do the ports that you're listening on make any difference if curl never connects to the right host?  And, obviously, the user never gets an HTTP header back if he doesn't connect to any server. – G-Man Aug 27 '15 at 23:28

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