Connections

    For ordinary command line usage, operating on a URL, these are details which
    are taken care of under the hood, and which you can mostly ignore. But at times
    you might find yourself wanting to tweak the specifics…

    Maybe you want the command to connect to your local
    server instead of the actual server.

    You can normally and easily do that by editing your hosts file (/etc/hosts
    on Linux and Unix systems) and adding, for example, 127.0.0.1 example.com to
    redirect the host to your localhost. However this edit requires admin access and
    it has the downside that it affects all other applications at the same time.

    The Host: header is the normal way an HTTP client tells the HTTP server which
    server it speaks to, as typically an HTTP server serves many different names
    using the same software instance.

    So, by passing in a custom modified Host: header you can have the
    server respond with the contents of the site even when you didn’t actually
    connect to that host name.

    For example, you run a test instance of your main site www.example.com on
    your local machine and you want to have curl ask for the index html:

    When setting a custom Host: header and using cookies, curl will extract the
    custom name and use that as host when matching cookies to send off.

    The Host: header is not enough when communicating with an HTTPS server. With
    HTTPS there’s a separate extension field in the TLS protocol called SNI
    (Server Name Indication) that lets the client tell the server the name of the
    server it wants to talk to. curl will only extract the SNI name to send from
    the given URL.

    Do you know better than the name resolver where curl should go? Then you can
    give an IP address to curl yourself! If you want to redirect port 80 access for
    example.com to instead reach your localhost:

    You can even specify multiple --resolve switches to provide multiple
    redirects of this sort, which can be handy if the URL you work with uses HTTP
    redirects or if you just want to have your command line work with multiple
    URLs.

    --resolve inserts the address into curl’s DNS cache, so it will effectively
    make curl believe that’s the address it got when it resolved the name.

    As a close relative to the --resolve option, the --connect-to option
    provides a minor variation. It allows you to specify a replacement name and
    port number for curl to use under the hood when a specific name and port
    number is used to connect.

    For example, suppose you have a single site called www.example.com that in turn
    is actually served by three different individual HTTP servers: load1, load2
    and load3, for load balancing purposes. In a typical normal procedure, curl
    resolves the main site and gets to speak to one of the load balanced servers
    (as it gets a list back and just picks one of them) and all is well. If you
    want to send a test request to one specific server out of the load balanced
    set (load1.example.com for example) you can instruct curl to do that.

    You can still use --resolve to accomplish this if you know the specific IP
    address of load1. But without having to first resolve and fix the IP address
    separately, you can tell curl:

    It redirects from a SOURCE NAME + SOURCE PORT to a DESTINATION NAME +
    DESTINATION PORT. curl will then resolve the load1.example.com name and
    connect, but in all other ways still assume it is talking to
    www.example.com.

    As should be detailed elsewhere in this book, curl may be built with several
    different name resolving backends. One of those backends is powered by the
    c-ares library and when curl is built to use c-ares, it gets a few extra
    superpowers that curl built to use other name resolve backends don’t get.
    Namely, it gains the ability to more specifically instruct what DNS servers to
    use and how that DNS traffic is using the network.

    With , you can specify exactly which DNS server curl should use
    instead of the default one. This lets you run your own experimental server that
    answers differently, or use a backup one if your regular one is unreliable or dead.

    With --dns-ipv4-addr and --dns-ipv6-addr you ask curl to “bind” its local
    end of the DNS communication to a specific IP address and with
    --dns-interface you can instruct curl to use a specific network interface to
    use for its DNS requests.

    These --dns-* options are very advanced and are only meant for people who know
    what they are doing and understand what these options do. But they offer very
    customizable DNS name resolution operations.

    curl will typically make a TCP connection to the host as an initial part of its
    network transfer. This TCP connection can fail or be very slow, if there are
    shaky network conditions or faulty remote servers.

    To reduce the impact on your scripts or other use, you can set the maximum time
    in seconds which curl will allow for the connection attempt. With
    --connect-timeout you tell curl the maximum time to allow for connecting,
    and if curl has not connected in that time it returns a failure.

    The connection timeout only limits the time curl is allowed to spend up
    until the moment it connects, so once the TCP connection has been established
    it can take longer time. See the Timeouts
    section for more on generic curl timeouts.

    The connection timeout can be specified as a decimal value for sub-second
    precision. For example, to allow 2781 milliseconds to connect:

    1. curl --connect-timeout 2.781 https://example.com/

    On machines with multiple network interfaces that are connected to multiple
    networks, there are situations where you can decide which network interface
    you would prefer the outgoing network traffic to use. Or which originating IP
    address (out of the multiple ones you have) to use in the communication.

    Tell curl which network interface, which IP address or even host name that you
    would like to “bind” your local end of the communication to, with the
    --interface option:

    A TCP connection is created between an IP address and a port number in the
    local end and an IP address and a port number in the remote end. The remote
    port number can be specified in the URL and usually helps identify which
    service you are targeting.

    The local port number is usually randomly assigned to your TCP connection
    by the network stack and you normally don’t have to think about it much further.
    However, in some circumstances you find yourself behind network equipment,
    firewalls or similar setups that put restrictions on what source port numbers
    that can be allowed to set up the outgoing connections.

    For situations like this, you can specify which local ports curl should
    bind the the connection to. You can specify a single port number to use, or a
    range of ports. We recommend using a range because ports are scarce
    resources and the exact one you want may already be in use. If you ask for a
    local port number (or range) that curl can’t obtain for you, it will exit with a
    failure.

    Also, on most operating systems you cannot bind to port numbers below 1024
    without having a higher privilege level (root) and we generally advise
    against running curl as root if you can avoid it.

    Ask curl to use a local port number between 4000 and 4200 when getting this
    HTTPS page:

    1. curl --local-port 4000-4200 https://example.com/

    TCP connections can be totally without traffic in either direction when they are
    not used. A totally idle connection can therefore not be clearly separated
    from a connection that has gone completely stale because of network or server
    issues.

    At the same time, lots of network equipment such as firewalls or NATs are
    keeping track of TCP connections these days, so that they can translate
    addresses, block “wrong” incoming packets, etc. These devices often count
    completely idle connections as dead after N minutes, where N varies
    between device to device but at times is as short as 10 minutes or even less.

    One way to help avoid a really slow connection (or an idle one) getting
    treated as dead and wrongly killed, is to make sure TCP keep alive is
    used. TCP keepalive is a feature in the TCP protocol that makes it send “ping
    frames” back and forth when it would otherwise be totally
    idle. It helps idle connections to detect breakage even when no traffic is
    moving over it, and helps intermediate systems not consider the connection dead.

    or change the interval to 5 minutes (300 seconds) with:

    1. curl --keepalive-time 300 https://example.com/