Simplified version of curl written in Go.

1. Supports a limited subset of curl options.
2. Supports some flags that curl does not. Read more about the new stuff.

• Why in the world you need another curl?
• How to install gocurl?
• How to use gocurl?
• New stuff
  • Encrypted ClientHello
  • Custom DNS servers
  • Oblivious HTTP
  • Experimental flags
    • Post-quantum cryptography
  • WebSocket support
• All command-line arguments

Curl is certainly awesome, but sometimes I need to have better control over what's happening on the inside and to be able to debug it. It seemed easier to me to implement the necessary parts of curl in Go.

Also, I'd like to be able to extend it with what fits my specific needs. Unfortunately, curl is a bit too huge for that now.

• Using homebrew:

  brew install ameshkov/tap/gocurl

• From source:

  go install github.com/ameshkov/gocurl@latest

• You can use a Docker image:

  docker run --rm ghcr.io/ameshkov/gocurl --help

• You can get a binary from the releases page.

Use it the same way you use original curl.

• gocurl https://httpbin.agrd.workers.dev/get make a GET request.
• gocurl -d "test" -v https://httpbin.agrd.workers.dev/post make a POST request with test data.
• gocurl -I https://httpbin.agrd.workers.dev/head make a HEAD request.
• gocurl -I --insecure https://expired.badssl.com/ do not verify TLS certificate.
• gocurl -I --http1.1 https://httpbin.agrd.workers.dev/head force use HTTP/1.1.
• gocurl -I --http2 https://httpbin.agrd.workers.dev/head force use HTTP/2.
• gocurl -I --http3 https://httpbin.agrd.workers.dev/head force use HTTP/3.
• gocurl -x socks5://user:pass@host:port https://httpbin.agrd.workers.dev/get use a proxy server.
• gocurl -I --tlsv1.3 https://tls-v1-2.badssl.com:1012/ force use TLS v1.3.
• gocurl -I --connect-to "httpbin.agrd.workers.dev:443:172.67.152.85:443" https://httpbin.agrd.workers.dev/head connect to the specified IP addresses.
• gocurl -I --resolve "httpbin.agrd.workers.dev:443:172.67.152.85" https://httpbin.agrd.workers.dev/head resolve the hostname to the specified IP address. Note, that unlike curl, gocurl ignores port in this option.
• gocurl -I --connect-timeout 3 http://10.255.255.1:9999/test set connection timeout to 3 seconds.

Also, you can use some new stuff that is not supported by curl.

• gocurl --json-output https://httpbin.agrd.workers.dev/get write output in machine-readable format (JSON).
• gocurl --tls-split-hello 5:50 https://httpbin.agrd.workers.dev/get split TLS ClientHello in two parts and make a 50ms delay after sending the first part.
• gocurl --tls-random "gyufwmGYeIiq0B4nUjEYu3NcqVdlHbIXhx74fq4terc=" https://httpbin.agrd.workers.dev/get use a custom TLS ClientHello random value.
• gocurl -v --ech https://crypto.cloudflare.com/cdn-cgi/trace enables support for ECH (Encrypted Client Hello) for the request. More on this below.
• gocurl --dns-servers "tls://dns.google" https://httpbin.agrd.workers.dev/get uses custom DNS-over-TLS server to resolve hostnames. More on this below.
• gocurl --experiment pq https://pq.cloudflareresearch.com/ enables post-quantum cryptography support for the request. More on this below.
• gocurl wss://httpbin.agrd.workers.dev/ws sends a WS upgrade request.
• gocurl -d "test message" wss://httpbin.agrd.workers.dev/ws establishes a WS connection, sends the first message through it and reads the response.

ECH or Encrypted Client Hello is a new standard that allows completely encrypting TLS Client Hello. Currently, the RFC is in the draft stage, but it is already supported by some big names like Cloudflare. gocurl supports ECH and provides several options to use it.

The simple option is just add --ech flag and see what happens:

gocurl -v --ech https://crypto.cloudflare.com/cdn-cgi/trace

In this case, gocurl will try to discover the ECH configuration from DNS records and then use them to establish the connection.

Instead of that, you may choose to supply your own configuration in the same base64-encoded format as used by the SVCB record:

# Send a type=https query and find ech record there.
% dig -t https crypto.cloudflare.com. +short
1 . alpn="http/1.1,h2" ipv4hint=162.159.137.85,162.159.138.85 ech=AEX+DQBBvgAgACARWS42g5NmDZo5pIpTWSwHzTwzdRKPdUW732QbyUeyDQAEAAEAAQASY2xvdWRmbGFyZS1lY2guY29tAAA= ipv6hint=2606:4700:7::a29f:8955,2606:4700:7::a29f:8a55

# You can now pass it to gocurl.
gocurl -v \
  --echconfig "AEX+DQBBvgAgACARWS42g5NmDZo5pIpTWSwHzTwzdRKPdUW732QbyUeyDQAEAAEAAQASY2xvdWRmbGFyZS1lY2guY29tAAA=" \
  https://crypto.cloudflare.com/cdn-cgi/trace

Interesting thing about ECH is that it may connect even if you use an expired configuration (see HelloRetryRequest in the RFC). It depends on both the server and the client implementation and does not work with Cloudflare at the moment.

Here's what happens under the hood:

1. gocurl resolves crypto.cloudflare.com IP address and connects to it.
2. It sends TLS ClientHello (outer) with encrypted inner ClientHello to that IP address. The ServerName field in the outer ClientHello is set to the one that is encoded in the ECH configuration (in this example it will be cloudflare-ech.com), and in the inner encrypted ClientHello it will be set to crypto.cloudflare.com.

You may want to configure a specific "client-facing" server instead and the way to do that is to use --connect-to. Let's send a request to cloudflare.com and use crypto.cloudflare.com as a client-facing server for that.

gocurl -v \
  --connect-to "cloudflare.com:443:crypto.cloudflare.com:443" \
  --echconfig "AEX+DQBBvgAgACARWS42g5NmDZo5pIpTWSwHzTwzdRKPdUW732QbyUeyDQAEAAEAAQASY2xvdWRmbGFyZS1lY2guY29tAAA=" \
  https://cloudflare.com/cdn-cgi/trace

For this command to work you may need to replace --echconfig with the current one discovered using DNS as was explained before.

Here's what happens now:

1. gocurl connects to crypto.cloudflare.com (client-facing relay).
2. Sends a TLS ClientHello with cloudflare-ech.com in the Server Name extension.
3. Establishes a TLS connection with cloudflare.com using the inner encrypted ClientHello.

gocurl allows using custom DNS servers to resolve hostnames when making requests. This can be achieved by using --dns-servers command-line argument. curl with c-ares also supports this command-line argument, but gocurl adds encrypted DNS support on top of it, it supports all popular DNS encryption protocols: DNS-over-QUIC, DNS-over-HTTPS, DNS-over-TLS and DNSCrypt.

You can specify multiple DNS servers, in this case gocurl will attempt to use them one by one until it receives a response or until all of them fail:

gocurl \
  --dns-servers "tls://dns.adguard-dns.com,tls://dns.google" \
  https://example.org/

• DNS-over-QUIC

  gocurl --dns-servers "quic://dns.adguard-dns.com" https://example.org/

• DNS-over-HTTPS

  gocurl --dns-servers "https://dns.adguard-dns.com/dns-query" https://example.org/

• DNS-over-TLS

  gocurl --dns-servers "tls://dns.adguard-dns.com" https://example.org/

• DNSCrypt

  gocurl \
      --dns-servers sdns://AQIAAAAAAAAAFDE3Ni4xMDMuMTMwLjEzMDo1NDQzINErR_JS3PLCu_iZEIbq95zkSV2LFsigxDIuUso_OQhzIjIuZG5zY3J5cHQuZGVmYXVsdC5uczEuYWRndWFyZC5jb20 \
      https://example.org/

Oblivious HTTP (OHTTP) is an IETF standard protocol that provides end-to-end encryption for HTTP requests and responses while hiding the client's identity from the target server. It works by routing encrypted requests through a relay and gateway, ensuring that:

• The relay sees who is making requests but not what is being requested.
• The gateway sees what is being requested but not who is making the request.
• The target server receives a normal HTTP request from the gateway.

This separation provides strong privacy guarantees, making OHTTP useful for privacy-sensitive applications.

gocurl has built-in support for OHTTP and can send requests through any OHTTP gateway by specifying two command-line arguments:

• --ohttp-gateway-url - URL of the OHTTP gateway where the encrypted request will be sent.
• --ohttp-keys-url - URL from which to retrieve the OHTTP KeyConfig needed to encrypt the request.

Here's how to make an OHTTP request to httpbin.agrd.workers.dev using a demo gateway:

gocurl -v \
  --ohttp-gateway-url "https://httpbin.agrd.workers.dev/ohttp/gateway" \
  --ohttp-keys-url "https://httpbin.agrd.workers.dev/ohttp/config" \
  https://httpbin.agrd.workers.dev/get

This command will:

1. Download the OHTTP KeyConfig from the keys URL.
2. Encrypt your request to https://httpbin.agrd.workers.dev/get using OHTTP.
3. Send the encrypted request to the gateway.
4. Receive the encrypted response from the gateway.
5. Decrypt and display the response.

You can also make POST requests through OHTTP:

gocurl -v \
  --ohttp-gateway-url "https://httpbin.agrd.workers.dev/ohttp/gateway" \
  --ohttp-keys-url "https://httpbin.agrd.workers.dev/ohttp/config" \
  -d "test data" \
  https://httpbin.agrd.workers.dev/post

One more example that uses a demo gateway from Oblivious Network:

gocurl -v \
  --ohttp-gateway-url "https://demo-gateway.oblivious.network/gateway" \
  --ohttp-keys-url "https://demo-gateway.oblivious.network/ohttp-configs" \
  https://httpbin.agrd.workers.dev/get

gocurl provides some initial support for WebSocket protocol. It may be extended in the future, see the corresponding Github issue.

• gocurl wss://httpbin.agrd.workers.dev/ws sends a WS upgrade request.
• gocurl -d "test message" wss://httpbin.agrd.workers.dev/ws establishes a WS connection, sends the first message through it and reads the response.

Experimental flags are added to gocurl whenever there's a feature that may be completely changed or removed in the future. Experiments can be enabled using the --experiment=<name[:value]> argument where name is the experiment name and value is an optional string value (the need for it depends on the actual experiment).

Post-quantum (PQ) cryptography has been designed to be secure against the threat of quantum computers. You can learn more about it from Cloudflare's blog post. gocurl supports it via the --experiment=pq flag.

Note, that it is not available for --http3 at the moment.

gocurl --experiment pq https://pq.cloudflareresearch.com/

Usage:
  gocurl [OPTIONS]

Application Options:
      --url=<URL>                                           URL the request will be made to. Can be specified without
                                                            any flags.
  -X, --request=<method>                                    HTTP method. GET by default.
  -d, --data=<data>                                         Sends the specified data to the HTTP server using content
                                                            type application/x-www-form-urlencoded.
  -H, --header=                                             Extra header to include in the request. Can be specified
                                                            multiple times.
  -x, --proxy=[protocol://username:password@]host[:port]    Use the specified proxy. The proxy string can be specified
                                                            with a protocol:// prefix.
      --connect-to=<HOST1:PORT1:HOST2:PORT2>                For a request to the given HOST1:PORT1 pair, connect to
                                                            HOST2:PORT2 instead. Can be specified multiple times.
      --connect-timeout=<seconds>                           Maximum time in seconds allowed for the connection phase.
  -I, --head                                                Fetch the headers only.
  -k, --insecure                                            Disables TLS verification of the connection.
      --tlsv1.3                                             Forces gocurl to use TLS v1.3 or newer.
      --tlsv1.2                                             Forces gocurl to use TLS v1.2 or newer.
      --tls-max=<VERSION>                                   (TLS) VERSION defines maximum supported TLS version. Can be
                                                            1.2 or 1.3. The minimum acceptable version is set by
                                                            tlsv1.2 or tlsv1.3.
      --ciphers=<space-separated list of ciphers>           Specifies which ciphers to use in the connection, see
                                                            https://go.dev/src/crypto/tls/cipher_suites.go for the full
                                                            list of available ciphers.
      --tls-servername=<HOSTNAME>                           Specifies the server name that will be sent in TLS
                                                            ClientHello
      --http1.1                                             Forces gocurl to use HTTP v1.1.
      --http2                                               Forces gocurl to use HTTP v2.
      --http3                                               Forces gocurl to use HTTP v3.
      --ech                                                 Enables ECH support for the request.
      --echgrease                                           Forces sending ECH grease in the ClientHello, but does not
                                                            try to resolve the ECH configuration.
      --echconfig=<base64-encoded data>                     ECH configuration to use for this request. Implicitly
                                                            enables --ech when specified.
  -4, --ipv4                                                This option tells gocurl to use IPv4 addresses only when
                                                            resolving host names.
  -6, --ipv6                                                This option tells gocurl to use IPv6 addresses only when
                                                            resolving host names.
      --dns-servers=<DNSADDR1,DNSADDR2>                     DNS servers to use when making the request. Supports
                                                            encrypted DNS: tls://, https://, quic://, sdns://
      --resolve=<[+]host:port:addr[,addr]...>               Provide a custom address for a specific host. port is
                                                            ignored by gocurl. '*' can be used instead of the host
                                                            name. Can be specified multiple times.
      --tls-split-hello=<CHUNKSIZE:DELAY>                   An option that allows splitting TLS ClientHello in two
                                                            parts in order to avoid common DPI systems detecting TLS.
                                                            CHUNKSIZE is the size of the first bytes before ClientHello
                                                            is split, DELAY is delay in milliseconds before sending the
                                                            second part.
      --tls-random=<base64>                                 Base64-encoded 32-byte TLS ClientHello random value.
      --json-output                                         Makes gocurl write machine-readable output in JSON format.
  -o, --output=<file>                                       Defines where to write the received data. If not set,
                                                            gocurl will write everything to stdout.
      --experiment=<name[:value]>                           Allows enabling experimental options. See the documentation
                                                            for available options. Can be specified multiple times.
      --ohttp-gateway-url=<URL>                             URL of the Oblivious HTTP gateway where the request should
                                                            be sent.
      --ohttp-keys-url=<URL>                                URL from which to retrieve Oblivious HTTP KeyConfig to use
                                                            for encrypting the request.
  -v, --verbose                                             Verbose output (optional).

Help Options:
  -h, --help                                                Show this help message