A1-http-connect-proxy-support.md proposed how gRPC supports TCP-level proxies via the HTTP CONNECT request, defined in RFC-2817.

This guide documents gRPC C-Core's default proxy mapper implementation.

HTTP Proxy

Case 1 in the proposal documents a use-case where all outbound traffic from an environment must go through a proxy. Configurations for such environments are usually performed using environment variables such as http_proxy. gRPC supports this by providing a default proxy mapper implementation that allows for overriding the server name (provided in the channel creation hostname) to resolve based on such configurations.

Enabling the HTTP Proxy

C-Core checks the following places to determine the HTTP proxy to use, stopping at the first one that is set:

1. GRPC_ARG_HTTP_PROXY channel arg
2. grpc_proxy environment variable
3. https_proxy environment variable
4. http_proxy environment variable

If none of the above are set, then no HTTP proxy will be used.

The allowed format is an RFC3986 URI string where the scheme is expected to be "http" and the authority portion is used to determine the proxy to be used. For example, for an HTTP proxy setting of http://username:password@proxy.google.com:443, username:password would be used as user credentials for proxy authentication as per RFC7617 and proxy.google.com:443 would be the host:port HTTP proxy target. If the port part of the authority is omitted, a default port of 443 is used. Note that user credential can also be omitted if the proxy does not need authentication.

Disabling HTTP Proxy

If an HTTP proxy is set, C-Core then checks the following places to exclude traffic destined to listed hosts from going through the proxy determined above, again stopping at the first one that is set:

1. no_grpc_proxy environment variable
2. no_proxyenvironment variable

If none of the above are set, then the previously found HTTP proxy is used.

The format takes a comma-separated list of names, and if any of these names matches as a suffix of the server host (provided in the channel target), then the proxy will not be used for that target. For example, with a grpc_proxy setting of proxy.google.com and a no_grpc_proxy setting of example.com, google.com, channel targets such as dns:///foo.google.com:50051 and bar.example.com:1234 will not use the proxy, but baz.googleapis.com:443 would still use the configured proxy proxy.google.com.

As of PR#31119, CIDR blocks are also supported in the list of names. For example, a no_proxy setting of 10.10.0.0/24 would not use the proxy for channel targets that mention IP addresses as the host between the range 10.10.0.0 to 10.10.0.255.

Disabling HTTP Proxy Channel-wide

The lookup and subsequent usage of an HTTP proxy for a specific channel can also be disabled by setting the channel arg GRPC_ARG_ENABLE_HTTP_PROXY to 0.

Address Proxy

Case 2 in the proposal documents a partially protected environment, where access to certain addresses must go through a proxy. Name resolution of protected servers works normally, and the proxy allows the CONNECT request to use an IP address instead of a hostname.

Only requests for certain hosts must go through the proxy. Requests to other servers work without the proxy. Custom logic is used to determine which hosts the proxy will be used for.

To use the address proxy, both of the following parameters need to be specified:

1. Address of the proxy can be specified using GRPC_ARG_ADDRESS_HTTP_PROXY channel argument or GRPC_ADDRESS_HTTP_PROXY environment variable. Value of the channel argument is preferred if both values are specified.

1. Comma-separated list of IP addresses and/or CIDR blocks that should be accessed through the proxy. This can be specified using the GRPC_ARG_ADDRESS_HTTP_PROXY_ENABLED_ADDRESSES channel argument or GRPC_ADDRESS_HTTP_PROXY_ENABLED_ADDRESSES environment variable. Value of the channel argument is preferred if both values are specified.