I did this

SOCKS5 Proxy IPv6 Compatibility Issue

Issue Summary

When using the curl command with a SOCKS5 proxy on an IPv6 device, users experience an issue when trying to access services like https://api.ipify.io. Specifically, the following sequence of events occurs:

1. The device queries the DNS to obtain the IPv6 address of api.ipify.io (note: the query bypasses the proxy DNS).
2. If the SOCKS5 proxy supports only IPv4, it responds with an error as defined by RFC 1928, Section 6:

   X'08' Address type not supported.
   

3. Upon receiving this response, the curl command should ideally attempt to retry the connection using IPv4. However, instead of this fallback behavior, curl exits with an error code, without attempting the IPv4 connection.

This leads to a situation where users must either:

• Manually specify the -4 flag to force IPv4 usage.
• Or, configure the proxy to provide DNS resolution.

Expected Behavior

The expected behavior is that when an IPv6 address is requested and the SOCKS5 proxy cannot handle it, curl should automatically attempt to resolve and connect using IPv4 instead. This fallback mechanism should be implemented either within the proxy server or within the client behavior.

Steps to Reproduce

1. Use the following curl command with a SOCKS5 proxy:

   curl -x socks5://username:password000@ip:port https://api.ipify.io

2. On an IPv6-only device, the proxy responds with an error indicating that the address type is not supported.
3. Instead of retrying with IPv4, the command exits with an error code.

Workaround

To circumvent this issue, users must:

• Specify the -4 flag in the curl command to force IPv4 resolution, or
• Ensure that the proxy DNS is configured to handle the resolution.

Proposed Solution

It is recommended to implement automatic fallback behavior for SOCKS5 proxies when dealing with mixed IPv4 and IPv6 requests. Specifically:

• The proxy should detect an unsupported address type and automatically retry the request using IPv4.
• Alternatively, this functionality could be integrated into the curl client to trigger the fallback mechanism.

References

• RFC 1928 - SOCKS Protocol Version 5: https://tools.ietf.org/html/rfc1928
• curl documentation: https://curl.se/docs/

I expected the following

No response

curl/libcurl version

all version.

operating system

all os.