IPv4 outbound_bind_addr not being applied for UDP traffic in ssserver

[luujenn]

We are using shadowsocks to proxy both TCP and UDP traffic. We want to use the outbound_bind_addr to send outbound traffic from ssserver from a different IP address. We are only using IPv4 in our environment.

We are seeing an issue where outbound UDP traffic from ssserver does not use the outbound_bind_addr when outbound_bind_addr is an IPv4 address. Instead, outbound UDP traffic uses the ssserver server IP. Outbound TCP traffic does use the outbound_bind_addr correctly.

It looks like this is happening because when a UDP socket is created, it expects the AddrFamily and the SocketAddr to be the same type for the bind_addr to be applied. But for UDP it looks like the AddrFamily is always IPv6, even though the outbound_bind_addr is IPv4. This causes the match statement for bind_addr assignment to not apply the configured outbound_bind_addr and it defaults to the ssserver server IP instead.

In our testing, if we use an IPv4-mapped IPv6 address for the outbound_bind_addr, UDP traffic successfully originates from the IPv6 outbound_bind_addr but TCP traffic is unsuccessful.

How can we enable an IPv4 outbound_bind_addr for TCP and UDP traffic?

An idea could be to make dual stack configuration configurable as a ssserver config option, so using only IPv4 or only IPv6 are options. Alternatively, maybe we could default to using IPv4 for outbound UDP connections if the outbound_bind_addr is an IPv4. If you can suggest a solution, we’re happy to push up a patch.

[zonyitoo]

A patch like this should fix your issue:

    let bind_addr = match (af, config.bind_local_addr) {
        (AddrFamily::Ipv4, Some(SocketAddr::V4(addr))) => addr.into(),
        (AddrFamily::Ipv4, Some(SocketAddr::V6(addr))) => {
            // IPv6 address must be a IPv4-mapped IPv6 address
            match addr.to_ipv4_mapped() {
                Some(addr) => addr.into(),
                None => return Err(Error::new(ErrorKind::InvalidInput, "Invalid IPv6 address")),
            }
        }

        (AddrFamily::Ipv6, Some(SocketAddr::V6(addr))) => addr.into(),
        (AddrFamily::Ipv6, Some(SocketAddr::V4(addr))) => addr.to_ipv6_mapped().into(),

        (AddrFamily::Ipv4, None) => SocketAddr::new(Ipv4Addr::UNSPECIFIED.into(), 0),
        (AddrFamily::Ipv6, None) => SocketAddr::new(Ipv6Addr::UNSPECIFIED.into(), 0),
    };

You could open an PR for this change. Remember to apply this change to all platforms in sys.

[luujenn]

Thanks @zonyitoo! We'll open a PR for this change using your recommendation.