I have a docker container with multiple network namespaces. Call them root, namespace1, namespace2.

The root container has an eni attached to it, with connectivity to the public internet.

Namespace 1 has two veth pairs:
veth_pair1 – connected to namespace2 with ip address address123.
veth_pair2 – connected to root with ip address234

In namespace 1, the ip route command shows:
default via address123 dev veth_pair1 (namespace2 ip address)
address234 dev veth_pair2

By default, traffic is routed over the default route through vethpair1 between namespace1 and namespace2.

What I want to do is under specific conditions, make http requests over the veth_pair2 (address234), while also using the dns lookup server of the root namespace.

Following two posts:

1. https://koraygocmen.medium.com/custom-dns-resolver-for-the-default-http-client-in-go-a1420db38a5d
2. How to bind an http.Client in Go to an IP Address

I constructed a custom DNS http client bound to the local address to try and make HTTP requests from the namespace1 container over the veth_pair2 through root container.

localTCPAddr, err := net.ResolveTCPAddr("tcp", "address234")
if err != nil {
  log.Printf("failed to resolve tcp local address with error %v", err)
  }

log.Printf("using local tcp address with any port: %v", localTCPAddr)

dialer := &net.Dialer{
    Resolver: &net.Resolver{
        PreferGo: true,
        Dial: func(ctx context.Context, network, address string) (net.Conn, error) {
            d := net.Dialer{
            LocalAddr: &localTCPAddr,
            Timeout: time.Duration(5000) * time.Millisecond,
              }
            return d.DialContext(ctx, "tcp", "DNS server address")
            },
            },
        }

dialContext := func(ctx context.Context, network, addr string) (net.Conn, error) {
        return dialer.DialContext(ctx, network, addr)
            }
        //
customHttpClient := &http.Client{
        Transport: &http.Transport{
        DialContext: dialContext,
        },
        }

However, when the docker container starts and the task to send the HTTP request out from the namespace1 over the root container gets called, I get the following error:

real addresses have been replaced with variables

dial tcp: lookup #redactedURI on {DNS address}:
  dial tcp {localAddress}:80->{customDNSserverAddress}: bind: cannot assign requested address

it seems from my research that the address is unavailable. but ive tried multiple iterations of the local address and ports, so im suspicious that its a different setup problem. any ideas?