How to Access the Underlying Socket of a net/http Response?

How to Access the Underlying Socket of a net/http Response

When working with HTTP connections in Go, there may be scenarios where developers need to access the underlying network socket. The net/http package provides a comprehensive way to handle HTTP requests and responses, but it doesn't directly expose the underlying socket. Here's how to retrieve the socket in a Go program.

Using Request Context (Go 1.13 and above)

With the release of Go 1.13, the net/http package introduced the ability to store the net.Conn in the request context. This provides a convenient and clean way to access the socket:

<code class="go">package main

import (
    "context"
    "net/http"
    "net"
    "log"
)

type contextKey struct {
    key string
}

var ConnContextKey = &contextKey{"http-conn"}

func SaveConnInContext(ctx context.Context, c net.Conn) (context.Context) {
    return context.WithValue(ctx, ConnContextKey, c)
}

func GetConn(r *http.Request) (net.Conn) {
    return r.Context().Value(ConnContextKey).(net.Conn)
}

func main() {
    http.HandleFunc("/", myHandler)

    server := http.Server{
        Addr: ":8080",
        ConnContext: SaveConnInContext,
    }
    server.ListenAndServe()
}

func myHandler(w http.ResponseWriter, r *http.Request) {
    conn := GetConn(r)
    ...
}</code>

Using ConnStateEvent for TCP Sockets

For servers listening on TCP ports, the net.Conn.RemoteAddr().String() is unique for each connection. Hence, it can be used as a key to a global map of connections:

<code class="go">package main

import (
    "net/http"
    "net"
    "fmt"
    "log"
)

var conns = make(map[string]net.Conn)

func ConnStateEvent(conn net.Conn, event http.ConnState) {
    if event == http.StateActive {
        conns[conn.RemoteAddr().String()] = conn
    } else if event == http.StateHijacked || event == http.StateClosed {
        delete(conns, conn.RemoteAddr().String())
    }
}

func GetConn(r *http.Request) (net.Conn) {
    return conns[r.RemoteAddr]
}

func main() {
    http.HandleFunc("/", myHandler)

    server := http.Server{
        Addr: ":8080",
        ConnState: ConnStateEvent,
    }
    server.ListenAndServe()
}

func myHandler(w http.ResponseWriter, r *http.Request) {
    conn := GetConn(r)
    ...
}</code>

Using ConnSaveListener for UNIX Sockets

For servers listening on UNIX sockets, net.Conn.RemoteAddr().String() always returns "@". To make it work, you can override net.Listener.Accept() and use it to override net.Conn.RemoteAddr().String(). Here's an example:

<code class="go">package main

import (
    "net/http"
    "net"
    "os"
    "golang.org/x/sys/unix"
    "fmt"
    "log"
)

func main() {
    http.HandleFunc("/", myHandler)

    listenPath := "/var/run/go_server.sock"
    l, err := NewUnixListener(listenPath)
    if err != nil {
        log.Fatal(err)
    }
    defer os.Remove(listenPath)

    server := http.Server{
        ConnState: ConnStateEvent,
    }
    server.Serve(NewConnSaveListener(l))
}

func myHandler(w http.ResponseWriter, r *http.Request) {
    conn := GetConn(r)
    if unixConn, isUnix := conn.(*net.UnixConn); isUnix {
        f, _ := unixConn.File()
        pcred, _ := unix.GetsockoptUcred(int(f.Fd()), unix.SOL_SOCKET, unix.SO_PEERCRED)
        f.Close()
        log.Printf("Remote UID: %d", pcred.Uid)
    }
}

var conns = make(map[string]net.Conn)

type connSaveListener struct {
    net.Listener
}

func NewConnSaveListener(wrap net.Listener) (net.Listener) {
    return connSaveListener{wrap}
}

func (self connSaveListener) Accept() (net.Conn, error) {
    conn, err := self.Listener.Accept()
    ptrStr := fmt.Sprintf("%d", &conn)
    conns[ptrStr] = conn
    return remoteAddrPtrConn{conn, ptrStr}, err
}

func GetConn(r *http.Request) (net.Conn) {
    return conns[r.RemoteAddr]
}

func ConnStateEvent(conn net.Conn, event http.ConnState) {
    if event == http.StateHijacked || event == http.StateClosed {
        delete(conns, conn.RemoteAddr().String())
    }
}

type remoteAddrPtrConn struct {
    net.Conn
    ptrStr string
}

func (self remoteAddrPtrConn) RemoteAddr() (net.Addr) {
    return remoteAddrPtr{self.ptrStr}
}

type remoteAddrPtr struct {
    ptrStr string
}

func (remoteAddrPtr) Network() (string) {
    return ""
}

func (self remoteAddrPtr) String() (string) {
    return self.ptrStr
}

func NewUnixListener(path string) (net.Listener, error) {
    if err := unix.Unlink(path); err != nil & os.IsNotExist(err) {
        return nil, err
    }
    mask := unix.Umask(0777)
    defer unix.Umask(mask)

    l, err := net.Listen("unix", path)
    if err != nil {
        return nil, err
    }

    if err := os.Chmod(path, 0660); err != nil {
        l.Close()
        return nil, err
    }

    return l, nil
}</code>

Conclusion

Accessing the underlying socket of an http.ResponseWriter can be achieved using the methods described above. The preferred approach depends on the specific requirements and the version of Go being used.

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