How Can I Securely Drop Privileges After Binding to a Port in Go (v1.7)?

Dropping Privileges in Golang (v1.7)

In Go, it is often desirable to execute a process with root privileges, bind to a port, and then drop privileges to a non-root user. However, due to limitations in Go v1.7, direct use of syscall.SetUid() is not supported.

To achieve this, there are several approaches to consider:

iptables Redirection:

Redesigning ports via iptables is one option. This method involves rerouting another port to port 80. However, this approach exposes security concerns, as any non-root process could potentially pose as the webserver.

Alternative Method:

A more secure approach utilizes Go's native capabilities. Here's a step-by-step guide:

1. Open the port.
2. Check the user ID (UID).
3. If the UID is 0 (root), obtain the desired user and UID.
4. Use glibc calls (setuid and setgid) to modify the process UID and group ID (GID).

This approach provides granularity in privilege management. The following code snippet demonstrates its implementation:

package main

import (
    "fmt"
    "io"
    "log"
    "net"

    "golang.org/x/sys/unix"
)

func main() {
    listener, err := net.Listen("tcp", "127.0.0.1:80")
    if err != nil {
        log.Fatal(err)
    }

    for {
        conn, err := listener.Accept()
        if err != nil {
            log.Fatal(err)
        }

        uid := unix.Getuid()
        if uid == 0 {
            fmt.Fprintf(conn, "Running as root, downgrading to non-root user")

            // Mock retrieving desired user and UID
            desiredUser := "www-data"
            uid = 33 // Example non-root UID

            if err := unix.Setuid(uid); err != nil {
                log.Fatal(err)
            }
            fmt.Fprintf(conn, "Successfully dropped privileges to user %s\n", desiredUser)
        }

        fmt.Fprintf(conn, "Hello, world!\n")

        // Serve requests
        io.Copy(conn, conn)

        // Close the connection
        conn.Close()
    }
}

By following these steps and leveraging Go's built-in capabilities, it is possible to securely drop privileges and execute the desired application with non-root permissions.

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