How Can I Securely Drop Privileges After Binding to a Privileged Port in Go?

Privilege Dropping in Go (v1.7): A Secure Approach

In the context of developing custom web servers using Go, it's essential to consider how to handle the privileges required to bind to ports typically reserved for root accounts. While setting up the server with root privileges is necessary to establish the necessary bindings, it's crucial to drop these privileges as soon as possible to prevent potential security vulnerabilities.

In Go's v1.7 release, the ability to directly manipulate privileges using syscall.SetUid() is not supported. This limitation poses a challenge for developers seeking a clean and secure solution to privilege dropping.

To address this issue, an alternative approach involves utilizing glibc calls to set the UID and GID of the process. By binding to the desired port and detecting the UID, developers can safely downgrade to a non-root user if the UID is initially 0. This strategy ensures that the server only operates with the reduced privileges once the binding is complete.

To illustrate this approach, consider the following code snippet:

import (
    "crypto/tls"
    "log"
    "net/http"
    "os/user"
    "strconv"
    "syscall"
)

func main() {
    ...
    listener, err := tls.Listen("tcp4", "127.0.0.1:445", &tlsconf)
    if err != nil {
        log.Fatalln("Error opening port:", err)
    }
    if syscall.Getuid() == 0 {
        log.Println("Running as root, downgrading to user www-data")
        ...
        cerr, errno := C.setgid(C.__gid_t(gid))
        if cerr != 0 {
            log.Fatalln("Unable to set GID due to error:", errno)
        }
        cerr, errno = C.setuid(C.__uid_t(uid))
        if cerr != 0 {
            log.Fatalln("Unable to set UID due to error:", errno)
        }
    }
    ...
    err = http.Serve(listener, nil)
    log.Fatalln(err)
}

This code demonstrates the complete process of opening a TLS-encrypted port, detecting and downgrading from root privileges, and serving HTTP requests using the lower-privileged user.

By adhering to this approach, developers can create secure custom web servers in Go while maintaining the necessary level of control over privileges and minimizing potential security risks.

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