github.com/psiphon-inc/goarista@v0.0.0-20160825065156-d002785f4c67/netns/netns.go (about)

     1  // Copyright (C) 2016  Arista Networks, Inc.
     2  // Use of this source code is governed by the Apache License 2.0
     3  // that can be found in the COPYING file.
     4  
     5  // Package netns provides a utility function that allows a user to
     6  // perform actions in a different network namespace
     7  package netns
     8  
     9  import (
    10  	"fmt"
    11  	"os"
    12  	"runtime"
    13  )
    14  
    15  const (
    16  	netNsRunDir = "/var/run/netns/"
    17  	selfNsFile  = "/proc/self/ns/net"
    18  )
    19  
    20  // Callback is a function that gets called in a given network namespace.
    21  // The user needs to check any errors from any calls inside this function.
    22  type Callback func()
    23  
    24  // File descriptor interface so we can mock for testing
    25  type handle interface {
    26  	close() error
    27  	fd() int
    28  }
    29  
    30  // The file descriptor associated with a network namespace
    31  type nsHandle int
    32  
    33  // setNsByName wraps setNs, allowing specification of the network namespace by name.
    34  // It returns the file descriptor mapped to the given network namespace.
    35  func setNsByName(nsName string) error {
    36  	netPath := netNsRunDir + nsName
    37  	handle, err := getNs(netPath)
    38  	if err != nil {
    39  		return fmt.Errorf("Failed to getNs: %s", err)
    40  	}
    41  	err = setNs(handle)
    42  	handle.close()
    43  	if err != nil {
    44  		return fmt.Errorf("Failed to setNs: %s", err)
    45  	}
    46  	return nil
    47  }
    48  
    49  // Do takes a function which it will call in the network namespace specified by nsName.
    50  // The goroutine that calls this will lock itself to its current OS thread, hop
    51  // namespaces, call the given function, hop back to its original namespace, and then
    52  // unlock itself from its current OS thread.
    53  // Do returns an error if an error occurs at any point besides in the invocation of
    54  // the given function.
    55  // The caller should check both the error of Do and any errors from the given function call.
    56  func Do(nsName string, cb Callback) error {
    57  	// If destNS is empty, the function is called in the caller's namespace
    58  	if nsName == "" {
    59  		cb()
    60  		return nil
    61  	}
    62  
    63  	// Get the file descriptor to the current namespace
    64  	currNsFd, err := getNs(selfNsFile)
    65  	if os.IsNotExist(err) {
    66  		return fmt.Errorf("File descriptor to current namespace does not exist: %s", err)
    67  	} else if err != nil {
    68  		return fmt.Errorf("Failed to open %s: %s", selfNsFile, err)
    69  	}
    70  	defer currNsFd.close()
    71  
    72  	runtime.LockOSThread()
    73  	defer runtime.UnlockOSThread()
    74  
    75  	// Jump to the new network namespace
    76  	if err := setNsByName(nsName); err != nil {
    77  		return fmt.Errorf("Failed to set the namespace to %s: %s", nsName, err)
    78  	}
    79  
    80  	// Call the given function
    81  	cb()
    82  
    83  	// Come back to the original namespace
    84  	if err = setNs(currNsFd); err != nil {
    85  		return fmt.Errorf("Failed to return to the original namespace: %s", err)
    86  	}
    87  
    88  	return nil
    89  }