Provide more information when packet capture fails

This change is motivated by user @deliciouslytyped and the comment left here: #119 (comment). Thanks! If the user mistypes the interface name, for example, termshark's stderr is misleading because it always refers the user to a link with information on how to set up tshark for non-root capture. To make this better, I now capture the stderr for the test process that is launched to test packet capture when termshark starts. If the process returns a non-zero code, I print that stderr on termshark's stderr. For example: $ termshark -i wlanfoo (The termshark UI will start when packets are detected on wlanfoo...) Cannot capture on device wlanfoo: exit status 1 (exit code 1) Standard error stream from the capture process: Starting termshark's custom live capture procedure. First, trying dumpcap command /usr/bin/dumpcap -i wlanfoo -a duration:1 Capturing on 'wlanfoo' dumpcap: The capture session could not be initiated on interface 'wlanfoo' (No such device exists). Please check that you have the proper interface or pipe specified. Retrying with capture command [/home/gcla/bin/tshark-slow -i wlanfoo -a duration:1] Capturing on 'wlanfoo' tshark: The capture session could not be initiated on interface 'wlanfoo' (No such device exists). Please check that you have the proper interface or pipe specified. 0 packets captured You might need: sudo setcap cap_net_raw,cap_net_admin eip dumpcap Or try running with sudo or as root. See https://termshark.io/no-root for more info.
gcla · Jul 4, 2022 · 786e63f · 786e63f
1 parent a9843d9
commit 786e63f
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Showing 3 changed files with 30 additions and 10 deletions.
diff --git a/cmd/termshark/termshark.go b/cmd/termshark/termshark.go
@@ -5,6  5,7 @@
 package main
 
 import (
 	"bytes"
 	"fmt"
 	"os"
 	"os/exec"
@@ -711,6  712,7 @@ func cmain() int {
 	//======================================================================
 
 	ifaceExitCode := 0
 	stderr := &bytes.Buffer{}
 	var ifaceErr error
 
 	// This is deferred until after the app is Closed - otherwise messages written to stdout/stderr are
@@ -722,6  724,19 @@ func cmain() int {
 				fmt.Fprintf(os.Stderr, ": %v", ifaceErr)
 			}
 			fmt.Fprintf(os.Stderr, " (exit code %d)\n", ifaceExitCode)
 			if stderr.Len() != 0 {
 				// The default capture bin is termshark itself, with a special environment
 				// variable set that causes it to try dumpcap, then tshark, in that order (for
 				// efficiency of capture, but falling back to tshark for extcap interfaces).
 				// But telling the user the capture process is "termshark" is misleading.
 				cbin := termshark.CaptureBin()
 				if cbin == "termshark" {
 					cbin = "the capture process"
 				}
 
 				fmt.Fprintf(os.Stderr, "Standard error stream from %s:\n", cbin)
 				fmt.Fprintf(os.Stderr, "\n%s\n", stderr.String())
 			}
 			if runtime.GOOS == "linux" && os.Geteuid() != 0 {
 				fmt.Fprintf(os.Stderr, "You might need: sudo setcap cap_net_raw,cap_net_admin eip %s\n", termshark.PrivilegedBin())
 				fmt.Fprintf(os.Stderr, "Or try running with sudo or as root.\n")
@@ -922,10  937,11 @@ func cmain() int {
 		ifaceExitCode = 0
 		for _, psrc := range psrcs {
 			if psrc.IsInterface() {
-				if ifaceExitCode, ifaceErr = termshark.RunForExitCode(
 				if ifaceExitCode, ifaceErr = termshark.RunForStderr(
 					termshark.CaptureBin(),
 					[]string{"-i", psrc.Name(), "-a", "duration:1"},
 					append(os.Environ(), "TERMSHARK_CAPTURE_MODE=1"),
 					stderr,
 				); ifaceExitCode != 0 {
 					return 1
 				}

diff --git a/system/dumpcapext.go b/system/dumpcapext.go
@@ -8,13  8,12 @@
 package system
 
 import (
 	"fmt"
 	"os"
 	"os/exec"
 	"regexp"
 	"strconv"
 	"syscall"
-
-	log "github.com/sirupsen/logrus"
 )
 
 //======================================================================
@@ -45,31  44,32 @@ func DumpcapExt(dumpcapBin string, tsharkBin string, args ...string) error {
 			if len(fdnum) == 2 {
 				fd, err := strconv.Atoi(fdnum[1])
 				if err != nil {
-					log.Warnf("Unexpected error parsing %s: %v", args[1], err)
 					fmt.Fprintf(os.Stderr, "Unexpected error parsing %s: %v\n", args[1], err)
 				} else {
 					err = Dup2(fd, 0)
 					if err != nil {
-						log.Warnf("Problem duplicating fd %d to 0: %v", fd, err)
-						log.Warnf("Will not try to replace argument %s to tshark", args[1])
 						fmt.Fprintf(os.Stderr, "Problem duplicating fd %d to 0: %v\n", fd, err)
 						fmt.Fprintf(os.Stderr, "Will not try to replace argument %s to tshark\n", args[1])
 					} else {
-						log.Infof("Replacing argument %s with - for tshark compatibility", args[1])
 						fmt.Fprintf(os.Stderr, "Replacing argument %s with - for tshark compatibility\n", args[1])
 						args[1] = "-"
 					}
 				}
 			}
 		}
 	}
 
 	fmt.Fprintf(os.Stderr, "Starting termshark's custom live capture procedure.\n")
 	dumpcapCmd := exec.Command(dumpcapBin, args...)
-	log.Infof("Starting dumpcap command %v", dumpcapCmd)
 	fmt.Fprintf(os.Stderr, "First, trying dumpcap command %v\n", dumpcapCmd)
 	dumpcapCmd.Stdin = os.Stdin
 	dumpcapCmd.Stdout = os.Stdout
 	dumpcapCmd.Stderr = os.Stderr
 	if dumpcapCmd.Run() != nil {
 		var tshark string
 		tshark, err = exec.LookPath(tsharkBin)
 		if err == nil {
-			log.Infof("Retrying with dumpcap command %v", append([]string{tshark}, args...))
 			fmt.Fprintf(os.Stderr, "Retrying with capture command %v\n", append([]string{tshark}, args...))
 			err = syscall.Exec(tshark, append([]string{tshark}, args...), os.Environ())
 		}
 	}

diff --git a/utils.go b/utils.go
@@ -299,14  299,18 @@ func TSharkPath() (string, *gowid.KeyValueError) {
 }
 
 func RunForExitCode(prog string, args []string, env []string) (int, error) {
 	return RunForStderr(prog, args, env, ioutil.Discard)
 }
 
 func RunForStderr(prog string, args []string, env []string, stderr io.Writer) (int, error) {
 	var err error
 	exitCode := -1 // default bad
 	cmd := exec.Command(prog, args...)
 	if env != nil {
 		cmd.Env = env
 	}
 	cmd.Stdout = ioutil.Discard
-	cmd.Stderr = ioutil.Discard
 	cmd.Stderr = stderr
 	err = cmd.Run()
 	if err != nil {
 		if exerr, ok := err.(*exec.ExitError); ok {