[Iftop-users] [PATCH] options: select interface that is IPv4 default route (Closes: #644998)

[Shawn Landden]

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=644998
---
 options.c | 77 ++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 76 insertions(+), 1 deletion(-)

diff --git a/options.c b/options.c
index a075357..efff2c7 100644
--- a/options.c
+++ b/options.c
@@ -6,8 +6,13 @@
 
 #include "config.h"
 
+#ifdef __linux__
+#define _GNU_SOURCE
+#endif
+
 #include <sys/types.h>
 
+#include <fcntl.h>
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
@@ -90,6 +95,72 @@ static int is_bad_interface_name(char *i) {
     return 0;
 }
 
+/* none of these errors are expected, so I think it is OK to print them */
+static char *get_interface_for_default_ipv4_route(void) {
+#ifdef __linux__
+    pid_t pid;
+    int fds[2];
+    int r;
+    char buf[4096];
+    char *p, *q;
+
+    r = pipe2(fds, O_CLOEXEC);
+    if (r < 0) {
+        fprintf(stderr, "get_interface_for_default_ipv4_route: pipe() failed: %s, continuing...", strerror(r));
+        return NULL;
+    }
+    pid = fork();
+    if (pid < 0) {
+        fprintf(stderr, "get_interface_for_default_ipv4_route: fork() failed: %s, continuing...", strerror(r));
+        return NULL;
+    } else if (pid == 0) {
+        /* child */
+        r = dup2(fds[0], STDOUT_FILENO);
+        if (r < 0) {
+            char buf[1] = {'\n'};
+
+            /* we have to write something as the other end is expecting something */
+            r = write(fds[0], &buf, sizeof(buf));
+            _exit(EXIT_FAILURE);
+        }
+
+        execlp("ip", /* add "-6", here to make default IPv6 route */ "route", NULL);
+        char buf[1] = {'\n'};
+
+        /* we have to write something as the other end is expecting something */
+        r = write(fds[0], &buf, sizeof(buf));
+        _exit(EXIT_FAILURE);
+    }
+    /* parent */
+    close(fds[1]);
+    r = read(fds[1], &buf, sizeof(buf));
+    if (r < 0) {
+        fprintf(stderr, "get_interface_for_default_ipv4_route: read() failed: %s, continuing...", strerror(r));
+        return NULL;
+    }
+
+    p = strstr((char *)&buf, "default via ");
+    if (!p)
+        return NULL;
+    p += strlen("default via ");
+    q = p;
+    for (;*p != '\n' && *p; p++)
+        ;
+    *p = '\0';
+    p = strstr(q, " dev ");
+    if (!p)
+        return NULL;
+    p += strlen(" dev ");
+    q = p;
+    for (;*p != ' ' && *p; p++)
+        ;
+    *p = '\0';
+    return xstrdup(q);
+#else
+    return NULL;
+#endif
+}
+
 /* This finds the first interface which is up and is not the loopback
  * interface or one of the interface types listed in bad_interface_names. */
 static char *get_first_interface(void) {
@@ -123,9 +194,13 @@ static char *get_first_interface(void) {
 
 void options_set_defaults() {
     char *s;
+
+    if (!options.interface)
+        options.interface = get_interface_for_default_ipv4_route();
     /* Should go through the list of interfaces, and find the first one which
      * is up and is not lo or dummy*. */
-    options.interface = get_first_interface();
+    if (!options.interface)
+        options.interface = get_first_interface();
     if (!options.interface)
         options.interface = "eth0";
 
-- 
2.17.1