What is the difference between 'ifconfig up eth0' and 'ifup eth0'?

Solution Unverified - Updated -

Environment

  • Red Hat Enterprise Linux (RHEL) 5

Issue

  • What is the difference between 'ifconfig up eth0' and 'ifup eth0'?

Resolution

  • An "ifconfig" command and "ifup" or "ifdown" command can activate network interfaces. "ifconfig" directly controls network interfaces, however "ifup" or "ifdown" executes "/etc/sysconfig/network-scripts/ifup-" or "ifdown-" scripts. The scripts use an ip command.

  • "ifconfig" and "ip" commands use ioctl() to activate or deactivate network interfaces. "ifconfig" flags network interfaces 'IFF_UP | IFF_RUNNING', however "ip" flags that 'IFF_UP'.

  • An "ifconfig up eth0" activates eth0 but does not setup IP addresses, however an "ifup eth0" setup IP addresses or other options based by an ifcfg-eth0, because the "ifup" uses an "ifup-eth*" script.

  • The ifup command will also configure any static routes that are configured in the network-scripts directory. The ifconfig command will not.

Diagnostic Steps

ifconfig.c::main()


 473         if (!strcmp(*spp, "up")) {
 474             goterr |= set_flag(ifr.ifr_name, (IFF_UP | IFF_RUNNING));
 475             spp++;
 476             continue;
 477         }
 478         if (!strcmp(*spp, "down")) {
 479             goterr |= clr_flag(ifr.ifr_name, IFF_UP);
 480             spp++;
 481             continue;
 482         }

ifconfig.c::set_flag()


 125 /* Set a certain interface flag. */
 126 static int set_flag(char *ifname, short flag)
 127 {
 128     struct ifreq ifr;
 129 
 130     safe_strncpy(ifr.ifr_name, ifname, IFNAMSIZ);
 131     if (ioctl(skfd, SIOCGIFFLAGS, &ifr) < 0) {
 132         fprintf(stderr, _("%s: unknown interface: %s\n"),
 133                 ifname, strerror(errno));
 134         return (-1);
 135     }
 136     safe_strncpy(ifr.ifr_name, ifname, IFNAMSIZ);
 137     ifr.ifr_flags |= flag;
 138     if (ioctl(skfd, SIOCSIFFLAGS, &ifr) < 0) {
 139         perror("SIOCSIFFLAGS");
 140         return -1;
 141     }
 142 
 143     return (0);
 144 }

ip/iplink.c::do_set()


254 static int do_set(int argc, char **argv)
255 {
:
267         while (argc > 0) {
268                 if (strcmp(*argv, "up") == 0) {
269                         mask |= IFF_UP;
270                         flags |= IFF_UP;
271                 } else if (strcmp(*argv, "down") == 0) {
272                         mask |= IFF_UP;
273                         flags &= ~IFF_UP;
:
409         if (mask)
410                 return do_chflags(dev, flags, mask);
411         return 0;

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