How to Build a Simple USB Network Card Monitor Daemon (ethcheck) on Linux

1. Implementing USB Network Card Monitoring

The article starts by noting that the open‑source tool ifplugd can monitor network interface status, but it only works when the interface is already registered in the system (visible via ifconfig -a). USB Ethernet adapters are registered only after being plugged in, and they may be unplugged at any time, so a custom solution is needed.

2. Design of the ethcheck Program

2.1 Program Flowchart

2.2 Daemonizing the Process

Because the program must run in the background and stay resident, it is turned into a daemon. The essential steps are fork, exit the parent, create a new session with setsid(), change the working directory to /tmp, reset the file‑creation mask, and close all open file descriptors.

void init_daemon(void) {
  int pid;
  int i;
  if (pid = fork())
    exit(0); // parent exits
  else if (pid < 0)
    exit(1); // fork failed
  setsid(); // become session leader
  chdir("/tmp");
  umask(0);
  for (i = 0; i < NOFILE; ++i) close(i);
  return;
}

2.3 Checking Whether a Specific Interface Exists

The program reads /proc/net/dev to see if the target interface name appears in the list.

cat /proc/net/dev
Inter-|   Receive ...
  lo:   26163 ...
  eth0: 285444708 ...

The following C functions parse that file, extract the interface name, and return 1 if the interface is present, 0 otherwise.

static char *interface_name_cut(char *buf, char **name) {
  while (*buf == ' ') buf++;
  *name = buf;
  char *stat = strrchr(buf, ':');
  *stat++ = '\0';
  return stat;
}

int check_interface_fromproc(char *interface) {
  FILE *fp = fopen(_PATH_PROC_NET_DEV, "r");
  if (fp == NULL) { printf("open proc file error
"); return -1; }
  char buf[PROCBUFSIZ];
  fgets(buf, PROCBUFSIZ, fp); // skip header lines
  fgets(buf, PROCBUFSIZ, fp);
  while (fgets(buf, PROCBUFSIZ, fp) != NULL) {
    char *name;
    interface_name_cut(buf, &name);
    if (strcmp(interface, name) == 0) return 1;
  }
  fclose(fp);
  return 0;
}

2.4 Handling the Case When the Interface Is Missing

If the interface is not found, the daemon sleeps for a short interval and then re‑checks /proc/net/dev again.

2.5 When the Interface Exists – Verify Its IP Address

When the interface is present, the program obtains its IP address using the ioctl() system call with the SIOCGIFADDR request.

int getLocalIp(const char *eth, char *ip) {
  struct ifreq ifr;
  struct sockaddr_in sin;
  int fd = socket(AF_INET, SOCK_STREAM, 0);
  if (fd < 0) return -1;
  strcpy(ifr.ifr_name, eth);
  if (ioctl(fd, SIOCGIFADDR, &ifr) < 0) { close(fd); return -1; }
  memcpy(&sin, &ifr.ifr_addr, sizeof(sin));
  snprintf(ip, IP_SIZE, "%s", inet_ntoa(sin.sin_addr));
  close(fd);
  return 0;
}

The retrieved address is stored in the buffer passed as ip.

2.6 Comparing the Obtained IP with the Desired One

If the current IP matches the target IP, the daemon simply sleeps for a configurable period before the next check.

2.7 Changing the IP When It Differs

If the IP does not match, the daemon invokes a pre‑written shell script if.sh to reconfigure the interface.

#!/bin/bash
IPADDR=192.168.40.8
ETHPORT=eth1
echo "ethcheck set" $ETHPORT $IPADDR
if [ $# -eq 2 ]; then
  if [ $1 = $ETHPORT ]; then
    if [ $2 = "up" ]; then
      ifconfig $ETHPORT $IPADDR
      sleep 1
      ip rule add from all lookup main pref 9000
      sleep 1
      echo 1 > /proc/sys/net/ipv4/ip_forward
      iptables -F
      echo "set $ETHPORT done"
    elif [ $2 = "down" ]; then
      echo "down"
    elif [ $2 = "disable" ]; then
      echo "disable"
    elif [ $2 = "error" ]; then
      echo "error"
    fi
  fi
fi

This script sets the interface IP, updates routing rules, enables IP forwarding, and flushes iptables as needed.