Enable and Use Promiscuous Mode on Linux: Commands and C Code

Introduction

During development a board needed to receive multicast packets from a customer module that resides in a different IP subnet. By default the NIC drops such packets, so the article explains how to capture traffic from other subnets by enabling the network‑card promiscuous mode.

Promiscuous Mode

Promiscuous mode (also called "promisc" or listening mode) allows a host to accept every Ethernet frame that passes the NIC, regardless of the destination MAC address. In normal (non‑promiscuous) mode the NIC only receives frames addressed to its own MAC, plus broadcast and multicast frames.

NIC Working Modes

Broadcast mode

Multicast mode

Unicast mode

Promiscuous mode

Setting Promiscuous Mode on Linux

Enable Promiscuous Mode

ifconfig eth0 promisc

Disable Promiscuous Mode

ifconfig eth0 -promisc

Query Promiscuous Mode

Use ip or ifconfig to check whether the interface shows the PROMISC flag.

Using ip

ip link show eth0

Using ifconfig

ifconfig eth0

Programmatic Enabling in C

The following C snippet demonstrates how to enable promiscuous mode programmatically using ioctl calls.

struct ifreq ethreq;
strncpy(ethreq.ifr_name, "eth0", IFNAMSIZ);
if (ioctl(sock_raw_fd, SIOCGIFFLAGS, &ethreq) != 0) {
    perror("ioctl");
    close(sock_raw_fd);
    exit(-1);
}
ethreq.ifr_flags |= IFF_PROMISC;
if (ioctl(sock_raw_fd, SIOCSIFFLAGS, &ethreq) != 0) {
    perror("ioctl");
    close(sock_raw_fd);
    exit(-1);
}