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Manual Reference Pages  - NETDEVICE (7)

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netdevice - Low level access to Linux network devices

CONTENTS

Synopsis
Description
Ioctls
Notes
Bugs

#include <sys/ioctl.h>
#include <net/if.h>

This man page describes the sockets interface which is used to configure network devices.

Linux supports some standard ioctls to configure network devices. They can be used on any socket’s file descriptor regardless of the family or type. They pass an ifreq structure:

struct ifreq {
    char ifr_name[IFNAMSIZ]; /* Interface name */
    union {
        struct sockaddr ifr_addr;
        struct sockaddr ifr_dstaddr;
        struct sockaddr ifr_broadaddr;
        struct sockaddr ifr_netmask;
        struct sockaddr ifr_hwaddr;
        short           ifr_flags;
        int             ifr_ifindex;
        int             ifr_metric;
        int             ifr_mtu;
        struct ifmap    ifr_map;
        char            ifr_slave[IFNAMSIZ];
        char            ifr_newname[IFNAMSIZ];
        char *          ifr_data;
    };
};

struct ifconf {
    int                ifc_len; /* size of buffer */
    union {           
        char *         ifc_buf; /* buffer address */
        struct ifreq * ifc_req; /* array of structures */
    }; 
}; 

Normally, the user specifies which device to affect by setting ifr_name to the name of the interface. All other members of the structure may share memory.

If an ioctl is marked as privileged then using it requires an effective user ID of 0 or the CAP_NET_ADMIN capability. If this is not the case EPERM will be returned.

SIOCGIFNAME
	Given the ifr_ifindex, return the name of the interface in ifr_name. This is the only ioctl which returns its result in ifr_name.
SIOCGIFINDEX
	Retrieve the interface index of the interface into ifr_ifindex.
SIOCGIFFLAGS, SIOCSIFFLAGS
	Get or set the active flag word of the device. ifr_flags contains a bitmask of the following values: Device flags IFF_UP Interface is running. IFF_BROADCAST Valid broadcast address set. IFF_DEBUG Internal debugging flag. IFF_LOOPBACK Interface is a loopback interface. IFF_POINTOPOINT Interface is a point-to-point link. IFF_RUNNING Resources allocated. IFF_NOARP No arp protocol, L2 destination address not set. IFF_PROMISC Interface is in promiscuous mode. IFF_NOTRAILERS Avoid use of trailers. IFF_ALLMULTI Receive all multicast packets. IFF_MASTER Master of a load balancing bundle. IFF_SLAVE Slave of a load balancing bundle. IFF_MULTICAST Supports multicast IFF_PORTSEL Is able to select media type via ifmap. IFF_AUTOMEDIA Auto media selection active. IFF_DYNAMIC The addresses are lost when the interface goes down. Setting the active flag word is a privileged operation, but any process may read it.
SIOCGIFMETRIC, SIOCSIFMETRIC
	Get or set the metric of the device using ifr_metric. This is currently not implemented; it sets ifr_metric to 0 if you attempt to read it and returns EOPNOTSUPP if you attempt to set it.
SIOCGIFMTU, SIOCSIFMTU
	Get or set the MTU (Maximum Transfer Unit) of a device using ifr_mtu. Setting the MTU is a privileged operation. Setting the MTU to too small values may cause kernel crashes.
SIOCGIFHWADDR, SIOCSIFHWADDR
	Get or set the hardware address of a device using ifr_hwaddr. The hardware address is specified in a struct sockaddr. sa_family contains the ARPHRD_* device type, sa_data the L2 hardware address starting from byte 0. Setting the hardware address is a privileged operation.
SIOCSIFHWBROADCAST
	Set the hardware broadcast address of a device from ifr_hwaddr. This is a privileged operation.
SIOCGIFMAP, SIOCSIFMAP
	Get or set the interface’s hardware parameters using ifr_map. Setting the parameters is a privileged operation. struct ifmap { unsigned long mem_start; unsigned long mem_end; unsigned short base_addr; unsigned char irq; unsigned char dma; unsigned char port; }; The interpretation of the ifmap structure depends on the device driver and the architecture.
SIOCADDMULTI, SIOCDELMULTI
	Add an address to or delete an address from the device’s link layer multicast filters using ifr_hwaddr. These are privileged operations. See also packet(7) for an alternative.
SIOCGIFTXQLEN, SIOCSIFTXQLEN
	Get or set the transmit queue length of a device using ifr_qlen. Setting the transmit queue length is a privileged operation.
SIOCSIFNAME
	Changes the name of the interface specified in ifr_name to ifr_newname. This is a privileged operation. It is only allowed when the interface is not up.
SIOCGIFCONF
	Return a list of interface (transport layer) addresses. This currently means only addresses of the AF_INET (IPv4) family for compatibility. The user passes a ifconf structure as argument to the ioctl. It contains a pointer to an array of ifreq structures in ifc_req and its length in bytes in ifc_len. The kernel fills the ifreqs with all current L3 interface addresses that are running: ifr_name contains the interface name (eth0:1 etc.), ifr_addr the address. The kernel returns with the actual length in ifc_len. If ifc_len is equal to the original length the buffer probably has overflowed and you should retry with a bigger buffer to get all addresses. When no error occurs the ioctl returns 0; otherwise -1. Overflow is not an error.

Most protocols support their own ioctls to configure protocol specific interface options. See the protocol man pages for a description. For configuring IP addresses see ip(7).

In addition some devices support private ioctls. These are not described here.

Strictly speaking, SIOCGIFCONF is IP specific and belongs in ip(7).

The names of interfaces with no addresses or that don’t have the IFF_RUNNING flag set can be found via /proc/net/dev.

Local IPv6 IP addresses can be found via /proc/net or via rtnetlink(7).

glibc 2.1 is missing the ifr_newname macro in net/if.h. Add the following to your program as a workaround:

#ifndef ifr_newname
#define ifr_newname     ifr_ifru.ifru_slave
#endif

capabilities(7), ip(7), proc(7), rtnetlink(7)