netdevice - Low level access to Linux network devices


#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.
 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.
 Retrieve the interface index of the interface into ifr_ifindex.
 Get or set the active flag word of the device. ifr_flags contains a bit mask of the following values:
Device flags
IFF_UPInterface is running.
IFF_BROADCASTValid broadcast address set.
IFF_DEBUGInternal debugging flag.
IFF_LOOPBACKInterface is a loopback interface.
IFF_POINTOPOINTInterface is a point-to-point link.
IFF_RUNNINGResources allocated.
IFF_NOARPNo arp protocol, L2 destination address not set.
IFF_PROMISCInterface is in promiscuous mode.
IFF_NOTRAILERSAvoid use of trailers.
IFF_ALLMULTIReceive all multicast packets.
IFF_MASTERMaster of a load balancing bundle.
IFF_SLAVESlave of a load balancing bundle.
IFF_MULTICASTSupports multicast
IFF_PORTSELIs able to select media type via ifmap.
IFF_AUTOMEDIAAuto media selection active.
IFF_DYNAMICThe addresses are lost when the interface goes down.
IFF_LOWER_UPDriver signals L1 up (since Linux 2.6.17)
IFF_DORMANTDriver signals dormant (since Linux 2.6.17)
IFF_ECHOEcho sent packets (since Linux 2.6.25)
Setting the active flag word is a privileged operation, but any process may read it.
 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.
 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.
 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.
 Set the hardware broadcast address of a device from ifr_hwaddr. This is a privileged operation.
 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.

 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.
 Get or set the transmit queue length of a device using ifr_qlen. Setting the transmit queue length is a privileged operation.
 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.
 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


proc(5), capabilities(7), ip(7), rtnetlink(7)


This page is part of release 3.23 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at

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