netdevice(7) Miscellaneous Information Manual netdevice(7)
NAME
netdevice - low-level access to Linux network devices
SYNOPSIS
#include <sys/ioctl.h>
#include <net/if.h>
DESCRIPTION
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. Most of them 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;
};
};
AF_INET6 is an exception. It passes an in6_ifreq structure:
struct in6_ifreq {
struct in6_addr ifr6_addr;
u32 ifr6_prefixlen;
int ifr6_ifindex; /* Interface index */
};
Normally, the user specifies which device to affect by setting ifr_name
to the name of the interface or ifr6_ifindex to the index of the inter-
face. All other members of the structure may share memory.
Ioctls
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 con-
tains a bit mask 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.
IFF_LOWER_UP Driver signals L1 up (since Linux 2.6.17)
IFF_DORMANT Driver signals dormant (since Linux 2.6.17)
IFF_ECHO Echo sent packets (since Linux 2.6.25)
Setting the active flag word is a privileged operation, but any process
may read it.
SIOCGIFPFLAGS
SIOCSIFPFLAGS
Get or set extended (private) flags for the device. ifr_flags
contains a bit mask of the following values:
Private flags
IFF_802_1Q_VLAN Interface is 802.1Q VLAN device.
IFF_EBRIDGE Interface is Ethernet bridging device.
IFF_SLAVE_INACTIVE Interface is inactive bonding slave.
IFF_MASTER_8023AD Interface is 802.3ad bonding master.
IFF_MASTER_ALB Interface is balanced-alb bonding master.
IFF_BONDING Interface is a bonding master or slave.
IFF_SLAVE_NEEDARP Interface needs ARPs for validation.
IFF_ISATAP Interface is RFC4214 ISATAP interface.
Setting the extended (private) interface flags is a privileged opera-
tion.
SIOCGIFADDR
SIOCSIFADDR
SIOCDIFADDR
Get, set, or delete the address of the device using ifr_addr, or
ifr6_addr with ifr6_prefixlen. Setting or deleting the interface
address is a privileged operation. For compatibility, SIOCGI-
FADDR returns only AF_INET addresses, SIOCSIFADDR accepts AF_INET
and AF_INET6 addresses, and SIOCDIFADDR deletes only AF_INET6 ad-
dresses. A AF_INET address can be deleted by setting it to zero
via SIOCSIFADDR.
SIOCGIFDSTADDR
SIOCSIFDSTADDR
Get or set the destination address of a point-to-point device us-
ing ifr_dstaddr. For compatibility, only AF_INET addresses are
accepted or returned. Setting the destination address is a priv-
ileged operation.
SIOCGIFBRDADDR
SIOCSIFBRDADDR
Get or set the broadcast address for a device using ifr_brdaddr.
For compatibility, only AF_INET addresses are accepted or re-
turned. Setting the broadcast address is a privileged operation.
SIOCGIFNETMASK
SIOCSIFNETMASK
Get or set the network mask for a device using ifr_netmask. For
compatibility, only AF_INET addresses are accepted or returned.
Setting the network mask is a privileged operation.
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_fam-
ily 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 allowed only
when the interface is not up.
SIOCGIFCONF
Return a list of interface (network layer) addresses. This cur-
rently means only addresses of the AF_INET (IPv4) family for com-
patibility. Unlike the others, this ioctl passes an ifconf
structure:
struct ifconf {
int ifc_len; /* size of buffer */
union {
char *ifc_buf; /* buffer address */
struct ifreq *ifc_req; /* array of structures */
};
};
If ifc_req is NULL, SIOCGIFCONF returns the necessary buffer size
in bytes for receiving all available addresses in ifc_len. Oth-
erwise, ifc_req contains a pointer to an array of ifreq struc-
tures to be filled with all currently active L3 interface ad-
dresses. ifc_len contains the size of the array in bytes.
Within each ifreq structure, ifr_name will receive the interface
name, and ifr_addr the address. The actual number of bytes
transferred is returned in ifc_len.
If the size specified by ifc_len is insufficient to store all the
addresses, the kernel will skip the exceeding ones and return
success. There is no reliable way of detecting this condition
once it has occurred. It is therefore recommended to either de-
termine the necessary buffer size beforehand by calling SIOCGIF-
CONF with ifc_req set to NULL, or to retry the call with a bigger
buffer whenever ifc_len upon return differs by less than
sizeof(struct ifreq) from its original value.
If an error occurs accessing the ifconf or ifreq structures,
EFAULT will be returned.
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 de-
scribed here.
NOTES
SIOCGIFCONF and the other ioctls that accept or return only AF_INET
socket addresses are IP-specific and perhaps should rather be documented
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.
AF_INET6 IPv6 addresses can be read from /proc/net/if_inet6 or via rt-
netlink(7). Adding a new IPv6 address and deleting an existing IPv6 ad-
dress can be done via SIOCSIFADDR and SIOCDIFADDR or via rtnetlink(7).
Retrieving or changing destination IPv6 addresses of a point-to-point
interface is possible only via rtnetlink(7).
BUGS
glibc 2.1 is missing the ifr_newname macro in <net/if.h>. Add the fol-
lowing to your program as a workaround:
#ifndef ifr_newname
#define ifr_newname ifr_ifru.ifru_slave
#endif
SEE ALSO
proc(5), capabilities(7), ip(7), rtnetlink(7)
Linux man-pages 6.9.1 2024-05-02 netdevice(7)
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