man7/netdevice.7

.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
.\"
.\" %%%LICENSE_START(VERBATIM_ONE_PARA)
.\" Permission is granted to distribute possibly modified copies
.\" of this page provided the header is included verbatim,
.\" and in case of nontrivial modification author and date
.\" of the modification is added to the header.
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.\"
.\" $Id: netdevice.7,v 1.10 2000/08/17 10:09:54 ak Exp $
.\"
.\" Modified, 2004-11-25, mtk, formatting and a few wording fixes
.\"
.\" Modified, 2011-11-02, <bidulock@openss7.org>, added many basic
.\"  but missing ioctls, such as SIOCGIFADDR.
.\"
.TH NETDEVICE  7 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
netdevice \- low-level access to Linux network devices
.SH SYNOPSIS
.nf
.B "#include <sys/ioctl.h>"
.B "#include <net/if.h>"
.fi
.SH DESCRIPTION
This man page describes the sockets interface which is used to configure
network devices.
.PP
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
.I ifreq
structure:
.PP
.in +4n
.EX
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;
    };
};
.EE
.in
.PP
.B AF_INET6
is an exception.
It passes an
.I in6_ifreq
structure:
.PP
.in +4n
.EX
struct in6_ifreq {
    struct in6_addr     ifr6_addr;
    u32                 ifr6_prefixlen;
    int                 ifr6_ifindex; /* Interface index */
};
.EE
.in
.PP
Normally, the user specifies which device to affect by setting
.I ifr_name
to the name of the interface or
.I ifr6_ifindex
to the index of the interface.
All other members of the structure may
share memory.
.SS Ioctls
If an ioctl is marked as privileged, then using it requires an effective
user ID of 0 or the
.B CAP_NET_ADMIN
capability.
If this is not the case,
.B EPERM
will be returned.
.TP
.B SIOCGIFNAME
Given the
.IR ifr_ifindex ,
return the name of the interface in
.IR ifr_name .
This is the only ioctl which returns its result in
.IR ifr_name .
.TP
.B SIOCGIFINDEX
Retrieve the interface index of the interface into
.IR ifr_ifindex .
.TP
.BR SIOCGIFFLAGS ", " SIOCSIFFLAGS
Get or set the active flag word of the device.
.I ifr_flags
contains a bit mask of the following values:
.\" Do not right adjust text blocks in tables
.na
.TS
tab(:);
c s
l l.
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:T{
No arp protocol, L2 destination address not set.
T}
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:T{
The addresses are lost when the interface goes down.
T}
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)
.TE
.ad
.PP
Setting the active flag word is a privileged operation, but any
process may read it.
.TP
.BR SIOCGIFPFLAGS ", " SIOCSIFPFLAGS
Get or set extended (private) flags for the device.
.I ifr_flags
contains a bit mask of the following values:
.TS
tab(:);
c s
l l.
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.
.TE
.PP
Setting the extended (private) interface flags is a privileged operation.
.TP
.BR SIOCGIFADDR ", " SIOCSIFADDR ", " SIOCDIFADDR
Get, set, or delete the address of the device using
.IR ifr_addr ,
or
.I ifr6_addr
with
.IR ifr6_prefixlen .
Setting or deleting the interface address is a privileged operation.
For compatibility,
.B SIOCGIFADDR
returns only
.B AF_INET
addresses,
.B SIOCSIFADDR
accepts
.B AF_INET
and
.B AF_INET6
addresses, and
.B SIOCDIFADDR
deletes only
.B AF_INET6
addresses.
A
.B AF_INET
address can be deleted by setting it to zero via
.BR SIOCSIFADDR .
.TP
.BR SIOCGIFDSTADDR ", " SIOCSIFDSTADDR
Get or set the destination address of a point-to-point device using
.IR ifr_dstaddr .
For compatibility, only
.B AF_INET
addresses are accepted or returned.
Setting the destination address is a privileged operation.
.TP
.BR SIOCGIFBRDADDR ", " SIOCSIFBRDADDR
Get or set the broadcast address for a device using
.IR ifr_brdaddr .
For compatibility, only
.B AF_INET
addresses are accepted or returned.
Setting the broadcast address is a privileged operation.
.TP
.BR SIOCGIFNETMASK ", " SIOCSIFNETMASK
Get or set the network mask for a device using
.IR ifr_netmask .
For compatibility, only
.B AF_INET
addresses are accepted or returned.
Setting the network mask is a privileged operation.
.TP
.BR SIOCGIFMETRIC ", " SIOCSIFMETRIC
Get or set the metric of the device using
.IR ifr_metric .
This is currently not implemented; it sets
.I ifr_metric
to 0 if you attempt to read it and returns
.B EOPNOTSUPP
if you attempt to set it.
.TP
.BR SIOCGIFMTU ", " SIOCSIFMTU
Get or set the MTU (Maximum Transfer Unit) of a device using
.IR ifr_mtu .
Setting the MTU is a privileged operation.
Setting the MTU to
too small values may cause kernel crashes.
.TP
.BR SIOCGIFHWADDR ", " SIOCSIFHWADDR
Get or set the hardware address of a device using
.IR ifr_hwaddr .
The hardware address is specified in a struct
.IR sockaddr .
.I sa_family
contains the ARPHRD_* device type,
.I sa_data
the L2 hardware address starting from byte 0.
Setting the hardware address is a privileged operation.
.TP
.B SIOCSIFHWBROADCAST
Set the hardware broadcast address of a device from
.IR ifr_hwaddr .
This is a privileged operation.
.TP
.BR SIOCGIFMAP ", " SIOCSIFMAP
Get or set the interface's hardware parameters using
.IR ifr_map .
Setting the parameters is a privileged operation.
.IP
.in +4n
.EX
struct ifmap {
    unsigned long   mem_start;
    unsigned long   mem_end;
    unsigned short  base_addr;
    unsigned char   irq;
    unsigned char   dma;
    unsigned char   port;
};
.EE
.in
.IP
The interpretation of the ifmap structure depends on the device driver
and the architecture.
.TP
.BR SIOCADDMULTI ", " SIOCDELMULTI
Add an address to or delete an address from the device's link layer
multicast filters using
.IR ifr_hwaddr .
These are privileged operations.
See also
.BR packet (7)
for an alternative.
.TP
.BR SIOCGIFTXQLEN ", " SIOCSIFTXQLEN
Get or set the transmit queue length of a device using
.IR ifr_qlen .
Setting the transmit queue length is a privileged operation.
.TP
.B SIOCSIFNAME
Changes the name of the interface specified in
.I ifr_name
to
.IR ifr_newname .
This is a privileged operation.
It is allowed only when the interface
is not up.
.TP
.B SIOCGIFCONF
Return a list of interface (network layer) addresses.
This currently
means only addresses of the
.B AF_INET
(IPv4) family for compatibility.
Unlike the others, this ioctl passes an
.I ifconf
structure:
.IP
.in +4n
.EX
struct ifconf {
    int               ifc_len; /* size of buffer */
    union {
        char         *ifc_buf; /* buffer address */
        struct ifreq *ifc_req; /* array of structures */
    };
};
.EE
.in
.IP
If
.I ifc_req
is NULL,
.B SIOCGIFCONF
returns the necessary buffer size in bytes
for receiving all available addresses in
.IR ifc_len .
Otherwise,
.I ifc_req
contains a pointer to an array of
.I ifreq
structures to be filled with all currently active L3 interface addresses.
.I ifc_len
contains the size of the array in bytes.
Within each
.I ifreq
structure,
.I ifr_name
will receive the interface name, and
.I ifr_addr
the address.
The actual number of bytes transferred is returned in
.IR ifc_len .
.IP
If the size specified by
.I 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 determine the necessary buffer size
beforehand by calling
.B SIOCGIFCONF
with
.I ifc_req
set to NULL, or to retry the call with a bigger buffer whenever
.I ifc_len
upon return differs by less than
.I sizeof(struct ifreq)
from its original value.
.IP
If an error occurs accessing the
.I ifconf
or
.I ifreq
structures,
.B EFAULT
will be returned.
.\" Slaving isn't supported in 2.2
.\" .
.\" .TP
.\" .BR SIOCGIFSLAVE ", " SIOCSIFSLAVE
.\" Get or set the slave device using
.\" .IR ifr_slave .
.\" Setting the slave device is a privileged operation.
.\" .PP
.\" FIXME . add amateur radio stuff.
.PP
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
.BR ip (7).
.PP
In addition, some devices support private ioctls.
These are not described here.
.SH NOTES
.B SIOCGIFCONF
and the other ioctls that accept or return only
.B AF_INET
socket addresses
are IP-specific and perhaps should rather be documented in
.BR ip (7).
.PP
The names of interfaces with no addresses or that don't have the
.B IFF_RUNNING
flag set can be found via
.IR /proc/net/dev .
.PP
.B AF_INET6
IPv6 addresses can be read from
.I /proc/net/if_inet6
or via
.BR rtnetlink (7).
Adding a new IPv6 address and deleting an existing IPv6 address
can be done via
.B SIOCSIFADDR
and
.B SIOCDIFADDR
or via
.BR rtnetlink (7).
Retrieving or changing destination IPv6 addresses of a point-to-point
interface is possible only via
.BR rtnetlink (7).
.SH BUGS
glibc 2.1 is missing the
.I ifr_newname
macro in
.IR <net/if.h> .
Add the following to your program as a workaround:
.PP
.in +4n
.EX
#ifndef ifr_newname
#define ifr_newname     ifr_ifru.ifru_slave
#endif
.EE
.in
.SH SEE ALSO
.BR proc (5),
.BR capabilities (7),
.BR ip (7),
.BR rtnetlink (7)