17.7 Interface Operations
As we showed in the previous section, the
SIOCGIFCONF request returns the name and a socket address
structure for each interface that is configured. There are a
multitude of other requests that we can then issue to set or get
all the other characteristics of the interface. The get version of these requests
(SIOCGxxx) is often issued by the netstat
program, and the set version
(SIOCSxxx) is often issued by the ifconfig
program. Any user can get the interface information, while it takes
superuser privileges to set the information.
These requests take or return an ifreq
structure whose address is specified as the third argument to
ioctl. The interface is always identified by its name:
le0, lo0, ppp0, etc. in the
ifr_name member.
Many of these requests use a socket address
structure to specify or return an IP address or address mask with
the application. For IPv4, the address or mask is contained in the
sin_addr member of an Internet socket address structure;
for IPv6, it is in the sin6_addr member of an IPv6 socket
address structure.
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SIOCGIFADDR
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Return the unicast address in the
ifr_addr member.
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SIOCSIFADDR
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Set the interface address from the
ifr_addr member. The initialization function for the
interface is also called.
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SIOCGIFFLAGS
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Return the interface flags in the
ifr_flags member. The names of the various flags are
IFF_xxx and are defined
by including the <net/if.h> header. The flags
indicate, for example, if the interface is up (IFF_UP), if
the interface is a point-to-point interface
(IFF_POINTOPOINT), if the interface supports broadcasting
(IFF_BROADCAST), and so on.
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SIOCSIFFLAGS
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Set the interface flags from the
ifr_flags member.
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SIOCGIFDSTADDR
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Return the point-to-point address in the
ifr_dstaddr member.
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SIOCSIFDSTADDR
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Set the point-to-point address from the
ifr_dstaddr member.
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SIOCGIFBRDADDR
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Return the broadcast address in the
ifr_broadaddr member. The application must first fetch the
interface flags and then issue the correct request:
SIOCGIFBRDADDR for a broadcast interface or
SIOCGIFDSTADDR for a point-to-point interface.
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SIOCSIFBRDADDR
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Set the broadcast address from the
ifr_broadaddr member.
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SIOCGIFNETMASK
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Return the subnet mask in the ifr_addr
member.
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SIOCSIFNETMASK
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Set the subnet mask from the ifr_addr
member.
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SIOCGIFMETRIC
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Return the interface metric in the
ifr_metric member. The interface metric is maintained by
the kernel for each interface but is used by the routing daemon
routed. The interface metric is added to the hop count (to
make an interface less favorable).
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SIOCSIFMETRIC
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Set the interface routing metric from the
ifr_metric member.
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In this section, we described the generic
interface requests. Many implementations have additional requests
as well.
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