NAME

"Socket::Netlink" - interface to Linux's "PF_NETLINK" socket family

SYNOPSIS

     use Socket;
     use Socket::Netlink qw( :DEFAULT pack_nlmsghdr unpack_nlmsghdr );

     socket( my $sock, PF_NETLINK, SOCK_RAW, 0 ) or die "socket: $!";

     send( $sock, pack_nlmsghdr( 18, NLMFREQUEST|NLMFDUMP, 0, 0,
          "\0\0\0\0\0\0\0\0" ), 0 )
        or die "send: $!";

     recv( $sock, my $buffer, 65536, 0 ) or die "recv: $!";

     printf "Received type=%d flags=%x:\n%v02x\n",
        ( unpack_nlmsghdr( $buffer ) )[ 0, 1, 4 ];

DESCRIPTION

This module contains the low-level constants and structure handling functions required to use Linux's "PF_NETLINK" socket family. It is suggested to use the high-level object interface to this instead; see IO::Socket::Netlink.

CONSTANTS

The following constants are exported

PF_NETLINK

The packet family (for "socket()" calls)

AF_NETLINK

The address family

ADDRESS FUNCTIONS

The following pair of functions operate on "AF_NETLINK" address structures. The meainings of the parameters are:

    pid     The unique endpoint number for this netlink socket. If given as
            0 to the "bind()" syscall, the kernel will allocate an endpoint
            number of the process's PID.

groups A 32-bit bitmask of the multicast groups to join.

$addr = pack_sockaddr_nl( $pid, $groups ) Returns a "sockaddr_nl" structure with the fields packed into it.

( $pid, $groups ) = unpack_sockaddr_nl( $addr ) Takes a "sockaddr_nl" structure and returns the unpacked fields from it.

STRUCTURE FUNCTIONS

The following function pairs operate on structure types used by netlink

$buffer = pack_nlmsghdr( $type, $flags, $seq, $pid, $body ) ( $type, $flags, $seq, $pid, $body, $morebuffer ) = unpack_nlmsghdr( $buffer ) Pack or unpack a "struct nlmsghdr" and its payload body.

Because a single netlink message can contain more than payload body, the "unpack_nlmsghdr" function will return the remaining buffer after unpacking the first message, in case there are others. If there are no more, the $morebuffer list element will not be returned.

     while( defined $buffer ) {
        ( my ( $type, $flags, $seq, $pid, $body ), $buffer ) = unpack_nlmsghdr( $buffer );
        ...
     }

There is no similar functionallity for "pack_nlmsghdr"; simply concatenate multiple results together to send more than one message.

$buffer = pack_nlmsgerr( $error, $msg ) ( $error, $msg ) = unpack_nlmsgerr( $buffer ) Pack or unpack a "struct nlmsgerr". The kernel expects or reports negative integers in its structures; these functions take or return normal positive error values suitable for use with $!.

$buffer = pack_nlattrs( %attrs )
%attrs = unpack_nlattrs( $buffer )
Pack or unpack a list of netlink attributes.

These functions take or return even-sized lists of "$type, $value" pairs. The type will be the number in the netlink attribute message, and the value will be a plain packed string buffer. It is the caller's responsibilty to further pack/unpack this buffer as appropriate for the specific type.

Because these functions take/return even-sized lists, they may be passed or returned into hashes.

SEE ALSO

• netlink(7) - netlink - Communication between kernel and userspace (AF_NETLINK)
• IO::Socket::Netlink - Object interface to "AF_NETLINK" domain sockets

AUTHOR

Paul Evans <leonerd@leonerd.org.uk>