man-pages/man3/rtnetlink.3

.\" This man page is Copyright (C) 1999 Andi Kleen <ak@muc.de>.
.\" 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.
.\" $Id: rtnetlink.3,v 1.2 1999/05/18 10:35:10 freitag Exp $
.TH RTNETLINK 3 1999-05-14 "Linux Man Page" "Linux Programmer's Manual"
.SH NAME
rtnetlink \- Macros to manipulate rtnetlink messages
.SH SYNOPSIS
.B #include <asm/types.h>
.br
.B #include <linux/netlink.h>
.br
.B #include <linux/rtnetlink.h>
.br
.B #include <sys/socket.h>

.BI "rtnetlink_socket = socket(PF_NETLINK, int " socket_type \
", NETLINK_ROUTE);"
.sp
.BI "int RTA_OK(struct rtattr *" rta ", int " rtabuflen );
.sp
.BI "void *RTA_DATA(struct rtattr *" rta );
.sp
.BI "unsigned int RTA_PAYLOAD(struct rtattr *" rta );
.sp
.BI "struct rtattr *RTA_NEXT(struct rtattr *" rta \
", unsigned int " rtabuflen );
.sp
.BI "unsigned int RTA_LENGTH(unsigned int " length );
.sp
.BI "unsigned int RTA_SPACE(unsigned int "length );
.SH DESCRIPTION
All
.BR rtnetlink (7)
messages consist of a
.BR netlink (7)
message header and appended attributes.
The attributes should be only
manipulated using the macros provided here.
.PP
.BI RTA_OK( rta ", " attrlen )
returns true if
.I rta
points to a valid routing attribute;
.I attrlen
is the  running length of the attribute buffer.
When not true then you must assume there are no more attributes in the
message, even if
.I attrlen
is non-zero.
.br
.BI RTA_DATA( rta )
returns a pointer to the start of this attribute's data.
.br
.BI RTA_PAYLOAD( rta )
returns the length of this attribute's data.
.br
.BI RTA_NEXT( rta ", " attrlen )
gets the next attribute after
.IR rta .
Calling this macro will update
.IR attrlen .
You should use
.B RTA_OK
to check for the validity of the returned pointer.
.br
.BI RTA_LENGTH( len )
returns the length which is required for
.I len
bytes of data plus the header.
.br
.BI RTA_SPACE( len )
returns the amount of space which will be needed in the message with
.I len
bytes of data.
.SH EXAMPLE
.\" FIXME would be better to use libnetlink in the EXAMPLE code here

Creating a rtnetlink message to set a MTU of a device.
.nf
    struct {
        struct nlmsghdr  nh;
        struct ifinfomsg if;
        char             attrbuf[512];
    } req;

    struct rtattr *rta;
    unsigned int mtu = 1000;

    int rtnetlink_sk = socket(PF_NETLINK, SOCK_DGRAM, NETLINK_ROUTE);

    memset(&req, 0, sizeof(req));
    req.nh.nlmsg_len = NLMSG_LENGTH(sizeof(struct ifinfomsg));
    req.nh.nlmsg_flags = NLM_F_REQUEST;
    req.nh.nlmsg_type = RTML_NEWLINK;
    req.if.ifi_family = AF_UNSPEC;
    req.if.ifi_index = INTERFACE_INDEX;
    req.if.ifi_change = 0xffffffff; /* ???*/
    rta = (struct rtattr *)(((char *) &req) +
                                  NLMSG_ALIGN(n->nlmsg_len));
    rta->rta_type = IFLA_MTU;
    rta->rta_len = sizeof(unsigned int);
    req.n.nlmsg_len = NLMSG_ALIGN(req.n.nlmsg_len) +
                                  RTA_LENGTH(sizeof(mtu));
    memcpy(RTA_DATA(rta), &mtu, sizeof(mtu));
    send(rtnetlink_sk, &req, req.n.nlmsg_len);
.fi
.SH BUGS
This manual page is lacking and incomplete.
.SH "SEE ALSO"
.BR netlink (3),
.BR netlink (7),
.BR rtnetlink (7)