RTNETLINK Section: Linux Programmer's Manual (3) Updated: 14 May 2025 Index   NAME rtnetlink - Macros to manipuate rtnetlink messages   SYNOPSIS   SYNOPSIS #include <asm/types.h> #include <linux/netlink.h> #include <linux/rtnetlink.h> #include <sys/socket.h> rtnetlink_socket = socket(PF_NETLINK, int socket_type, NETLINK_ROUTE); int RTA_OK(struct rtattr *rta, int rtabuflen); void *RTA_DATA(struct rtattr *rta); unsigned int RTA_PAYLOAD(struct rtattr *rta); struct rtattr *RTA_NEXT(struct rtattr *rta, unsigned int rtabuflen); unsigned int RTA_LENGTH(unsigned int length); unsigned int RTA_SPACE(unsigned int length);   DESCRIPTION All messages consist of a message header and appended attributes. The attributes should be only manipulated using the macros provided here. RTA_OK(rta, attrlen) returns true if rta points to a valid routing attribute; 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 attrlen is non-zero. RTA_DATA(rta) returns a pointer to the start of this attribute's data. RTA_PAYLOAD(rta) returns the length of this attribute's data. RTA_NEXT(rta, attrlen) gets the next attribute after rta. Calling this macro will update attrlen. You should use RTA_OK to check for the validity of the returned pointer. RTA_LENGTH(len) returns the length which is required for len bytes of data plus the header. RTA_SPACE(len) returns the amount of space which will be needed in the message with len bytes of data.   EXAMPLE Creating a rtnetlink message to set a MTU of a device. struct { struct nlmsghdr nh; struct ifinfomsg if; char attrbuf[512]; } req; struct rtattr *rta; unsigned int mtu = 1000; int rtnetlink_socket = 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_socket, &req, req.n.nlmsg_len);   SEE ALSO   Index NAME SYNOPSIS SYNOPSIS DESCRIPTION EXAMPLE SEE ALSO This document was created by using the manual pages. Time: 11:58:05 GMT, March 24, 2025