Appendix - Code Listing routeadd.c

The file included below summarises the code for adding a route to the kernel routing table, the purpose of the code is to show the flow of control, however it may not complete in all respects.

#include <stdio.h> #include <asm/types.h> #include <linux/netlink.h> #include <linux/rtnetlink.h> struct rtnl_handle { int fd; struct sockaddr_nl local; struct sockaddr_nl peer; __u32 seq; __u32 dump; }; // This code may not be complete in all respects, it just shows the flow of // control, for more detailed information refer to the code of iproute2 and // zebra packages. // This function is to open the netlink socket as the name suggests. int netlink_open(struct rtnl_handle* rth) { int addr_len; memset(rth, 0, sizeof(rth)); // Creating the netlink socket of family NETLINK_ROUTE rth->fd = socket(AF_NETLINK, SOCK_RAW, NETLINK_ROUTE); if (rth->fd < 0) { perror("cannot open netlink socket"); return -1; } memset(&rth->local, 0, sizeof(rth->local)); rth->local.nl_family = AF_NETLINK; rth->local.nl_groups = 0; // Binding the netlink socket if (bind(rth->fd, (struct sockaddr*)&rth->local, sizeof(rth->local)) < 0) { perror("cannot bind netlink socket"); return -1; } addr_len = sizeof(rth->local); if (getsockname(rth->fd, (struct sockaddr*)&rth->local, &addr_len) < 0) { perror("cannot getsockname"); return -1; } if (addr_len != sizeof(rth->local)) { fprintf(stderr, "wrong address lenght %d\n", addr_len); return -1; } if (rth->local.nl_family != AF_NETLINK) { fprintf(stderr, "wrong address family %d\n", rth->local.nl_family); return -1; } rth->seq = time(NULL); return 0; } // This function does the actual reading and writing to the netlink socket int rtnl_talk(struct rtnl_handle *rtnl, struct nlmsghdr *n, pid_t peer, unsigned groups, struct nlmsghdr *answer) { int status; struct nlmsghdr *h; struct sockaddr_nl nladdr; // Forming the iovector with the netlink packet. struct iovec iov = { (void*)n, n->nlmsg_len }; char buf[8192]; // Forming the message to be sent. struct msghdr msg = { (void*)&nladdr, sizeof(nladdr), &iov, 1, NULL, 0, 0 }; // Filling up the details of the netlink socket to be contacted in the // kernel. memset(&nladdr, 0, sizeof(nladdr)); nladdr.nl_family = AF_NETLINK; nladdr.nl_pid = peer; nladdr.nl_groups = groups; n->nlmsg_seq = ++rtnl->seq; if (answer == NULL) n->nlmsg_flags |= NLM_F_ACK; // Actual sending of the message, status contains success/failure status = sendmsg(rtnl->fd, &msg, 0); if (status < 0) return -1; } // This function forms the netlink packet to add a route to the kernel routing // table route_add(__u32* destination, __u32* gateway) { struct rtnl_handle rth; // structure of the netlink packet. struct { struct nlmsghdr n; struct rtmsg r; char buf[1024]; } req; char mxbuf[256]; struct rtattr * mxrta = (void*)mxbuf; unsigned mxlock = 0; memset(&req, 0, sizeof(req)); // Initialisation of a few parameters req.n.nlmsg_len = NLMSG_LENGTH(sizeof(struct rtmsg)); req.n.nlmsg_flags = NLM_F_REQUEST|NLM_F_CREATE; req.n.nlmsg_type = RTM_NEWROUTE; req.r.rtm_family = AF_INET; req.r.rtm_table = RT_TABLE_MAIN; req.r.rtm_protocol = RTPROT_BOOT; req.r.rtm_scope = RT_SCOPE_UNIVERSE; req.r.rtm_type = RTN_UNICAST; mxrta->rta_type = RTA_METRICS; mxrta->rta_len = RTA_LENGTH(0); // RTA_DST and RTA_GW are the two esential parameters for adding a route, // there are other parameters too which are not discussed here. For ipv4, // the length of the address is 4 bytes. addattr_l(&req.n, sizeof(req), RTA_DST, destination, 4); addattr_l(&req.n, sizeof(req), RTA_GATEWAY, gateway, 4); // opening the netlink socket to communicate with the kernel if (rtnl_open(&rth) < 0) { fprintf(stderr, "cannot open rtnetlink\n"); exit(1); } // sending the packet to the kernel. if (rtnl_talk(&rth, &req.n, 0, 0, NULL) < 0) exit(2); return 0; } // This is the utility function for adding the parameters to the packet. int addattr_l(struct nlmsghdr *n, int maxlen, int type, void *data, int alen) { int len = RTA_LENGTH(alen); struct rtattr *rta; if (NLMSG_ALIGN(n->nlmsg_len) + len > maxlen) return -1; rta = (struct rtattr*)(((char*)n) + NLMSG_ALIGN(n->nlmsg_len)); rta->rta_type = type; rta->rta_len = len; memcpy(RTA_DATA(rta), data, alen); n->nlmsg_len = NLMSG_ALIGN(n->nlmsg_len) + len; return 0; }