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[dragonfly.git] / usr.sbin / rarpd / rarpd.c

/*
 * Copyright (c) 1990, 1991, 1992, 1993, 1996
 *      The Regents of the University of California.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that: (1) source code distributions
 * retain the above copyright notice and this paragraph in its entirety, (2)
 * distributions including binary code include the above copyright notice and
 * this paragraph in its entirety in the documentation or other materials
 * provided with the distribution
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 *
 * @(#) Copyright (c) 1990, 1991, 1992, 1993, 1996 The Regents of the University of California.  All rights reserved.
 * $FreeBSD: src/usr.sbin/rarpd/rarpd.c,v 1.41 2004/08/07 04:28:54 imp Exp $
 * $DragonFly: src/usr.sbin/rarpd/rarpd.c,v 1.4 2004/12/18 22:48:05 swildner Exp $
 */

/*
 * rarpd - Reverse ARP Daemon
 *
 * Usage:       rarpd -a [-dfsv] [-t directory] [hostname]
 *              rarpd [-dfsv] [-t directory] interface [hostname]
 *
 * 'hostname' is optional solely for backwards compatibility with Sun's rarpd.
 * Currently, the argument is ignored.
 */
#include <sys/param.h>
#include <sys/file.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <sys/time.h>

#include <net/bpf.h>
#include <net/ethernet.h>
#include <net/if.h>
#include <net/if_types.h>
#include <net/if_dl.h>
#include <net/route.h>

#include <netinet/in.h>
#include <netinet/if_ether.h>

#include <arpa/inet.h>

#include <dirent.h>
#include <errno.h>
#include <ifaddrs.h>
#include <netdb.h>
#include <stdarg.h>
#include <stdio.h>
#include <string.h>
#include <syslog.h>
#include <stdlib.h>
#include <unistd.h>

/* Cast a struct sockaddr to a struct sockaddr_in */
#define SATOSIN(sa) ((struct sockaddr_in *)(sa))

#ifndef TFTP_DIR
#define TFTP_DIR "/tftpboot"
#endif

#define ARPSECS (20 * 60)               /* as per code in netinet/if_ether.c */
#define REVARP_REQUEST ARPOP_REVREQUEST
#define REVARP_REPLY ARPOP_REVREPLY

/*
 * The structure for each interface.
 */
struct if_info {
        struct if_info  *ii_next;
        int             ii_fd;                  /* BPF file descriptor */
        in_addr_t       ii_ipaddr;              /* IP address */
        in_addr_t       ii_netmask;             /* subnet or net mask */
        u_char          ii_eaddr[ETHER_ADDR_LEN];       /* ethernet address */
        char            ii_ifname[IF_NAMESIZE];
};

/*
 * The list of all interfaces that are being listened to.  rarp_loop()
 * "selects" on the descriptors in this list.
 */
struct if_info *iflist;

int verbose;                    /* verbose messages */
const char *tftp_dir = TFTP_DIR;        /* tftp directory */

int dflag;                      /* messages to stdout/stderr, not syslog(3) */
int sflag;                      /* ignore /tftpboot */

static  u_char zero[6];

static int      bpf_open(void);
static in_addr_t        choose_ipaddr(in_addr_t **, in_addr_t, in_addr_t);
static char     *eatoa(u_char *);
static int      expand_syslog_m(const char *fmt, char **newfmt);
static void     init(char *);
static void     init_one(struct ifaddrs *, char *, int);
static char     *intoa(in_addr_t);
static in_addr_t        ipaddrtonetmask(in_addr_t);
static void     logmsg(int, const char *, ...) __printflike(2, 3);
static int      rarp_bootable(in_addr_t);
static int      rarp_check(u_char *, u_int);
static void     rarp_loop(void);
static int      rarp_open(char *);
static void     rarp_process(struct if_info *, u_char *, u_int);
static void     rarp_reply(struct if_info *, struct ether_header *,
                in_addr_t, u_int);
static void     update_arptab(u_char *, in_addr_t);
static void     usage(void);

int
main(int argc, char *argv[])
{
        int op;
        char *ifname, *hostname, *name;

        int aflag = 0;          /* listen on "all" interfaces  */
        int fflag = 0;          /* don't fork */

        if ((name = strrchr(argv[0], '/')) != NULL)
                ++name;
        else
                name = argv[0];
        if (*name == '-')
                ++name;

        /*
         * All error reporting is done through syslog, unless -d is specified
         */
        openlog(name, LOG_PID | LOG_CONS, LOG_DAEMON);

        opterr = 0;
        while ((op = getopt(argc, argv, "adfst:v")) != -1)
                switch (op) {
                case 'a':
                        ++aflag;
                        break;

                case 'd':
                        ++dflag;
                        break;

                case 'f':
                        ++fflag;
                        break;

                case 's':
                        ++sflag;
                        break;

                case 't':
                        tftp_dir = optarg;
                        break;

                case 'v':
                        ++verbose;
                        break;

                default:
                        usage();
                        /* NOTREACHED */
                }
        argc -= optind;
        argv += optind;

        ifname = (aflag == 0) ? argv[0] : NULL;
        hostname = ifname ? argv[1] : argv[0];

        if ((aflag && ifname) || (!aflag && ifname == NULL))
                usage();

        init(ifname);

        if (!fflag) {
                if (daemon(0,0)) {
                        logmsg(LOG_ERR, "cannot fork");
                        exit(1);
                }
        }
        rarp_loop();
        return(0);
}

/*
 * Add to the interface list.
 */
static void
init_one(struct ifaddrs *ifa, char *target, int pass1)
{
        struct if_info *ii, *ii2;
        struct sockaddr_dl *ll;
        int family;

        family = ifa->ifa_addr->sa_family;
        switch (family) {
        case AF_INET:
                if (pass1)
                        /* Consider only AF_LINK during pass1. */
                        return;
                /* FALLTHROUGH */
        case AF_LINK:
                if (!(ifa->ifa_flags & IFF_UP) ||
                    (ifa->ifa_flags & (IFF_LOOPBACK | IFF_POINTOPOINT)))
                        return;
                break;
        default:
                return;
        }

        /* Don't bother going any further if not the target interface */
        if (target != NULL && strcmp(ifa->ifa_name, target) != 0)
                return;

        /* Look for interface in list */
        for (ii = iflist; ii != NULL; ii = ii->ii_next)
                if (strcmp(ifa->ifa_name, ii->ii_ifname) == 0)
                        break;

        if (pass1 && ii != NULL)
                /* We've already seen that interface once. */
                return;

        /* Allocate a new one if not found */
        if (ii == NULL) {
                ii = (struct if_info *)malloc(sizeof(*ii));
                if (ii == NULL) {
                        logmsg(LOG_ERR, "malloc: %m");
                        exit(1);
                }
                bzero(ii, sizeof(*ii));
                ii->ii_fd = -1;
                strlcpy(ii->ii_ifname, ifa->ifa_name, sizeof(ii->ii_ifname));
                ii->ii_next = iflist;
                iflist = ii;
        } else if (!pass1 && ii->ii_ipaddr != 0) {
                /*
                 * Second AF_INET definition for that interface: clone
                 * the existing one, and work on that cloned one.
                 * This must be another IP address for this interface,
                 * so avoid killing the previous configuration.
                 */
                ii2 = (struct if_info *)malloc(sizeof(*ii2));
                if (ii2 == NULL) {
                        logmsg(LOG_ERR, "malloc: %m");
                        exit(1);
                }
                memcpy(ii2, ii, sizeof(*ii2));
                ii2->ii_fd = -1;
                ii2->ii_next = iflist;
                iflist = ii2;

                ii = ii2;
        }

        switch (family) {
        case AF_INET:
                ii->ii_ipaddr = SATOSIN(ifa->ifa_addr)->sin_addr.s_addr;
                ii->ii_netmask = SATOSIN(ifa->ifa_netmask)->sin_addr.s_addr;
                if (ii->ii_netmask == 0)
                        ii->ii_netmask = ipaddrtonetmask(ii->ii_ipaddr);
                if (ii->ii_fd < 0)
                        ii->ii_fd = rarp_open(ii->ii_ifname);
                break;

        case AF_LINK:
                ll = (struct sockaddr_dl *)ifa->ifa_addr;
                if (ll->sdl_type == IFT_ETHER)
                        bcopy(LLADDR(ll), ii->ii_eaddr, 6);
                break;
        }
}
/*
 * Initialize all "candidate" interfaces that are in the system
 * configuration list.  A "candidate" is up, not loopback and not
 * point to point.
 */
static void
init(char *target)
{
        struct if_info *ii, *nii, *lii;
        struct ifaddrs *ifhead, *ifa;
        int error;

        error = getifaddrs(&ifhead);
        if (error) {
                logmsg(LOG_ERR, "getifaddrs: %m");
                exit(1);
        }
        /*
         * We make two passes over the list we have got.  In the first
         * one, we only collect AF_LINK interfaces, and initialize our
         * list of interfaces from them.  In the second pass, we
         * collect the actual IP addresses from the AF_INET
         * interfaces, and allow for the same interface name to appear
         * multiple times (in case of more than one IP address).
         */
        for (ifa = ifhead; ifa != NULL; ifa = ifa->ifa_next)
                init_one(ifa, target, 1);
        for (ifa = ifhead; ifa != NULL; ifa = ifa->ifa_next)
                init_one(ifa, target, 0);
        freeifaddrs(ifhead);

        /* Throw away incomplete interfaces */
        lii = NULL;
        for (ii = iflist; ii != NULL; ii = nii) {
                nii = ii->ii_next;
                if (ii->ii_ipaddr == 0 ||
                    bcmp(ii->ii_eaddr, zero, 6) == 0) {
                        if (lii == NULL)
                                iflist = nii;
                        else
                                lii->ii_next = nii;
                        if (ii->ii_fd >= 0)
                                close(ii->ii_fd);
                        free(ii);
                        continue;
                }
                lii = ii;
        }

        /* Verbose stuff */
        if (verbose)
                for (ii = iflist; ii != NULL; ii = ii->ii_next)
                        logmsg(LOG_DEBUG, "%s %s 0x%08x %s",
                            ii->ii_ifname, intoa(ntohl(ii->ii_ipaddr)),
                            (in_addr_t)ntohl(ii->ii_netmask), eatoa(ii->ii_eaddr));
}

static void
usage(void)
{
        fprintf(stderr, "%s\n%s\n",
            "usage: rarpd -a [-dfsv] [-t directory]",
            "       rarpd [-dfsv] [-t directory] interface");
        exit(1);
}

static int
bpf_open(void)
{
        int fd;
        int n = 0;
        char device[sizeof "/dev/bpf000"];

        /*
         * Go through all the minors and find one that isn't in use.
         */
        do {
                sprintf(device, "/dev/bpf%d", n++);
                fd = open(device, O_RDWR);
        } while ((fd == -1) && (errno == EBUSY));

        if (fd == -1) {
                logmsg(LOG_ERR, "%s: %m", device);
                exit(1);
        }
        return fd;
}

/*
 * Open a BPF file and attach it to the interface named 'device'.
 * Set immediate mode, and set a filter that accepts only RARP requests.
 */
static int
rarp_open(char *device)
{
        int fd;
        struct ifreq ifr;
        u_int dlt;
        int immediate;

        static struct bpf_insn insns[] = {
                BPF_STMT(BPF_LD|BPF_H|BPF_ABS, 12),
                BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, ETHERTYPE_REVARP, 0, 3),
                BPF_STMT(BPF_LD|BPF_H|BPF_ABS, 20),
                BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, REVARP_REQUEST, 0, 1),
                BPF_STMT(BPF_RET|BPF_K, sizeof(struct ether_arp) +
                         sizeof(struct ether_header)),
                BPF_STMT(BPF_RET|BPF_K, 0),
        };
        static struct bpf_program filter = {
                sizeof insns / sizeof(insns[0]),
                insns
        };

        fd = bpf_open();
        /*
         * Set immediate mode so packets are processed as they arrive.
         */
        immediate = 1;
        if (ioctl(fd, BIOCIMMEDIATE, &immediate) == -1) {
                logmsg(LOG_ERR, "BIOCIMMEDIATE: %m");
                exit(1);
        }
        strlcpy(ifr.ifr_name, device, sizeof(ifr.ifr_name));
        if (ioctl(fd, BIOCSETIF, (caddr_t)&ifr) == -1) {
                logmsg(LOG_ERR, "BIOCSETIF: %m");
                exit(1);
        }
        /*
         * Check that the data link layer is an Ethernet; this code won't
         * work with anything else.
         */
        if (ioctl(fd, BIOCGDLT, (caddr_t)&dlt) == -1) {
                logmsg(LOG_ERR, "BIOCGDLT: %m");
                exit(1);
        }
        if (dlt != DLT_EN10MB) {
                logmsg(LOG_ERR, "%s is not an ethernet", device);
                exit(1);
        }
        /*
         * Set filter program.
         */
        if (ioctl(fd, BIOCSETF, (caddr_t)&filter) == -1) {
                logmsg(LOG_ERR, "BIOCSETF: %m");
                exit(1);
        }
        return fd;
}

/*
 * Perform various sanity checks on the RARP request packet.  Return
 * false on failure and log the reason.
 */
static int
rarp_check(u_char *p, u_int len)
{
        struct ether_header *ep = (struct ether_header *)p;
        struct ether_arp *ap = (struct ether_arp *)(p + sizeof(*ep));

        if (len < sizeof(*ep) + sizeof(*ap)) {
                logmsg(LOG_ERR, "truncated request, got %u, expected %lu",
                                len, (u_long)(sizeof(*ep) + sizeof(*ap)));
                return 0;
        }
        /*
         * XXX This test might be better off broken out...
         */
        if (ntohs(ep->ether_type) != ETHERTYPE_REVARP ||
            ntohs(ap->arp_hrd) != ARPHRD_ETHER ||
            ntohs(ap->arp_op) != REVARP_REQUEST ||
            ntohs(ap->arp_pro) != ETHERTYPE_IP ||
            ap->arp_hln != 6 || ap->arp_pln != 4) {
                logmsg(LOG_DEBUG, "request fails sanity check");
                return 0;
        }
        if (bcmp((char *)&ep->ether_shost, (char *)&ap->arp_sha, 6) != 0) {
                logmsg(LOG_DEBUG, "ether/arp sender address mismatch");
                return 0;
        }
        if (bcmp((char *)&ap->arp_sha, (char *)&ap->arp_tha, 6) != 0) {
                logmsg(LOG_DEBUG, "ether/arp target address mismatch");
                return 0;
        }
        return 1;
}

/*
 * Loop indefinitely listening for RARP requests on the
 * interfaces in 'iflist'.
 */
static void
rarp_loop(void)
{
        u_char *buf, *bp, *ep;
        int cc, fd;
        fd_set fds, listeners;
        int bufsize, maxfd = 0;
        struct if_info *ii;

        if (iflist == NULL) {
                logmsg(LOG_ERR, "no interfaces");
                exit(1);
        }
        if (ioctl(iflist->ii_fd, BIOCGBLEN, (caddr_t)&bufsize) == -1) {
                logmsg(LOG_ERR, "BIOCGBLEN: %m");
                exit(1);
        }
        buf = malloc(bufsize);
        if (buf == NULL) {
                logmsg(LOG_ERR, "malloc: %m");
                exit(1);
        }

        while (1) {
                /*
                 * Find the highest numbered file descriptor for select().
                 * Initialize the set of descriptors to listen to.
                 */
                FD_ZERO(&fds);
                for (ii = iflist; ii != NULL; ii = ii->ii_next) {
                        FD_SET(ii->ii_fd, &fds);
                        if (ii->ii_fd > maxfd)
                                maxfd = ii->ii_fd;
                }
                listeners = fds;
                if (select(maxfd + 1, &listeners, NULL, NULL, NULL) == -1) {
                        /* Don't choke when we get ptraced */
                        if (errno == EINTR)
                                continue;
                        logmsg(LOG_ERR, "select: %m");
                        exit(1);
                }
                for (ii = iflist; ii != NULL; ii = ii->ii_next) {
                        fd = ii->ii_fd;
                        if (!FD_ISSET(fd, &listeners))
                                continue;
                again:
                        cc = read(fd, (char *)buf, bufsize);
                        /* Don't choke when we get ptraced */
                        if ((cc == -1) && (errno == EINTR))
                                goto again;

                        /* Loop through the packet(s) */
#define bhp ((struct bpf_hdr *)bp)
                        bp = buf;
                        ep = bp + cc;
                        while (bp < ep) {
                                u_int caplen, hdrlen;

                                caplen = bhp->bh_caplen;
                                hdrlen = bhp->bh_hdrlen;
                                if (rarp_check(bp + hdrlen, caplen))
                                        rarp_process(ii, bp + hdrlen, caplen);
                                bp += BPF_WORDALIGN(hdrlen + caplen);
                        }
                }
        }
#undef bhp
}

/*
 * True if this server can boot the host whose IP address is 'addr'.
 * This check is made by looking in the tftp directory for the
 * configuration file.
 */
static int
rarp_bootable(in_addr_t addr)
{
        struct dirent *dent;
        DIR *d;
        char ipname[9];
        static DIR *dd = NULL;

        sprintf(ipname, "%08X", (in_addr_t)ntohl(addr));

        /*
         * If directory is already open, rewind it.  Otherwise, open it.
         */
        if ((d = dd) != NULL)
                rewinddir(d);
        else {
                if (chdir(tftp_dir) == -1) {
                        logmsg(LOG_ERR, "chdir: %s: %m", tftp_dir);
                        exit(1);
                }
                d = opendir(".");
                if (d == NULL) {
                        logmsg(LOG_ERR, "opendir: %m");
                        exit(1);
                }
                dd = d;
        }
        while ((dent = readdir(d)) != NULL)
                if (strncmp(dent->d_name, ipname, 8) == 0)
                        return 1;
        return 0;
}

/*
 * Given a list of IP addresses, 'alist', return the first address that
 * is on network 'net'; 'netmask' is a mask indicating the network portion
 * of the address.
 */
static in_addr_t
choose_ipaddr(in_addr_t **alist, in_addr_t net, in_addr_t netmask)
{
        for (; *alist; ++alist)
                if ((**alist & netmask) == net)
                        return **alist;
        return 0;
}

/*
 * Answer the RARP request in 'pkt', on the interface 'ii'.  'pkt' has
 * already been checked for validity.  The reply is overlaid on the request.
 */
static void
rarp_process(struct if_info *ii, u_char *pkt, u_int len)
{
        struct ether_header *ep;
        struct hostent *hp;
        in_addr_t target_ipaddr;
        char ename[256];

        ep = (struct ether_header *)pkt;
        /* should this be arp_tha? */
        if (ether_ntohost(ename, (struct ether_addr *)&ep->ether_shost) != 0) {
                logmsg(LOG_ERR, "cannot map %s to name",
                        eatoa(ep->ether_shost));
                return;
        }

        if ((hp = gethostbyname(ename)) == NULL) {
                logmsg(LOG_ERR, "cannot map %s to IP address", ename);
                return;
        }

        /*
         * Choose correct address from list.
         */
        if (hp->h_addrtype != AF_INET) {
                logmsg(LOG_ERR, "cannot handle non IP addresses for %s",
                                                                ename);
                return;
        }
        target_ipaddr = choose_ipaddr((in_addr_t **)hp->h_addr_list,
                                      ii->ii_ipaddr & ii->ii_netmask,
                                      ii->ii_netmask);
        if (target_ipaddr == 0) {
                logmsg(LOG_ERR, "cannot find %s on net %s",
                       ename, intoa(ntohl(ii->ii_ipaddr & ii->ii_netmask)));
                return;
        }
        if (sflag || rarp_bootable(target_ipaddr))
                rarp_reply(ii, ep, target_ipaddr, len);
        else if (verbose > 1)
                logmsg(LOG_INFO, "%s %s at %s DENIED (not bootable)",
                    ii->ii_ifname,
                    eatoa(ep->ether_shost),
                    intoa(ntohl(target_ipaddr)));
}

/*
 * Poke the kernel arp tables with the ethernet/ip address combinataion
 * given.  When processing a reply, we must do this so that the booting
 * host (i.e. the guy running rarpd), won't try to ARP for the hardware
 * address of the guy being booted (he cannot answer the ARP).
 */
struct sockaddr_inarp sin_inarp = {
        sizeof(struct sockaddr_inarp), AF_INET, 0,
        {0},
        {0},
        0, 0
};
struct sockaddr_dl sin_dl = {
        sizeof(struct sockaddr_dl), AF_LINK, 0, IFT_ETHER, 0, 6,
        0, ""
};
struct {
        struct rt_msghdr rthdr;
        char rtspace[512];
} rtmsg;

static void
update_arptab(u_char *ep, in_addr_t ipaddr)
{
        int cc;
        struct sockaddr_inarp *ar, *ar2;
        struct sockaddr_dl *ll, *ll2;
        struct rt_msghdr *rt;
        int xtype, xindex;
        static pid_t pid;
        int r;
        static int seq;

        r = socket(PF_ROUTE, SOCK_RAW, 0);
        if (r == -1) {
                logmsg(LOG_ERR, "raw route socket: %m");
                exit(1);
        }
        pid = getpid();

        ar = &sin_inarp;
        ar->sin_addr.s_addr = ipaddr;
        ll = &sin_dl;
        bcopy(ep, LLADDR(ll), 6);

        /* Get the type and interface index */
        rt = &rtmsg.rthdr;
        bzero(rt, sizeof(rtmsg));
        rt->rtm_version = RTM_VERSION;
        rt->rtm_addrs = RTA_DST;
        rt->rtm_type = RTM_GET;
        rt->rtm_seq = ++seq;
        ar2 = (struct sockaddr_inarp *)rtmsg.rtspace;
        bcopy(ar, ar2, sizeof(*ar));
        rt->rtm_msglen = sizeof(*rt) + sizeof(*ar);
        errno = 0;
        if ((write(r, rt, rt->rtm_msglen) == -1) && (errno != ESRCH)) {
                logmsg(LOG_ERR, "rtmsg get write: %m");
                close(r);
                return;
        }
        do {
                cc = read(r, rt, sizeof(rtmsg));
        } while (cc > 0 && (rt->rtm_seq != seq || rt->rtm_pid != pid));
        if (cc == -1) {
                logmsg(LOG_ERR, "rtmsg get read: %m");
                close(r);
                return;
        }
        ll2 = (struct sockaddr_dl *)((u_char *)ar2 + ar2->sin_len);
        if (ll2->sdl_family != AF_LINK) {
                /*
                 * XXX I think this means the ip address is not on a
                 * directly connected network (the family is AF_INET in
                 * this case).
                 */
                logmsg(LOG_ERR, "bogus link family (%d) wrong net for %08X?\n",
                    ll2->sdl_family, ipaddr);
                close(r);
                return;
        }
        xtype = ll2->sdl_type;
        xindex = ll2->sdl_index;

        /* Set the new arp entry */
        bzero(rt, sizeof(rtmsg));
        rt->rtm_version = RTM_VERSION;
        rt->rtm_addrs = RTA_DST | RTA_GATEWAY;
        rt->rtm_inits = RTV_EXPIRE;
        rt->rtm_rmx.rmx_expire = time(0) + ARPSECS;
        rt->rtm_flags = RTF_HOST | RTF_STATIC;
        rt->rtm_type = RTM_ADD;
        rt->rtm_seq = ++seq;

        bcopy(ar, ar2, sizeof(*ar));

        ll2 = (struct sockaddr_dl *)((u_char *)ar2 + sizeof(*ar2));
        bcopy(ll, ll2, sizeof(*ll));
        ll2->sdl_type = xtype;
        ll2->sdl_index = xindex;

        rt->rtm_msglen = sizeof(*rt) + sizeof(*ar2) + sizeof(*ll2);
        errno = 0;
        if ((write(r, rt, rt->rtm_msglen) == -1) && (errno != EEXIST)) {
                logmsg(LOG_ERR, "rtmsg add write: %m");
                close(r);
                return;
        }
        do {
                cc = read(r, rt, sizeof(rtmsg));
        } while (cc > 0 && (rt->rtm_seq != seq || rt->rtm_pid != pid));
        close(r);
        if (cc == -1) {
                logmsg(LOG_ERR, "rtmsg add read: %m");
                return;
        }
}

/*
 * Build a reverse ARP packet and sent it out on the interface.
 * 'ep' points to a valid REVARP_REQUEST.  The REVARP_REPLY is built
 * on top of the request, then written to the network.
 *
 * RFC 903 defines the ether_arp fields as follows.  The following comments
 * are taken (more or less) straight from this document.
 *
 * REVARP_REQUEST
 *
 * arp_sha is the hardware address of the sender of the packet.
 * arp_spa is undefined.
 * arp_tha is the 'target' hardware address.
 *   In the case where the sender wishes to determine his own
 *   protocol address, this, like arp_sha, will be the hardware
 *   address of the sender.
 * arp_tpa is undefined.
 *
 * REVARP_REPLY
 *
 * arp_sha is the hardware address of the responder (the sender of the
 *   reply packet).
 * arp_spa is the protocol address of the responder (see the note below).
 * arp_tha is the hardware address of the target, and should be the same as
 *   that which was given in the request.
 * arp_tpa is the protocol address of the target, that is, the desired address.
 *
 * Note that the requirement that arp_spa be filled in with the responder's
 * protocol is purely for convenience.  For instance, if a system were to use
 * both ARP and RARP, then the inclusion of the valid protocol-hardware
 * address pair (arp_spa, arp_sha) may eliminate the need for a subsequent
 * ARP request.
 */
static void
rarp_reply(struct if_info *ii, struct ether_header *ep, in_addr_t ipaddr,
                u_int len)
{
        u_int n;
        struct ether_arp *ap = (struct ether_arp *)(ep + 1);

        update_arptab((u_char *)&ap->arp_sha, ipaddr);

        /*
         * Build the rarp reply by modifying the rarp request in place.
         */
        ap->arp_op = htons(REVARP_REPLY);

#ifdef BROKEN_BPF
        ep->ether_type = ETHERTYPE_REVARP;
#endif
        bcopy((char *)&ap->arp_sha, (char *)&ep->ether_dhost, 6);
        bcopy((char *)ii->ii_eaddr, (char *)&ep->ether_shost, 6);
        bcopy((char *)ii->ii_eaddr, (char *)&ap->arp_sha, 6);

        bcopy((char *)&ipaddr, (char *)ap->arp_tpa, 4);
        /* Target hardware is unchanged. */
        bcopy((char *)&ii->ii_ipaddr, (char *)ap->arp_spa, 4);

        /* Zero possible garbage after packet. */
        bzero((char *)ep + (sizeof(*ep) + sizeof(*ap)),
                        len - (sizeof(*ep) + sizeof(*ap)));
        n = write(ii->ii_fd, (char *)ep, len);
        if (n != len)
                logmsg(LOG_ERR, "write: only %d of %d bytes written", n, len);
        if (verbose)
                logmsg(LOG_INFO, "%s %s at %s REPLIED", ii->ii_ifname,
                    eatoa(ap->arp_tha),
                    intoa(ntohl(ipaddr)));
}

/*
 * Get the netmask of an IP address.  This routine is used if
 * SIOCGIFNETMASK doesn't work.
 */
static in_addr_t
ipaddrtonetmask(in_addr_t addr)
{
        addr = ntohl(addr);
        if (IN_CLASSA(addr))
                return htonl(IN_CLASSA_NET);
        if (IN_CLASSB(addr))
                return htonl(IN_CLASSB_NET);
        if (IN_CLASSC(addr))
                return htonl(IN_CLASSC_NET);
        logmsg(LOG_DEBUG, "unknown IP address class: %08X", addr);
        return htonl(0xffffffff);
}

/*
 * A faster replacement for inet_ntoa().
 */
static char *
intoa(in_addr_t addr)
{
        char *cp;
        u_int byte;
        int n;
        static char buf[sizeof(".xxx.xxx.xxx.xxx")];

        cp = &buf[sizeof buf];
        *--cp = '\0';

        n = 4;
        do {
                byte = addr & 0xff;
                *--cp = byte % 10 + '0';
                byte /= 10;
                if (byte > 0) {
                        *--cp = byte % 10 + '0';
                        byte /= 10;
                        if (byte > 0)
                                *--cp = byte + '0';
                }
                *--cp = '.';
                addr >>= 8;
        } while (--n > 0);

        return cp + 1;
}

static char *
eatoa(u_char *ea)
{
        static char buf[sizeof("xx:xx:xx:xx:xx:xx")];

        sprintf(buf, "%x:%x:%x:%x:%x:%x",
            ea[0], ea[1], ea[2], ea[3], ea[4], ea[5]);
        return (buf);
}

static void
logmsg(int pri, const char *fmt, ...)
{
        va_list v;
        FILE *fp;
        char *newfmt;

        va_start(v, fmt);
        if (dflag) {
                if (pri == LOG_ERR)
                        fp = stderr;
                else
                        fp = stdout;
                if (expand_syslog_m(fmt, &newfmt) == -1) {
                        vfprintf(fp, fmt, v);
                } else {
                        vfprintf(fp, newfmt, v);
                        free(newfmt);
                }
                fputs("\n", fp);
                fflush(fp);
        } else {
                vsyslog(pri, fmt, v);
        }
        va_end(v);
}

static int
expand_syslog_m(const char *fmt, char **newfmt)
{
        const char *str, *m;
        char *p, *np;

        p = strdup("");
        str = fmt;
        while ((m = strstr(str, "%m")) != NULL) {
                asprintf(&np, "%s%.*s%s", p, (int)(m - str),
                    str, strerror(errno));
                free(p);
                if (np == NULL) {
                        errno = ENOMEM;
                        return (-1);
                }
                p = np;
                str = m + 2;
        }
        
        if (*str != '\0') {
                asprintf(&np, "%s%s", p, str);
                free(p);
                if (np == NULL) {
                        errno = ENOMEM;
                        return (-1);
                }
                p = np;
        }

        *newfmt = p;
        return (0);
}