/* vi: set sw=4 ts=4: */
/*
 * RFC3927 ZeroConf IPv4 Link-Local addressing
 * (see <http://www.zeroconf.org/>)
 *
 * Copyright (C) 2003 by Arthur van Hoff (avh@strangeberry.com)
 * Copyright (C) 2004 by David Brownell
 *
 * Licensed under GPLv2 or later, see file LICENSE in this source tree.
 */

/*
 * ZCIP just manages the 169.254.*.* addresses.  That network is not
 * routed at the IP level, though various proxies or bridges can
 * certainly be used.  Its naming is built over multicast DNS.
 */
//config:config ZCIP
//config:       bool "zcip"
//config:       default y
//config:       select PLATFORM_LINUX
//config:       select FEATURE_SYSLOG
//config:       help
//config:         ZCIP provides ZeroConf IPv4 address selection, according to RFC 3927.
//config:         It's a daemon that allocates and defends a dynamically assigned
//config:         address on the 169.254/16 network, requiring no system administrator.
//config:
//config:         See http://www.zeroconf.org for further details, and "zcip.script"
//config:         in the busybox examples.

//applet:IF_ZCIP(APPLET(zcip, BB_DIR_SBIN, BB_SUID_DROP))

//kbuild:lib-$(CONFIG_ZCIP) += zcip.o

//#define DEBUG

// TODO:
// - more real-world usage/testing, especially daemon mode
// - kernel packet filters to reduce scheduling noise
// - avoid silent script failures, especially under load...
// - link status monitoring (restart on link-up; stop on link-down)

//usage:#define zcip_trivial_usage
//usage:       "[OPTIONS] IFACE SCRIPT"
//usage:#define zcip_full_usage "\n\n"
//usage:       "Manage a ZeroConf IPv4 link-local address\n"
//usage:     "\n        -f              Run in foreground"
//usage:     "\n        -q              Quit after obtaining address"
//usage:     "\n        -r 169.254.x.x  Request this address first"
//usage:     "\n        -l x.x.0.0      Use this range instead of 169.254"
//usage:     "\n        -v              Verbose"
//usage:     "\n"
//usage:     "\n$LOGGING=none           Suppress logging"
//usage:     "\n$LOGGING=syslog         Log to syslog"
//usage:     "\n"
//usage:     "\nWith no -q, runs continuously monitoring for ARP conflicts,"
//usage:     "\nexits only on I/O errors (link down etc)"

#include "libbb.h"
#include "common_bufsiz.h"
#include <netinet/ether.h>
#include <net/if.h>
#include <net/if_arp.h>
#include <linux/sockios.h>

#include <syslog.h>

/* We don't need more than 32 bits of the counter */
#define MONOTONIC_US() ((unsigned)monotonic_us())

struct arp_packet {
        struct ether_header eth;
        struct ether_arp arp;
} PACKED;

enum {
        /* 0-1 seconds before sending 1st probe */
        PROBE_WAIT = 1,
        /* 1-2 seconds between probes */
        PROBE_MIN = 1,
        PROBE_MAX = 2,
        PROBE_NUM = 3,          /* total probes to send */
        ANNOUNCE_INTERVAL = 2,  /* 2 seconds between announces */
        ANNOUNCE_NUM = 3,       /* announces to send */
        /* if probe/announce sees a conflict, multiply RANDOM(NUM_CONFLICT) by... */
        CONFLICT_MULTIPLIER = 2,
        /* if we monitor and see a conflict, how long is defend state? */
        DEFEND_INTERVAL = 10,
};

/* States during the configuration process. */
enum {
        PROBE = 0,
        ANNOUNCE,
        MONITOR,
        DEFEND
};

#define VDBG(...) do { } while (0)


enum {
        sock_fd = 3
};

struct globals {
        struct sockaddr iface_sockaddr;
        struct ether_addr our_ethaddr;
        uint32_t localnet_ip;
} FIX_ALIASING;
#define G (*(struct globals*)bb_common_bufsiz1)
#define INIT_G() do { setup_common_bufsiz(); } while (0)


/**
 * Pick a random link local IP address on 169.254/16, except that
 * the first and last 256 addresses are reserved.
 */
static uint32_t pick_nip(void)
{
        unsigned tmp;

        do {
                tmp = rand() & IN_CLASSB_HOST;
        } while (tmp > (IN_CLASSB_HOST - 0x0200));
        return htonl((G.localnet_ip + 0x0100) + tmp);
}

static const char *nip_to_a(uint32_t nip)
{
        struct in_addr in;
        in.s_addr = nip;
        return inet_ntoa(in);
}

/**
 * Broadcast an ARP packet.
 */
static void send_arp_request(
        /* int op, - always ARPOP_REQUEST */
        /* const struct ether_addr *source_eth, - always &G.our_ethaddr */
                                        uint32_t source_nip,
        const struct ether_addr *target_eth, uint32_t target_nip)
{
        enum { op = ARPOP_REQUEST };
#define source_eth (&G.our_ethaddr)

        struct arp_packet p;
        memset(&p, 0, sizeof(p));

        // ether header
        p.eth.ether_type = htons(ETHERTYPE_ARP);
        memcpy(p.eth.ether_shost, source_eth, ETH_ALEN);
        memset(p.eth.ether_dhost, 0xff, ETH_ALEN);

        // arp request
        p.arp.arp_hrd = htons(ARPHRD_ETHER);
        p.arp.arp_pro = htons(ETHERTYPE_IP);
        p.arp.arp_hln = ETH_ALEN;
        p.arp.arp_pln = 4;
        p.arp.arp_op = htons(op);
        memcpy(&p.arp.arp_sha, source_eth, ETH_ALEN);
        memcpy(&p.arp.arp_spa, &source_nip, 4);
        memcpy(&p.arp.arp_tha, target_eth, ETH_ALEN);
        memcpy(&p.arp.arp_tpa, &target_nip, 4);

        // send it
        // Even though sock_fd is already bound to G.iface_sockaddr, just send()
        // won't work, because "socket is not connected"
        // (and connect() won't fix that, "operation not supported").
        // Thus we sendto() to G.iface_sockaddr. I wonder which sockaddr
        // (from bind() or from sendto()?) kernel actually uses
        // to determine iface to emit the packet from...
        xsendto(sock_fd, &p, sizeof(p), &G.iface_sockaddr, sizeof(G.iface_sockaddr));
#undef source_eth
}

/**
 * Run a script.
 * argv[0]:intf argv[1]:script_name argv[2]:junk argv[3]:NULL
 */
static int run(char *argv[3], const char *param, uint32_t nip)
{
        int status;
        const char *addr = addr; /* for gcc */
        const char *fmt = "%s %s %s" + 3;

        argv[2] = (char*)param;

        VDBG("%s run %s %s\n", argv[0], argv[1], argv[2]);

        if (nip != 0) {
                addr = nip_to_a(nip);
                xsetenv("ip", addr);
                fmt -= 3;
        }
        bb_error_msg(fmt, argv[2], argv[0], addr);

        status = spawn_and_wait(argv + 1);
        if (status < 0) {
                bb_perror_msg("%s %s %s" + 3, argv[2], argv[0]);
                return -errno;
        }
        if (status != 0)
                bb_error_msg("script %s %s failed, exitcode=%d", argv[1], argv[2], status & 0xff);
        return status;
}

/**
 * Return milliseconds of random delay, up to "secs" seconds.
 */
static ALWAYS_INLINE unsigned random_delay_ms(unsigned secs)
{
        return (unsigned)rand() % (secs * 1000);
}

/**
 * main program
 */
int zcip_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int zcip_main(int argc UNUSED_PARAM, char **argv)
{
        char *r_opt;
        const char *l_opt = "169.254.0.0";
        int state;
        int nsent;
        unsigned opts;

        // Ugly trick, but I want these zeroed in one go
        struct {
                const struct ether_addr null_ethaddr;
                struct ifreq ifr;
                uint32_t chosen_nip;
                int conflicts;
                int timeout_ms; // must be signed
                int verbose;
        } L;
#define null_ethaddr (L.null_ethaddr)
#define ifr          (L.ifr         )
#define chosen_nip   (L.chosen_nip  )
#define conflicts    (L.conflicts   )
#define timeout_ms   (L.timeout_ms  )
#define verbose      (L.verbose     )

        memset(&L, 0, sizeof(L));
        INIT_G();

#define FOREGROUND (opts & 1)
#define QUIT       (opts & 2)
        // Parse commandline: prog [options] ifname script
        // exactly 2 args; -v accumulates and implies -f
        opt_complementary = "=2:vv:vf";
        opts = getopt32(argv, "fqr:l:v", &r_opt, &l_opt, &verbose);
#if !BB_MMU
        // on NOMMU reexec early (or else we will rerun things twice)
        if (!FOREGROUND)
                bb_daemonize_or_rexec(0 /*was: DAEMON_CHDIR_ROOT*/, argv);
#endif
        // Open an ARP socket
        // (need to do it before openlog to prevent openlog from taking
        // fd 3 (sock_fd==3))
        xmove_fd(xsocket(AF_PACKET, SOCK_PACKET, htons(ETH_P_ARP)), sock_fd);
        if (!FOREGROUND) {
                // do it before all bb_xx_msg calls
                openlog(applet_name, 0, LOG_DAEMON);
                logmode |= LOGMODE_SYSLOG;
        }
        bb_logenv_override();

        { // -l n.n.n.n
                struct in_addr net;
                if (inet_aton(l_opt, &net) == 0
                 || (net.s_addr & htonl(IN_CLASSB_NET)) != net.s_addr
                ) {
                        bb_error_msg_and_die("invalid network address");
                }
                G.localnet_ip = ntohl(net.s_addr);
        }
        if (opts & 4) { // -r n.n.n.n
                struct in_addr ip;
                if (inet_aton(r_opt, &ip) == 0
                 || (ntohl(ip.s_addr) & IN_CLASSB_NET) != G.localnet_ip
                ) {
                        bb_error_msg_and_die("invalid link address");
                }
                chosen_nip = ip.s_addr;
        }
        argv += optind - 1;

        /* Now: argv[0]:junk argv[1]:intf argv[2]:script argv[3]:NULL */
        /* We need to make space for script argument: */
        argv[0] = argv[1];
        argv[1] = argv[2];
        /* Now: argv[0]:intf argv[1]:script argv[2]:junk argv[3]:NULL */
#define argv_intf (argv[0])

        xsetenv("interface", argv_intf);

        // Initialize the interface (modprobe, ifup, etc)
        if (run(argv, "init", 0))
                return EXIT_FAILURE;

        // Initialize G.iface_sockaddr
        // G.iface_sockaddr is: { u16 sa_family; u8 sa_data[14]; }
        //memset(&G.iface_sockaddr, 0, sizeof(G.iface_sockaddr));
        //TODO: are we leaving sa_family == 0 (AF_UNSPEC)?!
        safe_strncpy(G.iface_sockaddr.sa_data, argv_intf, sizeof(G.iface_sockaddr.sa_data));

        // Bind to the interface's ARP socket
        xbind(sock_fd, &G.iface_sockaddr, sizeof(G.iface_sockaddr));

        // Get the interface's ethernet address
        //memset(&ifr, 0, sizeof(ifr));
        strncpy_IFNAMSIZ(ifr.ifr_name, argv_intf);
        xioctl(sock_fd, SIOCGIFHWADDR, &ifr);
        memcpy(&G.our_ethaddr, &ifr.ifr_hwaddr.sa_data, ETH_ALEN);

        // Start with some stable ip address, either a function of
        // the hardware address or else the last address we used.
        // we are taking low-order four bytes, as top-order ones
        // aren't random enough.
        // NOTE: the sequence of addresses we try changes only
        // depending on when we detect conflicts.
        {
                uint32_t t;
                move_from_unaligned32(t, ((char *)&G.our_ethaddr + 2));
                srand(t);
        }
        // FIXME cases to handle:
        //  - zcip already running!
        //  - link already has local address... just defend/update

        // Daemonize now; don't delay system startup
        if (!FOREGROUND) {
#if BB_MMU
                bb_daemonize(0 /*was: DAEMON_CHDIR_ROOT*/);
#endif
                bb_error_msg("start, interface %s", argv_intf);
        }

        // Run the dynamic address negotiation protocol,
        // restarting after address conflicts:
        //  - start with some address we want to try
        //  - short random delay
        //  - arp probes to see if another host uses it
        //    00:04:e2:64:23:c2 > ff:ff:ff:ff:ff:ff arp who-has 169.254.194.171 tell 0.0.0.0
        //  - arp announcements that we're claiming it
        //    00:04:e2:64:23:c2 > ff:ff:ff:ff:ff:ff arp who-has 169.254.194.171 (00:04:e2:64:23:c2) tell 169.254.194.171
        //  - use it
        //  - defend it, within limits
        // exit if:
        // - address is successfully obtained and -q was given:
        //   run "<script> config", then exit with exitcode 0
        // - poll error (when does this happen?)
        // - read error (when does this happen?)
        // - sendto error (in send_arp_request()) (when does this happen?)
        // - revents & POLLERR (link down). run "<script> deconfig" first
        if (chosen_nip == 0) {
 new_nip_and_PROBE:
                chosen_nip = pick_nip();
        }
        nsent = 0;
        state = PROBE;
        while (1) {
                struct pollfd fds[1];
                unsigned deadline_us = deadline_us;
                struct arp_packet p;
                int ip_conflict;
                int n;

                fds[0].fd = sock_fd;
                fds[0].events = POLLIN;
                fds[0].revents = 0;

                // Poll, being ready to adjust current timeout
                if (!timeout_ms) {
                        timeout_ms = random_delay_ms(PROBE_WAIT);
                        // FIXME setsockopt(sock_fd, SO_ATTACH_FILTER, ...) to
                        // make the kernel filter out all packets except
                        // ones we'd care about.
                }
                if (timeout_ms >= 0) {
                        // Set deadline_us to the point in time when we timeout
                        deadline_us = MONOTONIC_US() + timeout_ms * 1000;
                }

                VDBG("...wait %d %s nsent=%u\n",
                                timeout_ms, argv_intf, nsent);

                n = safe_poll(fds, 1, timeout_ms);
                if (n < 0) {
                        //bb_perror_msg("poll"); - done in safe_poll
                        return EXIT_FAILURE;
                }
                if (n == 0) { // timed out?
                        VDBG("state:%d\n", state);
                        switch (state) {
                        case PROBE:
                                // No conflicting ARP packets were seen:
                                // we can progress through the states
                                if (nsent < PROBE_NUM) {
                                        nsent++;
                                        VDBG("probe/%u %s@%s\n",
                                                        nsent, argv_intf, nip_to_a(chosen_nip));
                                        timeout_ms = PROBE_MIN * 1000;
                                        timeout_ms += random_delay_ms(PROBE_MAX - PROBE_MIN);
                                        send_arp_request(0, &null_ethaddr, chosen_nip);
                                        continue;
                                }
                                // Switch to announce state
                                nsent = 0;
                                state = ANNOUNCE;
                                goto send_announce;
                        case ANNOUNCE:
                                // No conflicting ARP packets were seen:
                                // we can progress through the states
                                if (nsent < ANNOUNCE_NUM) {
 send_announce:
                                        nsent++;
                                        VDBG("announce/%u %s@%s\n",
                                                        nsent, argv_intf, nip_to_a(chosen_nip));
                                        timeout_ms = ANNOUNCE_INTERVAL * 1000;
                                        send_arp_request(chosen_nip, &G.our_ethaddr, chosen_nip);
                                        continue;
                                }
                                // Switch to monitor state
                                // FIXME update filters
                                run(argv, "config", chosen_nip);
                                // NOTE: all other exit paths should deconfig...
                                if (QUIT)
                                        return EXIT_SUCCESS;
                                // fall through: switch to MONITOR
                        default:
                        // case DEFEND:
                        // case MONITOR: (shouldn't happen, MONITOR timeout is infinite)
                                // Defend period ended with no ARP replies - we won
                                timeout_ms = -1; // never timeout in monitor state
                                state = MONITOR;
                                continue;
                        }
                }

                // Packet arrived, or link went down.
                // We need to adjust the timeout in case we didn't receive
                // a conflicting packet.
                if (timeout_ms > 0) {
                        unsigned diff = deadline_us - MONOTONIC_US();
                        if ((int)(diff) < 0) {
                                // Current time is greater than the expected timeout time.
                                diff = 0;
                        }
                        VDBG("adjusting timeout\n");
                        timeout_ms = (diff / 1000) | 1; // never 0
                }

                if ((fds[0].revents & POLLIN) == 0) {
                        if (fds[0].revents & POLLERR) {
                                // FIXME: links routinely go down;
                                // this shouldn't necessarily exit.
                                bb_error_msg("iface %s is down", argv_intf);
                                if (state >= MONITOR) {
                                        // Only if we are in MONITOR or DEFEND
                                        run(argv, "deconfig", chosen_nip);
                                }
                                return EXIT_FAILURE;
                        }
                        continue;
                }

                // Read ARP packet
                if (safe_read(sock_fd, &p, sizeof(p)) < 0) {
                        bb_perror_msg_and_die(bb_msg_read_error);
                }

                if (p.eth.ether_type != htons(ETHERTYPE_ARP))
                        continue;
                if (p.arp.arp_op != htons(ARPOP_REQUEST)
                 && p.arp.arp_op != htons(ARPOP_REPLY)
                ) {
                        continue;
                }
#ifdef DEBUG
                {
                        struct ether_addr *sha = (struct ether_addr *) p.arp.arp_sha;
                        struct ether_addr *tha = (struct ether_addr *) p.arp.arp_tha;
                        struct in_addr *spa = (struct in_addr *) p.arp.arp_spa;
                        struct in_addr *tpa = (struct in_addr *) p.arp.arp_tpa;
                        VDBG("source=%s %s\n", ether_ntoa(sha), inet_ntoa(*spa));
                        VDBG("target=%s %s\n", ether_ntoa(tha), inet_ntoa(*tpa));
                }
#endif
                ip_conflict = 0;
                if (memcmp(&p.arp.arp_sha, &G.our_ethaddr, ETH_ALEN) != 0) {
                        if (memcmp(p.arp.arp_spa, &chosen_nip, 4) == 0) {
                                // A probe or reply with source_ip == chosen ip
                                ip_conflict = 1;
                        }
                        if (p.arp.arp_op == htons(ARPOP_REQUEST)
                         && memcmp(p.arp.arp_spa, &const_int_0, 4) == 0
                         && memcmp(p.arp.arp_tpa, &chosen_nip, 4) == 0
                        ) {
                                // A probe with source_ip == 0.0.0.0, target_ip == chosen ip:
                                // another host trying to claim this ip!
                                ip_conflict |= 2;
                        }
                }
                VDBG("state:%d ip_conflict:%d\n", state, ip_conflict);
                if (!ip_conflict)
                        continue;

                // Either src or target IP conflict exists
                if (state <= ANNOUNCE) {
                        // PROBE or ANNOUNCE
                        conflicts++;
                        timeout_ms = PROBE_MIN * 1000
                                + CONFLICT_MULTIPLIER * random_delay_ms(conflicts);
                        goto new_nip_and_PROBE;
                }

                // MONITOR or DEFEND: only src IP conflict is a problem
                if (ip_conflict & 1) {
                        if (state == MONITOR) {
                                // Src IP conflict, defend with a single ARP probe
                                VDBG("monitor conflict - defending\n");
                                timeout_ms = DEFEND_INTERVAL * 1000;
                                state = DEFEND;
                                send_arp_request(chosen_nip, &G.our_ethaddr, chosen_nip);
                                continue;
                        }
                        // state == DEFEND
                        // Another src IP conflict, start over
                        VDBG("defend conflict - starting over\n");
                        run(argv, "deconfig", chosen_nip);
                        conflicts = 0;
                        timeout_ms = 0;
                        goto new_nip_and_PROBE;
                }
                // Note: if we only have a target IP conflict here (ip_conflict & 2),
                // IOW: if we just saw this sort of ARP packet:
                //  aa:bb:cc:dd:ee:ff > xx:xx:xx:xx:xx:xx arp who-has <chosen_nip> tell 0.0.0.0
                // we expect _kernel_ to respond to that, because <chosen_nip>
                // is (expected to be) configured on this iface.
        } // while (1)
#undef argv_intf
}