/* 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.
 */

//#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        -v              Verbose"
//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 <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 {
/* 169.254.0.0 */
        LINKLOCAL_ADDR = 0xa9fe0000,

/* protocol timeout parameters, specified in seconds */
        PROBE_WAIT = 1,
        PROBE_MIN = 1,
        PROBE_MAX = 2,
        PROBE_NUM = 3,
        MAX_CONFLICTS = 10,
        RATE_LIMIT_INTERVAL = 60,
        ANNOUNCE_WAIT = 2,
        ANNOUNCE_NUM = 2,
        ANNOUNCE_INTERVAL = 2,
        DEFEND_INTERVAL = 10
};

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

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


enum {
        sock_fd = 3
};

struct globals {
        struct sockaddr saddr;
        struct ether_addr eth_addr;
} FIX_ALIASING;
#define G (*(struct globals*)&bb_common_bufsiz1)
#define saddr    (G.saddr   )
#define eth_addr (G.eth_addr)
#define INIT_G() do { } 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(void)
{
        unsigned tmp;

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

/**
 * Broadcast an ARP packet.
 */
static void arp(
        /* int op, - always ARPOP_REQUEST */
        /* const struct ether_addr *source_eth, - always &eth_addr */
                                        struct in_addr source_ip,
        const struct ether_addr *target_eth, struct in_addr target_ip)
{
        enum { op = ARPOP_REQUEST };
#define source_eth (&eth_addr)

        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_ip, sizeof(p.arp.arp_spa));
        memcpy(&p.arp.arp_tha, target_eth, ETH_ALEN);
        memcpy(&p.arp.arp_tpa, &target_ip, sizeof(p.arp.arp_tpa));

        // send it
        // Even though sock_fd is already bound to saddr, just send()
        // won't work, because "socket is not connected"
        // (and connect() won't fix that, "operation not supported").
        // Thus we sendto() to saddr. 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), &saddr, sizeof(saddr));
#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, struct in_addr *ip)
{
        int status;
        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 (ip) {
                addr = inet_ntoa(*ip);
                xsetenv("ip", addr);
                fmt -= 3;
        }
        bb_info_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 rand() % (secs * 1000);
}

/**
 * main program
 */
int zcip_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int zcip_main(int argc UNUSED_PARAM, char **argv)
{
        int state;
        char *r_opt;
        unsigned opts;

        // ugly trick, but I want these zeroed in one go
        struct {
                const struct in_addr null_ip;
                const struct ether_addr null_addr;
                struct in_addr ip;
                struct ifreq ifr;
                int timeout_ms; /* must be signed */
                unsigned conflicts;
                unsigned nprobes;
                unsigned nclaims;
                int ready;
                int verbose;
        } L;
#define null_ip    (L.null_ip   )
#define null_addr  (L.null_addr )
#define ip         (L.ip        )
#define ifr        (L.ifr       )
#define timeout_ms (L.timeout_ms)
#define conflicts  (L.conflicts )
#define nprobes    (L.nprobes   )
#define nclaims    (L.nclaims   )
#define ready      (L.ready     )
#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:v", &r_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;
        }
        if (opts & 4) { // -r n.n.n.n
                if (inet_aton(r_opt, &ip) == 0
                 || (ntohl(ip.s_addr) & IN_CLASSB_NET) != LINKLOCAL_ADDR
                ) {
                        bb_error_msg_and_die("invalid link address");
                }
        }
        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", NULL))
                return EXIT_FAILURE;

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

        // bind to the interface's ARP socket
        xbind(sock_fd, &saddr, sizeof(saddr));

        // get the interface's ethernet address
        //memset(&ifr, 0, sizeof(ifr));
        strncpy_IFNAMSIZ(ifr.ifr_name, argv_intf);
        xioctl(sock_fd, SIOCGIFHWADDR, &ifr);
        memcpy(&eth_addr, &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 *)&eth_addr + 2));
                srand(t);
        }
        if (ip.s_addr == 0)
                ip.s_addr = pick();

        // 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_info_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
        //  - arp announcements that we're claiming it
        //  - 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 arp()) (when does this happen?)
        // - revents & POLLERR (link down). run "<script> deconfig" first
        state = PROBE;
        while (1) {
                struct pollfd fds[1];
                unsigned deadline_us;
                struct arp_packet p;
                int source_ip_conflict;
                int target_ip_conflict;

                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.
                }
                // set deadline_us to the point in time when we timeout
                deadline_us = MONOTONIC_US() + timeout_ms * 1000;

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

                switch (safe_poll(fds, 1, timeout_ms)) {

                default:
                        //bb_perror_msg("poll"); - done in safe_poll
                        return EXIT_FAILURE;

                // timeout
                case 0:
                        VDBG("state = %d\n", state);
                        switch (state) {
                        case PROBE:
                                // timeouts in the PROBE state mean no conflicting ARP packets
                                // have been received, so we can progress through the states
                                if (nprobes < PROBE_NUM) {
                                        nprobes++;
                                        VDBG("probe/%u %s@%s\n",
                                                        nprobes, argv_intf, inet_ntoa(ip));
                                        arp(/* ARPOP_REQUEST, */
                                                        /* &eth_addr, */ null_ip,
                                                        &null_addr, ip);
                                        timeout_ms = PROBE_MIN * 1000;
                                        timeout_ms += random_delay_ms(PROBE_MAX - PROBE_MIN);
                                }
                                else {
                                        // Switch to announce state.
                                        state = ANNOUNCE;
                                        nclaims = 0;
                                        VDBG("announce/%u %s@%s\n",
                                                        nclaims, argv_intf, inet_ntoa(ip));
                                        arp(/* ARPOP_REQUEST, */
                                                        /* &eth_addr, */ ip,
                                                        &eth_addr, ip);
                                        timeout_ms = ANNOUNCE_INTERVAL * 1000;
                                }
                                break;
                        case RATE_LIMIT_PROBE:
                                // timeouts in the RATE_LIMIT_PROBE state mean no conflicting ARP packets
                                // have been received, so we can move immediately to the announce state
                                state = ANNOUNCE;
                                nclaims = 0;
                                VDBG("announce/%u %s@%s\n",
                                                nclaims, argv_intf, inet_ntoa(ip));
                                arp(/* ARPOP_REQUEST, */
                                                /* &eth_addr, */ ip,
                                                &eth_addr, ip);
                                timeout_ms = ANNOUNCE_INTERVAL * 1000;
                                break;
                        case ANNOUNCE:
                                // timeouts in the ANNOUNCE state mean no conflicting ARP packets
                                // have been received, so we can progress through the states
                                if (nclaims < ANNOUNCE_NUM) {
                                        nclaims++;
                                        VDBG("announce/%u %s@%s\n",
                                                        nclaims, argv_intf, inet_ntoa(ip));
                                        arp(/* ARPOP_REQUEST, */
                                                        /* &eth_addr, */ ip,
                                                        &eth_addr, ip);
                                        timeout_ms = ANNOUNCE_INTERVAL * 1000;
                                }
                                else {
                                        // Switch to monitor state.
                                        state = MONITOR;
                                        // link is ok to use earlier
                                        // FIXME update filters
                                        run(argv, "config", &ip);
                                        ready = 1;
                                        conflicts = 0;
                                        timeout_ms = -1; // Never timeout in the monitor state.

                                        // NOTE: all other exit paths
                                        // should deconfig ...
                                        if (QUIT)
                                                return EXIT_SUCCESS;
                                }
                                break;
                        case DEFEND:
                                // We won!  No ARP replies, so just go back to monitor.
                                state = MONITOR;
                                timeout_ms = -1;
                                conflicts = 0;
                                break;
                        default:
                                // Invalid, should never happen.  Restart the whole protocol.
                                state = PROBE;
                                ip.s_addr = pick();
                                timeout_ms = 0;
                                nprobes = 0;
                                nclaims = 0;
                                break;
                        } // switch (state)
                        break; // case 0 (timeout)

                // packets arriving, or link went down
                case 1:
                        // 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.
                                        // Should never happen.
                                        VDBG("missed an expected timeout\n");
                                        timeout_ms = 0;
                                } else {
                                        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 (ready) {
                                                run(argv, "deconfig", &ip);
                                        }
                                        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;
#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("%s recv arp type=%d, op=%d,\n",
                                        argv_intf, ntohs(p.eth.ether_type),
                                        ntohs(p.arp.arp_op));
                                VDBG("\tsource=%s %s\n",
                                        ether_ntoa(sha),
                                        inet_ntoa(*spa));
                                VDBG("\ttarget=%s %s\n",
                                        ether_ntoa(tha),
                                        inet_ntoa(*tpa));
                        }
#endif
                        if (p.arp.arp_op != htons(ARPOP_REQUEST)
                         && p.arp.arp_op != htons(ARPOP_REPLY))
                                continue;

                        source_ip_conflict = 0;
                        target_ip_conflict = 0;

                        if (memcmp(p.arp.arp_spa, &ip.s_addr, sizeof(struct in_addr)) == 0
                         && memcmp(&p.arp.arp_sha, &eth_addr, ETH_ALEN) != 0
                        ) {
                                source_ip_conflict = 1;
                        }
                        if (p.arp.arp_op == htons(ARPOP_REQUEST)
                         && memcmp(p.arp.arp_tpa, &ip.s_addr, sizeof(struct in_addr)) == 0
                         && memcmp(&p.arp.arp_tha, &eth_addr, ETH_ALEN) != 0
                        ) {
                                target_ip_conflict = 1;
                        }

                        VDBG("state = %d, source ip conflict = %d, target ip conflict = %d\n",
                                state, source_ip_conflict, target_ip_conflict);
                        switch (state) {
                        case PROBE:
                        case ANNOUNCE:
                                // When probing or announcing, check for source IP conflicts
                                // and other hosts doing ARP probes (target IP conflicts).
                                if (source_ip_conflict || target_ip_conflict) {
                                        conflicts++;
                                        if (conflicts >= MAX_CONFLICTS) {
                                                VDBG("%s ratelimit\n", argv_intf);
                                                timeout_ms = RATE_LIMIT_INTERVAL * 1000;
                                                state = RATE_LIMIT_PROBE;
                                        }

                                        // restart the whole protocol
                                        ip.s_addr = pick();
                                        timeout_ms = 0;
                                        nprobes = 0;
                                        nclaims = 0;
                                }
                                break;
                        case MONITOR:
                                // If a conflict, we try to defend with a single ARP probe.
                                if (source_ip_conflict) {
                                        VDBG("monitor conflict -- defending\n");
                                        state = DEFEND;
                                        timeout_ms = DEFEND_INTERVAL * 1000;
                                        arp(/* ARPOP_REQUEST, */
                                                /* &eth_addr, */ ip,
                                                &eth_addr, ip);
                                }
                                break;
                        case DEFEND:
                                // Well, we tried.  Start over (on conflict).
                                if (source_ip_conflict) {
                                        state = PROBE;
                                        VDBG("defend conflict -- starting over\n");
                                        ready = 0;
                                        run(argv, "deconfig", &ip);

                                        // restart the whole protocol
                                        ip.s_addr = pick();
                                        timeout_ms = 0;
                                        nprobes = 0;
                                        nclaims = 0;
                                }
                                break;
                        default:
                                // Invalid, should never happen.  Restart the whole protocol.
                                VDBG("invalid state -- starting over\n");
                                state = PROBE;
                                ip.s_addr = pick();
                                timeout_ms = 0;
                                nprobes = 0;
                                nclaims = 0;
                                break;
                        } // switch state
                        break; // case 1 (packets arriving)
                } // switch poll
        } // while (1)
#undef argv_intf
}