/* vi: set sw=4 ts=4: */
/*
 * udhcp client
 * Russ Dill <Russ.Dill@asu.edu> July 2001
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
 */
//applet:IF_UDHCPC(APPLET(udhcpc, BB_DIR_SBIN, BB_SUID_DROP))

//kbuild:lib-$(CONFIG_UDHCPC) += common.o packet.o signalpipe.o socket.o
//kbuild:lib-$(CONFIG_UDHCPC) += dhcpc.o
//kbuild:lib-$(CONFIG_FEATURE_UDHCPC_ARPING) += arpping.o
//kbuild:lib-$(CONFIG_FEATURE_UDHCP_RFC3397) += domain_codec.o

#include <syslog.h>
/* Override ENABLE_FEATURE_PIDFILE - ifupdown needs our pidfile to always exist */
#define WANT_PIDFILE 1
#include "common.h"
#include "dhcpd.h"
#include "dhcpc.h"

#include <netinet/if_ether.h>
#include <linux/filter.h>
#include <linux/if_packet.h>

#ifndef PACKET_AUXDATA
# define PACKET_AUXDATA 8
struct tpacket_auxdata {
        uint32_t tp_status;
        uint32_t tp_len;
        uint32_t tp_snaplen;
        uint16_t tp_mac;
        uint16_t tp_net;
        uint16_t tp_vlan_tci;
        uint16_t tp_padding;
};
#endif


/* "struct client_data_t client_data" is in bb_common_bufsiz1 */


#if ENABLE_LONG_OPTS
static const char udhcpc_longopts[] ALIGN1 =
        "clientid-none\0"  No_argument       "C"
        "vendorclass\0"    Required_argument "V" //deprecated
        "fqdn\0"           Required_argument "F"
        "interface\0"      Required_argument "i"
        "now\0"            No_argument       "n"
        "pidfile\0"        Required_argument "p"
        "quit\0"           No_argument       "q"
        "release\0"        No_argument       "R"
        "request\0"        Required_argument "r"
        "script\0"         Required_argument "s"
        "timeout\0"        Required_argument "T"
        "retries\0"        Required_argument "t"
        "tryagain\0"       Required_argument "A"
        "syslog\0"         No_argument       "S"
        "request-option\0" Required_argument "O"
        "no-default-options\0" No_argument   "o"
        "foreground\0"     No_argument       "f"
        USE_FOR_MMU(
        "background\0"     No_argument       "b"
        )
        "broadcast\0"      No_argument       "B"
        IF_FEATURE_UDHCPC_ARPING("arping\0"     Optional_argument "a")
        IF_FEATURE_UDHCP_PORT("client-port\0"   Required_argument "P")
        ;
#endif
/* Must match getopt32 option string order */
enum {
        OPT_C = 1 << 0,
        OPT_V = 1 << 1,
        OPT_F = 1 << 2,
        OPT_i = 1 << 3,
        OPT_n = 1 << 4,
        OPT_p = 1 << 5,
        OPT_q = 1 << 6,
        OPT_R = 1 << 7,
        OPT_r = 1 << 8,
        OPT_s = 1 << 9,
        OPT_T = 1 << 10,
        OPT_t = 1 << 11,
        OPT_S = 1 << 12,
        OPT_A = 1 << 13,
        OPT_O = 1 << 14,
        OPT_o = 1 << 15,
        OPT_x = 1 << 16,
        OPT_f = 1 << 17,
        OPT_B = 1 << 18,
/* The rest has variable bit positions, need to be clever */
        OPTBIT_B = 18,
        USE_FOR_MMU(             OPTBIT_b,)
        IF_FEATURE_UDHCPC_ARPING(OPTBIT_a,)
        IF_FEATURE_UDHCP_PORT(   OPTBIT_P,)
        USE_FOR_MMU(             OPT_b = 1 << OPTBIT_b,)
        IF_FEATURE_UDHCPC_ARPING(OPT_a = 1 << OPTBIT_a,)
        IF_FEATURE_UDHCP_PORT(   OPT_P = 1 << OPTBIT_P,)
};


/*** Script execution code ***/
struct dhcp_optitem {
        unsigned len;
        uint8_t code;
        uint8_t malloced;
        uint8_t *data;
        char *env;
};

/* get a rough idea of how long an option will be (rounding up...) */
static const uint8_t len_of_option_as_string[] ALIGN1 = {
        [OPTION_IP              ] = sizeof("255.255.255.255 "),
        [OPTION_IP_PAIR         ] = sizeof("255.255.255.255 ") * 2,
        [OPTION_STATIC_ROUTES   ] = sizeof("255.255.255.255/32 255.255.255.255 "),
        [OPTION_6RD             ] = sizeof("132 128 ffff:ffff:ffff:ffff:ffff:ffff:ffff:ffff 255.255.255.255 "),
        [OPTION_STRING          ] = 1,
        [OPTION_STRING_HOST     ] = 1,
#if ENABLE_FEATURE_UDHCP_RFC3397
        [OPTION_DNS_STRING      ] = 1, /* unused */
        /* Hmmm, this severely overestimates size if SIP_SERVERS option
         * is in domain name form: N-byte option in binary form
         * mallocs ~16*N bytes. But it is freed almost at once.
         */
        [OPTION_SIP_SERVERS     ] = sizeof("255.255.255.255 "),
#endif
//      [OPTION_BOOLEAN         ] = sizeof("yes "),
        [OPTION_U8              ] = sizeof("255 "),
        [OPTION_U16             ] = sizeof("65535 "),
//      [OPTION_S16             ] = sizeof("-32768 "),
        [OPTION_U32             ] = sizeof("4294967295 "),
        [OPTION_S32             ] = sizeof("-2147483684 "),
};

/* note: ip is a pointer to an IP in network order, possibly misaliged */
static int sprint_nip(char *dest, const char *pre, const uint8_t *ip)
{
        return sprintf(dest, "%s%u.%u.%u.%u", pre, ip[0], ip[1], ip[2], ip[3]);
}

/* really simple implementation, just count the bits */
static int mton(uint32_t mask)
{
        int i = 0;
        mask = ntohl(mask); /* 111110000-like bit pattern */
        while (mask) {
                i++;
                mask <<= 1;
        }
        return i;
}

#if ENABLE_FEATURE_UDHCPC_SANITIZEOPT
/* Check if a given name represents a valid DNS name */
/* See RFC1035, 2.3.1 */
/* We don't need to be particularly anal. For example, allowing _, hyphen
 * at the end, or leading and trailing dots would be ok, since it
 * can't be used for attacks. (Leading hyphen can be, if someone uses cmd "$hostname"
 * in the script: then hostname may be treated as an option)
 */
static int good_hostname(const char *name)
{
        if (*name == '-') /* Can't start with '-' */
                return 0;

        while (*name) {
                unsigned char ch = *name++;
                if (!isalnum(ch))
                        /* DNS allows only '-', but we are more permissive */
                        if (ch != '-' && ch != '_' && ch != '.')
                                return 0;
                // TODO: do we want to validate lengths against NS_MAXLABEL and NS_MAXDNAME?
        }
        return 1;
}
#else
# define good_hostname(name) 1
#endif

/* Create "opt_name=opt_value" string */
static NOINLINE char *xmalloc_optname_optval(const struct dhcp_optitem *opt_item, const struct dhcp_optflag *optflag, const char *opt_name)
{
        unsigned upper_length;
        int len, type, optlen;
        char *dest, *ret;
        uint8_t *option;

        option = opt_item->data;
        len = opt_item->len;
        type = optflag->flags & OPTION_TYPE_MASK;
        optlen = dhcp_option_lengths[type];
        upper_length = len_of_option_as_string[type]
                * ((unsigned)(len + optlen) / (unsigned)optlen);

        dest = ret = xmalloc(upper_length + strlen(opt_name) + 2);
        dest += sprintf(ret, "%s=", opt_name);

        while (len >= optlen) {
                switch (type) {
                case OPTION_IP:
                case OPTION_IP_PAIR:
                        dest += sprint_nip(dest, "", option);
                        if (type == OPTION_IP_PAIR)
                                dest += sprint_nip(dest, "/", option + 4);
                        break;
//              case OPTION_BOOLEAN:
//                      dest += sprintf(dest, *option ? "yes" : "no");
//                      break;
                case OPTION_U8:
                        dest += sprintf(dest, "%u", *option);
                        break;
//              case OPTION_S16:
                case OPTION_U16: {
                        uint16_t val_u16;
                        move_from_unaligned16(val_u16, option);
                        dest += sprintf(dest, "%u", ntohs(val_u16));
                        break;
                }
                case OPTION_S32:
                case OPTION_U32: {
                        uint32_t val_u32;
                        move_from_unaligned32(val_u32, option);
                        dest += sprintf(dest, type == OPTION_U32 ? "%lu" : "%ld", (unsigned long) ntohl(val_u32));
                        break;
                }
                /* Note: options which use 'return' instead of 'break'
                 * (for example, OPTION_STRING) skip the code which handles
                 * the case of list of options.
                 */
                case OPTION_STRING:
                case OPTION_STRING_HOST:
                        memcpy(dest, option, len);
                        dest[len] = '\0';
//TODO: it appears option 15 DHCP_DOMAIN_NAME is often abused
//by DHCP admins to contain a space-separated list of domains,
//not one domain name (presumably, to work as list of search domains,
//instead of using proper option 119 DHCP_DOMAIN_SEARCH).
//Currently, good_hostname() balks on strings containing spaces.
//Do we need to allow it? Only for DHCP_DOMAIN_NAME option?
                        if (type == OPTION_STRING_HOST && !good_hostname(dest))
                                safe_strncpy(dest, "bad", len);
                        return ret;
                case OPTION_STATIC_ROUTES: {
                        /* Option binary format:
                         * mask [one byte, 0..32]
                         * ip [big endian, 0..4 bytes depending on mask]
                         * router [big endian, 4 bytes]
                         * may be repeated
                         *
                         * We convert it to a string "IP/MASK ROUTER IP2/MASK2 ROUTER2"
                         */
                        const char *pfx = "";

                        while (len >= 1 + 4) { /* mask + 0-byte ip + router */
                                uint32_t nip;
                                uint8_t *p;
                                unsigned mask;
                                int bytes;

                                mask = *option++;
                                if (mask > 32)
                                        break;
                                len--;

                                nip = 0;
                                p = (void*) &nip;
                                bytes = (mask + 7) / 8; /* 0 -> 0, 1..8 -> 1, 9..16 -> 2 etc */
                                while (--bytes >= 0) {
                                        *p++ = *option++;
                                        len--;
                                }
                                if (len < 4)
                                        break;

                                /* print ip/mask */
                                dest += sprint_nip(dest, pfx, (void*) &nip);
                                pfx = " ";
                                dest += sprintf(dest, "/%u ", mask);
                                /* print router */
                                dest += sprint_nip(dest, "", option);
                                option += 4;
                                len -= 4;
                        }

                        return ret;
                }
                case OPTION_6RD:
                        /* Option binary format (see RFC 5969):
                         *  0                   1                   2                   3
                         *  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
                         * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                         * |  OPTION_6RD   | option-length |  IPv4MaskLen  |  6rdPrefixLen |
                         * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                         * |                           6rdPrefix                           |
                         * ...                        (16 octets)                        ...
                         * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                         * ...                   6rdBRIPv4Address(es)                    ...
                         * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                         * We convert it to a string
                         * "IPv4MaskLen 6rdPrefixLen 6rdPrefix 6rdBRIPv4Address..."
                         *
                         * Sanity check: ensure that our length is at least 22 bytes, that
                         * IPv4MaskLen <= 32,
                         * 6rdPrefixLen <= 128,
                         * 6rdPrefixLen + (32 - IPv4MaskLen) <= 128
                         * (2nd condition needs no check - it follows from 1st and 3rd).
                         * Else, return envvar with empty value ("optname=")
                         */
                        if (len >= (1 + 1 + 16 + 4)
                         && option[0] <= 32
                         && (option[1] + 32 - option[0]) <= 128
                        ) {
                                /* IPv4MaskLen */
                                dest += sprintf(dest, "%u ", *option++);
                                /* 6rdPrefixLen */
                                dest += sprintf(dest, "%u ", *option++);
                                /* 6rdPrefix */
                                dest += sprint_nip6(dest, /* "", */ option);
                                option += 16;
                                len -= 1 + 1 + 16;
                                *dest++ = ' ';
                                /* 6rdBRIPv4Address(es), use common IPv4 logic to process them */
                                type = OPTION_IP;
                                optlen = 4;
                                continue;
                        }

                        return ret;
#if ENABLE_FEATURE_UDHCP_RFC3397
                case OPTION_DNS_STRING:
                        /* unpack option into dest; use ret for prefix (i.e., "optname=") */
                        dest = dname_dec(option, len, ret);
                        if (dest) {
                                free(ret);
                                return dest;
                        }
                        /* error. return "optname=" string */
                        return ret;
                case OPTION_SIP_SERVERS:
                        /* Option binary format:
                         * type: byte
                         * type=0: domain names, dns-compressed
                         * type=1: IP addrs
                         */
                        option++;
                        len--;
                        if (option[-1] == 1) {
                                /* use common IPv4 logic to process IP addrs */
                                type = OPTION_IP;
                                optlen = 4;
                                continue;
                        }
                        if (option[-1] == 0) {
                                dest = dname_dec(option, len, ret);
                                if (dest) {
                                        free(ret);
                                        return dest;
                                }
                        }
                        return ret;
#endif
                } /* switch */

                /* If we are here, try to format any remaining data
                 * in the option as another, similarly-formatted option
                 */
                option += optlen;
                len -= optlen;
// TODO: it can be a list only if (optflag->flags & OPTION_LIST).
// Should we bail out/warn if we see multi-ip option which is
// not allowed to be such (for example, DHCP_BROADCAST)? -
                if (len < optlen /* || !(optflag->flags & OPTION_LIST) */)
                        break;
                *dest++ = ' ';
                *dest = '\0';
        } /* while */

        return ret;
}

static void optitem_unset_env_and_free(void *item)
{
        struct dhcp_optitem *opt_item = item;
        bb_unsetenv_and_free(opt_item->env);
        if (opt_item->malloced)
                free(opt_item->data);
        free(opt_item);
}

/* Used by static options (interface, siaddr, etc) */
static void putenvp(char *new_opt)
{
        struct dhcp_optitem *opt_item;

        opt_item = xzalloc(sizeof(*opt_item));
        /* opt_item->code = 0, so it won't appear in concat_option's lookup */
        /* opt_item->malloced = 0 */
        /* opt_item->data = NULL */
        opt_item->env = new_opt;
        llist_add_to(&client_data.envp, opt_item);
        log2(" %s", new_opt);
        putenv(new_opt);
}

/* Support RFC3396 Long Encoded Options */
static struct dhcp_optitem *concat_option(uint8_t *data, uint8_t len, uint8_t code)
{
        llist_t *item;
        struct dhcp_optitem *opt_item;

        /* Check if an option with the code already exists.
         * A possible optimization is to create a bitmap of all existing options in the packet,
         * and iterate over the option list only if they exist.
         * This will result in bigger code, and because dhcp packets don't have too many options it
         * shouldn't have a big impact on performance.
         */
        for (item = client_data.envp; item != NULL; item = item->link) {
                opt_item = (struct dhcp_optitem *)item->data;
                if (opt_item->code == code) {
                        /* This option was seen already, concatenate */
                        uint8_t *new_data;

                        new_data = xmalloc(len + opt_item->len);
                        memcpy(
                                mempcpy(new_data, opt_item->data, opt_item->len),
                                data, len
                        );
                        opt_item->len += len;
                        if (opt_item->malloced)
                                free(opt_item->data);
                        opt_item->malloced = 1;
                        opt_item->data = new_data;
                        return opt_item;
                }
        }

        /* This is a new option, add a new dhcp_optitem to the list */
        opt_item = xzalloc(sizeof(*opt_item));
        opt_item->code = code;
        /* opt_item->malloced = 0 */
        opt_item->data = data;
        opt_item->len = len;
        llist_add_to(&client_data.envp, opt_item);
        return opt_item;
}

static const char* get_optname(uint8_t code, const struct dhcp_optflag **dh)
{
        /* Find the option:
         * dhcp_optflags is sorted so we stop searching when dh->code >= code, which is faster
         * than iterating over the entire array.
         * Options which don't have a match in dhcp_option_strings[], e.g DHCP_REQUESTED_IP,
         * are located after the sorted array, so these entries will never be reached
         * and they'll count as unknown options.
         */
        for (*dh = dhcp_optflags; (*dh)->code && (*dh)->code < code; (*dh)++)
                continue;

        if ((*dh)->code == code)
                return nth_string(dhcp_option_strings, (*dh - dhcp_optflags));

        return NULL;
}

/* put all the parameters into the environment */
static void fill_envp(struct dhcp_packet *packet)
{
        uint8_t *optptr;
        struct dhcp_scan_state scan_state;
        char *new_opt;

        putenvp(xasprintf("interface=%s", client_data.interface));

        if (!packet)
                return;

        init_scan_state(packet, &scan_state);

        /* Iterate over the packet options.
         * Handle each option based on whether it's an unknown / known option.
         * Long options are supported in compliance with RFC 3396.
         */
        while ((optptr = udhcp_scan_options(packet, &scan_state)) != NULL) {
                const struct dhcp_optflag *dh;
                const char *opt_name;
                struct dhcp_optitem *opt_item;
                uint8_t code = optptr[OPT_CODE];
                uint8_t len = optptr[OPT_LEN];
                uint8_t *data = optptr + OPT_DATA;

                opt_item = concat_option(data, len, code);
                opt_name = get_optname(code, &dh);
                if (opt_name) {
                        new_opt = xmalloc_optname_optval(opt_item, dh, opt_name);
                        if (opt_item->code == DHCP_SUBNET && opt_item->len == 4) {
                                /* Generate extra envvar for DHCP_SUBNET, $mask */
                                uint32_t subnet;
                                move_from_unaligned32(subnet, opt_item->data);
                                putenvp(xasprintf("mask=%u", mton(subnet)));
                        }
                } else {
                        unsigned ofs;
                        new_opt = xmalloc(sizeof("optNNN=") + 1 + opt_item->len*2);
                        ofs = sprintf(new_opt, "opt%u=", opt_item->code);
                        bin2hex(new_opt + ofs, (char *)opt_item->data, opt_item->len)[0] = '\0';
                }
                log2(" %s", new_opt);
                putenv(new_opt);
                /* putenv will replace the existing environment variable in case of a duplicate.
                 * Free the previous occurrence (NULL if it's the first one).
                 */
                free(opt_item->env);
                opt_item->env = new_opt;
        }

        /* Export BOOTP fields. Fields we don't (yet?) export:
         * uint8_t op;      // always BOOTREPLY
         * uint8_t htype;   // hardware address type. 1 = 10mb ethernet
         * uint8_t hlen;    // hardware address length
         * uint8_t hops;    // used by relay agents only
         * uint32_t xid;
         * uint16_t secs;   // elapsed since client began acquisition/renewal
         * uint16_t flags;  // only one flag so far: bcast. Never set by server
         * uint32_t ciaddr; // client IP (usually == yiaddr. can it be different
         *                  // if during renew server wants to give us different IP?)
         * uint8_t chaddr[16]; // link-layer client hardware address (MAC)
         */
        /* Most important one: yiaddr as $ip */
        new_opt = xmalloc(sizeof("ip=255.255.255.255"));
        sprint_nip(new_opt, "ip=", (uint8_t *) &packet->yiaddr);
        putenvp(new_opt);

        if (packet->siaddr_nip) {
                /* IP address of next server to use in bootstrap */
                new_opt = xmalloc(sizeof("siaddr=255.255.255.255"));
                sprint_nip(new_opt, "siaddr=", (uint8_t *) &packet->siaddr_nip);
                putenvp(new_opt);
        }
        if (packet->gateway_nip) {
                /* IP address of DHCP relay agent */
                new_opt = xmalloc(sizeof("giaddr=255.255.255.255"));
                sprint_nip(new_opt, "giaddr=", (uint8_t *) &packet->gateway_nip);
                putenvp(new_opt);
        }
        if (!(scan_state.overload & FILE_FIELD) && packet->file[0]) {
                /* watch out for invalid packets */
                new_opt = xasprintf("boot_file=%."DHCP_PKT_FILE_LEN_STR"s", packet->file);
                putenvp(new_opt);
        }
        if (!(scan_state.overload & SNAME_FIELD) && packet->sname[0]) {
                /* watch out for invalid packets */
                new_opt = xasprintf("sname=%."DHCP_PKT_SNAME_LEN_STR"s", packet->sname);
                putenvp(new_opt);
        }
}

/* Call a script with env vars */
static void d4_run_script(struct dhcp_packet *packet, const char *name)
{
        char *argv[3];

        fill_envp(packet);

        /* call script */
        log1("executing %s %s", client_data.script, name);
        argv[0] = (char*) client_data.script;
        argv[1] = (char*) name;
        argv[2] = NULL;
        spawn_and_wait(argv);

        /* Free all allocated environment variables */
        llist_free(client_data.envp, optitem_unset_env_and_free);
        client_data.envp = NULL;
}

static void d4_run_script_deconfig(void)
{
        d4_run_script(NULL, "deconfig");
}

/*** Sending/receiving packets ***/

static ALWAYS_INLINE uint32_t random_xid(void)
{
        return rand();
}

/* Initialize the packet with the proper defaults */
static void init_packet(struct dhcp_packet *packet, char type)
{
        unsigned secs;

        /* Fill in: op, htype, hlen, cookie fields; message type option: */
        udhcp_init_header(packet, type);

        packet->xid = client_data.xid;

        client_data.last_secs = monotonic_sec();
        if (client_data.first_secs == 0)
                client_data.first_secs = client_data.last_secs;
        secs = client_data.last_secs - client_data.first_secs;
        packet->secs = (secs < 0xffff) ? htons(secs) : 0xffff;

        memcpy(packet->chaddr, client_data.client_mac, 6);
}

static void add_client_options(struct dhcp_packet *packet)
{
        int i, end, len;

        udhcp_add_simple_option(packet, DHCP_MAX_SIZE, htons(IP_UDP_DHCP_SIZE));

        /* Add a "param req" option with the list of options we'd like to have
         * from stubborn DHCP servers. Pull the data from the struct in common.c.
         * No bounds checking because it goes towards the head of the packet. */
        end = udhcp_end_option(packet->options);
        len = 0;
        for (i = 1; i < DHCP_END; i++) {
                if (client_data.opt_mask[i >> 3] & (1 << (i & 7))) {
                        packet->options[end + OPT_DATA + len] = i;
                        len++;
                }
        }
        if (len) {
                packet->options[end + OPT_CODE] = DHCP_PARAM_REQ;
                packet->options[end + OPT_LEN] = len;
                packet->options[end + OPT_DATA + len] = DHCP_END;
        }

        /* Request broadcast replies if we have no IP addr */
        if ((option_mask32 & OPT_B) && packet->ciaddr == 0)
                packet->flags |= htons(BROADCAST_FLAG);

        /* Add -x options if any */
        {
                struct option_set *curr = client_data.options;
                while (curr) {
                        udhcp_add_binary_option(packet, curr->data);
                        curr = curr->next;
                }
//              if (client_data.sname)
//                      strncpy((char*)packet->sname, client_data.sname, sizeof(packet->sname) - 1);
//              if (client_data.boot_file)
//                      strncpy((char*)packet->file, client_data.boot_file, sizeof(packet->file) - 1);
        }

        // This will be needed if we remove -V VENDOR_STR in favor of
        // -x vendor:VENDOR_STR
        //if (!udhcp_find_option(packet.options, DHCP_VENDOR, /*dhcpv6:*/ 0))
        //      /* not set, set the default vendor ID */
        //      ...add (DHCP_VENDOR, "udhcp "BB_VER) opt...
}

static void add_serverid_and_clientid_options(struct dhcp_packet *packet, uint32_t server)
{
        struct option_set *ci;

        udhcp_add_simple_option(packet, DHCP_SERVER_ID, server);

        /* RFC 2131 section 2:
         * If the client uses a 'client identifier' in one message,
         * it MUST use that same identifier in all subsequent messages.
         * section 3.1.6:
         * If the client used a 'client identifier' when it obtained the lease,
         * it MUST use the same 'client identifier' in the DHCPRELEASE message.
         */
        ci = udhcp_find_option(client_data.options, DHCP_CLIENT_ID, /*dhcpv6:*/ 0);
        if (ci)
                udhcp_add_binary_option(packet, ci->data);
}

/* RFC 2131
 * 4.4.4 Use of broadcast and unicast
 *
 * The DHCP client broadcasts DHCPDISCOVER, DHCPREQUEST and DHCPINFORM
 * messages, unless the client knows the address of a DHCP server.
 * The client unicasts DHCPRELEASE messages to the server. Because
 * the client is declining the use of the IP address supplied by the server,
 * the client broadcasts DHCPDECLINE messages.
 *
 * When the DHCP client knows the address of a DHCP server, in either
 * INIT or REBOOTING state, the client may use that address
 * in the DHCPDISCOVER or DHCPREQUEST rather than the IP broadcast address.
 * The client may also use unicast to send DHCPINFORM messages
 * to a known DHCP server. If the client receives no response to DHCP
 * messages sent to the IP address of a known DHCP server, the DHCP
 * client reverts to using the IP broadcast address.
 */

static int raw_bcast_from_client_data_ifindex(struct dhcp_packet *packet, uint32_t src_nip)
{
        return udhcp_send_raw_packet(packet,
                /*src*/ src_nip, CLIENT_PORT,
                /*dst*/ INADDR_BROADCAST, SERVER_PORT, MAC_BCAST_ADDR,
                client_data.ifindex);
}

static int bcast_or_ucast(struct dhcp_packet *packet, uint32_t ciaddr, uint32_t server)
{
        if (server)
                return udhcp_send_kernel_packet(packet,
                        ciaddr, CLIENT_PORT,
                        server, SERVER_PORT,
                        client_data.interface);
        return raw_bcast_from_client_data_ifindex(packet, ciaddr);
}

/* Broadcast a DHCP discover packet to the network, with an optionally requested IP */
/* NOINLINE: limit stack usage in caller */
static NOINLINE int send_discover(uint32_t requested)
{
        struct dhcp_packet packet;

        /* Fill in: op, htype, hlen, cookie, chaddr fields,
         * xid field, message type option:
         */
        init_packet(&packet, DHCPDISCOVER);

        if (requested)
                udhcp_add_simple_option(&packet, DHCP_REQUESTED_IP, requested);

        /* Add options: maxsize,
         * "param req" option according to -O, options specified with -x
         */
        add_client_options(&packet);

        bb_simple_info_msg("broadcasting discover");
        return raw_bcast_from_client_data_ifindex(&packet, INADDR_ANY);
}

/* Broadcast a DHCP request message */
/* RFC 2131 3.1 paragraph 3:
 * "The client _broadcasts_ a DHCPREQUEST message..."
 */
/* NOINLINE: limit stack usage in caller */
static NOINLINE int send_select(uint32_t server, uint32_t requested)
{
        struct dhcp_packet packet;
        struct in_addr temp_addr;
        char server_str[sizeof("255.255.255.255")];

/*
 * RFC 2131 4.3.2 DHCPREQUEST message
 * ...
 * If the DHCPREQUEST message contains a 'server identifier'
 * option, the message is in response to a DHCPOFFER message.
 * Otherwise, the message is a request to verify or extend an
 * existing lease. If the client uses a 'client identifier'
 * in a DHCPREQUEST message, it MUST use that same 'client identifier'
 * in all subsequent messages. If the client included a list
 * of requested parameters in a DHCPDISCOVER message, it MUST
 * include that list in all subsequent messages.
 */
        /* Fill in: op, htype, hlen, cookie, chaddr fields,
         * xid field, message type option:
         */
        init_packet(&packet, DHCPREQUEST);

        udhcp_add_simple_option(&packet, DHCP_REQUESTED_IP, requested);

        udhcp_add_simple_option(&packet, DHCP_SERVER_ID, server);

        /* Add options: maxsize,
         * "param req" option according to -O, options specified with -x
         */
        add_client_options(&packet);

        temp_addr.s_addr = server;
        strcpy(server_str, inet_ntoa(temp_addr));
        temp_addr.s_addr = requested;
        bb_info_msg("broadcasting select for %s, server %s",
                        inet_ntoa(temp_addr),
                        server_str
        );
        return raw_bcast_from_client_data_ifindex(&packet, INADDR_ANY);
}

/* Unicast or broadcast a DHCP renew message */
/* NOINLINE: limit stack usage in caller */
static NOINLINE int send_renew(uint32_t server, uint32_t ciaddr)
{
        struct dhcp_packet packet;

/*
 * RFC 2131 4.3.2 DHCPREQUEST message
 * ...
 * DHCPREQUEST generated during RENEWING state:
 *
 * 'server identifier' MUST NOT be filled in, 'requested IP address'
 * option MUST NOT be filled in, 'ciaddr' MUST be filled in with
 * client's IP address. In this situation, the client is completely
 * configured, and is trying to extend its lease. This message will
 * be unicast, so no relay agents will be involved in its
 * transmission.  Because 'giaddr' is therefore not filled in, the
 * DHCP server will trust the value in 'ciaddr', and use it when
 * replying to the client.
 */
        /* Fill in: op, htype, hlen, cookie, chaddr fields,
         * xid field, message type option:
         */
        init_packet(&packet, DHCPREQUEST);

        packet.ciaddr = ciaddr;

        /* Add options: maxsize,
         * "param req" option according to -O, options specified with -x
         */
        add_client_options(&packet);

        if (server) {
                struct in_addr temp_addr;
                temp_addr.s_addr = server;
                bb_info_msg("sending renew to server %s", inet_ntoa(temp_addr));
        } else {
                bb_simple_info_msg("broadcasting renew");
        }

        return bcast_or_ucast(&packet, ciaddr, server);
}

#if ENABLE_FEATURE_UDHCPC_ARPING
/* Broadcast a DHCP decline message */
/* NOINLINE: limit stack usage in caller */
static NOINLINE int send_decline(uint32_t server, uint32_t requested)
{
        struct dhcp_packet packet;

        /* Fill in: op, htype, hlen, cookie, chaddr, random xid fields,
         * message type option:
         */
        init_packet(&packet, DHCPDECLINE);

        /* DHCPDECLINE uses "requested ip", not ciaddr, to store offered IP */
        udhcp_add_simple_option(&packet, DHCP_REQUESTED_IP, requested);

        add_serverid_and_clientid_options(&packet, server);

        bb_simple_info_msg("broadcasting decline");
        return raw_bcast_from_client_data_ifindex(&packet, INADDR_ANY);
}
#endif

/* Unicast a DHCP release message */
static
ALWAYS_INLINE /* one caller, help compiler to use this fact */
int send_release(uint32_t server, uint32_t ciaddr)
{
        struct dhcp_packet packet;

        /* Fill in: op, htype, hlen, cookie, chaddr, random xid fields,
         * message type option:
         */
        init_packet(&packet, DHCPRELEASE);

        /* DHCPRELEASE uses ciaddr, not "requested ip", to store IP being released */
        packet.ciaddr = ciaddr;

        add_serverid_and_clientid_options(&packet, server);

        bb_info_msg("sending %s", "release");
        /* Note: normally we unicast here since "server" is not zero.
         * However, there _are_ people who run "address-less" DHCP servers,
         * and reportedly ISC dhcp client and Windows allow that.
         */
        return bcast_or_ucast(&packet, ciaddr, server);
}

/* Returns -1 on errors that are fatal for the socket, -2 for those that aren't */
/* NOINLINE: limit stack usage in caller */
static NOINLINE int d4_recv_raw_packet(struct dhcp_packet *dhcp_pkt, int fd)
{
        int bytes;
        struct ip_udp_dhcp_packet packet;
        uint16_t check;
        unsigned char cmsgbuf[CMSG_LEN(sizeof(struct tpacket_auxdata))];
        struct iovec iov;
        struct msghdr msg;
        struct cmsghdr *cmsg;

        /* used to use just safe_read(fd, &packet, sizeof(packet))
         * but we need to check for TP_STATUS_CSUMNOTREADY :(
         */
        iov.iov_base = &packet;
        iov.iov_len = sizeof(packet);
        memset(&msg, 0, sizeof(msg));
        msg.msg_iov = &iov;
        msg.msg_iovlen = 1;
        msg.msg_control = cmsgbuf;
        msg.msg_controllen = sizeof(cmsgbuf);
        for (;;) {
                bytes = recvmsg(fd, &msg, 0);
                if (bytes < 0) {
                        if (errno == EINTR)
                                continue;
                        log1s("packet read error, ignoring");
                        /* NB: possible down interface, etc. Caller should pause. */
                        return bytes; /* returns -1 */
                }
                break;
        }

        if (bytes < (int) (sizeof(packet.ip) + sizeof(packet.udp))) {
                log1s("packet is too short, ignoring");
                return -2;
        }

        if (bytes < ntohs(packet.ip.tot_len)) {
                /* packet is bigger than sizeof(packet), we did partial read */
                log1s("oversized packet, ignoring");
                return -2;
        }

        /* ignore any extra garbage bytes */
        bytes = ntohs(packet.ip.tot_len);

        /* make sure its the right packet for us, and that it passes sanity checks */
        if (packet.ip.protocol != IPPROTO_UDP
         || packet.ip.version != IPVERSION
         || packet.ip.ihl != (sizeof(packet.ip) >> 2)
         || packet.udp.dest != htons(CLIENT_PORT)
        /* || bytes > (int) sizeof(packet) - can't happen */
         || ntohs(packet.udp.len) != (uint16_t)(bytes - sizeof(packet.ip))
        ) {
                log1s("unrelated/bogus packet, ignoring");
                return -2;
        }

        /* verify IP checksum */
        check = packet.ip.check;
        packet.ip.check = 0;
        if (check != inet_cksum(&packet.ip, sizeof(packet.ip))) {
                log1s("bad IP header checksum, ignoring");
                return -2;
        }

        for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
                if (cmsg->cmsg_level == SOL_PACKET
                 && cmsg->cmsg_type == PACKET_AUXDATA
                ) {
                        /* some VMs don't checksum UDP and TCP data
                         * they send to the same physical machine,
                         * here we detect this case:
                         */
                        struct tpacket_auxdata *aux = (void *)CMSG_DATA(cmsg);
                        if (aux->tp_status & TP_STATUS_CSUMNOTREADY)
                                goto skip_udp_sum_check;
                }
        }

        /* verify UDP checksum. IP header has to be modified for this */
        memset(&packet.ip, 0, offsetof(struct iphdr, protocol));
        /* ip.xx fields which are not memset: protocol, check, saddr, daddr */
        packet.ip.tot_len = packet.udp.len; /* yes, this is needed */
        check = packet.udp.check;
        packet.udp.check = 0;
        if (check && check != inet_cksum(&packet, bytes)) {
                log1s("packet with bad UDP checksum received, ignoring");
                return -2;
        }
 skip_udp_sum_check:

        if (packet.data.cookie != htonl(DHCP_MAGIC)) {
                log1s("packet with bad magic, ignoring");
                return -2;
        }

        log2("received %s", "a packet");
        /* log2 because more informative msg for valid packets is printed later at log1 level */
        udhcp_dump_packet(&packet.data);

        bytes -= sizeof(packet.ip) + sizeof(packet.udp);
        memcpy(dhcp_pkt, &packet.data, bytes);
        return bytes;
}


/*** Main ***/

/* Values for client_data.listen_mode */
#define LISTEN_NONE   0
#define LISTEN_KERNEL 1
#define LISTEN_RAW    2

/* Values for client_data.state */
/* initial state: (re)start DHCP negotiation */
#define INIT_SELECTING  0
/* discover was sent, DHCPOFFER reply received */
#define REQUESTING      1

/* select/renew was sent, DHCPACK reply received */
#define BOUND           2
/* half of lease passed, want to renew it by sending unicast renew requests */
#define RENEWING        3
/* renew requests were not answered, lease is almost over, send broadcast renew */
#define REBINDING       4
/* manually requested renew (SIGUSR1) */
#define RENEW_REQUESTED 5
/* release, possibly manually requested (SIGUSR2) */
#define RELEASED        6

static int udhcp_raw_socket(int ifindex)
{
        int fd;
        struct sockaddr_ll sock;

        log2("opening raw socket on ifindex %d", ifindex);

        fd = xsocket(PF_PACKET, SOCK_DGRAM, htons(ETH_P_IP));
        /* ^^^^^
         * SOCK_DGRAM: remove link-layer headers on input (SOCK_RAW keeps them)
         * ETH_P_IP: want to receive only packets with IPv4 eth type
         */

        memset(&sock, 0, sizeof(sock)); /* let's be deterministic */
        sock.sll_family = AF_PACKET;
        sock.sll_protocol = htons(ETH_P_IP);
        sock.sll_ifindex = ifindex;
        /*sock.sll_hatype = ARPHRD_???;*/
        /*sock.sll_pkttype = PACKET_???;*/
        /*sock.sll_halen = ???;*/
        /*sock.sll_addr[8] = ???;*/
        xbind(fd, (struct sockaddr *) &sock, sizeof(sock));

#if 0 /* Several users reported breakage when BPF filter is used */
        if (CLIENT_PORT == 68) {
                /* Use only if standard port is in use */
                /*
                 *      I've selected not to see LL header, so BPF doesn't see it, too.
                 *      The filter may also pass non-IP and non-ARP packets, but we do
                 *      a more complete check when receiving the message in userspace.
                 *
                 * and filter shamelessly stolen from:
                 *
                 *      http://www.flamewarmaster.de/software/dhcpclient/
                 *
                 * There are a few other interesting ideas on that page (look under
                 * "Motivation").  Use of netlink events is most interesting.  Think
                 * of various network servers listening for events and reconfiguring.
                 * That would obsolete sending HUP signals and/or make use of restarts.
                 *
                 * Copyright: 2006, 2007 Stefan Rompf <sux@loplof.de>.
                 * License: GPL v2.
                 */
                static const struct sock_filter filter_instr[] = {
                        /* load 9th byte (protocol) */
                        BPF_STMT(BPF_LD|BPF_B|BPF_ABS, 9),
                        /* jump to L1 if it is IPPROTO_UDP, else to L4 */
                        BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, IPPROTO_UDP, 0, 6),
                        /* L1: load halfword from offset 6 (flags and frag offset) */
                        BPF_STMT(BPF_LD|BPF_H|BPF_ABS, 6),
                        /* jump to L4 if any bits in frag offset field are set, else to L2 */
                        BPF_JUMP(BPF_JMP|BPF_JSET|BPF_K, 0x1fff, 4, 0),
                        /* L2: skip IP header (load index reg with header len) */
                        BPF_STMT(BPF_LDX|BPF_B|BPF_MSH, 0),
                        /* load udp destination port from halfword[header_len + 2] */
                        BPF_STMT(BPF_LD|BPF_H|BPF_IND, 2),
                        /* jump to L3 if udp dport is CLIENT_PORT, else to L4 */
                        BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, 68, 0, 1),
                        /* L3: accept packet ("accept 0x7fffffff bytes") */
                        /* Accepting 0xffffffff works too but kernel 2.6.19 is buggy */
                        BPF_STMT(BPF_RET|BPF_K, 0x7fffffff),
                        /* L4: discard packet ("accept zero bytes") */
                        BPF_STMT(BPF_RET|BPF_K, 0),
                };
                static const struct sock_fprog filter_prog = {
                        .len = sizeof(filter_instr) / sizeof(filter_instr[0]),
                        /* casting const away: */
                        .filter = (struct sock_filter *) filter_instr,
                };
                /* Ignoring error (kernel may lack support for this) */
                if (setsockopt(fd, SOL_SOCKET, SO_ATTACH_FILTER, &filter_prog,
                                sizeof(filter_prog)) >= 0)
                        log1("attached filter to raw socket fd"); // log?
        }
#endif

        if (setsockopt_1(fd, SOL_PACKET, PACKET_AUXDATA) != 0) {
                if (errno != ENOPROTOOPT)
                        log1s("can't set PACKET_AUXDATA on raw socket");
        }

        return fd;
}

static void change_listen_mode(int new_mode)
{
        log1("entering listen mode: %s",
                new_mode != LISTEN_NONE
                        ? (new_mode == LISTEN_KERNEL ? "kernel" : "raw")
                        : "none"
        );

        client_data.listen_mode = new_mode;
        if (client_data.sockfd >= 0) {
                close(client_data.sockfd);
                client_data.sockfd = -1;
        }
        if (new_mode == LISTEN_KERNEL)
                client_data.sockfd = udhcp_listen_socket(/*INADDR_ANY,*/ CLIENT_PORT, client_data.interface);
        else if (new_mode != LISTEN_NONE)
                client_data.sockfd = udhcp_raw_socket(client_data.ifindex);
        /* else LISTEN_NONE: client_data.sockfd stays closed */
}

static void perform_release(uint32_t server_id, uint32_t requested_ip)
{
        char buffer[sizeof("255.255.255.255")];
        struct in_addr temp_addr;

        change_listen_mode(LISTEN_NONE);

        /* send release packet */
        if (client_data.state == BOUND
         || client_data.state == RENEWING
         || client_data.state == REBINDING
         || client_data.state == RENEW_REQUESTED
        ) {
                temp_addr.s_addr = server_id;
                strcpy(buffer, inet_ntoa(temp_addr));
                temp_addr.s_addr = requested_ip;
                bb_info_msg("unicasting a release of %s to %s",
                                inet_ntoa(temp_addr), buffer);
                client_data.xid = random_xid(); //TODO: can omit?
                send_release(server_id, requested_ip); /* unicast */
        }
        bb_simple_info_msg("entering released state");
/*
 * We can be here on: SIGUSR2,
 * or on exit (SIGTERM) and -R "release on quit" is specified.
 * Users requested to be notified in all cases, even if not in one
 * of the states above.
 */
        d4_run_script_deconfig();
        client_data.state = RELEASED;
}

#if BB_MMU
static void client_background(void)
{
        bb_daemonize(0);
        logmode &= ~LOGMODE_STDIO;
        /* rewrite pidfile, as our pid is different now */
        write_pidfile(client_data.pidfile);
}
#endif

//usage:#if defined CONFIG_UDHCP_DEBUG && CONFIG_UDHCP_DEBUG >= 1
//usage:# define IF_UDHCP_VERBOSE(...) __VA_ARGS__
//usage:#else
//usage:# define IF_UDHCP_VERBOSE(...)
//usage:#endif
//usage:#define udhcpc_trivial_usage
//usage:       "[-fbq"IF_UDHCP_VERBOSE("v")"RB]"IF_FEATURE_UDHCPC_ARPING(" [-a[MSEC]]")" [-t N] [-T SEC] [-A SEC|-n]\n"
//usage:       "        [-i IFACE]"IF_FEATURE_UDHCP_PORT(" [-P PORT]")" [-s PROG] [-p PIDFILE]\n"
//usage:       "        [-oC] [-r IP] [-V VENDOR] [-F NAME] [-x OPT:VAL]... [-O OPT]..."
//usage:#define udhcpc_full_usage "\n"
//usage:     "\n        -i IFACE        Interface to use (default "CONFIG_UDHCPC_DEFAULT_INTERFACE")"
//usage:        IF_FEATURE_UDHCP_PORT(
//usage:     "\n        -P PORT         Use PORT (default 68)"
//usage:        )
//usage:     "\n        -s PROG         Run PROG at DHCP events (default "CONFIG_UDHCPC_DEFAULT_SCRIPT")"
//usage:     "\n        -p FILE         Create pidfile"
//usage:     "\n        -B              Request broadcast replies"
//usage:     "\n        -t N            Send up to N discover packets (default 3)"
//usage:     "\n        -T SEC          Pause between packets (default 3)"
//usage:     "\n        -A SEC          Wait if lease is not obtained (default 20)"
//usage:        USE_FOR_MMU(
//usage:     "\n        -b              Background if lease is not obtained"
//usage:        )
//usage:     "\n        -n              Exit if lease is not obtained"
//usage:     "\n        -q              Exit after obtaining lease"
//usage:     "\n        -R              Release IP on exit"
//usage:     "\n        -f              Run in foreground"
//usage:     "\n        -S              Log to syslog too"
//usage:        IF_FEATURE_UDHCPC_ARPING(
//usage:     "\n        -a[MSEC]        Validate offered address with ARP ping"
//usage:        )
//usage:     "\n        -r IP           Request this IP address"
//usage:     "\n        -o              Don't request any options (unless -O is given)"
//usage:     "\n        -O OPT          Request option OPT from server (cumulative)"
//usage:     "\n        -x OPT:VAL      Include option OPT in sent packets (cumulative)"
//usage:     "\n                        Examples of string, numeric, and hex byte opts:"
//usage:     "\n                        -x hostname:bbox - option 12"
//usage:     "\n                        -x lease:3600 - option 51 (lease time)"
//usage:     "\n                        -x 0x3d:0100BEEFC0FFEE - option 61 (client id)"
//usage:     "\n                        -x 14:'\"dumpfile\"' - option 14 (shell-quoted)"
//usage:     "\n        -F NAME         Ask server to update DNS mapping for NAME"
//usage:     "\n        -V VENDOR       Vendor identifier (default 'udhcp VERSION')"
//usage:     "\n        -C              Don't send MAC as client identifier"
//usage:        IF_UDHCP_VERBOSE(
//usage:     "\n        -v              Verbose"
//usage:        )
//usage:     "\nSignals:"
//usage:     "\n        USR1    Renew lease"
//usage:     "\n        USR2    Release lease"

int udhcpc_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int udhcpc_main(int argc UNUSED_PARAM, char **argv)
{
        uint8_t *message;
        const char *str_V, *str_F, *str_r;
        IF_FEATURE_UDHCPC_ARPING(const char *str_a = "2000";)
        IF_FEATURE_UDHCP_PORT(char *str_P;)
        uint8_t *clientid_mac_ptr;
        llist_t *list_O = NULL;
        llist_t *list_x = NULL;
        int tryagain_timeout = 20;
        int discover_timeout = 3;
        int discover_retries = 3;
        uint32_t server_id = server_id; /* for compiler */
        uint32_t requested_ip = 0;
        int packet_num;
        int timeout; /* must be signed */
        int lease_remaining; /* must be signed */
        unsigned opt;
        IF_FEATURE_UDHCPC_ARPING(unsigned arpping_ms;)
        int retval;

        setup_common_bufsiz();

        /* Default options */
        IF_FEATURE_UDHCP_PORT(SERVER_PORT = 67;)
        IF_FEATURE_UDHCP_PORT(CLIENT_PORT = 68;)
        client_data.interface = CONFIG_UDHCPC_DEFAULT_INTERFACE;
        client_data.script = CONFIG_UDHCPC_DEFAULT_SCRIPT;
        client_data.sockfd = -1;
        str_V = "udhcp "BB_VER;

        /* Make sure fd 0,1,2 are open */
        /* Set up the signal pipe on fds 3,4 - must be before openlog() */
        udhcp_sp_setup();

        /* Parse command line */
        opt = getopt32long(argv, "^"
                /* O,x: list; -T,-t,-A take numeric param */
                "CV:F:i:np:qRr:s:T:+t:+SA:+O:*ox:*fB"
                USE_FOR_MMU("b")
                IF_FEATURE_UDHCPC_ARPING("a::")
                IF_FEATURE_UDHCP_PORT("P:")
                "v"
                "\0" IF_UDHCP_VERBOSE("vv") /* -v is a counter */
                , udhcpc_longopts
                , &str_V, &str_F
                , &client_data.interface, &client_data.pidfile /* i,p */
                , &str_r /* r */
                , &client_data.script /* s */
                , &discover_timeout, &discover_retries, &tryagain_timeout /* T,t,A */
                , &list_O
                , &list_x
                IF_FEATURE_UDHCPC_ARPING(, &str_a)
                IF_FEATURE_UDHCP_PORT(, &str_P)
                IF_UDHCP_VERBOSE(, &dhcp_verbose)
        );
        if (opt & OPT_F) {
                char *p;
                unsigned len;
                /* FQDN option format: [0x51][len][flags][0][0]<fqdn> */
                len = strlen(str_F);
                p = udhcp_insert_new_option(
                                &client_data.options, DHCP_FQDN,
                                len + 3, /*dhcp6:*/ 0);
                /* Flag bits: 0000NEOS
                 * S: 1 = Client requests server to update A RR in DNS as well as PTR
                 * O: 1 = Server indicates to client that DNS has been updated regardless
                 * E: 1 = Name is in DNS format, i.e. <4>host<6>domain<3>com<0>,
                 *    not "host.domain.com". Format 0 is obsolete.
                 * N: 1 = Client requests server to not update DNS (S must be 0 then)
                 * Two [0] bytes which follow are deprecated and must be 0.
                 */
                p[OPT_DATA + 0] = 0x1;
                /*p[OPT_DATA + 1] = 0; - xzalloc did it */
                /*p[OPT_DATA + 2] = 0; */
                memcpy(p + OPT_DATA + 3, str_F, len); /* do not store NUL byte */
        }
        if (opt & OPT_r)
                if (!inet_aton(str_r, (void*)&requested_ip))
                        bb_show_usage();
#if ENABLE_FEATURE_UDHCP_PORT
        if (opt & OPT_P) {
                CLIENT_PORT = xatou16(str_P);
                SERVER_PORT = CLIENT_PORT - 1;
        }
#endif
        IF_FEATURE_UDHCPC_ARPING(arpping_ms = xatou(str_a);)
        while (list_O) {
                char *optstr = llist_pop(&list_O);
                unsigned n = bb_strtou(optstr, NULL, 0);
                if (errno || n > 254) {
                        n = udhcp_option_idx(optstr, dhcp_option_strings);
                        n = dhcp_optflags[n].code;
                }
                client_data.opt_mask[n >> 3] |= 1 << (n & 7);
        }
        if (!(opt & OPT_o)) {
                unsigned i, n;
                for (i = 0; (n = dhcp_optflags[i].code) != 0; i++) {
                        if (dhcp_optflags[i].flags & OPTION_REQ) {
                                client_data.opt_mask[n >> 3] |= 1 << (n & 7);
                        }
                }
        }
        while (list_x) {
                char *optstr = xstrdup(llist_pop(&list_x));
                udhcp_str2optset(optstr, &client_data.options,
                                dhcp_optflags, dhcp_option_strings,
                                /*dhcpv6:*/ 0
                );
                free(optstr);
        }
        if (str_V[0] != '\0') {
                char *p;
                unsigned len = strnlen(str_V, 254);
                p = udhcp_insert_new_option(
                                &client_data.options, DHCP_VENDOR,
                                len, /*dhcp6:*/ 0);
                memcpy(p + OPT_DATA, str_V, len); /* do not store NUL byte */
        }

        clientid_mac_ptr = NULL;
        if (!(opt & OPT_C) && !udhcp_find_option(client_data.options, DHCP_CLIENT_ID, /*dhcpv6:*/ 0)) {
                /* not suppressed and not set, create default client ID */
                clientid_mac_ptr = udhcp_insert_new_option(
                                &client_data.options, DHCP_CLIENT_ID,
                                1 + 6, /*dhcp6:*/ 0);
                clientid_mac_ptr[OPT_DATA] = 1; /* type: ethernet */
                clientid_mac_ptr += OPT_DATA + 1; /* skip option code, len, ethernet */
        }

        /* Not really necessary (we redo it on every iteration)
         * but allows early (before daemonization) detection
         * of bad interface name.
         */
        if (udhcp_read_interface(client_data.interface,
                        &client_data.ifindex,
                        NULL,
                        client_data.client_mac)
        ) {
                return 1;
        }

#if !BB_MMU
        /* on NOMMU reexec (i.e., background) early */
        if (!(opt & OPT_f)) {
                bb_daemonize_or_rexec(0 /* flags */, argv);
                logmode = LOGMODE_NONE;
        }
#endif
        if (opt & OPT_S) {
                openlog(applet_name, LOG_PID, LOG_DAEMON);
                logmode |= LOGMODE_SYSLOG;
        }

        /* Create pidfile */
        write_pidfile(client_data.pidfile);
        /* Goes to stdout (unless NOMMU) and possibly syslog */
        bb_simple_info_msg("started, v"BB_VER);
        /* We want random_xid to be random... */
        srand(monotonic_us());

        client_data.state = INIT_SELECTING;
        d4_run_script_deconfig();
        packet_num = 0;
        timeout = 0;
        lease_remaining = 0;

        /* Main event loop. select() waits on signal pipe and possibly
         * on sockfd.
         * "continue" statements in code below jump to the top of the loop.
         */
        for (;;) {
                struct pollfd pfds[2];
                struct dhcp_packet packet;

                //bb_error_msg("sockfd:%d, listen_mode:%d", client_data.sockfd, client_data.listen_mode);

                /* Was opening raw or udp socket here
                 * if (client_data.listen_mode != LISTEN_NONE && client_data.sockfd < 0),
                 * but on fast network renew responses return faster
                 * than we open sockets. Thus this code is moved
                 * to change_listen_mode(). Thus we open listen socket
                 * BEFORE we send renew request (see "case BOUND:"). */

                udhcp_sp_fd_set(pfds, client_data.sockfd);

                retval = 0;
                /* If we already timed out, fall through with retval = 0, else... */
                if (timeout > 0) {
                        unsigned diff;

                        if (timeout > INT_MAX/1000)
                                timeout = INT_MAX/1000;
                        log1("waiting %u seconds", timeout);
                        diff = (unsigned)monotonic_sec();
                        retval = poll(pfds, 2, timeout * 1000);
                        diff = (unsigned)monotonic_sec() - diff;
                        lease_remaining -= diff;
                        if (lease_remaining < 0)
                                lease_remaining = 0;
                        timeout -= diff;
                        if (timeout < 0)
                                timeout = 0;

                        if (retval < 0) {
                                /* EINTR? A signal was caught, don't panic */
                                if (errno == EINTR) {
                                        continue;
                                }
                                /* Else: an error occurred, panic! */
                                bb_simple_perror_msg_and_die("poll");
                        }
                }

                /* If timeout dropped to zero, time to become active:
                 * resend discover/renew/whatever
                 */
                if (retval == 0) {
                        /* When running on a bridge, the ifindex may have changed
                         * (e.g. if member interfaces were added/removed
                         * or if the status of the bridge changed).
                         * Refresh ifindex and client_mac:
                         */
                        if (udhcp_read_interface(client_data.interface,
                                        &client_data.ifindex,
                                        NULL,
                                        client_data.client_mac)
                        ) {
                                goto ret0; /* iface is gone? */
                        }
                        if (clientid_mac_ptr)
                                memcpy(clientid_mac_ptr, client_data.client_mac, 6);

                        switch (client_data.state) {
                        case INIT_SELECTING:
                                if (!discover_retries || packet_num < discover_retries) {
                                        if (packet_num == 0) {
                                                change_listen_mode(LISTEN_RAW);
                                                client_data.xid = random_xid();
                                        }
                                        /* broadcast */
                                        send_discover(requested_ip);
                                        timeout = discover_timeout;
                                        packet_num++;
                                        continue;
                                }
 leasefail:
                                change_listen_mode(LISTEN_NONE);
                                d4_run_script(NULL, "leasefail");
#if BB_MMU /* -b is not supported on NOMMU */
                                if (opt & OPT_b) { /* background if no lease */
                                        bb_simple_info_msg("no lease, forking to background");
                                        client_background();
                                        /* do not background again! */
                                        opt = ((opt & ~(OPT_b|OPT_n)) | OPT_f);
                                        /* ^^^ also disables -n (-b takes priority over -n):
                                         * ifup's default udhcpc options are -R -n,
                                         * and users want to be able to add -b
                                         * (in a config file) to make it background
                                         * _and not exit_.
                                         */
                                } else
#endif
                                if (opt & OPT_n) { /* abort if no lease */
                                        bb_simple_info_msg("no lease, failing");
                                        retval = 1;
                                        goto ret;
                                }
                                /* Wait before trying again */
                                timeout = tryagain_timeout;
                                packet_num = 0;
                                continue;
                        case REQUESTING:
                                if (packet_num < 3) {
                                        /* send broadcast select packet */
                                        send_select(server_id, requested_ip);
                                        timeout = discover_timeout;
                                        packet_num++;
                                        continue;
                                }
                                /* Timed out, go back to init state.
                                 * "discover...select...discover..." loops
                                 * were seen in the wild. Treat them similarly
                                 * to "no response to discover" case */
                                client_data.state = INIT_SELECTING;
                                goto leasefail;
                        case BOUND:
                                /* 1/2 lease passed, enter renewing state */
                                client_data.state = RENEWING;
                                client_data.first_secs = 0; /* make secs field count from 0 */
                        got_SIGUSR1:
                                log1s("entering renew state");
                                change_listen_mode(LISTEN_KERNEL);
                                /* fall right through */
                        case RENEW_REQUESTED: /* in manual (SIGUSR1) renew */
                        case RENEWING:
                                if (packet_num == 0) {
                                        /* Send an unicast renew request */
                        /* Sometimes observed to fail (EADDRNOTAVAIL) to bind
                         * a new UDP socket for sending inside send_renew.
                         * I hazard to guess existing listening socket
                         * is somehow conflicting with it, but why is it
                         * not deterministic then?! Strange.
                         * Anyway, it does recover by eventually failing through
                         * into INIT_SELECTING state.
                         */
                                        if (send_renew(server_id, requested_ip) >= 0) {
                                                timeout = discover_timeout;
                                                packet_num++;
                                                continue;
                                        }
                                        /* else: error sending.
                                         * example: ENETUNREACH seen with server
                                         * which gave us bogus server ID 1.1.1.1
                                         * which wasn't reachable (and probably did not exist).
                                         */
                                } /* else: we had sent one packet, but got no reply */
                                log1s("no response to renew");
                                if (lease_remaining > 30) {
                                        /* Some lease time remains, try to renew later */
                                        change_listen_mode(LISTEN_NONE);
                                        goto BOUND_for_half_lease;
                                }
                                /* Enter rebinding state */
                                client_data.state = REBINDING;
                                log1s("entering rebinding state");
                                /* Switch to bcast receive */
                                change_listen_mode(LISTEN_RAW);
                                packet_num = 0;
                                /* fall right through */
                        case REBINDING:
                                /* Lease is *really* about to run out,
                                 * try to find DHCP server using broadcast */
                                if (lease_remaining > 0 && packet_num < 3) {
                                        /* send a broadcast renew request */
                                        send_renew(0 /*INADDR_ANY*/, requested_ip);
                                        timeout = discover_timeout;
                                        packet_num++;
                                        continue;
                                }
                                /* Timed out, enter init state */
                                change_listen_mode(LISTEN_NONE);
                                bb_simple_info_msg("lease lost, entering init state");
                                d4_run_script_deconfig();
                                client_data.state = INIT_SELECTING;
                                client_data.first_secs = 0; /* make secs field count from 0 */
                                timeout = 0;
                                packet_num = 0;
                                continue;
                        /* case RELEASED: */
                        }
                        /* RELEASED state (when we got SIGUSR2) ends up here.
                         * (wait for SIGUSR1 to re-init, or for TERM, etc)
                         */
                        timeout = INT_MAX;
                        continue; /* back to main loop */
                } /* if poll timed out */

                /* poll() didn't timeout, something happened */

                /* Is it a signal? */
                switch (udhcp_sp_read()) {
                case SIGUSR1:
                        if (client_data.state <= REQUESTING)
                                /* Initial negotiations in progress, do not disturb */
                                break;
                        if (client_data.state == REBINDING)
                                /* Do not go back from rebind to renew state */
                                break;

                        if (lease_remaining > 30) /* if renew fails, do not go back to BOUND */
                                lease_remaining = 30;
                        client_data.first_secs = 0; /* make secs field count from 0 */
                        packet_num = 0;

                        switch (client_data.state) {
                        case BOUND:
                        case RENEWING:
                                /* Try to renew/rebind */
                                client_data.state = RENEW_REQUESTED;
                                goto got_SIGUSR1;

                        case RENEW_REQUESTED:
                                /* Two SIGUSR1 received, start things over */
                                change_listen_mode(LISTEN_NONE);
                                d4_run_script_deconfig();

                        default:
                        /* case RELEASED: */
                                /* Wake from SIGUSR2-induced deconfigured state */
                                change_listen_mode(LISTEN_NONE);
                        }
                        client_data.state = INIT_SELECTING;
                        /* Kill any timeouts, user wants this to hurry along */
                        timeout = 0;
                        continue;
                case SIGUSR2:
                        perform_release(server_id, requested_ip);
                        /* ^^^ switches to LISTEN_NONE */
                        timeout = INT_MAX;
                        continue;
                case SIGTERM:
                        bb_info_msg("received %s", "SIGTERM");
                        goto ret0;
                }

                /* Is it a packet? */
                if (!pfds[1].revents)
                        continue; /* no */

                {
                        int len;

                        /* A packet is ready, read it */
                        if (client_data.listen_mode == LISTEN_KERNEL)
                                len = udhcp_recv_kernel_packet(&packet, client_data.sockfd);
                        else
                                len = d4_recv_raw_packet(&packet, client_data.sockfd);
                        if (len == -1) {
                                /* Error is severe, reopen socket */
                                bb_error_msg("read error: "STRERROR_FMT", reopening socket" STRERROR_ERRNO);
                                sleep(discover_timeout); /* 3 seconds by default */
                                change_listen_mode(client_data.listen_mode); /* just close and reopen */
                        }
                        if (len < 0)
                                continue;
                }

                if (packet.xid != client_data.xid) {
                        log1("xid %x (our is %x)%s",
                                (unsigned)packet.xid, (unsigned)client_data.xid,
                                ", ignoring packet"
                        );
                        continue;
                }

                /* Ignore packets that aren't for us */
                if (packet.hlen != 6
                 || memcmp(packet.chaddr, client_data.client_mac, 6) != 0
                ) {
//FIXME: need to also check that last 10 bytes are zero
                        log1("chaddr does not match%s", ", ignoring packet");
                        continue;
                }

                message = udhcp_get_option(&packet, DHCP_MESSAGE_TYPE);
                if (message == NULL) {
                        log1("no message type option%s", ", ignoring packet");
                        continue;
                }

                switch (client_data.state) {
                case INIT_SELECTING:
                        /* Must be a DHCPOFFER */
                        if (*message == DHCPOFFER) {
                                struct in_addr temp_addr;
                                uint8_t *temp;

/* What exactly is server's IP? There are several values.
 * Example DHCP offer captured with tchdump:
 *
 * 10.34.25.254:67 > 10.34.25.202:68 // IP header's src
 * BOOTP fields:
 * Your-IP 10.34.25.202
 * Server-IP 10.34.32.125   // "next server" IP
 * Gateway-IP 10.34.25.254  // relay's address (if DHCP relays are in use)
 * DHCP options:
 * DHCP-Message Option 53, length 1: Offer
 * Server-ID Option 54, length 4: 10.34.255.7       // "server ID"
 * Default-Gateway Option 3, length 4: 10.34.25.254 // router
 *
 * We think that real server IP (one to use in renew/release)
 * is one in Server-ID option. But I am not 100% sure.
 * IP header's src and Gateway-IP (same in this example)
 * might work too.
 * "Next server" and router are definitely wrong ones to use, though...
 */
/* We used to ignore packets without DHCP_SERVER_ID.
 * I've got user reports from people who run "address-less" servers.
 * They either supply DHCP_SERVER_ID of 0.0.0.0 or don't supply it at all.
 * They say ISC DHCP client supports this case.
 */
                                server_id = 0;
                                temp = udhcp_get_option32(&packet, DHCP_SERVER_ID);
                                if (!temp) {
                                        bb_simple_info_msg("no server ID, using 0.0.0.0");
                                } else {
                                        /* it IS unaligned sometimes, don't "optimize" */
                                        move_from_unaligned32(server_id, temp);
                                }
                                /*xid = packet.xid; - already is */
                                temp_addr.s_addr = requested_ip = packet.yiaddr;
                                log1("received offer of %s", inet_ntoa(temp_addr));

                                /* enter requesting state */
                                client_data.state = REQUESTING;
                                timeout = 0;
                                packet_num = 0;
                        }
                        continue;
                case REQUESTING:
                case RENEWING:
                case RENEW_REQUESTED:
                case REBINDING:
                        if (*message == DHCPACK) {
                                unsigned start;
                                struct in_addr temp_addr;
                                char server_str[sizeof("255.255.255.255")];
                                uint8_t *temp;

                                change_listen_mode(LISTEN_NONE);

                                temp_addr.s_addr = server_id;
                                strcpy(server_str, inet_ntoa(temp_addr));
                                temp_addr.s_addr = packet.yiaddr;

                                lease_remaining = 60 * 60;
                                temp = udhcp_get_option32(&packet, DHCP_LEASE_TIME);
                                if (temp) {
                                        uint32_t lease;
                                        /* it IS unaligned sometimes, don't "optimize" */
                                        move_from_unaligned32(lease, temp);
                                        lease_remaining = ntohl(lease);
                                }
                                /* Log message _before_ we sanitize lease */
                                bb_info_msg("lease of %s obtained from %s, lease time %u%s",
                                        inet_ntoa(temp_addr), server_str, (unsigned)lease_remaining,
                                        temp ? "" : " (default)"
                                );
                                /* paranoia: must not be too small and not prone to overflows */
                                /* NB: 60s leases _are_ used in real world
                                 * (temporary IPs while ISP modem initializes)
                                 * do not break this case by bumping it up.
                                 */
                                if (lease_remaining < 0) /* signed overflow? */
                                        lease_remaining = INT_MAX;
                                if (lease_remaining < 30)
                                        lease_remaining = 30;
                                requested_ip = packet.yiaddr;
#if ENABLE_FEATURE_UDHCPC_ARPING
                                if (opt & OPT_a) {
/* RFC 2131 3.1 paragraph 5:
 * "The client receives the DHCPACK message with configuration
 * parameters. The client SHOULD perform a final check on the
 * parameters (e.g., ARP for allocated network address), and notes
 * the duration of the lease specified in the DHCPACK message. At this
 * point, the client is configured. If the client detects that the
 * address is already in use (e.g., through the use of ARP),
 * the client MUST send a DHCPDECLINE message to the server and restarts
 * the configuration process..." */
                                        if (!arpping(requested_ip,
                                                        NULL,
                                                        (uint32_t) 0,
                                                        client_data.client_mac,
                                                        client_data.interface,
                                                        arpping_ms)
                                        ) {
                                                bb_simple_info_msg("offered address is in use "
                                                        "(got ARP reply), declining");
                                                client_data.xid = random_xid(); //TODO: can omit?
                                                send_decline(server_id, packet.yiaddr);

                                                if (client_data.state != REQUESTING)
                                                        d4_run_script_deconfig();
                                                client_data.state = INIT_SELECTING;
                                                client_data.first_secs = 0; /* make secs field count from 0 */
                                                requested_ip = 0;
                                                timeout = tryagain_timeout;
                                                packet_num = 0;
                                                continue; /* back to main loop */
                                        }
                                }
#endif
                                /* enter bound state */
                                start = monotonic_sec();
                                d4_run_script(&packet, client_data.state == REQUESTING ? "bound" : "renew");
                                lease_remaining -= (unsigned)monotonic_sec() - start;
                                if (lease_remaining < 0)
                                        lease_remaining = 0;
                                if (opt & OPT_q) { /* quit after lease */
                                        goto ret0;
                                }
                                /* future renew failures should not exit (JM) */
                                opt &= ~OPT_n;
#if BB_MMU /* NOMMU case backgrounded earlier */
                                if (!(opt & OPT_f)) {
                                        client_background();
                                        /* do not background again! */
                                        opt = ((opt & ~OPT_b) | OPT_f);
                                }
#endif
 BOUND_for_half_lease:
                                timeout = (unsigned)lease_remaining / 2;
                                client_data.state = BOUND;
                                /* make future renew packets use different xid */
                                /* client_data.xid = random_xid(); ...but why bother? */
                                packet_num = 0;
                                continue; /* back to main loop */
                        }
                        if (*message == DHCPNAK) {
                                /* If network has more than one DHCP server,
                                 * "wrong" server can reply first, with a NAK.
                                 * Do not interpret it as a NAK from "our" server.
                                 */
                                uint32_t svid = 0; /* we treat no server id as 0.0.0.0 */
                                uint8_t *temp = udhcp_get_option32(&packet, DHCP_SERVER_ID);
                                if (temp)
                                        move_from_unaligned32(svid, temp);
                                if (svid != server_id) {
                                        log1("received DHCP NAK with wrong"
                                                " server ID%s", ", ignoring packet");
                                        continue;
                                }
                                /* return to init state */
                                change_listen_mode(LISTEN_NONE);
                                bb_info_msg("received %s", "DHCP NAK");
                                d4_run_script(&packet, "nak");
                                if (client_data.state != REQUESTING)
                                        d4_run_script_deconfig();
                                sleep(3); /* avoid excessive network traffic */
                                client_data.state = INIT_SELECTING;
                                client_data.first_secs = 0; /* make secs field count from 0 */
                                requested_ip = 0;
                                timeout = 0;
                                packet_num = 0;
                        }
                        continue;
                /* case BOUND: - ignore all packets */
                /* case RELEASED: - ignore all packets */
                }
                /* back to main loop */
        } /* for (;;) - main loop ends */

 ret0:
        if (opt & OPT_R) /* release on quit */
                perform_release(server_id, requested_ip);
        retval = 0;
 ret:
        /*if (client_data.pidfile) - remove_pidfile has its own check */
                remove_pidfile(client_data.pidfile);
        return retval;
}