Busybox udhcpc：自動識別ip配置以及流程解析

 #
 # To config network
 #
udhcpc

BusyBox v1.27.2 (2020-08-25 12:39:58 CST) multi-call binary.

Usage: udhcpc [-fbqRB] [-a[MSEC]] [-t N] [-T SEC] [-A SEC/-n]
        [-i IFACE] [-s PROG] [-p PIDFILE]
        [-oC] [-r IP] [-V VENDOR] [-F NAME] [-x OPT:VAL]... [-O OPT]...        -i,--interface IFACE    Interface to use (default eth0)        -s,--script PROG        Run PROG at DHCP events (default /usr/share/udhcpc/default.script)        -p,--pidfile FILE       Create pidfile        -B,--broadcast          Request broadcast replies        -t,--retries N          Send up to N discover packets (default 3)        -T,--timeout SEC        Pause between packets (default 3)        -A,--tryagain SEC       Wait if lease is not obtained (default 20)        -n,--now                Exit if lease is not obtained        -q,--quit               Exit after obtaining lease        -R,--release            Release IP on exit        -f,--foreground         Run in foreground        -b,--background         Background if lease is not obtained        -S,--syslog             Log to syslog too        -a[MSEC],--arping[=MSEC] Validate offered address with ARP ping        -r,--request IP         Request this IP address        -o,--no-default-options Don't request any options (unless -O is given)
        -O,--request-option OPT Request option OPT from server (cumulative)        -x OPT:VAL              Include option OPT in sent packets (cumulative)
                                Examples of string, numeric, and hex byte opts:                                -x hostname:bbox - option 12
                                -x lease:3600 - option 51 (lease time)                                -x 0x3d:0100BEEFC0FFEE - option 61 (client id)        -F,--fqdn NAME          Ask server to update DNS mapping for NAME        -V,--vendorclass VENDOR Vendor identifier (default 'udhcp VERSION')        -C,--clientid-none      Don't send MAC as client identifierSignals:
        USR1    Renew lease
        USR2    Release lease

static int __init ip_auto_config(void)
{...    if (ic_myaddr == NONE ||...
        ic_first_dev->next) {
#ifdef IPCONFIG_DYNAMIC        if (ic_dynamic() < 0) {
            ic_close_devs();...        }#else /* !DYNAMIC */...#endif /* IPCONFIG_DYNAMIC */
    } else {...
    }...
}static int __init ic_dynamic(void)
{...#ifdef IPCONFIG_BOOTP    if (do_bootp)
        ic_bootp_init();------------------------初始化DHCP/BOOTP相關，主要是注冊處理從服務器接收到的DHCP/BOOTP回復。#endif#ifdef IPCONFIG_RARP    if (do_rarp)
        ic_rarp_init();#endif...    for (;;) {
#ifdef IPCONFIG_BOOTP        if (do_bootp && (d->able & IC_BOOTP))
            ic_bootp_send_if(d, jiffies - start_jiffies);---發(fā)送DHCP/BOOTP請求到d網(wǎng)絡設備。#endif...        if (!d->next) {
            jiff = jiffies + timeout;            while (time_before(jiffies, jiff) && !ic_got_reply)
                schedule_timeout_uninterruptible(1);
        }
#ifdef IPCONFIG_DHCP        /* DHCP isn't done until we get a DHCPACK. */
        if ((ic_got_reply & IC_BOOTP) &&
            (ic_proto_enabled & IC_USE_DHCP) &&
            ic_dhcp_msgtype != DHCPACK) {
            ic_got_reply = 0;            /* continue on device that got the reply */
            d = ic_dev;
            pr_cont(",");            continue;
        }#endif /* IPCONFIG_DHCP */...
    }...
}

/*
 *  Send DHCP/BOOTP request to single interface. */static void __init ic_bootp_send_if(struct ic_device *d, unsigned long jiffies_diff)
{    struct net_device *dev = d->dev;    struct sk_buff *skb;    struct bootp_pkt *b;    struct iphdr *h;    int hlen = LL_RESERVED_SPACE(dev);    int tlen = dev->needed_tailroom;    /* Allocate packet */
    skb = alloc_skb(sizeof(struct bootp_pkt) + hlen + tlen + 15,
            GFP_KERNEL);--------------------------分配skb，下面逐漸填充內容。    if (!skb)        return;
    skb_reserve(skb, hlen);
    b = (struct bootp_pkt *) skb_put(skb, sizeof(struct bootp_pkt));
    memset(b, 0, sizeof(struct bootp_pkt));    /* Construct IP header */
    skb_reset_network_header(skb);
    h = ip_hdr(skb);
    h->version = 4;
    h->ihl = 5;
    h->tot_len = htons(sizeof(struct bootp_pkt));
    h->frag_off = htons(IP_DF);
    h->ttl = 64;
    h->protocol = IPPROTO_UDP;
    h->daddr = htonl(INADDR_BROADCAST);
    h->check = ip_fast_csum((unsigned char *) h, h->ihl);    /* Construct UDP header */
    b->udph.source = htons(68);
    b->udph.dest = htons(67);
    b->udph.len = htons(sizeof(struct bootp_pkt) - sizeof(struct iphdr));    /* UDP checksum not calculated -- explicitly allowed in BOOTP RFC */

    /* Construct DHCP/BOOTP header */
    b->op = BOOTP_REQUEST;    if (dev->type < 256) /* check for false types */
        b->htype = dev->type;    else if (dev->type == ARPHRD_FDDI)
        b->htype = ARPHRD_ETHER;    else {
        pr_warn("Unknown ARP type 0x%04x for device %s\n", dev->type,
            dev->name);
        b->htype = dev->type; /* can cause undefined behavior */
    }    /* server_ip and your_ip address are both already zero per RFC2131 */
    b->hlen = dev->addr_len;
    memcpy(b->hw_addr, dev->dev_addr, dev->addr_len);
    b->secs = htons(jiffies_diff / HZ);
    b->xid = d->xid;    /* add DHCP options or BOOTP extensions */#ifdef IPCONFIG_DHCP    if (ic_proto_enabled & IC_USE_DHCP)
        ic_dhcp_init_options(b->exten, d);---------構建DHCP附加內容。    else#endif
        ic_bootp_init_ext(b->exten);    /* Chain packet down the line... */
    skb->dev = dev;
    skb->protocol = htons(ETH_P_IP);    if (dev_hard_header(skb, dev, ntohs(skb->protocol),
                dev->broadcast, dev->dev_addr, skb->len) < 0) {
        kfree_skb(skb);
        printk("E");        return;
    }    if (dev_queue_xmit(skb) < 0)-------------------發(fā)送skb。
        printk("E");
}

/* 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

 udhcpc_main( argc UNUSED_PARAM,  ** (udhcp_read_interface(client_config.& =...     pollfd pfds[== timeout -= 
         (tv > == poll(pfds, , tv < INT_MAX/ ? tv *  (retval < 
                 (errno ==+= (unsigned)monotonic_sec() -
         (retval ==              (udhcp_read_interface(client_config.& ret0; =  (!discover_retries || packet_num < (packet_num == ==++ BB_MMU /* -b is not supported on NOMMU */                 (opt & OPT_b) { = ((opt & ~OPT_b) |
                 (opt & OPT_n) { = ==  (packet_num < =++=== ; 
             RENEW_REQUESTED:  (timeout > >>= =
            
                 (timeout >  >>= == ; 
                = =; 

        
         (listen_mode == LISTEN_NONE || !pfds[; 
             (listen_mode === udhcp_recv_kernel_packet(&= udhcp_recv_raw_packet(& (len == -+= (unsigned)monotonic_sec() - (len <  (packet.xid !=
         (packet.hlen != 
         || memcmp(packet.chaddr, client_config.client_mac, ) != 
            log1(); 
            = udhcp_get_option(& (message ==
             (*message ==*= = udhcp_get_option(& (!=== = =  (*message ==*= udhcp_get_option(& (!=  * =
                     (lease_seconds < =  (lease_seconds >  / =  / 
                ===&packet, state == REQUESTING ?  : = (unsigned)monotonic_sec() -= lease_seconds /  ((unsigned)timeout <= already_waited_sec = = (opt & OPT_q) { 
                    &= ~ BB_MMU /* NOMMU case backgrounded earlier */                 (!(opt &= ((opt & ~OPT_b) |
                
                

                ;  (*message ==
                 (server_addr != *= udhcp_get_option(& (!
                             (svid !=, &packet,  (state !=); == ; = = = = 
         (opt & OPT_R) =

/* Call a script with a par file and env vars */static void udhcp_run_script(struct dhcp_packet *packet, const char *name)
{    char **envp, **curr;    char *argv[3];

    envp = fill_envp(packet);----------將options以及packet中的內容導出到shell環(huán)境變量中，下面執(zhí)行shell的過程中使用。    /* call script */
    log1("executing %s %s", client_config.script, name);
    argv[0] = (char*) client_config.script;
    argv[1] = (char*) name;
    argv[2] = NULL;
    spawn_and_wait(argv);    for (curr = envp; *curr; curr++) {
        log2(" %s", *curr);
        bb_unsetenv_and_free(*curr);
    }
    free(envp);
}/* put all the parameters into the environment */static char **fill_envp(struct dhcp_packet *packet)
{    int envc;    int i;    char **envp, **curr;    const char *opt_name;
    uint8_t *temp;
    uint8_t overload = 0;#define BITMAP unsigned#define BBITS (sizeof(BITMAP) * 8)#define BMASK(i) (1 << (i & (sizeof(BITMAP) * 8 - 1)))#define FOUND_OPTS(i) (found_opts[(unsigned)i / BBITS])
    BITMAP found_opts[256 / BBITS];

    memset(found_opts, 0, sizeof(found_opts));    /* We need 6 elements for:
     * "interface=IFACE"
     * "ip=N.N.N.N" from packet->yiaddr
     * "siaddr=IP" from packet->siaddr_nip (unless 0)
     * "boot_file=FILE" from packet->file (unless overloaded)
     * "sname=SERVER_HOSTNAME" from packet->sname (unless overloaded)
     * terminating NULL     */
    envc = 6;    /* +1 element for each option, +2 for subnet option: */
    if (packet) {        /* note: do not search for "pad" (0) and "end" (255) options *///TODO: change logic to scan packet _once_
        for (i = 1; i < 255; i++) {
            temp = udhcp_get_option(packet, i);            if (temp) {                if (i == DHCP_OPTION_OVERLOAD)
                    overload |= *temp;                else if (i == DHCP_SUBNET)
                    envc++; /* for $mask */
                envc++;                /*if (i != DHCP_MESSAGE_TYPE)*/
                FOUND_OPTS(i) |= BMASK(i);
            }
        }
    }
    curr = envp = xzalloc(sizeof(envp[0]) * envc);    *curr = xasprintf("interface=%s", client_config.interface);
    putenv(*curr++);    if (!packet)        return envp;    /* Most important one: yiaddr as $ip */
    *curr = xmalloc(sizeof("ip=255.255.255.255"));
    sprint_nip(*curr, "ip=", (uint8_t *) &packet->yiaddr);
    putenv(*curr++);    if (packet->siaddr_nip) {        /* IP address of next server to use in bootstrap */
        *curr = xmalloc(sizeof("siaddr=255.255.255.255"));
        sprint_nip(*curr, "siaddr=", (uint8_t *) &packet->siaddr_nip);
        putenv(*curr++);
    }    if (!(overload & FILE_FIELD) && packet->file[0]) {        /* watch out for invalid packets */
        *curr = xasprintf("boot_file=%."DHCP_PKT_FILE_LEN_STR"s", packet->file);
        putenv(*curr++);
    }    if (!(overload & SNAME_FIELD) && packet->sname[0]) {        /* watch out for invalid packets */
        *curr = xasprintf("sname=%."DHCP_PKT_SNAME_LEN_STR"s", packet->sname);
        putenv(*curr++);
    }    /* Export known DHCP options */
    opt_name = dhcp_option_strings;
    i = 0;    while (*opt_name) {
        uint8_t code = dhcp_optflags[i].code;
        BITMAP *found_ptr = &FOUND_OPTS(code);
        BITMAP found_mask = BMASK(code);        if (!(*found_ptr & found_mask))            goto next;        *found_ptr &= ~found_mask; /* leave only unknown options */
        temp = udhcp_get_option(packet, code);        *curr = xmalloc_optname_optval(temp, &dhcp_optflags[i], opt_name);---將optname和value導出到shell環(huán)境變量中。
        putenv(*curr++);        if (code == DHCP_SUBNET) {            /* Subnet option: make things like "$ip/$mask" possible */
            uint32_t subnet;
            move_from_unaligned32(subnet, temp);            *curr = xasprintf("mask=%u", mton(subnet));
            putenv(*curr++);
        }
 next:
        opt_name += strlen(opt_name) + 1;
        i++;
    }...    return envp;
}

#!/bin/sh

# udhcpc script edited by Tim Riker <Tim@Rikers.org>[ -z "$1" ] && echo "Error: should be called from udhcpc" && exit 1RESOLV_CONF="/etc/resolv.conf"[ -e $RESOLV_CONF ] || touch $RESOLV_CONF
[ -n "$broadcast" ] && BROADCAST="broadcast $broadcast"[ -n "$subnet" ] && NETMASK="netmask $subnet"case "$1" in    deconfig)        /sbin/ifconfig $interface up        /sbin/ifconfig $interface 0.0.0.0--------------------$interface是udhcpc指定的，首先up啟動設備；然后配置一個默認0.0.0.0地址。

        # drop info from this interface
        # resolv.conf may be a symlink to /tmp/, so take care
        TMPFILE=$(mktemp)
        grep -vE "# $interface\$" $RESOLV_CONF > $TMPFILE
        cat $TMPFILE > $RESOLV_CONF
        rm -f $TMPFILE        if [ -x /usr/sbin/avahi-autoipd ]; then            /usr/sbin/avahi-autoipd -k $interface
        fi
        ;;    leasefail|nak)        if [ -x /usr/sbin/avahi-autoipd ]; then            /usr/sbin/avahi-autoipd -wD $interface --no-chroot
        fi
        ;;    renew|bound)        if [ -x /usr/sbin/avahi-autoipd ]; then            /usr/sbin/avahi-autoipd -k $interface
        fi        /sbin/ifconfig $interface $ip $BROADCAST $NETMASK-----------根據(jù)udhcpc獲取的interface/ip/BROADCASR/NETMASK，通過ifconfig配置到interface的設備。        if [ -n "$router" ] ; then
            echo "deleting routers"
            while route del default gw 0.0.0.0 dev $interface 2> /dev/null; do
                :
            done            for i in $router ; do
                route add default gw $i dev $interface--------------router不為空的情況下，先刪除默認路由，重新建立路由。
            done
        fi

        # drop info from this interface
        # resolv.conf may be a symlink to /tmp/, so take care
        TMPFILE=$(mktemp)
        grep -vE "# $interface\$" $RESOLV_CONF > $TMPFILE
        cat $TMPFILE > $RESOLV_CONF
        rm -f $TMPFILE

        # prefer rfc3359 domain search list (option 119) if available        if [ -n "$search" ]; then
            search_list=$search
        elif [ -n "$domain" ]; then
            search_list=$domain
        fi

        [ -n "$search_list" ] &&
            echo "search $search_list # $interface" >> $RESOLV_CONF        for i in $dns ; do
            echo adding dns $i
            echo "nameserver $i # $interface" >> $RESOLV_CONF--------更新dns信息到/etc/resolv.conf中。
        done
        ;;
esac

HOOK_DIR="$0.d"for hook in "${HOOK_DIR}/"*; do
    [ -f "${hook}" -a -x "${hook}" ] || continue
    "${hook}" "${@}"done

exit 0

[2020-08-26 19:16:08] udhcpc: started, v1.27.2[2020-08-26 19:16:08] udhcpc: sending discover-------------------------------------發(fā)送DHCPDISCOVER到DHCP服務器，服務器會返回一個可用IP地址。[2020-08-26 19:16:10] udhcpc: sending select for 192.168.33.184--------------------發(fā)送DHCPREQUEST到DHCP服務器，是對DPCPOFFER的響應。[2020-08-26 19:16:10] udhcpc: lease of 192.168.33.184 obtained, lease time 86400---此時已經經過DHCP服務器分配IP，IP地址為192.168.33.184。然后調用default.script的bound或renew。[2020-08-26 19:16:10] deleting routers---------------------------------------------default.script中刪除路由。
[2020-08-26 19:16:10] adding dns 192.168.33.1--------------------------------------配置DNS。

   Message         Use   -------         ---

   DHCPDISCOVER -  Client broadcast to locate available servers.

   DHCPOFFER    -  Server to client in response to DHCPDISCOVER with
                   offer of configuration parameters.

   DHCPREQUEST  -  Client message to servers either (a) requesting
                   offered parameters from one server and implicitly
                   declining offers from all others, (b) confirming
                   correctness of previously allocated address after,
                   e.g., system reboot, or (c) extending the lease on a
                   particular network address.

   DHCPACK      -  Server to client with configuration parameters,
                   including committed network address.

   DHCPNAK      -  Server to client indicating client's notion of network
                   address is incorrect (e.g., client has moved to new
                   subnet) or client's lease as expired
   DHCPDECLINE  -  Client to server indicating network address is already                   in use.

   DHCPRELEASE  -  Client to server relinquishing network address and
                   cancelling remaining lease.

   DHCPINFORM   -  Client to server, asking only for local configuration
                   parameters; client already has externally configured
                   network address.

                          Table 2:  DHCP messages

                Server          Client          Server
            (not selected)                    (selected)

                  v               v               v                  |               |               |
                  |     Begins initialization     |
                  |               |               |
                  | _____________/|\____________  |
                  |/DHCPDISCOVER | DHCPDISCOVER  \|
                  |               |               |
              Determines          |          Determines
             configuration        |         configuration                  |               |               |
                  |\             |  ____________/ |
                  | \________    | /DHCPOFFER     |
                  | DHCPOFFER\   |/               |
                  |           \  |                |
                  |       Collects replies        |
                  |             \|                |
                  |     Selects configuration     |
                  |               |               |
                  | _____________/|\____________  |
                  |/ DHCPREQUEST  |  DHCPREQUEST\ |
                  |               |               |
                  |               |     Commits configuration                  |               |               |
                  |               | _____________/|
                  |               |/ DHCPACK      |
                  |               |               |
                  |    Initialization complete    |
                  |               |               |
                  .               .               .
                  .               .               .                  |               |               |
                  |      Graceful shutdown        |
                  |               |               |
                  |               |\ ____________ |
                  |               | DHCPRELEASE  \|
                  |               |               |
                  |               |        Discards lease                  |               |               |
                  v               v               v
     Figure 3: Timeline diagram of messages exchanged between DHCP
               client and servers when allocating a new network address