關鍵詞：udhcpc、DHCP、DNS、default.script、route、ifconfig等等。
udhcpc來自於Busybox，基於DHCP服務，達到自動配置IP的功能。
下面分別介紹如何配置udhcpc，以及udhcpc是如何達到自動配置IP功能的。
1. 配置udhcpc
1.1 配置kernel支持DHCP
進入Networking -> Networking options -> TCP/IP networking，選擇IP: DHCP support。
1.2 配置udhcpc
進入Networking Utilities -> udhcpc (DHCP client)。
1.3 啟動udhcpc
在/etc/init.d/rcS中啟動udhcpc。
#
# To config network
#
udhcpc
1.4 udhcpc使用說明
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 identifier Signals: USR1 Renew lease USR2 Release lease
2. 流程解析
整個udhcpc的框架是可執行文件udhcpc、腳本文件/usr/share/udhcpc/default.script、DNS配置文件/etc/resolv.conf。
入口是udhcpc，然后udhcpc調用shell腳本default.script中的deconfig/leasefail/bound/renew/nak等選項，resolv.conf存放DNS配置文件。
2.1 Kernel DHCP相關
DHCP在內核中是IP Autocofig dispatcher一部分，入口在ip_auto_config()。
內核負責將udhcpc發送的DHCP請求通過對應的網絡設備發送出去，並將接受到的數據解析返回給udhcpc。
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);---發送DHCP/BOOTP請求到d網絡設備。 #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 */ ... } ... }
ic_bootp_send_if()通過d指定的網絡設備發送socket DHCP/BOOTP請求。
/* * 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)-------------------發送skb。 printk("E"); }
2.2 udhcpc解析
和DHCP服務交互的狀態如下：
/* 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遵循DHCP協議和DHCP服務器進行交互，通過狀態機來解析從DHCP服務器獲取的packet。
int udhcpc_main(int argc UNUSED_PARAM, char **argv) { ... if (udhcp_read_interface(client_config.interface, &client_config.ifindex, NULL, client_config.client_mac)---------------------根據client_config.interface獲取對應的client_config.client_mac和client_config.ifindex。 ) { return 1; } ... /* Goes to stdout (unless NOMMU) and possibly syslog */ bb_error_msg("started, v"BB_VER); /* Set up the signal pipe */ udhcp_sp_setup(); /* We want random_xid to be random... */ srand(monotonic_us()); state = INIT_SELECTING; udhcp_run_script(NULL, "deconfig");--------執行CONFIG_UDHCPC_DEFAULT_SCRIPT指定腳本的deconfig選項。 change_listen_mode(LISTEN_RAW); ... for (;;) {---------------------------------udhcpc的主循環體。 int tv; struct pollfd pfds[2]; struct dhcp_packet packet; /* silence "uninitialized!" warning */ unsigned timestamp_before_wait = timestamp_before_wait; udhcp_sp_fd_set(pfds, sockfd); tv = timeout - already_waited_sec; retval = 0; /* If we already timed out, fall through with retval = 0, else... */ if (tv > 0) { log1("waiting on select %u seconds", tv); timestamp_before_wait = (unsigned)monotonic_sec(); retval = poll(pfds, 2, tv < INT_MAX/1000 ? tv * 1000 : INT_MAX); if (retval < 0) { /* EINTR? A signal was caught, don't panic */ if (errno == EINTR) { already_waited_sec += (unsigned)monotonic_sec() - timestamp_before_wait; continue; } /* Else: an error occurred, panic! */ bb_perror_msg_and_die("select"); } } /* If timeout dropped to zero, time to become active: * resend discover/renew/whatever */ if (retval == 0) { if (udhcp_read_interface(client_config.interface, &client_config.ifindex, NULL, client_config.client_mac) ) { goto ret0; /* iface is gone? */ } if (clientid_mac_ptr) memcpy(clientid_mac_ptr, client_config.client_mac, 6); /* We will restart the wait in any case */ already_waited_sec = 0; switch (state) { case INIT_SELECTING: if (!discover_retries || packet_num < discover_retries) { if (packet_num == 0) xid = random_xid(); /* broadcast */ send_discover(xid, requested_ip);-------------------------在網絡中廣播一個DHCP discover包到DHCP服務器，如果存在requested_ip則指定想要發現的ip地址。 timeout = discover_timeout; packet_num++; continue; } leasefail: udhcp_run_script(NULL, "leasefail");---------------------------執行default.script的leasefail命令。 #if BB_MMU /* -b is not supported on NOMMU */ if (opt & OPT_b) { /* background if no lease */ bb_error_msg("no lease, forking to background"); client_background(); /* do not background again! */ opt = ((opt & ~OPT_b) | OPT_f); } else #endif if (opt & OPT_n) { /* abort if no lease */ bb_error_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(xid, server_addr, requested_ip);------------------廣播一個DHCP request消息，表示選擇requested_ip作為。 timeout = discover_timeout; packet_num++; continue; } change_listen_mode(LISTEN_RAW); state = INIT_SELECTING; goto leasefail; case BOUND: /* 1/2 lease passed, enter renewing state */ state = RENEWING; client_config.first_secs = 0; /* make secs field count from 0 */ change_listen_mode(LISTEN_KERNEL);---------------------------------切換socket監聽模式為LISTEN_KERNEL。 log1("entering renew state"); /* fall right through */ case RENEW_REQUESTED: /* manual (SIGUSR1) renew */ case_RENEW_REQUESTED: case RENEWING: if (timeout > 60) { send_renew(xid, server_addr, requested_ip); timeout >>= 1; continue; } /* Timed out, enter rebinding state */ log1("entering rebinding state"); state = REBINDING; /* fall right through */ case REBINDING: /* Switch to bcast receive */ change_listen_mode(LISTEN_RAW); /* Lease is *really* about to run out, * try to find DHCP server using broadcast */ if (timeout > 0) { /* send a broadcast renew request */ send_renew(xid, 0 /*INADDR_ANY*/, requested_ip); timeout >>= 1; continue; } /* Timed out, enter init state */ bb_error_msg("lease lost, entering init state"); udhcp_run_script(NULL, "deconfig"); state = INIT_SELECTING; client_config.first_secs = 0; /* make secs field count from 0 */ /*timeout = 0; - already is */ packet_num = 0; continue; /* case RELEASED: */ } /* yah, I know, *you* say it would never happen */ timeout = INT_MAX; continue; /* back to main loop */ } /* if select timed out */ /* select() didn't timeout, something happened */ ... /* Is it a packet? */ if (listen_mode == LISTEN_NONE || !pfds[1].revents) continue; /* no */ { int len; /* A packet is ready, read it */ if (listen_mode == LISTEN_KERNEL) len = udhcp_recv_kernel_packet(&packet, sockfd); else len = udhcp_recv_raw_packet(&packet, sockfd);--------------------------接收socket消息，保存到packet中。 if (len == -1) { /* Error is severe, reopen socket */ bb_error_msg("read error: %s, reopening socket", strerror(errno)); sleep(discover_timeout); /* 3 seconds by default */ change_listen_mode(listen_mode); /* just close and reopen */ } /* If this packet will turn out to be unrelated/bogus, * we will go back and wait for next one. * Be sure timeout is properly decreased. */ already_waited_sec += (unsigned)monotonic_sec() - timestamp_before_wait; if (len < 0) continue; } if (packet.xid != xid) { log1("xid %x (our is %x), ignoring packet", (unsigned)packet.xid, (unsigned)xid); continue; } /* Ignore packets that aren't for us */ if (packet.hlen != 6 || memcmp(packet.chaddr, client_config.client_mac, 6) != 0 ) { //FIXME: need to also check that last 10 bytes are zero log1("chaddr does not match, ignoring packet"); // log2? continue; } message = udhcp_get_option(&packet, DHCP_MESSAGE_TYPE); if (message == NULL) { bb_error_msg("no message type option, ignoring packet"); continue; } switch (state) {--------------------------------------------------------------根據當前所處的state，對packaet進行處理。 case INIT_SELECTING:----------------------------------------------------------處於INIT_SELECTING時，獲取DHCP分配的IP地址。 /* Must be a DHCPOFFER */ if (*message == DHCPOFFER) { uint8_t *temp; server_addr = 0; temp = udhcp_get_option(&packet, DHCP_SERVER_ID); if (!temp) { bb_error_msg("no server ID, using 0.0.0.0"); } else { /* it IS unaligned sometimes, don't "optimize" */ move_from_unaligned32(server_addr, temp); } /*xid = packet.xid; - already is */ requested_ip = packet.yiaddr; /* enter requesting state */ state = REQUESTING;----------------------------------------------------跳轉到REQUESTING狀態。 timeout = 0; packet_num = 0; already_waited_sec = 0; } continue; case REQUESTING: case RENEWING: case RENEW_REQUESTED: case REBINDING: if (*message == DHCPACK) { unsigned start; uint32_t lease_seconds; struct in_addr temp_addr; uint8_t *temp; temp = udhcp_get_option(&packet, DHCP_LEASE_TIME); if (!temp) { bb_error_msg("no lease time with ACK, using 1 hour lease"); lease_seconds = 60 * 60; } else { /* it IS unaligned sometimes, don't "optimize" */ move_from_unaligned32(lease_seconds, temp); lease_seconds = ntohl(lease_seconds); /* paranoia: must not be too small and not prone to overflows */ if (lease_seconds < 0x10) lease_seconds = 0x10; if (lease_seconds > 0x7fffffff / 1000) lease_seconds = 0x7fffffff / 1000; }
... /* enter bound state */ temp_addr.s_addr = packet.yiaddr; bb_error_msg("lease of %s obtained, lease time %u", inet_ntoa(temp_addr), (unsigned)lease_seconds); requested_ip = packet.yiaddr;------------------------------------------更新requested_ip，即DHCP分配給本機的IP地址。 start = monotonic_sec(); udhcp_run_script(&packet, state == REQUESTING ? "bound" : "renew");----如果當前狀態時REQUESTING，執行default.script的bound；否則執行renew。 already_waited_sec = (unsigned)monotonic_sec() - start; timeout = lease_seconds / 2; if ((unsigned)timeout < already_waited_sec) { /* Something went wrong. Back to discover state */ timeout = already_waited_sec = 0; } state = BOUND; change_listen_mode(LISTEN_NONE); 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 /* make future renew packets use different xid */ /* xid = random_xid(); ...but why bother? */ continue; /* back to main loop */ } if (*message == DHCPNAK) {-----------------------------------------------錯誤情況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. */ if (server_addr != 0) { uint32_t svid; uint8_t *temp; temp = udhcp_get_option(&packet, DHCP_SERVER_ID); if (!temp) { non_matching_svid: log1("received DHCP NAK with wrong" " server ID, ignoring packet"); continue; } move_from_unaligned32(svid, temp); if (svid != server_addr) goto non_matching_svid; } /* return to init state */ bb_error_msg("received %s", "DHCP NAK"); udhcp_run_script(&packet, "nak");----------------------------------執行default.script的nak腳本。 if (state != REQUESTING)-------------------------------------------如果狀態時REQUESTING，則重新進行一次流程。 udhcp_run_script(NULL, "deconfig"); change_listen_mode(LISTEN_RAW); sleep(3); /* avoid excessive network traffic */ state = INIT_SELECTING; client_config.first_secs = 0; /* make secs field count from 0 */ requested_ip = 0; timeout = 0; packet_num = 0; already_waited_sec = 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_addr, requested_ip); retval = 0; ret: /*if (client_config.pidfile) - remove_pidfile has its own check */ remove_pidfile(client_config.pidfile); return retval; }
udhcp_run_script()是udhcpc和default.script的接口，通過udhcp_run_script()即可調用其中一部分腳本。
/* 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環境變量中，下面執行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; }
2.3 default.script腳本解析
default.script是udhcpc的延伸，方便靈活的實現udhcpc的功能。
#!/bin/sh # udhcpc script edited by Tim Riker <Tim@Rikers.org> [ -z "$1" ] && echo "Error: should be called from udhcpc" && exit 1 RESOLV_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
2.4 運行實例log
運行log如下：
[2020-08-26 19:16:08] udhcpc: started, v1.27.2
[2020-08-26 19:16:08] udhcpc: sending discover-------------------------------------發送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。
3. DHCP協議
參考：《Dynamic Host Configuration Protocol》
其中《3.1 Client-server interaction - allocating a network address》介紹了如何分配一個IP地址的流程。
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
4. 小結
整個自動配置IP功能，需要Kernel支持DHCP、udhcpc、default.script。
在rcS中啟動udhcpc，默認使用default.script腳本已達到自動配置ip、route等。