Busybox udhcpc：自动识别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相关，主要是注册处理从服务器接收到的DHCP/BOOTP回复。#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);---发送DHCP/BOOTP请求到d网络设备。#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，下面逐渐填充内容。 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);---------构建DHCP附加内容。 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)-------------------发送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);----------将options以及packet中的内容导出到shell环境变量中，下面执行shell的过程中使用。 /* 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 $ip */ *curr = xmalloc(sizeof("ip=255.255.255.255")); sprint_nip(*curr, "ip=", (uint8_t *) &packet->yiaddr); putenv(*curr++); if (packet->siaddr_nip) { /* IP address of next server to use in bootstrap */ *curr = xmalloc(sizeof("siaddr=255.255.255.255")); sprint_nip(*curr, "siaddr=", (uint8_t *) &packet->siaddr_nip); putenv(*curr++); } if (!(overload & FILE_FIELD) && packet->file[0]) { /* watch out for invalid packets */ *curr = xasprintf("boot_file=%."DHCP_PKT_FILE_LEN_STR"s", packet->file); putenv(*curr++); } if (!(overload & SNAME_FIELD) && packet->sname[0]) { /* watch out for invalid packets */ *curr = xasprintf("sname=%."DHCP_PKT_SNAME_LEN_STR"s", packet->sname); putenv(*curr++); } /* Export known DHCP options */ opt_name = dhcp_option_strings; i = 0; while (*opt_name) { uint8_t code = dhcp_optflags[i].code; BITMAP *found_ptr = &FOUND_OPTS(code); BITMAP found_mask = BMASK(code); if (!(*found_ptr & found_mask)) goto next; *found_ptr &= ~found_mask; /* leave only unknown options */ temp = udhcp_get_option(packet, code); *curr = xmalloc_optname_optval(temp, &dhcp_optflags[i], opt_name);---将optname和value导出到shell环境变量中。 putenv(*curr++); if (code == DHCP_SUBNET) { /* Subnet option: make things like "$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 [ -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启动设备；然后配置一个默认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 $NETMASK-----------根据udhcpc获取的interface/ip/BROADCASR/NETMASK，通过ifconfig配置到interface的设备。 if [ -n "$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 $interface--------------router不为空的情况下，先删除默认路由，重新建立路由。 done fi # drop info from this interface # resolv.conf may be a symlink to /tmp/, so take care TMPFILE=$(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-------------------------------------发送DHCPDISCOVER到DHCP服务器，服务器会返回一个可用IP地址。[2020-08-26 19:16:10] udhcpc: sending select for 192.168.33.184--------------------发送DHCPREQUEST到DHCP服务器，是对DPCPOFFER的响应。[2020-08-26 19:16:10] udhcpc: lease of 192.168.33.184 obtained, lease time 86400---此时已经经过DHCP服务器分配IP，IP地址为192.168.33.184。然后调用default.script的bound或renew。[2020-08-26 19:16:10] deleting routers---------------------------------------------default.script中删除路由。 [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