udhcpc来自于Busybox，基于DHCP服务，达到自动配置IP的功能。
下面分别介绍如何配置udhcpc，以及udhcpc是如何达到自动配置IP功能的。
1. 配置udhcpc1.1 配置kernel支持DHCP进入Networking -> Networking options -> TCP/IP networking，选择IP: DHCP support。
进入Networking Utilities -> udhcpc (DHCP client)。
在/etc/init.d/rcS中启动udhcpc。
# # 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
整个udhcpc的框架是可执行文件udhcpc、脚本文件/usr/share/udhcpc/default.script、DNS配置文件/etc/resolv.conf。
入口是udhcpc，然后udhcpc调用shell脚本default.script中的deconfig/leasefail/bound/renew/nak等选项，resolv.conf存放DNS配置文件。
2.1 Kernel DHCP相关DHCP在内核中是IP Autocofig dispatcher一部分，入口在ip_auto_config()。
内核负责将udhcpc发送的DHCP请求通过对应的网络设备发送出去，并将接受到的数据解析返回给udhcpc。
发布文章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 */...
}...
}
ic_bootp_send_if()通过d指定的网络设备发送socket DHCP/BOOTP请求。
/*
* 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");
}
2.2 udhcpc解析
和DHCP服务交互的状态如下：
/* 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协议和DHCP服务器进行交互，通过状态机来解析从DHCP服务器获取的packet。
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) =
udhcp_run_script()是udhcpc和default.script的接口，通过udhcp_run_script()即可调用其中一部分脚本。
/* 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);----------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;
}
default.script是udhcpc的延伸，方便灵活的实现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 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
运行log如下：
[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。
参考：《Dynamic Host Configuration Protocol》
其中《3.1 Client-server interaction - allocating a network address》介绍了如何分配一个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. 小结
整个自动配置IP功能，需要Kernel支持DHCP、udhcpc、default.script。
在rcS中启动udhcpc，默认使用default.script脚本已达到自动配置ip、route等。
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