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原创声明： 本文分析属于 tiandimz 所有，转载请注明出处！谢谢！
本文源码版本为 LWIP 1.4.0
源码下载地址
下面的代码是 dns.c 的全部代码！主要完成ipv4的客户端域名解析部分，不涉及复杂的服务器端的处理过程。
下面进行简要分析，由于LWIP是轻量级的IP协议栈，所以对于DNS的要求也很简单，主要是针对客户端测的实现，能够提供API接口，通过调用接口可以实现DNS查询请求，并能够解析DNS响应，并建立本地缓存！
主要涉及的文件有 dns.c dns.h netdb.c netdb.h ip_addr.h ip_addr.c
对于复杂的域名解析服务器的程序，可以参考 bind 9.
dns报文格式请见：
DNS报文格式
#include "lwip/opt.h" //LWIP的配置选项文件
#if LWIP_DNS /* don't build if not configured for use in lwipopts.h */
#include "lwip/udp.h"
#include "lwip/mem.h"
#include "lwip/memp.h"
#include "lwip/dns.h" //dns的头文件
#include <string.h>
/** DNS server IP address opendns 是一个开放dns服务，我们的请求也是发往这个服务器地址！*/
#ifndef DNS_SERVER_ADDRESS
#define DNS_SERVER_ADDRESS(ipaddr) (ip4_addr_set_u32(ipaddr, ipaddr_addr("208.67.222.222"))) /* resolver1.opendns.com */
#endif
/** DNS server port address */
#ifndef DNS_SERVER_PORT
#define DNS_SERVER_PORT 53 // DNS通过 udp承载，主要利用 53端口
#endif
/** DNS maximum number of retries when asking for a name, before "timeout". */
#ifndef DNS_MAX_RETRIES
#define DNS_MAX_RETRIES 4 //dns请求超时的重试最大次数 4次
#endif
/** DNS resource record max. TTL (one week as default) */
#ifndef DNS_MAX_TTL
#define DNS_MAX_TTL 604800 //源记录的最大生存时间
#endif
/* DNS protocol flags DNS的标记位 */
#define DNS_FLAG1_RESPONSE 0x80
#define DNS_FLAG1_OPCODE_STATUS 0x10
#define DNS_FLAG1_OPCODE_INVERSE 0x08
#define DNS_FLAG1_OPCODE_STANDARD 0x00
#define DNS_FLAG1_AUTHORATIVE 0x04
#define DNS_FLAG1_TRUNC 0x02
#define DNS_FLAG1_RD 0x01
#define DNS_FLAG2_RA 0x80
#define DNS_FLAG2_ERR_MASK 0x0f
#define DNS_FLAG2_ERR_NONE 0x00
#define DNS_FLAG2_ERR_NAME 0x03
/* DNS protocol states */
#define DNS_STATE_UNUSED 0
#define DNS_STATE_NEW 1
#define DNS_STATE_ASKING 2
#define DNS_STATE_DONE 3
#ifdef PACK_STRUCT_USE_INCLUDES
# include "arch/bpstruct.h"
#endif
PACK_STRUCT_BEGIN
/** DNS message header DNS的消息头部 根据DNS的RFC协议 1035 描述进行设计的，采用结构体和位域来实现*/
struct dns_hdr {
PACK_STRUCT_FIELD(u16_t id);
PACK_STRUCT_FIELD(u8_t flags1);
PACK_STRUCT_FIELD(u8_t flags2);
PACK_STRUCT_FIELD(u16_t numquestions);
PACK_STRUCT_FIELD(u16_t numanswers);
PACK_STRUCT_FIELD(u16_t numauthrr);
PACK_STRUCT_FIELD(u16_t numextrarr);
} PACK_STRUCT_STRUCT;
PACK_STRUCT_END
#ifdef PACK_STRUCT_USE_INCLUDES
# include "arch/epstruct.h"
#endif
#define SIZEOF_DNS_HDR 12
/** DNS query message structure.
No packing needed: only used locally on the stack. dns消息查询结构体 */
struct dns_query {
/* DNS query record starts with either a domain name or a pointer
to a name already present somewhere in the packet. */
u16_t type;
u16_t cls;
};
#define SIZEOF_DNS_QUERY 4
/** DNS answer message structure.
No packing needed: only used locally on the stack. DNS应答消息报文格式 */
struct dns_answer {
/* DNS answer record starts with either a domain name or a pointer
to a name already present somewhere in the packet. */
u16_t type;
u16_t cls;
u32_t ttl;
u16_t len;
};
#define SIZEOF_DNS_ANSWER 10
/** DNS table entry DNS的表格 */
struct dns_table_entry {
u8_t state;
u8_t numdns;
u8_t tmr;
u8_t retries;
u8_t seqno;
u8_t err;
u32_t ttl;
char name[DNS_MAX_NAME_LENGTH];//DNS_MAX_NAME_LENGTH 256，域名字符串最大长度 256
ip_addr_t ipaddr;
/* pointer to callback on DNS query done callback 函数指针*/
dns_found_callback found;
void *arg;
};
#if DNS_LOCAL_HOSTLIST
#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
/** Local host-list. For hostnames in this list, no
* external name resolution is performed */
static struct local_hostlist_entry *local_hostlist_dynamic; //本地的主机列表
#else /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
/** Defining this allows the local_hostlist_static to be placed in a different
* linker section (e.g. FLASH) */
#ifndef DNS_LOCAL_HOSTLIST_STORAGE_PRE
#define DNS_LOCAL_HOSTLIST_STORAGE_PRE static
#endif /* DNS_LOCAL_HOSTLIST_STORAGE_PRE */
/** Defining this allows the local_hostlist_static to be placed in a different
* linker section (e.g. FLASH) */
#ifndef DNS_LOCAL_HOSTLIST_STORAGE_POST
#define DNS_LOCAL_HOSTLIST_STORAGE_POST
#endif /* DNS_LOCAL_HOSTLIST_STORAGE_POST */
DNS_LOCAL_HOSTLIST_STORAGE_PRE struct local_hostlist_entry local_hostlist_static[]
DNS_LOCAL_HOSTLIST_STORAGE_POST = DNS_LOCAL_HOSTLIST_INIT;
#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
static void dns_init_local(); //dns初始化本地列表
#endif /* DNS_LOCAL_HOSTLIST */
/* forward declarations 函数声明包括：dns接受响应报文处理函数和dns检查本地列表函数*/
static void dns_recv(void *s, struct udp_pcb *pcb, struct pbuf *p, ip_addr_t *addr, u16_t port);
static void dns_check_entries(void);
/*-----------------------------------------------------------------------------
* Globales 一些全局变量
*----------------------------------------------------------------------------*/
/* DNS variables */
static struct udp_pcb *dns_pcb; //因为用UDP承载，所有需要在UDP 的pcb中记录
static u8_t dns_seqno; //
static struct dns_table_entry dns_table[DNS_TABLE_SIZE]; //DNS_TABLE_SIZE 为 4 ，只存4个历史域名记录
static ip_addr_t dns_servers[DNS_MAX_SERVERS]; //DNS_MAX_SERVERS为 2 ，主DNS和辅DNS
/** Contiguous buffer for processing responses 处理响应的缓存 */
static u8_t dns_payload_buffer[LWIP_MEM_ALIGN_BUFFER(DNS_MSG_SIZE)];
static u8_t* dns_payload;
/**
* Initialize the resolver: set up the UDP pcb and configure the default server
* (DNS_SERVER_ADDRESS).初始化解析，建立UDP pcb 配置默认的服务器地址
*/
void
dns_init()
{
ip_addr_t dnsserver; /* 声明dns服务器地址 */
dns_payload = (u8_t *)LWIP_MEM_ALIGN(dns_payload_buffer); /* 分配内存并对齐 */
/* initialize default DNS server address */
DNS_SERVER_ADDRESS(&dnsserver); /* 设置DNS服务器地址 */
LWIP_DEBUGF(DNS_DEBUG, ("dns_init: initializing\n")); /* LWIP的打印LOG语句 */
/* if dns client not yet initialized... */
if (dns_pcb == NULL) {
dns_pcb = udp_new(); /* 创建PCB，由 udp承载 */
if (dns_pcb != NULL) {
/* initialize DNS table not needed (initialized to zero since it is a
* global variable) */
LWIP_ASSERT("For implicit initialization to work, DNS_STATE_UNUSED needs to be 0",
DNS_STATE_UNUSED == 0);
/* initialize DNS client */
udp_bind(dns_pcb, IP_ADDR_ANY, 0); /* 绑定udp */
udp_recv(dns_pcb, dns_recv, NULL); /* 完成udp_recv与 dns_recv的映射*/
/* initialize default DNS primary server */
dns_setserver(0, &dnsserver); /* 设置首选 DNS服务器 */
}
}
#if DNS_LOCAL_HOSTLIST
dns_init_local(); /* 如果有本地缓存的话，就初始化本地列表 */
#endif
}
/**
* Initialize one of the DNS servers.
*
* @param numdns the index of the DNS server to set must be < DNS_MAX_SERVERS 索引：DNS服务器数组的索引号，不能大于上限MAX
* @param dnsserver IP address of the DNS server to set
*/
void
dns_setserver(u8_t numdns, ip_addr_t *dnsserver)
{
if ((numdns < DNS_MAX_SERVERS) && (dns_pcb != NULL) &&
(dnsserver != NULL) && !ip_addr_isany(dnsserver)) {
dns_servers[numdns] = (*dnsserver); /* 根据numdns，设置DNS服务器 */
}
}
/**
* Obtain one of the currently configured DNS server. 读dns配置信息
*
* @param numdns the index of the DNS server
* @return IP address of the indexed DNS server or "ip_addr_any" if the DNS
* server has not been configured.
*/
ip_addr_t
dns_getserver(u8_t numdns) /* 入参是索引号，如果有该服务器信息，就返回，否则，就返回IP_ADDR_ANY,*/
{
if (numdns < DNS_MAX_SERVERS) {
return dns_servers[numdns];
} else {
return *IP_ADDR_ANY;
}
}
/**
* The DNS resolver client timer - handle retries and timeouts and should
* be called every DNS_TMR_INTERVAL milliseconds (every second by default). 定时管理与重传管理
*/
void
dns_tmr(void)
{
if (dns_pcb != NULL) {
LWIP_DEBUGF(DNS_DEBUG, ("dns_tmr: dns_check_entries\n"));
dns_check_entries(); /* 定时管理与重传管理 */
}
}
#if DNS_LOCAL_HOSTLIST /* 如果配置了本地缓存列表，就要有本地的初始化 */
static void
dns_init_local()
{
#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC && defined(DNS_LOCAL_HOSTLIST_INIT) /* 如果是动态列表 */
int i;
struct local_hostlist_entry *entry;
/* Dynamic: copy entries from DNS_LOCAL_HOSTLIST_INIT to list */
struct local_hostlist_entry local_hostlist_init[] = DNS_LOCAL_HOSTLIST_INIT;
size_t namelen;
for (i = 0; i < sizeof(local_hostlist_init) / sizeof(struct local_hostlist_entry); i++) {
struct local_hostlist_entry *init_entry = &local_hostlist_init[i]; /* 把本地列表初始化的表项遍历读出来，存到缓存里 */
LWIP_ASSERT("invalid host name (NULL)", init_entry->name != NULL);
namelen = strlen(init_entry->name); /* 域名长度 */
LWIP_ASSERT("namelen <= DNS_LOCAL_HOSTLIST_MAX_NAMELEN", namelen <= DNS_LOCAL_HOSTLIST_MAX_NAMELEN);
entry = (struct local_hostlist_entry *)memp_malloc(MEMP_LOCALHOSTLIST); /* 分配内存，返回地址指针 */
LWIP_ASSERT("mem-error in dns_init_local", entry != NULL);
if (entry != NULL) {
entry->name = (char*)entry + sizeof(struct local_hostlist_entry);
MEMCPY((char*)entry->name, init_entry->name, namelen); /* 字符串拷贝*/
((char*)entry->name)[namelen] = 0;
entry->addr = init_entry->addr; /* 链表移动，指向下一个 */
entry->next = local_hostlist_dynamic;
local_hostlist_dynamic = entry;
}
}
#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC && defined(DNS_LOCAL_HOSTLIST_INIT) */
}
/**
* Scans the local host-list for a hostname.
*
* @param hostname Hostname to look for in the local host-list
* @return The first IP address for the hostname in the local host-list or
* IPADDR_NONE if not found.
*/
static u32_t
dns_lookup_local(const char *hostname) /* 正向解析，解析出IP地址，可能有多个IP地址，返回一个；如果没有，就返回IPADDR_NONE */
{
#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
struct local_hostlist_entry *entry = local_hostlist_dynamic;
while(entry != NULL) {
if(strcmp(entry->name, hostname) == 0) { /* 字符串匹配，遍历整个缓存链表，如果找到，就返回ip地址 */
return ip4_addr_get_u32(&entry->addr); /* 在此处可以做个判断，如果是v6地址，要返回v6地址，可以增加条件编译语句 */
}
entry = entry->next;
}
#else /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
int i;
for (i = 0; i < sizeof(local_hostlist_static) / sizeof(struct local_hostlist_entry); i++) {
if(strcmp(local_hostlist_static[i].name, hostname) == 0) {
return ip4_addr_get_u32(&local_hostlist_static[i].addr);
}
}
#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC */
return IPADDR_NONE; /* 否则，返回无 */
}
#if DNS_LOCAL_HOSTLIST_IS_DYNAMIC
/** Remove all entries from the local host-list for a specific hostname
* and/or IP addess
*
* @param hostname hostname for which entries shall be removed from the local
* host-list
* @param addr address for which entries shall be removed from the local host-list
* @return the number of removed entries
*/
int
dns_local_removehost(const char *hostname, const ip_addr_t *addr) /* 删除DNS本地的记录，类似删除链表节点操作，先指向下一个节点，然后释放*/
{
int removed = 0;
struct local_hostlist_entry *entry = local_hostlist_dynamic;
struct local_hostlist_entry *last_entry = NULL;
while (entry != NULL) {
if (((hostname == NULL) || !strcmp(entry->name, hostname)) &&
((addr == NULL) || ip_addr_cmp(&entry->addr, addr))) {
struct local_hostlist_entry *free_entry;
if (last_entry != NULL) {
last_entry->next = entry->next;
} else {
local_hostlist_dynamic = entry->next;
}
free_entry = entry;
entry = entry->next;
memp_free(MEMP_LOCALHOSTLIST, free_entry);
removed++;
} else {
last_entry = entry;
entry = entry->next;
}
}
return removed; /* 返回删除个数 */
}
/**
* Add a hostname/IP address pair to the local host-list.
* Duplicates are not checked.
*
* @param hostname hostname of the new entry
* @param addr IP address of the new entry
* @return ERR_OK if succeeded or ERR_MEM on memory error
*/
err_t
dns_local_addhost(const char *hostname, const ip_addr_t *addr) /* 向本地增加新的记录 */
{
struct local_hostlist_entry *entry;
size_t namelen;
LWIP_ASSERT("invalid host name (NULL)", hostname != NULL);
namelen = strlen(hostname);
LWIP_ASSERT("namelen <= DNS_LOCAL_HOSTLIST_MAX_NAMELEN", namelen <= DNS_LOCAL_HOSTLIST_MAX_NAMELEN);
entry = (struct local_hostlist_entry *)memp_malloc(MEMP_LOCALHOSTLIST);
if (entry == NULL) {
return ERR_MEM;
}
entry->name = (char*)entry + sizeof(struct local_hostlist_entry); /* 一段域名+长度 */
MEMCPY((char*)entry->name, hostname, namelen);
((char*)entry->name)[namelen] = 0; /* 以0为结束 */
ip_addr_copy(entry->addr, *addr);
entry->next = local_hostlist_dynamic;
local_hostlist_dynamic = entry;
return ERR_OK;
}
#endif /* DNS_LOCAL_HOSTLIST_IS_DYNAMIC*/
#endif /* DNS_LOCAL_HOSTLIST */
/**
* Look up a hostname in the array of known hostnames. 查询域名
*
* @note This function only looks in the internal array of known
* hostnames, it does not send out a query for the hostname if none
* was found. The function dns_enqueue() can be used to send a query
* for a hostname.
*
* @param name the hostname to look up
* @return the hostname's IP address, as u32_t (instead of ip_addr_t to
* better check for failure: != IPADDR_NONE) or IPADDR_NONE if the hostname
* was not found in the cached dns_table.
*/
static u32_t
dns_lookup(const char *name)
{
u8_t i;
#if DNS_LOCAL_HOSTLIST || defined(DNS_LOOKUP_LOCAL_EXTERN)
u32_t addr;
#endif /* DNS_LOCAL_HOSTLIST || defined(DNS_LOOKUP_LOCAL_EXTERN) */
#if DNS_LOCAL_HOSTLIST
if ((addr = dns_lookup_local(name)) != IPADDR_NONE) {
return addr; /* 如果查询本地域名，找到了，就返回 */
}
#endif /* DNS_LOCAL_HOSTLIST */
#ifdef DNS_LOOKUP_LOCAL_EXTERN
if((addr = DNS_LOOKUP_LOCAL_EXTERN(name)) != IPADDR_NONE) {
return addr;
}
#endif /* DNS_LOOKUP_LOCAL_EXTERN */
/* Walk through name list, return entry if found. If not, return NULL. */
for (i = 0; i < DNS_TABLE_SIZE; ++i) {
if ((dns_table[i].state == DNS_STATE_DONE) &&
(strcmp(name, dns_table[i].name) == 0)) { /* 遍历列表，字符串匹配查找 */
LWIP_DEBUGF(DNS_DEBUG, ("dns_lookup: \"%s\": found = ", name));
ip_addr_debug_print(DNS_DEBUG, &(dns_table[i].ipaddr));
LWIP_DEBUGF(DNS_DEBUG, ("\n"));
return ip4_addr_get_u32(&dns_table[i].ipaddr);
}
}
return IPADDR_NONE;
}
#if DNS_DOES_NAME_CHECK
/**
* Compare the "dotted" name "query" with the encoded name "response"
* to make sure an answer from the DNS server matches the current dns_table
* entry (otherwise, answers might arrive late for hostname not on the list
* any more).
*
* @param query hostname (not encoded) from the dns_table
* @param response encoded hostname in the DNS response
* @return 0: names equal; 1: names differ 比较，查询请求的域名和解析响应的域名是否相等，相同为0，不同为1
*/
static u8_t
dns_compare_name(unsigned char *query, unsigned char *response)
{
unsigned char n;
do {
n = *response++;
/** @see RFC 1035 - 4.1.4. Message compression */
if ((n & 0xc0) == 0xc0) {
/* Compressed name */
break;
} else {
/* Not compressed name */
while (n > 0) {
if ((*query) != (*response)) {
return 1;
}
++response;
++query;
--n;
};
++query;
}
} while (*response != 0);
return 0;
}
#endif /* DNS_DOES_NAME_CHECK */
/**
* Walk through a compact encoded DNS name and return the end of the name. 解析查询字符串的名字
*
* @param query encoded DNS name in the DNS server response
* @return end of the name
*/
static unsigned char *
dns_parse_name(unsigned char *query)
{
unsigned char n;
do {
n = *query++;
/** @see RFC 1035 - 4.1.4. Message compression */
if ((n & 0xc0) == 0xc0) {
/* Compressed name */
break;
} else {
/* Not compressed name */
while (n > 0) {
++query;
--n;
};
}
} while (*query != 0);
return query + 1;
}
/**
* Send a DNS query packet.
*
* @param numdns index of the DNS server in the dns_servers table
* @param name hostname to query
* @param id index of the hostname in dns_table, used as transaction ID in the
* DNS query packet
* @return ERR_OK if packet is sent; an err_t indicating the problem otherwise
*/
static err_t
dns_send(u8_t numdns, const char* name, u8_t id) /* 发送请求 */
{
err_t err;
struct dns_hdr *hdr;
struct dns_query qry;
struct pbuf *p;
char *query, *nptr;
const char *pHostname;
u8_t n;
LWIP_DEBUGF(DNS_DEBUG, ("dns_send: dns_servers[%"U16_F"] \"%s\": request\n",
(u16_t)(numdns), name));
LWIP_ASSERT("dns server out of array", numdns < DNS_MAX_SERVERS);
LWIP_ASSERT("dns server has no IP address set", !ip_addr_isany(&dns_servers[numdns]));
/* if here, we have either a new query or a retry on a previous query to process */
p = pbuf_alloc(PBUF_TRANSPORT, SIZEOF_DNS_HDR + DNS_MAX_NAME_LENGTH +
SIZEOF_DNS_QUERY, PBUF_RAM);
if (p != NULL) {
LWIP_ASSERT("pbuf must be in one piece", p->next == NULL);
/* fill dns header */
hdr = (struct dns_hdr*)p->payload;
memset(hdr, 0, SIZEOF_DNS_HDR);
hdr->id = htons(id);
hdr->flags1 = DNS_FLAG1_RD;
hdr->numquestions = PP_HTONS(1);
query = (char*)hdr + SIZEOF_DNS_HDR;
pHostname = name;
--pHostname;
/* convert hostname into suitable query format. */
do {
++pHostname;
nptr = query;
++query;
for(n = 0; *pHostname != '.' && *pHostname != 0; ++pHostname) {
*query = *pHostname;
++query;
++n;
}
*nptr = n;
} while(*pHostname != 0);
*query++='\0';
/* fill dns query */
qry.type = PP_HTONS(DNS_RRTYPE_A);
qry.cls = PP_HTONS(DNS_RRCLASS_IN);
SMEMCPY(query, &qry, SIZEOF_DNS_QUERY);
/* resize pbuf to the exact dns query */
pbuf_realloc(p, (u16_t)((query + SIZEOF_DNS_QUERY) - ((char*)(p->payload))));
/* connect to the server for faster receiving */
udp_connect(dns_pcb, &dns_servers[numdns], DNS_SERVER_PORT);
/* send dns packet */
err = udp_sendto(dns_pcb, p, &dns_servers[numdns], DNS_SERVER_PORT);
/* free pbuf */
pbuf_free(p);
} else {
err = ERR_MEM;
}
return err;
}
/**
* dns_check_entry() - see if pEntry has not yet been queried and, if so, sends out a query. 检查一个dns,根据state 来处理。
* Check an entry in the dns_table:
* - send out query for new entries
* - retry old pending entries on timeout (also with different servers)
* - remove completed entries from the table if their TTL has expired
*
* @param i index of the dns_table entry to check
*/
static void
dns_check_entry(u8_t i)
{
err_t err;
struct dns_table_entry *pEntry = &dns_table[i];
LWIP_ASSERT("array index out of bounds", i < DNS_TABLE_SIZE);
switch(pEntry->state) {
case DNS_STATE_NEW: { /* 一个新的dns */
/* initialize new entry */
pEntry->state = DNS_STATE_ASKING;
pEntry->numdns = 0;
pEntry->tmr = 1;
pEntry->retries = 0;
/* send DNS packet for this entry */
err = dns_send(pEntry->numdns, pEntry->name, i);
if (err != ERR_OK) {
LWIP_DEBUGF(DNS_DEBUG | LWIP_DBG_LEVEL_WARNING,
("dns_send returned error: %s\n", lwip_strerr(err)));
}
break;
}
case DNS_STATE_ASKING: { /* 正在询问 */
if (--pEntry->tmr == 0) {
if (++pEntry->retries == DNS_MAX_RETRIES) {
if ((pEntry->numdns+1<DNS_MAX_SERVERS) && !ip_addr_isany(&dns_servers[pEntry->numdns+1])) {
/* change of server */
pEntry->numdns++;
pEntry->tmr = 1;
pEntry->retries = 0;
break;
} else {
LWIP_DEBUGF(DNS_DEBUG, ("dns_check_entry: \"%s\": timeout\n", pEntry->name));
/* call specified callback function if provided */
if (pEntry->found)
(*pEntry->found)(pEntry->name, NULL, pEntry->arg); /* callback 函数 */
/* flush this entry */
pEntry->state = DNS_STATE_UNUSED;
pEntry->found = NULL;
break;
}
}
/* wait longer for the next retry */
pEntry->tmr = pEntry->retries;
/* send DNS packet for this entry */
err = dns_send(pEntry->numdns, pEntry->name, i);
if (err != ERR_OK) {
LWIP_DEBUGF(DNS_DEBUG | LWIP_DBG_LEVEL_WARNING,
("dns_send returned error: %s\n", lwip_strerr(err)));
}
}
break;
}
case DNS_STATE_DONE: {
/* if the time to live is nul */
if (--pEntry->ttl == 0) {
LWIP_DEBUGF(DNS_DEBUG, ("dns_check_entry: \"%s\": flush\n", pEntry->name));
/* flush this entry */
pEntry->state = DNS_STATE_UNUSED; /* 状态改变 */
pEntry->found = NULL;
}
break;
}
case DNS_STATE_UNUSED:
/* nothing to do */
break;
default:
LWIP_ASSERT("unknown dns_table entry state:", 0);
break;
}
}
/**
* Call dns_check_entry for each entry in dns_table - check all entries. 检查所有的dns
*/
static void
dns_check_entries(void)
{
u8_t i;
for (i = 0; i < DNS_TABLE_SIZE; ++i) {
dns_check_entry(i); /*循环调用*/
}
}
/**
* Receive input function for DNS response packets arriving for the dns UDP pcb. 接受响应处理，根据udp报文
*
* @params see udp.h
*/
static void
dns_recv(void *arg, struct udp_pcb *pcb, struct pbuf *p, ip_addr_t *addr, u16_t port)
{
/* 初始化局部变量 */
u16_t i;
char *pHostname;
struct dns_hdr *hdr;
struct dns_answer ans;
struct dns_table_entry *pEntry;
u16_t nquestions, nanswers;
/* 对入参的检查 */
LWIP_UNUSED_ARG(arg);
LWIP_UNUSED_ARG(pcb);
LWIP_UNUSED_ARG(addr);
LWIP_UNUSED_ARG(port);
/* is the dns message too big ? */
if (p->tot_len > DNS_MSG_SIZE) {
LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: pbuf too big\n"));
/* free pbuf and return */
goto memerr;
}
/* is the dns message big enough ? */
if (p->tot_len < (SIZEOF_DNS_HDR + SIZEOF_DNS_QUERY + SIZEOF_DNS_ANSWER)) {
LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: pbuf too small\n"));
/* free pbuf and return */
goto memerr;
}
/*以上是一些异常情况处理 */
/* copy dns payload inside static buffer for processing */
if (pbuf_copy_partial(p, dns_payload, p->tot_len, 0) == p->tot_len) {
/* The ID in the DNS header should be our entry into the name table. */
hdr = (struct dns_hdr*)dns_payload;
i = htons(hdr->id);
if (i < DNS_TABLE_SIZE) {
pEntry = &dns_table[i];
if(pEntry->state == DNS_STATE_ASKING) {
/* This entry is now completed. */
pEntry->state = DNS_STATE_DONE;
pEntry->err = hdr->flags2 & DNS_FLAG2_ERR_MASK;
/* We only care about the question(s) and the answers. The authrr
and the extrarr are simply discarded. */
nquestions = htons(hdr->numquestions); /* 主机字节序转换网络字节序 */
nanswers = htons(hdr->numanswers);
/* Check for error. If so, call callback to inform. */
if (((hdr->flags1 & DNS_FLAG1_RESPONSE) == 0) || (pEntry->err != 0) || (nquestions != 1)) {
LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": error in flags\n", pEntry->name));
/* call callback to indicate error, clean up memory and return */
goto responseerr;
}
#if DNS_DOES_NAME_CHECK /* 域名比较 */
/* Check if the name in the "question" part match with the name in the entry. */
if (dns_compare_name((unsigned char *)(pEntry->name), (unsigned char *)dns_payload + SIZEOF_DNS_HDR) != 0) {
LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": response not match to query\n", pEntry->name));
/* call callback to indicate error, clean up memory and return */
goto responseerr;
}
#endif /* DNS_DOES_NAME_CHECK */
/* Skip the name in the "question" part */
pHostname = (char *) dns_parse_name((unsigned char *)dns_payload + SIZEOF_DNS_HDR) + SIZEOF_DNS_QUERY; /* 解析域名 */
while (nanswers > 0) { /* 循环 */
/* skip answer resource record's host name */
pHostname = (char *) dns_parse_name((unsigned char *)pHostname);
/* Check for IP address type and Internet class. Others are discarded. */
SMEMCPY(&ans, pHostname, SIZEOF_DNS_ANSWER);
/* 字节序转换，读响应头里的类型，类，长度信息是否匹配*/
if((ans.type == PP_HTONS(DNS_RRTYPE_A)) && (ans.cls == PP_HTONS(DNS_RRCLASS_IN)) &&
(ans.len == PP_HTONS(sizeof(ip_addr_t))) ) {
/* read the answer resource record's TTL, and maximize it if needed */
pEntry->ttl = ntohl(ans.ttl); /* 字节序转换 */
if (pEntry->ttl > DNS_MAX_TTL) {
pEntry->ttl = DNS_MAX_TTL;
}
/* read the IP address after answer resource record's header */
SMEMCPY(&(pEntry->ipaddr), (pHostname+SIZEOF_DNS_ANSWER), sizeof(ip_addr_t));
LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": response = ", pEntry->name));
ip_addr_debug_print(DNS_DEBUG, (&(pEntry->ipaddr)));
LWIP_DEBUGF(DNS_DEBUG, ("\n"));
/* call specified callback function if provided */
if (pEntry->found) {
(*pEntry->found)(pEntry->name, &pEntry->ipaddr, pEntry->arg); /* callback函数 */
}
/* deallocate memory and return */
goto memerr;
} else {
pHostname = pHostname + SIZEOF_DNS_ANSWER + htons(ans.len);
}
--nanswers;
}
LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": error in response\n", pEntry->name));
/* call callback to indicate error, clean up memory and return */
goto responseerr;
}
}
}
/* deallocate memory and return */
goto memerr;
responseerr:
/* ERROR: call specified callback function with NULL as name to indicate an error */
if (pEntry->found) {
(*pEntry->found)(pEntry->name, NULL, pEntry->arg);
}
/* flush this entry */
pEntry->state = DNS_STATE_UNUSED;
pEntry->found = NULL;
memerr:
/* free pbuf 释放buffer */
pbuf_free(p);
return;
}
/**
* Queues a new hostname to resolve and sends out a DNS query for that hostname
*
* @param name the hostname that is to be queried
* @param found a callback founction to be called on success, failure or timeout
* @param callback_arg argument to pass to the callback function
* @return @return a err_t return code.
*/
static err_t
dns_enqueue(const char *name, dns_found_callback found, void *callback_arg) /* dns入队，加入一个新的域名*/
{
u8_t i;
u8_t lseq, lseqi;
struct dns_table_entry *pEntry = NULL;
size_t namelen;
/* search an unused entry, or the oldest one */
lseq = lseqi = 0;
for (i = 0; i < DNS_TABLE_SIZE; ++i) {
pEntry = &dns_table[i];
/* is it an unused entry ? */
if (pEntry->state == DNS_STATE_UNUSED)
break;
/* check if this is the oldest completed entry */
if (pEntry->state == DNS_STATE_DONE) {
if ((dns_seqno - pEntry->seqno) > lseq) {
lseq = dns_seqno - pEntry->seqno;
lseqi = i;
}
}
}
/* if we don't have found an unused entry, use the oldest completed one */
if (i == DNS_TABLE_SIZE) {
if ((lseqi >= DNS_TABLE_SIZE) || (dns_table[lseqi].state != DNS_STATE_DONE)) {
/* no entry can't be used now, table is full */
LWIP_DEBUGF(DNS_DEBUG, ("dns_enqueue: \"%s\": DNS entries table is full\n", name));
return ERR_MEM;
} else {
/* use the oldest completed one */
i = lseqi;
pEntry = &dns_table[i];
}
}
/* use this entry */
LWIP_DEBUGF(DNS_DEBUG, ("dns_enqueue: \"%s\": use DNS entry %"U16_F"\n", name, (u16_t)(i)));
/* fill the entry */
pEntry->state = DNS_STATE_NEW;
pEntry->seqno = dns_seqno++;
pEntry->found = found;
pEntry->arg = callback_arg;
namelen = LWIP_MIN(strlen(name), DNS_MAX_NAME_LENGTH-1);
MEMCPY(pEntry->name, name, namelen);
pEntry->name[namelen] = 0;
/* force to send query without waiting timer */
dns_check_entry(i);
/* dns query is enqueued */
return ERR_INPROGRESS;
}
/**
* Resolve a hostname (string) into an IP address. 通过域名字符串解析出IP地址
* NON-BLOCKING callback version for use with raw API!!!
*
* Returns immediately with one of err_t return codes: 返回有三种结果，一种ok,一种正在处理，一种无效参数
* - ERR_OK if hostname is a valid IP address string or the host
* name is already in the local names table.
* - ERR_INPROGRESS enqueue a request to be sent to the DNS server
* for resolution if no errors are present.
* - ERR_ARG: dns client not initialized or invalid hostname
* 四个入参，域名，地址，回调函数，回调函数入参
* @param hostname the hostname that is to be queried
* @param addr pointer to a ip_addr_t where to store the address if it is already
* cached in the dns_table (only valid if ERR_OK is returned!)
* @param found a callback function to be called on success, failure or timeout (only if
* ERR_INPROGRESS is returned!)
* @param callback_arg argument to pass to the callback function
* @return a err_t return code.
*/
err_t
dns_gethostbyname(const char *hostname, ip_addr_t *addr, dns_found_callback found,
void *callback_arg)
{
u32_t ipaddr;
/* not initialized or no valid server yet, or invalid addr pointer 入参有误时的处理
* or invalid hostname or invalid hostname length */
if ((dns_pcb == NULL) || (addr == NULL) ||
(!hostname) || (!hostname[0]) ||
(strlen(hostname) >= DNS_MAX_NAME_LENGTH)) {
return ERR_ARG;
}
#if LWIP_HAVE_LOOPIF
if (strcmp(hostname, "localhost")==0) { /* 查询是本机域名 */
ip_addr_set_loopback(addr);
return ERR_OK;
}
#endif /* LWIP_HAVE_LOOPIF */
/* host name already in octet notation? set ip addr and return ERR_OK */
ipaddr = ipaddr_addr(hostname);
if (ipaddr == IPADDR_NONE) {
/* already have this address cached? */
ipaddr = dns_lookup(hostname);
}
if (ipaddr != IPADDR_NONE) {
ip4_addr_set_u32(addr, ipaddr); /* 此处也可以返回v6地址 */
return ERR_OK;
}
/* queue query with specified callback */
return dns_enqueue(hostname, found, callback_arg); /* 入队*/
}
#endif /* LWIP_DNS */
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