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Added function documentation to dns.c, removed function documentation from dns.h (functions should be documented where they are implemented!), dns_table_entry doesn't have to be packed (slower, bigger code for most machines), converted dns_init to return void, dns_table is implicitly initialized to zero, dns_lookup can be static, dns_send returns correct error values, added some asserts, compacted dns_recv using some (ugly) gotos, compacted dns_gethostbyname by combining the same return value

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This commit is contained in:
goldsimon 2007-11-20 20:28:01 +00:00
parent f58515b51e
commit b3c52f574a
2 changed files with 199 additions and 174 deletions
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317
src/core/dns.c
View File

@ -190,10 +190,6 @@ PACK_STRUCT_END
# include "arch/epstruct.h"
#endif
#ifdef PACK_STRUCT_USE_INCLUDES
# include "arch/bpstruct.h"
#endif
PACK_STRUCT_BEGIN
/** DNS table entry */
struct dns_table_entry {
u8_t state;
@ -205,13 +201,11 @@ struct dns_table_entry {
u32_t ttl;
char name[DNS_MAX_NAME_LENGTH];
struct ip_addr ipaddr;
void (* found)(const char *name, struct ip_addr *ipaddr, void *arg); /* pointer to callback on DNS query done */
/* pointer to callback on DNS query done */
dns_found_callback found;
void *arg;
} PACK_STRUCT_STRUCT;
PACK_STRUCT_END
#ifdef PACK_STRUCT_USE_INCLUDES
# include "arch/epstruct.h"
#endif
};
/* forward declarations */
static void dns_recv(void *s, struct udp_pcb *pcb, struct pbuf *p, struct ip_addr *addr, u16_t port);
@ -232,15 +226,12 @@ static u8_t dns_payload[DNS_MSG_SIZE];
#endif /* (DNS_USES_STATIC_BUF == 1) */
/**
* Initialize the resolver and configure which DNS server to use for queries.
*
* param dnsserver A pointer to a 4-byte representation of the IP
* address of the DNS server to be configured.
* Initialize the resolver: set up the UDP pcb and configure the default server
* (DNS_SERVER_ADDRESS).
*/
err_t
void
dns_init()
{
u8_t i;
struct ip_addr dnsserver = {DNS_SERVER_ADDRESS};
LWIP_DEBUGF(DNS_DEBUG, ("dns_init: initializing\n"));
@ -250,11 +241,10 @@ dns_init()
dns_pcb = udp_new();
if (dns_pcb != NULL) {
/* initialize DNS table */
for (i=0; i<DNS_TABLE_SIZE; ++i) {
dns_table[i].state = DNS_STATE_UNUSED;
dns_table[i].found = 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);
@ -264,33 +254,43 @@ dns_init()
dns_setserver(0, &dnsserver);
}
}
return ERR_OK;
}
/**
* Initialize one if the DNS server.
* Initialize one of the DNS servers.
*
* @param numdns the index of the DNS server to set must be < DNS_MAX_SERVERS
* @param dnsserver IP address of the DNS server to set
*/
void
dns_setserver(u8_t numdns, struct ip_addr *dnsserver)
{
if ((numdns<DNS_MAX_SERVERS) && (dns_pcb != NULL) && (dnsserver != NULL) && (dnsserver->addr !=0 )) {
if ((numdns < DNS_MAX_SERVERS) && (dns_pcb != NULL) &&
(dnsserver != NULL) && (dnsserver->addr !=0 )) {
dns_servers[numdns] = (*dnsserver);
}
}
/**
* Obtain one of the currently configured DNS server.
* return IP address of one of the currently configured DNS server or "ip_addr_any"
* if the DNS server has not been configured.
*
* @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.
*/
struct ip_addr
dns_getserver(u8_t numdns)
{
return (((numdns<DNS_MAX_SERVERS) && (dns_pcb != NULL))?dns_servers[numdns]:(*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
* The DNS resolver client timer - handle retries and timeouts and should
* be called every DNS_TMR_INTERVAL miliseconds (every second by default).
*/
void
dns_tmr(void)
@ -304,25 +304,27 @@ dns_tmr(void)
/**
* Look up a hostname in the array of known hostnames.
*
* \note This function only looks in the internal array of known
* @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_query() can be used to send a query
* was found. The function dns_enqueue() can be used to send a query
* for a hostname.
*
* return A pointer to a 4-byte representation of the hostname's IP
* address, or NULL if the hostname was not found in the array of
* hostnames.
* @param name the hostname to look up
* @return the hostname's IP address, as u32_t (instead of struct ip_addr to
* better check for failure: != 0) or 0 if the hostname was not found
* in the cached dns_table.
*/
u32_t
static u32_t
dns_lookup(const char *name)
{
u8_t i;
/* 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) ) {
for (i = 0; i < DNS_TABLE_SIZE; ++i) {
if ((dns_table[i].state == DNS_STATE_DONE) &&
(strcmp(name, dns_table[i].name) == 0)) {
LWIP_DEBUGF(DNS_DEBUG, ("dns_lookup: \"%s\": found = ", name));
ip_addr_debug_print(DNS_DEBUG, (&(dns_table[i].ipaddr)));
ip_addr_debug_print(DNS_DEBUG, &(dns_table[i].ipaddr));
LWIP_DEBUGF(DNS_DEBUG, ("\n"));
return dns_table[i].ipaddr.addr;
}
@ -333,8 +335,14 @@ dns_lookup(const char *name)
#if DNS_DOES_NAME_CHECK
/**
* dns_compare_name: compare the "dotted" name "query" with the
* encoded name "response"
* 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
*/
static u8_t
dns_compare_name(unsigned char *query, unsigned char *response)
@ -344,13 +352,13 @@ dns_compare_name(unsigned char *query, unsigned char *response)
do {
n = *response++;
/** @see RFC 1035 - 4.1.4. Message compression */
if ((n & 0xc0)==0xc0) {
if ((n & 0xc0) == 0xc0) {
/* Compressed name */
break;
} else {
/* Not compressed name */
while(n > 0) {
if ((*query)!=(*response)) {
while (n > 0) {
if ((*query) != (*response)) {
return 1;
}
++response;
@ -359,15 +367,17 @@ dns_compare_name(unsigned char *query, unsigned char *response)
};
++query;
}
} while(*response != 0);
} while (*response != 0);
return 0;
}
#endif /* DNS_DOES_NAME_CHECK */
/**
* dns_parse_name() - walk through a compact encoded DNS name and return the end
* of the name.
* Walk through a compact encoded DNS name and return the end of the name.
*
* @param query encoded DNS name in the DNS server response
* @return end of the name
*/
static unsigned char *
dns_parse_name(unsigned char *query)
@ -377,27 +387,34 @@ dns_parse_name(unsigned char *query)
do {
n = *query++;
/** @see RFC 1035 - 4.1.4. Message compression */
if ((n & 0xc0)==0xc0) {
if ((n & 0xc0) == 0xc0) {
/* Compressed name */
break;
} else {
/* Not compressed name */
while(n > 0) {
while (n > 0) {
++query;
--n;
};
}
} while(*query != 0);
} while (*query != 0);
return query + 1;
}
/**
* dns_send
* 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)
{
err_t err;
struct dns_hdr *hdr;
struct dns_query *qry;
struct pbuf *p;
@ -405,18 +422,23 @@ dns_send(u8_t numdns, const char* name, u8_t id)
const char *pHostname;
u8_t n;
LWIP_DEBUGF(DNS_DEBUG, ("dns_send: dns_servers[%"U16_F"] \"%s\": request\n", (u16_t)(numdns), name));
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", dns_servers[numdns].addr != 0);
/* if here, we have either a new query or a retry on a previous query to process */
p = pbuf_alloc(PBUF_TRANSPORT, sizeof(struct dns_hdr)+DNS_MAX_NAME_LENGTH+sizeof(struct dns_query), PBUF_RAM);
if (p) {
p = pbuf_alloc(PBUF_TRANSPORT, sizeof(struct dns_hdr) + DNS_MAX_NAME_LENGTH +
sizeof(struct 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;
hdr = (struct dns_hdr*)p->payload;
memset(hdr, 0, sizeof(struct dns_hdr));
hdr->id = htons(id);
hdr->flags1 = DNS_FLAG1_RD;
hdr->numquestions = htons(1);
query = (char *)hdr + sizeof(struct dns_hdr);
query = (char*)hdr + sizeof(struct dns_hdr);
pHostname = name;
--pHostname;
@ -440,28 +462,36 @@ dns_send(u8_t numdns, const char* name, u8_t id)
qry->class = htons(DNS_RRCLASS_IN);
/* resize pbuf to the exact dns query */
pbuf_realloc(p, (query+sizeof(struct dns_query))-((char*)(p->payload)));
pbuf_realloc(p, (query + sizeof(struct dns_query)) - ((char*)(p->payload)));
/* send dns packet */
udp_sendto(dns_pcb, p, dns_servers+numdns, DNS_SERVER_PORT);
err = udp_sendto(dns_pcb, p, &dns_servers[numdns], DNS_SERVER_PORT);
/* free pbuf */
pbuf_free(p);
return ERR_OK;
} else {
err = ERR_MEM;
}
return ERR_BUF;
return err;
}
/**
* dns_check_entry() - see if pEntry has not yet been queried and, if so, sends out a query.
* 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)
{
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: {
@ -511,17 +541,22 @@ dns_check_entry(u8_t i)
if (--pEntry->ttl == 0) {
LWIP_DEBUGF(DNS_DEBUG, ("dns_check_entry: \"%s\": flush\n", pEntry->name));
/* flush this entry */
pEntry->state = DNS_STATE_UNUSED;
pEntry->found = NULL;
pEntry->state = DNS_STATE_UNUSED;
pEntry->found = NULL;
}
break;
}
case DNS_STATE_UNUSED:
/* nothing to do */
break;
default:
LWIP_ASSERT("unknown dns_table entry state:", 0);
break;
}
}
/**
* dns_check_entries() - Runs through the list of names to see if there are any
* that have not yet been queried and, if so, sends out a query.
* Call dns_check_entry for each entry in dns_table - check all entries.
*/
static void
dns_check_entries(void)
@ -534,7 +569,9 @@ dns_check_entries(void)
}
/**
* Callback for DNS responses
* Receive input function for DNS response packets arriving for the dns UDP pcb.
*
* @params see udp.h
*/
static void
dns_recv(void *s, struct udp_pcb *pcb, struct pbuf *p, struct ip_addr *addr, u16_t port)
@ -555,32 +592,32 @@ dns_recv(void *s, struct udp_pcb *pcb, struct pbuf *p, struct ip_addr *addr, u16
/* is the dns message too big ? */
if (p->tot_len > DNS_MSG_SIZE) {
LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: pbuf too big\n"));
pbuf_free(p);
return;
/* free pbuf and return */
goto memerr1;
}
/* is the dns message big enought ? */
if (p->tot_len < (sizeof(struct dns_hdr)+sizeof(struct dns_query)+sizeof(struct dns_answer))) {
/* is the dns message big enough ? */
if (p->tot_len < (sizeof(struct dns_hdr) + sizeof(struct dns_query) + sizeof(struct dns_answer))) {
LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: pbuf too small\n"));
pbuf_free(p);
return;
/* free pbuf and return */
goto memerr1;
}
#if (DNS_USES_STATIC_BUF == 2)
dns_payload = mem_malloc(p->tot_len);
if (dns_payload == NULL) {
LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: mem_malloc error\n"));
pbuf_free(p);
return;
/* free pbuf and return */
goto memerr1;
}
#endif /* (DNS_USES_STATIC_BUF == 2) */
/* 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;
hdr = (struct dns_hdr*)dns_payload;
i = htons(hdr->id);
if(i < DNS_TABLE_SIZE) {
if (i < DNS_TABLE_SIZE) {
pEntry = &dns_table[i];
if(pEntry->state == DNS_STATE_ASKING) {
/* This entry is now completed. */
@ -593,40 +630,18 @@ dns_recv(void *s, struct udp_pcb *pcb, struct pbuf *p, struct ip_addr *addr, u16
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)) {
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 specified callback function if provided */
if (pEntry->found)
(*pEntry->found)(pEntry->name, NULL, pEntry->arg);
/* flush this entry */
pEntry->state = DNS_STATE_UNUSED;
pEntry->found = NULL;
/* free pbuf */
pbuf_free(p);
#if (DNS_USES_STATIC_BUF == 2)
/* free dns buffer */
mem_free(dns_payload);
#endif /* (DNS_USES_STATIC_BUF == 2) */
return;
/* call callback to indicate error, clean up memory and return */
goto responseerr;
}
#if DNS_DOES_NAME_CHECK
/* Check if the name in the "question" part match with the name in the entry. */
if (dns_compare_name(pEntry->name, (unsigned char *)dns_payload + sizeof(struct dns_hdr)) != 0) {
LWIP_DEBUGF(DNS_DEBUG, ("dns_recv: \"%s\": response not match to query\n", pEntry->name));
/* call specified callback function if provided */
if (pEntry->found)
(*pEntry->found)(pEntry->name, NULL, pEntry->arg);
/* flush this entry */
pEntry->state = DNS_STATE_UNUSED;
pEntry->found = NULL;
/* free pbuf */
pbuf_free(p);
#if (DNS_USES_STATIC_BUF == 2)
/* free dns buffer */
mem_free(dns_payload);
#endif /* (DNS_USES_STATIC_BUF == 2) */
return;
/* call callback to indicate error, clean up memory and return */
goto responseerr;
}
#endif /* DNS_DOES_NAME_CHECK */
@ -651,15 +666,11 @@ dns_recv(void *s, struct udp_pcb *pcb, struct pbuf *p, struct ip_addr *addr, u16
ip_addr_debug_print(DNS_DEBUG, (&(pEntry->ipaddr)));
LWIP_DEBUGF(DNS_DEBUG, ("\n"));
/* call specified callback function if provided */
if (pEntry->found)
if (pEntry->found) {
(*pEntry->found)(pEntry->name, &pEntry->ipaddr, pEntry->arg);
/* free pbuf */
pbuf_free(p);
#if (DNS_USES_STATIC_BUF == 2)
/* free dns buffer */
mem_free(dns_payload);
#endif /* (DNS_USES_STATIC_BUF == 2) */
return;
}
/* deallocate memory and return */
goto memerr2;
} else {
pHostname = pHostname + sizeof(struct dns_answer) + htons(ans->len);
}
@ -669,17 +680,34 @@ dns_recv(void *s, struct udp_pcb *pcb, struct pbuf *p, struct ip_addr *addr, u16
}
}
}
/* free pbuf */
pbuf_free(p);
memerr2:
#if (DNS_USES_STATIC_BUF == 2)
/* free dns buffer */
mem_free(dns_payload);
#endif /* (DNS_USES_STATIC_BUF == 2) */
memerr1:
/* free pbuf */
pbuf_free(p);
return;
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;
/* deallocate memory and return */
goto memerr2;
}
/**
* Queues a name so that a question for the name will be sent out.
* param name - The hostname that is to be queried.
* 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 arg argument to pass to the callback function
* @return a DNS_RESULT (@see DNS_RESULT, @see enum dns_result)
*/
static DNS_RESULT
dns_enqueue(const char *name, void (*found)(const char *name, struct ip_addr *addr, void *arg), void *arg)
@ -698,7 +726,7 @@ dns_enqueue(const char *name, void (*found)(const char *name, struct ip_addr *ad
/* check if this is the oldest completed entry */
if (pEntry->state == DNS_STATE_DONE) {
if (dns_seqno - pEntry->seqno > lseq) {
if ((dns_seqno - pEntry->seqno) > lseq) {
lseq = dns_seqno - pEntry->seqno;
lseqi = i;
}
@ -722,11 +750,11 @@ dns_enqueue(const char *name, void (*found)(const char *name, struct ip_addr *ad
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 = arg;
strcpy( pEntry->name, name);
pEntry->state = DNS_STATE_NEW;
pEntry->seqno = dns_seqno++;
pEntry->found = found;
pEntry->arg = arg;
strcpy(pEntry->name, name);
/* force to send query without waiting timer */
dns_check_entry(i);
@ -736,35 +764,42 @@ dns_enqueue(const char *name, void (*found)(const char *name, struct ip_addr *ad
}
/**
* NON-BLOCKING callback version for use with raw API
* Resolve a hostname (string) into an IP address.
* NON-BLOCKING callback version for use with raw API!!!
*
* Returns immediately with one of DNS_RESULT return codes:
* - DNS_COMPLETE if hostname is a valid IP address string or the host
* name is already in the local names table.
* - DNS_REQUEST_QUEUED and queues a request to be sent to the DNS server
* for resolution if no errors are present.
*
* @param name the hostname that is to be queried
* @param addr pointer to a struct ip_addr where to store the address if it is already
* cached in the dns_table (only valid if DNS_COMPLETE is returned!)
* @param found a callback founction to be called on success, failure or timeout (only if
* DNS_QUERY_QUEUED is returned!)
* @param arg argument to pass to the callback function
* @return a DNS_RESULT (@see DNS_RESULT, @see enum dns_result)
*/
DNS_RESULT dns_gethostbyname(const char *hostname, struct ip_addr *addr,
void (*found)(const char *name, struct ip_addr *ipaddr, void *arg),
void *arg
)
DNS_RESULT dns_gethostbyname(const char *hostname, struct ip_addr *addr, dns_found_callback found,
void *callback_arg)
{
/* not initialized or no valid server yet, or invalid addr pointer */
if ((dns_pcb == NULL) || (addr == NULL))
/* not initialized or no valid server yet, or invalid addr pointer
* or invalid hostname or invalid hostname length */
if ((dns_pcb == NULL) || (addr == NULL) || (!hostname) || (!hostname[0]) ||
(strlen(hostname) >= DNS_MAX_NAME_LENGTH)) {
return DNS_QUERY_INVALID;
}
/* invalid hostname */
if ((!hostname) || (!hostname[0]))
return DNS_QUERY_INVALID;
/* invalid hostname length */
if (strlen(hostname) >= DNS_MAX_NAME_LENGTH)
return DNS_QUERY_INVALID;
/* host name already in octet notation? set ip addr and return COMPLETE */
if ((addr->addr = inet_addr(hostname)) != INADDR_NONE)
return DNS_COMPLETE;
/* already have this address cached? */
if ((addr->addr = dns_lookup(hostname)) != 0)
/* host name already in octet notation? set ip addr and return COMPLETE
* already have this address cached? */
if (((addr->addr = inet_addr(hostname)) != INADDR_NONE) ||
((addr->addr = dns_lookup(hostname)) != 0)) {
return DNS_COMPLETE;
}
/* queue query with specified callback */
return dns_enqueue(hostname, found, arg);
return dns_enqueue(hostname, found, callback_arg);
}
#endif /* LWIP_DNS */

56
src/include/lwip/dns.h
View File

@ -79,46 +79,36 @@
#define DNS_RRCLASS_HS 4 /* Hesiod [Dyer 87] */
#define DNS_RRCLASS_FLUSH 0x800 /* Flush bit */
/* enumerated list of possible result values returned by dns_gethostname() */
/** enumerated list of possible result values returned by dns_gethostname() */
typedef enum dns_result {
/** dns_table is filled with queries, try again later */
DNS_ERR_MEM,
/** invalid hostname, hostname too long,
* invalid arguments or dns module not initialized */
DNS_QUERY_INVALID,
/** the hostname was enqueued for query,
* found callback will be called when resolved */
DNS_QUERY_QUEUED,
/** the hostname was found in the cache and was directly returned,
* found callback will not be called */
DNS_COMPLETE
}DNS_RESULT;
} DNS_RESULT;
/* initializes the resolver */
err_t dns_init(void);
/* handles requests, retries and timeouts - call every DNS_TMR_INTERVAL tick */
void dns_tmr(void);
/* initializes DNS server IP address */
void dns_setserver(u8_t numdns, struct ip_addr *dnsserver);
/* returns configured DNS server IP address */
struct ip_addr dns_getserver(u8_t numdns);
/* resolves a host 'name' in ip address */
DNS_RESULT dns_gethostbyname(const char *hostName, struct ip_addr *addr,
void (*found)(const char *name, struct ip_addr *ipaddr, void *arg),
void *arg);
/* dns_gethostbyname() - Returns immediately with one of DNS_RESULT return codes
* Return value will be DNS_COMPLETE if hostName is a valid
* IP address string or the host name is already in the local
* names table. Returns DNS_REQUEST_QUEUED and queues a
* request to be sent to the DNS server for resolution if no
* errors are present.
*/
/* dns_found_func() - Callback which is invoked when a hostname is found.
* This function should be implemented by the application using the DNS resolver.
* param 'name' - pointer to the name that was looked up.
* param 'ipaddr' - pointer to a struct ip_addr containing the IP address of the
* hostname, or NULL if the name could not be found.
/** Callback which is invoked when a hostname is found.
* A function of this type must be implemented by the application using the DNS resolver.
* @param name pointer to the name that was looked up.
* @param ipaddr pointer to a struct ip_addr containing the IP address of the hostname,
* or NULL if the name could not be found (or on any other error).
* @param a user-specified callback argument passed to dns_gethostbyname
*/
typedef void (*dns_found_callback)(const char *name, struct ip_addr *ipaddr, void *arg);
void dns_init(void);
void dns_tmr(void);
void dns_setserver(u8_t numdns, struct ip_addr *dnsserver);
struct ip_addr dns_getserver(u8_t numdns);
DNS_RESULT dns_gethostbyname(const char *hostname, struct ip_addr *addr,
dns_found_callback found, void *callback_arg);
#endif /* LWIP_DNS */