Using the Domaing Name System

Using the Domaing Name System
with Examples in C

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Review

The Domain Name System (DNS):
- A distributed database that allows users to map human readable names to IP addresses
Interaction with the DNS:
- Handled by a resolver that first consults a local DNS server and then, if necessary, consults foreign DNS servers
Domain Names:
- Are hierarchical
- Portions are separated by '.'
- Left portion is most specific, right portion is DNS root

Review (cont.)

Byte Order:
- Network byte order is big-endian (i.e., most-significant-byte first)
Addresses:
- There are structures for both IPv4 and IPv6 addresses

DNS Forward Resolution: getaddrinfo()

int getaddrinfo(const char *host, const char *service, const struct addrinfo *hints, struct addrinfo **first)

Purpose:

Get the IP addresses for a given host (and service) name

Details:

`host`	The host name
`service`	The service name (or port number)
`hints`	Further criteria to use
`first`	The first IP address (in a linked structure)
Return	0 on success; non-zero on error

#define _POSIX_C_SOURCE >= 201112L
#include <netdb.h>

#include <sys/socket.h>

#include <sys/types.h>

DNS Forward Resolution (cont.)

struct addrinfo { int ai_flags; int ai_family; // AF_INET, AF_INET6 int ai_socktype; // SOCK_STREAM, SOCK_DGRAM int ai_protocol; // Protocol (name or number) socklen_t ai_addrlen; // sizeof(si_addr) struct sockaddr *ai_addr; // Socket address char *ai_canonname; // Canonical name of host (or NULL) struct addrinfo *ai_next; // Pointer to next struct in linked structure };

Note: ai_canonname will be NULL for all elements in the linked structure after the first.

DNS Forward Resolution (cont.): freeaddrinfo()

void freeaddrinfo(struct addrinfo *first)

Purpose:

Free the memory allocated by a call to getaddrinfo()

Details:

first The first element in the linked structure

#include <netdb.h>

#include <sys/socket.h>

DNS Forward Resolution (cont.)

unixexamples/dns/get_address.c

DNS Reverse Resolution: getnameinfo()

int getnameinfo(const struct sockaddr *address, socklen_t addrlen, char *host, socklen_t hostlen, char* service, socklen servlen, int flags)

Purpose:

Get the host name (and service) for a given IP address

Details:

`address`	The IP address of interest
`addrlen`	The length of the IP address
`host`	The corresponding host nam
`hostlen`	The maximum length of a host name
`service`	The corresponding service
`servlen`	The maximum length of a service name
`flags`	A bit mask of flags
Return	0 on success; non-zero on error

#define _GNU_SOURCE
#define _POSIX_C_SOURCE >= 201112L
#include <netdb.h>

#include <sys/socket.h>

DNS Forward Resolution (cont.)

unixexamples/dns/get_name.c

There's Always More to Learn