In this lab, you will learn and use getaddrinfo() for
looking up the IPv4 addresses of a domain name aka host name. You will
also finish with freeaddrinfo() for freeing the data
structure created by getaddrinfo().
getaddrinfo()getaddrinfo() takes many parameters because it can do
many things; the man page can be overwhelming at first sight. On the
bright side, it offers to completely compute all parameters you need for
socket(), bind(), and
connect().
You should read the man page, but here is an orientation:
int getaddrinfo(const char *host,
const char *service,
const struct addrinfo *hints,
struct addrinfo **res);host: The domain name, e.g.,
"mathlab.utsc.utoronto.ca"
service: If you intend a port number, this can be
the string form of that number, e.g., "80" for port number
80. Why string? Because many common port numbers have names, e.g., 80 is
the standard/default port for http, and you can give "http"
here. (See /etc/services for common ports and names.)
getaddrinfo() wants this because it offers to fill in
the port-number field in sockaddr_in (or
sockaddr_in6) for you later.
hints: Pointer to your own
struct addrinfo you have filled in for limiting the search,
e.g., IPv4-only, stream-only. (See below.)
res: getaddrinfo() wants to give you a
linked list containing the answers because in general there can be
multiple answers. (struct addrinfo is a list node.) It
wants to store the head pointer in your variable. So you provide the
address of your variable here to receive the head pointer.
The return int is 0 if no error, or one of the
EAI_* error codes if error (see man page).
In hints, the following 4 fields in
struct addrinfo are relevant, and the other fields should
be set to 0 (if number field) or NULL (if pointer field):
ai_family: AF_INET if you want
IPv4-only, AF_INET6 if IPv6-only, AF_UNSPEC if
you accept both.
ai_socktype: SOCK_STREAM if stream-only
(this course), SOCK_DGRAM if datagram-only, 0 if you accept
both.
ai_protocol: 0 in this course for stream-only.
(Similar to the protocol parameter of socket().)
ai_flags: Moar customizations. You may just use 0
for this lab. See the man page or the textbook for the full
menu.
In the struct addrinfo you receive via
res:
ai_family, ai_socktype,
ai_protocol: have actual resolved values you can pass to
socket().
ai_addr, ai_addrlen: pointer to
struct sockaddr and size, so you can pass to
bind() or connect(). In this lab, you give
ai_addr to print_inet4_addr() (provided in the
starter code).
ai_canonname: Not used in this lab. (See the man
page.)
ai_next: pointer to the next
struct addrinfo node.
Your job is to complete getaddr.c to call getaddrinfo()
and then use the provided print_inet4_addr() to print out
all IPv4 addresses found.
After that, use freeaddrinfo() to free the linked list.
I will use valgrind to check this!
Optional: If getaddrinfo() returns a non-zero number,
learn and use gai_strerror() for the corresponding error
message, print it to stderr, and exit.
Sample runs and outputs:
$ ./a.out utsc.utoronto.ca
142.1.96.30
$ ./a.out google.com | sort
142.250.139.100
142.250.139.101
142.250.139.102
142.250.139.113
142.250.139.138
142.250.139.139The second example shows a case of multiple answers. Although the
order can be different every time, piping to sort should
yield a reproducible result, as shown.