CSCB09 Summer Week 12 Lab

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.

Task

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, assuming your executable is ./getaddr :

$ ./getaddr mathlab.utsc.utoronto.ca
142.1.96.164
$ ./getaddr ageofempires.com | sort
20.112.250.133
20.231.239.246
20.236.44.162
20.70.246.20
20.76.201.171

The ageofempires.com 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.