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Question regarding creating the socket file for the AF_INET.

For AF_UNIX talking within the same host, we create socket file example /tmp/name_of_the_socket_file

How about AF_INET, does it requires socket file to be created?

AFAIK, I don't need to create socket file for AF_INET and I can communicate with simple client and server model.

My colleague told me I need to create socket file for the AF_INET with IP address as the file name, I don't understand that someone can clarify me?

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  • What do you mean by "create a socket file" ? Are you talking about compiled code or scripts? Or anything else? Do you have code examples to discuss as a basis?
    – user86969
    Commented Feb 28, 2018 at 13:36

1 Answer 1

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My colleague told me I need to create socket file for the AF_INET

Your colleague is wrong. Taking a look at manpages of bind(2) you can see that different socket types have rules:

The rules used in name binding vary between address families. Consult the manual entries in Section 7 for detailed information. For AF_INET, see ip(7); for AF_INET6, see ipv6(7); for AF_UNIX, see packet(7); for AF_X25, see x25(7); and for AF_NETLINK, see netlink(7).

You will see at ip(7) that there is no invocation involving AF_INET that can create a file. There is also this great IBM paper about this socket structure and some history about it. Legacy structure form(BSD 4.4/Unix 98):

struct sockaddr_in {
  uint8_t         sin_len;
  sa_family_t     sin_family;
  u_short         sin_port;
  struct in_addr  sin_addr;
  char            sin_zero[8];
};

Current struct:

struct sockaddr_in {
    sa_family_t    sin_family; /* address family: AF_INET */
    in_port_t      sin_port;   /* port in network byte order */
    struct in_addr sin_addr;   /* internet address */
};

/* Internet address. */
struct in_addr {
    uint32_t       s_addr;     /* address in network byte order */
};

See, no mention of file creation whatsoever.

Quoting other part of ip(7) manpages about how this kind of socket(AF_INET) works:

When a process wants to receive new incoming packets or connections, it should bind a socket to a local interface address using bind(2). In this case, only one IP socket may be bound to any given local (address, port) pair. When INADDR_ANY is specified in the bind call, the socket will be bound to all local interfaces. When listen(2) is called on an unbound socket, the socket is automatically bound to a random free port with the local address set to INADDR_ANY. When connect(2) is called on an unbound socket, the socket is automatically bound to a random free port or to a usable shared port with the local address set to INADDR_ANY.

--snip--

Address format

An IP socket address is defined as a combination of an IP interface address and a 16-bit port number. The basic IP protocol does not supply port numbers, they are implemented by higher level protocols like udp(7) and tcp(7). On raw sockets sin_port is set to the IP protocol.

But if you look at unix(7) manpages you will see some examples of AF_UNIX socket type and its basic scructure. There is even a field called sun_path that is the path for socket files(see Related Stuff below):

#define UNIX_PATH_MAX    108

struct sockaddr_un {
  sa_family_t sun_family;               /* AF_UNIX */
  char        sun_path[UNIX_PATH_MAX];  /* pathname */
};

tl,dr: AF_UNIX is for sockets(and they use files) while AF_INET is for binding to ip addresses, and creating communications on its various forms(unicast, multicast, broadcast...).

Related Stuff:

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  • Thanks, @nwildner, but my colleague is not convinced what I said. Now I have the correct info and it clears my confusion. Commented Feb 28, 2018 at 13:56
  • 1
    @LethalProgrammer , Well, is up to him now to show you any software at github that uses AF_INET to create local files ;) It also does not makes sense in my mind why i would create a FILE with an IP adress on its name, if i can use the localhost address or 127.0.0.1 to deal with internal connections using AF_INET if it is totally required by any sort of "internal software development law" :)
    – user34720
    Commented Feb 28, 2018 at 14:04
  • Unfortunately ibm.com/support/knowledgecenter/en/ssw_i5_54/rzab6/cafinet.htm is not available any more :(.
    – pevik
    Commented Jan 20, 2022 at 12:31

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