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发信人: georgehill (人生不定式), 信区: Linux
标 题: Implementing AF-independent application(转寄)
发信站: BBS 荔园晨风站 (Sat Sep 16 11:31:55 2000), 站内信件
【 以下文字转载自 georgehill 的信箱 】
【 原文由 georgehill.bbs@smth.org 所发表 】
发信人: hhuu (什么是水，就是water啊), 信区: FreeBSD
标 题: Implementing AF-independent application(转寄)
发信站: BBS 水木清华站 (Sat Sep 16 00:06:14 2000)
Implementing AF-independent application
Jun-ichiro itojun Itoh, KAME Project
$Id: index.html,v 1.18 1999/08/16 11:15:07 itojun Exp $
Introduction
By deployment of Internet Protocol Version 6 (IPv6), the application
programmers has to cope with socket connection with multiple address
families, i.e. AF_INET and AF_INET6. The document describes how a programmer
can handle those multiple address families at ease.
This document assumes that you are familiar with AF_INET socket programming.
You may want to refer RFC2553 and RFC2292.
If you find any mistakes, please let the author know. The document will be
updated right away. Thank you!
struct sockaddr_storage
draft-ietf-ipngwg-bsd-api-new-03 proposes struct sockaddr_storage. This is a
placeholder for all sockaddr-variant
structures. This is implemented like follows:
struct sockaddr_storage {
u_char ss_len;
u_char ss_family;
u_char padding[128 - 2];
};
You should use this structure to hold any of sockaddr-variant structures.
union sockunion
Alternatively, you may want to implement sockunion.h, with the following
content:
union sockunion {
struct sockinet {
u_char si_len;
u_char si_family;
u_short si_port;
} su_si;
struct sockaddr_in su_sin;
struct sockaddr_in6 su_sin6;
};
#define su_len su_si.si_len
#define su_family su_si.si_family
#define su_port su_si.si_port
NOTE: For better portability, struct sockaddr_storage should be used. union
sockunion is okay, but is not really portable enough.
Rules of thumb
1. avoid struct in_addr and struct in6_addr.
Since we are trying to implement AF-independent programs, all of the memory
structures that handle network address have to be
AF-independent. In that sense, we should avoid struct in_addr and in6_addr,
since they have no room to hold AF information.
Suppose you pass an network address to some function, foo(). If you use struct
in_addr or struct in6_addr, you will end up with
extra parameter to indicate address family, as below:
struct in_addr in4addr;
struct in6_addr in6addr;
/* IPv4 case */
foo(&in4addr, AF_INET);
/* IPv6 case */
foo(&in6addr, AF_INET6);
This way the network address and address family is will not live together, and
leads to bunch of if/switch statement and mistakes in
programming. Why don't we just use struct sockaddr_storage like below?
struct sockaddr_storage ss;
/* AF independent! */
foo(&ss);
Also, by near-future update to IPv6 basic socket API (RFC2553), sockaddr_in6
will include interface index for link-local scoped address,
as well as site index for site-local scoped address. Therefore, if your
application needs to handle scoped addresses, avoiding in6_addr (and
using sockaddr_in6) is a critical requirement.
2. use getaddrinfo() and getnameinfo() everywhere.
getaddrinfo() and getnameinfo() are new address independent variant that hides
every gory detail in name-to-address translation, or
vice versa. It implements functionalities for the following functions:
gethostbyname()
gethostbyaddr()
inet_ntop()
inet_pton()
getservbyname()
getservbyport()
These can perform DNS/hostname table lookup, though it can be turned off if you
want. getaddrinfo() can return multiple addresses, if a
host have multiple address with multiple address families, as below:
localhost. IN A 127.0.0.1
IN AAAA ::1
It can query hostname as well as service name/port at once. Therefore, we can
bury all the gory details about initializing sockaddr structure
into library function.
Anyway. inet_aton() can be written as follows:
int error;
char *name;
struct sockaddr_storage ss;
struct sockaddr *sa;
struct addrinfo hints;
struct addrinfo *res;
/*
* inet_aton() case.
* This cannot handle IPv6 addresses. Also, it cannot return
* multiple addresses.
*/
if (!inet_aton(name, &((struct sockaddr_in *)&ss)->sin_addr))
perror("inet_aton");
/* getaddrinfo() case. It can handle multiple addresses. */
memset(&hints, 0, sizeof(hints));
/* set-up hints structure */
hints.ai_family = PF_UNSPEC;
error = getaddrinfo(name, NULL, &hints, &res);
if (error)
perror(gai_strerror(error));
else {
while (res) {
sa = res->ai_addr;
/* do what you want */
res = res->ai_next;
}
}
inet_ntoa() can be written as follows:
int error;
char *name;
char namebuf[BUFSIZ];
struct sockaddr_storage ss;
/*
* inet_ntoa() case. This cannot handle IPv6 addresses.
* No way to pass the error status.
*/
name = inet_ntoa(((struct sockaddr_in *)&ss)->sin_addr);
/* getnameinfo() case. NI_NUMERICHOST avoids DNS lookup. */
error = getnameinfo((struct sockaddr *)&ss. ss.ss_len,
namebuf, sizeof(namebuf), NULL, 0, NI_NUMERICHOST);
if (error)
perror("getnameinfo");
name = namebuf;
gethostbyname() can be written as follows:
struct sockaddr *sa;
struct hostent *hp;
char *name;
int af;
struct addrinfo hints;
struct addrinfo *res;
/* gethostbyname() case. It is just for single AF denoted by "af". */
hp = gethostbyname2(name, af);
/*
* getaddrinfo() case. You can get IPv6 address and IPv4 address
* at the same time.
*/
memset(&hints, 0, sizeof(hints));
/* set-up hints structure */
hints.ai_family = PF_UNSPEC;
error = getaddrinfo(name, NULL, &hints, &res);
if (error)
perror(gai_strerror(error));
else {
while (res) {
sa = (struct sockaddr *)res->ai_addr;
/* do what you want */
res = res->ai_next;
}
}
Now, gethostbyaddr() can be written as follows:
struct sockaddr_storage ss;
struct sockaddr_in *sin;
struct sockaddr_in6 *sin6;
struct hostent *hp;
char *name;
/* gethostbyaddr() case. */
switch (ss.ss_family) {
case AF_INET:
sin = (struct sockaddr_in *)&ss;
hp = gethostbyaddr(&sin->sin_addr, sizeof(sin->sin_addr),
ss.ss_family);
break;
case AF_INET6:
sin6 = (struct sockaddr_in6 *)&ss;
hp = gethostbyaddr(&sin6->sin6_addr, sizeof(sin6->sin6_addr),
ss.ss_family);
break;
}
name = hp->h_name;
/* getnameinfo() case. NI_NUMERICHOST avoids DNS lookup. */
error = getnameinfo((struct sockadddr *)&ss, ss.ss_len,
namebuf, sizeof(namebuf), NULL, 0, 0);
if (error)
perror("getnameinfo");
name = namebuf;
Modifying servers called from inetd
To port your server that is called via inetd (for example, pop server), you
must rewrite the following portions:
1.all struct sockaddr_in has to be changed into struct sockaddr_storage. Be
sure to update cast operators and sizeof
operations as well. Pointers should be changed into struct sockaddr *.
2.rewrite struct/union field names.
3.inet_aton() and inet_ntoa() has to be changed to getaddrinfo() and
getnameinfo().
4.gethostbyname() and gethostbyaddr() has to be changed to getnameinfo() and
getaddrinfo().
The simplest server has no sockaddr-related code inside, it will just use
standard input. However, most servers has logging functionality,
which requires the address for the peer. It will be obtained by using
getpeername(). Therefore, you must rewrite address-to-name
translation part for peer address.
Use great care on sizeof operation to sockaddr. This kind of code is very
popular:
int slen;
struct sockaddr_in sin;
slen = sizeof(struct sockaddr_in);
getsockname(s, (struct sockaddr *)&sin, &slen);
If we simply modify the type of sin, we're doomed. You'll need to change sizeof
operation as well, like:
int slen;
struct sockaddr_storeage ss;
slen = sizeof(ss);
getsockname(s, (struct sockaddr *)&ss, &slen);
Modifying daemons
IPv4 daemons usually bind to IN_ADDR_ANY, that is, 0.0.0.0. To obtain this kind
of address in a AF-independent manner, you can use
AI_PASSIVE flag for getaddrinfo(). Mutliprotocol daemon may want to bind() to
all the addresses returned from getaddrinfo().
struct addrinfo hints;
struct addrinfo *res;
char *myservice;
memset(&hints, 0, sizeof(hints));
/* set-up hints structure */
hints.ai_family = PF_UNSPEC;
hints.ai_flags = AI_PASSIVE;
hints.ai_socktype = SOCK_STREAM;
error = getaddrinfo(NULL, myservice, &hints, &res);
if (error)
perror(gai_strerror(error));
else {
/*
* "res" has a chain of addrinfo structure filled with
* 0.0.0.0 (for IPv4), 0:0:0:0:0:0:0:0 (for IPv6) and alike,
* with port filled for "myservice".
*/
while (res) {
/* bind() and listen() to res->ai_addr */
}
}
Modifying clients
Client side program may want to connect to all resolved addresses, as telnet
program does (telnet tries to connect to all resolved addresses,
sequentially until connection is established).
struct addrinfo hints;
struct addrinfo *res;
char *server;
char *hisservice;
memset(&hints, 0, sizeof(hints));
/* set-up hints structure */
hints.ai_family = PF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
error = getaddrinfo(server, hisservice, &hints, &res);
if (error)
perror(gai_strerror(error));
else {
while (res) {
/* try to connect() to res->ai_addr */
if (success)
break;
}
}
/* whatever you would like to perform */
(more sections may come here, sorry...)
What about inet_ntop() and inet_pton()?
In previous sections, we talked almost nothing about the usage of inet_ntop()
and inet_pton(). This is because they are not very AF
independent. Since inet_ntoa() and inet_aton() are just for IPv4 addresses,
inet_ntop() and inet_pton() are described as
replacement, in RFC2553. They are defined as follows:
int inet_pton(int af, const char *src, void *dst);
const char *inet_ntop(int af, const void *src,
char *dst, size_t size);
inet_pton() and inet_ntop() assumes in_addr or in6_addr for handling addresses,
that are, as I wrote, something we would like to
avoid. If you got a some sockaddr-ish structure, you can get printable form of
address by the following statements.
struct sockaddr_storage ss;
char buf[BUFLEN];
switch (ss.ss_family) {
case AF_INET:
inet_ntop(ss.ss_family,
&((struct sockaddr_in *)&ss)->sin_addr, buf, BUFLEN);
break;
case AF_INET6:
inet_ntop(ss.ss_family,
&((struct sockaddr_in6 *)&ss)->sin6_addr, buf, BUFLEN);
break;
}
This requires extra conditional statement, since inet_ntop() is not written for
sockaddr structures. Worse, for converting printable form
into address, you need to konw the address family, prior to the call to
inet_pton(). You can perform error-and-retly loop but it is not a
very clean way of dealing with it.
struct sockaddr_storage ss;
struct sockaddr_in *sin;
struct sockaddr_in6 *sin6;
char *printable;
switch (ss.ss_family) {
case AF_INET:
sin = (struct sockaddr_in *)&ss;
inet_pton(af, printable, &sin->sin_addr));
break;
case AF_INET6:
inet_pton(af, printable, &sin6->sin6_addr);
break;
}
Answer: You should use getnameinfo whereever possible.
update history
November 1998: document struct sockaddr_storage.
December 1998: replace union sockunion into struct sockaddr_storage, as
struct sockaddr_storage is recommended.
July 1999: comment out most of union sockunion part. Thank Mr. Adam M.
Costello for comment.
--
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