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2.3 2.3 When you need to reserve room for a sockaddr (as for recvfrom(2)), use struct sockaddr_storage. It is specified that struct sockaddr_storage is big enough for any kind of sockaddrs. sockaddr_in6 is larger than sockaddr; therefore, if there is a possibility to hold sockaddr_in6 into a memory region, it is not sufficient to use sockaddr to reserve memory space. void foo(s, buf, siz) int s; char *buf; size_t siz; { struct sockaddr_storage ss; socklen_t sslen; sslen = sizeof(ss); recvfrom(s, buf, siz, (struct sockaddr *)&ss, &sslen); } There is another important reason for using sockaddr. Due to the scoped IPv6 addresses, the IPv6 address (128 bits) does not uniquely identify the peer. In Figure 2.1, from node B, we can see two nodes with fe80::1: one on Ethernet segment 1, another on Ethernet segment 2. To communicate with node A or node C, node B has to disambiguate between them with a link-local address specifying a 128bit address is not enough you need to specify the scope identification (in link-local case, specifying the outgoing interface is enough). sockaddr_in6 has a member named sin6_scope_id to disambiguate destinations between multiple scope zones. String representation of a scoped IPv6 address is augmented with scope identifier after % sign, such as fe80::1%ether1. Scope identification string (ether1 part) is implementation-dependent. getaddrinfo(3) will translate the string into a sin6_ scope_id value. Figure 2.1 Disambiguate the peer when there are multiple adjacent scope zones. Chapter
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18 2.3 Guidelines to Address-Family Independent Socket Programming 18 2.3 Guidelines to Address-Family Independent Socket Programming /* THIS IS A VERY BAD PRACTICE */ extern void foo(int); int in; in = htonl(0×7f000001); /* 127.0.0.1 */ foo(in); To handle IPv4 and IPv6 addresses, it is suggested you use sockaddrs, such as sockaddr_in or sockaddr_in6, always. With sockaddrs, the data contains the identification of address family, so we can pass around the address data and know which address family it belongs to. When passing pointers around, use struct sockaddr *, and let the called function handle it. extern int foo(struct sockaddr *); int main(argc, argv) int argc; char **argv; { struct sockaddr_in sin; /* setup sin */ foo((struct sockaddr *)&sin); } int foo(sa) struct sockaddr *sa; { switch (sa->sa_family) { case AF_INET: case AF_INET6: /* do something */ return 0; default: return -1; /*not supported*/ } }
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2.3 2.3 g 17 if (hp->h_length != sizeof(sin6.sin6_addr)) { fprintf(stderr, invalid address sizen ); exit(1); /*NOTREACHED*/ } memset(&sin6, 0, sizeof(sin6)); sin6.sin6_family = AF_INET6; salen = sin6.sin6_len = sizeof(struct sockaddr_in6); memcpy(&sin6.sin6_addr, hp->h_addr, sizeof(sin6.sin6_addr)); sin6.sin6_port = htons(80); /* connect to the peer */ if (connect(s, (struct sockaddr *)&sin6, salen) 0) { perror( connect ); exit(1); } 2.3 Guidelines to Address-Family Independent Socket Programming So, how can we make our program address-family independent? This section enumerates important tips to be followed to achieve this goal. 2.3.1 Using sockaddrs for address representation To support IPv4/v6 dual stack from your program, you first need to be able to handle IPv4 and IPv6 addresses in your program. Traditionally, IPv4-only programs used struct in_addr to hold IPv4 addresses. However, since the structure does not contain an identification of address family, the data is not self-contained. /* * this example is IPv4-only, and we cannot identify address family * from the data itself. foo() cannot distinguish the address * family of the given address. * inet_addr(3) is not recommended due to the lack of failure handling. */ extern void foo(void *); struct in_addr in; if (inet_aton( 127.0.0.1″, &in) != 1) { fprintf(stderr, could not translate addressn ); exit(1); } foo(&in); Novice programmers even mistakenly use int or u_int32_t to hold IPv4 addresses. This is not a portable way, since int can be of a different size (e.g., 64 bits), and from a Chapter 2
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16 2.2 Why Programs Need to Be Address-Family Independent? 16 2.2 Why Programs Need to Be Address-Family Independent? ; /*NOTREACHED*/ } /* DNS name lookup */ hp = gethostbyname(hostname); if (!hp) { fprintf(stderr, host not foundn ); exit(1); /*NOTREACHED*/ } if (hp->h_length != sizeof(sin.sin_addr)) { fprintf(stderr, invalid address sizen ); exit(1); /*NOTREACHED*/ } memset(&sin, 0, sizeof(sin)); sin.sin_family = AF_INET; salen = sin.sin_len = sizeof(struct sockaddr_in); memcpy(&sin.sin_addr, hp->h_addr, sizeof(sin.sin_addr)); sin.sin_port = htons(80); /* connect to the peer */ if (connect(s, (struct sockaddr *)&sin, salen) 0) { perror( connect ); exit(1); } Program 2.2 Program rewritten to support IPv6 with AF_INET6 hardcoded THIS METHOD IS NOT RECOMMENDED /* * AF_INET6 code - the book recommend AGAINST rewriting applications * like this. */ struct sockaddr_in6 sin6; socklen_t salen; struct hostent *hp; /* open the socket - IPv6 only, no IPv4 support */ s = socket(AF_INET6, SOCK_STREAM, IPPROTO_TCP); if (s < 0) { perror( socket ); exit(1); /*NOTREACHED*/ } /* DNS name lookup - does not support scope ID */ hp = gethostbyname2(hostname, AF_INET6); if (!hp) { fprintf(stderr, host not foundn ); exit(1); /*NOTREACHED*/ }
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2.2 2.2 ? 15 APIs such as gethostbyname2(3) do not provide support for scoped IPv6 addresses. Program 2.1 presents a program that hardcodes IPv4 assumptions. Bold portions depend on IPv4 or on IPv4 API assumptions. Other reading material may recommend to just replace AF_INET into AF_INET6 and sockaddr_in into sockaddr_in6, as in Program 2.2. However, the approach has multiple drawbacks. First, with gethostbyname2(3), you can only connect to IPv6 destinations, not IPv4 destinations. In an IPv4/v6 dual stack environment, FQDN can be resolved into multiple IPv4 addresses as well as multiple IPv6 addresses. Clients should try to contact all of them, not just the IPv6 ones. Second, IPv6 supports scoped IPv6 addresses, as discussed earlier. With the use of gethostbyname2(3), we cannot handle scoped IPv6 addresses, since gethostbyname2( 3) does not return scope identification. Third, by hardcoding AF_INET6 the code will work only on IPv6-enabled kernels, since a kernel without IPv6 support does not usually have AF_INET6 socket support. If you want to ship a single binary that works correctly on IPv4-only, IPv6only, and IPv4/v6 dual stack kernel without recompilation, address-family independence is needed. Fourth, the code is not future-proven. In the future, when a new protocol comes up, we would like to avoid rewriting exising applications. IPv6 transition is costly, so we would like to solve other problems together with the IPv6 transition; therefore, let us make sure we won t need to upgrade our networking code ever again. Finally, from our experience, by writing applications in an address-family independent manner, you can maintain higher portability and stability of your applications. Therefore, this book does not recommend hardcoding AF_INET6. Program 2.1 Original program, which is IPv4-only. /* * original code */ struct sockaddr_in sin; socklen_t salen; struct hostent *hp; /* open the socket */ s= socket(AF_INET, SOCK_STREAM, IPPROTO_TCP); if (s < 0) { perror( socket ); Chapter
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14 2.2 Why Programs Need to Be Address-Family Independent? u_int32_t sin6_flowinfo; /* IP6 flow information */ struct in6_addr sin6_addr; /* IP6 address*/ u_int32_t sin6_scope_id; /* scope zone index*/ }; To identify IPv6 peers on the socket API, we use a C structure called sockaddr_in6. For instance, to issue operations such as connect(2) on a socket created with AF_INET6 specified, we use sockaddr_in6. Compared with sockaddr_in, sockaddr_in6 adds two fields: sin6_flowinfo and sin6_scope_id. Standardization of sin6_flowinfo is not finished yet; therefore, this book does not go into its details. We discuss sin6_scope_id in detail later in the book. 2.2 Why Programs Need to Be Address-Family Independent? In this book we advocate address-family independent socket layer programming for IPv6 transition. By following the instructions in the book, your code will become independent from the address family (such as AF_INET or AF_INET6). Here are several reasons for taking this direction: To support the IPv4/v6 dual stack environment, programs must be able to handle both IPv4 and IPv6 properly. If you hardcode AF_INET or AF_INET6 into your programs, your program ends up not working properly in the IPv4/v6 dual stack environment. We would like to avoid rewriting network applications when a new protocol becomes available. It includes both the IP layer (as with IPv7 there are currently no plans, but we don t know about the future) as well as the transport/ session layer (similar to using SCTP instead of TCP). For instance, in some systems, it could be possible that your program becomes capable of supporting AppleTalk by using address-family independent APIs. We have enough tools for address-family independent programming, such as sockaddr_storage, getaddrinfo(3), and getnameinfo(3). If you hardcode address family into your program, your program will not function if the operating system kernel does not support the address family. With a program independent of address family, you can ship a single source/binary for any operating system kernel configuration. From my experience, it is cleaner and more portable to write a program this way than to write a program in an IPv6-only manner.
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2 2 2.1 AF_INET6: The Address Family for IPv6 As we have seen in Chapter 1, on the socket API we use a constant AF_INET to identify IPv4 sockets. Also, to identify IPv4 peers on the socket we have used C structure, called sockaddr_in. To handle IPv6 on the socket API, we use a constant called AF_INET6. The expression is as follows: s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP); This could be rewritten as: s = socket(AF_INET6, SOCK_STREAM, IPPROTO_TCP); to initialize an IPv6 socket into variable s. The following code shows the definition of sockaddr_in and sockaddr_in6: Definition of sockaddr_in: struct sockaddr_in { u_int8_t sin_len; /* length of sockaddr */ u_int8_t sin_family; /* address family */ u_int16_t sin_port; /* TCP/UDP port number */ struct in_addr sin_addr; /* IPv4 address */ int8_t sin_zero[8]; /* padding */ }; Definition of sockaddr_in6: struct sockaddr_in6 { u_int8_t sin6_len; /* length of this struct (socklen_t) */ u_int8_t sin6_family; /* AF_INET6 (sa_family_t) */ u_int16_t sin6_port; /* Transport layer port */ 13
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12 1.4 IPv6 Architecture from a Programmer s Point of View 12 1.4 IPv6 Architecture from a Programmer s Point of View Note: There was another kind of scoped address, site-local address, defined in the specification. However, it is soon to be deprecated so you do not need to worry about it. For more details, you may want to check other IPv6-related reading materials, such as those listed in the References.
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1.4 1.4 w 11 Figure 1.5 Variable-length subnet mask in IPv4. If we try to connect more nodes to subnet B on the diagram s left side, we have to renumber subnet B s network address to 10.1.2.16/28 to accommodate them. Figure 1.6 IPv6 uses a fixed 64-bit subnet mask. There is no need to renumber even when you connect more nodes to an IPv6 subnet. Cannot accommodate more nodes on subnet B Each IPv6 subnet can accommodate 2^64 nodes into multicast, and broadcast is no longer needed. For instance, to transmit a packet to all nodes on a specific broadcast medium, we use an IPv6 link-local all-nodes multicast address, which is ff02::1. IPv6 introduces anycast as a new communication model, which is one-to-one communication, where the destination node can be chosen from multiple nodes based on closeness from the source. In IPv4, with a private address as the only exception, unicast addresses are globally unique. In IPv6, there are scoped IPv6 addresses, namely, link-local IPv6 addresses. These addresses are defined to be unique across a given link. Link- local address is under the fe80::/10 prefix range. Since uniqueness of a link- Chapter
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10 1.4 IPv6 Architecture from a Programmer s Point of View into binary representation, such as struct in_addr (inet_pton(3)) and vice versa (inet_ntop(3)). 1.4 IPv6 Architecture from a Programmer s Point of View From a programer s point of view, IPv4 and IPv6 are almost exactly the same; we have an IP address (size differs: 32 bit and 128 bit) to identify nodes (actually network interfaces) and a TCP/UDP port number to identify services on the node. There are several points that programmers need to know: In both cases, users normally will use DNS names, rather than IP addresses, to identify the peer. For instance, users use http://www.example.com/ rather than http://10.2.3.4/. IPv4 addresses are presented as decimals separated by dots, such as 10.2.3.4. IPv6 addresses are presented as hexadecimals separated by colons, such as 3ffe:501:ffff:0:0:0:0:1. Two continuous colons can be used to mean continuous zeros for example, 3ffe:501:ffff:0:0:0:0:1 is equal to 3ffe:501:ffff::1. To avoid ambiguity with the separator for the port number, the numeric IPv6 address in a URL has to be wrapped with a square bracket: http:// [3ffe:501:ffff::1]:80/. Again, however, users won t, and shouldn t need to, use a numeric IPv6 address in URLs. DNS names should be used instead. In IPv4, we used variable-length subnet masks, such as /24 (netmask 0xffffff00), /28 (0xfffffff0), or /29 (0xfffffff8). Variable-length subnet mask was introduced to reduce IPv4 address space use; however, it has certain drawbacks: It limits how many devices you can connect to your subnet, and you will need to change subnet mask, or renumber the subnet, when the number of devices goes too high. In IPv6, we always use /64 as the subnet mask. Therefore, it is guaranteed that up to 264 devices can be connected to a given subnet. (See Figures 1.5 and 1.6.) In IPv4, a node normally has a single IPv4 address associated with it. In IPv6, it is normal to have multiple IP addresses onto a single node. More specifically, IPv6 addresses are assigned to interfaces, not to nodes. An interface can have multiple IPv6 addresses. In IPv4, there were three communication models: unicast, broadcast, and multicast. Unicast is for one-to-one communication, broadcast is for one-to-all communiation on a specific broadcast medium (e.g., an ethernet link), and multicast is for one-to-many communication with a specific set of nodes (within a multicast group). With IPv6, broadcast is deprecated and integrated
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