IP6(4)

✨ IP6(4) — Internet Protocol version 6 (IPv6) network layer ✨ 💖

okay so bestie, this is the ip6(4) manual page and it's literally the IPv6 network layer, no cap. 💅

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🌸 NAME 🌸

ip6 — Internet Protocol version 6 (IPv6) network layer

🎀 SYNOPSIS 🎀

#include <sys/socket.h>
#include <netinet/in.h>

int
socket(AF_INET6, SOCK_RAW, proto);

💖 DESCRIPTION 💖

okay so the IPv6 network layer? It's literally what the IPv6 protocol family uses to transport data, no cap. And get this: IPv6 packets contain an IPv6 header that is not provided as part of the payload contents when passed to an application. It's giving "not included", bestie. 😭

IPv6 header options affect the behavior of this protocol and may be used by high-level protocols (such as the tcp(4) and udp(4) protocols) as well as directly by “raw sockets”, which process IPv6 messages at a lower-level and may be useful for developing new protocols and special-purpose applications. Slay. 🔥

✨ Header ✨

Every single IPv6 packet starts with an IPv6 header. Periodt. 💅 When data received by the kernel are passed to the application, this header is not included in buffer, even when raw sockets are being used. Like, not there. 🙈

Likewise, when data are sent to the kernel for transmit from the application, the buffer is not examined for an IPv6 header: the kernel always constructs the header. It's giving "I got this". 👑

To directly access IPv6 headers from received packets and specify them as part of the buffer passed to the kernel, link-level access (bpf(4), for example) must instead be utilized. That's the vibe. 🌈

The header has the following definition, bestie:

             struct ip6_hdr {
                  union {
                       struct ip6_hdrctl {
                            uint32_t ip6_un1_flow;   /* 20 bits of flow ID */
                            uint16_t ip6_un1_plen;   /* payload length */
                            uint8_t  ip6_un1_nxt;    /* next header */
                            uint8_t  ip6_un1_hlim;   /* hop limit */
                       } ip6_un1;
                       uint8_t ip6_un2_vfc;  /* version and class */
                  } ip6_ctlun;
                  struct in6_addr ip6_src;   /* source address */
                  struct in6_addr ip6_dst;   /* destination address */
             } __packed;

             #define ip6_vfc         ip6_ctlun.ip6_un2_vfc
             #define ip6_flow        ip6_ctlun.ip6_un1.ip6_un1_flow
             #define ip6_plen        ip6_ctlun.ip6_un1.ip6_un1_plen
             #define ip6_nxt         ip6_ctlun.ip6_un1.ip6_un1_nxt
             #define ip6_hlim        ip6_ctlun.ip6_un1.ip6_un1_hlim
             #define ip6_hops        ip6_ctlun.ip6_un1.ip6_un1_hlim

All fields are in network-byte order. And any options you specify (see “Options” below) must also be specified in network-byte order. No exceptions. 🚫

ip6_flow specifies the flow ID. ip6_plen specifies the payload length. ip6_nxt specifies the type of the next header. ip6_hlim specifies the hop limit. Got it? 💖

The top 4 bits of ip6_vfc specify the class and the bottom 4 bits specify the version. It's giving bit-split realness. ✨

ip6_src and ip6_dst specify the source and destination addresses. Obviously. 💅

The IPv6 header may be followed by any number of extension headers that start with the following generic definition:

             struct ip6_ext {
                  uint8_t ip6e_nxt;
                  uint8_t ip6e_len;
             } __packed;

🦋 Options 🦋

IPv6 allows header options on packets to manipulate the behavior of the protocol. It's giving control freak, but make it networking. 😌 These options and other control requests are accessed with the getsockopt(2) and setsockopt(2) system calls at level IPPROTO_IPV6 and by using ancillary data in recvmsg(2) and sendmsg(2). Iconic. 👑 They can be used to access most of the fields in the IPv6 header and extension headers. No cap.

The following socket options are supported, bestie:

• IPV6_UNICAST_HOPS int * — Get or set the default hop limit header field for outgoing unicast datagrams sent on this socket. It's giving "how far do these unicast packets go?" 🦋
• IPV6_MULTICAST_IF u_int * — Get or set the interface from which multicast packets will be sent. For hosts with multiple interfaces, each multicast transmission is sent from the primary network interface. The interface is specified as its index as provided by if_nametoindex(3). A value of zero specifies the default interface. Periodt. 💅
• IPV6_MULTICAST_HOPS int * — Get or set the default hop limit header field for outgoing multicast datagrams sent on this socket. This option controls the scope of multicast datagram transmissions. 🌈

Datagrams with a hop limit of 1 are not forwarded beyond the local network. Multicast datagrams with a hop limit of zero will not be transmitted on any network but may be delivered locally if the sending host belongs to the destination group and if multicast loopback (see below) has not been disabled on the sending socket. Multicast datagrams with a hop limit greater than 1 may be forwarded to the other networks if a multicast router (such as mrouted(8) (ports/net/mrouted)) is attached to the local network. Slay. 🔥

• IPV6_MULTICAST_LOOP u_int * — Get or set the status of whether multicast datagrams will be looped back for local delivery when a multicast datagram is sent to a group to which the sending host belongs. It's giving "do I hear myself?" 🎧

This option improves performance for applications that may have no more than one instance on a single host (such as a router daemon) by eliminating the overhead of receiving their own transmissions. It should generally not be used by applications for which there may be more than one instance on a single host (such as a conferencing program) or for which the sender does not belong to the destination group (such as a time-querying program). So know your vibe. 💜

A multicast datagram sent with an initial hop limit greater than 1 may be delivered to the sending host on a different interface from that on which it was sent if the host belongs to the destination group on that other interface. The multicast loopback control option has no effect on such delivery. Wild, right? 😭

• IPV6_JOIN_GROUP struct ipv6_mreq * — Join a multicast group. A host must become a member of a multicast group before it can receive datagrams sent to the group. Like, you can't just show up uninvited. 🚪

             struct ipv6_mreq {
                     struct in6_addr ipv6mr_multiaddr;
                     unsigned int    ipv6mr_interface;
             };

ipv6mr_interface may be set to zeroes to choose the default multicast interface or to the index of a particular multicast-capable interface if the host is multihomed. Membership is associated with a single interface; programs running on multihomed hosts may need to join the same group on more than one interface. It's giving extra steps, but okay. 🙈

If the multicast address is unspecified (i.e., all zeroes), messages from all multicast addresses will be accepted by this group. Note that setting to this value requires superuser privileges. So like, you need the power, bestie. 👑

• IPV6_LEAVE_GROUP struct ipv6_mreq * — Drop membership from the associated multicast group. Memberships are automatically dropped when the socket is closed or when the process exits. So you don't even have to do anything, it's giving self-cleaning. 🧹
• IPV6_PORTRANGE int * — Get or set the allocation policy of ephemeral ports for when the kernel automatically binds a local address to this socket. The following values are available:
  • IPV6_PORTRANGE_DEFAULT — Use the regular range of non-reserved ports (varies, see ip(4)).
  • IPV6_PORTRANGE_HIGH — Use a high range (varies, see ip(4)).
  • IPV6_PORTRANGE_LOW — Use a low, reserved range (600-1023, see ip(4)).
• IPV6_PKTINFO int * — Get or set whether additional information about subsequent packets will be provided as ancillary data along with the payload in subsequent recvmsg(2) calls. The information is stored in the following structure in the ancillary data returned:

             struct in6_pktinfo {
                     struct in6_addr ipi6_addr;    /* src/dst IPv6 address */
                     unsigned int    ipi6_ifindex; /* send/recv if index */
             };

• IPV6_HOPLIMIT int * — Get or set whether the hop limit header field from subsequent packets will be provided as ancillary data along with the payload in subsequent recvmsg(2) calls. The value is stored as an int in the ancillary data returned. Simple. ✨
• IPV6_HOPOPTS int * — Get or set whether the hop-by-hop options from subsequent packets will be provided as ancillary data along with the payload in subsequent recvmsg(2) calls. The option is stored in the following structure in the ancillary data returned:

             struct ip6_hbh {
                     uint8_t ip6h_nxt;       /* next header */
                     uint8_t ip6h_len;       /* length in units of 8 octets */
             /* followed by options */
             } __packed;

The inet6_option_space() routine and family of routines may be used to manipulate this data. Iconic. 💅

This option requires superuser privileges. So like, you need the juice. 🧃

• IPV6_DSTOPTS int * — Get or set whether the destination options from subsequent packets will be provided as ancillary data along with the payload in subsequent recvmsg(2) calls. The option is stored in the following structure in the ancillary data returned:

             struct ip6_dest {
                     uint8_t ip6d_nxt;       /* next header */
                     uint8_t ip6d_len;       /* length in units of 8 octets */
             /* followed by options */
             } __packed;

The inet6_option_space() routine and family of routines may be used to manipulate this data. Iconic. 💅

This option requires superuser privileges. So like, you need the juice. 🧃

• IPV6_TCLASS int * — Get or set the value of the traffic class field used for outgoing datagrams on this socket. The value must be between -1 and 255. A value of -1 resets to the default value. So like, -1 is the 'undo' button. ↩️
• IPV6_RECVTCLASS int * — Get or set the status of whether the traffic class header field will be provided as ancillary data along with the payload in subsequent recvmsg(2) calls. The header field is stored as a single value of type int. Periodt. 💅
• IPV6_RTHDR int * — Get or set whether the routing header from subsequent packets will be provided as ancillary data along with the payload in subsequent recvmsg(2) calls. The header is stored in the following structure in the ancillary data returned:

struct ip6_rthdr {
        uint8_t ip6r_nxt;       /* next header */
        uint8_t ip6r_len;       /* length in units of 8 octets */
        uint8_t ip6r_type;      /* routing type */
        uint8_t ip6r_segleft;   /* segments left */
/* followed by routing-type-specific data */
} __packed;

The inet6_option_space() routine and its little family of routines may be used to manipulate this data. ✨

bestie heads UP 🚨 This option requires superuser privileges. no root, no routing header, periodt. 💅

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• IPV6_PKTOPTIONS struct cmsghdr * — get or set all the header options and extension headers at one time on the last packet sent or received on the socket. the whole squad, all at once 🫶

Everything has to fit inside the size of one mbuf (see mbuf(9)), so no squeezing your entire personality in there. Options are given as a series of cmsghdr structures followed by their matching values. cmsg_level gets set to IPPROTO_IPV6, cmsg_type gets set to one of the other values in this list, and the trailing data becomes the option value. iconic little trio

When you're setting options, if the length optlen you hand to setsockopt(2) is zero, every header option resets to its default values — a lil factory reset, we love a clean slate 🧼. Otherwise, that length should specify the size the whole series of control messages consumes.

And here's the plot twist: instead of using sendmsg(2) to specify option values, the ancillary data used in those calls that corresponds to the header options you want may be directly specified as the control message in the series of control messages you pass as the argument to setsockopt(2). two ways to do the same thing, slay

• IPV6_CHECKSUM int * — get or set the byte offset into a packet where the 16-bit checksum lives. When it's set, that byte offset is where incoming packets are expected to have the checksums of their data stored, and where outgoing packets will have the checksums of their data computed and stored by the kernel. 🌸

A value of -1 means no checksums get checked on incoming packets, and no checksums get computed or stored on outgoing packets. free and unbothered But ngl — the offset of the checksum for ICMPv6 sockets cannot be relocated or turned off. that one's locked in place, sorry bestie 💀

• IPV6_V6ONLY int * — get or set whether only IPv6 connections can be made to this socket. For wildcard sockets, this can restrict connections to IPv6 only. v6 only, no plus-ones allowed
• IPV6_USE_MIN_MTU int * — get or set whether the minimal IPv6 maximum transmission unit (MTU) size will be used, so that fragmentation doesn't happen for subsequent outgoing datagrams. no crumbs, no fragments ✂️
• IPV6_AUTH_LEVEL int * — get or set the ipsec(4) authentication level. 🔐
• IPV6_ESP_TRANS_LEVEL int * — get or set the ESP transport level.
• IPV6_ESP_NETWORK_LEVEL int * — get or set the ESP encapsulation level.
• IPV6_IPCOMP_LEVEL int * — get or set the ipcomp(4) level.

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The IPV6_PKTINFO, IPV6_HOPLIMIT, IPV6_HOPOPTS, IPV6_DSTOPTS, and IPV6_RTHDR options will return ancillary data along with payload contents in subsequent recvmsg(2) calls, with cmsg_level set to IPPROTO_IPV6 and cmsg_type set to the respective option name value (e.g., IPV6_HOPTLIMIT). they come bearing gifts 💝

These options may also be used directly as ancillary cmsg_type values in sendmsg(2) to set options on the packet being transmitted by the call. The cmsg_level value must be IPPROTO_IPV6. For these options, the ancillary data object value format is the same as the value returned as explained for each when received with recvmsg(2). symmetry!! we love to see it

okay important note bestie: using sendmsg(2) to specify options on particular packets works only on UDP and raw sockets. To manipulate header options for packets on TCP sockets, only the socket options may be used. TCP said no per-packet vibes 😭

In some cases, there are multiple APIs defined for manipulating an IPv6 header field. A good example is the outgoing interface for multicast datagrams, which can be set by the IPV6_MULTICAST_IF socket option, through the IPV6_PKTINFO option, and through the sin6_scope_id field of the socket address passed to the sendto(2) system call. three doors, same room, drama ensues

Resolving these conflicts is implementation dependent. This implementation determines the value like this, in priority order:

1. Options specified by using ancillary data (i.e., sendmsg(2)) are considered first. 👑
2. Options specified by using IPV6_PKTOPTIONS to set "sticky" options are considered second.
3. Options specified by using the individual, basic, and direct socket options (e.g., IPV6_UNICAST_HOPS) are considered third.
4. Options specified in the socket address supplied to sendto(2) are the last choice. picked last in gym class fr

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🌈 Multicasting 🌈

IPv6 multicasting is supported only on AF_INET6 sockets of type SOCK_DGRAM and SOCK_RAW, and only on networks where the interface driver supports multicasting. if your driver isn't multicasting, we're simply not multicasting

Socket options (see above) that manipulate membership of multicast groups and other multicast options include IPV6_MULTICAST_IF, IPV6_MULTICAST_HOPS, IPV6_MULTICAST_LOOP, IPV6_LEAVE_GROUP, and IPV6_JOIN_GROUP. 🎀

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🔥 Raw Sockets 🔥

Raw IPv6 sockets are connectionless and are normally used with the sendto(2) and recvfrom(2) calls — though the connect(2) call may be used to fix the destination address for future outgoing packets, so that send(2) may be used instead, and the bind(2) call may be used to fix the source address for future outgoing packets, instead of having the kernel choose a source address. set it and forget it energy ✨

By using connect(2) or bind(2), raw socket input gets constrained to only packets with their source address matching the socket destination address if connect(2) was used, and to packets with their destination address matching the socket source address if bind(2) was used. you matched, you're in

If the proto argument to socket(2) is zero, the default protocol (IPPROTO_RAW) is used for outgoing packets. For incoming packets, protocols recognized by the kernel are not passed to the application socket (e.g., tcp(4) and udp(4)) — except for some ICMPv6 messages. The ICMPv6 messages not passed to raw sockets include echo, timestamp, and address mask requests. If proto is non-zero, only packets with this protocol will be passed to the socket. gatekeeping, but make it protocol-level

IPv6 fragments are also not passed to application sockets until they have been reassembled. If reception of all packets is desired, link-level access (such as bpf(4)) must be used instead. no half-finished fragments in this house 💅

Outgoing packets automatically have an IPv6 header prepended to them (based on the destination address and the protocol number the socket was created with). Incoming packets are received by an application without the IPv6 header or any extension headers. headers stay in the group chat, not in your inbox

Outgoing packets will be fragmented automatically by the kernel if they are too large. Incoming packets will be reassembled before being sent to the raw socket, so packet fragments or fragment headers will never be seen on a raw socket. seamless, as she should be 🦋

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💖 EXAMPLES 💖

The following determines the hop limit on the next packet received:

struct iovec iov[2];
u_char buf[BUFSIZ];
struct cmsghdr *cm;
struct msghdr m;
int optval;
bool found;
u_char data[2048];

/* Create socket. */

(void)memset(&m, 0, sizeof(m));
(void)memset(&iov, 0, sizeof(iov));

iov[0].iov_base = data;         /* buffer for packet payload */
iov[0].iov_len = sizeof(data);  /* expected packet length */

m.msg_name = &from;             /* sockaddr_in6 of peer */
m.msg_namelen = sizeof(from);
m.msg_iov = iov;
m.msg_iovlen = 1;
m.msg_control = (caddr_t)buf;   /* buffer for control messages */
m.msg_controllen = sizeof(buf);

/*
 * Enable the hop limit value from received packets to be
 * returned along with the payload.
 */
optval = 1;
if (setsockopt(s, IPPROTO_IPV6, IPV6_HOPLIMIT, &optval,
    sizeof(optval)) == -1)
        err(1, "setsockopt");

found = false;
do {
        if (recvmsg(s, &m, 0) == -1)
                err(1, "recvmsg");
        for (cm = CMSG_FIRSTHDR(&m); cm != NULL;
             cm = CMSG_NXTHDR(&m, cm)) {
                if (cm->cmsg_level == IPPROTO_IPV6 &&
                    cm->cmsg_type == IPV6_HOPLIMIT &&
                    cm->cmsg_len == CMSG_LEN(sizeof(int))) {
                        found = true;
                        (void)printf("hop limit: %d\n",
                            *(int *)CMSG_DATA(cm));
                        break;
                }
        }
} while (!found);

study it, run it, become it. main character code fr fr 🫶

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💀 DIAGNOSTICS 💀

A socket operation may fail with one of the following errors returned:

• [EISCONN] — when trying to establish a connection on a socket which already has one, or when trying to send a datagram with the destination address specified and the socket is already connected. you're already taken, bestie
• [ENOTCONN] — when trying to send a datagram, but no destination address is specified, and the socket has not been connected. no address, no plans, no delivery
• [ENOBUFS] — when the system runs out of memory for an internal data structure. the system is full, respectfully 😭
• [EADDRNOTAVAIL] — when an attempt is made to create a socket with a network address for which no network interface exists. address said "I don't know her"
• [EACCES] — when an attempt is made to create a raw IPv6 socket by a non-privileged process. no root, no raw socket. the bouncer said no. 🚫

The following errors specific to IPv6 may occur when setting or getting header options:

• [EINVAL] — an unknown socket option name was given. who is she? never heard of her
• [EINVAL] — an ancillary data object was improperly formed. the vibes were off, structurally

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✨ SEE ALSO ✨

getsockopt(2), recv(2), send(2), setsockopt(2), socket(2), CMSG_DATA(3), if_nametoindex(3), bpf(4), icmp6(4), inet6(4), ip(4), netintro(4), tcp(4), udp(4)

• W. Stevens and M. Thomas, Advanced Sockets API for IPv6, RFC 2292, February 1998.
• S. Deering and R. Hinden, Internet Protocol, Version 6 (IPv6) Specification, RFC 2460, December 1998.
• R. Gilligan, S. Thomson, J. Bound, and W. Stevens, Basic Socket Interface Extensions for IPv6, RFC 2553, March 1999.
• W. Stevens, B. Fenner, and A. Rudoff, UNIX Network Programming, third edition.

the sacred texts 📚✨

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👑 STANDARDS 👑

Most of the socket options are defined in RFC 2292 or RFC 2553. The IPV6_V6ONLY socket option is defined in RFC 3493 Section 5.3. The IPV6_PORTRANGE socket option and the conflict resolution rule are not defined in the RFCs and should be considered implementation dependent. homegrown rules, handle with care 🎀

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and that's the whole page bestie!! 🦋 this one's from Debian, dated August 19, 2018, and it's the IP6(4) page — go forth and socket responsibly, love u, bye!! 💅💖🌸