v1.17.0
ADDED: * netx: ** docs updated ** added NOTES.adoc for more extensive info that doesn't need to be in the library docs ** Addr4in6Compat() ** Addr4in6Mapped() ** Addr4in6Mapped() ** HasSubnet() ** IP4MapPfx4to6() ** IP4MapPfx6to4() ** IPSetCombined() ** IPSetFrom() ** IPSetFromNative() ** IsAddr4Compat() ** IsPublic() ** MustIPSetCombined() ** MustIPSetFrom() ** MustIPSetFromNative() ** Pfx4in6Compat() (untested) ** Pfx4in6Mapped() (untested) ** UnmapAddr() ** UnmapPfx() ** Better support in other functions for IPv4-Compatible/IPv4-Mapped addressing
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brent saner
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= `netx` Notes
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Brent Saner <bts@square-r00t.net>
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Last rendered {localdatetime}
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:doctype: book
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:docinfo: shared
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:data-uri:
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:imagesdir: images
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:sectlinks:
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:sectnums:
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:sectnumlevels: 7
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:toc: preamble
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:toc2: left
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:idprefix:
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:toclevels: 7
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:source-highlighter: rouge
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:docinfo: shared
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[id="4in6"]
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== IPv4 Addresses as IPv6
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There are, effectively, two RFC-specified ways of representing an IPv4 address in IPv6 notation/address space/context.
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Per https://datatracker.ietf.org/doc/html/rfc4291#section-2.5.5[RFC 4291 § 2.5.5^]:
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* _IPv4-Compatible IPv6 Address_ (https://datatracker.ietf.org/doc/html/rfc4291#section-2.5.5.1[RFC 4291 § 2.5.5.1^]) -- `::/96`
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** e.g. `::192.0.2.10`
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* _IPv4-Mapped IPv6 Address_ (https://datatracker.ietf.org/doc/html/rfc4291#section-2.5.5.2[RFC 4291 § 2.5.5.2^]) -- `::ffff:0:0/96`
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** e.g. `::ffff:192.0.2.10`
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The former is considered deprecated, whereas the latter is considered the modern proper representation.
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Some naïve or legacy software, however, still use the former.
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An IPv6 address is *128 bits* (hence a prefix length/mask of `/128` representing a complete address), or *16 bytes* (`128 / 8`).
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An IPv4 address is *32 bits* (hence a prefix length/mask of `/32` representing a complete address), or *4 bytes* (`32 / 8`).
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This is why both the "IPv4-compatible" and "IPv4-mapped" IPv6 addresses are within a reserved prefix length of 96 bits (`/96`) - it leaves a remaining
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*32 bits* (`128 - 96`) for a complete IPv4 address.
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[IMPORTANT]
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====
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The IPv4-Compatible and IPv4-Mapped prefixes *are not* intended for routing!
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They are used for *syntactic/contextual* use.
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For routing/actual IPv4 <=> IPv6 translation, see https://datatracker.ietf.org/doc/html/rfc6052[RFC 6052^] and https://datatracker.ietf.org/doc/html/rfc8215[RFC 8215^].
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This library may add supporting functions for these _translation_ prefixes at a later time.
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====
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[id="4in6_addr"]
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=== Addresses
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While https://pkg.go.dev/net/netip#ParseAddr[`net/netip.ParseAddr`^] (or https://pkg.go.dev/net/netip#MustParseAddr[`net/netip.MustParseAddr`^]) will handle
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*parsing* the "IPv4-Compatible" format just fine, it will treat it *as* an IPv6 address -- not an "IPv6-wrapped/IPv6-mapped IPv4" address.
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[%collapsible]
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.For example...
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====
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[source,go]
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----
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package main
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import (
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"fmt"
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"net/netip"
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)
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func main() {
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fmt.Printf(
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"::192.0.2.10 -> %s\n",
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netip.MustParseAddr("::192.0.2.10").String(),
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)
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}
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----
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will print:
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[source,text]
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----
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::192.0.2.10 -> ::c000:20a
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----
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====
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For addresses, this is an easy enough fix:
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. Take the IPv6 as a byte slice, `b` (`b = ip4in6.AsSlice()`)
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. Slice out the last 4 bytes as `b2` (`b2 = b[12:16]`)
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. Parse `b2` as an IPv4 address (`ip4 = netip.FromSlice(b2)`)
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For the visual learners, the original address (`::192.0.2.10` or, more properly, `::c000:20a`) can be represented in multiple ways.
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[%collapsible]
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.For instance...
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====
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* A "canonical"/shorted IPv6 address: `::c000:20a`
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* A fully "exploded" IPv6 address: `0000:0000:0000:0000:0000:0000:c000:020a`
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* Hex bytes:
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** `00:00:00:00:00:00:00:00:00:00:00:00:C0:00:02:0A`
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** `0x000000000000000000000000C000020A`
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** Via `xxd(1)`:
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+
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[source,text]
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----
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# xxd -c4 -g 1
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00000000: 00 00 00 00 ....
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00000004: 00 00 00 00 ....
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00000008: 00 00 00 00 ....
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0000000c: c0 00 02 0a ....
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----
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** Via `hexdump(1)`:
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+
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[source,text]
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----
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# hexdump -v -e '"%08_ax: " 4/1 "0x%02X ""\n"'
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00000000: 0x00 0x00 0x00 0x00
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00000004: 0x00 0x00 0x00 0x00
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00000008: 0x00 0x00 0x00 0x00
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0000000c: 0xC0 0x00 0x02 0x0A
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----
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* A Go byte slice:
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** Either as explicit `byte`:
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+
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[source,go]
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----
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[]byte{
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0x00, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00,
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0x00, 0x00, 0x00, 0x00,
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0xC0, 0x00, 0x02, 0x0A,
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}
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----
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** Or as `uint8` (a `byte` is, quite literally, https://go.dev/ref/spec#Numeric_types[just a type alias for `uint8`^]):
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+
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[source,go]
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----
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[]byte{
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uint8(0), uint8(0), uint8(0), uint8(0),
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uint8(0), uint8(0), uint8(0), uint8(0),
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uint8(0), uint8(0), uint8(0), uint8(0),
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uint8(192), uint8(0), uint8(2), uint8(10),
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}
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----
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====
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When seeing it represented as a slice of `uint8`, some things should start to click mentally looking at the last
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four bytes.
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An IPv4 address can then be derived by taking the last four bytes (`[]byte{...}[12:16]`):
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* `[]byte{0xC0, 0x00, 0x02, 0x0A}`
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* `[]byte(uint8(192), uint8(0), uint8(2), uint8(10))`
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As you may have guessed by now, this is the byte format of the IPv4 address `192.0.2.10`.
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The `netx.UnmapAddr()` function handles this cleanly.
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'''
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If you instead prefer to think in _prefixing/masking_, the last four bytes are *32 bits* (`8 * 4`) -- the size of an IPv4 address.
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The "compatible" mapping is '::/96', and an IPv6 address is *128 bits* (*16 bytes*, `128 / 8`) total -- `128 - 96 == 32 bits`, or *4 bytes*.
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[id="4in6_pfx"]
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=== Prefixes
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Prefixes are a little more tricky.
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Prefixing for IPv4-Compatible/IPv4-Mapped IPv6 addresses works a bit oddly because generally the "4-in-6" addresses are
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meant to be purely representative - a way to represent IPv4 addresses in IPv6 implementations.
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But there are cases where one needs a *prefix* represented.
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This leads to the awkward case where an IPv4 prefix length (e.g. `/24`) needs to be represented as an IPv6 prefix length (e.g. `/120`).
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While this module (`netx`) does offer helper functions to calculate this (`netx.IP4MapPfx6to4()`, `netx.IP4MapPfx4to6()`),
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the `netx.UnmapPfx()` function is offered as a convenience function that handles the translation from an IPv6 representation
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of an IPv4 prefix into a fully "native" IPv4 prefix.
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[NOTE]
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====
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`netx.IP4MapPfx4to6(len4 uint8)` simply takes the prefix length `len4`, and (if `len4 <= 32`) adds `96`.
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`netx.IP4MapPfx6to4(len6 uint8)` simply takes the prefix length `len6`, and (if `len6 >= 96`) subtracts `96`.
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====
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This is achieved by:
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. Taking the address from the prefix (`ip4in6 = pfx4in6.Addr()`)
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.. And <<4in6_addr, converting to native IPv4>> (`ip4 = netx.UnmapAddr(ip4in6)`)
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. Taking the IPv6 prefix length (`len6 = pfx4in6.Bits()`)
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.. And converting it to an IPv4 prefix length (`len4 = netx.IP4MapPfx6to4(uint8(len6))`)
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. Then re-assembling (`pfx4 = netip.PrefixFrom(ip4, int(len4))`)
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[id="misc"]
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== Miscellaneous
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[id="misc_unspec"]
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=== Unspecified Address
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There are two "unspecified" addresses for each family:
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* IPv4
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** `0.0.0.0/0`
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** `0.0.0.0/32`
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* IPv6
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** `::/0`
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** `::/128`
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For each family, the `/0` prefix length
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