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Share IPv6 address with host OS by default

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Adds support for clat-v6-addr=shared and make this the default
behaviour. This makes the CLAT function share the address the host OS
uses for direct IPv6 connection towards the PLAT prefix, thus removing
the previous requirement for a secondary IPv6 address dedicated to the
CLAT function.

When using a shared address in this manner, enable connection tracking
marking by default (so that direct IPv6 connections from the host OS to
IPv4 destinations behind the PLAT keeps working) and disable Proxy-ND
(as there is no need for it, as the host OS kernel will handle NDP
interactions all on its own).

To use the previous default behaviour, use clat-v6-addr=derived.

Closes #25
Closes #46
This commit is contained in:
Tore Anderson
2025-02-02 12:06:36 +01:00
parent 03042228be
commit 33252dcb13
2 changed files with 81 additions and 37 deletions
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76
README.pod
View File

@@ -193,32 +193,41 @@ functional references to it in application-level payload, and so on.
The default address is one from I<RFC 7335>.
=item B<clat-v6-addr=ipv6-address> (default: auto-generated)
=item B<clat-v6-addr=[ipv6-address|shared|derived> (default: I<shared>)
The IPv6 address of the CLAT. Traffic to/from the B<clat-v4-addr> will be
translated into this address. When using B<clatd> as an SIIT-DC Edge Relay, you
will want to set this to the same IPv6 address in the Explicit Address Mapping
configured in the SIIT-DC Border Relay.
By default, B<clatd> will attempt to figure out which network device will be
used for traffic towards the PLAT, see if there is any SLAAC-based globally
scoped addresses on it (i.e., a /64 with '0xfffe' in the middle of the
Interface ID), and will if so substitute that '0xfffe' value with '0xc1a7'
("clat") to generate a CLAT IPv6 address.
When set to I<shared> (the default), B<clatd> will re-use and share the primary
IPv6 address used by the host OS when communicating with directly with the PLAT
prefix. This allows B<clatd> to operate without requiring a separate, dedicated
IPv6 address assigned to the CLAT function. While this mode is the most
compatible with various network deployments, it comes with certain trade-offs,
see the B<LIMITATIONS> section for more information.
If only a non-SLAAC global address is found on the PLAT-facing device,
B<clatd> will substitute its Interface ID with a random integer and use the
result as the CLAT IPv6 address. It will only do so if the prefix length is
/120 or smaller, as otherwise the risk of IID collisions is considered to be
too high. Note that on most Perl platforms, the I<rand()> function is limited
to 48 bits, which means that for longer IIDs, the least significant bits will
be all 0.
When set to I<derived>, B<clatd> will attempt to figure out which network
device will be used for traffic towards the PLAT, see if there is any
SLAAC-based globally scoped addresses on it (i.e., a /64 with '0xfffe' in the
middle of the Interface ID), and will if so substitute that '0xfffe' value with
'0xc1a7' ("clat") to generate a CLAT IPv6 address.
If only a non-SLAAC global address is found on the PLAT-facing device, B<clatd>
will substitute its Interface ID with a random integer and use the result as
the CLAT IPv6 address. It will only do so if the prefix length is /120 or
smaller, as otherwise the risk of IID collisions is considered to be too high.
Note that on most Perl platforms, the I<rand()> function is limited to 48 bits,
which means that for longer IIDs, the least significant bits will be all 0.
The I<derived> mode is not guaranteed to result in a working configuration, see
the B<LIMITATIONS> section for more information.
If multiple addresses are found in either category, the one that shares the
longest common prefix with the PLAT prefix will be preferred when deriving
the CLAT IPv6 address according to the algorithm described above.
=item B<ctmark> (default: 0)
=item B<ctmark> (default: I<0xc1a7> if I<clat-v6-addr=shared>, otherwise I<0>)
If set to a non-zero integer, nftables will be used to mark outgoing
connections through the CLAT with this connection tracking mark, and the Linux
@@ -282,7 +291,7 @@ DNS64 answers using the method in I<RFC 7050>.
The IPv6 translation prefix fallback. This is used if no plat-prefix is set
or auto detected.
=item B<proxynd-enable> (default: I<yes>)
=item B<proxynd-enable> (default: I<no> if I<clat-v6-addr=shared>, otherwise I<yes>)
Controls whether or not B<clatd> should add a Proxy-ND entry for the CLAT IPv6
address on the network device facing the PLAT. This is probably necessary
@@ -292,6 +301,10 @@ point-to-point links like PPP or 3GPP mobile broadband, as in those cases
IPv6 ND isn't used. However it doesn't hurt to add Proxy-ND entries in that
case, either.
If the CLAT is sharing an IPv6 address with the host OS, this is not necessary
as the host OS will be handling NDP anyway, so Proxy-ND does get enabled by
default when I<clat-v6-addr> is set to its default value I<shared>.
Any entries added wil be removed when B<clatd> is shutting down.
=item B<route-table> (default: I<0xc1a7>)
@@ -406,15 +419,25 @@ unset or 0, there is no default.
=head1 LIMITATIONS
If no IPv6 addresses on the PLAT-facing device are EUI-64-derived (e.g., when
using SLAAC with I<RFC 4941> or I<RFC 7217> privacy addressing or static
addresses), B<clatd> will generate and use an CLAT IPv6 address using a random
Interface ID from the same subnet prefix (if it is /120 or shorter).
I<RFC 6877> suggests DHCPv6 IA_PD should be attempted in this case instead, but
this isn't currently implemented.
When using B<clat-v6-addr=shared> (or when B<clat-v6-addr> is set to another
address assigned to a local interface) and B<ctmark> is set to 0, the host OS
will not be able to communicate bi-directionally with IPv4 destinations
directly through the PLAT (e.g., I<ping6 64:ff9b::192.0.2.1>). This is because
the response traffic will be routed back to the CLAT, and ultimately return to
the Linux kernel as an IPv4 packet, which does not match the outgoing IPv6
socket. Such direct communication is normal when using DNS64 synthesis for all
queries (as opposed to just I<ipv4only.arpa>).
When using B<clat-v6-addr=derived> and no IPv6 addresses on the PLAT-facing
device are EUI-64-derived (e.g., when using SLAAC with I<RFC 4941> or I<RFC
7217> privacy addressing or static addresses), B<clatd> will generate and use
an CLAT IPv6 address using a random Interface ID from the same subnet prefix
(if it is /120 or shorter). I<RFC 6877> suggests DHCPv6 IA_PD should be
attempted in this case instead, but this isn't currently implemented.
B<clatd> will not attempt to perform Duplicate Address Detection for the IPv6
address it generates. This is a violation of I<RFC 6877>.
address it generates when using B<clat-v6-addr=derived>. This is a violation of
I<RFC 6877>.
There is no guarantee that the generated CLAT IPv6 address is in fact usable,
as the network might block its use.
@@ -423,15 +446,6 @@ If the upstream network is using DHCPv6, B<clatd> will not be able to generate
a CLAT IPv6 address at all, due to the fact that DHCPv6-assigned addresses do
not carry a prefix length.
If B<clat-v6-addr> is set to an address assigned to a local interface and
B<ctmark> is not set, the host OS will not be able to communicate
bi-directionally with IPv4 destinations directly through the PLAT (e.g.,
I<ping6 64:ff9b::192.0.2.1>). This is because the response traffic will be
routed back to the CLAT, and ultimately return to the Linux kernel as an IPv4
packet, which does not match the outgoing IPv6 socket. Such direct
communication is normal when using DNS64 synthesis for all queries (as opposed
to just I<ipv4only.arpa>).
B<clatd> will not attempt to perform a connectivity check to a discovered PLAT
prefix before setting up the CLAT, as I<RFC 7050> suggest it should.

42
clatd
View File

@@ -38,17 +38,17 @@ $CFG{"script-up"} = undef; # sh script to run when starting up
$CFG{"script-down"} = undef; # sh script to run when shutting down
$CFG{"clat-dev"} = "clat"; # TUN interface name to use
$CFG{"clat-v4-addr"} = "192.0.0.1"; # from RFC 7335
$CFG{"clat-v6-addr"} = undef; # derive from existing SLAAC addr
$CFG{"clat-v6-addr"} = "shared"; # re-use primary address from host OS
$CFG{"dns64-servers"} = undef; # use system resolver by default
$CFG{"cmd-ip"} = "ip"; # assume in $PATH
$CFG{"cmd-nft"} = "nft"; # assume in $PATH
$CFG{"cmd-tayga"} = "tayga"; # assume in $PATH
$CFG{"ctmark"} = 0; # match ctmark for routing pkts to CLAT
$CFG{"ctmark"} = undef; # match ctmark for routing pkts to CLAT
$CFG{"forwarding-enable"} = 1; # enable ipv6 forwarding?
$CFG{"plat-dev"} = undef; # PLAT-facing device, default detect
$CFG{"plat-prefix"} = undef; # detect using DNS64 by default
$CFG{"plat-fallback-prefix"} = undef; # fallback prefix if no prefix is found
$CFG{"proxynd-enable"} = 1; # add proxy-nd entry for clat?
$CFG{"proxynd-enable"} = undef; # add proxy-nd entry for clat?
$CFG{"route-table"} = 0xc1a7; # add route to CLAT in this table
$CFG{"tayga-conffile"} = undef; # make a temporary one by default
$CFG{"tayga-v4-addr"} = "192.0.0.2"; # from RFC 7335
@@ -394,6 +394,7 @@ sub get_plat_prefix {
sub get_plat_dev {
d("get_plat_dev(): finding which network dev faces the PLAT");
my $plat_dev;
my $plat_dev_srcip;
my $plat_prefix = cfg("plat-prefix");
if(!$plat_prefix) {
err("get_plat_dev(): No PLAT prefix to work with");
@@ -406,9 +407,13 @@ sub get_plat_dev {
d("get_plat_dev(): Found PLAT-facing device: $1");
$plat_dev = $1;
}
if(/ src (\S+) /) {
d("get_plat_dev(): Found PLAT-facing device source IP: $1");
$plat_dev_srcip = $1;
}
}
close($fd) or err("get_plat_dev(): 'ip -6 route get $plat_prefix' failed");
return $plat_dev;
return ($plat_dev, $plat_dev_srcip);
}
@@ -766,9 +771,34 @@ if(!$CFG{"plat-prefix"}) {
}
p("Using PLAT (NAT64) prefix: $CFG{'plat-prefix'}");
}
$CFG{"plat-dev"} ||= get_plat_dev();
my @plat_dev = get_plat_dev();
$CFG{"plat-dev"} ||= $plat_dev[0];
p("Device facing the PLAT: ", $CFG{"plat-dev"});
$CFG{"clat-v6-addr"} ||= get_clat_v6_addr();
if($CFG{"clat-v6-addr"} eq "shared") {
if(!$plat_dev[1]) {
err("Could not determine source IP facing the PLAT, needed due to using ",
"clat-v6-addr=shared");
}
$CFG{"clat-v6-addr"} = $plat_dev[1];
if(!defined($CFG{"proxynd-enable"})) {
$CFG{"proxynd-enable"} = 0;
}
if(!defined($CFG{"ctmark"})) {
$CFG{"ctmark"} = 0xc1a7;
}
} elsif($CFG{"clat-v6-addr"} eq "derived") {
$CFG{"clat-v6-addr"} = get_clat_v6_addr();
}
# Fill defaults for proxynd-enable and ctmark for clat-v6-addr!=shared
if(!defined($CFG{"proxynd-enable"})) {
$CFG{"proxynd-enable"} = 1;
}
if(!defined($CFG{"ctmark"})) {
$CFG{"ctmark"} = 0;
}
p("Using CLAT IPv4 address: ", $CFG{"clat-v4-addr"});
p("Using CLAT IPv6 address: ", $CFG{"clat-v6-addr"});
if(!$CFG{"v4-defaultroute-advmss"} and cfgint("v4-defaultroute-mtu")) {