Partial

RFC 4787 - Network Address Translation (NAT) Behavioral Requirements for Unicast UDP

Every requirement this repository extracted from RFC 4787, the tests bound to it, and what a reader has verified about them. This summary is enrolled and gated by ./le rfc check.

Overview

Positive

what Ze has

Tested both ways0.0%0 of 13 gated MUSTs

a positive test proves Ze does what the requirement demands and a negative one proves it refuses what the requirement forbids

One polarity plus reason0.0%0 of 13 gated MUSTs

the requirement admits no counter-case, so one polarity plus a recorded reason is the whole proof available for it

One polarity, unexcused0.0%0 of 13 gated MUSTs

one direction is tested, the other is neither tested nor excused, and nothing states which

Partial proof; remaining gap0.0%0 of 13 gated MUSTs

scoped tests exist; the remaining obligation is unmet or unproven, with zero whole-requirement credit

Proven by a recorded break0.0%0 of 0 tagged units

a red was observed once under a recorded procedure, and the unit, the claim and the producer it rested on still hash to what was recorded. The break is not re-run. A test pair is not a proof until one has been observed

Neutral

measures that are neither good news nor bad

Gated MUSTs13of 29 this summary declares

MUST-level requirements the gate HOLDS. A population, not a result: the shares beside it are what says how Ze stands

Out of scope0of 13 gated MUSTs

an obligation that does not bind Ze. A {not-applicable} annotation says it never bound; a {feature-declined} annotation says its condition is an optional feature Ze does not offer, and quotes the RFC sentence that makes it optional. Scope, not coverage: it stays in the denominator every share on this page is taken over

Not applicable0.0%0 of 13 gated MUSTs

a {not-applicable} annotation says the obligation does not bind Ze, so no test is owed for it. It stays in the denominator every share here is taken over

Met below Ze53.8%7 of 13 gated MUSTs

a {lower-layer} annotation says a layer under Ze performs the behavior, on state Ze installs into that layer, and names the producer that installs it. The obligation binds Ze and is met; Ze proves none of it, because its own boundary carries no value the behavior reads

Optional feature declined0.0%0 of 13 gated MUSTs

a {feature-declined} annotation says the obligation is conditional on a feature the RFC makes optional and Ze does not offer, and it quotes the sentence that makes it optional. The condition is false, so nothing is owed and nothing is missing. It stays in the denominator every share here is taken over

Linux cannot do this0.0%0 of 13 gated MUSTs

Linux cannot provide the primitive this requirement needs. The obligation still binds Ze and remains unmet; it stays in every denominator and earns no proof credit

Negative

what Ze owes

No test at all46.2%6 of 13 gated MUSTs

no test carries the requirement id, whether or not a gap states why

The 9 shares marked as a part above are the whole of the 13 gated MUSTs: they add to 100%. Proven by a recorded break is a share of TAGGED UNITS, a different population, so it is not one of them.

How to read the colors

A color names what the measure MEANS, not how well Ze scores on it. Green is a good outcome at any value, red is a bad one, and neither a population nor a scope count is an outcome, so both take no color. The number under the label is what says how far Ze has got.

Card	Tone here	Why that color
Gated MUSTs	neutral	no color: a population is a scale, and a larger one is neither good news nor bad. It is the accounting total
Out of scope	neutral	no color: an obligation that never bound Ze is neither an achievement nor a failure, and counting it either way would be a claim
Tested both ways	ok	green at every value: a test pair is the outcome this gate exists to produce, and the share under the label is what says how far Ze has got
One polarity plus reason	ok	green at every value: where no counter-case exists, one polarity IS the complete answer, and a recorded reason is what the gate demands beside it
One polarity, unexcused	ok	green at zero, RED above it: half a proof with no reason for the other half
Partial proof; remaining gap	ok	green at zero, RED above it: a tested clause cannot prove the whole requirement
No test at all	bad	green at zero, RED above it: a binding obligation nothing exercises is a claim with nothing behind it, whether or not a reason is stated
Not applicable	neutral	no color: an obligation that never bound Ze is neither an achievement nor a failure, and counting it either way would be a claim
Met below Ze	neutral	no color: an obligation met below Ze is neither a test Ze wrote nor work Ze owes, and the two green shares above are what says how much Ze proves itself
Optional feature declined	neutral	no color: an obligation whose condition Ze never meets is neither an achievement nor a failure. The absent FEATURE is disclosed on the RFC's own status row, as an implementation gap a later scope decision can revisit
Linux cannot do this	neutral	neutral at every value: an evidenced platform limit is neither a proof nor a scope exemption
Proven by a recorded break	ok	green at every value: an observed break is the outcome the discrimination gate exists to produce. The denominator is TAGGED UNITS, not obligations, so this share is not one of the parts above
Audit verdicts	warn	RED on the first weak, wrong or unimplemented verdict, amber while a verdict is no longer current or a gated MUST is unjudged, green when every one is judged sound and current

At a glance

Field	Value
Public status	Partial
Enrolment	Enrolled
Requirements	29
Gated MUST-level	13
Not applicable, so out of scope	0
Declared gaps	6
Declared gaps a test demonstrates	0
Gated with no test	0
Nightly-only evidence	0
Test tags	0
Tagged units	0
Recorded audit verdicts	0
Discrimination records	0
Summary	`rfc/short/rfc4787.md`
Requirement shard	`rfc/requirements/rfc4787.md`
RFC text	`rfc/full/rfc4787.txt`

Enrolment

Enrolled: NAT behavioral requirements for unicast UDP (BCP 127). Every MUST-level row is classified: the behaviors Linux netfilter performs on the NAT rules ze installs are {lower-layer}, and the behaviors neither ze nor the Linux defaults provide (endpoint-independent mapping, the two-minute UDP timer, hairpinning, deterministic translation, address-conflict handling) are {gap}.

What the public ledger says

Status: Partial

What the ledger says is covered

Source NAT, destination NAT and masquerade are configured through the firewall YANG tree and lowered into nftables NAT chains
Linux nf_nat never overloads a port, refreshes a mapping on outbound traffic, keeps a mapping across ICMP, reassembles out-of-order fragments through nf_defrag, and fragments or answers DF=1 on the forwarding path.

What the ledger says remains

Endpoint-independent mapping is not guaranteed: nf_nat keeps the source port only while the translated tuple is unique and remaps per destination otherwise, masquerade { random; } randomizes per flow, and VPP NAT44-ED is endpoint-dependent (RFC4787-4.1-1, RFC4787-8-1). Ze sets no conntrack timeout, so a UDP flow seen in one direction expires after the Linux default of 30 seconds (RFC4787-4.3-1). No hairpinning is configured (RFC4787-6-1, RFC4787-6-2). A dynamically configured external address is not checked against the internal network (RFC4787-4.4-1).

Coverage

Bucket	Count	What it counts
Positive and negative tests	0	one part of the gated population
Annotated (including scoped evidence)	13	one part of the gated population
One polarity only	0	one part of the gated population
No test and no annotation	0	one part of the gated population
Partial proof; remaining gap (subset of annotated; zero whole-requirement credit)	0	an overlay: each of these is also counted by the part it falls in
Evidence that runs nightly only	0	an overlay: each of these is also counted by the part it falls in
Derived from other rows	0	outside the gated population: each asserts nothing and derives its state from the rows it names, which the parts above already count
Gated MUST-level requirements	13	every gated MUST falls in exactly one bucket above

Annotated (including scoped evidence) (13): RFC4787-4.1-1, RFC4787-4.2.1-1, RFC4787-4.3-1, RFC4787-4.3-2, RFC4787-4.4-1, RFC4787-6-1, RFC4787-6-2, RFC4787-8-1, RFC4787-9-1, RFC4787-10-1, RFC4787-10-2, RFC4787-11-1, RFC4787-11-2

Attainable ceiling

Mandatory ceiling: 6 of 13 (46.2%); excluded: Met by a lower layer: 7 of 13 (53.8%). Optional ceiling: 16 of 16 (100.0%). Lowest mandatory ceiling: rfc4787 6 of 13 (46.2%). Lowest optional ceiling: rfc4787 16 of 16 (100.0%). Ceilings describe current classifications, not an immutable Linux maximum. Not-applicable and feature-declined classifications can be revisited. Every gap remains work Ze owes.

Requirements

Requirement	Level	Section	Tests
`RFC4787-4.1-1`	REQ-1: A NAT MUST have an "Endpoint-Independent Mapping" behavior. (§4.1)
	MUST	4.1	positive`no test`no positive test negative`no test`no negative test {gap}: nf_nat preserves the internal source port only while the translated 5-tuple is unique and otherwise allocates another port for that destination, the masquerade random and fully-random flags ze lowers randomize per flow, and VPP NAT44-ED maps per endpoint; internal/plugins/firewall/nft/lower_linux.go lowerMasquerade
`RFC4787-4.2.1-1`	REQ-3: A NAT MUST NOT have a "Port assignment" behavior of "Port overloading". (§4.2.1)
	MUST NOT	4.2.1	positive`no test`no positive test negative`no test`no negative test {lower-layer}: Linux nf_nat; internal/plugins/firewall/nft/lower_linux.go::lowerMasquerade installs the masquerade expression and nf_nat's unique-tuple allocation never gives two internal endpoints one external tuple
`RFC4787-4.3-1`	REQ-5: A NAT UDP mapping timer MUST NOT expire in less than two minutes, unless REQ-5a applies. (§4.3)
	MUST NOT	4.3	positive`no test`no positive test negative`no test`no negative test {gap}: ze installs no conntrack timeout policy, so a UDP flow seen in one direction expires after Linux's nf_conntrack_udp_timeout default of 30 seconds; internal/plugins/firewall/nft/lower_linux.go
`RFC4787-4.3-2`	REQ-6: The NAT mapping Refresh Direction MUST have a "NAT Outbound refresh behavior" of "True". (§4.3)
	MUST	4.3	positive`no test`no positive test negative`no test`no negative test {lower-layer}: Linux nf_conntrack; internal/plugins/firewall/nft/lower_linux.go::lowerMasquerade installs the NAT rule and nf_conntrack refreshes the entry's timeout on every packet of the flow, outbound included
`RFC4787-4.4-1`	REQ-7: A NAT device whose external IP interface can be configured dynamically MUST either (1) automatically ensure that its internal network uses IP addresses that do not conflict with its external network, or (2) be able to translate and forward traffic between all internal nodes and all external nodes whose IP addresses numerically conflict with the internal network. (§4.4)
	MUST	4.4	positive`no test`no positive test negative`no test`no negative test {gap}: ze neither checks a DHCP- or PPP-learned external address against the internal subnets nor translates traffic to a conflicting external range
`RFC4787-6-1`	REQ-9: A NAT MUST support "Hairpinning". (§6)
	MUST	6	positive`no test`no positive test negative`no test`no negative test {gap}: ze generates no hairpin rule, and a packet from an internal host to another internal host's external mapping matches no conntrack reply tuple and is delivered to the NAT itself
`RFC4787-6-2`	a) A NAT Hairpinning behavior MUST be "External source IP address and port". (§6)
	MUST	6	positive`no test`no positive test negative`no test`no negative test {gap}: no hairpinning exists (RFC4787-6-1), so there is no hairpin source to translate
`RFC4787-8-1`	REQ-11: A NAT MUST have deterministic behavior, i.e., it MUST NOT change the NAT translation (Section 4) or the Filtering (Section 5) Behavior at any point in time, or under any particular conditions. (§8)
	MUST	8	positive`no test`no positive test negative`no test`no negative test {gap}: nf_nat preserves the port when it is free and switches to another port under contention, which changes the mapping behavior under load
`RFC4787-9-1`	Receipt of any sort of ICMP message MUST NOT destroy the NAT mapping. (§9)
	MUST NOT	9	positive`no test`no positive test negative`no test`no negative test {lower-layer}: Linux nf_conntrack; internal/plugins/firewall/nft/lower_linux.go::lowerMasquerade installs the NAT rule and nf_conntrack classifies an ICMP error as RELATED to the entry without deleting it
`RFC4787-10-1`	REQ-13: If the packet received on an internal IP address has DF=1, the NAT MUST send back an ICMP message "Fragmentation needed and DF set" to the host, as described in [RFC0792]. (§10)
	MUST	10	positive`no test`no positive test negative`no test`no negative test {lower-layer}: Linux IPv4 forwarding; internal/component/iface/dispatch.go::SetMTU sets the egress MTU and the kernel forwarding path answers an oversize DF=1 packet with ICMP Fragmentation Needed
`RFC4787-10-2`	If the packet has DF=0, the NAT MUST fragment the packet and SHOULD send the fragments in order. (§10)
	MUST	10	positive`no test`no positive test negative`no test`no negative test {lower-layer}: Linux IPv4 forwarding; internal/component/iface/dispatch.go::SetMTU sets the egress MTU and the kernel forwarding path fragments an oversize DF=0 packet in order
`RFC4787-11-1`	REQ-14: A NAT MUST support receiving in-order and out-of-order fragments, so it MUST have "Received Fragment Out of Order" behavior. (§11)
	MUST	11	positive`no test`no positive test negative`no test`no negative test {lower-layer}: Linux nf_defrag_ipv4; internal/plugins/firewall/nft/lower_linux.go::lowerMasquerade installs a NAT rule, which loads conntrack and its defragmentation hook, and the reassembly queue accepts fragments in any order
`RFC4787-11-2`	a) A NAT's out-of-order fragment processing mechanism MUST be designed so that fragmentation-based DoS attacks do not compromise the NAT's ability to process in-order and unfragmented IP packets. (§11)
	MUST	11	positive`no test`no positive test negative`no test`no negative test {lower-layer}: Linux IPv4 reassembly; internal/plugins/firewall/nft/lower_linux.go::lowerMasquerade installs the NAT rule, and the kernel bounds reassembly memory with ipfrag_high_thresh and expires queues after ipfrag_time while unfragmented packets bypass reassembly
`RFC4787-4.1-2`	REQ-2: It is RECOMMENDED that a NAT have an "IP address pooling" behavior of "Paired". (§4.1)
	RECOMMENDED	4.1	positive`no test`no positive test negative`no test`no negative test
`RFC4787-4.2.1-2`	If the host's source port was in the range 0-1023, it is RECOMMENDED the NAT's source port be in the same range. (§4.2.1)
	RECOMMENDED	4.2.1	positive`no test`no positive test negative`no test`no negative test
`RFC4787-4.2.1-3`	If the host's source port was in the range 1024-65535, it is RECOMMENDED that the NAT's source port be in that range. (§4.2.1)
	RECOMMENDED	4.2.1	positive`no test`no positive test negative`no test`no negative test
`RFC4787-4.2.2-1`	REQ-4: It is RECOMMENDED that a NAT have a "Port parity preservation" behavior of "Yes". (§4.2.2)
	RECOMMENDED	4.2.2	positive`no test`no positive test negative`no test`no negative test
`RFC4787-4.3-5`	c) A default value of five minutes or more for the NAT UDP mapping timer is RECOMMENDED. (§4.3)
	RECOMMENDED	4.3	positive`no test`no positive test negative`no test`no negative test
`RFC4787-5-1`	REQ-8: If application transparency is most important, it is RECOMMENDED that a NAT have an "Endpoint-Independent Filtering" behavior. (§5)
	RECOMMENDED	5	positive`no test`no positive test negative`no test`no negative test
`RFC4787-5-2`	If a more stringent filtering behavior is most important, it is RECOMMENDED that a NAT have an "Address-Dependent Filtering" behavior. (§5)
	RECOMMENDED	5	positive`no test`no positive test negative`no test`no negative test
`RFC4787-7-1`	To eliminate interference with UNSAF NAT traversal mechanisms and allow integrity protection of UDP communications, NAT ALGs for UDP-based protocols SHOULD be turned off. (§7)
	SHOULD	7	positive`no test`no positive test negative`no test`no negative test
`RFC4787-7-2`	a) If a NAT includes ALGs, it is RECOMMENDED that the NAT allow the NAT administrator to enable or disable each ALG separately. (§7)
	RECOMMENDED	7	positive`no test`no positive test negative`no test`no negative test
`RFC4787-9-2`	The NAT's default configuration SHOULD NOT filter ICMP messages based on their source IP address. (§9)
	SHOULD NOT	9	positive`no test`no positive test negative`no test`no negative test
`RFC4787-9-3`	Such ICMP messages SHOULD be rewritten by the NAT (specifically, the IP headers and the ICMP payload) and forwarded to the appropriate internal or external host. (§9)
	SHOULD	9	positive`no test`no positive test negative`no test`no negative test
`RFC4787-9-4`	b) It is RECOMMENDED that a NAT support ICMP Destination Unreachable messages. (§9)
	RECOMMENDED	9	positive`no test`no positive test negative`no test`no negative test
`RFC4787-4.3-3`	a) For specific destination ports in the well-known port range (ports 0-1023), a NAT MAY have shorter UDP mapping timers that are specific to the IANA-registered application running over that specific destination port. (§4.3)
	MAY	4.3	positive`no test`no positive test negative`no test`no negative test
`RFC4787-4.3-4`	b) The value of the NAT UDP mapping timer MAY be configurable. (§4.3)
	MAY	4.3	positive`no test`no positive test negative`no test`no negative test
`RFC4787-4.3-6`	a) The NAT mapping Refresh Direction MAY have a "NAT Inbound refresh behavior" of "True". (§4.3)
	MAY	4.3	positive`no test`no positive test negative`no test`no negative test
`RFC4787-5-3`	a) The filtering behavior MAY be an option configurable by the administrator of the NAT. (§5)
	MAY	5	positive`no test`no positive test negative`no test`no negative test

Gaps and untested MUSTs

Requirement	State	Reason
`RFC4787-4.1-1` REQ-1: A NAT MUST have an "Endpoint-Independent Mapping" behavior. (§4.1)	{gap}, no test	nf_nat preserves the internal source port only while the translated 5-tuple is unique and otherwise allocates another port for that destination, the masquerade random and fully-random flags ze lowers randomize per flow, and VPP NAT44-ED maps per endpoint; internal/plugins/firewall/nft/lower_linux.go lowerMasquerade
`RFC4787-4.2.1-1` REQ-3: A NAT MUST NOT have a "Port assignment" behavior of "Port overloading". (§4.2.1)	no test	no test carries this requirement id; annotated {lower-layer}: Linux nf_nat; internal/plugins/firewall/nft/lower_linux.go::lowerMasquerade installs the masquerade expression and nf_nat's unique-tuple allocation never gives two internal endpoints one external tuple
`RFC4787-4.3-1` REQ-5: A NAT UDP mapping timer MUST NOT expire in less than two minutes, unless REQ-5a applies. (§4.3)	{gap}, no test	ze installs no conntrack timeout policy, so a UDP flow seen in one direction expires after Linux's nf_conntrack_udp_timeout default of 30 seconds; internal/plugins/firewall/nft/lower_linux.go
`RFC4787-4.3-2` REQ-6: The NAT mapping Refresh Direction MUST have a "NAT Outbound refresh behavior" of "True". (§4.3)	no test	no test carries this requirement id; annotated {lower-layer}: Linux nf_conntrack; internal/plugins/firewall/nft/lower_linux.go::lowerMasquerade installs the NAT rule and nf_conntrack refreshes the entry's timeout on every packet of the flow, outbound included
`RFC4787-4.4-1` REQ-7: A NAT device whose external IP interface can be configured dynamically MUST either (1) automatically ensure that its internal network uses IP addresses that do not conflict with its external network, or (2) be able to translate and forward traffic between all internal nodes and all external nodes whose IP addresses numerically conflict with the internal network. (§4.4)	{gap}, no test	ze neither checks a DHCP- or PPP-learned external address against the internal subnets nor translates traffic to a conflicting external range
`RFC4787-6-1` REQ-9: A NAT MUST support "Hairpinning". (§6)	{gap}, no test	ze generates no hairpin rule, and a packet from an internal host to another internal host's external mapping matches no conntrack reply tuple and is delivered to the NAT itself
`RFC4787-6-2` a) A NAT Hairpinning behavior MUST be "External source IP address and port". (§6)	{gap}, no test	no hairpinning exists (RFC4787-6-1), so there is no hairpin source to translate
`RFC4787-8-1` REQ-11: A NAT MUST have deterministic behavior, i.e., it MUST NOT change the NAT translation (Section 4) or the Filtering (Section 5) Behavior at any point in time, or under any particular conditions. (§8)	{gap}, no test	nf_nat preserves the port when it is free and switches to another port under contention, which changes the mapping behavior under load
`RFC4787-9-1` Receipt of any sort of ICMP message MUST NOT destroy the NAT mapping. (§9)	no test	no test carries this requirement id; annotated {lower-layer}: Linux nf_conntrack; internal/plugins/firewall/nft/lower_linux.go::lowerMasquerade installs the NAT rule and nf_conntrack classifies an ICMP error as RELATED to the entry without deleting it
`RFC4787-10-1` REQ-13: If the packet received on an internal IP address has DF=1, the NAT MUST send back an ICMP message "Fragmentation needed and DF set" to the host, as described in [RFC0792]. (§10)	no test	no test carries this requirement id; annotated {lower-layer}: Linux IPv4 forwarding; internal/component/iface/dispatch.go::SetMTU sets the egress MTU and the kernel forwarding path answers an oversize DF=1 packet with ICMP Fragmentation Needed
`RFC4787-10-2` If the packet has DF=0, the NAT MUST fragment the packet and SHOULD send the fragments in order. (§10)	no test	no test carries this requirement id; annotated {lower-layer}: Linux IPv4 forwarding; internal/component/iface/dispatch.go::SetMTU sets the egress MTU and the kernel forwarding path fragments an oversize DF=0 packet in order
`RFC4787-11-1` REQ-14: A NAT MUST support receiving in-order and out-of-order fragments, so it MUST have "Received Fragment Out of Order" behavior. (§11)	no test	no test carries this requirement id; annotated {lower-layer}: Linux nf_defrag_ipv4; internal/plugins/firewall/nft/lower_linux.go::lowerMasquerade installs a NAT rule, which loads conntrack and its defragmentation hook, and the reassembly queue accepts fragments in any order
`RFC4787-11-2` a) A NAT's out-of-order fragment processing mechanism MUST be designed so that fragmentation-based DoS attacks do not compromise the NAT's ability to process in-order and unfragmented IP packets. (§11)	no test	no test carries this requirement id; annotated {lower-layer}: Linux IPv4 reassembly; internal/plugins/firewall/nft/lower_linux.go::lowerMasquerade installs the NAT rule, and the kernel bounds reassembly memory with ipfrag_high_thresh and expires queues after ipfrag_time while unfragmented packets bypass reassembly

Proof state

A tagged unit reads unproven where no discrimination record exists for it: nothing in this tree has been observed to break it, so the claim its tag makes is unproven.