draft-ietf-dnsext-dnsproxy-03.txt   draft-ietf-dnsext-dnsproxy-04.txt 
DNSEXT R. Bellis DNSEXT R. Bellis
Internet-Draft Nominet UK Internet-Draft Nominet UK
Intended status: BCP March 3, 2009 Intended status: BCP April 15, 2009
Expires: September 4, 2009 Expires: October 17, 2009
DNS Proxy Implementation Guidelines DNS Proxy Implementation Guidelines
draft-ietf-dnsext-dnsproxy-03 draft-ietf-dnsext-dnsproxy-04
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
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and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on September 4, 2009. This Internet-Draft will expire on October 17, 2009.
Copyright Notice Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info). publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. The Transparency Principle . . . . . . . . . . . . . . . . . . 3 3. The Transparency Principle . . . . . . . . . . . . . . . . . . 3
4. Protocol Conformance . . . . . . . . . . . . . . . . . . . . . 4 4. Protocol Conformance . . . . . . . . . . . . . . . . . . . . . 4
4.1. Unexpected Flags and Data . . . . . . . . . . . . . . . . 4 4.1. Unexpected Flags and Data . . . . . . . . . . . . . . . . 4
4.2. Label Compression . . . . . . . . . . . . . . . . . . . . 4 4.2. Label Compression . . . . . . . . . . . . . . . . . . . . 4
4.3. Unknown Resource Record Types . . . . . . . . . . . . . . 5 4.3. Unknown Resource Record Types . . . . . . . . . . . . . . 5
4.4. Packet Size Limits . . . . . . . . . . . . . . . . . . . . 5 4.4. Packet Size Limits . . . . . . . . . . . . . . . . . . . . 5
4.4.1. TCP Transport . . . . . . . . . . . . . . . . . . . . 5 4.4.1. TCP Transport . . . . . . . . . . . . . . . . . . . . 6
4.4.2. Extension Mechanisms for DNS (EDNS0) . . . . . . . . . 6 4.4.2. Extension Mechanisms for DNS (EDNS0) . . . . . . . . . 6
4.4.3. IP Fragmentation . . . . . . . . . . . . . . . . . . . 6 4.4.3. IP Fragmentation . . . . . . . . . . . . . . . . . . . 6
4.5. Secret Key Transaction Authentication for DNS (TSIG) . . . 7 4.5. Secret Key Transaction Authentication for DNS (TSIG) . . . 7
5. DHCP's Interaction with DNS . . . . . . . . . . . . . . . . . 7 5. DHCP's Interaction with DNS . . . . . . . . . . . . . . . . . 7
5.1. Domain Name Server (DHCP Option 6) . . . . . . . . . . . . 7 5.1. Domain Name Server (DHCP Option 6) . . . . . . . . . . . . 8
5.2. Domain Name (DHCP Option 15) . . . . . . . . . . . . . . . 8 5.2. Domain Name (DHCP Option 15) . . . . . . . . . . . . . . . 8
5.3. DHCP Leases . . . . . . . . . . . . . . . . . . . . . . . 8 5.3. DHCP Leases . . . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6.1. Forgery Resilience . . . . . . . . . . . . . . . . . . . . 9 6.1. Forgery Resilience . . . . . . . . . . . . . . . . . . . . 9
6.2. Interface Binding . . . . . . . . . . . . . . . . . . . . 10 6.2. Interface Binding . . . . . . . . . . . . . . . . . . . . 10
6.3. Packet Filtering . . . . . . . . . . . . . . . . . . . . . 10 6.3. Packet Filtering . . . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
8. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . . 11 10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . . 12 10.2. Informative References . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction 1. Introduction
Recent research ([SAC035], [DOTSE]) has found that many commonly-used Research has found ([SAC035], [DOTSE]) that many commonly-used
broadband gateways (and similar devices) contain DNS proxies which broadband gateways (and similar devices) contain DNS proxies which
are incompatible in various ways with current DNS standards. are incompatible in various ways with current DNS standards.
These proxies are usual simple DNS forwarders, but do not usually These proxies are usually simple DNS forwarders, but typically do not
have any caching capabilities. The proxy serves as a convenient have any caching capabilities. The proxy serves as a convenient
default DNS resolver for clients on the LAN, but relies on an default DNS resolver for clients on the LAN, but relies on an
upstream resolver (e.g. at an ISP) to perform recursive DNS lookups. upstream resolver (e.g. at an ISP) to perform recursive DNS lookups.
This document describes the incompatibilities that have been Note that to ensure full DNS protocol interoperability it is
discovered and offers guidelines to implementors on how to provide preferred that client stub resolvers should communicate directly with
maximum interoperability. full-feature upstream recursive resolvers wherever possible.
That notwithstanding, this document describes the incompatibilities
that have been discovered and offers guidelines to implementors on
how to provide better interoperability in those cases where the
client must use the broadband gateway's DNS proxy.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. The Transparency Principle 3. The Transparency Principle
It is not considered practical for a simple DNS proxy to implement It is not considered practical for a simple DNS proxy to implement
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might be implemented might be implemented
o it would substantially complicate the configuration UI of the o it would substantially complicate the configuration UI of the
device device
Furthermore some modern DNS protocol extensions (see e.g. EDNS0, Furthermore some modern DNS protocol extensions (see e.g. EDNS0,
below) are intended to be used as "hop-by-hop" mechanisms. If the below) are intended to be used as "hop-by-hop" mechanisms. If the
DNS proxy is considered to be such a "hop" in the resolution chain, DNS proxy is considered to be such a "hop" in the resolution chain,
then for it to function correctly, it would need to be fully then for it to function correctly, it would need to be fully
compliant with all such mechanisms. compliant with all such mechanisms.
Research [SAC035] has shown that the more actively a proxy [SAC035] shows that the more actively a proxy participates in the DNS
participates in the DNS protocol then the more likely it is that it protocol then the more likely it is that it will somehow interfere
will somehow interfere with the flow of messages between the DNS with the flow of messages between the DNS client and the upstream
client and the upstream recursive resolvers. recursive resolvers.
The role of the proxy should therefore be no more and no less than to The role of the proxy should therefore be no more and no less than to
receive DNS requests from clients on the LAN side, forward those receive DNS requests from clients on the LAN side, forward those
verbatim to one of the known upstream recursive resolvers on the WAN verbatim to one of the known upstream recursive resolvers on the WAN
side, and ensure that the whole response is returned verbatim to the side, and ensure that the whole response is returned verbatim to the
original client. original client.
It is RECOMMENDED that proxies should be as transparent as possible, It is RECOMMENDED that proxies should be as transparent as possible,
such that any "hop-by-hop" mechanisms or newly introduced protocol such that any "hop-by-hop" mechanisms or newly introduced protocol
extensions operate as if the proxy were not there. extensions operate as if the proxy were not there.
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Similarly all responses MUST be proxied regardless of the values of Similarly all responses MUST be proxied regardless of the values of
the TYPE and CLASS fields of any Resource Record therein. the TYPE and CLASS fields of any Resource Record therein.
4.4. Packet Size Limits 4.4. Packet Size Limits
[RFC1035] specifies that the maximum size of the DNS payload in a UDP [RFC1035] specifies that the maximum size of the DNS payload in a UDP
packet is 512 octets. Where the required portions of a response packet is 512 octets. Where the required portions of a response
would not fit inside that limit the DNS server MUST set the would not fit inside that limit the DNS server MUST set the
"TrunCation" (TC) bit in the DNS response header to indicate that "TrunCation" (TC) bit in the DNS response header to indicate that
truncation has occurred. There are however two standard mechanisms truncation has occurred. There are however two standard mechanisms
(described below) for transporting responses larger than 512 octets. (described in Section 4.4.1 and Section 4.4.2) for transporting
responses larger than 512 octets.
Many proxies have been observed to truncate all responses at 512 Many proxies have been observed to truncate all responses at 512
octets, and others at a packet size related to the WAN MTU, in either octets, and others at a packet size related to the WAN MTU, in either
case doing so without correctly setting the TC bit. case doing so without correctly setting the TC bit.
Other proxies have been observed to remove the TC bit in server Other proxies have been observed to remove the TC bit in server
responses which correctly had the TC bit set by the server. responses which correctly had the TC bit set by the server.
If a DNS response is truncated but the TC bit is not set then client If a DNS response is truncated but the TC bit is not set then client
failures may result, in particular a naive DNS client library might failures may result. In particular a naive DNS client library might
suffer crashes due to reading beyond the end of the data actually suffer crashes due to reading beyond the end of the data actually
received. received.
Since UDP packets larger than 512 octets are now expected in normal Since UDP packets larger than 512 octets are now expected in normal
operation, proxies SHOULD NOT truncate UDP packets that exceed that operation, proxies SHOULD NOT truncate UDP packets that exceed that
size. See Section 4.4.3 for recommendations for packet sizes size. See Section 4.4.3 for recommendations for packet sizes
exceeding the WAN MTU. exceeding the WAN MTU.
If a proxy must unilaterally truncate a response then the proxy MUST If a proxy must unilaterally truncate a response then the proxy MUST
set the TC bit. Similarly, proxies MUST NOT remove the TC bit from set the TC bit. Similarly, proxies MUST NOT remove the TC bit from
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the client. the client.
However some proxies have been observed to either reject (with a However some proxies have been observed to either reject (with a
FORMERR response code) or black-hole any packet containing an OPT RR. FORMERR response code) or black-hole any packet containing an OPT RR.
As per Section 4.1 proxies SHOULD NOT refuse to proxy such packets. As per Section 4.1 proxies SHOULD NOT refuse to proxy such packets.
4.4.3. IP Fragmentation 4.4.3. IP Fragmentation
Support for UDP packet sizes exceeding the WAN MTU depends on the Support for UDP packet sizes exceeding the WAN MTU depends on the
gateway's algorithm for handling fragmented IP packets. Several gateway's algorithm for handling fragmented IP packets. Several
options are possible: methods are possible:
1. fragments are dropped 1. fragments are dropped
2. fragments are forwarded individually as they're received 2. fragments are forwarded individually as they're received
3. complete packets are reassembled on the gateway, and then re- 3. complete packets are reassembled on the gateway, and then re-
fragmented (if necessary) as they're forwarded to the client fragmented (if necessary) as they're forwarded to the client
Method 1 above will cause compatibility problems with EDNS0 unless
Option 1 above will cause compatibility problems with EDNS0 unless
the DNS client is configured to advertise an EDNS0 buffer size the DNS client is configured to advertise an EDNS0 buffer size
limited to 28 octets less than the MTU. Note that RFC 2671 does limited to 28 octets less than the MTU. Note that RFC 2671 does
recommend that the path MTU should be taken into account when using recommend that the path MTU should be taken into account when using
EDNS0. EDNS0.
Also, whilst the EDNS0 specification allows for a buffer size of up Also, whilst the EDNS0 specification allows for a buffer size of up
to 65535 octets, most common DNS server implementations do not to 65535 octets, most common DNS server implementations do not
support a buffer size above 4096 octets. support a buffer size above 4096 octets.
Therefore proxies SHOULD (whichever of options 2 or 3 above is in Therefore (irrespective of which of the methods above is in use)
use) be capable of forwarding UDP packets up to a payload size of at proxies SHOULD be capable of forwarding UDP packets up to a payload
least 4096 octets. size of at least 4096 octets.
NB: in theory IP fragmentation may also occur if the LAN MTU is NB: in theory IP fragmentation may also occur if the LAN MTU is
smaller than the WAN MTU, although the author has not observed such a smaller than the WAN MTU, although the author has not observed such a
configuration in use on any residential broadband service. configuration in use on any residential broadband service.
4.5. Secret Key Transaction Authentication for DNS (TSIG) 4.5. Secret Key Transaction Authentication for DNS (TSIG)
[RFC2845] defines TSIG, which is a mechanism for authenticating DNS [RFC2845] defines TSIG, which is a mechanism for authenticating DNS
requests and responses at the packet level. requests and responses at the packet level.
Any modifications made to the DNS portions of a TSIG-signed query or Any modifications made to the DNS portions of a TSIG-signed query or
response packet (with the exception of the Query ID) will cause a response packet (with the exception of the Query ID) will cause a
TSIG authentication failure. TSIG authentication failure.
DNS proxies MUST implement Section 4.7 of [RFC2845] and either DNS proxies MUST implement Section 4.7 of [RFC2845] and either
forward packets unchanged (as recommended above) or fully implement forward packets unchanged (as recommended above) or fully implement
TSIG. TSIG.
As per Section 4.3, DNS proxies SHOULD be capable of proxying packets As per Section 4.3, DNS proxies MUST be capable of proxying packets
containing TKEY [RFC2930] Resource Records. containing TKEY [RFC2930] Resource Records.
NB: any DNS proxy (such as those commonly found in WiFi hotspot NB: any DNS proxy (such as those commonly found in WiFi hotspot
"walled gardens") which transparently intercepts all DNS queries, and "walled gardens") which transparently intercepts all DNS queries, and
which returns unsigned responses to signed queries, will also cause which returns unsigned responses to signed queries, will also cause
TSIG authentication failures. TSIG authentication failures.
5. DHCP's Interaction with DNS 5. DHCP's Interaction with DNS
Whilst this document is primarily about DNS proxies, most consumers Whilst this document is primarily about DNS proxies, most consumers
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Most gateways default to supplying their own IP address in the DHCP Most gateways default to supplying their own IP address in the DHCP
"Domain Name Server" option [RFC2132]. The net result is that "Domain Name Server" option [RFC2132]. The net result is that
without explicit re-configuration many DNS clients will by default without explicit re-configuration many DNS clients will by default
send queries to the gateway's DNS proxy. This is understandable send queries to the gateway's DNS proxy. This is understandable
behaviour given that the correct upstream settings are not usually behaviour given that the correct upstream settings are not usually
known at boot time. known at boot time.
Most gateways learn their own DNS settings via values supplied by an Most gateways learn their own DNS settings via values supplied by an
ISP via DHCP or PPP over the WAN interface. However whilst many ISP via DHCP or PPP over the WAN interface. However whilst many
gateways do allow the end-user to override those values, some gateways do allow the device administrator to override those values,
gateways only use those end-user supplied values to affect the some gateways only use those supplied values to affect the proxy's
proxy's own forwarding function, and do not offer these values via own forwarding function, and do not offer these values via DHCP.
DHCP.
When using such a device the only way to avoid using the DNS proxy is When using such a device the only way to avoid using the DNS proxy is
to hard-code the required values in the client operating system. to hard-code the required values in the client operating system.
This may be acceptable for a desktop system but it is inappropriate This may be acceptable for a desktop system but it is inappropriate
for mobile devices which are regularly used on many different for mobile devices which are regularly used on many different
networks. networks.
As per Section 3, end-users SHOULD be able to send their DNS queries As per Section 3, end-users SHOULD be able to send their DNS queries
directly to specified upstream resolvers, ideally without hard-coding directly to specified upstream resolvers, ideally without hard-coding
those settings in their stub resolver. those settings in their stub resolver.
It is therefore RECOMMENDED that gateways SHOULD support end-user It is therefore RECOMMENDED that gateways SHOULD support device
configuration of values for the "Domain Name Server" DHCP option. administrator configuration of values for the "Domain Name Server"
DHCP option.
5.2. Domain Name (DHCP Option 15) 5.2. Domain Name (DHCP Option 15)
A significant amount of traffic to the DNS Root Name Servers is for A significant amount of traffic to the DNS Root Name Servers is for
invalid top-level domain names, and some of that traffic can be invalid top-level domain names, and some of that traffic can be
attributed to particular equipment vendors whose firmware defaults attributed to particular equipment vendors whose firmware defaults
this DHCP option to specific values. this DHCP option to specific values.
Since no standard exists for a "local" scoped domain name suffix it Since no standard exists for a "local" scoped domain name suffix it
is RECOMMENDED that the default value for this option SHOULD be is RECOMMENDED that the default value for this option SHOULD be
empty, and that this option SHOULD NOT be sent to clients when no empty, and that this option MUST NOT be sent to clients when no value
value is configured. is configured.
5.3. DHCP Leases 5.3. DHCP Leases
It is noted that some DHCP servers in broadband gateways by default It is noted that some DHCP servers in broadband gateways by default
offer their own IP address for the "Domain Name Server" option (as offer their own IP address for the "Domain Name Server" option (as
describe above) but then automatically start offering the upstream described above) but then automatically start offering the upstream
settings once they've been learnt over the WAN interface. servers' addresses once they've been learnt over the WAN interface.
In general this behaviour is highly desirable, but the effect for the In general this behaviour is highly desirable, but the effect for the
end-user is that the settings used depend on whether the DHCP lease end-user is that the settings used depend on whether the DHCP lease
was obtained before or after the WAN link was established. was obtained before or after the WAN link was established.
If the DHCP lease is obtained whilst the WAN link is down then the If the DHCP lease is obtained whilst the WAN link is down then the
DHCP client (and hence the DNS client) will not receive the correct DHCP client (and hence the DNS client) will not receive the correct
values until the DHCP lease is renewed. values until the DHCP lease is renewed.
Whilst no specific recommendations are given here, vendors may wish Whilst no specific recommendations are given here, vendors may wish
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If a DNS proxy is running on a broadband gateway with NAT that is If a DNS proxy is running on a broadband gateway with NAT that is
compliant with [RFC4787] then it SHOULD also follow the compliant with [RFC4787] then it SHOULD also follow the
recommendations in Section 10 of [RFC5452] concerning how long DNS recommendations in Section 10 of [RFC5452] concerning how long DNS
state is kept. state is kept.
6.2. Interface Binding 6.2. Interface Binding
Some gateways have been observed to have their DNS proxy listening on Some gateways have been observed to have their DNS proxy listening on
both internal (LAN) and external (WAN) interfaces. In this both internal (LAN) and external (WAN) interfaces. In this
configuration it is possible for the proxy to be used to mount configuration it is possible for the proxy to be used to mount
reflector attacks as described in [RFC5358] reflector attacks as described in [RFC5358].
The DNS proxy in a gateway SHOULD NOT by default be accessible from The DNS proxy in a gateway SHOULD NOT by default be accessible from
the WAN interfaces of the device. the WAN interfaces of the device.
6.3. Packet Filtering 6.3. Packet Filtering
The Transparency and Robustness Principles are not entirely The Transparency and Robustness Principles are not entirely
compatible with the Deep Packet Inspection features of security compatible with the deep packet inspection features of security
appliances such as firewalls which are intended to protect systems on appliances such as firewalls which are intended to protect systems on
the inside of a network from rogue traffic. the inside of a network from rogue traffic.
However a clear distinction may be made between traffic that is However a clear distinction may be made between traffic that is
intrinsically malformed and that which merely contains unexpected intrinsically malformed and that which merely contains unexpected
data. data.
Examples of malformed packets which MAY be dropped include: Examples of malformed packets which MAY be dropped include:
o invalid compression pointers (i.e. those that point outside of the o invalid compression pointers (i.e. those that point outside of the
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the original request, it is RECOMMENDED that proxies SHOULD instead the original request, it is RECOMMENDED that proxies SHOULD instead
return a suitable DNS error response to the client (i.e. SERVFAIL) return a suitable DNS error response to the client (i.e. SERVFAIL)
instead of dropping the packet completely. instead of dropping the packet completely.
7. IANA Considerations 7. IANA Considerations
This document requests no IANA actions. This document requests no IANA actions.
8. Change Log 8. Change Log
NB: to be removed by the RFC Editor before publication.
draft-ietf-dnsproxy-04 - post WGLC
Introduction expanded
Section 5.2 - changed SHOULD to MUST
Section 4.5 - changed SHOULD to MUST (Alex Bligh)
Editorial nits (from Andrew Sullivan, Alfred Hones)
Clarificaton on end-user vs device administrator (Alan Barrett,
Paul Selkirk)
draft-ietf-dnsproxy-03 draft-ietf-dnsproxy-03
Editorial nits and mention of LAN MTU (from Alex Bligh) Editorial nits and mention of LAN MTU (from Alex Bligh)
draft-ietf-dnsproxy-02 draft-ietf-dnsproxy-02
Changed "router" to "gateway" throughout (David Oran) Changed "router" to "gateway" throughout (David Oran)
Updated forgery resilience reference Updated forgery resilience reference
Elaboration on bypassability (from Nicholas W.) Elaboration on bypassability (from Nicholas W.)
Elaboration on NAT source port randomisation (from Nicholas W.) Elaboration on NAT source port randomisation (from Nicholas W.)
Mention of using short DHCP leases while the WAN link is down Mention of using short DHCP leases while the WAN link is down
(from Ralph Droms) (from Ralph Droms)
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