100 lines
4.2 KiB
ReStructuredText
100 lines
4.2 KiB
ReStructuredText
Frequently Asked Questions
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==========================
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.. _faq-dns:
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DNS does not work on the nodes
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Gluon nodes will ignore the DNS server on the WAN port for everything except
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the mesh VPN, which can lead to confusion.
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All normal services on the nodes exclusively use the DNS server on the mesh
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interface. This DNS server must be announced in router advertisements (using
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*radvd* or a similar software) from one or more central servers in meshes based
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on *batman-adv*. If your mesh does not have global IPv6 connectivity, you can setup
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your *radvd* not to announce a default route by setting the *default lifetime* to 0;
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in this case, the *radvd* is only used to announce the DNS server.
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.. _faq-mtu:
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What is a good MTU on the mesh-vpn
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Setting the MTU on the transport interface requires careful consideration, as
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setting it too low will cause excessive fragmentation and setting it too high
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may leave peers with a broken tunnel due to packet loss.
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Consider these key values:
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- Payload: Allow for the transport of IPv6 packets, by adhering to the minimum MTU
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of 1280 Byte specified in RFC 2460
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- and configure `MSS clamping`_ accordingly,
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- and announce your link MTU via Router Advertisments and DHCP
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.. _MSS clamping: https://www.tldp.org/HOWTO/Adv-Routing-HOWTO/lartc.cookbook.mtu-mss.html
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- Encapsulation: Account for the overhead created by the configured mesh protocol
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encapsulating the payload, which is
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- up to 32 Byte (14 Byte Ethernet + 18 Byte batadv) for batman-adv compat v15 (v2014.0 and later)
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- up to 28 Byte (14 Byte Ethernet + 14 Byte batadv) for batman-adv compat v14 (v2011.3.0 until and including v2013.4.0)
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- PMTU: What MTU does the path between your gateway and each of its peers support?
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For reference, the complete MTU stack looks like this:
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.. image:: mtu-diagram_v5.png
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Minimum MTU
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-----------
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Calculcate the minimum transport MTU by adding the encapsulation overhead to the
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minimum payload MTU required. This is the lowest recommended value, since going
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lower would cause unnecessary fragmentation for clients which respect the announced
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link MTU.
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Example: Our network currently uses batman-adv v15, it therefore requires up
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to 32 Bytes of encapsulation overhead on top of the minimal link MTU required for
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transporting IPv6.::
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\ 1312 1294 1280 0
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\---------+-----------------+-------------+----------------------------------+
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\TAP | batadv v15 | Ethernet | Payload |
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\-------+-----------------+-------------+----------------------------------+
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\ ^
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MTU_LOW = 1280 Byte + 14 Byte + 18 Byte = 1312 Byte
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Maximum MTU
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-----------
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Calculating the maximum transport MTU is interesting, because it increases the
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throughput, by allowing larger payloads to be transported, but also more difficult
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as you have to take into account the tunneling overhead and each peers PMTU, which
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varies between providers.
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The underlying reasons are mostly PPPoE, Tunneling and IPv6 transition technologies
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like DS-Lite.
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Example: The peer with the smallest MTU on your network is behind DS-Lite and can
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transport IPv4 packets up to 1436 Bytes in size. Your tunnel uses IPv4 (20 Byte),
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UDP (8 Byte), Fastd (24 byte) and you require TAP (14 Byte) for Layer 2 (Ethernet)
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Tunneling.::
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1436 1416 1408 1384 1370 \
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+-------------------+--------+-----------------------+-------------+------\
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| IP | UDP | Fastd | TAP | bat\
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+-------------------+--------+-----------------------+-------------+--------\
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^ \
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MTU_HIGH = 1436 Byte - 20 Byte - 8 Byte - 24 Byte - 14 Byte = 1370 Byte
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Conclusion
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----------
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Determining the maximum MTU can be a tedious process, especially since the PMTU
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of peers could change at any time. The general recommendation for maximized
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compatibility is therefore the minimum MTU of 1312 Byte, which works well with
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all combinations of IPv4, IPv6, batman-adv compat v14 and v15.
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