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Raise the overlay MTU to 1280: below that Linux disables IPv6

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The setup recipe failed with a missing sysctl directory and "RTNETLINK
answers: Invalid argument". The cause was the default MTU of 1100.

IPv6 requires a minimum MTU of 1280 (RFC 8200) and Linux enforces it by
tearing IPv6 down on any interface below it: the per-device
/proc/sys/net/ipv6/conf entries disappear and an address can no longer be
assigned. Evidence on the test host: every interface at 1280 or above has
an IPv6 conf directory, every interface below it (1230, 1100) has none.

So the overlay MTU is now 1280, which is also the floor. A smaller value is
refused when the plugin opens, naming the reason, rather than surfacing as
an obscure netlink error after the user has already run four commands.

That leaves no slack against the other constraint: a packet needs mtu + 32
bytes of transport datagram, so 1312. A direct QUIC path offers roughly
1380 and fits; a relayed path may not, so the plugin now reports the exact
numbers when a link cannot carry a full-size packet, instead of only
counting silent drops.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This commit is contained in:
tsunagi
2026-09-21 12:41:57 +01:00
co-authored by Claude Opus 5
parent 38beb762d8
commit d2e336f2f9
6 changed files with 123 additions and 17 deletions
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README.md
+9 -3
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@@ -90,7 +90,7 @@ Within a few seconds both print something like:
--- status ---
control: 1 peer(s), 0 dial failure(s), 0 handshake failure(s)
wireguard: tsunkkcp43lmdje on fd15:1d9e:fa21:f201:…/64 mtu 1100, 1/1 tunnel(s) established
wireguard: tsunkkcp43lmdje on fd15:1d9e:fa21:f201:…/64 mtu 1280, 1/1 tunnel(s) established
4jO4kx9Z fd15:1d9e:fa21:f201:… handshake 3s ago tx=0 rx=0 dropped=0 path=Direct via Ip(…)
```
@@ -123,15 +123,21 @@ tsunagi tun-setup --network lab --secret "$SECRET"
```
```text
# Interface tsunjwc6dcrtmo5, address fd80:1210:f724:f620:d1bb:f982:3b6e:19bd/64, mtu 1100
# Interface tsunjwc6dcrtmo5, address fd80:1210:f724:f620:d1bb:f982:3b6e:19bd/64, mtu 1280
# Run once as root; then run `tsunagi up` as ab.
sudo ip tuntap add dev tsunjwc6dcrtmo5 mode tun user ab
sudo ip link set dev tsunjwc6dcrtmo5 mtu 1100 up
sudo ip link set dev tsunjwc6dcrtmo5 mtu 1280 up
sudo sysctl -qw net.ipv6.conf.tsunjwc6dcrtmo5.keep_addr_on_down=1
sudo ip -6 address add fd80:1210:f724:f620:d1bb:f982:3b6e:19bd/64 dev tsunjwc6dcrtmo5 nodad
```
The MTU is 1280 because that is the minimum IPv6 requires (RFC 8200). Linux
disables IPv6 entirely on an interface below it — the per-device
`/proc/sys/net/ipv6` entries vanish and `ip -6 address add` fails with
`Invalid argument` — so a smaller MTU cannot work at all. The agent refuses
one rather than letting it fail later.
The order and the last two lines are not decoration. A persistent TUN
interface has **no carrier** until a process attaches to it, and Linux flushes
IPv6 addresses from an interface that loses carrier unless