From dd0b318a44c73fb5afbed6c754e65fe304e5a3e2 Mon Sep 17 00:00:00 2001
From: Jack O'Sullivan <jackos1998@gmail.com>
Date: Sat, 22 Aug 2026 14:09:01 +0100
Subject: [PATCH] nixos/castle: Set lan-lo to the standard MTU

lan-lo is a VLAN on et100g, which carries hi's jumbo frames, so it
inherited 9000 rather than the 1500 the lo VLAN runs at. MSS clamping
hid this from TCP; UDP without working PMTUD was silently dropped.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
---
 nixos/boxes/home/castle/default.nix | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/nixos/boxes/home/castle/default.nix b/nixos/boxes/home/castle/default.nix
index 9cdee57..fe1ec6d 100644
--- a/nixos/boxes/home/castle/default.nix
+++ b/nixos/boxes/home/castle/default.nix
@@ -198,6 +198,9 @@ in
               ];
               "45-lan-lo" = {
                 matchConfig.Name = "lan-lo";
+                # The parent carries hi's jumbo frames, but lo runs at the standard MTU;
+                # without this the VLAN would inherit the parent's larger one.
+                linkConfig.MTUBytes = "1500";
                 networkConfig = {
                   DHCP = "ipv4";
                   IPv6AcceptRA = true;