隨意更改家中的 MTU 值會導致 Docker Bridge 掛掉
你好，我是無能。
在家裡的 Docker 環境中，不知為何無法連線到伺服器主機的位址，正覺得奇怪時，發現原因出在修改了路由器端的 MTU 值。即使執行 curl 等指令會回傳 200 OK，但實際用瀏覽器開啟時，卻會像圖片中那樣卡在中間，永遠停在載入畫面，讓人有點困擾。
回想最近是否改動了什麼，才想起修改過路由器端的 MTU 值，將其全部改回 1500 後就恢復正常了。
這是因為 WireGuard 會稍微調降 MTU，而且根據 Reddit 上的說法，在 Starlink 環境下將 MTU 值改為 1472 比較好，我當作一種「偏方」又再調降了一點，結果這就是主因。由於在 compose.yml 中將網路設定為使用主機網路的 network: host 就能解決問題，我原本以為是 Docker Bridge 引起的原因，但其實只是相關聯而已。
$ ip link show | grep -E "br-|docker"
5: br-13aad139d7db: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default
6: br-2dc434eeb46d: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default
7: br-45fa093100aa: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default
8: br-46e6bb59b194: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default
9: br-5267070e10e9: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default
10: br-645605dcd9b3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default
11: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN mode DEFAULT group default
12: br-2c8c384b95cd: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default
13: br-5819ab3dabfb: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default
14: br-632b4dfffca7: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default
15: br-9243101a1b11: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default
16: br-2612402b7376: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default
17: br-91f1adfffccc: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default
18: br-ce4a3c384d09: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default
19: br-f3c4b8d6015b: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP mode DEFAULT group default
20: veth7edbc95@if2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br-9243101a1b11 state UP mode DEFAULT group default
21: vethf27ed21@if2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br-f3c4b8d6015b state UP mode DEFAULT group default
23: veth64aeed9@if2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br-f3c4b8d6015b state UP mode DEFAULT group default
24: vethf09e0cb@if2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br-13aad139d7db state UP mode DEFAULT group default
25: vethe7f07de@if2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br-632b4dfffca7 state UP mode DEFAULT group default
26: veth83acc53@if2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br-f3c4b8d6015b state UP mode DEFAULT group default
27: veth817b530@if2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br-45fa093100aa state UP mode DEFAULT group default
28: vethd9335e3@if2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br-91f1adfffccc state UP mode DEFAULT group default
29: vethfe66e15@if2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br-645605dcd9b3 state UP mode DEFAULT group default
30: veth2efd851@if2: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br-632b4dfffca7 state UP mode DEFAULT group default確實仔細一看，全部都變成了 MTU 1500。
說到底，因為 Docker Bridge 是虛擬 NIC，現在回想起來，理所當然也得同樣修改 MTU 值，但我當時完全沒意識到這點，徹底忽略了。
因此，我將所有的 MTU 值改回 1500，再觀察一段時間。