Start kamal bridge network with custom MTU

[dankimio]

I'm relatively new to Docker and networking, and I'm having an issue where requests to some external hosts from within a Docker container fail.

For example, when I run $ curl -v https://github.com, the request times out with the following error:

*   Trying 140.82.121.4:443...
* Connected to github.com (140.82.121.4) port 443 (#0)
* ALPN: offers h2,http/1.1
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
*  CAfile: /etc/ssl/certs/ca-certificates.crt
*  CApath: /etc/ssl/certs
* Recv failure: Connection reset by peer
* OpenSSL SSL_connect: Connection reset by peer in connection to github.com:443 
* Closing connection 0
curl: (35) Recv failure: Connection reset by peer

while requests to other hosts (google.com, apple.com) succeed.

One solution I found suggests lowering the MTU (link 1, link 2).

Running $ ip a | grep mtu returns

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
2: ens3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1450 qdisc pfifo_fast state UP group default qlen 1000
3: docker0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN group default 
4: br-057b96d3de83: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default 
8: veth7261868@if7: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br-057b96d3de83 state UP group default 
10: veth2f584f4@if9: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br-057b96d3de83 state UP group default 
16: veth9c457f8@if15: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br-057b96d3de83 state UP group default 
18: veth66d763f@if17: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue master br-057b96d3de83 state UP group default 

The MTU value for docker0 and br- is higher than ens3's. I've found a way to set docker0's mtu to 1450 by overriding a value either in in /etc/docker/daemon.json or /lib/systemd/system/docker.service. However, the Kamal network still uses an MTU of 1500. Is there a way to set the MTU of the kamal bridge network to a lower value? Or could something else cause this issue?

$ docker network ls
NETWORK ID     NAME      DRIVER    SCOPE
45647e21c6a2   bridge    bridge    local
0123447ba8e8   host      host      local
057b96d3de83   kamal     bridge    local
66a66f821b25   none      null      local

[ryoikarashi]

I also encountered the exact same issue.

As a workaround, I was able to resolve this issue by SSHing into the remote server and creating a customized kamal network within the docker-setup hook.

#!/bin/sh
ssh root@<REMOTE_SERVER_IP> docker network  create --driver=bridge --opt com.docker.network.driver.mtu=1450 kamal

And here is the result of docker network inspect kamal in the remote server.

[
    {
        "Name": "kamal",
		...
        "Options": {
            "com.docker.network.driver.mtu": "1450"
        },
        ...
    }
]