docker-compose コンテナ内で通信ができない

### 解決したいこと

次のような構成でバックエンドと通信ができない。(Host is unreachable)

- CentOS 8.2 
- docker-ce
- docker-compose

```
   192.168.10.100
browser -- nginx[80] ------ backend container[80]
           (front)          (webmail)
           192.168.203.2    192.168.203.9
             
```

nginxからbackend containerに通信できない。(curlコマンドで試しても同様)
ただし、pingは通る。

### 解決方法

publicゾーン(default)に、マスカレードを有効にする。
※試行錯誤の結果なので、これが正しいかは未確認です...

```
sudo firewall-cmd --zone=public --add-masquerade --permanent
sudo firewall-cmd --reload
sudo systemctl restart docker
```

参考文献
https://forums.docker.com/t/docker-connection-refused-between-two-containers/68040/12

解決したいことの詳細情報
===========

以下は現象が発生したコードや環境の詳細情報です。
解決できた方法は、前述の項目を参照ください。

### 発生している問題・エラー

```
[user@centos8 mailu]$ sudo docker-compose exec front sh
/ # curl -LsS http://192.168.203.9
curl: (7) Failed to connect to 192.168.203.9 port 80: Host is unreachable
/ # ping 192.168.203.9
PING 192.168.203.6 (192.168.203.9): 56 data bytes
64 bytes from 192.168.203.9: seq=0 ttl=64 time=0.103 ms
64 bytes from 192.168.203.9: seq=1 ttl=64 time=0.094 ms
^C
--- 192.168.203.9 ping statistics ---
2 packets transmitted, 2 packets received, 0% packet loss
round-trip min/avg/max = 0.094/0.098/0.103 ms
```

### 該当するソースコード

https://setup.mailu.io/1.7/ から自動生成したものに対象変更を加えたもの。

```sh:mailu.env
# Mailu main configuration file
#
# This file is autogenerated by the configuration management wizard for compose flavor.
# For a detailed list of configuration variables, see the documentation at
# https://mailu.io

###################################
# Common configuration variables
###################################

# Set to a randomly generated 16 bytes string
SECRET_KEY=SECRETAAAAAAAAAA

# Subnet of the docker network. This should not conflict with any networks to which your system is connected. (Internal and external!)
SUBNET=192.168.203.0/24

# Main mail domain
DOMAIN=example.org

# Hostnames for this server, separated with comas
HOSTNAMES=example.org

# Postmaster local part (will append the main mail domain)
POSTMASTER=admin

# Choose how secure connections will behave (value: letsencrypt, cert, notls, mail, mail-letsencrypt)
TLS_FLAVOR=notls

# Authentication rate limit (per source IP address)
AUTH_RATELIMIT=10/minute;1000/hour

# Opt-out of statistics, replace with "True" to opt out
DISABLE_STATISTICS=False

###################################
# Optional features
###################################

# Expose the admin interface (value: true, false)
ADMIN=true

# Choose which webmail to run if any (values: roundcube, rainloop, none)
WEBMAIL=rainloop

# Dav server implementation (value: radicale, none)
WEBDAV=none

# Antivirus solution (value: clamav, none)
ANTIVIRUS=none

###################################
# Mail settings
###################################

# Message size limit in bytes
# Default: accept messages up to 50MB
# Max attachment size will be 33% smaller
MESSAGE_SIZE_LIMIT=50000000

# Networks granted relay permissions
# Use this with care, all hosts in this networks will be able to send mail without authentication!
RELAYNETS=192.168.10.0/24

# Will relay all outgoing mails if configured
RELAYHOST=

# Fetchmail delay
FETCHMAIL_DELAY=600

# Recipient delimiter, character used to delimiter localpart from custom address part
RECIPIENT_DELIMITER=+

# DMARC rua and ruf email
DMARC_RUA=admin
DMARC_RUF=admin

# Welcome email, enable and set a topic and body if you wish to send welcome
# emails to all users.
WELCOME=false
WELCOME_SUBJECT=Welcome to your new email account
WELCOME_BODY=Welcome to your new email account, if you can read this, then it is configured properly!

# Maildir Compression
# choose compression-method, default: none (value: bz2, gz)
COMPRESSION=
# change compression-level, default: 6 (value: 1-9)
COMPRESSION_LEVEL=

###################################
# Web settings
###################################

# Path to redirect / to
WEBROOT_REDIRECT=/webmail

# Path to the admin interface if enabled
WEB_ADMIN=/webmin

# Path to the webmail if enabled
WEB_WEBMAIL=/webmail

# Website name
SITENAME=Mailu

# Linked Website URL
WEBSITE=https://mailu.io



###################################
# Advanced settings
###################################

# Log driver for front service. Possible values:
# json-file (default)
# journald (On systemd platforms, useful for Fail2Ban integration)
# syslog (Non systemd platforms, Fail2Ban integration. Disables `docker-compose log` for front!)
# LOG_DRIVER=json-file

# Docker-compose project name, this will prepended to containers names.
COMPOSE_PROJECT_NAME=mailu

# Default password scheme used for newly created accounts and changed passwords
# (value: BLF-CRYPT, SHA512-CRYPT, SHA256-CRYPT, MD5-CRYPT, CRYPT)
PASSWORD_SCHEME=BLF-CRYPT

# Header to take the real ip from
REAL_IP_HEADER=

# IPs for nginx set_real_ip_from (CIDR list separated by commas)
REAL_IP_FROM=

# choose wether mailu bounces (no) or rejects (yes) mail when recipient is unknown (value: yes, no)
REJECT_UNLISTED_RECIPIENT=

# Log level threshold in start.py (value: CRITICAL, ERROR, WARNING, INFO, DEBUG, NOTSET)
LOG_LEVEL=WARNING

###################################
# Database settings
###################################
DB_FLAVOR=sqlite
```

```yaml:docker-compose.yml
# This file is auto-generated by the Mailu configuration wizard.
# Please read the documentation before attempting any change.
# Generated for compose flavor

version: '3'

services:

  # External dependencies
  redis:
    image: redis:alpine
    restart: always
    volumes:
      - "/usr/local/docker/mailu/redis:/data"

  # Core services
  front:
    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX:-}nginx:${MAILU_VERSION:-1.7}
    restart: always
    env_file: mailu.env
    logging:
      driver: json-file
    ports:
      - "192.168.10.100:80:80"
      - "::1:80:80"
      - "192.168.10.100:443:443"
      - "::1:443:443"
      - "192.168.10.100:25:25"
      - "::1:25:25"
      - "192.168.10.100:465:465"
      - "::1:465:465"
      - "192.168.10.100:587:587"
      - "::1:587:587"
      - "192.168.10.100:110:110"
      - "::1:110:110"
      - "192.168.10.100:995:995"
      - "::1:995:995"
      - "192.168.10.100:143:143"
      - "::1:143:143"
      - "192.168.10.100:993:993"
      - "::1:993:993"
    volumes:
      - "/usr/local/docker/mailu/certs:/certs"
      - "/usr/local/docker/mailu/overrides/nginx:/overrides"

  admin:
    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX:-}admin:${MAILU_VERSION:-1.7}
    restart: always
    env_file: mailu.env
    ports:
      - 192.168.10.100:8080:80
      - 80
    volumes:
      - "/usr/local/docker/mailu/data:/data"
      - "/usr/local/docker/mailu/dkim:/dkim"
    depends_on:
      - redis

  imap:
    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX:-}dovecot:${MAILU_VERSION:-1.7}
    restart: always
    env_file: mailu.env
    volumes:
      - "/usr/local/docker/mailu/mail:/mail"
      - "/usr/local/docker/mailu/overrides:/overrides"
    depends_on:
      - front

  smtp:
    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX:-}postfix:${MAILU_VERSION:-1.7}
    restart: always
    env_file: mailu.env
    volumes:
      - "/usr/local/docker/mailu/overrides:/overrides"
    depends_on:
      - front

  antispam:
    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX:-}rspamd:${MAILU_VERSION:-1.7}
    restart: always
    env_file: mailu.env
    volumes:
      - "/usr/local/docker/mailu/filter:/var/lib/rspamd"
      - "/usr/local/docker/mailu/dkim:/dkim"
      - "/usr/local/docker/mailu/overrides/rspamd:/etc/rspamd/override.d"
    depends_on:
      - front

  # Optional services

  webdav:
    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX:-}radicale:${MAILU_VERSION:-1.7}
    restart: always
    env_file: mailu.env
    volumes:
      - "/usr/local/docker/mailu/dav:/data"


  # Webmail
  webmail:
    image: ${DOCKER_ORG:-mailu}/${DOCKER_PREFIX:-}rainloop:${MAILU_VERSION:-1.7}
    restart: always
    env_file: mailu.env
    volumes:
      - "/usr/local/docker/mailu/webmail:/data"
    depends_on:
      - imap


networks:
  default:
    driver: bridge
    ipam:
      driver: default
      config:
        - subnet: 192.168.203.0/24
    # 以下はお試し
    driver_opts:
      com.docker.network.bridge.enable_icc: "true"
      com.docker.network.bridge.enable_ip_masquerade: "true"

```



### 自分で試したこと

- enable_iccが有効になっていないため、コンテナ間で通信ができない？
    - 前述の設定の通り設定したが、結果は同じだった(通信できない)
    - Enable or Disable Inter Container Connectivity [docker bridge-driver-options](https://docs.docker.com/engine/reference/commandline/network_create/#bridge-driver-options)

- docker0ネットワークをtrustedゾーンへ移動
    - 結果は同じだった(通信できない)

- CentOS8で、docker-ceでは通信できない?!
    - [stackoverflow](https://stackoverflow.com/questions/60464748/docker-containers-noroutetohostexception-host-is-unreachable)
    - CentOS7では同じソースで正しく動作するとのこと


### 環境

```shell-session:env
[user@centos8 mailu]$ cat /etc/centos-release
CentOS Linux release 8.2.2004 (Core)
[user@centos8 mailu]$ sudo docker system info
Client:
 Debug Mode: false

Server:
 Containers: 8
  Running: 8
  Paused: 0
  Stopped: 0
 Images: 10
 Server Version: 19.03.13
 Storage Driver: overlay2
  Backing Filesystem: xfs
  Supports d_type: true
  Native Overlay Diff: true
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 8fba4e9a7d01810a393d5d25a3621dc101981175
 runc version: dc9208a3303feef5b3839f4323d9beb36df0a9dd
 init version: fec3683
 Security Options:
  seccomp
   Profile: default
 Kernel Version: 4.18.0-193.19.1.el8_2.x86_64
 Operating System: CentOS Linux 8 (Core)
 OSType: linux
 Architecture: x86_64
 CPUs: 2
 Total Memory: 1.784GiB
 Name: centos8
 ID: EENH:WZZJ:FROE:QNUP:QGVT:RRIN:RUR3:THIT:F22Q:IMVS:4GEL:DYIC
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false
[user@centos8 mailu]$ sudo docker-compose --version
docker-compose version 1.27.4, build 40524192
```

```shell-session:docker-inspect
[user@centos8 ~]$ sudo docker inspect  mailu_default
[
    {
        "Name": "mailu_default",
        "Id": "27df81922313814a564c7a551a0c8cf3ea21603dba58002e73ba8f944e565c20",
        "Created": "2020-09-27T21:02:05.901818833-04:00",
        "Scope": "local",
        "Driver": "bridge",
        "EnableIPv6": false,
        "IPAM": {
            "Driver": "default",
            "Options": null,
            "Config": [
                {
                    "Subnet": "192.168.203.0/24"
                }
            ]
        },
        "Internal": false,
        "Attachable": true,
        "Ingress": false,
        "ConfigFrom": {
            "Network": ""
        },
        "ConfigOnly": false,
        "Containers": {
            "1facc50c2feb49fca0033ed6642a6610c9bf79e616627fc957eacbb0638a5545": {
                "Name": "mailu_antispam_1",
                "EndpointID": "db33492d3378ecee9c057343493e6f1666f5166db94ed0c29830fb6821bc0cf3",
                "MacAddress": "02:42:c0:a8:cb:06",
                "IPv4Address": "192.168.203.6/24",
                "IPv6Address": ""
            },
            "3a4cd121bc28240d46699040f407afdd0702b7c31e8272f7da7a80c743ba7357": {
                "Name": "mailu_front_1",
                "EndpointID": "2588705b19c4d8b79c429d2cda66881fa93545a704f7763c912a764676185893",
                "MacAddress": "02:42:c0:a8:cb:02",
                "IPv4Address": "192.168.203.2/24",
                "IPv6Address": ""
            },
            "51c2da8ef177eb6a33d3e67af20aba5d791703a2d705e307ab9a458a7c4c4209": {
                "Name": "mailu_webmail_1",
                "EndpointID": "eb3d2b060aed84eb814d9d52dbe808a7073a52bb87d7dc7f335cbfe23bc03c28",
                "MacAddress": "02:42:c0:a8:cb:09",
                "IPv4Address": "192.168.203.9/24",
                "IPv6Address": ""
            },
            "80f0b528ea72121e8281a7734264989d2740bf5c89b7df866dc219b6016d52f5": {
                "Name": "mailu_admin_1",
                "EndpointID": "6770a86d9d3678ca98af052b9fc3e8fb2daf046166f8dbf819af5b7e7d49a1a5",
                "MacAddress": "02:42:c0:a8:cb:05",
                "IPv4Address": "192.168.203.5/24",
                "IPv6Address": ""
            },
            "95f81670bd10572ccaf6f778228bab6a808b5bcc74061a09c44090359b89b58e": {
                "Name": "mailu_redis_1",
                "EndpointID": "593ddd4e0ff2082ee9243b9eff0b4cc8301ace0438e84c0af02cc9f775a07eae",
                "MacAddress": "02:42:c0:a8:cb:04",
                "IPv4Address": "192.168.203.4/24",
                "IPv6Address": ""
            },
            "bbeb23a885a4e41ac47a25bc1c296b817f6eeff9baf2d23b26f4ac9513f04cae": {
                "Name": "mailu_smtp_1",
                "EndpointID": "b4e3b3c2e3676069039660ff5aab6529ed224a343d3eca60262cfea3341b6b4a",
                "MacAddress": "02:42:c0:a8:cb:08",
                "IPv4Address": "192.168.203.8/24",
                "IPv6Address": ""
            },
            "bc78cf9b396530c65ff202acb9e925f236e9e655a7e35cf1159d502b653f2d05": {
                "Name": "mailu_imap_1",
                "EndpointID": "c92a0e46b68bb2f594dfb7395a6e6d206d0497c2b30103395edcafa547153c10",
                "MacAddress": "02:42:c0:a8:cb:07",
                "IPv4Address": "192.168.203.7/24",
                "IPv6Address": ""
            },
            "f25bec8296de7375d78c190b8fbe8dc2ed1eff3d3adbe53655428341f53b3da1": {
                "Name": "mailu_webdav_1",
                "EndpointID": "0154fd24c909d460830539cfe55fbb8b9c201abc17b1d2ae5cbbd49964e59e1a",
                "MacAddress": "02:42:c0:a8:cb:03",
                "IPv4Address": "192.168.203.3/24",
                "IPv6Address": ""
            }
        },
        "Options": {
            "com.docker.network.bridge.enable_icc": "true",
            "com.docker.network.bridge.enable_ip_masquerade": "true"
        },
        "Labels": {
            "com.docker.compose.network": "default",
            "com.docker.compose.project": "mailu",
            "com.docker.compose.version": "1.27.4"
        }
    }
]
```