Linode Ubuntu环境下Flask/Nginx配置问题：www子域名500错误

问题背景 #

在Linode计算实例上部署了基于Flask/Nginx/Gunicorn/Supervisor的简易微博站点，主域名domain.com可正常访问，但访问www.domain.com时出现500错误，Nginx日志提示如下：

rewrite or internal redirection cycle while internally redirecting to "/index.html", client: 2603:7000:a700:cc1:c867:5aae:ff32:199c, server: www.domain.com, request: "GET / HTTP/1.1", host: "www.domain.com"

当前生效的Nginx配置：

server {
    server_name domain.com www.domain.com;

    location / {
        proxy_pass http://127.0.0.1:8000;
        proxy_set_header Host $host;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    }

    listen [::]:443 ssl ipv6only=on; # managed by Certbot
    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/domain.com/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/domain.com/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot
}

server {
    if ($host = domain.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot

    listen [::]:80;
    listen 80;
    server_name domain.com;
    return 404; # managed by Certbot
}

问题原因 #

1. Flask主机名限制：Flask应用未配置允许www.domain.com作为请求Host头，当Nginx将www.domain.com的请求代理到Gunicorn时，Flask直接返回500错误。
2. Nginx错误页循环：Certbot生成的options-ssl-nginx.conf文件中默认包含error_page规则，会将500错误重定向到/index.html；而/index.html又会被Nginx代理回Flask，Flask再次返回500，形成无限循环。
3. HTTP跳转缺失：80端口的Nginx配置未处理www.domain.com的HTTP请求，导致http://www.domain.com直接返回404，影响访问体验。

解决步骤 #

步骤1：修改Flask配置，允许www域名 #

在Flask应用的配置代码中，将www.domain.com添加到允许的主机列表：

# 推荐方式：使用ALLOWED_HOSTS
app.config['ALLOWED_HOSTS'] = ['domain.com', 'www.domain.com']

# 备选方式：若使用SERVER_NAME（注意：会影响路由和子域名解析，需谨慎）
# app.config['SERVER_NAME'] = 'domain.com'

修改完成后，重启Supervisor或Gunicorn使配置生效。

步骤2：修复Nginx的HTTP跳转配置 #

更新80端口的server块，添加www.domain.com的HTTP到HTTPS跳转规则：

server {
    if ($host = domain.com) {
        return 301 https://$host$request_uri;
    } # managed by Certbot

    # 添加www域名的HTTP跳转规则
    if ($host = www.domain.com) {
        return 301 https://$host$request_uri;
    }

    listen [::]:80;
    listen 80;
    server_name domain.com www.domain.com; # 更新server_name包含www域名
    return 404; # managed by Certbot
}

步骤3：验证配置并重启Nginx #

先检查Nginx配置语法是否正确：

sudo nginx -t

确认无错误后，重启Nginx：

sudo systemctl restart nginx

内容的提问来源于stack exchange，提问作者goon