Alpine 3.18搭配cURL 8.2.1不遵循.localhost域名的/etc/hosts配置

问题描述 #

将Docker镜像更新至Alpine 3.18后，应用无法通过cURL向同一主机上的其他容器发送HTTP请求——此前通过Docker Compose的extra_hosts参数实现该功能。调试发现Alpine自带的cURL 8.2.1似乎无视/etc/hosts中的extra_hosts配置，但ping命令可正常解析域名。

环境配置 #

docker-compose.yml：

version: "3.8"

services:
    test:
        image: alpine:3.18
        extra_hosts:
            - example.localhost:host-gateway

调试步骤及结果 #

1. 启动容器：

docker-compose run test sh

2. 安装cURL：

apk add curl

3. 查看cURL版本：

/ # curl --version
curl 8.2.1 (aarch64-alpine-linux-musl) libcurl/8.2.1 OpenSSL/3.1.2 zlib/1.2.13 brotli/1.0.9 libidn2/2.3.4 nghttp2/1.55.1
Release-Date: 2023-07-26
Protocols: dict file ftp ftps gopher gophers http https imap imaps mqtt pop3 pop3s rtsp smb smbs smtp smtps telnet tftp ws wss
Features: alt-svc AsynchDNS brotli HSTS HTTP2 HTTPS-proxy IDN IPv6 Largefile libz NTLM NTLM_WB SSL threadsafe TLS-SRP UnixSockets

4. 执行cURL请求（失败）：

/ # curl -v example.localhost
* processing: example.localhost
*   Trying [::1]:80...
* Immediate connect fail for ::1: Address not available
*   Trying 127.0.0.1:80...
* connect to 127.0.0.1 port 80 failed: Connection refused
* Failed to connect to example.localhost port 80 after 0 ms: Couldn't connect to server
* Closing connection
curl: (7) Failed to connect to example.localhost port 80 after 0 ms: Couldn't connect to server

5. 确认/etc/hosts配置正确：

/ # cat /etc/hosts
127.0.0.1       localhost
::1     localhost ip6-localhost ip6-loopback
fe00::0 ip6-localnet
ff00::0 ip6-mcastprefix
ff02::1 ip6-allnodes
ff02::2 ip6-allrouters
192.168.65.254  example.localhost
172.20.0.3      76a7dfa28161

6. ping命令正常解析：

/ # ping example.localhost
PING example.localhost (192.168.65.254): 56 data bytes
64 bytes from 192.168.65.254: seq=0 ttl=62 time=2.224 ms
64 bytes from 192.168.65.254: seq=1 ttl=62 time=0.731 ms
64 bytes from 192.168.65.254: seq=2 ttl=62 time=0.322 ms
^C
--- example.localhost ping statistics ---
3 packets transmitted, 3 packets received, 0% packet loss
round-trip min/avg/max = 0.322/1.092/2.224 ms

原因分析 #

cURL 8.0+版本默认启用了AsynchDNS特性，在Alpine的musl libc环境下，异步DNS解析会绕过本地/etc/hosts文件，直接发起DNS查询，导致无法读取extra_hosts配置的映射关系。而ping命令使用系统同步DNS解析，会优先读取/etc/hosts，因此可以正常解析。

解决方案 #

临时解决（单命令生效） #

在cURL请求中添加--no-asynch-dns参数，禁用异步DNS解析，强制使用同步解析读取/etc/hosts：

curl -v --no-asynch-dns example.localhost

或者使用--resolve参数直接指定域名的IP映射，无需依赖/etc/hosts：

curl -v --resolve example.localhost:80:192.168.65.254 example.localhost

永久解决（全局生效） #

在容器中创建cURL配置文件，默认禁用异步DNS解析：

1. 创建/etc/curlrc文件：

echo "--no-asynch-dns" > /etc/curlrc

2. 后续所有cURL请求都会自动应用该配置，无需额外添加参数。

内容的提问来源于stack exchange，提问作者Maarten de Graaf