This dockerfile builds a Squid 4 instance and includes all the necessary tooling to run it as a MITM (man-in-the-middle) SSL proxy.

There's a number of reasons to do this - the big one being optimizing caching and delivery of objects during docker builds which might be downloading them from SSL protected endpoints.

It will require you to generate your own CA and set it as trusted.

The resulting docker image uses the following configuration environment variables:

• HTTP_PORT Default: 3128
• ICP_PORT If set, enables ICP on the given port for all users.
• HTCP_PORT If set, enables HTCP on the given port for all users.
• MITM_PROXY If set, tries to enable MITM SSL proxy functionality (requires CERT and KEY)
• MITM_CERT If set, the given PEM certificate is copied and used as the CA authority for MITM'ing connections.
• MITM_KEY If set, the given PEM certificate is copied and used as the signing key for the MITM CA.
• VISIBLE_HOSTNAME Default: docker-squid4 Should be set to a unique value if you are chaining multiple proxy servers.
• MAX_CACHE_SIZE Default: 40000 Cache size in megabytes. The cache defaults to /var/cache/squid4. You should mount a volume here to make it persistent.
• MAX_OBJECT_SIZE Default "1536 MB" Maximum object size to store in the cache. This is set high as one of my typical use cases is proxying distribution images.
• MEM_CACHE_SIZE Default: "128 MB" Default memory cache size. I've no real clue what this should be, but RAM is plentiful so I like to keep it fairly large.
• CACHE_PEERx Cache peers for the squid instance may be specified with multiple CACHE_PEER environment variables. The suffix of each is used to determine ordering by the unix sort function.
• EXTRA_CONFIGx Extra non-specific configuration lines to be appended after the main body of the configuration file. This is a good place for custom ACL parameters.
• CONFIG_DISABLE Default no If set to yes then squid configuration templating is disabled entirely, allowing bind mounting the configuration file in manually instead. The certificate and SSL setup still runs normally.
• DISABLE_CACHE Default `` If set to yes then squid configuration templating removes all `cache_dir` lines, setting squid to memory only cache.
• TLS_OPTIONS Default NO_SSLv3,NO_TLSv1 Allow overriding the default tls_outgoing_options supplied to OpenSSL. These are safe defaults, but if you're in a really broken environment might not be usable.

By default squid in SSL MITM mode treats cache_peer entries quite differently. Because squid unwraps the CONNECT statement when bumping an SSL connection, but does not rewrap it when communicating with peers, it requires all peers to connect with SSL as well. This breaks compatibility with simple minded proxies.

To work around this, proxychains-ng (proxychains4 internally) is built and included in this image. If you need to use an upstream proxy with a MITM squid4, you should launch the image in proxychains mode which intercepts squids direct outbound connections and redirects them via CONNECT requests. This also adds SOCKS4 and SOCKS5 proxy support if so desired.

proxychains is configured with the following environment variables. As with the others above, CONFIG_DISABLE prevents overwriting templated files.

• PROXYCHAIN Default none. If set to yes then squid will be launched with proxychains. You should specify some proxies when doing this.
• PROXYCHAIN_PROXYx Upstream proxies to be passed to the proxy chan config file. The suffix (x) determines the order in which they are templated into the configuration file. The format is a space separated string like "http 127.0.0.1 3129"
• PROXYCHAIN_TYPE Default strict_chain. Can be strict_chain or dynamic_chain sensibly within this image. In strict_chain mode, all proxies must be up. In dynamic_chain mode proxies are used in order, but skipped if down. Disable configuration and bind a configuration file to /etc/proxychains.conf if you need more flexibility.
• PROXYCHAIN_DNS Default none. When set to yes, turns on the proxy_dns option for Proxychains.

In some corporate environments, its not possible to get reliable DNS outbound service and proxychains-ng's DNS support won't be able to provide for Squid4 to actually work. To address this, configuration is included to setup and use DNS-over-HTTPS.

The idea of the DNS-over-HTTPS client is that it will use your local proxy and network access to provide DNS service to Squid4.

• DNS_OVER_HTTPS Default no. If yes then enables and starts the DNS_OVER_HTTPS service.
• DNS_OVER_HTTPS_LISTEN_ADDR Default 127.0.0.153:53. Squid doesn't support changing the port, so keep this in mind.
• DNS_OVER_HTTPS_SERVER Default https://dns.google.com/resolve. AFAIK there's no other options for this at the moment.
• DNS_OVER_HTTPS_NO_PROXY Default ``. List of DNS suffixes to not ever proxy via DNS_OVER_HTTPS.
• DNS_OVER_HTTPS_PREFIX_SERVER Default ``. Normal DNS server to try resolving first against.
• DNS_OVER_HTTPS_SUFFIX_SERVER Default ``. Normal DNS server to try resolving last against.

Since the DNS-over-HTTPS daemon is a separate Go binary, you may also need to specify your internal proxy as an upstream to allow it to contact the HTTPS DNS server - do this by passing the standard http_proxy and https_proxy parameters. Most likely these will be the same as your PROXYCHAIN_PROXYx directives (and probably only the 1).

The following command line will get you up and running quickly. It presumes you've generated a suitable CA certificate and are intending to use the proxy as a local MITM on your machine:

sudo mkdir -p /srv/squid/cache
docker run -it -p 3128:127.0.0.1:3128 --rm \
    -v /srv/squid/cache:/var/cache/squid4 \
    -v /etc/ssl/certs:/etc/ssl/certs:ro \ 
    -v /etc/ssl/private/local_mitm.pem:/local-mitm.pem:ro \
    -v /etc/ssl/certs/local_mitm.pem:/local-mitm.crt:ro \
    -e MITM_CERT=/local-mitm.crt \
    -e MITM_KEY=/local-mitm.pem \
    -e MITM_PROXY=yes \
    squid

Note that it doesn't really matter where we mount the certificate - the image launch script makes a copy as root to avoid messing with permissions anyway.

This is an example of a systemd unit file to persistly start squid4:

[Unit]
Description=Squid4 Docker Container
Documentation=http://wiki.squid.org
After=network.target docker.service
Requires=docker.service

[Service]
ExecStartPre=-/usr/bin/docker kill squid4
ExecStartPre=-/usr/bin/docker rm squid4
ExecStart=/usr/bin/docker run --net=host --rm \
    -v /srv/squid/cache:/var/cache/squid4 \
    -v /etc/ssl/certs:/etc/ssl/certs:ro \
    -v /etc/ssl/private/local_mitm.pem:/local_mitm.pem:ro \
    -v /etc/ssl/certs/local_mitm.pem:/local_mitm.crt:ro \
    -e MITM_KEY=/local_mitm.pem \
    -e MITM_CERT=/local_mitm.crt \
    -e MITM_PROXY=yes \
    --name squid4 \
    squid

[Install]
WantedBy=multi-user.target