redirector

redirector

A simple HTTP server to redirect traffic from one place to another.

Configuration

Add a configuration file at config.json (or another location set in the REDIRECTOR_CONFIG environment variable) with content like:

{
 "listen_address": ":8080",
 "metrics_address": ":9091",
 "default_response": {
  "code": 421,
  "body": "421 Misdirected Request\n\nTarget URI does not match an origin for which the server has been configured.\n",
  "headers": {
   "Connection":   "close",
   "Content-Type": "text/plain"
  },
  "log_hits": true
 },
 "domains": {
  "example.com": {
   "match_subdomains": true,
   "rewrites": [
    {
     "regexp": "^(.*)$",
     "replacement": "https://www.example.com$1",
     "code": 301,
     "log_hits": true
    }
   ]
  }
 }
}

For default_response, using a code of 0 will result in the connection being immediately closed if possible. If that is not possible at runtime, the headers and body will be used but with an HTTP status code of 500.

Each domain may also define a default_response key which matches if no rewrites match.

All regular expressions use re2 syntax.

For a given rewrite, replacement may include variables like $1 where the number will be replaced with the corresponding matched sub-pattern with that index. Replacing with named sub-patterns is not currently supported, and attempting to use a non-numeric variable will cause validation of configuration to fail. To insert a literal $, use $$.

Rewrites are applied in order, and only the first matching rewrite is applied. If there are duplicate domains, only the first matching domain is used.

If match_subdomains is true, all subdomains (including nested subdomains e.g. a.b.example.com for example.com) will be matched. It is an error to set match_subdomains to true if a matching subdomain is also elsewhere defined (e.g. you cannot do {"example.com": { "match_subdomains": true }, "www.example.com": {}).

Domains must be lowercase ASCII (i.e. in punycode if required). Domains may include a port after a colon (e.g. example.com:8080), but will be matched against the Host header directly, so use of :80 or :443 is not recommended as most clients do not include that in the Host header when using HTTP(S) on those ports.

Hosting

An example fly.toml is provided for use on Fly.io, which is where I host this for myself. I am not affiliated with Fly.io and they are, to my knowledge, not aware of me or this project.

An appropriate health check responder is included in config.example.json, matching the health check in fly.example.toml.

It is also necessary to obtain TLS certificates for each hosted domain. You can do this with a command like:

for domain in $(\
    jq -r \
        '.domains | del(."health-check.internal") | to_entries | map(if .value.match_subdomains then "*." + .key else .key end) | join("\n")' \
        config.json \
    ); do \
        echo Replace this with whatever you want, maybe like fly certs show "$domain" \
    ; done

When using $domain in the inner part of the loop there, it is important to ensure it is surrounded by double quotes.