A variation on the libvmod-redis based URL shortener, now doing a curveball to make the redirect cacheable.

default.vcl

import redis;

backend default {
    .host = "127.0.0.1";
    .port = "8080";
}

backend self {
    .host = "127.0.0.1";
    .port = "80";
}

sub vcl_recv {
    if (req.url ~ "(?i)^/l/") {
        set req.backend = self;
        if (req.url ~ "(?i)^/l/r/") {
            // Looks like a cache miss, and successful lookup, return a redirect now
            // (should check for localhost origin?)
            error 666 regsub(req.url, "^(?i)^/l/r/([\d\D]*)$", "\1");
        }
    }
}

sub vcl_error {
    if (obj.status == 666) {
        // Returning a redirect, so the self-generated response can be cached
        set obj.status = 302;
        set obj.http.Location = obj.response;
        set obj.response = "Found";
        return (deliver);
    }
}

sub vcl_miss {
    if (req.url ~ "(?i)^/l/") {
        // First try and make this URL into a Redis lookup command
        set req.http.Redis-Command = regsub(req.url, 
            "(?i)^/l/([a-z0-9\-_]{2})([a-z0-9\-_]{2,30}=?=?)$", 
            "HGET short:\1 \2");
        if (req.url == req.http.Redis-Command) {
           // Looks like the url doesn't match the pattern, so it's 404 Not Found
           unset req.http.Redis-Command;
           error 404 "Not Found";
        }

        // Issue the Redis command
        set bereq.url = redis.call(req.http.Redis-Command);
        unset req.http.Redis-Command;
        if (bereq.url) {
            // It returned something useful, use it!
            set bereq.url = "/l/r/" + bereq.url;
            return (fetch);
        }
        // Redis didn't return what we were looking for, so it's 404 Not Found
        error 404 "Not Found";
    }
}