JWT Validation in HAProxy - jwt

Question

I have an HAProxy configured to accept requests to *.mysubdomain.com. The HAProxy will parse the subdomain (prod or dev from prod.mysubdomain.com or dev.mysubdomain.com) and forward to the correct backend. Two backends exist, one for prod and one for dev. Each backend contains two server entries pointing towards Marathon LB instances on each subdomain.
The subdomains require a JWT cookie for authentication on the backend. I have the public key to check the validity of the JWT, but would like to do so in the HAProxy. Is there a way to add my own code to perform the JWT validity check within the HAProxy configuration?
The HAProxy configuration file is as follows:
global
maxconn 256
defaults
mode http
timeout connect 5000ms
timeout client 50000ms
timeout server 50000ms
frontend http-in
bind *:80
mode http
# Returns true when one of the headers contains one of the strings either isolated or delimited by dots. This is used to perform domain name matching.
acl host_dev hdr_dom(host) -i dev
acl host_prod hdr_dom(host) -i prod
acl jwtPresent req.cook(JWT) -m found
use_backend prod_domain if jwtPresent host_prod
use_backend dev_domain if jwtPresent host_dev
default_backend prod_domain
backend prod_domain
balance roundrobin
server prodDomain1 "${MARATHON_LB_PROD_1}" maxconn 32 check
server prodDomain2 "${MARATHON_LB_PROD_2}" maxconn 32 check
backend dev_domain
balance roundrobin
server devDomain1 "${MARATHON_LB_DEV_1}" maxconn 32 check
server devDomain2 "${MARATHON_LB_DEV_2}" maxconn 32 check

Answer 1

HAProxy can act as an API gateway and validate JWT tokens against a public key. They have written a blog post and provided sample code to show you how.
The post is here: https://www.haproxy.com/blog/using-haproxy-as-an-api-gateway-part-2-authentication/
The sample lua code is here: https://github.com/haproxytech/haproxy-lua-jwt

Answer 2

As the other answer pointed out, you have to use Lua script. You can use existing implementations from lua-resty-jwt or Kong.
Notes:
Those code-bases are not concise. A simple copy & paste won't work. So you have to extract the bare minimum you need.
You can't have dependencies in your Lua script. Only plain vanilla Lua. So you have to get rid of all require statements.
The tricky part is the HMAC implementation.
Avoid any I/O operation in your Lua script, e.g. file, database, network operations.
It's not an easy undertaking. Good luck! It's something worth sharing.

Answer 3

As far as I could tell, HAProxy does not have the functionality to perform the logic for validating the JWT. Instead, I implemented a script in Lua for haproxy.cfg to call to perform the validation:
global
maxconn 256
lua-load /choose_backend.lua
defaults
mode http
timeout connect 5000ms
timeout client 50000ms
timeout server 50000ms
frontend http-in
bind *:80
http-request set-header X-SSL-Client-DN %{+Q}[ssl_c_s_dn]
http-request set-var(txn.backend_name) lua.backend_select()
use_backend %[var(txn.backend_name)]
backend prod_domain
balance roundrobin
server prodDomain1 "${MARATHON_LB_PROD_1}" maxconn 32 check
server prodDomain2 "${MARATHON_LB_PROD_2}" maxconn 32 check
backend dev_domain
balance roundrobin
server devDomain1 "${MARATHON_LB_DEV_1}" maxconn 32 check
server devDomain2 "${MARATHON_LB_DEV_2}" maxconn 32 check