diff --git a/src/cas/service.ts b/src/cas/service.ts
new file mode 100644
index 0000000..d763336
--- /dev/null
+++ b/src/cas/service.ts
@@ -0,0 +1,24 @@
+import { CAS_HOST, CAS_EXTERNAL_SERVICES } from "~/utils/constants";
+
+/**
+ * @param cas_token token that can be found using `cas_login` function.
+ * @param service_url the service URL to authenticate to.
+ * @returns the URL to authenticate to said service.
+ */
+export const cas_service = async (cas_token: string, service_url: typeof CAS_EXTERNAL_SERVICES[keyof typeof CAS_EXTERNAL_SERVICES]): Promise<string> => {
+  const uri = new URL(CAS_HOST + "/cas/login");
+  uri.searchParams.set("service", service_url);
+  uri.searchParams.set("gateway", "true");
+
+  const response = await fetch(uri.href, {
+    headers: { "Cookie": "lemonldap=" + cas_token },
+    // prevent redirects since we want to
+    // return the redirection url.
+    redirect: "manual"
+  });
+
+  const redirection = response.headers.get("location");
+  if (!redirection) throw new Error("No redirection found.");
+
+  return redirection;
+};
diff --git a/src/utils/constants.ts b/src/utils/constants.ts
index b62b708..12b7773 100644
--- a/src/utils/constants.ts
+++ b/src/utils/constants.ts
@@ -1,3 +1,5 @@
+export const CAS_HOST = "https://cas.unilim.fr";
+
 export const PROD_ENDPOINT = "https://apis.unilim.fr";
 export const DEV_ENDPOINT = "https://apis-dev.unilim.fr";
 export const BIOME_PROD_ENDPOINT = "https://biome.unilim.fr";
@@ -6,3 +8,9 @@ export const BIOME_DEV_ENDPOINT = "https://biome-dev.unilim.fr";
 export const OAUTH2_CODE_CHALLENGE_METHOD = "plain";
 export const OAUTH2_CODE_CHALLENGE = Math.random().toString(36).substring(2);
 export const OAUTH2_STATE = Math.random().toString(36).substring(2);
+
+export const CAS_EXTERNAL_SERVICES = {
+  IUT_COMMUNITY_MOODLE: "https://community-iut.unilim.fr/login/index.php?authCAS=CAS",
+  COMMUNITIES_MOODLE: "https://communities.unilim.fr/login/index.php?authCAS=CAS",
+  GRADING: "https://inscription.unilim.fr/gestion/etudiant/rvn/"
+} as const;