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internal/authn/authn.go
554bcfc63721b22faf8bca78d5518d76e24a4d20
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| 1 | // Package authn owns gsg's browser session endpoints. Login trusts the |
| 2 | // identity header that the reverse proxy sets after forward_auth; logout |
| 3 | // only clears gsg's own cookie. Keeping both here means main.go composes |
| 4 | // listeners rather than implementing authentication. |
| 5 | // |
| 6 | // The two handlers are returned separately on purpose: login must only ever |
| 7 | // be mounted on the admin listener, while logout belongs on the public one. |
| 8 | // A single combined handler would make that distinction a calling |
| 9 | // convention instead of a structural one. |
| 10 | package authn |
| 11 | |
| 12 | import ( |
| 13 | "fmt" |
| 14 | "net/http" |
| 15 | "net/url" |
| 16 | "strings" |
| 17 | |
| 18 | "gsg/internal/session" |
| 19 | ) |
| 20 | |
| 21 | // Config is the small amount of state the session endpoints need. |
| 22 | type Config struct { |
| 23 | Sessions *session.Manager |
| 24 | UserHeader string // e.g. "X-Authentik-Username"; defaults to that |
| 25 | } |
| 26 | |
| 27 | // Handlers are mounted individually by the caller. |
| 28 | type Handlers struct { |
| 29 | Login http.Handler |
| 30 | Logout http.Handler |
| 31 | } |
| 32 | |
| 33 | // NewHandlers builds the login and logout endpoints. |
| 34 | func NewHandlers(cfg Config) (Handlers, error) { |
| 35 | if cfg.Sessions == nil { |
| 36 | return Handlers{}, fmt.Errorf("authn: no session manager") |
| 37 | } |
| 38 | if cfg.UserHeader == "" { |
| 39 | cfg.UserHeader = "X-Authentik-Username" |
| 40 | } |
| 41 | return Handlers{ |
| 42 | Login: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { login(cfg, w, r) }), |
| 43 | Logout: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) { logout(cfg, w, r) }), |
| 44 | }, nil |
| 45 | } |
| 46 | |
| 47 | // login mints a session from the proxy-supplied identity. Reaching this |
| 48 | // handler at all is the authorization decision: the admin listener is the |
| 49 | // only place it is mounted, and Caddy's forward_auth runs before it. |
| 50 | func login(cfg Config, w http.ResponseWriter, r *http.Request) { |
| 51 | user := r.Header.Get(cfg.UserHeader) |
| 52 | if user == "" { |
| 53 | http.Error(w, "missing identity header", http.StatusUnauthorized) |
| 54 | return |
| 55 | } |
| 56 | http.SetCookie(w, cfg.Sessions.Issue(user)) |
| 57 | http.Redirect(w, r, safeNext(r.URL.Query().Get("next")), http.StatusSeeOther) |
| 58 | } |
| 59 | |
| 60 | // logout needs no round trip through the identity provider; it only clears |
| 61 | // the cookie this process issued. |
| 62 | func logout(cfg Config, w http.ResponseWriter, r *http.Request) { |
| 63 | http.SetCookie(w, cfg.Sessions.Clear()) |
| 64 | http.Redirect(w, r, "/", http.StatusSeeOther) |
| 65 | } |
| 66 | |
| 67 | // safeNext returns next if it is a same-site relative path, else "/". next |
| 68 | // comes from a query parameter on a login redirect; without this check it |
| 69 | // would be an open redirect (?next=https://evil.example) off a trusted |
| 70 | // domain, right after proving who the user is. |
| 71 | func safeNext(next string) string { |
| 72 | if next == "" || !strings.HasPrefix(next, "/") || strings.HasPrefix(next, "//") { |
| 73 | return "/" |
| 74 | } |
| 75 | if u, err := url.Parse(next); err != nil || u.Scheme != "" || u.Host != "" { |
| 76 | return "/" |
| 77 | } |
| 78 | return next |
| 79 | } |