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kubernetes.go
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kubernetes.go
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package oidc
import (
"encoding/base64"
"encoding/json"
"errors"
"fmt"
"io"
"net/http"
"net/url"
"os"
"strings"
"time"
log "github.com/sirupsen/logrus"
"golang.org/x/oauth2"
)
type KubernetesDetails struct {
ProviderUrl string
ClientId string
Scopes []string
}
func AuthenticateKubernetes(config KubernetesDetails) (*TokenCredentials, error) {
tokenSource := FunctionTokenSource{
GetToken: func() (*oauth2.Token, error) {
log.Info("Getting new authentication token")
kubernetesToken, err := getKubernetesToken()
if err != nil {
return nil, err
}
c := &http.Client{}
resp, err := c.PostForm(config.ProviderUrl+"/connect/token",
url.Values{
"client_id": {config.ClientId},
"scope": {strings.Join(config.Scopes, " ")},
"grant_type": {"kubernetes"},
"client_assertion": {kubernetesToken},
})
if err != nil {
return nil, err
}
defer resp.Body.Close()
if resp.StatusCode == 400 {
var errResp oauthErrorResponse
err = json.NewDecoder(resp.Body).Decode(&errResp)
if err != nil {
return nil, err
}
return nil, errors.New(errResp.Error)
} else if resp.StatusCode != 200 {
return nil, makeErrorForHTTPResponse(resp)
}
body, err := io.ReadAll(resp.Body)
if err != nil {
return nil, err
}
token, err := parseOIDCToken(body)
if err != nil {
return nil, err
}
return token, nil
},
}
return &TokenCredentials{TokenSource: oauth2.ReuseTokenSource(nil, &tokenSource)}, nil
}
/**
* parseOIDCToken takes a JSON OIDC response and returns a correctly set OIDC token struct.
*
* See https://github.com/golang/oauth2/blob/8227340efae7cbdad9f68d6dff2b2c3306714564/jwt/jwt.go#L150
*/
func parseOIDCToken(body []byte) (*oauth2.Token, error) {
// tokenRes is the JSON response body.
var tokenRes struct {
AccessToken string `json:"access_token"`
TokenType string `json:"token_type"`
ExpiresIn int64 `json:"expires_in"` // relative seconds from now
}
if err := json.Unmarshal(body, &tokenRes); err != nil {
return nil, fmt.Errorf("kubernetes flow: cannot fetch token: %v", err)
}
token := &oauth2.Token{
AccessToken: tokenRes.AccessToken,
TokenType: tokenRes.TokenType,
}
if secs := tokenRes.ExpiresIn; secs > 0 {
token.Expiry = time.Now().Add(time.Duration(secs) * time.Second)
}
if expiry, err := extractExpiry(tokenRes.AccessToken); err != nil {
if token.Expiry.IsZero() || token.Expiry.After(*expiry) {
token.Expiry = *expiry
}
}
return token, nil
}
/**
* extractExpiry retrieves the expiry time from the OIDC JWT
*
* A modified version of golang.org/x/oauth2/jws
*/
func extractExpiry(payload string) (*time.Time, error) {
s := strings.Split(payload, ".")
if len(s) < 2 {
return nil, errors.New("kubernetes flow: invalid token received")
}
decoded, err := base64.RawURLEncoding.DecodeString(s[1])
if err != nil {
return nil, err
}
var c struct {
Exp int64 `json:"exp"`
}
err = json.Unmarshal(decoded, &c)
if err != nil {
return nil, err
}
if c.Exp == 0 {
return nil, fmt.Errorf("kubernetes flow: expiration time in JWT shouldn't be 0")
}
ret := time.Unix(c.Exp, 0)
return &ret, nil
}
func getKubernetesToken() (string, error) {
fromEnv := os.Getenv("K8S_SERVICEACCOUNT_TOKEN")
if fromEnv != "" {
return fromEnv, nil
}
fromFile, err := os.ReadFile("/var/run/secrets/kubernetes.io/serviceaccount/token")
if err != nil {
return "", err
}
return string(fromFile), nil
}