Add project files

ssh.go

@@ -0,0 +1,119 @@
+package main
+
+// Copyright (c) 2024 Julian Müller (ChaoticByte)
+// License: MIT
+
+import (
+	"bufio"
+	"fmt"
+	"log"
+	"slices"
+	"sync"
+
+	"github.com/gliderlabs/ssh"
+)
+
+var sessionListMutex = &sync.Mutex{}
+var sessionList = []*ssh.Session{}
+
+func PubkeyAuth(ctx ssh.Context, key ssh.PublicKey) bool {
+	// verify public key for auth
+	u := ctx.User()
+	pubkey := config.Clients[u]
+	if pubkey == "" { // username not in config.Clients
+		log.Printf("Authentication failure: Unknown user %s", u)
+		return false
+	}
+	k, _, _, _, err := ssh.ParseAuthorizedKey([]byte(pubkey))
+	if err != nil {
+		log.Printf("Authentication failure: Could not parse public key for %s", u)
+	}
+	if ssh.KeysEqual(k, key) {
+		return true // bassd
+	} else {
+		log.Printf("Authentication failure: Invalid public key for user %s", u)
+		return false
+	}
+}
+
+func AddToSessionList(s *ssh.Session) int {
+	// add new session to list
+	var idx int = -1
+	sessionListMutex.Lock()
+	defer sessionListMutex.Unlock()
+	lenClientList := len(sessionList)
+	// try to reuse an existing nil position in the slice
+	for i := range(lenClientList) {
+		if sessionList[i] == nil {
+			// we found one!
+			sessionList[i] = s
+			idx = i
+			break
+		}
+	}
+	if idx == -1 {
+		// we have to append
+		sessionList = append(sessionList, s)
+		idx = lenClientList
+	}
+	return idx
+}
+
+func RemoveFromSessionList(idx int) {
+	sessionListMutex.Lock()
+	defer sessionListMutex.Unlock()
+	// we have to set it to nil instead of constructing a new
+	// slice, so that the other session handlers have still
+	// the correct index for their own removal
+	sessionList[idx] = nil // session associated with the pointer reference should be freed automatically
+}
+
+func BroadcastLine(line []byte) {
+	if logFlag { log.Printf("%s", line) } // output message
+	sessions := sessionList // create snapshot of client list
+	for i := range(len(sessions)) {
+		session := sessions[i]
+		if session != nil {
+			(*session).Write(line)
+		}
+	}
+}
+
+func HandleSession(session ssh.Session) {
+	// get username
+	user := session.Context().User()
+	msgPrefix := []byte(fmt.Sprintf("%s: ", user))
+	// Add client to list
+	sessionListIdx := AddToSessionList(&session)
+	defer func () { // cleanup tasks
+		RemoveFromSessionList(sessionListIdx)
+		log.Printf("User %s disconnected.\n", user)
+		BroadcastLine([]byte(fmt.Sprintf("[disconnected] %s\n", user)))
+	}()
+	// helo
+	log.Printf("User %s connected.\n", user)
+	BroadcastLine([]byte(fmt.Sprintf("[connected] %s\n", user)))
+	// IO
+	linebuf := bufio.NewReader(session)
+	for {
+		line, err := linebuf.ReadBytes('\n')
+		if err != nil {
+			if err.Error() != "EOF" {
+				log.Println("An error occurred! ", user, " ", err)
+			}
+			break // disconnect
+		} else {
+			BroadcastLine(slices.Concat(msgPrefix, line))
+		}
+	}
+}
+
+func RunServer() {
+	log.Printf("Starting SSH server on %s:%v", config.Host, config.Port)
+	log.Fatal(ssh.ListenAndServe(
+		fmt.Sprintf("%s:%v", config.Host, config.Port),
+		HandleSession,
+		ssh.HostKeyFile(privateKeyFilepath),
+		ssh.PublicKeyAuth(PubkeyAuth),
+		ssh.NoPty()))
+}