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// net/ipv4/tcp_input.c
// tcp_v4_rcv -> tcp_v4_do_rcv -> tcp_rcv_state_process -> tcp_v4_conn_request -> tcp_conn_request
int tcp_conn_request(struct request_sock_ops *rsk_ops,
const struct tcp_request_sock_ops *af_ops,
struct sock *sk, struct sk_buff *skb)
{
...
struct request_sock *req;
bool want_cookie = false;
...
// 判断是否启用 syncookies
// tcp_syncookies=2 就是强制启用 syncookies，忽略 syn backlog
if ((net->ipv4.sysctl_tcp_syncookies == 2 ||
inet_csk_reqsk_queue_is_full(sk)) && !isn) {
want_cookie = tcp_syn_flood_action(sk, rsk_ops->slab_name);
if (!want_cookie)
goto drop;
}
...
// 处理 syn 包的 struct sock 对象
// !want_cookie 是不将 req 跟 listener 相互绑定起来，响应 synack 后会释放掉 req 对象，从而减少服务器的资源占用
req = inet_reqsk_alloc(rsk_ops, sk, !want_cookie);
if (!req)
goto drop;
req->syncookie = want_cookie;
...
if (want_cookie) {
isn = cookie_init_sequence(af_ops, sk, skb, &req->mss);
if (!tmp_opt.tstamp_ok)
inet_rsk(req)->ecn_ok = 0;
}
...
if (!want_cookie) {
tcp_reqsk_record_syn(sk, req, skb);
fastopen_sk = tcp_try_fastopen(sk, skb, req, &foc, dst);
}
if (fastopen_sk) {
af_ops->send_synack(fastopen_sk, dst, &fl, req,
&foc, TCP_SYNACK_FASTOPEN, skb);
...
} else {
tcp_rsk(req)->tfo_listener = false;
if (!want_cookie) // 不启用 syncookies，则将 req 加入到 syn backlog
inet_csk_reqsk_queue_hash_add(sk, req,
tcp_timeout_init((struct sock *)req));
// 发送 synack 包
af_ops->send_synack(sk, dst, &fl, req, &foc,
!want_cookie ? TCP_SYNACK_NORMAL :
TCP_SYNACK_COOKIE,
skb);
if (want_cookie) {
// 发送 synack 包后，释放掉 req
// 当发生 synflood 时，释放 req 是为了减少服务器资源占用
reqsk_free(req);
return 0;
}
}
...
return 0;
...
}
// net/ipv4/tcp_input.c
struct request_sock *inet_reqsk_alloc(const struct request_sock_ops *ops,
struct sock *sk_listener,
bool attach_listener)
{
struct request_sock *req = reqsk_alloc(ops, sk_listener,
attach_listener);
...
return req;
}
// include/net/request_sock.h
static inline struct request_sock *
reqsk_alloc(const struct request_sock_ops *ops, struct sock *sk_listener,
bool attach_listener)
{
struct request_sock *req;
req = kmem_cache_alloc(ops->slab, GFP_ATOMIC | __GFP_NOWARN);
if (!req)
return NULL;
req->rsk_listener = NULL;
if (attach_listener) { // syncookie 启用时，attach_listener 为 false
...
req->rsk_listener = sk_listener;
}
...
return req;
}
// net/ipv4/tcp_ipv4.c
// af_ops->send_synack
static int tcp_v4_send_synack(const struct sock *sk, struct dst_entry *dst,
struct flowi *fl,
struct request_sock *req,
struct tcp_fastopen_cookie *foc,
enum tcp_synack_type synack_type,
struct sk_buff *syn_skb)
{
...
int err = -1;
struct sk_buff *skb;
...
skb = tcp_make_synack(sk, dst, req, foc, synack_type, syn_skb);
if (skb) {
...
}
return err;
}
/// net/ipv4/tcp_output.c
/**
* tcp_make_synack - 分配一个 skb 并构建一个 SYNACK 包
*/
struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
struct request_sock *req,
struct tcp_fastopen_cookie *foc,
enum tcp_synack_type synack_type,
struct sk_buff *syn_skb)
{
...
struct sk_buff *skb;
...
skb = alloc_skb(MAX_TCP_HEADER, GFP_ATOMIC);
...
switch (synack_type) {
case TCP_SYNACK_NORMAL:
skb_set_owner_w(skb, req_to_sk(req));
break;
case TCP_SYNACK_COOKIE:
// synflood 攻击时，skb 对象不绑定 socket 是为了减少服务器资源占用
break;
case TCP_SYNACK_FASTOPEN:
skb_set_owner_w(skb, (struct sock *)sk);
break;
}
...
return skb;
}
|
