From f5fff5dc8a7a3f395b0525c02ba92c95d42b7390 Mon Sep 17 00:00:00 2001 From: Tom Quetchenbach Date: Sun, 21 Sep 2008 00:21:51 -0700 Subject: [PATCH] tcp: advertise MSS requested by user I'm trying to use the TCP_MAXSEG option to setsockopt() to set the MSS for both sides of a bidirectional connection. man tcp says: "If this option is set before connection establishment, it also changes the MSS value announced to the other end in the initial packet." However, the kernel only uses the MTU/route cache to set the advertised MSS. That means if I set the MSS to, say, 500 before calling connect(), I will send at most 500-byte packets, but I will still receive 1500-byte packets in reply. This is a bug, either in the kernel or the documentation. This patch (applies to latest net-2.6) reduces the advertised value to that requested by the user as long as setsockopt() is called before connect() or accept(). This seems like the behavior that one would expect as well as that which is documented. I've tried to make sure that things that depend on the advertised MSS are set correctly. Signed-off-by: Tom Quetchenbach Signed-off-by: David S. Miller --- net/ipv4/tcp_ipv4.c | 4 ++++ net/ipv4/tcp_output.c | 13 ++++++++++--- 2 files changed, 14 insertions(+), 3 deletions(-) diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c index 3dfbc21..44aef1c 100644 --- a/net/ipv4/tcp_ipv4.c +++ b/net/ipv4/tcp_ipv4.c @@ -1364,6 +1364,10 @@ struct sock *tcp_v4_syn_recv_sock(struct sock *sk, struct sk_buff *skb, tcp_mtup_init(newsk); tcp_sync_mss(newsk, dst_mtu(dst)); newtp->advmss = dst_metric(dst, RTAX_ADVMSS); + if (tcp_sk(sk)->rx_opt.user_mss && + tcp_sk(sk)->rx_opt.user_mss < newtp->advmss) + newtp->advmss = tcp_sk(sk)->rx_opt.user_mss; + tcp_initialize_rcv_mss(newsk); #ifdef CONFIG_TCP_MD5SIG diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c index 8f9793a..c3d58ee 100644 --- a/net/ipv4/tcp_output.c +++ b/net/ipv4/tcp_output.c @@ -2232,6 +2232,7 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst, struct sk_buff *skb; struct tcp_md5sig_key *md5; __u8 *md5_hash_location; + int mss; skb = sock_wmalloc(sk, MAX_TCP_HEADER + 15, 1, GFP_ATOMIC); if (skb == NULL) @@ -2242,13 +2243,17 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst, skb->dst = dst_clone(dst); + mss = dst_metric(dst, RTAX_ADVMSS); + if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < mss) + mss = tp->rx_opt.user_mss; + if (req->rcv_wnd == 0) { /* ignored for retransmitted syns */ __u8 rcv_wscale; /* Set this up on the first call only */ req->window_clamp = tp->window_clamp ? : dst_metric(dst, RTAX_WINDOW); /* tcp_full_space because it is guaranteed to be the first packet */ tcp_select_initial_window(tcp_full_space(sk), - dst_metric(dst, RTAX_ADVMSS) - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0), + mss - (ireq->tstamp_ok ? TCPOLEN_TSTAMP_ALIGNED : 0), &req->rcv_wnd, &req->window_clamp, ireq->wscale_ok, @@ -2258,8 +2263,7 @@ struct sk_buff *tcp_make_synack(struct sock *sk, struct dst_entry *dst, memset(&opts, 0, sizeof(opts)); TCP_SKB_CB(skb)->when = tcp_time_stamp; - tcp_header_size = tcp_synack_options(sk, req, - dst_metric(dst, RTAX_ADVMSS), + tcp_header_size = tcp_synack_options(sk, req, mss, skb, &opts, &md5) + sizeof(struct tcphdr); @@ -2333,6 +2337,9 @@ static void tcp_connect_init(struct sock *sk) if (!tp->window_clamp) tp->window_clamp = dst_metric(dst, RTAX_WINDOW); tp->advmss = dst_metric(dst, RTAX_ADVMSS); + if (tp->rx_opt.user_mss && tp->rx_opt.user_mss < tp->advmss) + tp->advmss = tp->rx_opt.user_mss; + tcp_initialize_rcv_mss(sk); tcp_select_initial_window(tcp_full_space(sk), -- 2.7.4