From 9a550198d4243228b1258bf865044951eee27af3 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Thu, 1 Oct 2026 15:56:33 +0000 Subject: [PATCH] ipv4: use zero IPID for atomic datagrams on connected sockets ip_select_ident_segs() uses the per-socket private generator for connected sockets, even for packets with IP_DF set. This was historically done to work around buggy Windows95/2000 VJ header compression implementations, dropping every other packet in a TCP stream when the IP ID field did not change. RFC 6864 section 4.2 states that "Originating sources MAY set the IPv4 ID field of atomic datagrams to any value". Packets with IP_DF set and skb->ignore_df cleared can not be fragmented, neither locally (ip_fragment() refuses to do so) nor by routers on the path. Set their IPID to zero, like we already do for unconnected sockets and in ip_build_and_send_pkt(). FreeBSD also does the same by default (net.inet.ip.rfc6864 = 1). For GSO packets, segments get IP IDs 0, 1, ..., n-1 from GSO/TSO. This does not hurt GRO on receivers, because TCP already forces a PSH flag on every TSO packet since commit 051ba67447de ("tcp: force a PSH flag on TSO packets"), so GRO already flushes at TSO packet boundaries. Connected sockets still use their private generator for packets without IP_DF, or with skb->ignore_df set. Update the inet_id kernel-doc accordingly. This avoids an atomic operation on a shared cache line for connected UDP sockets using IP_PMTUDISC_DO/IP_PMTUDISC_PROBE, and TCP no longer touches inet->inet_id in the fast path. Minor side effects: IP IDs can no longer be used to distinguish network duplicates from TCP retransmits, or to correlate packet captures taken at different points. OS fingerprints will also change. Signed-off-by: Eric Dumazet Reviewed-by: David Ahern Reviewed-by: Kuniyuki Iwashima Link: https://patch.msgid.link/20261001155633.2562504-1-edumazet@kernel.org Signed-off-by: Jakub Kicinski --- include/net/inet_sock.h | 2 +- include/net/ip.h | 15 +++++++++------ 2 files changed, 10 insertions(+), 7 deletions(-) diff --git a/include/net/inet_sock.h b/include/net/inet_sock.h index 7cdcbed3e5cb..24cbabc24582 100644 --- a/include/net/inet_sock.h +++ b/include/net/inet_sock.h @@ -207,7 +207,7 @@ struct rtable; * @inet_saddr - Sending source * @uc_ttl - Unicast TTL * @inet_sport - Source port - * @inet_id - ID counter for DF pkts + * @inet_id - ID counter for non atomic pkts * @tos - TOS * @mc_ttl - Multicasting TTL * @uc_index - Unicast outgoing device index diff --git a/include/net/ip.h b/include/net/ip.h index 6a3e8271a73b..604018cbf0f4 100644 --- a/include/net/ip.h +++ b/include/net/ip.h @@ -584,6 +584,13 @@ static inline void ip_select_ident_segs(struct net *net, struct sk_buff *skb, { struct iphdr *iph = ip_hdr(skb); + /* RFC 6864: the IPv4 ID of atomic datagrams has no meaning. + * DF packets without ignore_df can not be fragmented. + */ + if ((iph->frag_off & htons(IP_DF)) && !skb->ignore_df) { + iph->id = 0; + return; + } /* We had many attacks based on IPID, use the private * generator as much as we can. */ @@ -603,12 +610,8 @@ static inline void ip_select_ident_segs(struct net *net, struct sk_buff *skb, iph->id = htons(val); return; } - if ((iph->frag_off & htons(IP_DF)) && !skb->ignore_df) { - iph->id = 0; - } else { - /* Unfortunately we need the big hammer to get a suitable IPID */ - __ip_select_ident(net, iph, segs); - } + /* Unfortunately we need the big hammer to get a suitable IPID */ + __ip_select_ident(net, iph, segs); } static inline void ip_select_ident(struct net *net, struct sk_buff *skb,