Hello,
On Thu, 2 Jul 2026, Yizhou Zhao wrote:
> When an ICMP Fragmentation Needed error is received for a tunneled IPVS
> connection, ip_vs_in_icmp() recomputes the MTU that the original packet
> can use by subtracting the tunnel overhead from the reported next-hop
> MTU.
>
> The current code always subtracts sizeof(struct iphdr), which is only
> the IPIP overhead. For GUE and GRE tunnels, ipvs_udp_decap() and
> ipvs_gre_decap() already compute the additional tunnel header length,
> but that value is scoped to the decapsulation block and is lost before
> the ICMP_FRAG_NEEDED handling. As a result, the ICMP error sent back to
> the client advertises an MTU that is too large, so PMTUD can fail to
> converge for GUE/GRE-tunneled real servers.
>
> With a reported next-hop MTU of 1400, a GUE tunnel currently returns
> 1380 to the client. The correct value is 1368:
>
> 1400 - sizeof(struct iphdr) - sizeof(struct udphdr) -
> sizeof(struct guehdr)
>
> Hoist the tunnel header length into the main ip_vs_in_icmp() scope and
> subtract sizeof(struct iphdr) + ulen in the Fragmentation Needed path.
> The IPIP path keeps ulen as 0, so its existing 1400 - 20 = 1380 result
> is unchanged.
>
> Fixes: 508f744c0de3 ("ipvs: strip udp tunnel headers from icmp errors")
> Cc: [email protected]
> Reported-by: Yizhou Zhao <[email protected]>
> Reported-by: Yuxiang Yang <[email protected]>
> Reported-by: Ao Wang <[email protected]>
> Reported-by: Xuewei Feng <[email protected]>
> Reported-by: Qi Li <[email protected]>
> Reported-by: Ke Xu <[email protected]>
> Assisted-by: Claude-Code:GLM-5.2
> Signed-off-by: Yizhou Zhao <[email protected]>
Looks good to me for the nf tree, thanks!
Acked-by: Julian Anastasov <[email protected]>
> ---
> Changes in v2:
> - Use the short first hunk context so patch applies without fuzz.
> - Adjust Assisted-by to checkpatch's agent-name format.
> - Suggested by Julian.
> - Link to v1:
> https://lore.kernel.org/netdev/[email protected]/
> ---
> net/netfilter/ipvs/ip_vs_core.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/net/netfilter/ipvs/ip_vs_core.c b/net/netfilter/ipvs/ip_vs_core.c
> index d40b404c1bf6..906f2c361676 100644
> --- a/net/netfilter/ipvs/ip_vs_core.c
> +++ b/net/netfilter/ipvs/ip_vs_core.c
> @@ -1767,6 +1767,7 @@ ip_vs_in_icmp(struct netns_ipvs *ipvs, struct sk_buff
> *skb, int *related,
> bool tunnel, new_cp = false;
> union nf_inet_addr *raddr;
> char *outer_proto = "IPIP";
> + int ulen = 0;
>
> *related = 1;
>
> @@ -1831,7 +1832,6 @@ ip_vs_in_icmp(struct netns_ipvs *ipvs, struct sk_buff
> *skb, int *related,
> /* Error for our tunnel must arrive at LOCAL_IN */
> (skb_rtable(skb)->rt_flags & RTCF_LOCAL)) {
> __u8 iproto;
> - int ulen;
>
> /* Non-first fragment has no UDP/GRE header */
> if (unlikely(cih->frag_off & htons(IP_OFFSET)))
> @@ -1936,8 +1936,8 @@ ip_vs_in_icmp(struct netns_ipvs *ipvs, struct sk_buff
> *skb, int *related,
> if (dest_dst)
> mtu = dst_mtu(dest_dst->dst_cache);
> }
> - if (mtu > 68 + sizeof(struct iphdr))
> - mtu -= sizeof(struct iphdr);
> + if (mtu > 68 + sizeof(struct iphdr) + ulen)
> + mtu -= sizeof(struct iphdr) + ulen;
> info = htonl(mtu);
> }
> /* Strip outer IP, ICMP and IPIP/UDP/GRE, go to IP header of
Regards
--
Julian Anastasov <[email protected]>
