Re: [PATCH net] ipvs: fix MTU check for GSO packets in tunnel mode

[Julian Anastasov]

	Hello,

On Wed, 1 Apr 2026, Yingnan Zhang wrote:

> Currently, IPVS skips MTU checks for GSO packets by excluding them with
> the !skb_is_gso(skb) condition. This creates problems when IPVS tunnel
> mode encapsulates GSO packets with IPIP headers.
> 
> The issue manifests in two ways:
> 
> 1. MTU violation after encapsulation:
>    When a GSO packet passes through IPVS tunnel mode, the original MTU
>    check is bypassed. After adding the IPIP tunnel header, the packet
>    size may exceed the outgoing interface MTU, leading to unexpected
>    fragmentation at the IP layer.
> 
> 2. Fragmentation with problematic IP IDs:
>    When net.ipv4.vs.pmtu_disc=1 and a GSO packet with multiple segments
>    is fragmented after encapsulation, each segment gets a sequentially
>    incremented IP ID (0, 1, 2, ...). This happens because:
> 
>    a) The GSO packet bypasses MTU check and gets encapsulated
>    b) At __ip_finish_output, the oversized GSO packet is split into
>       separate SKBs (one per segment), with IP IDs incrementing
>    c) Each SKB is then fragmented again based on the actual MTU
> 
>    This sequential IP ID allocation differs from the expected behavior
>    and can cause issues with fragment reassembly and packet tracking.
> 
> Fix this by removing the GSO packet exception from the MTU check and
> properly validating GSO packets using skb_gso_validate_network_len().
> This function correctly validates whether the GSO segments will fit
> within the MTU after segmentation. If validation fails, send an ICMP
> Fragmentation Needed message to enable proper PMTU discovery.
> 
> Fixes: 4cdd34084d53 ("netfilter: nf_conntrack_ipv6: improve fragmentation handling")
> Signed-off-by: Yingnan Zhang <342144303@xxxxxx>
> ---
>  net/netfilter/ipvs/ip_vs_xmit.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/net/netfilter/ipvs/ip_vs_xmit.c b/net/netfilter/ipvs/ip_vs_xmit.c
> index 3601eb86d..82f2e7a32 100644
> --- a/net/netfilter/ipvs/ip_vs_xmit.c
> +++ b/net/netfilter/ipvs/ip_vs_xmit.c
> @@ -232,8 +232,15 @@ static inline bool ensure_mtu_is_adequate(struct netns_ipvs *ipvs, int skb_af,
>  			return true;
>  
>  		if (unlikely(ip_hdr(skb)->frag_off & htons(IP_DF) &&
> -			     skb->len > mtu && !skb_is_gso(skb) &&
> +			     skb->len > mtu &&
>  			     !ip_vs_iph_icmp(ipvsh))) {
> +			if (skb_is_gso(skb)) {
> +				if (skb_gso_validate_network_len(skb, mtu))
> +					return true;

	Should we add the same function call in
__mtu_check_toobig_v6() for IPv6 ? Comparing it with
net/ipv6/ip6_output.c:ip6_pkt_too_big()...

> +				icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED, htonl(mtu));
> +				IP_VS_DBG(1, "frag needed for %pI4\n", &ip_hdr(skb)->saddr);
> +				return false;
> +			}
>  			icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
>  				  htonl(mtu));
>  			IP_VS_DBG(1, "frag needed for %pI4\n",
> -- 
> 2.51.0

Regards

--
Julian Anastasov <ja@xxxxxx>