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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 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:
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Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. 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