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| -rw-r--r-- | doc/payload-expression.txt | 16 |
1 files changed, 16 insertions, 0 deletions
diff --git a/doc/payload-expression.txt b/doc/payload-expression.txt index 06538832..505cc0aa 100644 --- a/doc/payload-expression.txt +++ b/doc/payload-expression.txt @@ -134,6 +134,14 @@ Destination address | ipv4_addr |====================== +Careful with matching on *ip length*: If GRO/GSO is enabled, then the Linux +kernel might aggregate several packets into one big packet that is larger than +MTU. Moreover, if GRO/GSO maximum size is larger than 65535 (see man ip-link(8), +specifically gro_ipv6_max_size and gso_ipv6_max_size), then *ip length* might +be 0 for such jumbo packets. *meta length* allows you to match on the packet +length including the IP header size. If you want to perform heuristics on the +*ip length* field, then disable GRO/GSO. + ICMP HEADER EXPRESSION ~~~~~~~~~~~~~~~~~~~~~~ [verse] @@ -244,6 +252,14 @@ Destination address | ipv6_addr |======================= +Careful with matching on *ip6 length*: If GRO/GSO is enabled, then the Linux +kernel might aggregate several packets into one big packet that is larger than +MTU. Moreover, if GRO/GSO maximum size is larger than 65535 (see man ip-link(8), +specifically gro_ipv6_max_size and gso_ipv6_max_size), then *ip6 length* might +be 0 for such jumbo packets. *meta length* allows you to match on the packet +length including the IP header size. If you want to perform heuristics on the +*ip6 length* field, then disable GRO/GSO. + .Using ip6 header expressions ----------------------------- # matching if first extension header indicates a fragment |