netstack: parse incoming packet headers up-front

Netstack has traditionally parsed headers on-demand as a packet moves up the
stack. This is conceptually simple and convenient, but incompatible with
iptables, where headers can be inspected and mangled before even a routing
decision is made.

This changes header parsing to happen early in the incoming packet path, as soon
as the NIC gets the packet from a link endpoint. Even if an invalid packet is
found (e.g. a TCP header of insufficient length), the packet is passed up the
stack for proper stats bookkeeping.

PiperOrigin-RevId: 315179302

2 files changed, 12 insertions, 0 deletions

diff --git a/pkg/tcpip/header/ipv4.go b/pkg/tcpip/header/ipv4.go
index 76839eb92..62ac932bb 100644
--- a/pkg/tcpip/header/ipv4.go
+++ b/pkg/tcpip/header/ipv4.go
@@ -159,6 +159,11 @@ func (b IPv4) Flags() uint8 {
 	return uint8(binary.BigEndian.Uint16(b[flagsFO:]) >> 13)
 }
 
+// More returns whether the more fragments flag is set.
+func (b IPv4) More() bool {
+	return b.Flags()&IPv4FlagMoreFragments != 0
+}
+
 // TTL returns the "TTL" field of the ipv4 header.
 func (b IPv4) TTL() uint8 {
 	return b[ttl]
diff --git a/pkg/tcpip/header/ipv6_extension_headers.go b/pkg/tcpip/header/ipv6_extension_headers.go
index 2c4591409..3499d8399 100644
--- a/pkg/tcpip/header/ipv6_extension_headers.go
+++ b/pkg/tcpip/header/ipv6_extension_headers.go
@@ -354,6 +354,13 @@ func (b IPv6FragmentExtHdr) ID() uint32 {
 	return binary.BigEndian.Uint32(b[ipv6FragmentExtHdrIdentificationOffset:])
 }
 
+// IsAtomic returns whether the fragment header indicates an atomic fragment. An
+// atomic fragment is a fragment that contains all the data required to
+// reassemble a full packet.
+func (b IPv6FragmentExtHdr) IsAtomic() bool {
+	return !b.More() && b.FragmentOffset() == 0
+}
+
 // IPv6PayloadIterator is an iterator over the contents of an IPv6 payload.
 //
 // The IPv6 payload may contain IPv6 extension headers before any upper layer