|Set outer TOS bit to 0.|
|Egress||Drop outer TOS bit.|
With IFF_LINK1, gif will copy ECN bits ( 0x02 and 0x01 on IPv4 TOS byte or IPv6 traffic class byte) on egress and ingress, as follows:
|Copy TOS bits except for ECN CE (masked with 0xfe) from inner to outer. Set ECN CE bit to 0.|
|Egress||Use inner TOS bits with some change. If outer ECN CE bit is 1, enable ECN CE bit on the inner.|
Note that the ECN friendly behavior violates RFC2893. This should be used in mutual agreement with the peer.
A malicious party may try to circumvent security filters by using tunnelled packets. For better protection, gif performs both martian and ingress filtering against the outer source address on egress. Note that martian/ingress filters are in no way complete. You may want to secure your node by using packet filters. Ingress filtering can break tunnel operation in an asymmetrically routed network. It can be turned off by IFF_LINK2 bit.
By default, gif tunnels may not be nested. This behavior may be modified at runtime by setting the sysctl(8) variable net.link.gif.max_nesting to the desired level of nesting. Additionally, gif tunnels are restricted to one per pair of end points. Parallel tunnels may be enabled by setting the sysctl(8) variable net.link.gif.parallel_tunnels to 1.
gre(4), inet(4), inet6(4), ifconfig(8)
.Rs Transition Mechanisms for IPv6 Hosts and Routers
.Rs IPsec Interactions with ECN
The gif device first appeared in the WIDE hydrangea IPv6 kit.
There are many tunnelling protocol specifications, all defined differently from each other. The gif device may not interoperate with peers which are based on different specifications, and are picky about outer header fields. For example, you cannot usually use gif to talk with IPsec devices that use IPsec tunnel mode.
The current code does not check if the ingress address (outer source address) configured in the gif interface makes sense. Make sure to specify an address which belongs to your node. Otherwise, your node will not be able to receive packets from the peer, and it will generate packets with a spoofed source address.
If the outer protocol is IPv4, gif does not try to perform path MTU discovery for the encapsulated packet (DF bit is set to 0).
If the outer protocol is IPv6, path MTU discovery for encapsulated packets may affect communication over the interface. The first bigger-than-pmtu packet may be lost. To avoid the problem, you may want to set the interface MTU for gif to 1240 or smaller, when the outer header is IPv6 and the inner header is IPv4.
The gif device does not translate ICMP messages for the outer header into the inner header.
In the past, gif had a multi-destination behavior, configurable via IFF_LINK0 flag. The behavior is obsolete and is no longer supported.