For historical reasons, we were always reluctant to reverse path filtering, since configuration changes were tricky to evaluate for a larger userbase, IPFire permits a number of complex scenarios, and due to limited resources.
As a compromise, this patch suggests to enable Loose Reverse Path Filtering, as specified in RFC 3704 (section 2.4), to gain at least some security achievement on this end.
To quote from that:
Loose Reverse Path Forwarding (Loose RPF) is algorithmically similar to strict RPF, but differs in that it checks only for the existence of a route (even a default route, if applicable), not where the route points to. Practically, this could be considered as a "route presence check" ("loose RPF is a misnomer in a sense because there is no "reverse path" check in the first place).
The questionable benefit of Loose RPF is found in asymmetric routing situations: a packet is dropped if there is no route at all, such as to "Martian addresses" or addresses that are not currently routed, but is not dropped if a route exists.
There is no legitimate reason why we cannot enable this: If IPFire receives a packet on some interface it cannot route on _any_ interface at all, there is no sense in processing it.
While testing this change, I was unable to produce a situation where it actually causes any harm. In theory, it shouldn't do so anyways.
In the future, we will hopefully be able to set these sysctl's to "1", using Strict Reverse Path Filtering, as specified in RFC 3704 (section 2.2). Doing so was found to work fine in my testing environment as well, but there is no asymmetric routing in place there.
Signed-off-by: Peter Müller peter.mueller@ipfire.org --- config/etc/sysctl.conf | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/config/etc/sysctl.conf b/config/etc/sysctl.conf index bc2d21c93..c8c775d13 100644 --- a/config/etc/sysctl.conf +++ b/config/etc/sysctl.conf @@ -12,13 +12,13 @@ net.ipv4.tcp_syn_retries = 3 net.ipv4.tcp_synack_retries = 3
net.ipv4.conf.default.arp_filter = 1 -net.ipv4.conf.default.rp_filter = 0 +net.ipv4.conf.default.rp_filter = 2 net.ipv4.conf.default.accept_redirects = 0 net.ipv4.conf.default.accept_source_route = 0 net.ipv4.conf.default.log_martians = 1
net.ipv4.conf.all.arp_filter = 1 -net.ipv4.conf.all.rp_filter = 0 +net.ipv4.conf.all.rp_filter = 2 net.ipv4.conf.all.accept_redirects = 0 net.ipv4.conf.all.accept_source_route = 0 net.ipv4.conf.all.log_martians = 1