I was trying to find out if certain Linux machines allowed source routing or not. To do this, it should be enough to just take a peek into /proc/sys/net/ipv4/conf/*/accept_source_route. But, as it seems, the results are contradictory. Look at what I have on my workstation:

root@brabham:/proc/sys/net/ipv4/conf# for IF in * ; do echo -n "$IF: " ; cat $IF/accept_source_route ; done
all: 0
default: 1
eth0: 1
lo: 1
vboxnet0: 1

Now, if you look up how the settings in the "all" directory actually work, you'll see that a few talk about that, and those who do don't give much detail. This always rings a bell in my head, as it is a symptom that nobody actually knows exactly how it works, and those who do just don't write enough 🙂 E.g., a source writes:

The /proc/sys/net/ipv4/conf/ directory allows each system interface to be configured in different ways, including the use of default settings for unconfigured devices (in the /proc/sys/net/ipv4/conf/default/ subdirectory) and settings that override all special configurations (in the /proc/sys/net/ipv4/conf/all/ subdirectory).

Now, the last sentence does not make sense if you read it carefully, because that would make the per-interface subdirectories useless. In fact, if this was really the case, you could not, e.g., accept source routing on one interface and disable it on all the others, because the value in "all" would have priority.

So after some research I tried to ask to the most Linux informed person I know, one that I can safely mention without having to check the sources. And he told me that the "all" subdirectory was born as a shortcut to help changing the configuration on all interfaces simultaneously, but then evolved in an incoherent fashion, to the point that different settings work in different way (e.g.: one setting in all may override the per-interface setting, while others will do a boolean OR between all and the interface-specific value, while others again will do an AND). If you want to know how it goes in a specific case, you need to read the kernel code.

OK, now I have the confirmation that /proc/sys/net/ipv4/conf/*/accept_source_route is a minefield, but I did not make a step forward. In my specific case, that is the accept_source_route setting, how does it work?

Well, it turns out that it's a logical AND between "all" and the interface-specific value. Good to know.

Thanks Francesco 😉