[pkg-dhcp-devel] Bug#668626: [isc-dhcp-client] rfc3442 classless routes: breaks routing if gateway is a local IP

[Lars Kruse]

Package: isc-dhcp-client
Version: 4.1.1-P1-9
Severity: normal
Tags: patch

--- Please enter the report below this line. ---

Hi,

the current implementation of the "rfc3442-classless-routes" exit hook breaks
the routing in my local setup. The attached patch fixes this issue for me.
Here is my setup ...

Consider four hosts:

A/B/C:
  three routers in front of different sub-nets (netA, netB, netC); their WAN
  interfaces are configured via DHCP (provided by G)
G:
  central router - delivering DHCP to A, B and C

I want to announce A, B and C as routers for their respective sub-nets via DHCP
to the other routers. This reduces problems with ICMP redirect and simplifies
routing in general (for me).


Here is the problem:

A adds the following routes via the rfc3442 exit hook:
 netA via A
 netB via B
 netC via C
the route to itself ("netA via A") competes with A's interface based route
(something along "netA dev eth0"). The result: the hosts in netA can't talk to A
anymore, since A uses the new "netA via A" route (looping -> packets are lost)
The attached patch skips the routes that point to the current router itself.


Just in case you need some real-life numbers: the following example is taken
from a host with the local IPs 172.16.19.1 ("lan") and 172.16.23.13 ("wan").

root at router-lardia:~# route -n
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         172.16.23.1     0.0.0.0         UG    0      0        0 wan
172.16.16.0     172.16.23.1     255.255.240.0   UG    0      0        0 wan
172.16.17.0     172.16.23.11    255.255.255.0   UG    0      0        0 wan
172.16.18.0     172.16.23.12    255.255.255.0   UG    0      0        0 wan
172.16.19.0     172.16.23.13    255.255.255.0   UG    0      0        0 wan
172.16.19.0     0.0.0.0         255.255.255.0   U     0      0        0 lan

The second to last line (gateway 172.16.23.13) is the crucial one: it prevents
packets to the local "lan" network (172.16.19.0/24) from being delivered via
"lan".
Essentially the router does not communicate via the "lan" interface at all.

The respective rfc3442 dhcp option (distributed by the central gateway) is the
following:
 dhcp-option=121,172.16.16.0/20,172.16.23.1, \
    172.16.17.0/24,172.16.23.11, \
    172.16.18.0/24,172.16.23.12, \
    172.16.19.0/24,172.16.23.13


The attached patch compares the rfc3442 "gateway" with all local IP addresses
(retrieved via "ip addr show").
Basically it says: "never add a route that points to yourself".

What do you think about this change? Is this a relevant use-case?

cheers,
Lars
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