When configured with --server=<address>@<interface> or similar (e.g. through dbus), dnsmasq configures a fixed UDP port for all outgoing queries to the specified upstream DNS server. If an attacker is able to discover the opened port through other means (e.g. port scanning, guessing or sniffing), he would just have to guess the random transmission ID to forge a reply and get it accepted by dnsmasq. This flaw could be abused to perform a DNS Cache Poisoning attack.
Name: Petr Mensik (Red Hat)
When dns=dnsmasq is set in NetworkManager.conf, NetworkManager configures dnsmasq with the equivalent of --server=<address>@<interface>, enabling the preconditions to trigger this issue.
To exploit this flaw, an attacker has to be able to:
1) initiate a DNS query to the dnsmasq server
2) forge a reply with the upstream DNS server as source IP
3) make the forged reply arrive before the real one
4) know, through other means, the fixed outgoing port used by dnsmasq
5) guess the transmission ID, composed of 16bits
In a not-vulnerable configuration, an attacker would have to guess the right combination of (outgoing-port;transmission ID). Due to this flaw however, he just needs to find out the outgoing port through other means, like sniffing, scanning, etc. Once the outgoing port is discovered, it can be used to guess the transmission ID in a quicker way, due to the port being reused for all queries going through the same interface.
The flaw can be prevented by removing `--server=<address>@<interface>` option or by removing the directive `server=<address>@<interface>`. If dnsmasq is being run through NetworkManager, please be aware that NetworkManager automatically configures dnsmasq to use the `server=<address>@<interface>` directive, thus in this case the only way to prevent the flaw is to remove `dns=dnsmasq` from /etc/NetworkManager/NetworkManager.conf file.
If the `server=<address>@<interface>` must be kept active, the impact of this flaw can be reduced by disabling the dnsmasq cache by adding `--cache-size=0` when calling dnsmasq or by adding a line with `cache-size=0` to the dnsmasq configuration file (/etc/dnsmasq.conf by default). If dnsmasq is being run through NetworkManager, create a new file in /etc/NetworkManager/dnsmasq.d/ and add `cache-size=0` to it.
By disabling the cache, you may experience a performance loss in your environment due to all DNS queries being forwarded to the upstream servers. Please evaluate if the mitigation is appropriate for the system’s environment before applying.
Created dnsmasq tracking bugs for this issue:
Affects: fedora-all [bug 1939965]
dnsmasq 2.85, which fixes this issue, announced: https://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2021q2/014962.html