25.07 RC - no default gateway being set if default route is set to a gateway group and the Tier 1 member interface is down

luckman212

@stephenw10 Interesting workaround you posted above, I will try it!

@marcosm To answer the question, "what exactly is the problem that needs to be solved that necessitates avoiding a route?", my answer would be:

Adding a static route to a monitor IP can (not will) cause 2 main problems:

Problem 1

Users who try to access a service (DNS, HTTP etc) hosted on that IP will always use that one specific gateway. This gateway might be:

slow
expensive
both 1 & 2
administratively down or limited (e.g. 4G with data cap)
blocked at the far side by firewall rules

People using the network, who are likely unaware of such a configuration, will not understand why certain things are slow or broken, and simply complain. These users might be business users, or worse– family (wife, kids etc).

Problem 2

As a network administrator, having such a static route in place makes troubleshooting certain things difficult. For example, using 8.8.8.8 as a monitor IP means that you can't perform DNS lookups to Google DNS without adding more layers of complexity to your setup such as static LAN IPs and firewall rules to redirect DNS queries (as mentioned in the clever solution by Denny above).

One simple example:

WAN2 is a backup connection (LTE, metered) with monitor IP 8.8.8.8
A user joins the network and, being a savvy user, has their DNS server hard-coded to 8.8.8.8
Savvy user makes a lot of DNS requests
All of that traffic egresses WAN2
Company receives a $100 mobile data bill for exceeding their data cap for the month

This might seem to be an extreme example but, it has happened to me.

stephenw10

Mmm, a good solution here would be some anycast ping targets that aren't DNS servers. But using DNS servers there is really convenient!

dennypage

Effectively, @luckman212’s request is for a static route that only applies to IGMP echos originating from the firewall itself.

marcosm

@dennypage FWIW that doesn't happen currently even with pf. The route-to rule is based on the interface's source address with any destination that's not in the interface's subnet. Still, a rule can be created that applies to the correct traffic.

Given the feedback, it sounds like the issue isn't that a route should not exist, but rather some route is needed to allow pf to force the traffic. That's effectively the workaround @stephenw10 showed. Any potential undesired behavior from that kind of solution needs to be considered.

dennypage

@stephenw10 said in 25.07 RC - no default gateway being set if default route is set to a gateway group and the Tier 1 member interface is down:

Mmm, a good solution here would be some anycast ping targets that aren't DNS servers. But using DNS servers there is really convenient!

Convenient yes, but from time to time, Google and others get annoyed with everyone using their DNS servers as monitor targets and put temporary blocks in place. I generally recommend people to use regional routers in their ISP instead.

luckman212

@dennypage Exactly! I had written a script called hopfinder that I mentioned farther up, which already does this successfully & automatically for the FIOS connection, where traceroute works properly. On the LTE network, no such luck so I've resorted to querying the RDAP database (which has a nice parseable JSON output) for /32 hosts in T-mobile's network, and then iterating over a handful of them to find a few with the lowest latency. "it works" but the script takes about 45 seconds from start to finish, so not something to run every day, but once a week seems about right.

I'm planning to publish the updated script soon, trying to decide if it's worth making into a full package with a GUI.

SteveITS

Would a workaround for the fees be to block from LAN to 8.8.8.8 with a policy routing rule? Or would the static route override that? (haven't looked, just brainstorming)

FWIW since it was mentioned above, pfBlocker can block DoT, which it has tucked under "DNSBL SafeSearch." Though as I've mentioned elsewhere I know that at least the Dish DVR video on demand "app" (though not the DVR software) is hardcoded to use Google DoT, I think it was.

Patch

@marcosm said in 25.07 RC - no default gateway being set if default route is set to a gateway group and the Tier 1 member interface is down:

Let's keep in focus the following: what exactly is the problem that needs to be solved

Probably my understanding of how the system works but from my perspective it’s the required choices for monitoring

the monitoring address must be very reliable as a failure at that site will result in a failure of the monitored interface.so probably needs to be a site with extensive redundancy and probably with more than one option as even major sites like Microsoft go down and assuming otherwise results on extensive secondary failure. Using ISP routing hardware can also be problematic as when this hardware is heavily loaded pings maybe ignored which would result in secondary failure of the monitored interface.
The current approach requires a static route resulting in all traffic to the monitoring site to always use that specific interface. For a major site doing so over rides system interface loading objectives. Which suggests a monitoring site should be almost never used by real users traffic so some back water site.
The above requirements result in diametrically opposite choices. This raises the question of why this choice is actually required. Normal load balancing requires dynamically allocating traffic to one of several interfaces depending on current loading. Surely interface selection for interface monitoring should operate at this level.
imo ideally users would specify a pool of monitoring addresses. For general internet interfaces the monitoring pool for all interfaces would probably be the same however when monitoring a particular interface, that particular interface would be used.

luckman212

@Patch Nice summary, it conjures up memories of this 8-year-old idea (and bounty) of mine: dpinger multiple targets - aka gwmond

dennypage

@Patch said in 25.07 RC - no default gateway being set if default route is set to a gateway group and the Tier 1 member interface is down:

ideally users would specify a pool of monitoring addresses.

The idea of monitoring multiple addresses has been discussed at length previously.