I’ve been having trouble with my Asus router randomly losing connection to the internet.
In researching what to do to fix it, I ran across AsusWRT Merlin firmware, which you can install on Asus routers instead of their stock firmware.
What’s the general opinion on this? My first thought is don’t trust third party firmware for your router. But I see it’s fairly popular.
You didn’t mention your specific model, but Asus is well known to actually use a version of open source router firmware, so they kind of have to also support loading 3rd party open source firmware. It’s hard to know who would have better support for the specific hardware in your router. (Presumably Asus knows best how to implement the support for the features they advertised, but, on the other hand, many of those features are a part of the WiFi chipset, and anyone who gets the specs could do it too.)
I would say there would be little harm in trying it, but make sure you understand how to recover back to officially supported firmware in case things don’t go well for you. (Certainly Asus is not going to support any firmware but their own.)
I have been using dd-wrt and openwrt for years on my systems. They are nice, upgradeable, full of features. The fact that you can upgrade them on your own is extremely important since you can get security and bug fixes and not depend on a vendor that may not be interested in supporting your gear.
They might not be always stable though. One other additional sideffect is the sometimes routers have proprietary compression chips or network management algorithms that increase your bandwidth, these are not always supported by third party firmware.
Having said that, the routers that can be shipped with official versions of openwrt, such as Asuswrt, will probably be fine.
As I understand it, Merlin is just the ASUS firmware (at least the open source stuff) with some extra stuff added. So if ASUS fixes something, eventually Merlin will get that fix. (And sometimes, ASUS will take something from Merlin)
“Safe” is kinda vague, what are your specific concerns? Reliability, security vulnerabilities, or trustworthy developers?
I used dd-wrt for years back when I had the old Linksys WRT-54 routers but I can’t speak for it on anything modern. The concept is still the same though, you’re trading the manufacturer’s guarantee for extra features.
I am using dd-wrt as we speak (or type) its fine, OpenWrt is also fine and safe. I did update a few days ago my dd-wrt from a 2016 release to the latest 11/2020 and got all the security fixes etc.
It works even better than before and my various systems have better wifi coverage.
Having said that, AsusWRT and Merlin are based on Tomato which is a different codebase originally called HyperWRT.
Looks like Asus has adopted it and is using it for its routers.
Thanks for the replies! Not having researched this before, I was wondering how common it is to run off-brand firmware. I hadn’t realized it was open source before, so that eases my mind somewhat.
If it’s no more reliable than the Asus firmware, I probably won’t install.
The specific problem I’m trying to solve: my RT-AC66U B1 is requiring reboots more frequently than I think is normal. Every few days.
Yeah, I feel your pain, I have been there as well. I have experience with dd-wrt and openwrt and I can tell you that overall they are reliable. You may get a build with bugs though, I have had an openwrt that was not working very well with the wireless module of one of my routers.
The good thing is that you can always change builds if they are unreliable and downgrade or upgrade.
Another good thing is that if one of the open source firmware works and its not a low end model, it is very likely that it will be supported by another open source firmware. I think this is fairly common.
I have not rebooted any of my routers for ages 
Hmmm… I have an Asus router that reboots itself (kernel crashes). When it was new, it was very reliable, then after some firmware updates it became very bad. It was so bad that it was rebooting itself several times a day. The most recent firmware update (last summer) seems to have made it more stable. I still get the same symptoms (null pointer in kernel) but much less often. (Maybe once in 3 months.) When I was googling the symptoms, I was finding the same problems reported in the open source firmware, so I never bothered to try it. I’ve posted here before about my woes, so if you’re interested, you can read here:
I wish the router would reboot itself!
Instead, it loses the internet, so anyone in my house still connected to it is suddenly no longer online. Misleading, since they’re still connected to the network.
So I have to get out of my chair, go to turn-it-off-turn-it-on-again and it works. At least I’m getting exercise.
Have you looked at the logs at all? It sounds like it’s got some sort of memory leak that certain traffic exercises and it eventually dies. (That is what I also think was the cause of my problem, but it was a kernel memory allocation that eventually fails, causing the kernel the null pointer issue, which causes the reboot.) My router has a feature where-in you can schedule a reboot, which should supposedly be a workaround for problems related to memory leaks… just in my case there isn’t really a great time to schedule it. (I’m a night owl and up all night and the others in the household are day people who need a reliable internet during the day to do their work from home jobs.)
Haven’t looked at the logs – I might not know how to read them, but I’ll give it a try.
Router does have a scheduled reboot function. I played with it once, but will try scheduling every day to see if it helps.