this post was submitted on 16 Aug 2023
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No route to host until I ping and then it works fine? (lemmy.nz)
submitted 1 year ago* (last edited 1 year ago) by luthis@lemmy.nz to c/linux@lemmy.ml
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Probably should find a linux networking specific community for this one..

I have a strange issue that feels very familiar, like I've fixed it before, but I can't remember how.

I try to rtsp to security cam:

ffplay rtsp://user:password@192.168.19.137:554/h264Preview_01_main

And I get a no route:

Connection to tcp://192.168.19.137:554?timeout=0 failed: No route to host

rtsp://user:password@192.168.19.137:554/h264Preview_01_main: No route to host

Strange, I'm in the same subnet 192.168.19.129/24, and it worked a few days ago.

Check ping:

ping 192.168.19.137

PING 192.168.19.137 (192.168.19.137) 56(84) bytes of data.

64 bytes from 192.168.19.137: icmp_seq=1 ttl=64 time=5.69 ms

Of course.. So I run the command again;

ffplay rtsp://user:password@192.168.19.137:554/h264Preview_01_main

And now it works.

I could bandaid by crontabbing a ping every hour or something, but I would really like to know why I'm getting a 'no route' until I ping.

My routing table is pretty basic:

default via 192.168.19.1 dev enp4s0 proto dhcp src 192.168.19.129 metric 100

default via 192.168.19.1 dev enp4s0 proto dhcp src 192.168.19.129 metric 1002

172.17.0.0/16 dev docker0 proto kernel scope link src 172.17.0.1 linkdown

172.18.0.0/16 dev br-68c1e0344e27 proto kernel scope link src 172.18.0.1 linkdown

192.168.19.0/24 dev enp4s0 proto dhcp scope link src 192.168.19.129 metric 1002

192.168.19.1 dev enp4s0 proto dhcp scope link src 192.168.19.129 metric 1024

And I don't think I have any rules in firewall for LAN.

Any ideas?

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[–] Snowplow8861@lemmus.org 3 points 1 year ago

I can guess at some things but let me first start with what I think is happening:

You have a gateway set. Your device sends a broadcast arp message asking 'who has ip ' and the device with that ip is supposed to send back 'me with this mac address!'.

That device is either sending it so slowly that your machine says 'I can't go past the gateway, the gateway isn't responding' which in your error message is no route to host.

Assuming that you have no custom manual network route in play.

So things that can cause that are usually link layer and layer two issues and sometimes duplicate IPs. Two devices with the gateway ip.

You should watch your mac address table and arp table (arp a) and watch if the router gateway disappears or changes Mac addresses.