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Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).

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NTP Packet blocked... nftables [solved] (programming.dev)

submitted 2 days ago* (last edited 2 days ago) by Corsair@programming.dev to c/linux@lemmy.ml

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Hi,

I want to use openntpd to sync my clock

I'm using

ntpd -ds

I see in my firewall that the dns resolution is working, and I get a server IP from the pool 👍

but anyhow I get

ntp engine ready no reply received in time, skipping initial time setting
no reply from x.x.x.x received in time, next query 300s

Weird my nftable config file should allow it:

# extract
chain OUT {
type filter hook output priority 0; policy drop;
udp dport 123 accept
}

chain IN {
type filter hook input priority 0; policy drop;
ct state established, related accept
}

Any ideas, or which lemmy community to cross-post ?

Thanks.

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[–] InnerScientist@lemmy.world 1 points 2 days ago* (last edited 2 days ago)

Update your nftables rulefile or use nft commands to update your firewall to the following:

# extract
chain OUT {
    type filter hook output priority 0; policy drop;
    udp dport 123 accept

    limit rate 3/second log prefix "Nftables Blocked: OUT: "
}

chain IN {
    type filter hook input priority 0; policy drop;
    ct state established, related accept

    limit rate 3/second log prefix "Nftables Blocked: IN: "
}

Blocked pakets will show up in the kernel log (dmesg/journalcl)

If you want more information on why it is blocked then enable nftrace for those packets

nft add rule inet/ip/ip6 tablename OUT udp dport 123 meta nftrace set 1
nft add rule inet/ip/ip6 tablename IN udp dport 123 meta nftrace set 1
nft monitor trace

nft add rule inet/ip/ip6 tablename OUT meta nftrace set 1

Or maybe even

nft add rule inet/ip/ip6 tablename PREROUTING udp dport 123 meta nftrace set 1

Additionally you can use tcpdump -i to show network packets before they enter the firewall, there you should be able to tell what it's a trying to do.