[PR #2808] [client] Use the prerouting chain to mark for masquerading to support older systems #18300

New Issue

saavagebueno · 2026-08-05T04:08:21-04:00

saavagebueno commented

2026-08-05 04:08:21 -04:00

Original Pull Request: https://github.com/netbirdio/netbird/pull/2808

State: closed
Merged: Yes

Describe your changes

Replace dynamic postrouting rules with input interface with dynamic prerouting rules + static postrouting rules.
Older versions, such as Ubuntu 20.04, don't support matching on the input interface in the postrouting chain. As a result the masquerade rules didn't match at all.

Following an example with route 10.20.1.0/24

Before:

nftables:

table ip netbird {
        chain netbird-rt-postrouting {
                type nat hook postrouting priority srcnat - 1; policy accept;
                iifname "wt0" oifname != "lo" ip daddr 10.20.1.0/24 counter packets 0 bytes 0 masquerade
                oifname "wt0" iifname != "lo" ip saddr 10.20.1.0/24 counter packets 0 bytes 0 masquerade
        }
}

iptables:

-A POSTROUTING -j NETBIRD-RT-NAT
-A NETBIRD-RT-NAT -d 10.20.1.0/24 -i wt0 ! -o lo -j MASQUERADE
-A NETBIRD-RT-NAT -s 10.20.1.0/24 ! -i lo -o wt0 -j MASQUERADE

After:

nftables:

table ip netbird {
        chain netbird-rt-postrouting {
                type nat hook postrouting priority srcnat - 1; policy accept;
                meta mark 0x0001bd11 oifname != "lo" counter packets 0 bytes 0 masquerade
                meta mark 0x0001bd12 oifname "wt0" counter packets 0 bytes 0 masquerade
        }

        chain netbird-rt-prerouting {
                type filter hook prerouting priority mangle; policy accept;
                ct state new iifname "wt0" ip daddr 10.20.1.0/24 meta mark set 0x0001bd11
                ct state new iifname != "wt0" ip saddr 10.20.1.0/24 meta mark set 0x0001bd12
        }
}

iptables:


*mangle
-A PREROUTING -j NETBIRD-RT-PRE
-A NETBIRD-RT-PRE -d 10.20.1.0/24 -i wt0 -m conntrack --ctstate NEW -j MARK --set-xmark 0x1bd11/0xffffffff
-A NETBIRD-RT-PRE -s 10.20.1.0/24 ! -i wt0 -m conntrack --ctstate NEW -j MARK --set-xmark 0x1bd12/0xffffffff

*nat
-A POSTROUTING -j NETBIRD-RT-NAT
-A NETBIRD-RT-NAT ! -o lo -m mark --mark 0x1bd11 -j MASQUERADE
-A NETBIRD-RT-NAT -o wt0 -m mark --mark 0x1bd12 -j MASQUERADE

Issue ticket number and link

#2752

Checklist

Is it a bug fix
Is a typo/documentation fix
Is a feature enhancement
It is a refactor
Created tests that fail without the change (if possible)
Extended the README / documentation, if necessary

**Original Pull Request:** https://github.com/netbirdio/netbird/pull/2808 **State:** closed **Merged:** Yes --- ## Describe your changes Replace dynamic postrouting rules with input interface with dynamic prerouting rules + static postrouting rules. Older versions, such as Ubuntu 20.04, don't support matching on the input interface in the postrouting chain. As a result the masquerade rules didn't match at all. Following an example with route `10.20.1.0/24` ### Before: #### nftables: ``` table ip netbird { chain netbird-rt-postrouting { type nat hook postrouting priority srcnat - 1; policy accept; iifname "wt0" oifname != "lo" ip daddr 10.20.1.0/24 counter packets 0 bytes 0 masquerade oifname "wt0" iifname != "lo" ip saddr 10.20.1.0/24 counter packets 0 bytes 0 masquerade } } ``` #### iptables: ``` -A POSTROUTING -j NETBIRD-RT-NAT -A NETBIRD-RT-NAT -d 10.20.1.0/24 -i wt0 ! -o lo -j MASQUERADE -A NETBIRD-RT-NAT -s 10.20.1.0/24 ! -i lo -o wt0 -j MASQUERADE ``` ### After: #### nftables: ``` table ip netbird { chain netbird-rt-postrouting { type nat hook postrouting priority srcnat - 1; policy accept; meta mark 0x0001bd11 oifname != "lo" counter packets 0 bytes 0 masquerade meta mark 0x0001bd12 oifname "wt0" counter packets 0 bytes 0 masquerade } chain netbird-rt-prerouting { type filter hook prerouting priority mangle; policy accept; ct state new iifname "wt0" ip daddr 10.20.1.0/24 meta mark set 0x0001bd11 ct state new iifname != "wt0" ip saddr 10.20.1.0/24 meta mark set 0x0001bd12 } } ``` #### iptables: ``` *mangle -A PREROUTING -j NETBIRD-RT-PRE -A NETBIRD-RT-PRE -d 10.20.1.0/24 -i wt0 -m conntrack --ctstate NEW -j MARK --set-xmark 0x1bd11/0xffffffff -A NETBIRD-RT-PRE -s 10.20.1.0/24 ! -i wt0 -m conntrack --ctstate NEW -j MARK --set-xmark 0x1bd12/0xffffffff *nat -A POSTROUTING -j NETBIRD-RT-NAT -A NETBIRD-RT-NAT ! -o lo -m mark --mark 0x1bd11 -j MASQUERADE -A NETBIRD-RT-NAT -o wt0 -m mark --mark 0x1bd12 -j MASQUERADE ``` ## Issue ticket number and link #2752 ### Checklist - [x] Is it a bug fix - [ ] Is a typo/documentation fix - [ ] Is a feature enhancement - [ ] It is a refactor - [ ] Created tests that fail without the change (if possible) - [ ] Extended the README / documentation, if necessary

saavagebueno added the pull-request label 2026-08-05 04:08:21 -04:00

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Reference: DYNR/netbird#18300