mjhjmtu

I just discovered accidentally that cat /proc/sys/net/ipv4/ip_forward returns 1. I can't remember having enabled net.ipv4.ip_forward anytime ago, but since my system runs for several years now, I can't exclude that either.

I don't expect my 4.14.3-1-ARCH having net.ipv4.ip_forward = 1 by default, so in what place could this be enabled? I know about /etc/sysctl.d, but none of the files in there specifies IP forwarding. It is persistent across reboots, though, so where could it have been enabled? Thanks!

Edit: My Interfaces are:

$ ip link
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
 link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: wlp4s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DORMANT group default qlen 1000
 link/ether a4:34:d9:a2:33:7f brd ff:ff:ff:ff:ff:ff
3: enp0s31f6: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc fq_codel state DOWN mode DEFAULT group default qlen 1000
 link/ether 54:ee:75:8e:ef:37 brd ff:ff:ff:ff:ff:ff
4: pan1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UNKNOWN mode DEFAULT group default qlen 1000
 link/ether 2e:02:14:5b:d8:21 brd ff:ff:ff:ff:ff:ff

Edit 2: Contents of my filter-iptable:

# iptables -t filter -L
Chain INPUT (policy ACCEPT)
target prot opt source destination 

Chain FORWARD (policy ACCEPT)
target prot opt source destination 
ACCEPT all -- anywhere anywhere 
ACCEPT all -- anywhere anywhere 
ACCEPT all -- anywhere anywhere 

Chain OUTPUT (policy ACCEPT)
target prot opt source destination

Edit 3: I stumbled upon a connection called pan1 in NetworkManager, which is always connected and probably corresponds to the network interface pan1. Interestingly, it is in bridge-mode. Unfortunately, I can't imagine having created this connection by hand, and I can't find where it comes from (grep -rn "pan1" /etc/NetworkManager/system-connections/ gives no output). My first suspect, removing all paired bluetooth devices, brought no avail. Here's the connection's contents:

$ nmcli connection show pan1
connection.id: pan1
connection.uuid: fc8c690c-3874-48df-8ccb-98a0fc698604
connection.stable-id: --
connection.type: bridge
connection.interface-name: pan1
connection.autoconnect: no
connection.autoconnect-priority: 0
connection.autoconnect-retries: -1 (default)
connection.auth-retries: -1
connection.timestamp: 1512614672
connection.read-only: no
connection.permissions: --
connection.zone: --
connection.master: --
connection.slave-type: --
connection.autoconnect-slaves: -1 (default)
connection.secondaries: --
connection.gateway-ping-timeout: 0
connection.metered: unknown
connection.lldp: default
ipv4.method: manual
ipv4.dns: --
ipv4.dns-search: --
ipv4.dns-options: ""
ipv4.dns-priority: 100
ipv4.addresses: 10.228.230.1/24
ipv4.gateway: --
ipv4.routes: --
ipv4.route-metric: -1
ipv4.route-table: 0 (unspec)
ipv4.ignore-auto-routes: no
ipv4.ignore-auto-dns: no
ipv4.dhcp-client-id: --
ipv4.dhcp-timeout: 0 (default)
ipv4.dhcp-send-hostname: yes
ipv4.dhcp-hostname: --
ipv4.dhcp-fqdn: --
ipv4.never-default: no
ipv4.may-fail: yes
ipv4.dad-timeout: -1 (default)
ipv6.method: ignore
ipv6.dns: --
ipv6.dns-search: --
ipv6.dns-options: ""
ipv6.dns-priority: 100
ipv6.addresses: --
ipv6.gateway: --
ipv6.routes: --
ipv6.route-metric: -1
ipv6.route-table: 0 (unspec)
ipv6.ignore-auto-routes: no
ipv6.ignore-auto-dns: no
ipv6.never-default: no
ipv6.may-fail: yes
ipv6.ip6-privacy: -1 (unknown)
ipv6.addr-gen-mode: stable-privacy
ipv6.dhcp-send-hostname: yes
ipv6.dhcp-hostname: --
ipv6.token: --
bridge.mac-address: --
bridge.stp: no
bridge.priority: 32768
bridge.forward-delay: 0
bridge.hello-time: 1
bridge.max-age: 19
bridge.ageing-time: 299
bridge.group-forward-mask: 0
bridge.multicast-snooping: yes
proxy.method: none
proxy.browser-only: no
proxy.pac-url: --
proxy.pac-script: --
GENERAL.NAME: pan1
GENERAL.UUID: fc8c690c-3874-48df-8ccb-98a0fc698604
GENERAL.DEVICES: pan1
GENERAL.STATE: activated
GENERAL.DEFAULT: no
GENERAL.DEFAULT6: no
GENERAL.SPEC-OBJECT: --
GENERAL.VPN: no
GENERAL.DBUS-PATH: /org/freedesktop/NetworkManager/ActiveConnection/2
GENERAL.CON-PATH: /org/freedesktop/NetworkManager/Settings/62
GENERAL.ZONE: --
GENERAL.MASTER-PATH: --
IP4.ADDRESS[1]: 10.228.230.1/24
IP4.GATEWAY: --
IP4.ROUTE[1]: dst = 10.228.230.0/24, nh = 0.0.0.0, mt = 0
IP6.GATEWAY: --