问题：
服务器进程可由 LAN 上的设备或服务器本身访问，但不能由 LAN 外部访问（由于服务器确实接收数据包，因此端口转发配置正确）。
数据包跟踪显示，对于远程连接，SYN 已收到，但 SYN/ACK 从未发回，但对于 LAN 连接，SYN/ACK 会发回。
值得一提的是，还运行了一个 SSH 服务器；它可以从本地和远程连接，所以我不太明白端口 5555 进程发生了什么。
我花了大量的时间来尝试解决这个问题，如能得到任何反馈我将非常感谢！
提前感谢您看完这篇文章，我知道这是一篇很长的文字。
细节：
请注意：出于隐私原因，下面的一些值（例如 IP 地址）已被更改。
我有一个在所有接口上监听端口 5555 的进程。
# netstat -l
Proto Recv-Q Send-Q Local Address Foreign Address State
tcp 0 0 *:5555 *:* LISTEN
...snip...
我还连接了一个 OpenVPN 客户端，所有互联网流量都通过它转发。这是必需的，除调试外不能关闭。
# ip route list
0.0.0.0/1 via 10.1.1.123 dev tun0
default via 192.168.0.1 dev enp0s14 onlink
10.1.0.1 via 10.1.1.123 dev tun0
10.1.1.123 dev tun0 proto kernel scope link src 10.1.1.10
94.102.56.181 via 192.168.0.1 dev enp0s14
128.0.0.0/1 via 10.1.1.123 dev tun0
192.168.0.0/24 dev enp0s14 proto kernel scope link src 192.168.0.101
iptables配置为仅允许通过 VPN 进行互联网通信。允许 LAN 通信，并允许某些服务，例如 SSH 和我的服务器进程（端口 5555）。其他所有服务均被丢弃。
iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT DROP
...snip...
iptables -A INPUT -j ACCEPT -s 192.168.0.0/24 -i enp0s14
iptables -A OUTPUT -j ACCEPT -d 192.168.0.0/24 -o enp0s14
...snip...
iptables -A INPUT -j ACCEPT -p tcp -d 192.168.0.0/24 --dport 5555 -i enp0s14
iptables -A OUTPUT -j ACCEPT -p tcp -s 192.168.0.0/24 --sport 5555 -o enp0s14
...snip...
当我运行时，tcptrack我可以看到我的远程连接进来。
# tcptrack -d -i enp0s14
Client Server State Idle A Speed
123.123.123.123:53250 192.168.0.101:5555 SYN_SENT 32s 0 B/s
运行tcpdump显示 SYN 已收到但从未发送。
# tcpdump -i enp0s14 port 5555 -vv -n
tcpdump: listening on enp0s14, link-type EN10MB (Ethernet), capture size 262144 bytes
15:46:40.375191 IP (tos 0x0, ttl 48, id 31281, offset 0, flags [DF], proto TCP (6), length 60)
123.123.123.123.42901 > 192.168.0.101.5555: Flags [S], cksum 0x2e65 (correct), seq 808682189, win 14600, options [mss 1460,sackOK,TS val 3488166776 ecr 0,nop,wscale 7], length 0
15:46:47.235302 IP (tos 0x0, ttl 48, id 3275, offset 0, flags [DF], proto TCP (6), length 60)
123.123.123.123.41151 > 192.168.0.101.5555: Flags [S], cksum 0xa8d0 (correct), seq 2596314499, win 14600, options [mss 1460,sackOK,TS val 1660719432 ecr 0,nop,wscale 7], length 0
15:46:48.234346 IP (tos 0x0, ttl 48, id 3276, offset 0, flags [DF], proto TCP (6), length 60)
123.123.123.123.41151 > 192.168.0.101.5555: Flags [S], cksum 0xa4e8 (correct), seq 2596314499, win 14600, options [mss 1460,sackOK,TS val 1660720432 ecr 0,nop,wscale 7], length 0
...snip...
真正奇怪的是，检查iptables规则的数据包数表明，随着数据包数的增加，INPUT 和 OUTPUT 链都可以很好地接受外部连接。
# iptables -vxnL | grep 5555
99 5940 ACCEPT tcp -- enp0s14 * 0.0.0.0/0 192.168.0.0/24 tcp dpt:5555
235 11980 ACCEPT tcp -- * enp0s14 192.168.0.0/24 0.0.0.0/0 tcp spt:5555
...connection attempt...
# iptables -vxnL | grep 5555
103 6180 ACCEPT tcp -- enp0s14 * 0.0.0.0/0 192.168.0.0/24 tcp dpt:5555
239 12184 ACCEPT tcp -- * enp0s14 192.168.0.0/24 0.0.0.0/0 tcp spt:5555
以下是我的sysctl变量，希望它们有帮助：
# sysctl -a | grep "net\.\(core\|ipv4\)"
net.core.bpf_jit_enable = 0
net.core.busy_poll = 0
net.core.busy_read = 0
net.core.default_qdisc = pfifo_fast
net.core.dev_weight = 64
net.core.flow_limit_cpu_bitmap = 0
net.core.flow_limit_table_len = 4096
net.core.max_skb_frags = 17
net.core.message_burst = 10
net.core.message_cost = 5
net.core.netdev_budget = 300
net.core.netdev_max_backlog = 65536
net.core.netdev_tstamp_prequeue = 1
net.core.optmem_max = 25165824
net.core.rmem_default = 16777216
net.core.rmem_max = 16777216
net.core.rps_sock_flow_entries = 0
net.core.somaxconn = 4096
net.core.tstamp_allow_data = 1
net.core.warnings = 0
net.core.wmem_default = 16777216
net.core.wmem_max = 16777216
net.core.xfrm_acq_expires = 30
net.core.xfrm_aevent_etime = 10
net.core.xfrm_aevent_rseqth = 2
net.core.xfrm_larval_drop = 1
net.ipv4.cipso_cache_bucket_size = 10
net.ipv4.cipso_cache_enable = 1
net.ipv4.cipso_rbm_optfmt = 0
net.ipv4.cipso_rbm_strictvalid = 1
net.ipv4.conf.all.accept_local = 0
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.all.arp_accept = 0
net.ipv4.conf.all.arp_announce = 0
net.ipv4.conf.all.arp_filter = 0
net.ipv4.conf.all.arp_ignore = 0
net.ipv4.conf.all.arp_notify = 0
net.ipv4.conf.all.bootp_relay = 0
net.ipv4.conf.all.disable_policy = 0
net.ipv4.conf.all.disable_xfrm = 0
net.ipv4.conf.all.force_igmp_version = 0
net.ipv4.conf.all.forwarding = 0
net.ipv4.conf.all.igmpv2_unsolicited_report_interval = 10000
net.ipv4.conf.all.igmpv3_unsolicited_report_interval = 1000
net.ipv4.conf.all.ignore_routes_with_linkdown = 0
net.ipv4.conf.all.log_martians = 0
net.ipv4.conf.all.mc_forwarding = 0
net.ipv4.conf.all.medium_id = 0
net.ipv4.conf.all.promote_secondaries = 0
net.ipv4.conf.all.proxy_arp = 0
net.ipv4.conf.all.proxy_arp_pvlan = 0
net.ipv4.conf.all.route_localnet = 0
net.ipv4.conf.all.rp_filter = 1
net.ipv4.conf.all.secure_redirects = 1
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.all.shared_media = 1
net.ipv4.conf.all.src_valid_mark = 0
net.ipv4.conf.all.tag = 0
net.ipv4.conf.default.accept_local = 0
net.ipv4.conf.default.accept_redirects = 1
net.ipv4.conf.default.accept_source_route = 1
net.ipv4.conf.default.arp_accept = 0
net.ipv4.conf.default.arp_announce = 0
net.ipv4.conf.default.arp_filter = 0
net.ipv4.conf.default.arp_ignore = 0
net.ipv4.conf.default.arp_notify = 0
net.ipv4.conf.default.bootp_relay = 0
net.ipv4.conf.default.disable_policy = 0
net.ipv4.conf.default.disable_xfrm = 0
net.ipv4.conf.default.force_igmp_version = 0
net.ipv4.conf.default.forwarding = 0
net.ipv4.conf.default.igmpv2_unsolicited_report_interval = 10000
net.ipv4.conf.default.igmpv3_unsolicited_report_interval = 1000
net.ipv4.conf.default.ignore_routes_with_linkdown = 0
net.ipv4.conf.default.log_martians = 0
net.ipv4.conf.default.mc_forwarding = 0
net.ipv4.conf.default.medium_id = 0
net.ipv4.conf.default.promote_secondaries = 0
net.ipv4.conf.default.proxy_arp = 0
net.ipv4.conf.default.proxy_arp_pvlan = 0
net.ipv4.conf.default.route_localnet = 0
net.ipv4.conf.default.rp_filter = 1
net.ipv4.conf.default.secure_redirects = 1
net.ipv4.conf.default.send_redirects = 1
net.ipv4.conf.default.shared_media = 1
net.ipv4.conf.default.src_valid_mark = 0
net.ipv4.conf.default.tag = 0
net.ipv4.conf.enp0s14.accept_local = 0
net.ipv4.conf.enp0s14.accept_redirects = 1
net.ipv4.conf.enp0s14.accept_source_route = 1
net.ipv4.conf.enp0s14.arp_accept = 0
net.ipv4.conf.enp0s14.arp_announce = 0
net.ipv4.conf.enp0s14.arp_filter = 0
net.ipv4.conf.enp0s14.arp_ignore = 0
net.ipv4.conf.enp0s14.arp_notify = 0
net.ipv4.conf.enp0s14.bootp_relay = 0
net.ipv4.conf.enp0s14.disable_policy = 0
net.ipv4.conf.enp0s14.disable_xfrm = 0
net.ipv4.conf.enp0s14.force_igmp_version = 0
net.ipv4.conf.enp0s14.forwarding = 0
net.ipv4.conf.enp0s14.igmpv2_unsolicited_report_interval = 10000
net.ipv4.conf.enp0s14.igmpv3_unsolicited_report_interval = 1000
net.ipv4.conf.enp0s14.ignore_routes_with_linkdown = 0
net.ipv4.conf.enp0s14.log_martians = 0
net.ipv4.conf.enp0s14.mc_forwarding = 0
net.ipv4.conf.enp0s14.medium_id = 0
net.ipv4.conf.enp0s14.promote_secondaries = 0
net.ipv4.conf.enp0s14.proxy_arp = 0
net.ipv4.conf.enp0s14.proxy_arp_pvlan = 0
net.ipv4.conf.enp0s14.route_localnet = 0
net.ipv4.conf.enp0s14.rp_filter = 1
net.ipv4.conf.enp0s14.secure_redirects = 1
net.ipv4.conf.enp0s14.send_redirects = 1
net.ipv4.conf.enp0s14.shared_media = 1
net.ipv4.conf.enp0s14.src_valid_mark = 0
net.ipv4.conf.enp0s14.tag = 0
net.ipv4.conf.lo.accept_local = 0
net.ipv4.conf.lo.accept_redirects = 1
net.ipv4.conf.lo.accept_source_route = 1
net.ipv4.conf.lo.arp_accept = 0
net.ipv4.conf.lo.arp_announce = 0
net.ipv4.conf.lo.arp_filter = 0
net.ipv4.conf.lo.arp_ignore = 0
net.ipv4.conf.lo.arp_notify = 0
net.ipv4.conf.lo.bootp_relay = 0
net.ipv4.conf.lo.disable_policy = 1
net.ipv4.conf.lo.disable_xfrm = 1
net.ipv4.conf.lo.force_igmp_version = 0
net.ipv4.conf.lo.forwarding = 0
net.ipv4.conf.lo.igmpv2_unsolicited_report_interval = 10000
net.ipv4.conf.lo.igmpv3_unsolicited_report_interval = 1000
net.ipv4.conf.lo.ignore_routes_with_linkdown = 0
net.ipv4.conf.lo.log_martians = 0
net.ipv4.conf.lo.mc_forwarding = 0
net.ipv4.conf.lo.medium_id = 0
net.ipv4.conf.lo.promote_secondaries = 0
net.ipv4.conf.lo.proxy_arp = 0
net.ipv4.conf.lo.proxy_arp_pvlan = 0
net.ipv4.conf.lo.route_localnet = 0
net.ipv4.conf.lo.rp_filter = 0
net.ipv4.conf.lo.secure_redirects = 1
net.ipv4.conf.lo.send_redirects = 1
net.ipv4.conf.lo.shared_media = 1
net.ipv4.conf.lo.src_valid_mark = 0
net.ipv4.conf.lo.tag = 0
net.ipv4.conf.tun0.accept_local = 0
net.ipv4.conf.tun0.accept_redirects = 1
net.ipv4.conf.tun0.accept_source_route = 1
net.ipv4.conf.tun0.arp_accept = 0
net.ipv4.conf.tun0.arp_announce = 0
net.ipv4.conf.tun0.arp_filter = 0
net.ipv4.conf.tun0.arp_ignore = 0
net.ipv4.conf.tun0.arp_notify = 0
net.ipv4.conf.tun0.bootp_relay = 0
net.ipv4.conf.tun0.disable_policy = 0
net.ipv4.conf.tun0.disable_xfrm = 0
net.ipv4.conf.tun0.force_igmp_version = 0
net.ipv4.conf.tun0.forwarding = 0
net.ipv4.conf.tun0.igmpv2_unsolicited_report_interval = 10000
net.ipv4.conf.tun0.igmpv3_unsolicited_report_interval = 1000
net.ipv4.conf.tun0.ignore_routes_with_linkdown = 0
net.ipv4.conf.tun0.log_martians = 0
net.ipv4.conf.tun0.mc_forwarding = 0
net.ipv4.conf.tun0.medium_id = 0
net.ipv4.conf.tun0.promote_secondaries = 0
net.ipv4.conf.tun0.proxy_arp = 0
net.ipv4.conf.tun0.proxy_arp_pvlan = 0
net.ipv4.conf.tun0.route_localnet = 0
net.ipv4.conf.tun0.rp_filter = 1
net.ipv4.conf.tun0.secure_redirects = 1
net.ipv4.conf.tun0.send_redirects = 1
net.ipv4.conf.tun0.shared_media = 1
net.ipv4.conf.tun0.src_valid_mark = 0
net.ipv4.conf.tun0.tag = 0
net.ipv4.fwmark_reflect = 0
net.ipv4.icmp_echo_ignore_all = 0
net.ipv4.icmp_echo_ignore_broadcasts = 1
net.ipv4.icmp_errors_use_inbound_ifaddr = 0
net.ipv4.icmp_ignore_bogus_error_responses = 1
net.ipv4.icmp_msgs_burst = 50
net.ipv4.icmp_msgs_per_sec = 1000
net.ipv4.icmp_ratelimit = 1000
net.ipv4.icmp_ratemask = 6168
net.ipv4.igmp_link_local_mcast_reports = 1
net.ipv4.igmp_max_memberships = 20
net.ipv4.igmp_max_msf = 10
net.ipv4.igmp_qrv = 2
net.ipv4.inet_peer_maxttl = 600
net.ipv4.inet_peer_minttl = 120
net.ipv4.inet_peer_threshold = 65664
net.ipv4.ip_default_ttl = 64
net.ipv4.ip_dynaddr = 0
net.ipv4.ip_early_demux = 1
net.ipv4.ip_forward = 0
net.ipv4.ip_forward_use_pmtu = 0
net.ipv4.ip_local_port_range = 32768 60999
net.ipv4.ip_local_reserved_ports =
net.ipv4.ip_no_pmtu_disc = 0
net.ipv4.ip_nonlocal_bind = 0
net.ipv4.ipfrag_high_thresh = 4194304
net.ipv4.ipfrag_low_thresh = 3145728
net.ipv4.ipfrag_max_dist = 64
net.ipv4.ipfrag_secret_interval = 0
net.ipv4.ipfrag_time = 30
net.ipv4.neigh.default.anycast_delay = 100
net.ipv4.neigh.default.app_solicit = 0
net.ipv4.neigh.default.base_reachable_time_ms = 30000
net.ipv4.neigh.default.delay_first_probe_time = 5
net.ipv4.neigh.default.gc_interval = 30
net.ipv4.neigh.default.gc_stale_time = 60
net.ipv4.neigh.default.gc_thresh1 = 128
net.ipv4.neigh.default.gc_thresh2 = 512
net.ipv4.neigh.default.gc_thresh3 = 1024
net.ipv4.neigh.default.locktime = 100
net.ipv4.neigh.default.mcast_resolicit = 0
net.ipv4.neigh.default.mcast_solicit = 3
net.ipv4.neigh.default.proxy_delay = 80
net.ipv4.neigh.default.proxy_qlen = 64
net.ipv4.neigh.default.retrans_time_ms = 1000
net.ipv4.neigh.default.ucast_solicit = 3
net.ipv4.neigh.default.unres_qlen = 31
net.ipv4.neigh.default.unres_qlen_bytes = 65536
net.ipv4.neigh.enp0s14.anycast_delay = 100
net.ipv4.neigh.enp0s14.app_solicit = 0
net.ipv4.neigh.enp0s14.base_reachable_time_ms = 30000
net.ipv4.neigh.enp0s14.delay_first_probe_time = 5
net.ipv4.neigh.enp0s14.gc_stale_time = 60
net.ipv4.neigh.enp0s14.locktime = 100
net.ipv4.neigh.enp0s14.mcast_resolicit = 0
net.ipv4.neigh.enp0s14.mcast_solicit = 3
net.ipv4.neigh.enp0s14.proxy_delay = 80
net.ipv4.neigh.enp0s14.proxy_qlen = 64
net.ipv4.neigh.enp0s14.retrans_time_ms = 1000
net.ipv4.neigh.enp0s14.ucast_solicit = 3
net.ipv4.neigh.enp0s14.unres_qlen = 31
net.ipv4.neigh.enp0s14.unres_qlen_bytes = 65536
net.ipv4.neigh.lo.anycast_delay = 100
net.ipv4.neigh.lo.app_solicit = 0
net.ipv4.neigh.lo.base_reachable_time_ms = 30000
net.ipv4.neigh.lo.delay_first_probe_time = 5
net.ipv4.neigh.lo.gc_stale_time = 60
net.ipv4.neigh.lo.locktime = 100
net.ipv4.neigh.lo.mcast_resolicit = 0
net.ipv4.neigh.lo.mcast_solicit = 3
net.ipv4.neigh.lo.proxy_delay = 80
net.ipv4.neigh.lo.proxy_qlen = 64
net.ipv4.neigh.lo.retrans_time_ms = 1000
net.ipv4.neigh.lo.ucast_solicit = 3
net.ipv4.neigh.lo.unres_qlen = 31
net.ipv4.neigh.lo.unres_qlen_bytes = 65536
net.ipv4.neigh.tun0.anycast_delay = 100
net.ipv4.neigh.tun0.app_solicit = 0
net.ipv4.neigh.tun0.base_reachable_time_ms = 30000
net.ipv4.neigh.tun0.delay_first_probe_time = 5
net.ipv4.neigh.tun0.gc_stale_time = 60
net.ipv4.neigh.tun0.locktime = 100
net.ipv4.neigh.tun0.mcast_resolicit = 0
net.ipv4.neigh.tun0.mcast_solicit = 3
net.ipv4.neigh.tun0.proxy_delay = 80
net.ipv4.neigh.tun0.proxy_qlen = 64
net.ipv4.neigh.tun0.retrans_time_ms = 1000
net.ipv4.neigh.tun0.ucast_solicit = 3
net.ipv4.neigh.tun0.unres_qlen = 31
net.ipv4.neigh.tun0.unres_qlen_bytes = 65536
net.ipv4.ping_group_range = 1 0
net.ipv4.route.error_burst = 1250
net.ipv4.route.error_cost = 250
net.ipv4.route.gc_elasticity = 8
net.ipv4.route.gc_interval = 60
net.ipv4.route.gc_min_interval = 0
net.ipv4.route.gc_min_interval_ms = 500
net.ipv4.route.gc_thresh = -1
net.ipv4.route.gc_timeout = 300
net.ipv4.route.max_size = 2147483647
net.ipv4.route.min_adv_mss = 256
net.ipv4.route.min_pmtu = 552
net.ipv4.route.mtu_expires = 600
net.ipv4.route.redirect_load = 5
net.ipv4.route.redirect_number = 9
net.ipv4.route.redirect_silence = 5120
net.ipv4.tcp_abort_on_overflow = 0
net.ipv4.tcp_adv_win_scale = 1
net.ipv4.tcp_allowed_congestion_control = cubic reno
net.ipv4.tcp_app_win = 31
net.ipv4.tcp_autocorking = 1
net.ipv4.tcp_available_congestion_control = cubic reno
net.ipv4.tcp_base_mss = 1024
net.ipv4.tcp_challenge_ack_limit = 100
net.ipv4.tcp_congestion_control = cubic
net.ipv4.tcp_dsack = 1
net.ipv4.tcp_early_retrans = 3
net.ipv4.tcp_ecn = 2
net.ipv4.tcp_ecn_fallback = 1
net.ipv4.tcp_fack = 1
net.ipv4.tcp_fastopen = 1
net.ipv4.tcp_fastopen_key = 00000000-00000000-00000000-00000000
net.ipv4.tcp_fin_timeout = 15
net.ipv4.tcp_frto = 2
net.ipv4.tcp_fwmark_accept = 0
net.ipv4.tcp_invalid_ratelimit = 500
net.ipv4.tcp_keepalive_intvl = 15
net.ipv4.tcp_keepalive_probes = 5
net.ipv4.tcp_keepalive_time = 300
net.ipv4.tcp_limit_output_bytes = 262144
net.ipv4.tcp_low_latency = 0
net.ipv4.tcp_max_orphans = 32768
net.ipv4.tcp_max_reordering = 300
net.ipv4.tcp_max_syn_backlog = 256
net.ipv4.tcp_max_tw_buckets = 32768
net.ipv4.tcp_mem = 65536 131072 262144
net.ipv4.tcp_min_rtt_wlen = 300
net.ipv4.tcp_min_tso_segs = 2
net.ipv4.tcp_moderate_rcvbuf = 1
net.ipv4.tcp_mtu_probing = 0
net.ipv4.tcp_no_metrics_save = 0
net.ipv4.tcp_notsent_lowat = -1
net.ipv4.tcp_orphan_retries = 0
net.ipv4.tcp_pacing_ca_ratio = 120
net.ipv4.tcp_pacing_ss_ratio = 200
net.ipv4.tcp_probe_interval = 600
net.ipv4.tcp_probe_threshold = 8
net.ipv4.tcp_recovery = 1
net.ipv4.tcp_reordering = 3
net.ipv4.tcp_retrans_collapse = 1
net.ipv4.tcp_retries1 = 3
net.ipv4.tcp_retries2 = 15
net.ipv4.tcp_rfc1337 = 0
net.ipv4.tcp_rmem = 4096 524288 16777216
net.ipv4.tcp_sack = 1
net.ipv4.tcp_slow_start_after_idle = 1
net.ipv4.tcp_stdurg = 0
net.ipv4.tcp_syn_retries = 6
net.ipv4.tcp_synack_retries = 5
net.ipv4.tcp_syncookies = 0
net.ipv4.tcp_thin_dupack = 0
net.ipv4.tcp_thin_linear_timeouts = 0
net.ipv4.tcp_timestamps = 0
net.ipv4.tcp_tso_win_divisor = 3
net.ipv4.tcp_tw_recycle = 0
net.ipv4.tcp_tw_reuse = 0
net.ipv4.tcp_window_scaling = 0
net.ipv4.tcp_wmem = 4096 524288 16777216
net.ipv4.tcp_workaround_signed_windows = 0
net.ipv4.udp_mem = 65536 131072 262144
net.ipv4.udp_rmem_min = 4096
net.ipv4.udp_wmem_min = 4096
net.ipv4.xfrm4_gc_thresh = 2147483647
我已尝试根据这些服务器故障答案做一些事情：
为什么服务器不会发送 SYN/ACK 数据包来响应 SYN 数据包
- 将
net.ipv4.tcp_timestamps和设置net.ipv4.tcp_window_scaling为0
- 混合设置
net.ipv4.tcp_tw_reuse和net.ipv4.tcp_tw_recycle0
然而，这些都无法解决这个问题。
我感觉 VPN 可能以某种方式妨碍了进程，但即使禁用 VPN，我也无法从网络外部访问该进程。
任何想法都将不胜感激！
答案1
我自己找到了解决方案，部分归功于@symcbean 的评论，它引发了我的思考。
事实证明，无论出于何种原因，Linux 都不会将响应数据包路由回它们所连接的接口。为了解决这个问题，我们设置了高级路由。
ip rule add fwmark 1 table mycustomtable
ip route add table mycustomtable 0.0.0.0/0 via 192.168.0.1 dev enp0s14
然后我使用一条iptables规则来标记数据包：
iptables -t mangle -A OUTPUT -j MARK --set-mark 1 -s 192.168.0.0/24 -p tcp --sport 5555
rp_filter令人困惑（和恼人）的是，除非您将sysctl 变量设置为限制较少的值，否则这些都不起作用。我使用的是“2”，但您也可以使用“0”。
sysctl -w net.ipv4.conf.enp0s14.rp_filter=2
希望这能帮助到那些同样处境的人。