简介

今天去定位一个nodeport的问题，发现curl 127.0.0.1:32000 访问nodeport的时候会规律的hang住，本来以为是后端服务的问题，但是curl管理ip:nodeport 是正常的。这个就奇怪了，深入研究了下发现 ipvs模式下是不支持这样访问的，如果想使用 localhost: 得使用iptables模式。

下面是一个歪果仁的解释

After a little dig into the kernel, failing to connect localhost: can be explained.

Assume we are visiting http://127.0.0.1:, every packet will first pass through ip_vs_nat_xmit. Then after some check, it runs to https://github.com/torvalds/linux/blob/v4.18/net/netfilter/ipvs/ip_vs_xmit.c#L756. __ip_vs_get_out_rt is used to search for route to remote server, k8s pods in our case. Take a deeper look at __ip_vs_get_out_rt, after validating route cache or finding route via do_output_route4, it comes to crosses_local_route_boundary to judge whether the searched route can pass cross-local-route-boundary check.

Copy the code of crosses_local_route_boundary here and go deeper.

static inline bool crosses_local_route_boundary(int skb_af, struct sk_buff *skb,
						int rt_mode,
						bool new_rt_is_local)
{
	bool rt_mode_allow_local = !!(rt_mode & IP_VS_RT_MODE_LOCAL);
	bool rt_mode_allow_non_local = !!(rt_mode & IP_VS_RT_MODE_NON_LOCAL);
	bool rt_mode_allow_redirect = !!(rt_mode & IP_VS_RT_MODE_RDR);
	bool source_is_loopback;
	bool old_rt_is_local;

#ifdef CONFIG_IP_VS_IPV6
	/* omit ipv6 */
#endif
	{
		source_is_loopback = ipv4_is_loopback(ip_hdr(skb)->saddr);
		old_rt_is_local = skb_rtable(skb)->rt_flags & RTCF_LOCAL;
	}

	if (unlikely(new_rt_is_local)) {
		if (!rt_mode_allow_local)
			return true;
		if (!rt_mode_allow_redirect && !old_rt_is_local)
			return true;
	} else {
		if (!rt_mode_allow_non_local)
			return true;
		if (source_is_loopback)
			return true;
	}
	return false;
}

For nat mode of IPVS, rt_mode is assigned as IP_VS_RT_MODE_LOCAL | IP_VS_RT_MODE_NON_LOCAL | IP_VS_RT_MODE_RDR as https://github.com/torvalds/linux/blob/v4.18/net/netfilter/ipvs/ip_vs_xmit.c#L757-L759 indicates and new_rt_is_local is 0 due to https://github.com/torvalds/linux/blob/v4.18/net/netfilter/ipvs/ip_vs_xmit.c#L363.

source_is_loopback will be true because source address ip_hdr(skb)->saddr is 127.0.0.1. The five booleans defined in crosses_local_route_boundary will all be true in this case. So we finally fall into here

if (source_is_loopback)
return true;
and never pass the cross-local-route-boundary check.

解决方案

目前看如果想这样使用1是修改内核，这个可能比较难实现，另一个方案是修改路由表，修改源ip的地址，也是一种曲线救国的方式

sudo ip route change 127.0.0.1 dev lo proto  kernel scope host src <node ip> table local

引用

https://github.com/kubernetes/kubernetes/issues/96879
https://github.com/kubernetes/kubernetes/issues/67730
https://serverfault.com/questions/851221/what-is-the-local-routing-table-used-for