Kubernetes 之 Nameserver limits were exceeded

1、问题描述

最近查看kubernetes 的events，发现了有两个节点经常出现下面的信息：

DNSConfigForming  Nameserver limits were exceeded, some nameservers have been omitted, the applied nameserver line is: 192.96.0.10 10.2.5.1 10.2.5.2

这个是kubelet打出来的，字面意思很好理解，就是nameserver 超出限制，超出的将被忽略。那限制是多少呢？so带着这个疑问就去github kubernetes源码一探究竟。

首先确定知道这个是kubelet里面的events，找到dns的相关源码：kubernetes/pkg/kubelet/network/dns/dns.go

func (c *Configurer) formDNSNameserversFitsLimits(nameservers []string, pod *v1.Pod) []string {
    if len(nameservers) > validation.MaxDNSNameservers {
        nameservers = nameservers[0:validation.MaxDNSNameservers]
        log := fmt.Sprintf("Nameserver limits were exceeded, some nameservers have been omitted, the applied nameserver line is: %s", strings.Join(nameservers, " "))
        c.recorder.Event(pod, v1.EventTypeWarning, "DNSConfigForming", log)
        klog.Error(log)
    }
    return nameservers
}

找到上面的函数，有一个validation.MaxDNSNameservers，所以kubelet读取resolv.conf里面的nameserver是有数量限制的，最大值就是validation.MaxDNSNameservers，我们现在去找validation的源码。在上面的import里面

找到k8s.io/kubernetes/pkg/apis/core/validation，找到validation这个package的源码：

const (
    // Limits on various DNS parameters. These are derived from
    // restrictions in Linux libc name resolution handling.
    // Max number of DNS name servers.
    MaxDNSNameservers = 3
    // Max number of domains in search path.
    MaxDNSSearchPaths = 6
    // Max number of characters in search path.
    MaxDNSSearchListChars = 256
)