我在应用程序中的一个特殊应用程序端口上调用 ::connect() ，一般情况下它工作正常，但是，在两台特定机器之间，从一台机器到另一台机器，它会失败并显示 EHOSTUNREACH，这意味着“没有到主机的路由”。
如果我可以毫无问题地在端口 22 上进行 ssh，那么对于该特定机器对，::connect() 总是失败，这可能会发生什么情况？
以详细模式运行 ssh 会产生：
[localMachine ~] ssh -v -p 22 remoteMachine
OpenSSH_3.9p1, OpenSSL 0.9.7a Feb 19 2003
debug1: Reading configuration data /etc/ssh/ssh_config
debug1: Applying options for *
debug1: Connecting to remoteMachine [10.34.49.107] port 22.
debug1: Connection established.
debug1: identity file /home/WilliamKF/.ssh/identity type -1
debug1: identity file /home/WilliamKF/.ssh/id_rsa type 1
debug1: identity file /home/WilliamKF/.ssh/id_dsa type -1
debug1: Remote protocol version 2.0, remote software version OpenSSH_4.3
debug1: match: OpenSSH_4.3 pat OpenSSH*
debug1: Enabling compatibility mode for protocol 2.0
debug1: Local version string SSH-2.0-OpenSSH_3.9p1
debug1: SSH2_MSG_KEXINIT sent
debug1: SSH2_MSG_KEXINIT received
debug1: kex: server->client aes128-cbc hmac-md5 none
debug1: kex: client->server aes128-cbc hmac-md5 none
debug1: SSH2_MSG_KEX_DH_GEX_REQUEST(1024<1024<8192) sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_GROUP
debug1: SSH2_MSG_KEX_DH_GEX_INIT sent
debug1: expecting SSH2_MSG_KEX_DH_GEX_REPLY
debug1: Host 'remoteMachine' is known and matches the RSA host key.
debug1: Found key in /home/WilliamKF/.ssh/known_hosts:47
debug1: ssh_rsa_verify: signature correct
debug1: SSH2_MSG_NEWKEYS sent
debug1: expecting SSH2_MSG_NEWKEYS
debug1: SSH2_MSG_NEWKEYS received
debug1: SSH2_MSG_SERVICE_REQUEST sent
debug1: SSH2_MSG_SERVICE_ACCEPT received
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Next authentication method: gssapi-with-mic
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Authentications that can continue: publickey,gssapi-with-mic,password
debug1: Next authentication method: publickey
debug1: Trying private key: /home/WilliamKF/.ssh/identity
debug1: Offering public key: /home/WilliamKF/.ssh/id_rsa
debug1: Server accepts key: pkalg ssh-rsa blen 149
debug1: read PEM private key done: type RSA
debug1: Authentication succeeded (publickey).
debug1: channel 0: new [client-session]
debug1: Entering interactive session.
这是客户端的功能：
void // virtual
Sender::connectTCP()
{
// First build the feedback channel's socket & make it reuseable
// so we don't get the nasty message.
if (0 > (setFbSocket(socket(AF_INET, SOCK_STREAM, 0)))) {
THROW_ERR("failed to create the command socket: ");
}
setSocketOptions();
// Build the localIp address and bind it to the feedback socket.
// Although it's not traditional for a client to bind the sending socket
// to a the local address, we do it to prevent connect() from using an
// ephemeral port which (our site's firewall may block). Also build the
// remoteIp address.
buildAddr(getTCPcommandLocalAddr(), getLocalHost().c_str(),
getLocFbPort());
deepBind(getFbSocket(), getTCPcommandLocalAddr());
buildAddr(getTCPcommandRemoteAddr(), getRemoteHost().c_str(),
getRemFbPort());
// Connect to the receiver at the remote addr. Make multiple attempts
// when we get a connection refused errno (ECONNREFUSED). ECONNREFUSED
// means no one is listening at the other end ... which my be the result
// of a race condition (i.e., we're calling connect before the server has
// gotten to listen.)
const int timeoutMinutes = 5;
const int timeoutSeconds = timeoutMinutes * 60;
int conCount = timeoutSeconds;
while ((conCount > 0) &&
(0 > ::connect(getFbSocket(),
(sockaddr*)&getTCPcommandRemoteAddr(),
sizeof(sockaddr)))) {
switch (errno) {
case ECONNREFUSED: {
::sleep(1);
--conCount;
// Warn every 15 seconds, but don't warn at 5 minutes exactly.
if ((conCount % 15) == 0 && conCount) {
clog << "Warning: The server connection"
<< " has been refused for "
<< timeFromSeconds(timeoutSeconds - conCount)
<< ", will continue to retry for up to "
<< timeoutMinutes << " minutes.\n"
<< "Perhaps ports " << getRemFbPort() << " and "
<< getRemDataPort()
<<
" are not being routed properly to the server or alternatively "
"perhaps nothing on the server is listening to those ports?\n";
}
continue;
}
case EHOSTUNREACH: {
clog << "Error: Command connect failed: No route to host '"
<< getRemoteHost() << "'." << endl;
throw;
}
default: {
clog << "Error: Command connect failed: "
<< strerror(errno) << endl;
throw;
}
}
}
if (conCount == 0) {
clog << "Error: Command connect"
<< " continually refused after retrying for " << timeoutMinutes
<< " minutes: "
<< strerror(errno) << endl;
throw;
}
setCmdBlocking();
setDataBlocking();
setFbIsConn(true);
clog << "Application has connected to "
<< getRemoteHost() << ":" << getRemFbPort() << endl;
}
您是否会在目标端口上遇到防火墙过滤？您是否正在尝试连接到端口 22 或 sshd 正在运行的端口？或者其他端口？
看来您正在显式绑定套接字的客户端 - 查看您绑定的地址是否恰好无法从服务器框中访问（例如由于路由问题）。为了能够从潜在的防火墙问题（即防火墙允许端口 22，但拒绝您的应用程序端口）中看出这一点，请尝试 telnet 到目标主机：端口，而不是 ssh 到端口 22。
名称解析有什么奇怪的吗？ （您可能会被主机文件绊倒吗？被 ssh 忽略的 DNS 中的 AAAA (IPv6) 记录绊倒？ssh_config？）可能值得在详细模式下运行 ssh 来查看它连接到哪个主机。